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ffe82432-4883-4adc-b03b-937c1baf5090.74 | **2.2 Molecular descriptors**
*Cheminformatics and Its Applications*
Computational exploration of NPs has increased in recent years, giving greater relevance to studies that include structural diversity metrics calculated with parameters based on distances such as Euclidean distance, Manhattan distances, and Cosine distance. Other criteria are based on circular fingerprint (ECFP-4, ECFP-6) [22–24, 38–45] and fingerprint based on substructure (MACCS, PubChem) [22–24, 39–45]. Another metric used in NPs is the comparison by similarity that uses the
*Biological endpoints and targets in which natural products from Panama present bioactivity.*
In this study, the molecular scaffolds of natural products have been obtained using the Murcko method [22–24, 50–57]. Meanwhile, the molecular complexity is
) [23], fraction of chiral centers (FCC) [23], and globularity [22–24, 58–63]. An update of the Natural Products Database from the University of Panama (UPMA) containing 454 compounds (Unpublished data) has been evaluated against different therapeutic targets such as cytotoxicity bioassay in cell lines, antifungal assay in vitro, parasites of tropical diseases (*Leishmania* sp., *Plasmodium falciparum*, and *Trypanosoma cruzi*), and the bioassay against HIV-1 virus, demonstrating an inhibitor effect on protease, reverse transcriptase, nuclear factor NFkappaB, and Tat protein affecting the viral replication. These are the most significant biological targets in which the natural products from Panama present bioactivity. The values
hybridized carbons
Tanimoto index/Tanimoto coefficient [22–24, 45–49].
**natural products from Panama**
**2.1 Preparation curated and processing of data set**
frequently evaluated by descriptors in 2D such as fraction of sp3
of their biological activities are represented as percentages in **Figure 1**.
**2. Application of chemoinformatic antimalarial databases: case of**
In this chapter, we present a chemoinformatic analysis of natural products with antimalarial activities (in vitro), expressed as pIC50 against sensitive and resistant
**84**
(Fsp3
**Figure 1.**
The descriptors of physicochemical properties, hydrogen bond acceptors (HBAs), hydrogen bond donors (HBDs), number of rotatable bonds (NRBs), the octanol/water partition coefficient (logP), topological polar surface area (TPSA),
#### **Table 1.**
*Databases analyzed with chemoinformatic tools.*
and molecular weight (MW), or others such as molar refractivity, are important physicochemical parameters for quantitative structure-activity relationship (QSAR) analysis. These molecular descriptors are based on Lipinski's rule and Verger's rule regarding the prediction of the pharmacological similarity of orally active pharmacological potential [65–67]. The statistical analysis of the physicochemical properties was realized with RStudio Software 1.0.136 AGPL [68].
### **2.3 3D visualization of chemical space of compounds with antimalarial activity**
PCAs were done with MOE software [64], and the dominant characteristics are expressed as covariance and visualized with the corresponding 2D or 3D graphic score plot with DataWarrior program v. 5.0 [69]. **Figures 2**–**8** showed the distribution of different compounds with antimalarial activities in the chemical spaces.
In **Figures 2**–**8** we observed that NPs, drugs, and synthetic compounds occupy, in general, similar chemical space and are overlapping in most of the evaluated databases.
### **2.4 Molecular diversity based on fingerprints**
Three binary molecular fingerprints were calculated with RStudio package rcdk: Extended connectivity fingerprints with diameter 4 (ECFP-4) for similarity searching, molecular access system (MACCS) keys of 166 bits for determining similarity and molecular diversity, and PubChem keys of 881 bits for encoding molecular fragment information [42–44]. The similarity of fingerprints by structural pairs of compounds was calculated with the Tanimoto coefficient and analyzed with the cumulative distribution function (CDF). This approach has been used to calculate, measure, and represent the molecular variety of compound data sets [23].
**Figures 9**–**11** show the CDFs of the pairwise similarity of the different data sets evaluated with Tanimoto coefficient and ECPF-4, MACCS keys, and PubChem fingerprints, respectively.
**87**
**Figure 4.**
**Figure 3.**
*Chemoinformatic Approach: The Case of Natural Products of Panama*
**Figures 9**–**11** provide information on the structural diversity of the six databases. Similar approach has been previously published [23]; the curves obtained with ECFP-4 did not prove to be a suitable fingerprint representation for these data sets. In the three similarity graphs based on fingerprints, it is shown that the database of natural products with antimalarial activity, OMS, and MMV has the
In **Tables 2**–**4**, the statistical values of the pairwise Tanimoto similarity with the data sets analyzed are shown. In these tables, CHEMBL and DrugBank databases
lowest molecular diversity, while GSK DB was the most diverse.
*3D visualization of the chemical space of synthetic compounds.*
are excluded from our analysis, due to the small amount of data.
*3D visualization of the chemical spaces of natural products and GNF DBs.*
*DOI: http://dx.doi.org/10.5772/intechopen.87779*
**Figure 2.**
*3D visualization of the chemical space of natural product databases.*
*Chemoinformatic Approach: The Case of Natural Products of Panama DOI: http://dx.doi.org/10.5772/intechopen.87779*
| doab | 2025-04-07T04:13:04.429006 | 20-4-2021 18:19 | {
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ffe82432-4883-4adc-b03b-937c1baf5090.75 | **Figure 3.**
*Cheminformatics and Its Applications*
and molecular weight (MW), or others such as molar refractivity, are important physicochemical parameters for quantitative structure-activity relationship (QSAR) analysis. These molecular descriptors are based on Lipinski's rule and Verger's rule regarding the prediction of the pharmacological similarity of orally active pharmacological potential [65–67]. The statistical analysis of the physicochemical proper-
**2.3 3D visualization of chemical space of compounds with antimalarial activity**
PCAs were done with MOE software [64], and the dominant characteristics are expressed as covariance and visualized with the corresponding 2D or 3D graphic score plot with DataWarrior program v. 5.0 [69]. **Figures 2**–**8** showed the distribution of different compounds with antimalarial activities in the chemical spaces. In **Figures 2**–**8** we observed that NPs, drugs, and synthetic compounds occupy, in general, similar chemical space and are overlapping in most of the evaluated databases.
Three binary molecular fingerprints were calculated with RStudio package rcdk: Extended connectivity fingerprints with diameter 4 (ECFP-4) for similarity searching, molecular access system (MACCS) keys of 166 bits for determining similarity and molecular diversity, and PubChem keys of 881 bits for encoding molecular fragment information [42–44]. The similarity of fingerprints by structural pairs of compounds was calculated with the Tanimoto coefficient and analyzed with the cumulative distribution function (CDF). This approach has been used to calculate,
**Figures 9**–**11** show the CDFs of the pairwise similarity of the different data sets evaluated with Tanimoto coefficient and ECPF-4, MACCS keys, and PubChem
measure, and represent the molecular variety of compound data sets [23].
ties was realized with RStudio Software 1.0.136 AGPL [68].
**2.4 Molecular diversity based on fingerprints**
fingerprints, respectively.
**86**
**Figure 2.**
*3D visualization of the chemical space of natural product databases.*
*3D visualization of the chemical space of synthetic compounds.*
**Figures 9**–**11** provide information on the structural diversity of the six databases. Similar approach has been previously published [23]; the curves obtained with ECFP-4 did not prove to be a suitable fingerprint representation for these data sets. In the three similarity graphs based on fingerprints, it is shown that the database of natural products with antimalarial activity, OMS, and MMV has the lowest molecular diversity, while GSK DB was the most diverse.
In **Tables 2**–**4**, the statistical values of the pairwise Tanimoto similarity with the data sets analyzed are shown. In these tables, CHEMBL and DrugBank databases are excluded from our analysis, due to the small amount of data.
*3D visualization of the chemical spaces of natural products and GNF DBs.*
**Figure 5.** *3D visualization of the chemical spaces of natural products and TCMDC DBs.*
#### **Figure 6.**
*3D visualization of the chemical spaces of natural products and DBK DBs.*
| doab | 2025-04-07T04:13:04.429247 | 20-4-2021 18:19 | {
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ffe82432-4883-4adc-b03b-937c1baf5090.76 | **2.5 Molecular scaffolds: content and diversity**
| doab | 2025-04-07T04:13:04.429313 | 20-4-2021 18:19 | {
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ffe82432-4883-4adc-b03b-937c1baf5090.77 | *2.5.1 Scaffold content*
Murcko scaffolds were calculated with the program Molecular Equivalent Indices (MEQI) [50, 51] and DataWarrior program [69]. MEQI has been used to obtain the codes corresponding to the chemotypes most frequently analyzed in the databases. [23, 45, 52–55]. The distribution and diversity of the molecular scaffolds present in the data sets were calculated and analyzed using the cyclic system
**89**
**Figure 8.**
*3D visualization of the chemical spaces of all databases.*
**Figure 7.**
*Chemoinformatic Approach: The Case of Natural Products of Panama*
retrieval (CSR) curves [42]. These curves were obtained by plotting the fraction of scaffold and the fraction of compounds that contain cyclic systems [43, 44]. **Table 5** indicates that the MMV DB (0.491) was the most diverse in scaffold content taken as reference the F50 values compared to the data set from GSK (0.183), NPs (0.168), and GNF (0.161), respectively. CSR curves on **Figure 12** further confirm the relative scaffold variety of the eight databases. The analysis of area under curve (AUC) metrics associated with the CSR curves is reported in **Table 5**. The CSR curves showed that MMV has more variety in scaffold content with AUC value of 0.507. In contrast OSM, NPs, GNF, GSK, St. Jude, and CHEMBL were the least diverse (e.g., AUC scores of 0.745, 0.712, 0.705, 0.698, 0.655 and 0.607,
*3D visualization of the chemical spaces of natural products, OSM and St. Jude.*
*DOI: http://dx.doi.org/10.5772/intechopen.87779*
*Chemoinformatic Approach: The Case of Natural Products of Panama DOI: http://dx.doi.org/10.5772/intechopen.87779*
**Figure 7.**
*Cheminformatics and Its Applications*
**88**
*2.5.1 Scaffold content*
**Figure 6.**
**Figure 5.**
**2.5 Molecular scaffolds: content and diversity**
*3D visualization of the chemical spaces of natural products and DBK DBs.*
*3D visualization of the chemical spaces of natural products and TCMDC DBs.*
Murcko scaffolds were calculated with the program Molecular Equivalent Indices (MEQI) [50, 51] and DataWarrior program [69]. MEQI has been used to obtain the codes corresponding to the chemotypes most frequently analyzed in the databases. [23, 45, 52–55]. The distribution and diversity of the molecular scaffolds present in the data sets were calculated and analyzed using the cyclic system
*3D visualization of the chemical spaces of natural products, OSM and St. Jude.*
retrieval (CSR) curves [42]. These curves were obtained by plotting the fraction of scaffold and the fraction of compounds that contain cyclic systems [43, 44].
**Table 5** indicates that the MMV DB (0.491) was the most diverse in scaffold content taken as reference the F50 values compared to the data set from GSK (0.183), NPs (0.168), and GNF (0.161), respectively. CSR curves on **Figure 12** further confirm the relative scaffold variety of the eight databases. The analysis of area under curve (AUC) metrics associated with the CSR curves is reported in **Table 5**. The CSR curves showed that MMV has more variety in scaffold content with AUC value of 0.507. In contrast OSM, NPs, GNF, GSK, St. Jude, and CHEMBL were the least diverse (e.g., AUC scores of 0.745, 0.712, 0.705, 0.698, 0.655 and 0.607,
**Figure 8.**
*3D visualization of the chemical spaces of all databases.*
respectively). The CSR curves provide information on the diversity of the most frequent scaffolds in all databases.
## *2.5.2 Shannon entropy (SE) and scaled Shannon entropy (SSE)*
The Shannon entropy has been adapted to measure the scaffold diversity based on the (**N**) number of most recurrent scaffolds [70]. The scaled Shannon entropy is a normalized value that measures the most common chemotypes present in a
**Figure 9.**
*Curve for cumulative frequency distribution (CFD) based on ECFP-4.*
**Figure 10.** *Curve for cumulative frequency distribution based on MACCS keys.*
#### **Figure 11.**
*Curve for cumulative frequency distribution based on PubChem.*
database. Thus, SSE closer to 1 indicates higher scaffold diversity, while SSE closer to zero (0) indicates lower diversity. In this study, we calculated the SSE for values ranging from **N** = 10 to **N** = 40.
**91**
**Table 3.**
**Figure 12.**
**Table 2.**
*Chemoinformatic Approach: The Case of Natural Products of Panama*
*Cyclic system retrieval curves for all databases evaluated in this study.*
*The statistical values of the similarity of the Tanimoto coefficient with ECFP-4.*
**Similarity ECFP-4/Tanimoto coefficient**
**Similarity MACCS keys/Tanimoto coefficient**
**Figure 13** shows a histogram with the distribution of the 40 most populated scaffolds in NPAs. The histogram includes the corresponding chemotype code. The comparison of the scaffolds of the NPAs allowed the identification of the 68MBD
**DBs Min. 1st Qu. Median Mean 3rd Qu. Max.** GSK 0.07813 0.25682 0.33333 0.37009 0.45581 0.92683 NPs 0.00000 0.34426 0.43636 0.44673 0.54545 1.00000 OSM 0.00000 0.34483 0.43636 0.44693 0.54545 1.00000 MMV 0.00000 0.34483 0.43636 0.44677 0.54412 1.00000 ST JUDE 0.00000 0.33333 0.41250 0.42313 0.50000 1.00000 GNF 0.00000 0.31746 0.39437 0.39999 0.47619 1.00000
**DBs Min. 1st Qu. Median Mean 3rd Qu. Max.** GSK 0.01724 0.05789 0.08844 0.11490 0.12245 0.82353 NPs 0.00000 0.07826 0.09910 0.10565 0.12389 1.00000 OSM 0.00000 0.07826 0.09917 0.10607 0.12397 1.00000 MMV 0.00000 0.07826 0.09924 0.10615 0.12403 1.00000 ST JUDE 0.00000 0.08197 0.10345 0.10980 0.12857 1.00000 GNF 0.00000 0.08209 0.10345 0.10772 0.12739 1.00000
chemotype as one of the most active compounds in this database.
*The statistical values of the similarity of the Tanimoto coefficient with MACCS keys.*
*DOI: http://dx.doi.org/10.5772/intechopen.87779*
*Chemoinformatic Approach: The Case of Natural Products of Panama DOI: http://dx.doi.org/10.5772/intechopen.87779*
#### **Figure 12.** *Cyclic system retrieval curves for all databases evaluated in this study.*
#### **Table 2.**
*Cheminformatics and Its Applications*
frequent scaffolds in all databases.
respectively). The CSR curves provide information on the diversity of the most
The Shannon entropy has been adapted to measure the scaffold diversity based on the (**N**) number of most recurrent scaffolds [70]. The scaled Shannon entropy is a normalized value that measures the most common chemotypes present in a
database. Thus, SSE closer to 1 indicates higher scaffold diversity, while SSE closer to zero (0) indicates lower diversity. In this study, we calculated the SSE for values
*2.5.2 Shannon entropy (SE) and scaled Shannon entropy (SSE)*
**90**
**Figure 11.**
**Figure 10.**
**Figure 9.**
ranging from **N** = 10 to **N** = 40.
*Curve for cumulative frequency distribution based on MACCS keys.*
*Curve for cumulative frequency distribution (CFD) based on ECFP-4.*
*Curve for cumulative frequency distribution based on PubChem.*
*The statistical values of the similarity of the Tanimoto coefficient with ECFP-4.*
#### **Table 3.**
*The statistical values of the similarity of the Tanimoto coefficient with MACCS keys.*
**Figure 13** shows a histogram with the distribution of the 40 most populated scaffolds in NPAs. The histogram includes the corresponding chemotype code. The comparison of the scaffolds of the NPAs allowed the identification of the 68MBD chemotype as one of the most active compounds in this database.
#### **Table 4.**
*The statistical values of the similarity of the Tanimoto coefficient with PubChem.*
*M = number of molecules in the BD, N = number of chemotypes or substructures, FN/M = chemotype diversity fraction, NSING = singleton number, FNSING/M = singleton fraction between total molecules, FNSING/N = fraction of singleton among total chemotypes, AUC = area under the curve, F50 = fraction of chemotype required to recover 50% of the molecules.*
#### **Table 5.**
*Summary of the scaffold diversity of the eight databases analyzed in this work.*
| doab | 2025-04-07T04:13:04.429338 | 20-4-2021 18:19 | {
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"title": "Cheminformatics and its Applications",
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ffe82432-4883-4adc-b03b-937c1baf5090.78 | *2.5.3 Molecular complexity and flexibility*
The structural descriptors used to quantify fraction of sp3 hybridized carbons (Fsp3 ) [23, 58, 63, 70], fraction of chiral centers (CCF) [23, 59, 63, 70], fraction of aromatic atoms (Faro-atm), globularity [60], principal moments of inertia (PMI), normalized principal moments of inertia ratio (NRP) [61, 62], molecular complexity, shape index of Kier, and molecular flexibility were calculated with DataWarrior program [69] and MOE 2018.0101 [64]. **Figures 14**–**19** showed the descriptors utilized to evaluate the complexity and the molecular flexibility.
**Tables 6**–**8** summarize the statistics of the distribution of Fsp3 , FCC, and Faroatm of NPs and reference data sets. These results indicate that the NP data set has the largest complexity molecular in Fsp3 (0.63) and CCF (0.16) and a low distribution of Faro-atm (0.67–0.78). In contrast, GNF, MMV, St. Jude, and GSK DBs are very similar in these three metrics with values between 0.25 and 0.37, 0.27 and 0.37, and 0.014 and 0.025, respectively. In contrast, the structural flexibility was evaluated with the index of form presenting all databases in the range of 0.41–0.58 indicating that many of the compounds present sphericity and intermediate molecular flexibility (data not presented).
**93**
**Figure 15.**
**Figure 13.**
**Figure 14.**
*Distribution of the fraction of sp3*
*Chemoinformatic Approach: The Case of Natural Products of Panama*
The descriptors globularity, PMI, and NRP did not prove to be suitable metrics to measure and differentiate the molecular complexity in the data sets evaluated. This is because the corresponding values computed for all data sets were very low
*Distribution of the fraction of chiral centers in different databases.*
*Scaled Shannon entropy of the most frequent scaffolds with values ranging from 10 to 40 in natural products.*
*hybridized carbons in different databases.*
*DOI: http://dx.doi.org/10.5772/intechopen.87779*
*Chemoinformatic Approach: The Case of Natural Products of Panama DOI: http://dx.doi.org/10.5772/intechopen.87779*
*Cheminformatics and Its Applications*
**Similarity PubChem/Tanimoto coefficient**
**DBs Number of**
**Table 4.**
**Compounds (M)**
**Unique chemotypes (N)**
*The statistical values of the similarity of the Tanimoto coefficient with PubChem.*
*2.5.3 Molecular complexity and flexibility*
*chemotype required to recover 50% of the molecules.*
the largest complexity molecular in Fsp3
flexibility (data not presented).
The structural descriptors used to quantify fraction of sp3
*Summary of the scaffold diversity of the eight databases analyzed in this work.*
inertia (PMI), normalized principal moments of inertia ratio (NRP)
*M = number of molecules in the BD, N = number of chemotypes or substructures, FN/M = chemotype diversity fraction, NSING = singleton number, FNSING/M = singleton fraction between total molecules, FNSING/N = fraction of singleton among total chemotypes, AUC = area under the curve, F50 = fraction of*
**Tables 6**–**8** summarize the statistics of the distribution of Fsp3
) [23, 58, 63, 70], fraction of chiral centers (CCF) [23, 59, 63, 70],
**FN/M NSING FNSING/M FNSING/**
NPs 1298 629 0.4846 400 0.3082 0.6359 0.7125 0.1685 DBK 5 5 1.0000 5 1.0000 1.0000 0.4800 0.4000 CHEMBL 24 18 0.7500 16 0.6667 0.8889 0.6072 0.3333 OSM 89 39 0.4382 27 0.3034 0.6923 0.7453 0.1025 MMV 124 122 0.9839 120 0.9677 0.9836 0.5079 0.4918 St. JUDE 915 479 0.5235 325 0.3552 0.6785 0.6551 0.2474 GNF 4860 3229 0.6644 2690 0.5535 0.8331 0.7054 0.1615 GSK 12,463 6703 0.5378 5009 0.4019 0.7473 0.6982 0.1837
**DBs Min. 1st Qu. Median Mean 3rd Qu. Max.** GSK 0.08125 0.24500 0.37555 0.40263 0.54002 1.00000 NPs 0.03684 0.32298 0.43802 0.46184 0.58621 1.00000 OSM 0.03684 0.32340 0.43902 0.46253 0.58730 1.00000 MMV 0.03684 0.32444 0.44033 0.46321 0.58791 1.00000 ST JUDE 0.03684 0.38224 0.47143 0.47624 0.56195 1.00000 GNF 0.00000 0.40598 0.48117 0.47800 0.55446 1.00000
**NS**
**AUC F50**
fraction of aromatic atoms (Faro-atm), globularity [60], principal moments of
[61, 62], molecular complexity, shape index of Kier, and molecular flexibility were calculated with DataWarrior program [69] and MOE 2018.0101 [64]. **Figures 14**–**19** showed the descriptors utilized to evaluate the complexity and
atm of NPs and reference data sets. These results indicate that the NP data set has
tion of Faro-atm (0.67–0.78). In contrast, GNF, MMV, St. Jude, and GSK DBs are very similar in these three metrics with values between 0.25 and 0.37, 0.27 and 0.37, and 0.014 and 0.025, respectively. In contrast, the structural flexibility was evaluated with the index of form presenting all databases in the range of 0.41–0.58 indicating that many of the compounds present sphericity and intermediate molecular
hybridized car-
, FCC, and Faro-
(0.63) and CCF (0.16) and a low distribu-
**92**
bons (Fsp3
**Table 5.**
the molecular flexibility.
**Figure 13.** *Scaled Shannon entropy of the most frequent scaffolds with values ranging from 10 to 40 in natural products.*
**Figure 14.** *Distribution of the fraction of sp3 hybridized carbons in different databases.*
#### **Figure 15.**
*Distribution of the fraction of chiral centers in different databases.*
The descriptors globularity, PMI, and NRP did not prove to be suitable metrics to measure and differentiate the molecular complexity in the data sets evaluated. This is because the corresponding values computed for all data sets were very low
**Figure 16.**
*Distribution of the fraction of aromatic atoms (Faro-atm) in different databases.*
#### **Figure 17.**
*Shape index distribution of different databases.*
#### **Figure 18.**
*Distribution of the molecular flexibility in different databases.*
**95**
**Table 8.**
similar metrics [23, 63, 71].
*Distribution of fraction of aromatic atoms.*
*Chemoinformatic Approach: The Case of Natural Products of Panama*
**)**
**DBs Min 1qst median mean 3qrt max dev.st** NPs 0.000 0.481 0.636 0.656 0.833 2.000 0.254 CHEMBL 0.167 0.342 0.536 0.621 0.627 1.333 0.374 MMV 0.000 0.167 0.300 0.316 0.402 0.800 0.190 OSM 0.000 0.174 0.255 0.277 0.338 0.893 0.145 DBK 0.250 0.438 0.519 0.463 0.545 0.565 0.175 GNF 0.000 0.227 0.364 0.377 0.500 2.667 0.207 STJUDE 0.000 0.222 0.333 0.353 0.471 1.136 0.178 GSK 0.000 0.250 0.375 0.372 0.500 1.500 0.180
**DBs min 1qst median mean 3qrt max dev.st** NPs 0.000 0.033 0.139 0.161 0.267 0.656 0.145 CHEMBL 0.000 0.000 0.036 0.128 0.141 0.533 0.192 MMV 0.000 0.000 0.000 0.014 0.000 0.111 0.028 OSM 0.000 0.000 0.000 0.008 0.000 0.286 0.035 DBK 0.000 0.000 0.019 0.020 0.040 0.043 0.024 GNF 0.000 0.000 0.000 0.025 0.040 0.556 0.053 STJUDE 0.000 0.000 0.000 0.024 0.045 0.217 0.037 GSK 0.000 0.000 0.000 0.017 0.034 0.500 0.033
**DBs min 1qst median mean 3qrt max dev.st** NPs 0.000 0.000 0.324 0.341 0.600 1.133 0.294 CHEMBL 0.000 0.299 0.556 0.509 0.690 1.091 0.321 MMV 0.261 0.682 0.826 0.817 0.956 1.429 0.230 OSM 0.000 0.677 0.733 0.786 0.860 1.500 0.232 DBK 0.538 0.591 0.733 0.720 0.862 0.875 0.171 GNF 0.000 0.522 0.667 0.670 0.818 1.714 0.235 STJUDE 0.000 0.553 0.712 0.708 0.857 1.556 0.216 GSK 0.000 0.571 0.706 0.713 0.857 1.400 0.208
(close to zero) and did not differentiate the data sets (data not shown). The large molecular complexity of NPs measured is in agreement with previous studies using
*DOI: http://dx.doi.org/10.5772/intechopen.87779*
**hybridized atoms (Fsp3**
*in different databases.*
**Fraction of chiral centers (CCF)**
*Distribution of FCC in different databases.*
**Fraction of aromatic atoms (Faro-atm)**
**Fraction of sp3**
**Table 6.** *Distribution of Fsp3*
**Table 7.**
**Figure 19.**
*Distribution of the molecular complexity in different databases.*
### *Chemoinformatic Approach: The Case of Natural Products of Panama DOI: http://dx.doi.org/10.5772/intechopen.87779*
#### **Table 6.**
*Cheminformatics and Its Applications*
*Distribution of the fraction of aromatic atoms (Faro-atm) in different databases.*
**Figure 16.**
**Figure 17.**
**Figure 18.**
*Shape index distribution of different databases.*
*Distribution of the molecular flexibility in different databases.*
*Distribution of the molecular complexity in different databases.*
**94**
**Figure 19.**
*Distribution of Fsp3 in different databases.*
#### **Table 7.**
*Distribution of FCC in different databases.*
#### **Table 8.**
*Distribution of fraction of aromatic atoms.*
(close to zero) and did not differentiate the data sets (data not shown). The large molecular complexity of NPs measured is in agreement with previous studies using similar metrics [23, 63, 71].
| doab | 2025-04-07T04:13:04.429736 | 20-4-2021 18:19 | {
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} |
ffe82432-4883-4adc-b03b-937c1baf5090.79 | **3. Activity landscape modeling**
The methods of modeling the landscape based on properties of the compounds (property landscape modeling (PLM)) is at the interface between experimental sciences and computational chemistry, being a frequent strategy to systematically describe the structure-property relationships (SPR) of the compound data set [72]. PLM have been used in medicinal chemistry in the stages of drug discovery with a quantitative, descriptive, and statistical approach to activity cliffs [72–74]. Structure-activity relationships (SARs), using the concept of modeling the activity landscape (activity landscape modeling ALM), are an increasing common practice in the drug discovery process to identify the activity cliffs, guide the optimization of compound hits, and to avoid the deleterious effects of the activity cliffs in the studies of the classic models of QSAR and in the search of structural similarity. In this
**Figure 20.** *Structural similarity compared with activity cliffs in NPAs.*
**97**
**Figure 22.**
*plotter.*
*Chemoinformatic Approach: The Case of Natural Products of Panama*
research we analyze, through the web tool Activity Landscape Plotter (ALP) [72], a set of data from NPs from Panama with antimalarial activity against four strains of
*SAS maps of compounds with antimalarial activity ((a), (b), and (c)) through the web tool activity landscape*
*DOI: http://dx.doi.org/10.5772/intechopen.87779*
**Figure 21.**
*Structural similarity compared with activity cliffs in GSK and Novartis (GNF).*
*Cheminformatics and Its Applications*
**3. Activity landscape modeling**
The methods of modeling the landscape based on properties of the compounds (property landscape modeling (PLM)) is at the interface between experimental sciences and computational chemistry, being a frequent strategy to systematically describe the structure-property relationships (SPR) of the compound data set [72]. PLM have been used in medicinal chemistry in the stages of drug discovery with a quantitative, descriptive, and statistical approach to activity cliffs [72–74]. Structure-activity relationships (SARs), using the concept of modeling the activity landscape (activity landscape modeling ALM), are an increasing common practice in the drug discovery process to identify the activity cliffs, guide the optimization of compound hits, and to avoid the deleterious effects of the activity cliffs in the studies of the classic models of QSAR and in the search of structural similarity. In this
**96**
**Figure 21.**
**Figure 20.**
*Structural similarity compared with activity cliffs in NPAs.*
*Structural similarity compared with activity cliffs in GSK and Novartis (GNF).*
research we analyze, through the web tool Activity Landscape Plotter (ALP) [72], a set of data from NPs from Panama with antimalarial activity against four strains of
**Figure 22.**
*SAS maps of compounds with antimalarial activity ((a), (b), and (c)) through the web tool activity landscape plotter.*
*Plasmodium falciparum* in the erythrocyte gametocyte stage (**Figures 20** and **24**).
The generation and comparison of structure-activity pairs, by structure-activity similarity maps (SAS map). The SAS map has been used to link up structure and biological activity, based on a systematic pairwise comparison of all the compounds in a data set analyzed. We compare the values of structure-activity similarity, the activity difference, and structure-activity landscape index (SALI) to find the pairs of compounds with high molecular similarity and the activity difference that are located in the upper right quadrant of the SAS map (activity cliffs) [72–76]. **Figures 17**–**21** show SAS map in NP of Panama, NP published, GSK, and GNF. In SAS maps, data points are colored by density (**Figure 22**).
The SAS maps using the molecular fingerprints EFCP-4, MACCS keys, and PubChem led to the identification of a total of 26 pairs of compounds with structure-activity similarity ratios >0.50 and structure-activity landscape index values varying between 0.3 and 5.0. The web application Activity Landscape Plotter [72] is a tool that allows us to perform QSAR. The SAS generated represent 55 natural products isolated in Panama with antimalarial activity which were analyzed and compared the biological activities against strains of *Plasmodium falciparum* sensitive, resistant and multiresistant. The analysis with the parameters the (SAS / Tanimoto index / ECFP-4), a total of twenty-six pairs of compounds showed similarity values greater than 70%, sixteen pairs greater than 80% and only two pairs of compounds gave a similarity greater than 85%. While with activity cliffs, only three pairs of compounds show structural similarity correlated with the values of pIC50 activity [72, 77].
SAS maps are color-coded according to their intensity and we observe that most pairs of compounds with antimalarial activity show an intense red color. A nalyzed are located in the region of little structural similarity, indicating that the natural products have high structural diversity and low difference in activity, attributed to having similar functional groups in their molecules.
**99**
interest.
*Chemoinformatic Approach: The Case of Natural Products of Panama*
DAS maps represent the pairwise activity differences for each possible pair of compounds in an evaluated data set, against two biological targets. These maps permitted to differentiate if a structural modification can increase or decrease the
With this web application, we have carried out a QSAR study in a fast, simple, and easily interpretable way, obtaining three natural products as leading computational compounds for their optimization as *Plasmodium falciparum* blockers,
The chemoinformatic analysis of the 20,364 compounds (1312 NPs and 19,052 synthetic (MMV, OSM, GNF, St. Jude, GSK, CHEMBL, and DrugBank)) indicates that so many natural products and synthetic products (S) share the same chemical space showing molecules that have similar structural properties. NPs present a greater diversity based on fingerprint than the synthetic compounds. Also,
and greater complexity, while synthetic products contain a greater proportion of aromatic atoms. Finally, concerning the properties related to cyclicity, relative shape, and flexibility, all have very similar values, which could explain the antimalarial activity of computationally determined compound hits in this work against *Plasmodium falciparum*-sensitive (3D7, D6, poW, D10) and chloroquine-resistant
The DAO acknowledges the SNI 2018 awards from SENACYT of Panama.
The authors declare that there are no financial or commercial conflicts of
hybridization
*DOI: http://dx.doi.org/10.5772/intechopen.87779*
activity under one target or other (**Figure 23**).
*Antimalarial compounds in NPs from Panama.*
**4. Conclusion**
**Figure 24.**
strains (W2, Dd).
**Acknowledgements**
**Conflict of interest**
which exhibit a gametocidal activity [78] (**Figure 24**).
NPs have a higher proportion of chiral carbons and atoms with sp3
**Figure 23.** *DAS map with MACCS key fingerprint.*
*Chemoinformatic Approach: The Case of Natural Products of Panama DOI: http://dx.doi.org/10.5772/intechopen.87779*
**Figure 24.** *Antimalarial compounds in NPs from Panama.*
DAS maps represent the pairwise activity differences for each possible pair of compounds in an evaluated data set, against two biological targets. These maps permitted to differentiate if a structural modification can increase or decrease the activity under one target or other (**Figure 23**).
With this web application, we have carried out a QSAR study in a fast, simple, and easily interpretable way, obtaining three natural products as leading computational compounds for their optimization as *Plasmodium falciparum* blockers, which exhibit a gametocidal activity [78] (**Figure 24**).
### **4. Conclusion**
*Cheminformatics and Its Applications*
*Plasmodium falciparum* in the erythrocyte gametocyte stage (**Figures 20** and **24**). The generation and comparison of structure-activity pairs, by structure-activity similarity maps (SAS map). The SAS map has been used to link up structure and biological activity, based on a systematic pairwise comparison of all the compounds in a data set analyzed. We compare the values of structure-activity similarity, the activity difference, and structure-activity landscape index (SALI) to find the pairs of compounds with high molecular similarity and the activity difference that are located in the upper right quadrant of the SAS map (activity cliffs) [72–76]. **Figures 17**–**21** show SAS map in NP of Panama, NP published, GSK, and GNF. In
The SAS maps using the molecular fingerprints EFCP-4, MACCS keys, and PubChem led to the identification of a total of 26 pairs of compounds with structure-activity similarity ratios >0.50 and structure-activity landscape index values varying between 0.3 and 5.0. The web application Activity Landscape Plotter [72] is a tool that allows us to perform QSAR. The SAS generated represent 55 natural products isolated in Panama with antimalarial activity which were analyzed and compared the biological activities against strains of *Plasmodium falciparum* sensitive, resistant and multiresistant. The analysis with the parameters the (SAS / Tanimoto index / ECFP-4), a total of twenty-six pairs of compounds showed similarity values greater than 70%, sixteen pairs greater than 80% and only two pairs of compounds gave a similarity greater than 85%. While with activity cliffs, only three pairs of compounds show structural similarity correlated with the values of pIC50 activity
SAS maps are color-coded according to their intensity and we observe that most pairs of compounds with antimalarial activity show an intense red color. A nalyzed are located in the region of little structural similarity, indicating that the natural products have high structural diversity and low difference in activity, attributed to
SAS maps, data points are colored by density (**Figure 22**).
having similar functional groups in their molecules.
**98**
**Figure 23.**
*DAS map with MACCS key fingerprint.*
[72, 77].
The chemoinformatic analysis of the 20,364 compounds (1312 NPs and 19,052 synthetic (MMV, OSM, GNF, St. Jude, GSK, CHEMBL, and DrugBank)) indicates that so many natural products and synthetic products (S) share the same chemical space showing molecules that have similar structural properties. NPs present a greater diversity based on fingerprint than the synthetic compounds. Also, NPs have a higher proportion of chiral carbons and atoms with sp3 hybridization and greater complexity, while synthetic products contain a greater proportion of aromatic atoms. Finally, concerning the properties related to cyclicity, relative shape, and flexibility, all have very similar values, which could explain the antimalarial activity of computationally determined compound hits in this work against *Plasmodium falciparum*-sensitive (3D7, D6, poW, D10) and chloroquine-resistant strains (W2, Dd).
### **Acknowledgements**
The DAO acknowledges the SNI 2018 awards from SENACYT of Panama.
### **Conflict of interest**
The authors declare that there are no financial or commercial conflicts of interest.
*Cheminformatics and Its Applications*
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ffe82432-4883-4adc-b03b-937c1baf5090.80 | **Author details**
Dionisio A. Olmedo1 \* and José L. Medina-Franco2
1 CIFLORPAN Center for Pharmacognostic Research on Panamanian Flora, College of Pharmacy, University of Panama, Panama City, Panama
2 DIFACQUIM Research Group, Department of Pharmacy, School of Chemistry, National Autonomous University of Mexico (UNAM), Mexico City, Mexico
\*Address all correspondence to: [email protected]
© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
**101**
*Chemoinformatic Approach: The Case of Natural Products of Panama*
Acta. 2013;**1830**:3670-3695. DOI: 10.1016/j.bbagen.2013.02.008
Diversity-oriented asymmetric synthesis. Synthesis. 2014;**46**:2099- 2121. DOI: 10.1055/s-0033-1341247
[11] van Hattum H, Waldmann H. Biology-oriented synthesis: Harnessing the power of evolution. Journal of the American Chemical Society. 2014;**136**:11853-11859. DOI: 10.1021/
[12] Welsch ME, Snyder SA, Stockwell BR. Privileged scaffolds for library design and drug discovery. Current Opinion in Chemical Biology. 2010;**14**:347-361. DOI: 10.1016/j.cbpa
[13] Wetzel S, Bon RS, Kumar K, Waldmann H. Biology-oriented synthesis. Angewandte Chemie (International Ed. in English). 2011;**50**:10800-10826. DOI: 10.1002/
[14] Ertl P, Roggo R, Schuffenhauer A, Natural Product-likeness A. Score and its application for prioritization of compound libraries. Journal of Chemical Information and Modeling.
2008;**48**:68-74. DOI: 10.1021/
[15] Mang C, Jakupovic S, Schunk S, Ambrosi H-D, Schwarz O, Jakupovic J. Natural products in combinatorial chemistry: An andrographolide-based library. Journal of Combinatorial Chemistry. 2006;**8**(2):268-274. DOI:
[16] Wach JY, Gademann K. Reduce to the maximum: Truncated natural products as powerful modulators of biological processes. Synlett. 2012;**23**: 163-170. DOI: 10.1055/s-0031-1290125
[17] Feher M, Schmidt JM. Property distribution: Differences between
ja505861d
anie.201007004
ci700286x
10.1021/cc050143n
[10] Sen S, Prabhu G, Bathula C, Hati S.
*DOI: http://dx.doi.org/10.5772/intechopen.87779*
[1] Newman DJ, Cragg GM. Natural products as sources of new drugs from 1981 to 2014. Journal of Natural Products. 2016;**9**:629-661. DOI: 10.1021/
[2] Newman DJ, Cragg GM. Natural products as sources of new drugs over the 30 years from 1981 to 2010. Journal of Natural Products. 2016;**75**:311-335.
[3] Newman DJ, Cragg GM. Natural products as sources of new drugs over the last 25 years. Journal of Natural Products. 2007;**70**:461-477. DOI:
[4] Thomford NE, Senthebane DA, Rowe A, Munro D, Seele P, Maroyi A, et al. Natural products for drug discovery in the 21st century:
Innovations for novel drug discovery. International Journal of Molecular Sciences. 2018;**19**:1578. DOI: 10.3390/
[5] Gurnani N, Mehta D, Gupta M, Mehta BK. Natural products: Source of potential drugs. African Journal of Basic & Applied Sciences. 2014;**6**:171-186. DOI: 10.5829/idosi.ajbas.2014.6.6.21983
[6] Hong J. Role of natural product diversity in chemical biology. Current
[7] Schreiber SL. Organic chemistry: Molecular diversity by design. Nature. 2009;**457**:153-154. DOI: 10.1038/457153a
Opinion in Chemical Biology. 2011;**15**:350-354. DOI: 10.1016/j.
[8] Schneider G, Grabowski K. Properties and architecture of drugs and natural products revisited. Current Chemical Biology. 2007;**1**:115-127. DOI:
10.2174/2212796810701010115
[9] Cragg GM, Newman DJ. Natural products: A continuing source of novel drug leads. Biochimica et Biophysica
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ijms19061578
*Chemoinformatic Approach: The Case of Natural Products of Panama DOI: http://dx.doi.org/10.5772/intechopen.87779*
| doab | 2025-04-07T04:13:04.430540 | 20-4-2021 18:19 | {
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ffe82432-4883-4adc-b03b-937c1baf5090.82 | Drug Design and Develpment by Chemical Tools
*Cheminformatics and Its Applications*
[76] Garcia-Sanchez MO, Cruz-Monteagudo M, Medina-Franco JL.
computational chemistry on physics quantitative structure-epigenetic activity relationship. In: Lezcznski J, Roy K, editors. Advances in QSAR Modeling: Application in Pharmaceutical, Chemical, Foods, Agricultural and Environmental Science, 24. Gewerbestrasse 11, 6330 Cham, Switzerland: Springer Nature. Springer International Publishing AG;
[77] Medina-Franco JL. Scanning structure–activity relationships with structure–activity similarity and related maps: From consensus activity cliffs to selectivity switches. Journal of Chemical Information and Modeling. 2012;**52**(10):2485-2493. DOI: 10.1021/
[78] Kiszewski AE. Blocking plasmodium falciparum malaria transmission with drugs: The gametocytocidal and sporontocidal properties of current and prospective antimalarials. Pharmaceuticals. 2011;**4**(1):44-68. DOI: 10.3390/
Challenges and advances in
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ci300362x
ph4010044
**106**
**109**
**Chapter 7**
*Azhar Rasul*
**1. Introduction**
Applications
functionality or therapeutic effects [1].
silico ADMET profiling [9].
of CPP-based therapeutics.
Prologue: Cheminformatics and Its
Cheminformatics is a field of information technology that uses informational and computational techniques to provide a deeper understanding and solutions of problems of chemistry. Cheminformatics strategies originally emerged as vehicle in drug discovery where large libraries of compounds are evaluated for specific
Drug discovery is a highly systematic multistep procedure for the identification of new medicines [2]. Chemical toolsets including chemical probes, RNAi, and chemoproteomics have helped scientists to identify and validate novel druggable targets for therapeutic interventions [3–6]. Target validation is of pivotal importance in determining the suitability of a new target for further clinical evaluation. Following the process of target validation, hit identification and lead discovery process involves establishment of high throughput screening (HTS) systems as well as development of chemical tool compound libraries [2]. The next critical phase of the drug discovery process is pharmacokinetics and pharmacodynamic profiling of lead compounds [7] and investigation of Absorption, Distribution, Metabolism, Excretion and Toxicity (ADMET) properties [8]. Various critical steps in drug discovery involve the applications of cheminformatics such as compound selection, virtual library generation, in silico-based screening, HTS, HTS data mining, and in
In addition, cheminformatics have also helped the scientists to develop and optimize delivery of molecules to intracellular targets for therapeutic implications
Conjugation of therapeutic entities with peptide delivery molecules, especially cell-penetrating peptides (CPPs), has the potential to increase the therapeutic efficacy by enhancing the ability of therapeutics to reach specific intracellular targets [11]. Preclinical evaluations of CPP-mediated therapeutics have shown promising results in disease models that also prompted clinical trials in some cases. These outcomes have, thus, opened new perspectives for CPPs in the development of well-tolerated and specifically targeted human therapies [12]. Thus, insights into current approaches and potential of CPP-based drug delivery systems are presented for greater understanding of readers about powerful promises and clinical efficacy
**Cheminformatics and its applications** presents the applications of two fields, chemical biology and bioinformatics, in drug discovery, thus, providing comprehensive description of modern technologies such as structure-based drug design, molecular docking, high throughput screening, and pharmaceutical profiling, which are all critical steps for the development of successful marketable drugs. With the invention of advanced and modern techniques in bioinformatics, the process of drug discovery has become faster and economical. Bioinformatics-based
[10], thus, provided solutions for various unmet medical needs.
| doab | 2025-04-07T04:13:04.430915 | 20-4-2021 18:19 | {
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ffe82432-4883-4adc-b03b-937c1baf5090.84 | Prologue: Cheminformatics and Its Applications
*Azhar Rasul*
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ffe82432-4883-4adc-b03b-937c1baf5090.85 | **1. Introduction**
Cheminformatics is a field of information technology that uses informational and computational techniques to provide a deeper understanding and solutions of problems of chemistry. Cheminformatics strategies originally emerged as vehicle in drug discovery where large libraries of compounds are evaluated for specific functionality or therapeutic effects [1].
Drug discovery is a highly systematic multistep procedure for the identification of new medicines [2]. Chemical toolsets including chemical probes, RNAi, and chemoproteomics have helped scientists to identify and validate novel druggable targets for therapeutic interventions [3–6]. Target validation is of pivotal importance in determining the suitability of a new target for further clinical evaluation. Following the process of target validation, hit identification and lead discovery process involves establishment of high throughput screening (HTS) systems as well as development of chemical tool compound libraries [2]. The next critical phase of the drug discovery process is pharmacokinetics and pharmacodynamic profiling of lead compounds [7] and investigation of Absorption, Distribution, Metabolism, Excretion and Toxicity (ADMET) properties [8]. Various critical steps in drug discovery involve the applications of cheminformatics such as compound selection, virtual library generation, in silico-based screening, HTS, HTS data mining, and in silico ADMET profiling [9].
In addition, cheminformatics have also helped the scientists to develop and optimize delivery of molecules to intracellular targets for therapeutic implications [10], thus, provided solutions for various unmet medical needs.
Conjugation of therapeutic entities with peptide delivery molecules, especially cell-penetrating peptides (CPPs), has the potential to increase the therapeutic efficacy by enhancing the ability of therapeutics to reach specific intracellular targets [11]. Preclinical evaluations of CPP-mediated therapeutics have shown promising results in disease models that also prompted clinical trials in some cases. These outcomes have, thus, opened new perspectives for CPPs in the development of well-tolerated and specifically targeted human therapies [12]. Thus, insights into current approaches and potential of CPP-based drug delivery systems are presented for greater understanding of readers about powerful promises and clinical efficacy of CPP-based therapeutics.
**Cheminformatics and its applications** presents the applications of two fields, chemical biology and bioinformatics, in drug discovery, thus, providing comprehensive description of modern technologies such as structure-based drug design, molecular docking, high throughput screening, and pharmaceutical profiling, which are all critical steps for the development of successful marketable drugs. With the invention of advanced and modern techniques in bioinformatics, the process of drug discovery has become faster and economical. Bioinformatics-based
### *Cheminformatics and Its Applications*
computational techniques have provided platform for large-scale screening of small molecules and chemical biology has served pharmaceutics for the validation of obtained data from computer-aided techniques, thus, both of the fields go hand in hand to revolutionize the field of drug discovery. Keeping in view of the emerging trends on cheminformatics in drug discovery, this book is designed to enable scientists to understand the fundamentals of drug discovery. Beginning with the highlights of the historical timeline of drug discovery, this book simply and succinctly educates its readers about screening methods, medicinal chemistry strategies in drug design, lead generation, testing the bioactivity of leads, lead optimization, clinical trial basics, as well as challenges of drug discovery such as cell-penetrating peptides and acceleration of chemical tool discovery by academic collaborations. This book will provide a clearer picture of cheminformatics and its applications and will be useful for scientific community working in the arena of drug discovery. Several recent developments are also overviewed, which will make it valuable for academicians and scientists.
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ffe82432-4883-4adc-b03b-937c1baf5090.86 | **Author details**
Azhar Rasul Department of Zoology, Government College University Faisalabad, Pakistan
\*Address all correspondence to: [email protected]
© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
**111**
2002;**7**:566-600
*Prologue: Cheminformatics and Its Applications DOI: http://dx.doi.org/10.5772/intechopen.92650*
[1] Wishart DS. Introduction to
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[4] Bantscheff M, Drewes G.
[6] Moustakim M, Felce SL, Zaarour N, Farnie G, McCann FE, Brennan PE. Target identification using chemical probes. Methods in Enzymology. 2018;**610**:27-58
Pharmacology. 2014;**5**:174
[7] Tuntland T, Ethell B, Kosaka T, Blasco F, Zang RX, Jain M, et al. Implementation of pharmacokinetic and pharmacodynamic strategies in early research phases of drug discovery and development at Novartis Institute of Biomedical Research. Frontiers in
[8] Wang J, Skolnik S. Recent advances in physicochemical and ADMET profiling in drug discovery. Chemistry & Biodiversity. 2009;**6**:1887-1899
[9] Xu J, Hagler A. Chemoinformatics and drug discovery. Molecules.
[10] Gautam A, Chaudhary K, Kumar R, Sharma A, Kapoor P, Tyagi A, et al. In
Chemoproteomic approaches to drug target identification and drug profiling. Bioorganic & Medicinal Chemistry.
[5] Bunnage ME, Chekler EL, Jones LH. Target validation using chemical probes. Nature Chemical Biology.
cheminformatics. Current Protocols in Bioinformatics. 2016;**53**:14 11 11-14 11 21 silico approaches for designing highly effective cell penetrating peptides. Journal of Translational Medicine.
[11] Derakhshankhah H, Jafari S. Cell penetrating peptides: A concise review with emphasis on biomedical
Pharmacotherapy. 2018;**108**:1090-1096
[12] Guidotti G, Brambilla L, Rossi D. Cell-penetrating peptides: From basic research to clinics. Trends in Pharmacological Sciences.
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*Prologue: Cheminformatics and Its Applications DOI: http://dx.doi.org/10.5772/intechopen.92650*
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ffe82432-4883-4adc-b03b-937c1baf5090.89 | Accelerating Chemical Tool Discovery by Academic Collaborative Models
*Bahne Stechmann and Wolfgang Fecke*
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ffe82432-4883-4adc-b03b-937c1baf5090.90 | **Abstract**
The development of chemical tool compounds becomes increasingly important for chemical biology research projects in many disciplines of life sciences. In addition, they form essential parts in both academic and industrial drug discovery efforts. The required expertise and technology platforms for the identification and optimization of these potent and target-selective small molecules often exceed the capabilities of academic groups and smaller companies. Over the years, several initiatives were created all over the world which address this issue by either creating networks or consortia of academic institutes, public-private partnerships with industry, or even dedicated new research infrastructures for chemical biology. Several of these organizations and their different access models will be described. We will focus in particular on the model of EU-OPENSCREEN ERIC, a new European Research Infrastructure which was founded in 2018 and consists of more than 20 partner institutes from eight countries.
**Keywords:** academic consortia, chemical tools, drug, pharmacological screening
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} |
ffe82432-4883-4adc-b03b-937c1baf5090.91 | **1. Introduction**
In the last decade, the interdisciplinary field of chemical biology has emerged from the need to better understand the role of proteins or signaling pathways in cellular systems and whole organisms than it was previously feasible with more classical genetic tools or methods. Rather than changing the levels of proteins, or blocking completely their expression or activity, by deleting or overexpressing their respective DNA or RNA sequences, it is now becoming more and more possible to precisely modulate their function in a time- and concentration-dependent manner using potent, selective and cell-permeable chemical compounds. Although the relevance of these so-called chemical tool compounds or probes for solving basic mechanistical questions in life sciences is indisputable [1], their role often extends into the fields of pharmacology and molecular medicine. In fact, chemical tools are playing an important role in the validation of newly identified drug targets in pharmaceutical companies, and might even serve as starting points for the development of new therapeutics.
Despite recent technological advances in areas such as cryo-electron microscopy (Cryo-EM) [2], the major approach for identifying bioactive substances is still the systematic testing of compound collections, often comprising many thousands or
even millions of individual substances, with target- or pathway-specific biological assays which are designed to produce reproducible biological activities with high signal-to-noise ratios under experimental conditions which are fast, miniaturized and therefore cost-effective [3]. This approach is technically and logistically challenging and, in the past, could only be performed by large pharmaceutical companies. In addition to experienced personnel, it requires large facilities with often expensive equipment for compound storage, automated liquid handling and sensitive detection of biological reactions. In recent years, however, this picture started to change. In the wake of the sequencing of the human genome, mostly larger academic institutions started to create their own screening and translational drug discovery centers because many new potential drug targets were suddenly becoming available for which a solid understanding of their physiological roles and molecular mechanisms were missing. At the same time, pharmaceutical companies faced increased pressures due to high drug development costs, often resulting in down-sized research budgets and cost cutting exercises combined with a general trend of becoming risk-averse towards innovative drug targets with potential high failure rates [4]. As a result, many experienced industrial 'drug hunters' found employment in academic chemical probe discovery centers, supporting their efforts and helping to alleviate some of the initial issues these centers faced [5].
In this chapter we describe some of these new initiatives which were created to develop chemical tool compounds outside of the traditional pharmaceutical industry, highlighting their particular strengths, challenges and access models for the mostly academic scientific community.
| doab | 2025-04-07T04:13:04.431668 | 20-4-2021 18:19 | {
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ffe82432-4883-4adc-b03b-937c1baf5090.92 | **2. Developing chemical probes in academic networks**
At the beginning of the 21st century, academic institutions first began to implement dedicated assay development and screening centers which were soon followed by reports on the systematic testing of small molecule compound libraries in the US [6]. Comparable efforts in Europe's research institutes immediately received much attention, fostering collaborations between chemistry and biology groups and the establishment of academic screening platforms of diverse size. However, single platforms alone could not support comprehensively the needs of academic or industrial users due to limited chemical diversity of their compound collections and/or limited technical capabilities, and big pharma platforms were at that time not open to academic users. Pooling and coordination of public resources and expertise became imperative. Therefore, the efforts in the US were replicated with similar initiatives in countries such as France (Chimiothèque Nationale), Germany (ChemBioNet), Spain (ChemBioBank) and several others. Some years later, long-term cooperations between academic centers from different countries as well as public-private partnerships were established. We will describe some of these initiatives in more detail and will put particular emphasis on the collaborative model of the youngest organization for chemical biology, the EU-OPENSCREEN ERIC.
#### **2.1 The molecular libraries program (MLP) in the US**
The large, NIH-funded MLP was created in 2004 with the ambitious goal of creating a small molecule probe for every human protein in order to define the functions of genes, cells, and whole organisms in health and disease. The three components of the initiative were essentially: (a) a network of comprehensive and specialized screening centers plus specialized chemistry centers, (b) several cheminformatics approaches which included also a newly created
**115**
*Accelerating Chemical Tool Discovery by Academic Collaborative Models*
**2.2 The chemical biology consortium Sweden (CBCS)**
public compound database called PubChem with assay metadata, and (c) initiatives to generally advance technologies in the fields of chemical diversity, assay development and screening [6]. The aim was always to publish the new chemical probes and associated data immediately so that compounds could be used by the academic scientific community not only for basic research questions, but also for mechanistic validation of potentially disease-relevant drug targets and drug
Individual scientists could apply for funding to the NIH for their assay development and screening projects. Successfully, peer-reviewed projects were taken on board by one of the MLP centers, and high-throughput screens were conducted with a library which, by the end of the program, consisted of 390.000 compounds. About 5% of these molecules with often novel scaffolds were delivered by the academic synthetic chemistry community. In many cases, further chemical optimization yielded probes against protein targets which were deemed challenging or even 'undruggable'. Overall, during the 10-year period of the program, a total of 375 chemical tool compounds were developed against a broad range of target classes. 18 of these compounds were considered sufficiently interesting to serve as starting points for the development of therapeutics against a total of eight disease targets or target classes, and were licensed to biotech and pharmaceutical companies [7]. In light of the investment into the MLP it is debatable whether the ratio of probes to drug candidates can be regarded as a success or a disappointment but it certainly highlights the difficulties that chemical biologists are facing when they want to keep up with the speed of biological discoveries while translating academic findings into
Although much smaller than the MLP in the US, this example of a national consortium can highlight very well the particular strengths of a focused organization with only a few members. CBCS, with two nodes at the Karolinska Institutet and Umeå University, was founded as a non-for-profit research infrastructure for chemical biology in 2010 [8] by researchers from Biovitrum (former Pharmacia and Upjohn) and became an integrated platform of SciLifeLab, an already existing national centre for molecular biosciences, in 2013 [9]. The combined platform can investigate both chemical and genetic perturbations in biological systems. CBCS wants to enable high level basic research with open access publications while at the same time linking up academic and industrial groups. Complementary to CBCS, SciLifeLab offers a dedicated platform for drug discovery and development, with the clear goal of accelerating projects with translational potential. After nearly 10 years of operation, the consortium has produced more than 130 co-authored publications and 11 patent applications while scientific data provided the basis of
Users are encouraged to discuss in more detail project proposals with the CBCS staff prior to the submission of the official application. A proposal template, user agreements and estimated costs of typical screening and chemistry projects are available online. Project proposals are evaluated by an independent 'Project Review Committee' (PRC), which meets biannually. Prioritized projects may be subsidized, with the remaining costs covered by the applicant. Implemented projects are periodically re-evaluated by the Project Review Committee as they progress to pre-defined milestones. A project plan for a so-called "large collaborative project" may run over a maximum of 2 years for which the user is expected to cover the costs for all reagents and consumables, including a compound access fee for plating of library compounds.
There are also "small collaborative projects" which involve only limited CBCS
*DOI: http://dx.doi.org/10.5772/intechopen.91138*
development.
therapeutics.
six start-up companies [10].
#### *Accelerating Chemical Tool Discovery by Academic Collaborative Models DOI: http://dx.doi.org/10.5772/intechopen.91138*
*Cheminformatics and Its Applications*
mostly academic scientific community.
even millions of individual substances, with target- or pathway-specific biological assays which are designed to produce reproducible biological activities with high signal-to-noise ratios under experimental conditions which are fast, miniaturized and therefore cost-effective [3]. This approach is technically and logistically challenging and, in the past, could only be performed by large pharmaceutical companies. In addition to experienced personnel, it requires large facilities with often expensive equipment for compound storage, automated liquid handling and sensitive detection of biological reactions. In recent years, however, this picture started to change. In the wake of the sequencing of the human genome, mostly larger academic institutions started to create their own screening and translational drug discovery centers because many new potential drug targets were suddenly becoming available for which a solid understanding of their physiological roles and molecular mechanisms were missing. At the same time, pharmaceutical companies faced increased pressures due to high drug development costs, often resulting in down-sized research budgets and cost cutting exercises combined with a general trend of becoming risk-averse towards innovative drug targets with potential high failure rates [4]. As a result, many experienced industrial 'drug hunters' found employment in academic chemical probe discovery centers, supporting their efforts
and helping to alleviate some of the initial issues these centers faced [5].
**2. Developing chemical probes in academic networks**
organization for chemical biology, the EU-OPENSCREEN ERIC.
The large, NIH-funded MLP was created in 2004 with the ambitious goal of creating a small molecule probe for every human protein in order to define the functions of genes, cells, and whole organisms in health and disease. The three components of the initiative were essentially: (a) a network of comprehensive and specialized screening centers plus specialized chemistry centers, (b) several cheminformatics approaches which included also a newly created
**2.1 The molecular libraries program (MLP) in the US**
In this chapter we describe some of these new initiatives which were created to develop chemical tool compounds outside of the traditional pharmaceutical industry, highlighting their particular strengths, challenges and access models for the
At the beginning of the 21st century, academic institutions first began to implement dedicated assay development and screening centers which were soon followed by reports on the systematic testing of small molecule compound libraries in the US [6]. Comparable efforts in Europe's research institutes immediately received much attention, fostering collaborations between chemistry and biology groups and the establishment of academic screening platforms of diverse size. However, single platforms alone could not support comprehensively the needs of academic or industrial users due to limited chemical diversity of their compound collections and/or limited technical capabilities, and big pharma platforms were at that time not open to academic users. Pooling and coordination of public resources and expertise became imperative. Therefore, the efforts in the US were replicated with similar initiatives in countries such as France (Chimiothèque Nationale), Germany (ChemBioNet), Spain (ChemBioBank) and several others. Some years later, long-term cooperations between academic centers from different countries as well as public-private partnerships were established. We will describe some of these initiatives in more detail and will put particular emphasis on the collaborative model of the youngest
**114**
public compound database called PubChem with assay metadata, and (c) initiatives to generally advance technologies in the fields of chemical diversity, assay development and screening [6]. The aim was always to publish the new chemical probes and associated data immediately so that compounds could be used by the academic scientific community not only for basic research questions, but also for mechanistic validation of potentially disease-relevant drug targets and drug development.
Individual scientists could apply for funding to the NIH for their assay development and screening projects. Successfully, peer-reviewed projects were taken on board by one of the MLP centers, and high-throughput screens were conducted with a library which, by the end of the program, consisted of 390.000 compounds. About 5% of these molecules with often novel scaffolds were delivered by the academic synthetic chemistry community. In many cases, further chemical optimization yielded probes against protein targets which were deemed challenging or even 'undruggable'. Overall, during the 10-year period of the program, a total of 375 chemical tool compounds were developed against a broad range of target classes. 18 of these compounds were considered sufficiently interesting to serve as starting points for the development of therapeutics against a total of eight disease targets or target classes, and were licensed to biotech and pharmaceutical companies [7]. In light of the investment into the MLP it is debatable whether the ratio of probes to drug candidates can be regarded as a success or a disappointment but it certainly highlights the difficulties that chemical biologists are facing when they want to keep up with the speed of biological discoveries while translating academic findings into therapeutics.
| doab | 2025-04-07T04:13:04.431895 | 20-4-2021 18:19 | {
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ffe82432-4883-4adc-b03b-937c1baf5090.93 | **2.2 The chemical biology consortium Sweden (CBCS)**
Although much smaller than the MLP in the US, this example of a national consortium can highlight very well the particular strengths of a focused organization with only a few members. CBCS, with two nodes at the Karolinska Institutet and Umeå University, was founded as a non-for-profit research infrastructure for chemical biology in 2010 [8] by researchers from Biovitrum (former Pharmacia and Upjohn) and became an integrated platform of SciLifeLab, an already existing national centre for molecular biosciences, in 2013 [9]. The combined platform can investigate both chemical and genetic perturbations in biological systems. CBCS wants to enable high level basic research with open access publications while at the same time linking up academic and industrial groups. Complementary to CBCS, SciLifeLab offers a dedicated platform for drug discovery and development, with the clear goal of accelerating projects with translational potential. After nearly 10 years of operation, the consortium has produced more than 130 co-authored publications and 11 patent applications while scientific data provided the basis of six start-up companies [10].
Users are encouraged to discuss in more detail project proposals with the CBCS staff prior to the submission of the official application. A proposal template, user agreements and estimated costs of typical screening and chemistry projects are available online. Project proposals are evaluated by an independent 'Project Review Committee' (PRC), which meets biannually. Prioritized projects may be subsidized, with the remaining costs covered by the applicant. Implemented projects are periodically re-evaluated by the Project Review Committee as they progress to pre-defined milestones. A project plan for a so-called "large collaborative project" may run over a maximum of 2 years for which the user is expected to cover the costs for all reagents and consumables, including a compound access fee for plating of library compounds. There are also "small collaborative projects" which involve only limited CBCS
support for maximal 2 weeks, e.g. a short-term access to a specialized instrument such as an imaging plate reader. For these projects, no PRC application is required but they are undertaken with a "first come—first served" policy based on available resources [10]. In line with the open access data policy of the CBCS, the applicant and the CBCS agree upon a clear publication strategy before the implementation of the project. The target user group of CBCS are academic researchers at Swedish research institutions, who aim to develop chemical probes on a collaborative basis.
It is worth looking in more detail into the services CBCS can offer to their academic customers. The consortium assists in assay development for both biochemical and cell-based assays, gives access to the SciLifeLab compound collection and provides medicinal and computational chemistry expertise for hit validation and optimization. This model is very similar to the service offerings of the much larger European research infrastructure EU-OPENSCREEN which is being discussed below. In addition, mechanism-of-action studies can be performed with often specialized technologies such as cellular thermal shift assays (CETSA) [11]. In fact, the development of CETSA is a good example on how an expert consortium such as CBCS can impact and further develop disrupting technologies in collaboration with local academic groups and commercial partners (here: Pelago Biosciences). Starting life as a low throughput assay, CETSA is now amenable to high throughput screening [12]. Scientists usually come to the CBCS with the concept for a biological assay and first experimental data. They have then the chance to work further on the assay in the CBCS laboratories under guidance of their expert scientists, enabling in parallel scientific services and the education of users [10].
The CBCS compound collection consists of more than 200.000 compounds with high chemical diversity which are routinely quality controlled. While many of these compounds were donated by the pharmaceutical company Biovitrum, the library was further expanded with sets from commercial vendors and donations by other biotech companies. Importantly, the strategy has always been to build a modular collection of sub-libraries which can be adapted to the needs of each academic screening project, based mainly on assay throughput and cost per data point. For instance, in addition to a diverse primary screening set of 35.000 compounds, there are also focused libraries for particular target classes such as kinases, G-protein coupled receptors, agrochemicals etc., as well as a set of approved drugs [10]. This is very different to the concept of EU-OPENSCREEN which offers a high throughput screening set of 100.000 commercial compounds to their users, with the goal to have that set screened in almost all projects so that each compound becomes associated with "positive" and "negative" screening data from as many projects as possible (see below).
Overall, between 2010 and 2018 more than 400 collaborative projects with 236 individual users in Sweden were discussed. User interest grew continuously during these 8 years, currently leading to approximately one new project discussion per week. About 25% of discussions result in large project PRC applications while others obtain small project limited support, all documented in, on average, 20 publications per year [10].
#### **2.3 Public private initiatives: SGC and ELF**
In industry, chemical tool compounds play an important role as pharmaceutical modulators of novel drug targets. Typically, they are being used for testing a particular disease hypothesis and for validating the chemical tractability of newly discovered candidate proteins or signaling pathways for which otherwise comparatively little information is available. Sometimes their properties are even
**117**
cal companies.
*Accelerating Chemical Tool Discovery by Academic Collaborative Models*
instance epigenetic and other transcriptional modulators.
sufficient to act as starting points for drug discovery programs. The development of compounds with required potency and, most importantly, selectivity towards individual members of a protein class can be a formidable task even for larger pharmaceutical or biotech companies. It came therefore as no surprise that in 2009 several industrial partners decided to collaborate in a pre-competitive manner and initiated a public-private partnership (PPP) with leading academic institutes in the field of chemical biology. The aim was to develop high-quality chemical tool compounds for families of understudied proteins of potential therapeutic value, for
The chosen academic partners in that particular PPP were the universities of Oxford and Toronto which had already formed the so-called Structural Genomics Consortium (SGC) in 2004 with the goal of determining the three-dimensional structures of proteins with therapeutic relevance. The SGC advocates open access partnerships between industry and academia and is committed to make their chemical tool compounds available without any restrictions. In the last 10 years, and with financial support by several pharmaceutical companies, more than 50 chemical probes in the areas of epigenetics and kinase signaling were developed [13, 14]. Furthermore, seven pharmaceutical companies made their chemical tool compounds from older research programs available to the scientific community, including protocols, controls and associated data [15]. Efforts are now underway, under the umbrella of the Innovative Medicines Initiative (IMI), to expand the initial collection of compounds further by focusing not only on the protein classes which were selected in the past but also on the development of new technologies, making the identification and profiling of tool compounds generally faster and
Another PPP initiative supported by the IMI is the European Lead Factory (ELF) [17] which is a consortium of 20 partners, currently among them the universities of Oxford and Dundee while several other universities, research organizations and companies in the UK, Netherlands and Germany were former partners. The project was launched in 2013 and came to an end in 2018, with a follow-up five-year project funded in the same year [18]. During its lifetime, the ELF established a selection of about 550.000 compounds which are generally not commercially available. 300.000 of these were donated by seven participating pharmaceutical companies, while the rest was synthesized by medicinal chemistry partner companies during the last 5 years. Both the compound management facility in the UK and the high throughput screening center in the Netherlands were formerly part of pharmaceutical companies and able to perform screening operations and chemistry services such as hit optimization and modeling according to industry standards. The Oxford Biotechnology group of the SGC was selected as a key contributor of 3D co-crystal structures which are essential for compound optimization. During the lifetime of the project, more than 80 drug discovery programs across most therapeutic areas were pursued. By March 2018, two partnering deals between the respective project owner and one of the pharmaceutical company partners had emerged. Importantly, the ELF protects the IP rights of their academic collaborators against the pharmaceutical companies, ensuring that the academic researchers can always search for external partners in case that no development deal between them and one of the ELF industry partners could be fixed. This was one of the main concerns when the
It remains to be seen though if and how academic groups really benefit from these ambitious initiatives, especially when own research interests show little overlap with the essentially commercial interests of the participating pharmaceuti-
*DOI: http://dx.doi.org/10.5772/intechopen.91138*
more cost-effective [16].
project started in 2013 [19].
#### *Accelerating Chemical Tool Discovery by Academic Collaborative Models DOI: http://dx.doi.org/10.5772/intechopen.91138*
*Cheminformatics and Its Applications*
support for maximal 2 weeks, e.g. a short-term access to a specialized instrument such as an imaging plate reader. For these projects, no PRC application is required but they are undertaken with a "first come—first served" policy based on available resources [10]. In line with the open access data policy of the CBCS, the applicant and the CBCS agree upon a clear publication strategy before the implementation of the project. The target user group of CBCS are academic researchers at Swedish research institutions, who aim to develop chemical probes on a collaborative basis. It is worth looking in more detail into the services CBCS can offer to their academic customers. The consortium assists in assay development for both biochemical and cell-based assays, gives access to the SciLifeLab compound collection and provides medicinal and computational chemistry expertise for hit validation and optimization. This model is very similar to the service offerings of the much larger European research infrastructure EU-OPENSCREEN which is being discussed below. In addition, mechanism-of-action studies can be performed with often specialized technologies such as cellular thermal shift assays (CETSA) [11]. In fact, the development of CETSA is a good example on how an expert consortium such as CBCS can impact and further develop disrupting technologies in collaboration with local academic groups and commercial partners (here: Pelago Biosciences). Starting life as a low throughput assay, CETSA is now amenable to high throughput screening [12]. Scientists usually come to the CBCS with the concept for a biological assay and first experimental data. They have then the chance to work further on the assay in the CBCS laboratories under guidance of their expert scientists, enabling in
parallel scientific services and the education of users [10].
The CBCS compound collection consists of more than 200.000 compounds with high chemical diversity which are routinely quality controlled. While many of these compounds were donated by the pharmaceutical company Biovitrum, the library was further expanded with sets from commercial vendors and donations by other biotech companies. Importantly, the strategy has always been to build a modular collection of sub-libraries which can be adapted to the needs of each academic screening project, based mainly on assay throughput and cost per data point. For instance, in addition to a diverse primary screening set of 35.000 compounds, there are also focused libraries for particular target classes such as kinases, G-protein coupled receptors, agrochemicals etc., as well as a set of approved drugs [10]. This is very different to the concept of EU-OPENSCREEN which offers a high throughput screening set of 100.000 commercial compounds to their users, with the goal to have that set screened in almost all projects so that each compound becomes associated with "positive" and "negative" screening data from as many projects as
Overall, between 2010 and 2018 more than 400 collaborative projects with 236 individual users in Sweden were discussed. User interest grew continuously during these 8 years, currently leading to approximately one new project discussion per week. About 25% of discussions result in large project PRC applications while others obtain small project limited support, all documented in, on average, 20 publica-
In industry, chemical tool compounds play an important role as pharmaceutical modulators of novel drug targets. Typically, they are being used for testing a particular disease hypothesis and for validating the chemical tractability of newly discovered candidate proteins or signaling pathways for which otherwise comparatively little information is available. Sometimes their properties are even
**116**
possible (see below).
tions per year [10].
**2.3 Public private initiatives: SGC and ELF**
sufficient to act as starting points for drug discovery programs. The development of compounds with required potency and, most importantly, selectivity towards individual members of a protein class can be a formidable task even for larger pharmaceutical or biotech companies. It came therefore as no surprise that in 2009 several industrial partners decided to collaborate in a pre-competitive manner and initiated a public-private partnership (PPP) with leading academic institutes in the field of chemical biology. The aim was to develop high-quality chemical tool compounds for families of understudied proteins of potential therapeutic value, for instance epigenetic and other transcriptional modulators.
The chosen academic partners in that particular PPP were the universities of Oxford and Toronto which had already formed the so-called Structural Genomics Consortium (SGC) in 2004 with the goal of determining the three-dimensional structures of proteins with therapeutic relevance. The SGC advocates open access partnerships between industry and academia and is committed to make their chemical tool compounds available without any restrictions. In the last 10 years, and with financial support by several pharmaceutical companies, more than 50 chemical probes in the areas of epigenetics and kinase signaling were developed [13, 14]. Furthermore, seven pharmaceutical companies made their chemical tool compounds from older research programs available to the scientific community, including protocols, controls and associated data [15]. Efforts are now underway, under the umbrella of the Innovative Medicines Initiative (IMI), to expand the initial collection of compounds further by focusing not only on the protein classes which were selected in the past but also on the development of new technologies, making the identification and profiling of tool compounds generally faster and more cost-effective [16].
Another PPP initiative supported by the IMI is the European Lead Factory (ELF) [17] which is a consortium of 20 partners, currently among them the universities of Oxford and Dundee while several other universities, research organizations and companies in the UK, Netherlands and Germany were former partners. The project was launched in 2013 and came to an end in 2018, with a follow-up five-year project funded in the same year [18]. During its lifetime, the ELF established a selection of about 550.000 compounds which are generally not commercially available. 300.000 of these were donated by seven participating pharmaceutical companies, while the rest was synthesized by medicinal chemistry partner companies during the last 5 years. Both the compound management facility in the UK and the high throughput screening center in the Netherlands were formerly part of pharmaceutical companies and able to perform screening operations and chemistry services such as hit optimization and modeling according to industry standards. The Oxford Biotechnology group of the SGC was selected as a key contributor of 3D co-crystal structures which are essential for compound optimization. During the lifetime of the project, more than 80 drug discovery programs across most therapeutic areas were pursued. By March 2018, two partnering deals between the respective project owner and one of the pharmaceutical company partners had emerged. Importantly, the ELF protects the IP rights of their academic collaborators against the pharmaceutical companies, ensuring that the academic researchers can always search for external partners in case that no development deal between them and one of the ELF industry partners could be fixed. This was one of the main concerns when the project started in 2013 [19].
It remains to be seen though if and how academic groups really benefit from these ambitious initiatives, especially when own research interests show little overlap with the essentially commercial interests of the participating pharmaceutical companies.
| doab | 2025-04-07T04:13:04.432351 | 20-4-2021 18:19 | {
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ffe82432-4883-4adc-b03b-937c1baf5090.94 | **2.4 The European research infrastructure consortium (ERIC) EU-OPENSCREEN**
EU-OPENSCREEN [20] is a community-driven, bottom-up initiative in Europe, which brings together 21 partner sites, i.e. technology platforms and research groups at various universities and research institutions, to form an open-access research infrastructure for chemical biology and early drug discovery. Instead of building an ivory tower, the aim of EU-OPENSCREEN is to establish and operate an infrastructure that facilitates and encourages the engagement with the broader scientific community. In the framework of EU-OPENSCREEN, the partner sites and external researchers collaboratively develop novel tool compounds (or chemical 'probes') that allow researchers to interrogate and study fundamental cellular processes, such as signaling or metabolic pathways.
EU-OPENSCREEN is one of 55 research infrastructures listed on the current ESFRI (European Strategy Forum on Research Infrastructures) Roadmap [21] as an 'ESFRI Landmark Project', demonstrating the relevance for the European scientific community and the European Research Area (ERA). It is jointly funded by the research ministries of eight countries (the Czech Republic, Denmark, Finland, Germany, Latvia, Norway, Poland, Spain) and the European Commission. Since April 2018, it operates a European, not-for-profit organization ('European Research Infrastructure Consortium'), which is based in Berlin, Germany, and is legally independent from any university or research institute. EU-OPENSCREEN, and the European Research Infrastructures in general, promote open science and open innovation [22].
Many technology platforms at universities and research institutes predominantly work with the colleagues at their hosting institution. Larger European initiatives often engage with scientists from Western European countries, where these initiatives are based. Reaching out to, and encourage the active participation of, scientists from regions, which are often underrepresented in chemical biology and early drug discovery research, requires a different approach. Through its distributed network of partner sites across its member countries, EU-OPENSCREEN aims to have a more balanced engagement of local science communities. In each member country, a local partner establishes and coordinates a national network—e.g. CZ-OPENSCREEN in the Czech Republic, PL-OPENSCREEN in Poland, NOR-OPENSCREEN in Norway, Drug Discovery and Chemical Biology Consortium (DDCB) in Finland, ChemBioNet in Germany—to raise awareness about the initiative and to efficiently encourage scientists at the local level to participate.
| doab | 2025-04-07T04:13:04.433103 | 20-4-2021 18:19 | {
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ffe82432-4883-4adc-b03b-937c1baf5090.95 | *2.4.1 The research infrastructure*
The EU-OPENSCREEN infrastructure provides open-access to compound libraries, assay development and screening facilities, and medicinal chemistry and informatics platforms. It provides training and serves as a platform for industry engagement.
## *2.4.1.1 Compound collection*
The EU-OPENSCREEN compound collection is a diversity library, which has been designed in a collaborative effort of several partner sites. The library is jointly used by affiliated EU-OPENSCREEN partner sites for primary screening against biological targets solicited from external researchers who developed the appropriate assays. During the design of the library, 100,000 commercially available
**119**
projects.
*2.4.1.2 Database*
*Accelerating Chemical Tool Discovery by Academic Collaborative Models*
compounds were selected, with an emphasis on chemical stability, absence of reactive compounds, screening-compliant physico-chemical properties, and maximal diversity/coverage of chemical space. Furthermore, EU-OPENSCREEN crowdsources compounds from external chemists worldwide, in a federated approach through its national chemical biology networks. This collection of academic compounds will, over time, add increasing uniqueness to the EU-OPENSCREEN compound collection. The ambitious goal is to gather up to 40,000 compounds over the next years and to realize the vision of a truly European compound collection. In this context, the EU-OPENSCREEN compound collection will be dynamic and expanding. In analogy to the 'FAIR' (FAIR stands for findability, accessibility, interoperability, and reusability) data principles (described below), structural compound information and quality control data will be available online in an interoperable format (interoperability), unique identifier codes for each compound will be employed (findability), quality control will ensure the identity and purity of the compounds (reproducibility), and their distribution partner sites where they are accessible to external scientists and used in screening projects (accessibility). All compounds of the collections are carefully characterized and annotated for basic physico-chemical (e.g. solubility, light absorbance and fluorescence) and biological properties (e.g. cytotoxicity, antibiotic activity) by 'profiling' them in a standard panel of assays. These bioprofiling data increase the reliability and reproducibility of screening results, and identify compounds with properties that could potentially perturb specific bioassay read-out technologies (e.g. auto-fluorescence, luciferase inhibition, etc.) in order to reduce false-positive results. For chemists who provide compounds to be incorporated in the compound collection, these profiling data are an important incentive, in addition to the bioactivity data from the screening
The jointly used compound collection is stored centrally by the Compound Collection Management Facility (CCMF) in Berlin, Germany, and aliquots are distributed to the affiliated EU-OPENSCREEN partner sites, which are located in the eight EU-OPENSCREEN member countries. The CCMF is responsible for the acquisition, selection, maintenance and storage of the central collection and quality-controls of the compounds. The CCMF provides the screening and bioprofiling sites with copies of the compound collection (including, where necessary,
In many cases, primary screening data—even from publicly funded programs are not openly accessible by the scientific community. While private organizations, contract research organizations (CROs) and many public-private partnerships do not reveal data on a routine basis, EU-OPENSCREEN is committed to maximizing the re-use and impact of generated bioactivity data for the benefit of the wider scientific community. Therefore, EU-OPENSCREEN's ECBD adheres to the FAIR principles [23] and is closely linked to the ChEMBL [24] database, which will raise the discoverability and re-use of EU-OPENSCREEN's data. Via ECBD and ChEMBL, database users will be drawn from across the global biological and chemical science communities, both from academia and industry. Together with other European life sciences-research infrastructures, EU-OPENSCREEN partners also contribute towards the optimization of technological implementation, integration and interoperability of data and tools within the European Open Science Cloud (EOSC) and participate in the Horizon 2020-funded 'EOSC-Life' project (www.eosc-life.eu/). Another initiative, to which the EU-OPENSCREEN partner
cherry-picking for confirmatory and counter-screening activities).
*DOI: http://dx.doi.org/10.5772/intechopen.91138*
#### *Accelerating Chemical Tool Discovery by Academic Collaborative Models DOI: http://dx.doi.org/10.5772/intechopen.91138*
compounds were selected, with an emphasis on chemical stability, absence of reactive compounds, screening-compliant physico-chemical properties, and maximal diversity/coverage of chemical space. Furthermore, EU-OPENSCREEN crowdsources compounds from external chemists worldwide, in a federated approach through its national chemical biology networks. This collection of academic compounds will, over time, add increasing uniqueness to the EU-OPENSCREEN compound collection. The ambitious goal is to gather up to 40,000 compounds over the next years and to realize the vision of a truly European compound collection. In this context, the EU-OPENSCREEN compound collection will be dynamic and expanding. In analogy to the 'FAIR' (FAIR stands for findability, accessibility, interoperability, and reusability) data principles (described below), structural compound information and quality control data will be available online in an interoperable format (interoperability), unique identifier codes for each compound will be employed (findability), quality control will ensure the identity and purity of the compounds (reproducibility), and their distribution partner sites where they are accessible to external scientists and used in screening projects (accessibility). All compounds of the collections are carefully characterized and annotated for basic physico-chemical (e.g. solubility, light absorbance and fluorescence) and biological properties (e.g. cytotoxicity, antibiotic activity) by 'profiling' them in a standard panel of assays. These bioprofiling data increase the reliability and reproducibility of screening results, and identify compounds with properties that could potentially perturb specific bioassay read-out technologies (e.g. auto-fluorescence, luciferase inhibition, etc.) in order to reduce false-positive results. For chemists who provide compounds to be incorporated in the compound collection, these profiling data are an important incentive, in addition to the bioactivity data from the screening projects.
The jointly used compound collection is stored centrally by the Compound Collection Management Facility (CCMF) in Berlin, Germany, and aliquots are distributed to the affiliated EU-OPENSCREEN partner sites, which are located in the eight EU-OPENSCREEN member countries. The CCMF is responsible for the acquisition, selection, maintenance and storage of the central collection and quality-controls of the compounds. The CCMF provides the screening and bioprofiling sites with copies of the compound collection (including, where necessary, cherry-picking for confirmatory and counter-screening activities).
| doab | 2025-04-07T04:13:04.433311 | 20-4-2021 18:19 | {
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ffe82432-4883-4adc-b03b-937c1baf5090.96 | *2.4.1.2 Database*
*Cheminformatics and Its Applications*
**EU-OPENSCREEN**
innovation [22].
**2.4 The European research infrastructure consortium (ERIC)**
processes, such as signaling or metabolic pathways.
encourage scientists at the local level to participate.
*2.4.1 The research infrastructure*
*2.4.1.1 Compound collection*
EU-OPENSCREEN [20] is a community-driven, bottom-up initiative in Europe,
EU-OPENSCREEN is one of 55 research infrastructures listed on the current ESFRI (European Strategy Forum on Research Infrastructures) Roadmap [21] as an 'ESFRI Landmark Project', demonstrating the relevance for the European scientific community and the European Research Area (ERA). It is jointly funded by the research ministries of eight countries (the Czech Republic, Denmark, Finland, Germany, Latvia, Norway, Poland, Spain) and the European Commission. Since April 2018, it operates a European, not-for-profit organization ('European Research Infrastructure Consortium'), which is based in Berlin, Germany, and is legally independent from any university or research institute. EU-OPENSCREEN, and the European Research Infrastructures in general, promote open science and open
Many technology platforms at universities and research institutes predominantly work with the colleagues at their hosting institution. Larger European initiatives often engage with scientists from Western European countries, where these initiatives are based. Reaching out to, and encourage the active participation of, scientists from regions, which are often underrepresented in chemical biology and early drug discovery research, requires a different approach. Through its distributed network of partner sites across its member countries, EU-OPENSCREEN aims to have a more balanced engagement of local science communities. In each member country, a local partner establishes and coordinates a national network—e.g. CZ-OPENSCREEN in the Czech Republic, PL-OPENSCREEN in Poland, NOR-OPENSCREEN in Norway, Drug Discovery and Chemical Biology Consortium (DDCB) in Finland, ChemBioNet in Germany—to raise awareness about the initiative and to efficiently
The EU-OPENSCREEN infrastructure provides open-access to compound libraries, assay development and screening facilities, and medicinal chemistry and informatics platforms. It provides training and serves as a platform for industry
The EU-OPENSCREEN compound collection is a diversity library, which has been designed in a collaborative effort of several partner sites. The library is jointly used by affiliated EU-OPENSCREEN partner sites for primary screening against biological targets solicited from external researchers who developed the appropriate assays. During the design of the library, 100,000 commercially available
which brings together 21 partner sites, i.e. technology platforms and research groups at various universities and research institutions, to form an open-access research infrastructure for chemical biology and early drug discovery. Instead of building an ivory tower, the aim of EU-OPENSCREEN is to establish and operate an infrastructure that facilitates and encourages the engagement with the broader scientific community. In the framework of EU-OPENSCREEN, the partner sites and external researchers collaboratively develop novel tool compounds (or chemical 'probes') that allow researchers to interrogate and study fundamental cellular
**118**
engagement.
In many cases, primary screening data—even from publicly funded programs are not openly accessible by the scientific community. While private organizations, contract research organizations (CROs) and many public-private partnerships do not reveal data on a routine basis, EU-OPENSCREEN is committed to maximizing the re-use and impact of generated bioactivity data for the benefit of the wider scientific community. Therefore, EU-OPENSCREEN's ECBD adheres to the FAIR principles [23] and is closely linked to the ChEMBL [24] database, which will raise the discoverability and re-use of EU-OPENSCREEN's data. Via ECBD and ChEMBL, database users will be drawn from across the global biological and chemical science communities, both from academia and industry. Together with other European life sciences-research infrastructures, EU-OPENSCREEN partners also contribute towards the optimization of technological implementation, integration and interoperability of data and tools within the European Open Science Cloud (EOSC) and participate in the Horizon 2020-funded 'EOSC-Life' project (www.eosc-life.eu/). Another initiative, to which the EU-OPENSCREEN partner
Fraunhofer IME actively contributes, is the Innovative Medicines Initiative (IMI) funded 'FAIRplus' project (https://fairplus-project.eu/), which aims to facilitate the application of FAIR principles to data from certain IMI projects and datasets from pharmaceutical companies.
The ECBD is the central database for the integration of screening data from EU-OPENSCREEN projects with advanced search, analysis, and visualization tools. There will be three levels of data management and access: First, bioactivity data generation of compounds in screening projects, implemented at the individual EU-OPENSCREEN screening sites, using assays provided by the external collaboration partners; second, the integration of these screening datasets from partner sites into the ECBD; and, third, public dissemination of the data through established databases like ChEMBL [24] and PubChem [25, 26]. The ECBD is hosted by Petr Bartunek, the coordinator of CZ-OPENSCREEN, and his team at the Institute of Molecular Genetics of the ASCR in Prague, Czech Republic, who have developed the open data resource Probes & Drugs portal [27] as well as other databases such as the Zebrabase [28]. The e-infrastructure CESNET provides cloud-based hosting, backup and security.
An important aspect in the context of integrating complex and diverse screening data, when dealing with datasets from various affiliated, but legally independent sites that jointly use the compound collection, is the implementation of harmonized data standards and data curation. The ECBD adheres to well-established ontologies and identifiers, for example, the BioAssay Ontology (BAO) [29] for the classification and description of assays, which are commonly used by other similar open data repositories, such as ChEMBL or PubChem BioAssay. Only officially accredited partner sites have permission to upload data into the ECBD and uploaded data will be curated both automatically (e.g. file format, column values) as well as manually (e.g. data inspection) by the ECBD team. In case of ambiguities, the ECBD team contacts the data provider to resolve the issue. The ECBD team provides user support and help desk functions. Webinars on data deposition, the use of ECBD for data searching, visualizations and analysis are planned and dedicated workshops will be organized to demonstrate database users all ECBD capabilities and to share best practices in the community.
A grace period of up to 3 years between the completion of the primary screen and data publication in the EU-OPENSCREEN database is provided, during which the bioactivity datasets are not publicly accessible. This grace period allows for follow-up studies, publication in peer-review scientific journals and securing of intellectual property.
Assay development and screening facilities, and medicinal chemistry groups: EU-OPENSCREEN's affiliated screening partner sites implement the EU-OPENSCREEN high-throughput screening (HTS) and High-content screening (HCS) projects by using the EU-OPENSCREEN chemical compound collection, in collaboration with the external assay developer. They have been operational as local groups collaborating with external researchers over the past years, even before the EU-OPENSCREEN ERIC has been established. A recent publication showcases several successful projects, which have been realized by individual partner sites, as an example of the capabilities and expertise within the research infrastructure [20]. The chemistry groups have an excellent, proven track record in medicinal chemistry and hit-to-lead/tool optimization. As part of the collaborations with external researchers, they provide services ranging from the re-synthesis of hit compounds and chemical optimization by synthesis of focused libraries containing structurally similar analogues, elaboration of structure activity relationships (SAR), and NMR and TOF-LC-MS analytics.
**121**
*Accelerating Chemical Tool Discovery by Academic Collaborative Models*
*2.4.2 Access to the research infrastructure for external researchers*
hit-to-lead/tool optimization, with an external chemist.
**Second**, organic and medicinal chemists and pharmacologists who seek to expose their compounds to a large number of screens, and thereby a wide range of biological targets. They provide their compounds to EU-OPENSCREEN, so that their compounds are 'bio-profiled' and tested as part of the screening collection at the EU-OPENSCREEN partner sites. As chemists often have only limited opportunities to systematically annotate their compounds, their incentive to provide their compounds to EU-OPENSCREEN is the possibility to identify novel biological activities of their compounds. A similar approach to crowd-sourcing academic compounds has been applied over more than a decade within the French Chimiothèque
main groups of researchers who will benefit from EU-OPENSCREEN:
The EU-OPENSCREEN partner sites have been operational as local screening platforms for many years. During this time, they predominantly work with their colleagues from the hosting institution and university. By working with the same collaborators over a longer time period, both sides could, over the time, increasingly gain practical experience and build a knowledge base, for example, in developing miniaturized, robust assays which are amendable to screening large compound collections. One of the aims of EU-OPENSCREEN is to enable as-yet under-served and under-represented user communities, which, by definition, did not yet have the opportunity to gain practical experience in these areas. Therefore, EU-OPENSCREEN will offer training courses, for example in assay development and other aspects of high-throughput screening. Furthermore, staff exchanges at established partner sites for scientists from prospective sites in countries that are not yet members of EU-OPENSCREEN promote convergence in technical capacities.
External scientists have open access to a chemical library, assay development and screening facilities, medicinal chemistry and informatics platforms. There are three
**First**, molecular and cell biologists, biochemists, microbiologists, plant biologists etc. who develop assays which are amendable to screening and are interested in developing a chemical 'tool' compound for their biological target or pathway of interest to answer a biological question or, in the case of disease-relevant targets, develop new therapeutic approaches to addressing unmet medical needs for patients. These scientists benefit from the open access to the screening capabilities of EU-OPENSCREEN's screening partner sites. They are encouraged to contact and consult the central office of EU-OPENSCREEN, which acts as a single point of contact for external scientists, prior to submitting a project proposal. Depending on the proposal and project requirements, the central office identifies one or more partner sites within the network, which offer the appropriate technology and expertise. The technical feasibility and scientific novelty will be evaluated. After the project proposal has been accepted, the project is initiated in collaboration with a partner site by transferring the assay onto the screening platform. This process often involves further optimization and miniaturization into a 384-well or 1536-well plate format, with the external scientist, who developed the assay, being actively involved in this process at the screening facility. After the screening of the EU-OPENSCREEN compound collection at the EU-OPENSCREEN screening site, data analysis and hit validation, a list with the validated hits will be available to the assay developer. The validated hits will be further optimized either with an EU-OPENSCREEN chemistry site or, if the assay provider already has an established collaboration for the
*DOI: http://dx.doi.org/10.5772/intechopen.91138*
*2.4.1.3 Training*
| doab | 2025-04-07T04:13:04.433683 | 20-4-2021 18:19 | {
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ffe82432-4883-4adc-b03b-937c1baf5090.97 | *2.4.1.3 Training*
*Cheminformatics and Its Applications*
pharmaceutical companies.
backup and security.
best practices in the community.
intellectual property.
and TOF-LC-MS analytics.
Fraunhofer IME actively contributes, is the Innovative Medicines Initiative (IMI) funded 'FAIRplus' project (https://fairplus-project.eu/), which aims to facilitate the application of FAIR principles to data from certain IMI projects and datasets from
The ECBD is the central database for the integration of screening data from EU-OPENSCREEN projects with advanced search, analysis, and visualization tools. There will be three levels of data management and access: First, bioactivity data generation of compounds in screening projects, implemented at the individual EU-OPENSCREEN screening sites, using assays provided by the external collaboration partners; second, the integration of these screening datasets from partner sites into the ECBD; and, third, public dissemination of the data through established databases like ChEMBL [24] and PubChem [25, 26]. The ECBD is hosted by Petr Bartunek, the coordinator of CZ-OPENSCREEN, and his team at the Institute of Molecular Genetics of the ASCR in Prague, Czech Republic, who have developed the open data resource Probes & Drugs portal [27] as well as other databases such as the Zebrabase [28]. The e-infrastructure CESNET provides cloud-based hosting,
An important aspect in the context of integrating complex and diverse screening data, when dealing with datasets from various affiliated, but legally independent sites that jointly use the compound collection, is the implementation of harmonized data standards and data curation. The ECBD adheres to well-established ontologies and identifiers, for example, the BioAssay Ontology (BAO) [29] for the classification and description of assays, which are commonly used by other similar open data repositories, such as ChEMBL or PubChem BioAssay. Only officially accredited partner sites have permission to upload data into the ECBD and uploaded data will be curated both automatically (e.g. file format, column values) as well as manually (e.g. data inspection) by the ECBD team. In case of ambiguities, the ECBD team contacts the data provider to resolve the issue. The ECBD team provides user support and help desk functions. Webinars on data deposition, the use of ECBD for data searching, visualizations and analysis are planned and dedicated workshops will be organized to demonstrate database users all ECBD capabilities and to share
A grace period of up to 3 years between the completion of the primary screen and data publication in the EU-OPENSCREEN database is provided, during which the bioactivity datasets are not publicly accessible. This grace period allows for follow-up studies, publication in peer-review scientific journals and securing of
Assay development and screening facilities, and medicinal chemistry groups: EU-OPENSCREEN's affiliated screening partner sites implement the EU-OPENSCREEN high-throughput screening (HTS) and High-content screening (HCS) projects by using the EU-OPENSCREEN chemical compound collection, in collaboration with the external assay developer. They have been operational as local groups collaborating with external researchers over the past years, even before the EU-OPENSCREEN ERIC has been established. A recent publication showcases several successful projects, which have been realized by individual partner sites, as an example of the capabilities and expertise within the research infrastructure [20]. The chemistry groups have an excellent, proven track record in medicinal chemistry and hit-to-lead/tool optimization. As part of the collaborations with external researchers, they provide services ranging from the re-synthesis of hit compounds and chemical optimization by synthesis of focused libraries containing structurally similar analogues, elaboration of structure activity relationships (SAR), and NMR
**120**
The EU-OPENSCREEN partner sites have been operational as local screening platforms for many years. During this time, they predominantly work with their colleagues from the hosting institution and university. By working with the same collaborators over a longer time period, both sides could, over the time, increasingly gain practical experience and build a knowledge base, for example, in developing miniaturized, robust assays which are amendable to screening large compound collections. One of the aims of EU-OPENSCREEN is to enable as-yet under-served and under-represented user communities, which, by definition, did not yet have the opportunity to gain practical experience in these areas. Therefore, EU-OPENSCREEN will offer training courses, for example in assay development and other aspects of high-throughput screening. Furthermore, staff exchanges at established partner sites for scientists from prospective sites in countries that are not yet members of EU-OPENSCREEN promote convergence in technical capacities.
| doab | 2025-04-07T04:13:04.434371 | 20-4-2021 18:19 | {
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ffe82432-4883-4adc-b03b-937c1baf5090.98 | *2.4.2 Access to the research infrastructure for external researchers*
External scientists have open access to a chemical library, assay development and screening facilities, medicinal chemistry and informatics platforms. There are three main groups of researchers who will benefit from EU-OPENSCREEN:
**First**, molecular and cell biologists, biochemists, microbiologists, plant biologists etc. who develop assays which are amendable to screening and are interested in developing a chemical 'tool' compound for their biological target or pathway of interest to answer a biological question or, in the case of disease-relevant targets, develop new therapeutic approaches to addressing unmet medical needs for patients. These scientists benefit from the open access to the screening capabilities of EU-OPENSCREEN's screening partner sites. They are encouraged to contact and consult the central office of EU-OPENSCREEN, which acts as a single point of contact for external scientists, prior to submitting a project proposal. Depending on the proposal and project requirements, the central office identifies one or more partner sites within the network, which offer the appropriate technology and expertise. The technical feasibility and scientific novelty will be evaluated. After the project proposal has been accepted, the project is initiated in collaboration with a partner site by transferring the assay onto the screening platform. This process often involves further optimization and miniaturization into a 384-well or 1536-well plate format, with the external scientist, who developed the assay, being actively involved in this process at the screening facility. After the screening of the EU-OPENSCREEN compound collection at the EU-OPENSCREEN screening site, data analysis and hit validation, a list with the validated hits will be available to the assay developer. The validated hits will be further optimized either with an EU-OPENSCREEN chemistry site or, if the assay provider already has an established collaboration for the hit-to-lead/tool optimization, with an external chemist.
**Second**, organic and medicinal chemists and pharmacologists who seek to expose their compounds to a large number of screens, and thereby a wide range of biological targets. They provide their compounds to EU-OPENSCREEN, so that their compounds are 'bio-profiled' and tested as part of the screening collection at the EU-OPENSCREEN partner sites. As chemists often have only limited opportunities to systematically annotate their compounds, their incentive to provide their compounds to EU-OPENSCREEN is the possibility to identify novel biological activities of their compounds. A similar approach to crowd-sourcing academic compounds has been applied over more than a decade within the French Chimiothèque
Nationale [30]. Another, more recent opportunity for chemists to screen their compounds is the CO-ADD (Community for Open Antimicrobial Drug Discovery) [31, 32] initiative, where chemists can test their compounds for antimicrobial activity against ESKAPE pathogens. These initiatives demonstrate that the prospect of receiving bioactivity data is a strong incentive for chemists to donate, and disclose the structure and associated bioactivity data of, their compounds.
**Third**, database users who use EU-OPENSCREEN's European Chemical Biology Database (ECBD) to access the bioactivity datasets generated during the screening projects. Importantly, the data will also be accessible through other established open data repositories including the ChEMBL database. Assay providers who screen the EU-OPENSCREEN compound library benefit from the ECBD for their own projects by having access to the public bioactivity data from previous projects, and at the same time, they also contribute to worldwide efforts on open science.
### *2.4.2.1 Access policy and procedure*
The democratization of access to state-of-the-art technology platforms, resources and expertise is the key objective of all European research infrastructure. Importantly, as a European open access research infrastructure, a common access policy and procedure is applied across its network of partner sites. EU-OPENSCREEN is accessible to researchers from academia and industry worldwide. The access to EU-OPENSCREEN by external researchers is in line with the 'European Charter for Access to Research Infrastructures—Principles and Guidelines for Access and Related Services' [33] published by the European Commission in 2016. The charter's guidelines describe three access modes, by which access to research infrastructures may be provided—these are excellence-driven, marketdriven and wide access. Excellence-driven access is provided to the majority of scientists who developed an assay and collaborate with EU-OPENSCREEN to implement a screening and/or hit optimization project as well as to chemists who provide their compounds to be incorporated in the EU-OPENSCREEN compound collection. Scientists who use the ECBD will be provided wide access to the bioactivity data.
### **3. Conclusions**
In this book chapter, we described various academic collaboration models which aim to accelerate chemical too discovery. These initiatives differ in many aspects, for example in structure (e.g. individual academic research groups, public-private partnerships, research infrastructures; single-site vs. distributed/multinational), operational model (e.g. closed consortia, open-access research infrastructures), user communities, funding model (e.g. institutional funding, third-party funding over a defined funding period, long-term funding by member countries), access and data publication policies. Each of these initiatives complement each other and supports academic chemical biology and drug discovery.
### **Acknowledgements**
The authors would like to thank Ronald Frank (senior advisor of EU-OPENSCREEN, Berlin) and Anna-Lena Gustavsson (Head of the CBCS node at Karolinska Institutet, Stockholm) for ideas and information related to the content of the manuscript.
**123**
**Author details**
Bahne Stechmann and Wolfgang Fecke\* EU-OPENSCREEN ERIC, Berlin, Germany
provided the original work is properly cited.
\*Address all correspondence to: [email protected]
© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,
*Accelerating Chemical Tool Discovery by Academic Collaborative Models*
ASCR Academy of Sciences of the Czech Republic
ECBD European Chemical Biology Database
NIH National Institutes of Health NMR Nuclear magnetic resonance
CESNET association of universities of the Czech Republic and
ChEMBL chemical database of bioactive molecules with drug-like
ESKAPE acronym encompassing the names of six bacterial patho-
TOF-LC-MS Time-of-flight liquid chromatography mass spectroscopy
the Czech Academy of Sciences, operating the national e-infrastructure for science, research and education
properties, maintained by the European Bioinformatics Institute of the European Molecular Biology Laboratory
gens commonly associated with antimicrobial resistance
*DOI: http://dx.doi.org/10.5772/intechopen.91138*
The authors declare no conflict of interest.
**Conflict of interest**
**Abbreviations**
*Accelerating Chemical Tool Discovery by Academic Collaborative Models DOI: http://dx.doi.org/10.5772/intechopen.91138*
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ffe82432-4883-4adc-b03b-937c1baf5090.99 | **Conflict of interest**
*Cheminformatics and Its Applications*
*2.4.2.1 Access policy and procedure*
Nationale [30]. Another, more recent opportunity for chemists to screen their compounds is the CO-ADD (Community for Open Antimicrobial Drug Discovery) [31, 32] initiative, where chemists can test their compounds for antimicrobial activity against ESKAPE pathogens. These initiatives demonstrate that the prospect of receiving bioactivity data is a strong incentive for chemists to donate, and disclose
**Third**, database users who use EU-OPENSCREEN's European Chemical Biology Database (ECBD) to access the bioactivity datasets generated during the screening projects. Importantly, the data will also be accessible through other established open data repositories including the ChEMBL database. Assay providers who screen the EU-OPENSCREEN compound library benefit from the ECBD for their own projects by having access to the public bioactivity data from previous projects, and at the
the structure and associated bioactivity data of, their compounds.
same time, they also contribute to worldwide efforts on open science.
The democratization of access to state-of-the-art technology platforms, resources and expertise is the key objective of all European research infrastructure. Importantly, as a European open access research infrastructure, a common access policy and procedure is applied across its network of partner sites. EU-OPENSCREEN is accessible to researchers from academia and industry worldwide. The access to EU-OPENSCREEN by external researchers is in line with the 'European Charter for Access to Research Infrastructures—Principles and Guidelines
for Access and Related Services' [33] published by the European Commission in 2016. The charter's guidelines describe three access modes, by which access to research infrastructures may be provided—these are excellence-driven, marketdriven and wide access. Excellence-driven access is provided to the majority of scientists who developed an assay and collaborate with EU-OPENSCREEN to implement a screening and/or hit optimization project as well as to chemists who provide their compounds to be incorporated in the EU-OPENSCREEN compound collection. Scientists who use the ECBD will be provided wide access to the bioactivity data.
In this book chapter, we described various academic collaboration models which aim to accelerate chemical too discovery. These initiatives differ in many aspects, for example in structure (e.g. individual academic research groups, public-private partnerships, research infrastructures; single-site vs. distributed/multinational), operational model (e.g. closed consortia, open-access research infrastructures), user communities, funding model (e.g. institutional funding, third-party funding over a defined funding period, long-term funding by member countries), access and data publication policies. Each of these initiatives complement each other and
supports academic chemical biology and drug discovery.
The authors would like to thank Ronald Frank (senior advisor of
EU-OPENSCREEN, Berlin) and Anna-Lena Gustavsson (Head of the CBCS node at Karolinska Institutet, Stockholm) for ideas and information related to the content
**122**
**3. Conclusions**
**Acknowledgements**
of the manuscript.
The authors declare no conflict of interest.
| doab | 2025-04-07T04:13:04.434823 | 20-4-2021 18:19 | {
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ffe82432-4883-4adc-b03b-937c1baf5090.101 | **Author details**
Bahne Stechmann and Wolfgang Fecke\* EU-OPENSCREEN ERIC, Berlin, Germany
\*Address all correspondence to: [email protected]
© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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ffe82432-4883-4adc-b03b-937c1baf5090.104 | Chemical Biology Toolsets for Drug Discovery and Target Identification
*Ammara Riaz, Azhar Rasul, Iqra Sarfraz, Javaria Nawaz, Ayesha Sadiqa, Rabia Zara, Samreen Gul Khan and Zeliha Selamoglu*
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ffe82432-4883-4adc-b03b-937c1baf5090.105 | **Abstract**
Chemical biology is the scientific discipline that deals with the application of chemical techniques and often small molecules produced through synthetic chemistry, to the manipulation and study of biological systems. Its working framework ranges from simple chemical entities to complex drugs by employing the principles of biological origin. This chapter particularly focuses on the principles and working models of chemical biology to discover new drug leads. Drug discovery is an extensive and multifaceted complex process. Chemical biology uses both natural and synthetic compounds with the best therapeutic potential and verifies them by employing the best possible chemical toolsets. Screening of compounds is done by the use of phenotypic as well as the target-based screening to identify and characterize the potent hits. After the identification of target, it is characterized, and validated by extensive testing. The next step is the validation of hits obtained, and lead compounds are tested in clinical trials before introducing them for commercial application.
**Keywords:** chemical biology, drug discovery, target identification, target validation, phenotypic screening
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ffe82432-4883-4adc-b03b-937c1baf5090.106 | **1. Introduction to chemical biology and history**
Chemical biology flourished as a discipline of science which makes use of several aspects of chemistry to understand biology [1]. Chemical biology includes a wide range of fundamental problems related to the understanding of complex biological processes by the development of synthetic frameworks to generate selective and active lead compounds [2].
The roots of history of chemical biology lie in the emergence of chemistry and biology as separate disciplines. Chemical biology flourished as a separate discipline of science because of newer challenges and questions for the study of chemical methods employed on living bodies. This branch of study is concerned with advanced molecular concepts of biology harnessed to the use of chemical entities. In spite of the newness of this concept, the history of chemical biology extends up to two centuries, considering the foundations of chemistry and biology. Here only a brief account of history of chemical biology is discussed. Joseph Priestley
discovered nitrous oxide gas in 1772 and incubated the mice with "airs" (the gases discovered till that time). He used 10 gases including nitrous oxide on experimental mice. His experiments on mice faced a strong mass discontent from Americans who showed a sympathetic behavior towards animal rights. Thus, the first chemical biologist fell a prey to angry mob due to his experiment on mice [3].
Afterwards, another chemist, Humphry Davy, worked (1778–1829) on the newly isolated and unfamiliar gases at that time. Frightened by the previous experiment, Humphry completely omitted the use of mice and decided to carry out the research on himself. It was not a matter of surprise that one of the gases, carbon monoxide, proved fatal for the scientist, but the pleasant effect of nitrous oxide made him name this gas, "the laughing gas." He also investigated the use of this gas in medical surgeries. Samuel Taylor also documented this gas as a pleasure-making gas [4], but the practical use of this gas in medicine was described in 1844 by an American
**129**
*Chemical Biology Toolsets for Drug Discovery and Target Identification*
dentist, Horace Wells [5]. In 1998, three scientists, namely, Ferid Murad, Robert Furchgott, and Louis Ignarro, won Nobel Prize for the demonstration of significant
to lay the basis of chemical biology by carrying out his research on vitalism. He prepared urea from inorganic chemicals and rejected the famous "vital force theory" in 1828 [7]. The next important event in the history of chemical biology "cellular imaging" was revolutionized by utilizing the chemical approaches during the nineteenth century. John Hershel invented the cyanotype process which was brought into practice by Anna Atkins to prepare delicate botanical specimens. This noble lady also published her book entitled as *Photographs of British Algae: Cyanotype Impressions* [8]. Ehrlich (1854–1915) is thought to be the pioneer of the earliest forms of chemotherapy and drug therapy. He carried out numerous experiments on aniline based dyes and proposed the idea of "magic bullets." He said that these magic bullets are capable of targeting specific pathogens. He discovered a chemical compound Salvarsan, a drug used against syphilis. This compound is also called as Ehrlich's 606th compound, it was named so because of the successful compound he discovered after 605 failed target compounds. The discovery of this compound paved a way for the discovery of new chemical entities or the new "magic bullets" [6, 9] (**Figure 1**). Chemical biology flourished as an eminent scientific discipline due to significant
contributions of Koehler (pioneer of various chemical screening approaches), Saghatelian (discovery and characterization of lipids and peptides), Wang (use of chemoproteomics in determination of electrophilically lipidated cellular proteins),
Chemical probes are the small molecules which bind to the specific targeted sites and initiate their cellular activities. These archetypal tools act as highly valued reagents for molecular- and genetic-level biological research. Chemical probes are helpful in the
Many tools have been involved in target validation since the 1980s. Target identification and validation are long procedures. They were mainly based on structureactivity relationship. The drug discovery system becomes the most important approach towards the targeted cells [11]. Traditional antisense and RNA interference (RNAi) technologies are the robust tools used in multidimensional phases to discover and validate the potential drug targets. This approach elaborates the potentially selective cleavage of a targeted messenger RNA. This targeting technique enables the
Induced protein degradation is an event-driven approach which depends on drug binding and eliminating the target protein after tagging it. This approach is gaining attention in recent times because of the selective degradation of the target proteins.
accurate investigation of biological pathways and their associated targets [10].
researchers to explore the protein-based expression on phenotypes [12].
and Patti and Northen (metabolomics analysis) [1].
**2. Chemical biology tools**
**2.2 Antisense and RNAi technologies**
**2.3 Protein degradation strategies**
*2.3.1 Induced protein degradation*
**2.1 Chemical probes**
Wöhler is a well-known scientist in the history of chemical biology. He attempted
*DOI: http://dx.doi.org/10.5772/intechopen.91732*
role of nitric acid in cell signaling [6].
**Figure 1.** *History of chemical biology with its eminent events.*
#### *Chemical Biology Toolsets for Drug Discovery and Target Identification DOI: http://dx.doi.org/10.5772/intechopen.91732*
dentist, Horace Wells [5]. In 1998, three scientists, namely, Ferid Murad, Robert Furchgott, and Louis Ignarro, won Nobel Prize for the demonstration of significant role of nitric acid in cell signaling [6].
Wöhler is a well-known scientist in the history of chemical biology. He attempted to lay the basis of chemical biology by carrying out his research on vitalism. He prepared urea from inorganic chemicals and rejected the famous "vital force theory" in 1828 [7]. The next important event in the history of chemical biology "cellular imaging" was revolutionized by utilizing the chemical approaches during the nineteenth century. John Hershel invented the cyanotype process which was brought into practice by Anna Atkins to prepare delicate botanical specimens. This noble lady also published her book entitled as *Photographs of British Algae: Cyanotype Impressions* [8].
Ehrlich (1854–1915) is thought to be the pioneer of the earliest forms of chemotherapy and drug therapy. He carried out numerous experiments on aniline based dyes and proposed the idea of "magic bullets." He said that these magic bullets are capable of targeting specific pathogens. He discovered a chemical compound Salvarsan, a drug used against syphilis. This compound is also called as Ehrlich's 606th compound, it was named so because of the successful compound he discovered after 605 failed target compounds. The discovery of this compound paved a way for the discovery of new chemical entities or the new "magic bullets" [6, 9] (**Figure 1**).
Chemical biology flourished as an eminent scientific discipline due to significant contributions of Koehler (pioneer of various chemical screening approaches), Saghatelian (discovery and characterization of lipids and peptides), Wang (use of chemoproteomics in determination of electrophilically lipidated cellular proteins), and Patti and Northen (metabolomics analysis) [1].
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ffe82432-4883-4adc-b03b-937c1baf5090.108 | **2.1 Chemical probes**
*Cheminformatics and Its Applications*
discovered nitrous oxide gas in 1772 and incubated the mice with "airs" (the gases discovered till that time). He used 10 gases including nitrous oxide on experimental mice. His experiments on mice faced a strong mass discontent from Americans who showed a sympathetic behavior towards animal rights. Thus, the first chemical
Afterwards, another chemist, Humphry Davy, worked (1778–1829) on the newly isolated and unfamiliar gases at that time. Frightened by the previous experiment, Humphry completely omitted the use of mice and decided to carry out the research on himself. It was not a matter of surprise that one of the gases, carbon monoxide, proved fatal for the scientist, but the pleasant effect of nitrous oxide made him name this gas, "the laughing gas." He also investigated the use of this gas in medical surgeries. Samuel Taylor also documented this gas as a pleasure-making gas [4], but the practical use of this gas in medicine was described in 1844 by an American
biologist fell a prey to angry mob due to his experiment on mice [3].
**128**
**Figure 1.**
*History of chemical biology with its eminent events.*
Chemical probes are the small molecules which bind to the specific targeted sites and initiate their cellular activities. These archetypal tools act as highly valued reagents for molecular- and genetic-level biological research. Chemical probes are helpful in the accurate investigation of biological pathways and their associated targets [10].
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ffe82432-4883-4adc-b03b-937c1baf5090.109 | **2.2 Antisense and RNAi technologies**
Many tools have been involved in target validation since the 1980s. Target identification and validation are long procedures. They were mainly based on structureactivity relationship. The drug discovery system becomes the most important approach towards the targeted cells [11]. Traditional antisense and RNA interference (RNAi) technologies are the robust tools used in multidimensional phases to discover and validate the potential drug targets. This approach elaborates the potentially selective cleavage of a targeted messenger RNA. This targeting technique enables the researchers to explore the protein-based expression on phenotypes [12].
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ffe82432-4883-4adc-b03b-937c1baf5090.111 | *2.3.1 Induced protein degradation*
Induced protein degradation is an event-driven approach which depends on drug binding and eliminating the target protein after tagging it. This approach is gaining attention in recent times because of the selective degradation of the target proteins.
Drug discovery based on small molecules focuses on the loss of function of proteins due to the already-occupied binding sites ultimately making the proteins unable to target. In this approach, there is a need of high drug exposure in vivo to avoid target inhibition conditions which may lead to potentially harmful side effects of that drug. Proteolysis-targeting chimeras (PROTACS) use the cellular quality control setup to degrade the selective proteins as their targets. This protein degradation system reduces the quantity of drug to be exposed to the living systems which are to be used for halting the protein functions. These proteins may belong to regulatory proteins, transcription factors, and scaffolding proteins [13, 14].
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ffe82432-4883-4adc-b03b-937c1baf5090.112 | *2.3.2 Chemoproteomics*
Chemoproteomics is employed as a chemical tool for target identification. It can be used to investigate the signal transductions. This particular field of study has flourished as a key technology to characterize the action mechanism of chemical probes and drugs which can act as pharmacological modulators, hence validating the cellular targets of several therapeutic drug candidates. Chemoproteomics can be further characterized as affinity- and activity-based chemical proteomics [15]. In some cases when probe development is a difficult task, multiple kinase inhibitors are used for targeting the kinome effectively [16].
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ffe82432-4883-4adc-b03b-937c1baf5090.113 | **3. Drug discovery**
Drug discovery is a hectic multistep procedure comprising of highly systematic approaches to identify, and characterize different compounds leading towards the development of hits and validate them extensively via utilization of chemical toolsets to attain the status of a commercial therapeutic drug status. The important steps of drug discovery are mentioned in **Figure 2**.
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ffe82432-4883-4adc-b03b-937c1baf5090.114 | **3.1 Screening**
There are two fundamental approaches which can be used for the purpose of drug discovery, namely, phenotypic screening and target-based screening.
## **Figure 2.**
*A diagram representing the summary of key notes regarding drug discovery from natural products.*
**131**
**Table 1.**
*Chemical Biology Toolsets for Drug Discovery and Target Identification*
The first one looks at the effects of phenotype that the compound induces on cell, tissue or whole organism, and the second one evaluates the effects of a compound
In the early twentieth century, drug development started with the advancements in pharmacology and synthetic and therapeutic chemistry. In the 1950s and 1960s, enzyme kinetics has provided methods for accurate computation of compound's
Between 1999 and 2008, the US Food and Drug Administration (FDA) approved new drug discovery approaches. During this period, 75 small molecules were discovered and analyzed. Out of these, 28 drugs were discovered through phenotypic
"Alemtuzumab" was the first antibody that was been obtained by using hybridoma technology in combination with phenotypic identification. It was previously reported against relapse of multiple sclerosis and chronic lymphocytic leukemia (CLL). The CD44 antigen (cell surface glycoprotein) antagonist, RG7356, was isolated with the help of function F.I.R.S.T™ platform. Therefore, functional assays antibodies were used to check effects on cell signaling, proliferation, and programmed cell death [19]. Large combinatorial antibody libraries are the sources of human monoclonal antibodies, successfully used in medical and phenotypic screening. For example, BI-505 was isolated by using F.I.R.S.T™ platform. Improved versions of antibodies were ultimately used in simulation studies of tumor cell death assay and for selective B-lymphoma cell surface binding. Soon after the isolation of BI-505, its molecular target was identified as ICAM-1, which were found to be involved in apoptosis of
By using phenotypic screening technology, patients can increase their effective antibody response like B-cell repertoire. For example, from a healthcare worker, anti-respiratory syncytial virus (RSV) antibody, D25, was isolated. On the virus coat, D25 neutralizes RSV, and perfusion structure of the F protein was expressed which was not identified by target-based screening [21]. The use of phenotypic
**duration**
8–10 days [22]
13 days [23]
**References**
selection, and 17 drugs were identified by target dependent selection [18].
B-lymphoma cells. BI-505 has a broad antimyeloma activity [20].
**Disease Cells Assay type Time**
analyses
Cytochemical and
migration assay
immunohistochemical staining
Immunofluorescence staining for in vitro, Western blot, FACs, ELISA, in vitro biochemical kinase assay,
PC12 Protease release assay 48 hours [26]
Cell viability assay 48 hours [24]
Western blots 18–24 days [25]
RT-PCR assay 48 hours [27]
screening in various experiments is outlined in **Table 1**.
Breast cancer MCF7-RFP
Cystic fibrosis Bronchial
Idiopathic pulmonary fibrosis
Respiratory papillomatosis
Huntington's disease
Familial dysautonomia MDA-RFP
Alveolar epithelial type II cells
Lung tumor cells
epithelial cell
Neural crest precursors
*Phenotypic screening used in some experiments.*
*DOI: http://dx.doi.org/10.5772/intechopen.91732*
effectiveness and enzyme competence [17].
on a purified target protein.
*3.1.1 Phenotypic screening*
The first one looks at the effects of phenotype that the compound induces on cell, tissue or whole organism, and the second one evaluates the effects of a compound on a purified target protein.
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ffe82432-4883-4adc-b03b-937c1baf5090.115 | *3.1.1 Phenotypic screening*
*Cheminformatics and Its Applications*
*2.3.2 Chemoproteomics*
**3. Drug discovery**
**3.1 Screening**
Drug discovery based on small molecules focuses on the loss of function of proteins due to the already-occupied binding sites ultimately making the proteins unable to target. In this approach, there is a need of high drug exposure in vivo to avoid target inhibition conditions which may lead to potentially harmful side effects of that drug. Proteolysis-targeting chimeras (PROTACS) use the cellular quality control setup to degrade the selective proteins as their targets. This protein degradation system reduces the quantity of drug to be exposed to the living systems which are to be used for halting the protein functions. These proteins may belong to regulatory
Chemoproteomics is employed as a chemical tool for target identification. It can be used to investigate the signal transductions. This particular field of study has flourished as a key technology to characterize the action mechanism of chemical probes and drugs which can act as pharmacological modulators, hence validating the cellular targets of several therapeutic drug candidates. Chemoproteomics can be further characterized as affinity- and activity-based chemical proteomics [15]. In some cases when probe development is a difficult task, multiple kinase inhibitors
Drug discovery is a hectic multistep procedure comprising of highly systematic
approaches to identify, and characterize different compounds leading towards the development of hits and validate them extensively via utilization of chemical toolsets to attain the status of a commercial therapeutic drug status. The important
There are two fundamental approaches which can be used for the purpose of drug discovery, namely, phenotypic screening and target-based screening.
*A diagram representing the summary of key notes regarding drug discovery from natural products.*
proteins, transcription factors, and scaffolding proteins [13, 14].
are used for targeting the kinome effectively [16].
steps of drug discovery are mentioned in **Figure 2**.
**130**
**Figure 2.**
In the early twentieth century, drug development started with the advancements in pharmacology and synthetic and therapeutic chemistry. In the 1950s and 1960s, enzyme kinetics has provided methods for accurate computation of compound's effectiveness and enzyme competence [17].
Between 1999 and 2008, the US Food and Drug Administration (FDA) approved new drug discovery approaches. During this period, 75 small molecules were discovered and analyzed. Out of these, 28 drugs were discovered through phenotypic selection, and 17 drugs were identified by target dependent selection [18].
"Alemtuzumab" was the first antibody that was been obtained by using hybridoma technology in combination with phenotypic identification. It was previously reported against relapse of multiple sclerosis and chronic lymphocytic leukemia (CLL). The CD44 antigen (cell surface glycoprotein) antagonist, RG7356, was isolated with the help of function F.I.R.S.T™ platform. Therefore, functional assays antibodies were used to check effects on cell signaling, proliferation, and programmed cell death [19].
Large combinatorial antibody libraries are the sources of human monoclonal antibodies, successfully used in medical and phenotypic screening. For example, BI-505 was isolated by using F.I.R.S.T™ platform. Improved versions of antibodies were ultimately used in simulation studies of tumor cell death assay and for selective B-lymphoma cell surface binding. Soon after the isolation of BI-505, its molecular target was identified as ICAM-1, which were found to be involved in apoptosis of B-lymphoma cells. BI-505 has a broad antimyeloma activity [20].
By using phenotypic screening technology, patients can increase their effective antibody response like B-cell repertoire. For example, from a healthcare worker, anti-respiratory syncytial virus (RSV) antibody, D25, was isolated. On the virus coat, D25 neutralizes RSV, and perfusion structure of the F protein was expressed which was not identified by target-based screening [21]. The use of phenotypic screening in various experiments is outlined in **Table 1**.
### **Table 1.**
*Phenotypic screening used in some experiments.*
### *3.1.2 Target-based screening*
Target-based screening of natural compounds and synthetic chemicals is being considered as a significant innovation for anticancer drug development [28]. In 2007, Lysine demethylase 5B (KDM5B) and Histone demethylase were recognized, which are liable for the removal of H3K4me2/3 activation marker. Thus, for cancer therapy, KDM5B is regarded as a promising drug target, but the elevated levels of KDM5B were found in many human cancers [29].
The respiratory chain of *Streptococcus agalactiae* consists of two enzymes; type 2-NADH dehydrogenase (NDH-2) and cytochrome *bd* oxygen reductase. *S. agalactiae* is considered as the primary cause of sepsis and meningitis in neonates as well as considerable cause of pneumonia and urinary tract infection [30]. The difference between phenotypic and target-based screening is shown in **Figure 3**.
Some of the target-based screening methods are mentioned as follows.
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ffe82432-4883-4adc-b03b-937c1baf5090.116 | *3.1.2.1 Mass spectrometry-based method*
Mass spectrometry is known to be a highly efficient technique for the identification and structural characterization of natural products derived from herbal medicine [31].
Target-based method relies on mass spectrometry to search for active compounds, and this technology can be used for identification, structural characterization, quantitative elemental analysis, tracking of key intermediate compounds in a chemical reaction, analysis of pharmaceuticals and metabolites, and elucidation of unknown structures in drug development. All these achievements can be finally used in various applications like pharmaceutics (drug developments, pharmacokinetics, metabolic pathways), clinical screening, etc. On the basis of MS data information of compounds, the UniFi™ platform has been built for more detailed analysis of structures [32].
### *3.1.2.2 Liquid chromatography-mass spectrometry (LC-MS)*
LC-MS is an analytical technique for separating different complex mixtures into their components using liquid chromatography. These assays check the correct synthesis, purity, various physical and chemical properties like their volatility and active functionalities present in the newly synthesized chemical entities [33]. During drug discovery, LC-MS hyphenated technique is used for seperation and structural characterization of compounds [34].
#### **Figure 3.**
*The action potential of phenotypic as well as target-based screening of compounds to validate the hits and leads from natural and synthetic compounds.*
**133**
*Chemical Biology Toolsets for Drug Discovery and Target Identification*
GC-MS is another hyphenated technique for the identification and structure elucidation of unknown compounds derived from natural products [35]. For example, by using GC-MS technique, comprising a gas chromatograph (GC) coupled to a mass spectrometer (MS), complex components of natural oils mixtures may be separated, identified, and quantified, e.g., oils extracted from Apiaceae family (*Anethum graveolens*, *Carum carvi*, *Cuminum cyminum*, *Coriandrum sativum*, *Pimpinella anisum*, *Daucus carota*, *Apium graveolens*, *Foeniculum vulgare*, and *Ammi visnaga*). As a result of this separation technique, petroselinic acid was the major fatty acid from all other palmitic, palmitoleic, stearic, petroselinic, linoleic, linolinic, and arachidic acids [36].
*3.1.2.4 Ultra-performance liquid chromatography-mass spectrometry (UPLC-MS)*
Currently, UPLC-MS is one of the most adaptable hyphenated techniques. Proteomics and metabolomics have proved to be useful concepts for understanding the causes of different diseases. This technology aims to seperate and identify proteins and metabolites for cellular signaling pathways and to discover biomarkers for screening and diagnosis as well as determining response to a specific treatment [37]. For example, vancomycin (VCM) is clinically used for the treatment of human intracranial infections. The treatment concentration of vanomycin greatly varies among the patients. UPLC-MS technique was developed and used for the analysis of
Among the common techniques of metabolomics, NMR has evolved the most. Unlike mass spectroscopy, NMR is also used for quantitative analysis, but it does not require extra steps for sample preparation [39]. It is commonly used to analyze the 3D structures of biomacromolecules and their interactions. It has been proved a valuable tool for the reliable identification of small molecules that bind to proteins and for hit-to-lead optimization. Mainly, NMR spectroscopy is suitable for the analysis of bulk metabolites [40]. NMR has been used for analyzing the structure of protein, nucleic acid, and small molecule [41]. NMR has been proven to be a useful tool in target-based drug discovery in the step of hit identification and lead optimization [42]. For example, NMR spectroscopy is used to understand the structure of G-quadruplexes, which are noncanonical, four standard nucleic acids with consecutive sequences of guanines [43].
Isothermal titration calorimetry (ITC) is the only technique which is currently available for the direct determination of enthalpy, **Δ***H*, of a ligand binding to a protein [44]. Thermodynamic evaluation might be useful to provide information about specificity, agonist versus antagonist effects of ligands, and other important properties [45]. Fragment-based drug discovery (FBDD) is an approach of particular interest and relevance here. Fragments are molecules smaller than typical drugs, and they generally bind with lower affinity than conventional drug screening hits [46]. Measuring the contributions of enthalpy and entropy to the free energy of binding provides information that can be useful in fragment elaboration and subsequent medicinal chemistry work [47]. ITC is a uniquely powerful tool for characterization of the thermodynamics of test compounds binding to target proteins. Interaction between the compound and protein leads to release or uptake of small amounts of heat, while the mixture is held at a close approximation to
*3.1.2.3 Gas chromatography-mass spectrometry (GC-MS)*
*DOI: http://dx.doi.org/10.5772/intechopen.91732*
VCM in human cerebrospinal fluid [38].
*3.1.2.5 Nuclear magnetic resonance spectroscopy (NMR)*
*3.1.2.6 Thermal shift or calorimetry-based method*
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ffe82432-4883-4adc-b03b-937c1baf5090.117 | *3.1.2.3 Gas chromatography-mass spectrometry (GC-MS)*
*Cheminformatics and Its Applications*
KDM5B were found in many human cancers [29].
*3.1.2.2 Liquid chromatography-mass spectrometry (LC-MS)*
structural characterization of compounds [34].
*3.1.2.1 Mass spectrometry-based method*
analysis of structures [32].
Target-based screening of natural compounds and synthetic chemicals is being considered as a significant innovation for anticancer drug development [28]. In 2007, Lysine demethylase 5B (KDM5B) and Histone demethylase were recognized, which are liable for the removal of H3K4me2/3 activation marker. Thus, for cancer therapy, KDM5B is regarded as a promising drug target, but the elevated levels of
Mass spectrometry is known to be a highly efficient technique for the identification and structural characterization of natural products derived from herbal medicine [31]. Target-based method relies on mass spectrometry to search for active compounds, and this technology can be used for identification, structural characterization, quantitative elemental analysis, tracking of key intermediate compounds in a chemical reaction, analysis of pharmaceuticals and metabolites, and elucidation of unknown structures in drug development. All these achievements can be finally used in various applications like pharmaceutics (drug developments, pharmacokinetics, metabolic pathways), clinical screening, etc. On the basis of MS data information of compounds, the UniFi™ platform has been built for more detailed
LC-MS is an analytical technique for separating different complex mixtures into their components using liquid chromatography. These assays check the correct synthesis, purity, various physical and chemical properties like their volatility and active functionalities present in the newly synthesized chemical entities [33]. During drug discovery, LC-MS hyphenated technique is used for seperation and
*The action potential of phenotypic as well as target-based screening of compounds to validate the hits and leads*
The respiratory chain of *Streptococcus agalactiae* consists of two enzymes; type 2-NADH dehydrogenase (NDH-2) and cytochrome *bd* oxygen reductase. *S. agalactiae* is considered as the primary cause of sepsis and meningitis in neonates as well as considerable cause of pneumonia and urinary tract infection [30]. The difference between phenotypic and target-based screening is shown in **Figure 3**. Some of the target-based screening methods are mentioned as follows.
*3.1.2 Target-based screening*
**132**
**Figure 3.**
*from natural and synthetic compounds.*
GC-MS is another hyphenated technique for the identification and structure elucidation of unknown compounds derived from natural products [35]. For example, by using GC-MS technique, comprising a gas chromatograph (GC) coupled to a mass spectrometer (MS), complex components of natural oils mixtures may be separated, identified, and quantified, e.g., oils extracted from Apiaceae family (*Anethum graveolens*, *Carum carvi*, *Cuminum cyminum*, *Coriandrum sativum*, *Pimpinella anisum*, *Daucus carota*, *Apium graveolens*, *Foeniculum vulgare*, and *Ammi visnaga*). As a result of this separation technique, petroselinic acid was the major fatty acid from all other palmitic, palmitoleic, stearic, petroselinic, linoleic, linolinic, and arachidic acids [36].
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ffe82432-4883-4adc-b03b-937c1baf5090.118 | *3.1.2.4 Ultra-performance liquid chromatography-mass spectrometry (UPLC-MS)*
Currently, UPLC-MS is one of the most adaptable hyphenated techniques. Proteomics and metabolomics have proved to be useful concepts for understanding the causes of different diseases. This technology aims to seperate and identify proteins and metabolites for cellular signaling pathways and to discover biomarkers for screening and diagnosis as well as determining response to a specific treatment [37]. For example, vancomycin (VCM) is clinically used for the treatment of human intracranial infections. The treatment concentration of vanomycin greatly varies among the patients. UPLC-MS technique was developed and used for the analysis of VCM in human cerebrospinal fluid [38].
| doab | 2025-04-07T04:13:04.437018 | 20-4-2021 18:19 | {
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ffe82432-4883-4adc-b03b-937c1baf5090.119 | *3.1.2.5 Nuclear magnetic resonance spectroscopy (NMR)*
Among the common techniques of metabolomics, NMR has evolved the most. Unlike mass spectroscopy, NMR is also used for quantitative analysis, but it does not require extra steps for sample preparation [39]. It is commonly used to analyze the 3D structures of biomacromolecules and their interactions. It has been proved a valuable tool for the reliable identification of small molecules that bind to proteins and for hit-to-lead optimization. Mainly, NMR spectroscopy is suitable for the analysis of bulk metabolites [40]. NMR has been used for analyzing the structure of protein, nucleic acid, and small molecule [41]. NMR has been proven to be a useful tool in target-based drug discovery in the step of hit identification and lead optimization [42]. For example, NMR spectroscopy is used to understand the structure of G-quadruplexes, which are noncanonical, four standard nucleic acids with consecutive sequences of guanines [43].
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ffe82432-4883-4adc-b03b-937c1baf5090.120 | *3.1.2.6 Thermal shift or calorimetry-based method*
Isothermal titration calorimetry (ITC) is the only technique which is currently available for the direct determination of enthalpy, **Δ***H*, of a ligand binding to a protein [44]. Thermodynamic evaluation might be useful to provide information about specificity, agonist versus antagonist effects of ligands, and other important properties [45]. Fragment-based drug discovery (FBDD) is an approach of particular interest and relevance here. Fragments are molecules smaller than typical drugs, and they generally bind with lower affinity than conventional drug screening hits [46]. Measuring the contributions of enthalpy and entropy to the free energy of binding provides information that can be useful in fragment elaboration and subsequent medicinal chemistry work [47]. ITC is a uniquely powerful tool for characterization of the thermodynamics of test compounds binding to target proteins. Interaction between the compound and protein leads to release or uptake of small amounts of heat, while the mixture is held at a close approximation to
#### **Figure 4.**
*Comparison between the advantages and disadvantages of target-based and phenotypic screening based upon the different features such as molecular target of disease, its mechanism of action, confirmation methods, SAR optimization methods, and hypothesis limitation.*
constant temperature [48]. Thermal shift screening methods has allowed to identify compounds that interact with *Trypanosoma brucei* choline kinase (TBCK) and inhibit TBCK, a validated drug target against African sleeping sickness [49].
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ffe82432-4883-4adc-b03b-937c1baf5090.121 | *3.1.2.7 Affinity-based methods*
The methods regarding affinity-based immobilized proteins have vital role in understanding the connections between small molecules and their biological targets [50]. Affinity-based technologies are divided into two groups: (1) direct detection of noncovalent macromolecule-ligand complex and (2) indirect detection of noncovalent macromolecule-ligand complex. The negative aspect of this approach is that it recognizes chemical entities basically based on their binding affinities for a target irrespective of whether or not the biological function of the target is affected. In the late 1980s, matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ESI) techniques were used to analyze proteins and nucleic acids. Both phenotypic screening and target-based screening are comparable to each other in terms of benefits and drawbacks. This fact has been illustrated in **Figure 4**.
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ffe82432-4883-4adc-b03b-937c1baf5090.122 | **4. Target identification and characterization**
Target identification and elucidation of its action mechanism have played vital roles in probing small molecules and drug discovery. Target identification has been based on biological and technologically advanced cell-based assays [51].
#### **4.1 Disease association and target validation**
Identification of the molecules and their underlying pathophysiological mechanisms contribute towards the discovery of targets that can be modulated therapeutically [52]. Each drug target is linked to a disease using integrated genome-wide data from a broad range of data sources. The target validation reveals the evidence that associates a target with a disease [53].
**135**
*Chemical Biology Toolsets for Drug Discovery and Target Identification*
gies have been developed for validation of the drugs.
Bioactive small molecules are preferred as lead structures for the target validation. These small molecules isolated from phenotypic screen play a crucial role in chemical biology [54, 55]. Many genomic, proteomic, and bioinformatic technolo-
To identify the selective potent drugs, the first step is to find the protein interference. In signal transductions, protein-protein interactions are involved in the complex cellular networks that govern the different processes [56]. The deregulated transcription factors are involved in playing significant roles in human pathological abnormalities, but the complicated nature of protein-protein networks has made the transcription-targeted therapeutics impractical. Recent technological advancements are the ray of hope regarding the modulation of
Exosomes are highly adequate for drug carriers as a cell-based model. Due to the association of multiple proteins with cellular membranes, the exosomes are well-known in cell to cell communication, and they are the novel approach for the delivery of potent drugs. Exosome-based drug technique is applied for a variety of
Drug target discovery and validation demand complicated and expensive frameworks which may pose heavy financial load on pharmaceutical industry. Target validation is referred to as the direct involvement of a certain molecular target in pathological
The following approaches are used in target validation during the discovery and
Firstly access the antibody fitness towards a specific target. Then, standardized procedures are obligatory to ensure the quality of the sample in test procedures; hence, utilizing only a single approach will not work in all situations [59]. Mass spectrometry is used to identify the validation of the antibody. This type of technique confirms the validity for antibodies or their fragments against the targets. The antibody is able to bind to its natural antigen in cell lysates among thousands of
CETSA is used to assess the capability of a ligand to bind with its targets (cells or tissue samples). The basis of this method lies on the ligand-induced thermodynamic
conformity; hence, its reversal or inflection may have a therapeutic effect [12].
other proteins, DNA, RNA, and other cellular components [60].
disorders such as cancer and various neurodegenerative disorders [58].
*DOI: http://dx.doi.org/10.5772/intechopen.91732*
**4.2 Bioactive small molecules**
**4.3 Protein interactions**
protein interaction networks [57].
**5.1 Approaches to target validation**
*5.1.2 Cellular thermal shift assay (CETSA)*
**5. Target validation**
development of drug.
*5.1.1 Antibodies*
**4.4 Cell-based models and target validation**
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ffe82432-4883-4adc-b03b-937c1baf5090.123 | **4.2 Bioactive small molecules**
*Cheminformatics and Its Applications*
*3.1.2.7 Affinity-based methods*
*optimization methods, and hypothesis limitation.*
**Figure 4.**
**4. Target identification and characterization**
**4.1 Disease association and target validation**
associates a target with a disease [53].
constant temperature [48]. Thermal shift screening methods has allowed to identify compounds that interact with *Trypanosoma brucei* choline kinase (TBCK) and inhibit TBCK, a validated drug target against African sleeping sickness [49].
*Comparison between the advantages and disadvantages of target-based and phenotypic screening based upon the different features such as molecular target of disease, its mechanism of action, confirmation methods, SAR*
The methods regarding affinity-based immobilized proteins have vital role in understanding the connections between small molecules and their biological targets [50]. Affinity-based technologies are divided into two groups: (1) direct detection of noncovalent macromolecule-ligand complex and (2) indirect detection of noncovalent macromolecule-ligand complex. The negative aspect of this approach is that it recognizes chemical entities basically based on their binding affinities for a target irrespective of whether or not the biological function of the target is affected. In the late 1980s, matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ESI) techniques were used to analyze proteins and nucleic acids. Both phenotypic screening and target-based screening are comparable to each other in terms of benefits and drawbacks. This fact has been illustrated in **Figure 4**.
Target identification and elucidation of its action mechanism have played vital roles in probing small molecules and drug discovery. Target identification has been
Identification of the molecules and their underlying pathophysiological mechanisms contribute towards the discovery of targets that can be modulated therapeutically [52]. Each drug target is linked to a disease using integrated genome-wide data from a broad range of data sources. The target validation reveals the evidence that
based on biological and technologically advanced cell-based assays [51].
**134**
Bioactive small molecules are preferred as lead structures for the target validation. These small molecules isolated from phenotypic screen play a crucial role in chemical biology [54, 55]. Many genomic, proteomic, and bioinformatic technologies have been developed for validation of the drugs.
| doab | 2025-04-07T04:13:04.437472 | 20-4-2021 18:19 | {
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ffe82432-4883-4adc-b03b-937c1baf5090.124 | **4.3 Protein interactions**
To identify the selective potent drugs, the first step is to find the protein interference. In signal transductions, protein-protein interactions are involved in the complex cellular networks that govern the different processes [56]. The deregulated transcription factors are involved in playing significant roles in human pathological abnormalities, but the complicated nature of protein-protein networks has made the transcription-targeted therapeutics impractical. Recent technological advancements are the ray of hope regarding the modulation of protein interaction networks [57].
| doab | 2025-04-07T04:13:04.437520 | 20-4-2021 18:19 | {
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"section_idx": 124
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ffe82432-4883-4adc-b03b-937c1baf5090.125 | **4.4 Cell-based models and target validation**
Exosomes are highly adequate for drug carriers as a cell-based model. Due to the association of multiple proteins with cellular membranes, the exosomes are well-known in cell to cell communication, and they are the novel approach for the delivery of potent drugs. Exosome-based drug technique is applied for a variety of disorders such as cancer and various neurodegenerative disorders [58].
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"section_idx": 125
} |
ffe82432-4883-4adc-b03b-937c1baf5090.126 | **5. Target validation**
Drug target discovery and validation demand complicated and expensive frameworks which may pose heavy financial load on pharmaceutical industry. Target validation is referred to as the direct involvement of a certain molecular target in pathological conformity; hence, its reversal or inflection may have a therapeutic effect [12].
## **5.1 Approaches to target validation**
The following approaches are used in target validation during the discovery and development of drug.
| doab | 2025-04-07T04:13:04.437572 | 20-4-2021 18:19 | {
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ffe82432-4883-4adc-b03b-937c1baf5090.127 | *5.1.1 Antibodies*
Firstly access the antibody fitness towards a specific target. Then, standardized procedures are obligatory to ensure the quality of the sample in test procedures; hence, utilizing only a single approach will not work in all situations [59]. Mass spectrometry is used to identify the validation of the antibody. This type of technique confirms the validity for antibodies or their fragments against the targets. The antibody is able to bind to its natural antigen in cell lysates among thousands of other proteins, DNA, RNA, and other cellular components [60].
| doab | 2025-04-07T04:13:04.437601 | 20-4-2021 18:19 | {
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ffe82432-4883-4adc-b03b-937c1baf5090.128 | *5.1.2 Cellular thermal shift assay (CETSA)*
CETSA is used to assess the capability of a ligand to bind with its targets (cells or tissue samples). The basis of this method lies on the ligand-induced thermodynamic
#### *Cheminformatics and Its Applications*
stabilization of target proteins. The compound-treated cell lysates and intact cells were heated to different temperatures, and in the soluble fractions, the target protein was separated from destabilized protein and detected by Western blotting. SPROX is a method of target validation based on identification of ligand-induced stabilization of target proteins. It evaluates the levels of methionine oxidation of target proteins [61].
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ffe82432-4883-4adc-b03b-937c1baf5090.129 | *5.1.3 Drug affinity responsive target stability (DARTS)*
DARTS has been used for the identification of the targeted proteins. It is based on ligand binding interaction with proteins forming a complex which changes the structural stability of target protein. There alteration is measured by SDS page/ liquid chromatography. DARTS is also involved in the analysis of the low affinity interactions [61].
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ffe82432-4883-4adc-b03b-937c1baf5090.130 | **6. Hit generation**
Hit identification is considered as the significant bottleneck for lead generation success and for new medicines. An example for random hit identification is physical and biochemical testing [62]. The journey of a compound from the hit status to lead status follows a series of steps which have been briefly illustrated in **Figure 5**. The figure describes a note of possible techniques which could be utilized for the selection of lead compounds and proceeding them through lead optimization preclinical and clinical phase trials.
#### **Figure 5.**
*A diagram elaborating the significant steps of lead optimization proceeding to clinical phase of natural compounds.*
**137**
*Chemical Biology Toolsets for Drug Discovery and Target Identification*
Pharmaceutical companies are facing constant economic pressure to bring efficacy in drug discovery and development process. Lists of compounds obtained after hit optimization are further subjected to refining process in order to find out the lead compounds that can be analyzed for production at commercial scale. During this "hitto-lead" refining process, many compounds are dropped out due to inadequate absorption, distribution, metabolism, excretion, and toxicity/ADMET characteristics [63]. Refining of hit compounds to lead compound is done through the process of secondary screening. Almost 50% of all drug candidates thin out during optimiza-
There are many approaches available for the discovery and development of drug which might follow different pathways to optimize the compounds into bioavailable drugs. All these pathways must have a common origin; they all begin with a lead compound. It is necessary to go through the phylogeny study of all the compounds because there are some properties like solubility, target affinity, toxicity, ease of synthesis, and bioavailability, all of which are highly dependent on the initial lead
A rational approach is used to select lead drug candidate after optimization of hit compounds. There are many methods which can be used for screening of compounds. Selection of techniques depends upon the source of hit compounds and types of their solvents as well. The following techniques are useful in selection.
Quantitative structure-activity relationship model is used to compare chemical structures by using database of prior selected active compounds. Different software like ChemBioOffice Ultra 1.11 is used to generate two-dimensional and threedimensional structures. The results of QSAR can be validated by using statistical
It is called as Bayesian approach. It provides with proficient understanding of shape features, hydrophobic nature, and electrostatic properties of the compounds. All of these features lie under the structure–activity relationship of selected compounds from hits. Structure data analysis of SAR is obtained in 3D form. Other results are obtained in diverse type of interrelated biochemical data, i.e., average of activities and region explored analysis. The results obtained from average activity show a common part in active compounds, and region explored data exhibit the
It is a powerful method which is used to find out the proportion of ligands with high affinity to target proteins. The compounds which are found to have low ligand binding ability are eliminated, and the compounds with high ligand ability move forward to the precision of compounds. FBDD consists of the techniques such as NMR, SAR, X-ray crystallography, and surface plasmon resonance (SPR).
approaches like correlation coefficient and regression coefficient [66].
*DOI: http://dx.doi.org/10.5772/intechopen.91732*
tion and preclinical and clinical trials [64].
selection and the method of identification [65].
**7.1 Techniques of lead selection**
*7.1.1 QSAR model development*
*7.1.2 Visualization of SAR activity*
areas of fully explored compounds [67].
*7.1.3 Fragment-based drug discovery*
**7. Development of lead drug**
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ffe82432-4883-4adc-b03b-937c1baf5090.131 | **7. Development of lead drug**
*Cheminformatics and Its Applications*
*5.1.3 Drug affinity responsive target stability (DARTS)*
target proteins [61].
interactions [61].
**6. Hit generation**
and clinical phase trials.
stabilization of target proteins. The compound-treated cell lysates and intact cells were heated to different temperatures, and in the soluble fractions, the target protein was separated from destabilized protein and detected by Western blotting. SPROX is a method of target validation based on identification of ligand-induced stabilization of target proteins. It evaluates the levels of methionine oxidation of
DARTS has been used for the identification of the targeted proteins. It is based on ligand binding interaction with proteins forming a complex which changes the structural stability of target protein. There alteration is measured by SDS page/ liquid chromatography. DARTS is also involved in the analysis of the low affinity
Hit identification is considered as the significant bottleneck for lead generation success and for new medicines. An example for random hit identification is physical and biochemical testing [62]. The journey of a compound from the hit status to lead status follows a series of steps which have been briefly illustrated in **Figure 5**. The figure describes a note of possible techniques which could be utilized for the selection of lead compounds and proceeding them through lead optimization preclinical
*A diagram elaborating the significant steps of lead optimization proceeding to clinical phase of natural*
**136**
**Figure 5.**
*compounds.*
Pharmaceutical companies are facing constant economic pressure to bring efficacy in drug discovery and development process. Lists of compounds obtained after hit optimization are further subjected to refining process in order to find out the lead compounds that can be analyzed for production at commercial scale. During this "hitto-lead" refining process, many compounds are dropped out due to inadequate absorption, distribution, metabolism, excretion, and toxicity/ADMET characteristics [63].
Refining of hit compounds to lead compound is done through the process of secondary screening. Almost 50% of all drug candidates thin out during optimization and preclinical and clinical trials [64].
There are many approaches available for the discovery and development of drug which might follow different pathways to optimize the compounds into bioavailable drugs. All these pathways must have a common origin; they all begin with a lead compound. It is necessary to go through the phylogeny study of all the compounds because there are some properties like solubility, target affinity, toxicity, ease of synthesis, and bioavailability, all of which are highly dependent on the initial lead selection and the method of identification [65].
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ffe82432-4883-4adc-b03b-937c1baf5090.132 | **7.1 Techniques of lead selection**
A rational approach is used to select lead drug candidate after optimization of hit compounds. There are many methods which can be used for screening of compounds. Selection of techniques depends upon the source of hit compounds and types of their solvents as well. The following techniques are useful in selection.
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ffe82432-4883-4adc-b03b-937c1baf5090.133 | *7.1.1 QSAR model development*
Quantitative structure-activity relationship model is used to compare chemical structures by using database of prior selected active compounds. Different software like ChemBioOffice Ultra 1.11 is used to generate two-dimensional and threedimensional structures. The results of QSAR can be validated by using statistical approaches like correlation coefficient and regression coefficient [66].
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ffe82432-4883-4adc-b03b-937c1baf5090.134 | *7.1.2 Visualization of SAR activity*
It is called as Bayesian approach. It provides with proficient understanding of shape features, hydrophobic nature, and electrostatic properties of the compounds. All of these features lie under the structure–activity relationship of selected compounds from hits. Structure data analysis of SAR is obtained in 3D form. Other results are obtained in diverse type of interrelated biochemical data, i.e., average of activities and region explored analysis. The results obtained from average activity show a common part in active compounds, and region explored data exhibit the areas of fully explored compounds [67].
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ffe82432-4883-4adc-b03b-937c1baf5090.135 | *7.1.3 Fragment-based drug discovery*
It is a powerful method which is used to find out the proportion of ligands with high affinity to target proteins. The compounds which are found to have low ligand binding ability are eliminated, and the compounds with high ligand ability move forward to the precision of compounds. FBDD consists of the techniques such as NMR, SAR, X-ray crystallography, and surface plasmon resonance (SPR).
## *7.1.3.1 X-ray crystallography*
It can ascertain the binding sites and modes of ligand binding to protein [68].
### *7.1.3.2 Surface plasmon resonance (SPR)*
Surface plasmon resonance is known as a nonlabel technology that can identify, screen, and quantify intermolecular interactions in actual time. It is applied to quantify binding affinities. SPR-dependent biosensors work by detecting the ligands and immobilized target molecular interactions and supply appropriate information on kinetics of biomolecular interactions. The output information can be utilized to provide comprehensive functional data on binding actions such as specificity, kinetics, concentration, and affinity [69]. Scientific literature study revealed Biacore tools as mainly used SPR technology at commercial levels [70].
#### **7.2 Preclinical trials**
In the last 2 years, different methodologies based on high-throughput screening and their combinations with chemistry have been developed in order to manufacture versatile compounds by limiting the resources. Among these methodologies, several other in vitro and in silico supplementary approaches have also come forward for the identification and potential evaluation of these compounds as lead candidate validation. Those compounds which are selected as "hits" during this screening procedure are further analyzed and subjected to in vivo toxicity and efficacy profiling. During preclinical stage of drug development, simple formulation approaches are favored. Combinatorial chemistry and high-throughput approaches have been appraised in several publications [71].
PLOTs are preclinical lead optimization technologies that should be rapid enough to edge with high-throughput discovery screenings without causing further delay and should be predictive and cost-effective. PLOT platform usually comprised of in vitro systems, small and acquiescent to mechanization, and that is why it is easy to achieve the mandatory throughput with minimum use of compound use [72].
#### *7.2.1 Tools of preclinical drug development*
Selection of methodology and tools for selection of preclinical drug candidates is a rigorous process. Sequential approach of preclinical to clinical is practiced to sort out the long list of target selected compounds. This streamline strategy provides with deeper understanding of action of the drug prior to its progress to the next steps [73].
## *7.2.2 Pharmacokinetics and pharmacodynamics (PK/PD) during preclinical drug evaluation*
Pharmacodynamics involves the study of effect of drug in dose- and timedependent manner. Pharmacokinetics is the study of absorption, metabolization, distribution, and excretion of a drug over time. PK/PD is a program at early phase of lead drug development which acts as a bridge between drug discovery and preclinical drug development. This stage set aims for further development activities, and information obtained at this stage act as a key to subsequent steps.
**139**
*Chemical Biology Toolsets for Drug Discovery and Target Identification*
a.It provides potency-based intrinsic activity of the compound rather than dose.
b.It characterizes the compounds on the basis of dose concentration and effect
c.It allows the investigation of tolerance phenomenon of compounds on the basis
Optimization of a drug is a multifaceted process. It usually involves various types of screening methods which tend to find out the metabolism and pharmaco-
This is the final stage of preclinical trials; after this the optimized drug is further processed towards the clinical trial. Absorption, distribution, metabolism, and excretion screening is performed at this stage. The primary goal of ADME is to develop a competitive drug with adequate safety avoiding PK failure
Ideal properties of a drug in ADME testing involve the good oral bioavailability, blood clearance and volume of convenient dosing, and low potential of drug-drug interaction. All of these properties are assessed at early stage of drug discovery [76].
Drug effect is a parameter which determines the concentration of a drug which do not cause any harm at the site of action. In other words at this stage, toxicity of a drug is tested to find out the minimum safe dosage potency. In vitro DRUGeff testing of all compounds show interaction with the target treatment, until a small portion of dose gets to select according to biophase levels. Concentration of treatment dose maximization per unit of biophase acts as a key objective for lead optimization. The drugs qualifying this test enter into the clinical phase [77].
The final step of drug discovery and development is referred to as the clinical trial. At this stage, the data regarding safety and efficacy of the new drug must be proven by application to humans directly in different phases. After the successful trials, research data is sent to the FDA for approval for commercial manufacturing
The first phase of clinical trial normally takes several weeks to some months. At this stage application of optimized drug is tested on a small group of volunteers.
It is necessary because of the following reasons:
kinetic properties of selected compounds or drugs [75].
*DOI: http://dx.doi.org/10.5772/intechopen.91732*
of physiological parameters [74].
relationship.
**7.3 Lead optimization**
*7.3.1 ADME*
in clinical phase.
*7.3.3 DRUGeff*
*7.3.2 ADME properties*
**7.4 Clinical phase of drug discovery**
and marketing (**Figure 6**) [78].
*7.4.1 Clinical phase I*
It is necessary because of the following reasons:
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ffe82432-4883-4adc-b03b-937c1baf5090.136 | **7.3 Lead optimization**
Optimization of a drug is a multifaceted process. It usually involves various types of screening methods which tend to find out the metabolism and pharmacokinetic properties of selected compounds or drugs [75].
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ffe82432-4883-4adc-b03b-937c1baf5090.137 | *7.3.1 ADME*
*Cheminformatics and Its Applications*
*7.1.3.2 Surface plasmon resonance (SPR)*
have been appraised in several publications [71].
*7.2.1 Tools of preclinical drug development*
It can ascertain the binding sites and modes of ligand binding to protein [68].
Surface plasmon resonance is known as a nonlabel technology that can identify, screen, and quantify intermolecular interactions in actual time. It is applied to quantify binding affinities. SPR-dependent biosensors work by detecting the ligands and immobilized target molecular interactions and supply appropriate information on kinetics of biomolecular interactions. The output information can be utilized to provide comprehensive functional data on binding actions such as specificity, kinetics, concentration, and affinity [69]. Scientific literature study revealed Biacore tools as mainly used SPR technology at commercial
In the last 2 years, different methodologies based on high-throughput screening and their combinations with chemistry have been developed in order to manufacture versatile compounds by limiting the resources. Among these methodologies, several other in vitro and in silico supplementary approaches have also come forward for the identification and potential evaluation of these compounds as lead candidate validation. Those compounds which are selected as "hits" during this screening procedure are further analyzed and subjected to in vivo toxicity and efficacy profiling. During preclinical stage of drug development, simple formulation approaches are favored. Combinatorial chemistry and high-throughput approaches
PLOTs are preclinical lead optimization technologies that should be rapid enough to edge with high-throughput discovery screenings without causing further delay and should be predictive and cost-effective. PLOT platform usually comprised of in vitro systems, small and acquiescent to mechanization, and that is why it is easy to achieve the mandatory throughput with minimum use of
Selection of methodology and tools for selection of preclinical drug candidates is a rigorous process. Sequential approach of preclinical to clinical is practiced to sort out the long list of target selected compounds. This streamline strategy provides with deeper understanding of action of the drug prior to its progress to the next steps [73].
*7.2.2 Pharmacokinetics and pharmacodynamics (PK/PD) during preclinical drug*
Pharmacodynamics involves the study of effect of drug in dose- and timedependent manner. Pharmacokinetics is the study of absorption, metabolization, distribution, and excretion of a drug over time. PK/PD is a program at early phase of lead drug development which acts as a bridge between drug discovery and preclinical drug development. This stage set aims for further development activities, and information obtained at this stage act as a key to
*7.1.3.1 X-ray crystallography*
levels [70].
**7.2 Preclinical trials**
compound use [72].
*evaluation*
subsequent steps.
**138**
This is the final stage of preclinical trials; after this the optimized drug is further processed towards the clinical trial. Absorption, distribution, metabolism, and excretion screening is performed at this stage. The primary goal of ADME is to develop a competitive drug with adequate safety avoiding PK failure in clinical phase.
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ffe82432-4883-4adc-b03b-937c1baf5090.138 | *7.3.2 ADME properties*
Ideal properties of a drug in ADME testing involve the good oral bioavailability, blood clearance and volume of convenient dosing, and low potential of drug-drug interaction. All of these properties are assessed at early stage of drug discovery [76].
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ffe82432-4883-4adc-b03b-937c1baf5090.139 | *7.3.3 DRUGeff*
Drug effect is a parameter which determines the concentration of a drug which do not cause any harm at the site of action. In other words at this stage, toxicity of a drug is tested to find out the minimum safe dosage potency. In vitro DRUGeff testing of all compounds show interaction with the target treatment, until a small portion of dose gets to select according to biophase levels. Concentration of treatment dose maximization per unit of biophase acts as a key objective for lead optimization. The drugs qualifying this test enter into the clinical phase [77].
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ffe82432-4883-4adc-b03b-937c1baf5090.140 | **7.4 Clinical phase of drug discovery**
The final step of drug discovery and development is referred to as the clinical trial. At this stage, the data regarding safety and efficacy of the new drug must be proven by application to humans directly in different phases. After the successful trials, research data is sent to the FDA for approval for commercial manufacturing and marketing (**Figure 6**) [78].
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ffe82432-4883-4adc-b03b-937c1baf5090.141 | *7.4.1 Clinical phase I*
The first phase of clinical trial normally takes several weeks to some months. At this stage application of optimized drug is tested on a small group of volunteers.
**Figure 6.** *The journey of potential leads from preclinical to clinical trials.*
They may or may not get paid for their participation in drug trial studies. This mini trial is useful in determining the absorption and side effect of drug in relation to its dose concentration [17].
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ffe82432-4883-4adc-b03b-937c1baf5090.142 | *7.4.2 Clinical phase II*
The second phase of clinical trial may last up to 2 years. It is a totally randomized study which involves the application of drug on a relatively large group of patients. This trial study is divided into two groups of patients, one receiving experimental drug and the other receiving placebo. Sometimes it may be named as a blind application trial. This type of random application of drug allows investigators and pharmaceutics to prove the success and safety of drug to the FDA with comparative information [79].
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ffe82432-4883-4adc-b03b-937c1baf5090.143 | *7.4.3 Clinical phase III*
It is a large-scale testing of drugs on hundreds of patients. This third stage testing provides with a more thorough understanding and effectiveness of useful drugs to the FDA and pharmaceutical companies. The pharmaceutical company can request for the approval for commercial synthesis of drug after phase III is completed [80].
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ffe82432-4883-4adc-b03b-937c1baf5090.144 | *7.4.4 Clinical phase IV*
After the approval of a drug for commercial consumption, clinical phase IV trials are used as post marketing surveillance trials. This trial system is based upon the various objectives at commercial levels, i.e., the comparison of newly approved and already-available drugs in market, to evaluate the chronic effects on patients' quality of life and to estimate the economical comparison of newly approved and already-present drugs as well as the traditional system of medication [81].
**141**
**Author details**
**Conflict of interest**
, Azhar Rasul1
Authors have no conflict of interest.
, Samreen Gul Khan2
University Faisalabad, Faisalabad, Pakistan
provided the original work is properly cited.
\*Address all correspondence to: [email protected]
Faisalabad, Faisalabad, Pakistan
University, Nigde, Turkey
\*, Iqra Sarfraz1
the medical practitioners to use the best among the rest drugs discovered.
, Javaria Nawaz1
and Zeliha Selamoglu3
1 Department of Zoology, Faculty of Life Sciences, Government College University
2 Department of Chemistry, Faculty of Physical Sciences, Government College
3 Department of Medical Biology, Faculty of Medicine, Nigde Ömer Halisdemir
© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,
, Ayesha Sadiqa1
,
Ammara Riaz1
Rabia Zara1
*Chemical Biology Toolsets for Drug Discovery and Target Identification*
Chemical biology is an emerging field of science which particularly focuses on the research in biological systems by employing the chemicals and related chemoinformatic tools. This field of study is working well in combination with medicinal and combinatorial chemistry to seek the cure of incurable and life-threatening human pathologies. This chapter illustrated the significant techniques and chemical setups which can be employed to testify the chemical as well as biological aspects of natural and synthetic compounds before introducing them as therapeutic drugs in the field of medicine. There is an ultimate need of the hour to seek for the newer and better drugs which are safer, cheaper, and more effective than the already existing therapeutics. This field of study is flourishing at a very fast pace, and it is anticipated that it will provide better treatment options and strategies in future for
*DOI: http://dx.doi.org/10.5772/intechopen.91732*
**8. Conclusion and future perspectives**
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ffe82432-4883-4adc-b03b-937c1baf5090.145 | **8. Conclusion and future perspectives**
Chemical biology is an emerging field of science which particularly focuses on the research in biological systems by employing the chemicals and related chemoinformatic tools. This field of study is working well in combination with medicinal and combinatorial chemistry to seek the cure of incurable and life-threatening human pathologies. This chapter illustrated the significant techniques and chemical setups which can be employed to testify the chemical as well as biological aspects of natural and synthetic compounds before introducing them as therapeutic drugs in the field of medicine. There is an ultimate need of the hour to seek for the newer and better drugs which are safer, cheaper, and more effective than the already existing therapeutics. This field of study is flourishing at a very fast pace, and it is anticipated that it will provide better treatment options and strategies in future for the medical practitioners to use the best among the rest drugs discovered.
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ffe82432-4883-4adc-b03b-937c1baf5090.146 | **Conflict of interest**
*Cheminformatics and Its Applications*
dose concentration [17].
*The journey of potential leads from preclinical to clinical trials.*
*7.4.2 Clinical phase II*
**Figure 6.**
information [79].
completed [80].
*7.4.4 Clinical phase IV*
*7.4.3 Clinical phase III*
They may or may not get paid for their participation in drug trial studies. This mini trial is useful in determining the absorption and side effect of drug in relation to its
The second phase of clinical trial may last up to 2 years. It is a totally randomized study which involves the application of drug on a relatively large group of patients. This trial study is divided into two groups of patients, one receiving experimental drug and the other receiving placebo. Sometimes it may be named as a blind application trial. This type of random application of drug allows investigators and pharmaceutics to prove the success and safety of drug to the FDA with comparative
It is a large-scale testing of drugs on hundreds of patients. This third stage testing provides with a more thorough understanding and effectiveness of useful drugs to the FDA and pharmaceutical companies. The pharmaceutical company can request for the approval for commercial synthesis of drug after phase III is
After the approval of a drug for commercial consumption, clinical phase IV trials are used as post marketing surveillance trials. This trial system is based upon the various objectives at commercial levels, i.e., the comparison of newly approved and already-available drugs in market, to evaluate the chronic effects on patients' quality of life and to estimate the economical comparison of newly approved and
already-present drugs as well as the traditional system of medication [81].
**140**
Authors have no conflict of interest.
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ffe82432-4883-4adc-b03b-937c1baf5090.147 | **Author details**
Ammara Riaz1 , Azhar Rasul1 \*, Iqra Sarfraz1 , Javaria Nawaz1 , Ayesha Sadiqa1 , Rabia Zara1 , Samreen Gul Khan2 and Zeliha Selamoglu3
1 Department of Zoology, Faculty of Life Sciences, Government College University Faisalabad, Faisalabad, Pakistan
2 Department of Chemistry, Faculty of Physical Sciences, Government College University Faisalabad, Faisalabad, Pakistan
3 Department of Medical Biology, Faculty of Medicine, Nigde Ömer Halisdemir University, Nigde, Turkey
\*Address all correspondence to: [email protected]
© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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ffe82432-4883-4adc-b03b-937c1baf5090.149 | **Chapter 10**
*Cheminformatics and Its Applications*
[71] Ramstrom O, Lehn JM. Drug discovery by dynamic combinatorial libraries. Nature Reviews. Drug
[72] Atterwill CK, Wing MG. In vitro preclinical lead optimisation technologies (PLOTs) in pharmaceutical development. Toxicology Letters.
[73] Boger E, Friden M. Physiologically
pharmacodynamic modeling accurately predicts the better bronchodilatory effect of inhaled versus oral salbutamol dosage forms. Journal of Aerosol Medicine and Pulmonary Drug
[74] Ekblom M, Hammarlund-Udenaes M,
White RE, Njoroge FG. Lead optimization
hepatitis C virus (HCV) protease inhibitor SCH 503034. Perspectives in Medicinal
Paalzow L. Modeling of tolerance development and rebound effect during different intravenous administrations of morphine to rats. Journal of Pharmacology and Experimental Therapeutics. 1993;**266**:244-252
[75] Cheng KC, Korfmacher WA,
in discovery drug metabolism and pharmacokinetics/case study: The
[76] Balani SK, Miwa GT, Gan LS, Wu JT, Lee FW. Strategy of utilizing in vitro and in vivo ADME tools for lead optimization and drug candidate selection. Current Topics in Medicinal
Chemistry. 2005;**5**:1033-1038
[77] Braggio S, Montanari D, Rossi T, Ratti E. Drug efficiency: A new concept to guide lead optimization programs towards the selection of better clinical
Chemistry. 2007;**1**:1-9
Discovery. 2002;**1**:26-36
2002;**127**:143-151
based pharmacokinetic/
Delivery. 2019;**32**:1-12
[70] Kukanskis K, Elkind J, Melendez J, Murphy T, Miller G, Garner H. Detection of DNA hybridization using the TISPR-1 surface plasmon resonance biosensor. Analytical Biochemistry. 1999;**274**:7-17
candidates. Expert Opinion on Drug
[79] Sartori SB, Singewald N. Novel pharmacological targets in drug development for the treatment of anxiety and anxiety-related disorders. Pharmacology & Therapeutics.
[80] Regan D, Garcia K, Thamm D. Clinical, pathological, and ethical considerations for the conduct of clinical trials in dogs with naturally occurring cancer: A comparative approach to accelerate translational drug development. ILAR Journal.
[81] Stephenson N, Shane E, Chase J, Rowland J, Ries D, Justice N, et al. Survey of machine learning techniques in drug discovery. Current Drug Metabolism. 2019;**20**:185-193
[78] Swann PG, Shapiro MA. Regulatory considerations for development of bioanalytical assays for biotechnology products. Bioanalysis. 2011;**3**:597-603
Discovery. 2010;**5**:609-618
2019;**204**:107402
2018;**59**:99-110
**146**
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ffe82432-4883-4adc-b03b-937c1baf5090.150 | Artificial Intelligence-Based Drug Design and Discovery
*Yu-Chen Lo, Gui Ren, Hiroshi Honda and Kara L. Davis*
| doab | 2025-04-07T04:13:04.439381 | 20-4-2021 18:19 | {
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ffe82432-4883-4adc-b03b-937c1baf5090.151 | **Abstract**
The drug discovery process from hit-to-lead has been a challenging task that requires simultaneously optimizing numerous factors from maximizing compound activity, efficacy to minimizing toxicity and adverse reactions. Recently, the advance of artificial intelligence technique enables drugs to be efficiently purposed *in silico* prior to chemical synthesis and experimental evaluation. In this chapter, we present fundamental concepts of artificial intelligence and their application in drug design and discovery. The emphasis will be on machine learning and deep learning, which demonstrated extensive utility in many branches of computer-aided drug discovery including de novo drug design, QSAR (Quantitative Structure–Activity Relationship) analysis, drug repurposing and chemical space visualization. We will demonstrate how artificial intelligence techniques can be leveraged for developing chemoinformatics pipelines and presented with real-world case studies and practical applications in drug design and discovery. Finally, we will discuss limitations and future direction to guide this rapidly evolving field.
**Keywords:** artificial intelligence, chemoinformatics, data mining, drug discovery
## **1. Introduction**
The path of drug discovery from small molecule ligands to drugs that can be utilized clinically has been a long and arduous process. Starting with a hit compound, the drugs need to be evaluated through multiple *in vitro* and cell-based assays to improve the mechanism of actions followed by mouse models to demonstrate appropriate *in vivo* and transport properties. Mechanistically, the drugs not only need to exert enough binding affinity to the disease targets, but also necessitate proper transport through multiple physiological barriers to enable access to these targets. Other problems like chemical toxicity, often induced by off-targets interactions with unintended proteins as well as pharmacogenetic, where genetic variation influences drug responses all need to be considered in drug design. Therefore, these multifaceted problems in drug discovery often posed significant challenges for drug designers. Recently, the rise of artificial intelligence approach saw potential solutions to these challenges. A sub-umbrella of artificial intelligence called machine-learning has taken a central stage in many R&D sectors of pharmaceutical companies that allows drugs to be developed more efficiently and at the same time mitigate the cost associated with the required experiments [1]. Given some observations of chemical data, machine learning can be used to construct a predictor by learning compound properties from extracted features of compound structures and interactions. Because this approach does not require a mechanistic
understanding of how drugs behave, many compound properties like binding affinity and other transport and toxicity problems can be accurately forecasted in this way before they are synthesized [2]. Furthermore, by simultaneously tackling the Pharmacokinetics/Pharmacodynamics (PK/PD) problems using artificial intelligence, we can expect that the effort and time required to bring a drug from bench to bedside can be substantially reduced. In this regard, the artificial intelligence approach has now become an essential tool to facilitate the drug discovery process.
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ffe82432-4883-4adc-b03b-937c1baf5090.153 | **2.1 Chemical formats**
To facilitate the discussion on artificial intelligence and machine learning in drug discovery and design, it is necessary to understand the type of format and data presentation commonly used for chemical compounds in chemoinformatics. Chemoinformatics is a broad field that studying the application of computers in storing, processing and analyzing chemical data. The field already has more than 30 years of development with focuses on subjects such as chemical representation, chemical descriptors analysis, library design, QSAR analysis and computer-aided drug design [3]. Along with these developments, several popular chemical data formats for data processing has been proposed. Intuitively, the chemical compound is best represented by graphs, also known as "chemical graph" or "molecular graph" where nodes represent atoms and edges represent bonds. The molecular graph is useful for distinguishing different structural isomers but does not contain 3D conformation of the molecules. To store 2D or 3D coordinates of compounds, chemical file formats such as Structure Data Format (SDF), MDL (Molfile), and Protein Data Bank (PDB) formats can be used. In contrast to the PDB file that simply store structural data, the SDF format provides additional advantages of recording descriptors and other chemical properties thus offers better functionality for cheminformatics analysis. Due to the limited memory capacity for handling large compound database, several chemical line notations have also been introduced. One such format is the simplified molecular-input line-entry system (SMILES) format pioneered by Weininger et al [4]. Other linear notations include Wiswesser line notation (WLN), ROSDAL, and SYBYL Line Notation (SLN). Instead of recording compound coordinates directly, the SMILES format store compound structure using simpler ASCII codes. While memory-efficient, there is no unique strings for representing chemical compound particularly for large and structurally complex molecules. To address this, canonical SMILES was proposed that applied the Morgan algorithm for consistent labeling and ordering of chemical structures [5]. Another limitation is the loss of coordinate information and necessitate structural generation programs like PRODRG to predict native molecular geometry [6]. Recently, the need to exchange chemical data over the world wide web (WWW) also saw the development of chemical markup language (CML) similar to the XML format. Despite the development of multiple chemical file formats, many commercial and open source packages have allowed convenient file format conversion using Obabel and RDKit softwares [7, 8].
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ffe82432-4883-4adc-b03b-937c1baf5090.154 | **2.2 Chemical representations**
The ability to represent chemical compounds by machine-learning features that fully captured wide ranges of chemical and physical properties of the target molecule has been an active area of research in chemoinformatics and chemical biology
**149**
*Artificial Intelligence-Based Drug Design and Discovery DOI: http://dx.doi.org/10.5772/intechopen.89012*
properties of several microtubule destabilizing agents [19].
Besides chemical descriptors, the chemical fingerprint is another important chemical representation where the compounds are represented by a binary vector indicating the presence or absence of chemical features [20]. Common 2D chemical fingerprints include path-based fingerprint which detected all possible linear paths consisting of bonds and atoms of a structure given certain bond lengths. For a given pattern, several bits in a bit string is set. While path-based fingerprints like ECFP (Extended Connectivity Fingerprint) have a higher specificity, the potential limitation is "bit collision" where the number of possible patterns exceeds the bit capacity resulting in multiple patterns mapped to the same set of bits. Another type of fingerprint is substructure fingerprints. In the substructure fingerprint like (Molecular ACCess System) MACCS keys, the substructures are predefined and each bit in a bit string is set for specific chemical patterns. Although bit collision is less of an issue, the requirement to encompass all fragment space within a bit string often demands a larger memory size. Recently, the proposal of circular fingerprints represents the state-of-the-art in chemical fingerprint development [21]. In the circular fingerprint, each layer's feature is constructed by applying a fixed hash function to the concatenated features of the neighborhood in the previous layer and the results from the hashed function were mapped to bit string representing
[9, 10]. These chemical features, also known as chemical descriptors, provide the ability to extract essential characteristic of the compound and offer the possibility of developing predictor that can classify novel structures with similar properties. Broadly speaking, the chemical descriptors can be classified as 0D, 1D, 2D, 3D, and 4D [11]. 0D and 1D descriptors like molecular mass, atom number counts can be easily extracted from the molecular formula but does not provide much discriminatory power for compound classification. In practice, 2D and 3D chemical descriptors are the most commonly used molecular features for cheminformatics analysis [12]. Since chemical compound can be viewed as different arrangements of atoms and chemical bond, 2D descriptors can be generated from the molecular graph based on different connectivity of the molecules. Notable 2D descriptors include Weiner index, Balaban index, Randic index and others [1]. Beyond 2D descriptors, 3D descriptors leverage information from molecular surfaces, volumes, and shapes to provide a higher level of chemical representation. The dependency of ligand conformations also prompts the development of 4D descriptors, which accounts for different conformations of the molecules generated over a trajectory from the molecular dynamics simulation [13]. However, the requirement of correct 3D conformation makes 3D and 4D descriptors limited in several aspects. Another type of high dimensional descriptors is molecular interaction field (MIF) developed by Goodford and colleagues [14]. The MIF aims to capture the molecular environment of the ligand based on several properties by placing probes in a rectangular grid surround the target compound. At each grid point, hypothetical probes corresponding to different types of energetic interactions (hydrophobic, electrostatic) were evaluated. The comparison of MIF of compounds enables the identification of critical functional groups for kinase drug-target interactions and drug design [15]. Furthermore, correlating these field values to compound activity enable comparative molecular field analysis (CoMFA), an extended form of 3D-QSAR [16]. Altman's group at Stanford University took a different approach by inspecting ligand environment using amino acid microenvironment. This Feature-based approach lead to direct applications in pocket similarity comparison for identifying novel microtubule binding activity of several anti-estrogenic compounds as well as kinase off-target binding activity [17, 18]. Chemical descriptors can likewise be generated based on the biological phenotypes. For example, drug-induced cell cycle profile changes of compound have been recently utilized to identify DNA-targeting
#### *Artificial Intelligence-Based Drug Design and Discovery DOI: http://dx.doi.org/10.5772/intechopen.89012*
*Cheminformatics and Its Applications*
**2.1 Chemical formats**
**2. Chemoinformatic for drug discovery**
understanding of how drugs behave, many compound properties like binding affinity and other transport and toxicity problems can be accurately forecasted in this way before they are synthesized [2]. Furthermore, by simultaneously tackling the Pharmacokinetics/Pharmacodynamics (PK/PD) problems using artificial intelligence, we can expect that the effort and time required to bring a drug from bench to bedside can be substantially reduced. In this regard, the artificial intelligence approach has now become an essential tool to facilitate the drug discovery process.
To facilitate the discussion on artificial intelligence and machine learning in drug discovery and design, it is necessary to understand the type of format and data presentation commonly used for chemical compounds in chemoinformatics. Chemoinformatics is a broad field that studying the application of computers in storing, processing and analyzing chemical data. The field already has more than 30 years of development with focuses on subjects such as chemical representation, chemical descriptors analysis, library design, QSAR analysis and computer-aided drug design [3]. Along with these developments, several popular chemical data formats for data processing has been proposed. Intuitively, the chemical compound is best represented by graphs, also known as "chemical graph" or "molecular graph" where nodes represent atoms and edges represent bonds. The molecular graph is useful for distinguishing different structural isomers but does not contain 3D conformation of the molecules. To store 2D or 3D coordinates of compounds, chemical file formats such as Structure Data Format (SDF), MDL (Molfile), and Protein Data Bank (PDB) formats can be used. In contrast to the PDB file that simply store structural data, the SDF format provides additional advantages of recording descriptors and other chemical properties thus offers better functionality for cheminformatics analysis. Due to the limited memory capacity for handling large compound database, several chemical line notations have also been introduced. One such format is the simplified molecular-input line-entry system (SMILES) format pioneered by Weininger et al [4]. Other linear notations include Wiswesser line notation (WLN),
ROSDAL, and SYBYL Line Notation (SLN). Instead of recording compound coordinates directly, the SMILES format store compound structure using simpler ASCII codes. While memory-efficient, there is no unique strings for representing chemical compound particularly for large and structurally complex molecules. To address this, canonical SMILES was proposed that applied the Morgan algorithm for consistent labeling and ordering of chemical structures [5]. Another limitation is the loss of coordinate information and necessitate structural generation programs like PRODRG to predict native molecular geometry [6]. Recently, the need to exchange chemical data over the world wide web (WWW) also saw the development of chemical markup language (CML) similar to the XML format. Despite the development of multiple chemical file formats, many commercial and open source packages have allowed convenient file format conversion using Obabel and RDKit
The ability to represent chemical compounds by machine-learning features that fully captured wide ranges of chemical and physical properties of the target molecule has been an active area of research in chemoinformatics and chemical biology
**148**
softwares [7, 8].
**2.2 Chemical representations**
[9, 10]. These chemical features, also known as chemical descriptors, provide the ability to extract essential characteristic of the compound and offer the possibility of developing predictor that can classify novel structures with similar properties. Broadly speaking, the chemical descriptors can be classified as 0D, 1D, 2D, 3D, and 4D [11]. 0D and 1D descriptors like molecular mass, atom number counts can be easily extracted from the molecular formula but does not provide much discriminatory power for compound classification. In practice, 2D and 3D chemical descriptors are the most commonly used molecular features for cheminformatics analysis [12]. Since chemical compound can be viewed as different arrangements of atoms and chemical bond, 2D descriptors can be generated from the molecular graph based on different connectivity of the molecules. Notable 2D descriptors include Weiner index, Balaban index, Randic index and others [1]. Beyond 2D descriptors, 3D descriptors leverage information from molecular surfaces, volumes, and shapes to provide a higher level of chemical representation. The dependency of ligand conformations also prompts the development of 4D descriptors, which accounts for different conformations of the molecules generated over a trajectory from the molecular dynamics simulation [13]. However, the requirement of correct 3D conformation makes 3D and 4D descriptors limited in several aspects. Another type of high dimensional descriptors is molecular interaction field (MIF) developed by Goodford and colleagues [14]. The MIF aims to capture the molecular environment of the ligand based on several properties by placing probes in a rectangular grid surround the target compound. At each grid point, hypothetical probes corresponding to different types of energetic interactions (hydrophobic, electrostatic) were evaluated. The comparison of MIF of compounds enables the identification of critical functional groups for kinase drug-target interactions and drug design [15]. Furthermore, correlating these field values to compound activity enable comparative molecular field analysis (CoMFA), an extended form of 3D-QSAR [16]. Altman's group at Stanford University took a different approach by inspecting ligand environment using amino acid microenvironment. This Feature-based approach lead to direct applications in pocket similarity comparison for identifying novel microtubule binding activity of several anti-estrogenic compounds as well as kinase off-target binding activity [17, 18]. Chemical descriptors can likewise be generated based on the biological phenotypes. For example, drug-induced cell cycle profile changes of compound have been recently utilized to identify DNA-targeting properties of several microtubule destabilizing agents [19].
Besides chemical descriptors, the chemical fingerprint is another important chemical representation where the compounds are represented by a binary vector indicating the presence or absence of chemical features [20]. Common 2D chemical fingerprints include path-based fingerprint which detected all possible linear paths consisting of bonds and atoms of a structure given certain bond lengths. For a given pattern, several bits in a bit string is set. While path-based fingerprints like ECFP (Extended Connectivity Fingerprint) have a higher specificity, the potential limitation is "bit collision" where the number of possible patterns exceeds the bit capacity resulting in multiple patterns mapped to the same set of bits. Another type of fingerprint is substructure fingerprints. In the substructure fingerprint like (Molecular ACCess System) MACCS keys, the substructures are predefined and each bit in a bit string is set for specific chemical patterns. Although bit collision is less of an issue, the requirement to encompass all fragment space within a bit string often demands a larger memory size. Recently, the proposal of circular fingerprints represents the state-of-the-art in chemical fingerprint development [21]. In the circular fingerprint, each layer's feature is constructed by applying a fixed hash function to the concatenated features of the neighborhood in the previous layer and the results from the hashed function were mapped to bit string representing
specific substructures. A modified version of the circular fingerprint, known as graph convolution fingerprint, has recently been proposed where the hashed function is replaced by a differential neural network and a local filter is applied to each atom and neighborhoods similar to that of a convolution neural network. Many of the mentioned fingerprints has been implemented by several open source chemoinformatics package such as Chemoinformatics Development Kit (CDK) and RDKit and saw wide applications in compound database search and other computer-aided drug discovery tasks [22].
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ffe82432-4883-4adc-b03b-937c1baf5090.155 | **3. Artificial intelligence in drug discovery**
The rise of artificial intelligence and, in particular, machine learning and deep learning has given rise to a tsunami of applications in drug discovery and design [23, 24]. Here, we provide an overview of machine learning concepts and techniques commonly applied for chemoinformatics analysis. In a nutshell, machine learning aims to build predictive models based on several features derived from the chemical data, many of which are measured experimentally, such as lipophilicity, water solubility while others are purely theoretical, such as chemical descriptors and molecular fields derived from the chemical graph or 3D structure data. With chemical features on one hand, on the other hand of the equation is the properties that the model intended to learn, which can take on categorical or continuous values and usually pertaining to compound activity in question. Given every pair of features and labels, the model can be trained by identifying an optimal set of parameters that minimizes certain objective functions. Following the training phase, the best model can then be applied to predict the properties of new compounds (**Figure 1**).
Although machine learning has just recently gained in popularity, its application in chemistry is not new. The pioneering work of Alexander Crum-Brown and Thomas Fraser in elucidating the effects of different alkaloids on muscle paralysis results in the proposal of the first general equation for a structure–activity relationship, which intended to bridge biological activity as a function of chemical structure [25]. Early QSAR models such as Hansch analysis were mostly linear or quadratic model of physicochemical parameters that required extensive experimental measurement. This model was succeeded by the Free-Wilson model, which considers the parameters generated from the chemical structure and is more closely resemble the QSAR model in use today. Machine learning techniques in cheminformatics analysis can be broadly classified as supervised learning, unsupervised learning, and reinforcement learning. However, new learning algorithms through a combination of these approaches are continuing being developed. Many of these approaches have already found wide application in QSAR/QSPR prediction, de novo drug design, drug repurposing, and retrosynthetic planning [26–28].
## **3.1 Supervised learning**
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ffe82432-4883-4adc-b03b-937c1baf5090.156 | *3.1.1 Linear regression analysis*
Supervised learning has a long history of development in QSAR analysis [29]. The supervised learning task can include classification, to determine whether a compound class belong to a certain class label, or regression, to predict the bioactivity of a compound over a continuous range of values. A well-known supervised learning approach is the linear regression model, and often the first-line method for exploratory data analysis among statistician. The goal of linear regression is to find
**151**
significant issue.
**Figure 1.**
*to predict activity of new compounds.*
*Artificial Intelligence-Based Drug Design and Discovery DOI: http://dx.doi.org/10.5772/intechopen.89012*
a linear function such that a fitted line that minimizes the distance to the outcome variables. When the logistic function is applied to the linear model, the model can also be applicable for binary classification. A direct extension of linear regression is polynomial regression that model relationships between independent and independent variable as high-degree polynomial of the same or different combination of chemical features. In the case of model underfitting, polynomial regression provides a useful alternative for feature augmentation for the linear model. Both linear and polynomial regression formed the basis of classical Hansch and Free-Wilson analysis [30]. Interestingly, today's situation is completely reversed. With the rapid explosion of chemical descriptors and fingerprints available at chemoinformatician's disposal, twin curse of dimensionality and collinearity has now become a
*Chemoinformatics prediction using artificial intelligence. Starting with a compound, the chemical feature is extracted from the compound 2D graph. The chemical features then serve as input for the machine learning model and trained based on the compound activity. The trained model with fitted parameters can then be used*
Several approaches have been developed to tackle high dimensional data. One potential solution is to exhaustively explore all the possible combination of features to identify the best subset of predictors. However, this approach is inevitably
#### **Figure 1.**
*Cheminformatics and Its Applications*
drug discovery tasks [22].
pounds (**Figure 1**).
**3.1 Supervised learning**
*3.1.1 Linear regression analysis*
**3. Artificial intelligence in drug discovery**
specific substructures. A modified version of the circular fingerprint, known as graph convolution fingerprint, has recently been proposed where the hashed function is replaced by a differential neural network and a local filter is applied to each atom and neighborhoods similar to that of a convolution neural network. Many of the mentioned fingerprints has been implemented by several open source chemoinformatics package such as Chemoinformatics Development Kit (CDK) and RDKit and saw wide applications in compound database search and other computer-aided
The rise of artificial intelligence and, in particular, machine learning and deep learning has given rise to a tsunami of applications in drug discovery and design [23, 24]. Here, we provide an overview of machine learning concepts and techniques commonly applied for chemoinformatics analysis. In a nutshell, machine learning aims to build predictive models based on several features derived from the chemical data, many of which are measured experimentally, such as lipophilicity, water solubility while others are purely theoretical, such as chemical descriptors and molecular fields derived from the chemical graph or 3D structure data. With chemical features on one hand, on the other hand of the equation is the properties that the model intended to learn, which can take on categorical or continuous values and usually pertaining to compound activity in question. Given every pair of features and labels, the model can be trained by identifying an optimal set of parameters that minimizes certain objective functions. Following the training phase, the best model can then be applied to predict the properties of new com-
Although machine learning has just recently gained in popularity, its application in chemistry is not new. The pioneering work of Alexander Crum-Brown and Thomas Fraser in elucidating the effects of different alkaloids on muscle paralysis results in the proposal of the first general equation for a structure–activity relationship, which intended to bridge biological activity as a function of chemical structure [25]. Early QSAR models such as Hansch analysis were mostly linear or quadratic model of physicochemical parameters that required extensive experimental measurement. This model was succeeded by the Free-Wilson model, which considers the parameters generated from the chemical structure and is more closely resemble the QSAR model in use today. Machine learning techniques in cheminformatics analysis can be broadly classified as supervised learning, unsupervised learning, and reinforcement learning. However, new learning algorithms through a combination of these approaches are continuing being developed. Many of these approaches have already found wide application in QSAR/QSPR prediction, de novo
drug design, drug repurposing, and retrosynthetic planning [26–28].
Supervised learning has a long history of development in QSAR analysis [29]. The supervised learning task can include classification, to determine whether a compound class belong to a certain class label, or regression, to predict the bioactivity of a compound over a continuous range of values. A well-known supervised learning approach is the linear regression model, and often the first-line method for exploratory data analysis among statistician. The goal of linear regression is to find
**150**
*Chemoinformatics prediction using artificial intelligence. Starting with a compound, the chemical feature is extracted from the compound 2D graph. The chemical features then serve as input for the machine learning model and trained based on the compound activity. The trained model with fitted parameters can then be used to predict activity of new compounds.*
a linear function such that a fitted line that minimizes the distance to the outcome variables. When the logistic function is applied to the linear model, the model can also be applicable for binary classification. A direct extension of linear regression is polynomial regression that model relationships between independent and independent variable as high-degree polynomial of the same or different combination of chemical features. In the case of model underfitting, polynomial regression provides a useful alternative for feature augmentation for the linear model. Both linear and polynomial regression formed the basis of classical Hansch and Free-Wilson analysis [30]. Interestingly, today's situation is completely reversed. With the rapid explosion of chemical descriptors and fingerprints available at chemoinformatician's disposal, twin curse of dimensionality and collinearity has now become a significant issue.
Several approaches have been developed to tackle high dimensional data. One potential solution is to exhaustively explore all the possible combination of features to identify the best subset of predictors. However, this approach is inevitably
computationally infeasible for large feature space. To solve this, heuristic approach like forward and backward feature selection were developed where each feature was added to the predictors in a stepwise manner and only features that contribute greatest to the fit are kept [31]. An alternative approach for feature selection is dimensional reduction where a smaller set of uncorrelated features can be created as a combination of a larger set of correlated variables. One commonly used dimensional reduction technique is principal component analysis (PCA) that identifies new variables with the largest variances in the dataset [32]. Recently, variable shrinkage method like regularization and evolutionary algorithm has allowed feature selection during the model fitting phase. In the model regularization step, a penalty term is introduced to the objective function to control model complexity. The lasso regularization is one such approach that used an L1 penalty term to constraint objective function along the parameter axis, thus enable effective elimination of redundant features [33]. The evolutionary algorithm is another feature selection approach that encodes features as genes and through successive combination, the algorithm identifies the best set of features measured by a fitness score. Recently, elastic net combines penalties of the lasso and ridge regression and shows promise in variable selection when the number of predictors (*p*) is much bigger than the number of observations (*n*) [34]. Although linear regression analysis formed the backbone of early QSAR analysis, the simple linear assumption of feature vector space is a major limitation for modeling more complex system.
## *3.1.2 Artificial neural network and deep learning*
The requirement to parameterize the QSAR model in a non-linear way saw the widespread application of artificial neural network (ANN) in the chemoinformatic analysis. The ANN, first developed by Bernard Widrow of Stanford University in the 1950s, is inspired by the architecture of a human brain, which consisting of multiple layers of interconnecting nodes analogous to biological neurons. The early neural network model is called "perceptron" that consists of a single layer of inputs and a single layer of output neurons connected by different weights and activation functions [35]. However, it was soon recognized that the one-layer perceptron cannot correctly solve the XOR logical relationship [36]. This limitation prompts the development of multi-layer perceptron, where additional hidden layers were introduced into the model and the weights were estimated using the backpropagation algorithm [37]. As a direct extension of ANN, several deep learning techniques like deep neural network (DNN) has been introduced to process high dimensional data as well as unstructured data for machine vision and natural language processing (NLP). In multiple studies, DNN outperformed several classical machine learning methods in predicting biological activity, solubility, ADMET properties and compound toxicity [38, 39].
To handle high-dimensional data, several feature extraction and dimension reduction mechanisms has been integrated into diverse deep learning frameworks (**Figure 2**). In particular, the convolution neural network is a popular deep learning framework for imaging analysis [40]. A convolution neural network consists of convolution layers, max-pooling layers, and fully connected multilayer perceptron. The purpose of the convolution and max-pooling layer is to extracted local recurring patterns from the image data to fit the input dimension of the fully connected layers. This utility has recently been extended for protein structure analysis in the 3D-CNN approach where protein structures are treated as 3D images [41]. Other deep learning approaches include autoencoder and embedding representation. Autoencoder (AE) is a data-driven approach to obtain a latent presentation of high dimensional data using a smaller set of hidden neurons [42, 43]. An autoencoder
**153**
**Figure 2.**
*autoencoder (AE) and recurrent neural network (RNN).*
*Artificial Intelligence-Based Drug Design and Discovery DOI: http://dx.doi.org/10.5772/intechopen.89012*
found in the original training set.
*3.1.3 Instance-based learning*
consists of encoder and decoder. In the encoding step, the input signal is forward propagated to smaller and smaller sets of hidden layers thus effective map the data to low dimensional space. The training is achieved so that the hidden layers can propagate back to a larger set of output nodes to recover the original signal. A specific form of AE called variational AE (VAE) has recently been applied to de-novo drug design application where latent space was first constructed from the ZINC database from which novel compounds can be recovered by sampling such subspace [44]. In the context of NLP, word embedding such as word2vec implementation is a dimensional reduction technique to learn word presentation that preserves the similarity between data in low-dimension. This formulation has been extended to identify chemical representation in the analogous mol2vec program [45]. The requirement to model sequential data also prompted the development of recurrent neural networks (RNN). The RNN is a variant of artificial neural network where the output from the previous state is used as input for the current state. Therefore, this formulation has a classical analogy to the hidden Markov model (HMM), a type of belief network. RNN has been applied for de novo molecule design by "memorizing" from SMILES string in sequential order and generated novel SMILES by sampling from the underlying probability distribution [46]. By tuning the sampling parameters, it is found that RNN can oftentimes generated valid SMILES string not
In contrast to parametrized learning that required extensive efforts in model tuning and parameter estimation, instance-based learning, also known as memorybased learning, is a different type of machine learning strategy that generates hypothesis from the training data directly [47]. Therefore, the model complexity
*Deep learning architectures for drug discovery. Four common types of deep learning network for supervised and supervised learning including deep neural network (DNN), convolutional neural network (CNN),*
### *Artificial Intelligence-Based Drug Design and Discovery DOI: http://dx.doi.org/10.5772/intechopen.89012*
*Cheminformatics and Its Applications*
*3.1.2 Artificial neural network and deep learning*
and compound toxicity [38, 39].
computationally infeasible for large feature space. To solve this, heuristic approach like forward and backward feature selection were developed where each feature was added to the predictors in a stepwise manner and only features that contribute greatest to the fit are kept [31]. An alternative approach for feature selection is dimensional reduction where a smaller set of uncorrelated features can be created as a combination of a larger set of correlated variables. One commonly used dimensional reduction technique is principal component analysis (PCA) that identifies new variables with the largest variances in the dataset [32]. Recently, variable shrinkage method like regularization and evolutionary algorithm has allowed feature selection during the model fitting phase. In the model regularization step, a penalty term is introduced to the objective function to control model complexity. The lasso regularization is one such approach that used an L1 penalty term to constraint objective function along the parameter axis, thus enable effective elimination of redundant features [33]. The evolutionary algorithm is another feature selection approach that encodes features as genes and through successive combination, the algorithm identifies the best set of features measured by a fitness score. Recently, elastic net combines penalties of the lasso and ridge regression and shows promise in variable selection when the number of predictors (*p*) is much bigger than the number of observations (*n*) [34]. Although linear regression analysis formed the backbone of early QSAR analysis, the simple linear assumption of feature vector space is a major limitation for modeling more complex system.
The requirement to parameterize the QSAR model in a non-linear way saw the widespread application of artificial neural network (ANN) in the chemoinformatic analysis. The ANN, first developed by Bernard Widrow of Stanford University in the 1950s, is inspired by the architecture of a human brain, which consisting of multiple layers of interconnecting nodes analogous to biological neurons. The early neural network model is called "perceptron" that consists of a single layer of inputs and a single layer of output neurons connected by different weights and activation functions [35]. However, it was soon recognized that the one-layer perceptron cannot correctly solve the XOR logical relationship [36]. This limitation prompts the development of multi-layer perceptron, where additional hidden layers were introduced into the model and the weights were estimated using the backpropagation algorithm [37]. As a direct extension of ANN, several deep learning techniques like deep neural network (DNN) has been introduced to process high dimensional data as well as unstructured data for machine vision and natural language processing (NLP). In multiple studies, DNN outperformed several classical machine learning methods in predicting biological activity, solubility, ADMET properties
To handle high-dimensional data, several feature extraction and dimension reduction mechanisms has been integrated into diverse deep learning frameworks (**Figure 2**). In particular, the convolution neural network is a popular deep learning framework for imaging analysis [40]. A convolution neural network consists of convolution layers, max-pooling layers, and fully connected multilayer perceptron. The purpose of the convolution and max-pooling layer is to extracted local recurring patterns from the image data to fit the input dimension of the fully connected layers. This utility has recently been extended for protein structure analysis in the 3D-CNN approach where protein structures are treated as 3D images [41]. Other deep learning approaches include autoencoder and embedding representation. Autoencoder (AE) is a data-driven approach to obtain a latent presentation of high dimensional data using a smaller set of hidden neurons [42, 43]. An autoencoder
**152**
consists of encoder and decoder. In the encoding step, the input signal is forward propagated to smaller and smaller sets of hidden layers thus effective map the data to low dimensional space. The training is achieved so that the hidden layers can propagate back to a larger set of output nodes to recover the original signal. A specific form of AE called variational AE (VAE) has recently been applied to de-novo drug design application where latent space was first constructed from the ZINC database from which novel compounds can be recovered by sampling such subspace [44]. In the context of NLP, word embedding such as word2vec implementation is a dimensional reduction technique to learn word presentation that preserves the similarity between data in low-dimension. This formulation has been extended to identify chemical representation in the analogous mol2vec program [45]. The requirement to model sequential data also prompted the development of recurrent neural networks (RNN). The RNN is a variant of artificial neural network where the output from the previous state is used as input for the current state. Therefore, this formulation has a classical analogy to the hidden Markov model (HMM), a type of belief network. RNN has been applied for de novo molecule design by "memorizing" from SMILES string in sequential order and generated novel SMILES by sampling from the underlying probability distribution [46]. By tuning the sampling parameters, it is found that RNN can oftentimes generated valid SMILES string not found in the original training set.
| doab | 2025-04-07T04:13:04.440498 | 20-4-2021 18:19 | {
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ffe82432-4883-4adc-b03b-937c1baf5090.157 | *3.1.3 Instance-based learning*
In contrast to parametrized learning that required extensive efforts in model tuning and parameter estimation, instance-based learning, also known as memorybased learning, is a different type of machine learning strategy that generates hypothesis from the training data directly [47]. Therefore, the model complexity
#### **Figure 2.**
*Deep learning architectures for drug discovery. Four common types of deep learning network for supervised and supervised learning including deep neural network (DNN), convolutional neural network (CNN), autoencoder (AE) and recurrent neural network (RNN).*
is highly dependent on the size and quality of the dataset. Notable instance-based learning method includes the k-Nearest Neighbor (kNN) prediction, commonly known as "guilt-by-association" or "like-predicts-like". In the kNN algorithm, a majority voting rule is applied to predict the properties of a given data, based on the k nearest neighbor within certain metric distance [48]. Using this approach, the properties of the data can be inferred from the dominant properties shared among its nearest neighbors. In the field cheminformatics, chemical similarity principle is a direct application of kNN where the similarity between chemical structures can be used to infer similar biological activity [49]. For analyzing large compound set, chemical similarity networks, or chemical space networks, can be used to identify chemical subtypes and estimate chemical diversity [50, 51]. Furthermore, the similarity concept is commonly applied in computational chemical database search to identify similar compounds from a lead series [52]. A major limitation of kNN is the correct determination of the number of nearest neighbors since that too high or low of such parameter can lead to either high false positive and false negative rates.
In the case of binary classification, such as compound activity discrimination, support vector machine (SVM) is a popular non-parametrized machine learning model [53]. For given binary data labels, SVM intended to find a hyperplane such that it has the largest distance (margin) to the nearest training data point of two classes. Furthermore, kernel trick allows mapping data points to high dimensional feature space that are linearly inseparable. For multilabel classification problems, other instance-learning models such as radial basis neural network (RBNN), decision trees and Bayesian learning are generally applicable [54]. In RBNN, several radial basis functions, which often depict as bell shape regions over the feature space, are used to approximate the distribution of the data set. Other approaches like decision tree, such as the Classification And Regression Tree (CART) algorithm, can also be applied for multi-variable classification and regression and has been used to differentiate active estrogen compound from inactives [55]. In the decision tree model, the algorithm provides explanations for the observed pattern by identifying predictors that maximize the homogeneity of the dataset through successive binary partitions (splits). The Bayesian classifier is yet another powerful supervised learning approach that predicts future events based on past observations known as prior. In essence, Bayes' theorem allows the incorporation of prior probability distributions to generate posterior probabilities. In the case of multi-variable classification, a special form of Bayesian learner known as the naïve Bayes learner greatly simplify the computational complexity with independence assumption between features. PASS Online is an example of a Bayesian approach to predict over 4000 kinds of biological activity, including pharmacological effects, mechanisms of action, toxic and adverse effects [56]. In another study, DRABAL, a novel multiple label classification method that incorporates structure learning of a Bayesian network, was developed for processing more than 1.4 million interactions of over 400,000 compounds and analyze the existing relationships between five large HTS assays from the PubChem BioAssay Database [57].
While instance-based learning encompasses a diverse set of methodology and present unique advantages in constantly adapting to new data, this approach is nevertheless limited by the memory storage requirement and, as the dataset grows, data navigation becomes increasingly inefficient. To address this, data pre-segmentation technique such as KD tree is a common approach for instance reduction and memory complexity improvement [58]. In another aspect, the ability to assemble different classifiers into a meta-classifier that will potentially have superior generalization performance than individual classifier also led to the development of ensemble learning. The ensemble learning algorithm can include models that combine multiple types of classifier or sub-sample data from a single
**155**
*Artificial Intelligence-Based Drug Design and Discovery DOI: http://dx.doi.org/10.5772/intechopen.89012*
well as for virtual screening application [62, 63].
scaffold-hopping, and repurposing [65].
**3.2 Unsupervised learning**
*3.2.1 Clustering*
model. A notable example of ensemble learning is the random forest algorithm, which combines multiple decision trees and makes predictions via a majority voting
Given a compound dataset, unsupervised learning can include tasks such as detecting subpopulation to determine the number of chemotypes to estimate chemical diversity and chemical space visualization. Putting in a broader perspective, the purpose of unsupervised learning is to understand the underlying pattern of the datasets. Another important problem stem from unsupervised learning is the ability to define appropriate metrics that can be used to quantify the similarity of data distributed over feature space. These metrics can be useful for chemometrics application including measuring the similarity between pairs of compounds.
For unsupervised clustering, one popular approach is K-means clustering [60]. K-means clustering aims to partition the dataset into K-centroid. This is achieved by constantly minimizing the within-cluster distances and updating new centroids until the location of the K-centroids converges. K-means clustering has the advantage of operating at linear time but does not guarantee convergence to a global minimum. Another limitation is the requirement of a pre-determined number of clusters, which may not correspond to the optimal clusters for the data. To identify the optimal k values, one solution is called the "elbow method", which determine a k value with the largest change in the sum of distances as the k value increases. One study applied K-means clustering to estimate the diversity of compounds that inhibit cytochrome 3A4 activity [61]. Besides K-mean clustering, conventional clustering like hierarchical clustering is also commonly used. Hierarchical clustering can include agglomerative clustering, which merges smaller data objects to form larger clusters or divisive clustering, which generate smaller clusters by splitting from a large cluster. The hierarchical clustering has been demonstrated for their ability to classify large compound and enrich ICE inhibitors from specific clusters as
Although hierarchical clustering is suitable for initial exploratory analysis, it is limited by several shortcomings such as high space and time complexity and lack of robustness to noise. Supervised clustering using artificial networks include the self-organization map (SOM), also known as Kohonen network [64]. The purpose of SOM is to transform the input signal into a two-dimensional map (topological map) where input features that are similar to each other are mapped to similar regions of the map. The learning algorithm is achieved by competitive learning through a discriminant function that determines the closest (winning) neuron. During each training iteration, the winning neuron has its weight updated such that it moves closer to the corresponding input vector until the position of each neuron converges. The advantages of SOM are the ability to directly visualize the highdimensional data on low dimensional grid. Furthermore, the neural network makes SOM more robust to the noisy data and reduces the time complexity to the linear range. SOMs cover such diverse fields of drug discovery as screening library design,
Recently, manifold learning has gained tremendous traction due to the ability to perform dimensional reduction while preserving inter-point distances in lower dimension space for large-scale data visualization. Manifold learning algorithm includes ISOMAP, which build a sparse graph for high dimensional data and
rule for compound activity classification and QSAR modeling [59].
model. A notable example of ensemble learning is the random forest algorithm, which combines multiple decision trees and makes predictions via a majority voting rule for compound activity classification and QSAR modeling [59].
| doab | 2025-04-07T04:13:04.441252 | 20-4-2021 18:19 | {
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ffe82432-4883-4adc-b03b-937c1baf5090.158 | **3.2 Unsupervised learning**
Given a compound dataset, unsupervised learning can include tasks such as detecting subpopulation to determine the number of chemotypes to estimate chemical diversity and chemical space visualization. Putting in a broader perspective, the purpose of unsupervised learning is to understand the underlying pattern of the datasets. Another important problem stem from unsupervised learning is the ability to define appropriate metrics that can be used to quantify the similarity of data distributed over feature space. These metrics can be useful for chemometrics application including measuring the similarity between pairs of compounds.
| doab | 2025-04-07T04:13:04.441827 | 20-4-2021 18:19 | {
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"section_idx": 158
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ffe82432-4883-4adc-b03b-937c1baf5090.159 | *3.2.1 Clustering*
*Cheminformatics and Its Applications*
is highly dependent on the size and quality of the dataset. Notable instance-based learning method includes the k-Nearest Neighbor (kNN) prediction, commonly known as "guilt-by-association" or "like-predicts-like". In the kNN algorithm, a majority voting rule is applied to predict the properties of a given data, based on the k nearest neighbor within certain metric distance [48]. Using this approach, the properties of the data can be inferred from the dominant properties shared among its nearest neighbors. In the field cheminformatics, chemical similarity principle is a direct application of kNN where the similarity between chemical structures can be used to infer similar biological activity [49]. For analyzing large compound set, chemical similarity networks, or chemical space networks, can be used to identify chemical subtypes and estimate chemical diversity [50, 51]. Furthermore, the similarity concept is commonly applied in computational chemical database search to identify similar compounds from a lead series [52]. A major limitation of kNN is the correct determination of the number of nearest neighbors since that too high or low of such parameter can lead to either high false positive and false negative rates. In the case of binary classification, such as compound activity discrimination, support vector machine (SVM) is a popular non-parametrized machine learning model [53]. For given binary data labels, SVM intended to find a hyperplane such that it has the largest distance (margin) to the nearest training data point of two classes. Furthermore, kernel trick allows mapping data points to high dimensional feature space that are linearly inseparable. For multilabel classification problems, other instance-learning models such as radial basis neural network (RBNN), decision trees and Bayesian learning are generally applicable [54]. In RBNN, several radial basis functions, which often depict as bell shape regions over the feature space, are used to approximate the distribution of the data set. Other approaches like decision tree, such as the Classification And Regression Tree (CART) algorithm, can also be applied for multi-variable classification and regression and has been used to differentiate active estrogen compound from inactives [55]. In the decision tree model, the algorithm provides explanations for the observed pattern by identifying predictors that maximize the homogeneity of the dataset through successive binary partitions (splits). The Bayesian classifier is yet another powerful supervised learning approach that predicts future events based on past observations known as prior. In essence, Bayes' theorem allows the incorporation of prior probability distributions to generate posterior probabilities. In the case of multi-variable classification, a special form of Bayesian learner known as the naïve Bayes learner greatly simplify the computational complexity with independence assumption between features. PASS Online is an example of a Bayesian approach to predict over 4000 kinds of biological activity, including pharmacological effects, mechanisms of action, toxic and adverse effects [56]. In another study, DRABAL, a novel multiple label classification method that incorporates structure learning of a Bayesian network, was developed for processing more than 1.4 million interactions of over 400,000 compounds and analyze the existing relationships between five large HTS
**154**
assays from the PubChem BioAssay Database [57].
While instance-based learning encompasses a diverse set of methodology and present unique advantages in constantly adapting to new data, this approach is nevertheless limited by the memory storage requirement and, as the dataset grows, data navigation becomes increasingly inefficient. To address this, data pre-segmentation technique such as KD tree is a common approach for instance reduction and memory complexity improvement [58]. In another aspect, the ability to assemble different classifiers into a meta-classifier that will potentially have superior generalization performance than individual classifier also led to the development of ensemble learning. The ensemble learning algorithm can include models that combine multiple types of classifier or sub-sample data from a single
For unsupervised clustering, one popular approach is K-means clustering [60]. K-means clustering aims to partition the dataset into K-centroid. This is achieved by constantly minimizing the within-cluster distances and updating new centroids until the location of the K-centroids converges. K-means clustering has the advantage of operating at linear time but does not guarantee convergence to a global minimum. Another limitation is the requirement of a pre-determined number of clusters, which may not correspond to the optimal clusters for the data. To identify the optimal k values, one solution is called the "elbow method", which determine a k value with the largest change in the sum of distances as the k value increases. One study applied K-means clustering to estimate the diversity of compounds that inhibit cytochrome 3A4 activity [61]. Besides K-mean clustering, conventional clustering like hierarchical clustering is also commonly used. Hierarchical clustering can include agglomerative clustering, which merges smaller data objects to form larger clusters or divisive clustering, which generate smaller clusters by splitting from a large cluster. The hierarchical clustering has been demonstrated for their ability to classify large compound and enrich ICE inhibitors from specific clusters as well as for virtual screening application [62, 63].
Although hierarchical clustering is suitable for initial exploratory analysis, it is limited by several shortcomings such as high space and time complexity and lack of robustness to noise. Supervised clustering using artificial networks include the self-organization map (SOM), also known as Kohonen network [64]. The purpose of SOM is to transform the input signal into a two-dimensional map (topological map) where input features that are similar to each other are mapped to similar regions of the map. The learning algorithm is achieved by competitive learning through a discriminant function that determines the closest (winning) neuron. During each training iteration, the winning neuron has its weight updated such that it moves closer to the corresponding input vector until the position of each neuron converges. The advantages of SOM are the ability to directly visualize the highdimensional data on low dimensional grid. Furthermore, the neural network makes SOM more robust to the noisy data and reduces the time complexity to the linear range. SOMs cover such diverse fields of drug discovery as screening library design, scaffold-hopping, and repurposing [65].
Recently, manifold learning has gained tremendous traction due to the ability to perform dimensional reduction while preserving inter-point distances in lower dimension space for large-scale data visualization. Manifold learning algorithm includes ISOMAP, which build a sparse graph for high dimensional data and
identify the shortest distance that best preserves the original distance matrix in low dimensional space [66]. While ISOMAP requires very few parameters, the approach is nevertheless computational expensive due to an expensive dense matrix eigen-reduction process. More efficient approaches such as Locally Linear Embedding (LLE) has been proposed for QSAR analysis [67]. LLE assumes that the high dimensional structure can be approximated by a linear structure that preserves the local relationship with neighbors. A related approach is t-distributed stochastic neighbor embedding (tSNE), which relies on the pair-wise probability distribution of data points to preserve local distance [68].
### *3.2.2 Similarity*
The ability to measure data similarity is as important as the ability to discern the number of categories from a dataset. One approach for measuring data similarity is by determining the distance of two data points in the high-dimensional feature space. Intuitively, the similarity between two data points is inversely related to the measured distance between them. Commonly used distance metrics include Euclidean distance, Manhattan distance, Chebyshev distance [60]. All of these metrics is a specialized form of Minkowski distance, a generalized distance metrics defined in the norm space. Other important similarity measures such as the cosine similarity and Pearson's correlation coefficient, are commonly used to measure gene expression data or word embedding vector, when the magnitude of the vector is not essential. For binary features, metrics that measured shared bits between vectors can be used. For example, Tanimoto index, also known as the Jaccard coefficient, is one of the most commonly used metrics to measuring the similarity between two fingerprints in many cheminformatics applications. Tanimoto index has been extended to measure the similarity of 3D molecular volume and pharmacophore, such as those generated from the ligand structural alignment [69]. A generalized form of similarity metric is the kernel such as RBF or Gaussian kernel, which is a function that maps a pair of input vectors to high dimensional space and is an effective approach to tackle non-linearly separable case for discriminating analysis. The selection of an optimal similarity metrics can be achieved by clustering analysis, including comparing the clustering result and assess the quality of the clusters by different similarity measures.
#### **3.3 Reinforcement learning**
Reinforcement Learning came into the spotlight from the famous chess competition between professional chess player and AlphaGo that demonstrated the ability of AI to outcompete human intelligence [70]. Differ from supervised and unsupervised learning, the reinforcement learning focused on optimization of rewards and the output is dependent on the sequence of input. A basic reinforcement learning is modeled based on the Markov decision process and consists of a set of environment and agent state, a set of actions and transitional probability between states. At each time step, the agent interacts with the environment with a chosen action and a given reward. Several learning strategies have been developed to guide the action in each state. The most well-known algorithm is called the Q-learning algorithm [71]. The Q-learning predicts an expected reward of an action in a given state and as the agent interacts with the environment, the Q value function becomes progressively better at approximate the value of an action in a given state. Another approach for guiding the action for reinforcement learning is called policy learning, which aims to create a map that suggests the best action for a given state. The policy can be constructed using a deep neural network. Recently, deep Q-network (DQN) has been
**157**
*Artificial Intelligence-Based Drug Design and Discovery DOI: http://dx.doi.org/10.5772/intechopen.89012*
**4. Conclusion**
constructed that approximate the Q value-functions using a deep neural network [72]. One recent example of using deep reinforcement learning in de novo design is demonstrated by the ReLeaSE (Reinforcement Learning for Structural Evolution), which integrates both predictive and generative model for targeted library design based on SMILES string. The generative model is used to generate chemically feasible compound while the predictive model is then used to forecast the desired properties. The ReLeaSE method can be used to design chemical libraries with a bias toward structural complexity or toward compounds with a specific range of physical properties as well as inhibitory activity against Janus protein kinase 2 [73].
The path of drug discovery from small molecule ligand to drug that can be utilized clinically is a long and arduous process. The fundamental concept of artificial intelligence and the application in drug design and discovery presented will facilitate this process. In particular, the machine learning and deep learning, which demonstrated great utility in many branches of computer-aided drug discovery like
In this chapter, we presented the fundamental concept of artificial intelligence and their application in drug design and discovery. We first focused on chemoinformatics, a broad field that studying the application of computers in storing, processing, and analyzing chemical data. This field already has more than 30 years of development with focuses on subjects ranging from chemical representation, chemical descriptors analysis, library design, QSAR analysis, and retrosynthetic planning. We then discussed how artificial intelligence techniques can be leveraged for developing more effective chemoinformatics pipelines and presented with realworld case studies. From the algorithmic aspects, we mentioned three major class of machine learning algorithms including supervised learning, unsupervised learning, and reinforcement learning, each with their own strength and weakness as well as
As AI techniques gradually become indispensable tools for drug designer to solve their day-to-day problems, an emerging trend is to learn how to flexibly integrate these algorithms in the computational pipelines suitable for the problem at hand. For example, the process can start with an unsupervised learning to discerning the number of chemotypes followed by a supervised learning approach to predict multi-target activities. Furthermore, with the increasing computational power, deep learning network with increasing number layers and complexity will be also developed. Another potential development is the marriage between chemical big data and AI to mine the chemical "universe" for drug screening applications. The potential extensibility of AI in drug discovery and design is virtually boundless and
de novo drug design, QSAR analysis, chemical space visualization.
cover different areas of chemoinformatic applications.
awaits drug designer to further explore this exciting field.
constructed that approximate the Q value-functions using a deep neural network [72]. One recent example of using deep reinforcement learning in de novo design is demonstrated by the ReLeaSE (Reinforcement Learning for Structural Evolution), which integrates both predictive and generative model for targeted library design based on SMILES string. The generative model is used to generate chemically feasible compound while the predictive model is then used to forecast the desired properties. The ReLeaSE method can be used to design chemical libraries with a bias toward structural complexity or toward compounds with a specific range of physical properties as well as inhibitory activity against Janus protein kinase 2 [73].
| doab | 2025-04-07T04:13:04.441857 | 20-4-2021 18:19 | {
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} |
ffe82432-4883-4adc-b03b-937c1baf5090.160 | **4. Conclusion**
*Cheminformatics and Its Applications*
*3.2.2 Similarity*
different similarity measures.
**3.3 Reinforcement learning**
of data points to preserve local distance [68].
identify the shortest distance that best preserves the original distance matrix in low dimensional space [66]. While ISOMAP requires very few parameters, the approach is nevertheless computational expensive due to an expensive dense matrix eigen-reduction process. More efficient approaches such as Locally Linear Embedding (LLE) has been proposed for QSAR analysis [67]. LLE assumes that the high dimensional structure can be approximated by a linear structure that preserves the local relationship with neighbors. A related approach is t-distributed stochastic neighbor embedding (tSNE), which relies on the pair-wise probability distribution
The ability to measure data similarity is as important as the ability to discern the number of categories from a dataset. One approach for measuring data similarity is by determining the distance of two data points in the high-dimensional feature space. Intuitively, the similarity between two data points is inversely related to the measured distance between them. Commonly used distance metrics include Euclidean distance, Manhattan distance, Chebyshev distance [60]. All of these metrics is a specialized form of Minkowski distance, a generalized distance metrics defined in the norm space. Other important similarity measures such as the cosine similarity and Pearson's correlation coefficient, are commonly used to measure gene expression data or word embedding vector, when the magnitude of the vector is not essential. For binary features, metrics that measured shared bits between vectors can be used. For example, Tanimoto index, also known as the Jaccard coefficient, is one of the most commonly used metrics to measuring the similarity between two fingerprints in many cheminformatics applications. Tanimoto index has been extended to measure the similarity of 3D molecular volume and pharmacophore, such as those generated from the ligand structural alignment [69]. A generalized form of similarity metric is the kernel such as RBF or Gaussian kernel, which is a function that maps a pair of input vectors to high dimensional space and is an effective approach to tackle non-linearly separable case for discriminating analysis. The selection of an optimal similarity metrics can be achieved by clustering analysis, including comparing the clustering result and assess the quality of the clusters by
Reinforcement Learning came into the spotlight from the famous chess competition between professional chess player and AlphaGo that demonstrated the ability of AI to outcompete human intelligence [70]. Differ from supervised and unsupervised learning, the reinforcement learning focused on optimization of rewards and the output is dependent on the sequence of input. A basic reinforcement learning is modeled based on the Markov decision process and consists of a set of environment and agent state, a set of actions and transitional probability between states. At each time step, the agent interacts with the environment with a chosen action and a given reward. Several learning strategies have been developed to guide the action in each state. The most well-known algorithm is called the Q-learning algorithm [71]. The Q-learning predicts an expected reward of an action in a given state and as the agent interacts with the environment, the Q value function becomes progressively better at approximate the value of an action in a given state. Another approach for guiding the action for reinforcement learning is called policy learning, which aims to create a map that suggests the best action for a given state. The policy can be constructed using a deep neural network. Recently, deep Q-network (DQN) has been
**156**
The path of drug discovery from small molecule ligand to drug that can be utilized clinically is a long and arduous process. The fundamental concept of artificial intelligence and the application in drug design and discovery presented will facilitate this process. In particular, the machine learning and deep learning, which demonstrated great utility in many branches of computer-aided drug discovery like de novo drug design, QSAR analysis, chemical space visualization.
In this chapter, we presented the fundamental concept of artificial intelligence and their application in drug design and discovery. We first focused on chemoinformatics, a broad field that studying the application of computers in storing, processing, and analyzing chemical data. This field already has more than 30 years of development with focuses on subjects ranging from chemical representation, chemical descriptors analysis, library design, QSAR analysis, and retrosynthetic planning. We then discussed how artificial intelligence techniques can be leveraged for developing more effective chemoinformatics pipelines and presented with realworld case studies. From the algorithmic aspects, we mentioned three major class of machine learning algorithms including supervised learning, unsupervised learning, and reinforcement learning, each with their own strength and weakness as well as cover different areas of chemoinformatic applications.
As AI techniques gradually become indispensable tools for drug designer to solve their day-to-day problems, an emerging trend is to learn how to flexibly integrate these algorithms in the computational pipelines suitable for the problem at hand. For example, the process can start with an unsupervised learning to discerning the number of chemotypes followed by a supervised learning approach to predict multi-target activities. Furthermore, with the increasing computational power, deep learning network with increasing number layers and complexity will be also developed. Another potential development is the marriage between chemical big data and AI to mine the chemical "universe" for drug screening applications. The potential extensibility of AI in drug discovery and design is virtually boundless and awaits drug designer to further explore this exciting field.
*Cheminformatics and Its Applications*
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ffe82432-4883-4adc-b03b-937c1baf5090.161 | **Author details**
Yu-Chen Lo1,3\*, Gui Ren2 , Hiroshi Honda2 and Kara L. Davis3
1 Bioengineering, Stanford University, Stanford, CA, USA
2 Bioengineering, Northwestern Polytechnic University, Fremont, CA, USA
3 Pediatrics, Bass Center for Childhood Cancer, Stanford School of Medicine, Stanford, CA, USA
\*Address all correspondence to: [email protected]
© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
**159**
*Artificial Intelligence-Based Drug Design and Discovery DOI: http://dx.doi.org/10.5772/intechopen.89012*
Cheminformatics. 2011;**3**:33. DOI:
[8] Lovric M, Molero JM, Kern R. PySpark and RDKit: Moving towards big data in cheminformatics. Molecular Informatics. 2019;**38**(6):e1800082. DOI:
[9] Gupta A, Kumar V, Aparoy P. Role of topological, electronic, geometrical, constitutional and quantum chemical based descriptors in QSAR: mPGES-1 as a case study. Current Topics in Medicinal Chemistry. 2018;**18**(13):1075- 1090. DOI: 10.2174/15680266186661807
[10] Haggarty SJ, Clemons PA, Wong JC, Schreiber SL. Mapping chemical space using molecular descriptors and chemical genetics: Deacetylase inhibitors. Combinatorial Chemistry & High Throughput Screening.
[11] Sykora VJ, Leahy DE. Chemical descriptors library (CDL): A generic, open source software library for chemical informatics. Journal of Chemical Information and Modeling. 2008;**48**(10):1931-1942. DOI: 10.1021/
[12] Nettles JH, Jenkins JL, Bender A, Deng Z, Davies JW, Glick M. Bridging chemical and biological space: "Target fishing" using 2D and 3D molecular descriptors. Journal of Medicinal Chemistry. 2006;**49**(23):6802-6810.
DOI: 10.1021/jm060902w
10.1021/ci0340714
[13] Pan D, Tseng Y, Hopfinger AJ. Quantitative structure-based design: Formalism and application of receptordependent RD-4D-QSAR analysis to a set of glucose analogue inhibitors of glycogen phosphorylase. Journal of Chemical Information and Computer Sciences. 2003;**43**(5):1591-1607. DOI:
10.1186/1758-2946-3-33
10.1002/minf.201800082
19164149
2004;**7**(7):669-676
ci800135h
**References**
drudis.2018.05.010
s00216-005-0065-y
ci00057a005
[1] Lo YC, Rensi SE, Torng W, Altman RB. Machine learning in chemoinformatics and drug discovery. Drug Discovery Today. 2018;**23**(8):1538-1546. DOI: 10.1016/j.
[2] Idakwo G, Luttrell J, Chen M,
[3] Gasteiger J. Chemoinformatics: A new field with a long tradition. Analytical and Bioanalytical Chemistry. 2006;**384**(1):57-64. DOI: 10.1007/
[4] Weininger D. SMILES, a chemical language and information system. 1. Introduction to methodology and encoding rules. Journal of Chemical Information and Computer Sciences. 1988;**28**(1):31-36. DOI: 10.1021/
[5] O'Boyle NM. Towards a universal SMILES representation—A standard method to generate canonical SMILES
[6] Schuttelkopf AW, van Aalten DM. PRODRG: A tool for high-throughput crystallography of protein-ligand complexes. Acta Crystallographica. Section D, Biological Crystallography. 2004;**60**(Pt 8):1355-1363. DOI: 10.1107/
[7] O'Boyle NM, Banck M, James CA,
Morley C, Vandermeersch T, Hutchison GR. Open babel: An open chemical toolbox. Journal of
based on the InChI. Journal of Cheminformatics. 2012;**4**(1):22. DOI:
10.1186/1758-2946-4-22
S0907444904011679
Hong H, Zhou Z, Gong P, et al. A review on machine learning methods for in silico toxicity prediction. Journal of Environmental Science and Health. Part C, Environmental Carcinogenesis & Ecotoxicology Reviews. 2018;**36**(4):169-191. DOI: 10.1080/10590501.2018.1537118
*Artificial Intelligence-Based Drug Design and Discovery DOI: http://dx.doi.org/10.5772/intechopen.89012*
| doab | 2025-04-07T04:13:04.442825 | 20-4-2021 18:19 | {
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ffe82432-4883-4adc-b03b-937c1baf5090.164 | Cell-Penetrating Peptides: A Challenge for Drug Delivery
*Sonia Aroui and Abderraouf Kenani*
| doab | 2025-04-07T04:13:04.443082 | 20-4-2021 18:19 | {
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ffe82432-4883-4adc-b03b-937c1baf5090.165 | **Abstract**
Cell-penetrating peptide (CPP) is a term that describes relatively short amphipathic and cationic peptides (7–30 amino acid residues) with rapid translocation across the cell membrane. They can be used to deliver molecular bioactive cargoes due to their efficacy in cellular internalization and also to their low cytotoxicity. In this review we provide an overview of the current approaches and describe the potential of CPP-based drug delivery systems and indicate their powerful promise for clinical efficacy.
**Keywords:** cell-penetrating peptides, drugs
| doab | 2025-04-07T04:13:04.443114 | 20-4-2021 18:19 | {
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ffe82432-4883-4adc-b03b-937c1baf5090.166 | **1. Introduction**
A novel approach to overcome cell membrane impermeability and to deliver a large variety of particles and macromolecules into cells has been recently emerged, which is called cell-penetrating peptides (CPPs), also known as protein transduction domains (PTDs) [1, 2]. CPPs are generally short (up to 30 amino acids in length) water-soluble, cationic, and/or amphipathic peptides which make them promising vectors for therapeutic delivery, leading to a considerable amount of research focused on the intracellular delivery of drugs [3–5]. There are two principal types of CPPs that have been utilized for this purpose: (i) cationic CPPs, composed of short sequence of amino acids (arginine, lysine, and histidine). The indicated amino acids give the cationic charge to the peptide and permit its interaction with anionic motifs on the plasma membrane by a receptor-independent mechanism. (ii) amphipathic peptides, which have lipophilic and hydrophilic tails that are responsible for a direct peptide translocation mechanism across the plasma membrane [6].
The most important characteristic of CPPs is that they are able to translocate the plasma membrane at low micromolar concentrations in vivo and in vitro without using any receptors and without causing any significant membrane damage [7, 8]. Other benefits of using CPPs for therapeutic delivery are the absence of toxicity as compared to other cytoplasmic delivery devices, such as liposomes, polymers, etc. [6]. The mechanism for the CPP-facilitated cellular uptake remains not clear and depends on cargo and cellular type [9]. Due to its high density of basic amino acid residues (Arg and Lys), the large charge at physiological pH excludes the passive diffusion of CPPs across the lipid bilayer. Furthermore, it seems that classical uptake mechanisms such as protein-based receptors and transporters are not involved. On the contrary, endocytosis was shown as a common uptake mechanism, but is controversial at the same time. For example, in a number of reports, CPP
uptake was not inhibited at 4°C or in the presence of inhibitors of endocytosis; in contrast, a capture of CPPs in the endocytotic vesicles was observed when soluble heparin sulfate was added [9, 10]. Many other studies indicate that aggregation of the cell surface glycosaminoglycan heparan sulfate (HS) is an important element in the uptake mechanism [2]. The challenge of the strategy using CPPs should take into consideration the size, stability, nonspecific versus specific associations, and potency versus toxicity that all play an important role for the selection of delivery systems [5].
| doab | 2025-04-07T04:13:04.443176 | 20-4-2021 18:19 | {
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ffe82432-4883-4adc-b03b-937c1baf5090.167 | **2. History and origin of CPPs**
The CPPs are initially discovered in 1965 when it was observed that histones and cationic polyamines such as polylysine stimulate the uptake of albumin by tumor cells in culture. It was shown that the conjugation of polylysine to albumin and other proteins enhances their transport into cells. Moreover, a comparison study of different homopolymers of cationic amino acids demonstrates that medium-length polymers of arginine enter cells more effective than similarlength polymers composed of lysine, ornithine, or histidine [11]. In 1988, it was discovered that the human immunodeficiency virus type 1 (HIV-1) encoded trans-acting activator of transcription (Tat) peptide which also translocates cell membranes and gains intracellular mammalian cells [12, 13]. Covalently the conjugation of Tat peptide to proteins or fluorescent markers allowed these molecules to gain into the cell. A few years later, another discovery was followed when polycationic peptide of natural (VP22 and AntP) and synthetic origin (transportan) was used for the delivery of genes, proteins, small exogenous peptide, or even nanoparticles. Furthermore, it was demonstrated that small domains in these peptides are often responsible for cellular entry [14]. Thus, these translocation sequences could be shortened to a few amino acids in comparison with the first Tat peptide, without affecting cell penetration efficiency [13]. Since that time, the list of synthetic CPPs has increased sharply, and the number continues to rise (**Table 1**). In the last decade, another peptide was described named maurocalcine (MCa), a 33 amino acid residue peptide that has been isolated from the venom of the Tunisian chactid scorpion *Scorpio maurus palmatus.* It folds according to an "inhibitor cystine knot" (ICK) motif and contains three disulfide bridges connected by the following pattern: C1–C4, C2–C5, and C3– C6 [15]. MCa acts on ryanodine receptors resulting in pharmacological activation. These receptors are calcium channels located in the membrane of the endoplasmic reticulum. They control Ca2+ release from internal stores and therefore a large number of cell functions [16, 17].
This peptide possesses vector properties when coupled to fluorescent streptavidin. This complex was shown to enter various cell types within minutes and in all cell types tested, a common feature of CPPs. A variety of mutants of MCa were then designed in order to unravel the most active residues for its pharmacological and penetration activities (**Figure 1**) [18, 19].
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ffe82432-4883-4adc-b03b-937c1baf5090.168 | **3. Therapeutic applications of CPPs**
#### **3.1 CPP-cargo complex internalization mechanisms**
Two distinct advances were shown to be used to bind CPPs to molecular cargoes. One process is non-covalently which connect CPP to its cargoes using electrostatic
**167**
**Peptide** **Protein transduction domain**
Tat48-60 Pénétratin **Chimeric peptides**
Transportan
Pep-1
MPG CADY **Peptide models**
(Arg)x
MAP **Natural CPP**
Maurocalcine
**Table 1.**
(RRRRR)X KLALKLALKALKAALKA
GDCLPHLKLCKENKDCCSKKCKRRGTNIEKRCR
*Examples of four classes of CPPs and delivered cargoes. The list of cargo is not exhaustive and given for illustration. X = 7, 8, or 9 arginine residues.*
*Scorpio maurus palmatus*
Synthetic peptide
Synthetic peptide
GRKKRRQRRRPPQ
RQIKIWFQNRRMKWKK
GWTLNSAGYLLGKINLKALAALAKKIL
KETWWETWWTEWSQPKKKRKV
GALFLGFLGAAGSTMGAWSQPKKKRKV
GLWRALWRLLRSLWRLLWRA
Galanin + Mastoparan
Rich domain of tryptophan + *spacer* + domain derived from virus SV40-NLS sequence of T antigène
Hydrophobic motif derived from HIV-1 gp41 + *linker + domain derived* from virus SV40-NLS sequence of T antigène
Dérived from PPTG11, variant of JTS1 fusion protéin
siARN
siARN, Cyclosporine A
Natural CPPs
Doxorubicin
**Sequence**
**Origin**
VIH-1 *Drosophila Antennapedia homeodomain*
**Cargoes** ADN, peptide, PKC inhibitor
HSP20 phosphopeptide
Protéine, PNA
Enzyme siARN, oligo-nucléotides
*Cell-Penetrating Peptides: A Challenge for Drug Delivery DOI: http://dx.doi.org/10.5772/intechopen.91684*
**1.** *Examples of four classes of CPPs and delivered cargoes. The list of cargo is not exhaustive and given for illustration. X = 7, 8, or 9 arginine residues.*
## *Cell-Penetrating Peptides: A Challenge for Drug Delivery DOI: http://dx.doi.org/10.5772/intechopen.91684*
*Cheminformatics and Its Applications*
**2. History and origin of CPPs**
systems [5].
uptake was not inhibited at 4°C or in the presence of inhibitors of endocytosis; in contrast, a capture of CPPs in the endocytotic vesicles was observed when soluble heparin sulfate was added [9, 10]. Many other studies indicate that aggregation of the cell surface glycosaminoglycan heparan sulfate (HS) is an important element in the uptake mechanism [2]. The challenge of the strategy using CPPs should take into consideration the size, stability, nonspecific versus specific associations, and potency versus toxicity that all play an important role for the selection of delivery
The CPPs are initially discovered in 1965 when it was observed that histones and cationic polyamines such as polylysine stimulate the uptake of albumin by tumor cells in culture. It was shown that the conjugation of polylysine to albumin and other proteins enhances their transport into cells. Moreover, a comparison study of different homopolymers of cationic amino acids demonstrates that medium-length polymers of arginine enter cells more effective than similarlength polymers composed of lysine, ornithine, or histidine [11]. In 1988, it was discovered that the human immunodeficiency virus type 1 (HIV-1) encoded trans-acting activator of transcription (Tat) peptide which also translocates cell membranes and gains intracellular mammalian cells [12, 13]. Covalently the conjugation of Tat peptide to proteins or fluorescent markers allowed these molecules to gain into the cell. A few years later, another discovery was followed when polycationic peptide of natural (VP22 and AntP) and synthetic origin (transportan) was used for the delivery of genes, proteins, small exogenous peptide, or even nanoparticles. Furthermore, it was demonstrated that small domains in these peptides are often responsible for cellular entry [14]. Thus, these translocation sequences could be shortened to a few amino acids in comparison with the first Tat peptide, without affecting cell penetration efficiency [13]. Since that time, the list of synthetic CPPs has increased sharply, and the number continues to rise (**Table 1**). In the last decade, another peptide was described named maurocalcine (MCa), a 33 amino acid residue peptide that has been isolated from the venom of the Tunisian chactid scorpion *Scorpio maurus palmatus.* It folds according to an "inhibitor cystine knot" (ICK) motif and contains three disulfide bridges connected by the following pattern: C1–C4, C2–C5, and C3– C6 [15]. MCa acts on ryanodine receptors resulting in pharmacological activation. These receptors are calcium channels located in the membrane of the endoplasmic reticulum. They control Ca2+ release from internal stores and therefore a large number of cell
This peptide possesses vector properties when coupled to fluorescent streptavidin. This complex was shown to enter various cell types within minutes and in all cell types tested, a common feature of CPPs. A variety of mutants of MCa were then designed in order to unravel the most active residues for its pharmacological and
Two distinct advances were shown to be used to bind CPPs to molecular cargoes. One process is non-covalently which connect CPP to its cargoes using electrostatic
**166**
functions [16, 17].
penetration activities (**Figure 1**) [18, 19].
**3. Therapeutic applications of CPPs**
**3.1 CPP-cargo complex internalization mechanisms**
**167**
**Figure 1.**
*Example of origin of four CPPs: Maurocalcine, penetratin, tat, and polyarginine. Maurocalcine, penetratin, and tat are derived from natural sequences, but polyarginine was produced by de novo conception in order to obtain a good cellular penetration.*
interactions, such as MPG and Pep-1, amphipathic peptides carriers, which link to cargoes beyond any cross linking or chemical changes [20]. The second approach is more frequent and uses a covalent relation between the two compounds. This means has been widely used by different teams and has demonstrated positive advances, especially with TAT, penetratin, or polyarginines [21].
Various mechanisms for CPP internalization have been suggested, but the exact one is still not well known. Yet, many data approve that the energy-dependent tool (endocytosis) and the energy-independent mechanism (direct translocation) or both are involved in the cellular uptake progress [22].
For direct penetration, various mechanisms have been described: the carpet-like model (membrane destabilization) [23] and the pore formation model (barrelstave) [24]. Positively charged CPPs interact with negatively charged membrane components like phospholipid bilayer or heparan sulfate. Such interaction is dwelling on the first stage of all of these mechanisms, followed by destabilization of the membrane and finished by crossing of the CPP on the lipid membrane.
For endocytosis transduction or cellular digestion, pinocytosis, phagocytosis, and receptor-mediated endocytosis have been reported [25, 26]. A sum-up of CPP transduction systems is shown in **Figure 2**. In pinocytosis, the plasma membrane absorbs solutes, while in phagocytosis it takes great particles. In clathrin-mediated endocytosis, clathrin and also caveolin, which are receptor-mediated endocytosis and cover the intracellular part of the biomembranes, possess a key role in the uptake mechanism. These protein structures are pivotal for the membrane invagination and for the construction of the vesicles after bounding the extracellular molecule to the membrane receptor. Clathrin has a great diameter in comparison with caveolin-coated vesicles and was also considered as a selective route for the translocation of compounds into cells through specific receptors on the surface of the cell [27].
Many determinants influence the internalization process, such as the nature of CPP or the cell type, the cargo, and the experimental conditions (temperature and pH) [22].
**169**
*Cell-Penetrating Peptides: A Challenge for Drug Delivery DOI: http://dx.doi.org/10.5772/intechopen.91684*
**3.2 Delivery of chemotherapeutic agents**
**Figure 2.**
*CPP translocation mechanisms.*
intact to the cytosol of every cancer cell, sparing healthy cells.
additional therapeutic avenues for ischemic diseases [31].
Chemotherapy used for treatment of cancer has a lot of defects because of the toxicity of the drugs to normal healthy cells and also to resistance developed by tumor cells to the anticancer drug [28]. The major inconveniences with used cancer chemotherapy are the absence of specificity target to tumor cells and thus poor antitumor effect. The challenge in cancer therapy is to know how to deliver a drug
It was shown that polyarginines carry cargoes that exceed 500 Da by molecular electroporation across the cell membrane which may solve part of the drug delivery problem [29]. However, the use of well-chosen linkers and anions can help target cancer cells and contribute to successful conjugation process. For example, the CXC chemokine receptor 4 (CXCR4) is overexpressed in different types of cancer, including prostate, breast, colon, and small-cell lung cancer. Snyder et al. linked the CXCR4 receptor ligand, DV3, to two transducible anticancer peptides: a p53-activating peptide (DV3-TATp53C′) and a cyclin-dependent kinase 2 antagonist peptide (DV3-TAT-RxL). Treatment of tumor cells expressing the CXCR4 receptor with either the DV3-TATp53C′ or DV3-TAT-RxL targeted peptides resulted in an enhancement of tumor cell killing compared with treatment with nontargeted parental peptides [30]. Furthermore, hypoxia-inducible factor-1 (HIF), the transcription factor central to oxygen homeostasis, is regulated via the oxygendependent degradation domains (ODD) of its α isoforms (HIFα). The amino- and carboxyl-terminal sequences of ODD (NODD and CODD) were fused to TAT and injected into sponges implanted subcutaneously (s.c.) in mice by William et al. They demonstrated that this injection causes a markedly accelerated local angiogenic response and induction of glucose transporter-1 gene expression, thus opening
In some cancer cells, such as melanoma (common eye cancers in adults), p53 seems to be inhibited by overexpression of HDM2. A transducible peptide that inhibits HDM2 and Bcl-2 for their ability to induce tumor-specific apoptosis in
*Cell-Penetrating Peptides: A Challenge for Drug Delivery DOI: http://dx.doi.org/10.5772/intechopen.91684*
*Cheminformatics and Its Applications*
*Example of origin of four CPPs: Maurocalcine, penetratin, tat, and polyarginine. Maurocalcine, penetratin, and tat are derived from natural sequences, but polyarginine was produced by de novo conception in order to*
interactions, such as MPG and Pep-1, amphipathic peptides carriers, which link to cargoes beyond any cross linking or chemical changes [20]. The second approach is more frequent and uses a covalent relation between the two compounds. This means has been widely used by different teams and has demonstrated positive advances,
Various mechanisms for CPP internalization have been suggested, but the exact one is still not well known. Yet, many data approve that the energy-dependent tool (endocytosis) and the energy-independent mechanism (direct translocation) or
For direct penetration, various mechanisms have been described: the carpet-like
For endocytosis transduction or cellular digestion, pinocytosis, phagocytosis, and receptor-mediated endocytosis have been reported [25, 26]. A sum-up of CPP transduction systems is shown in **Figure 2**. In pinocytosis, the plasma membrane absorbs solutes, while in phagocytosis it takes great particles. In clathrin-mediated endocytosis, clathrin and also caveolin, which are receptor-mediated endocytosis and cover the intracellular part of the biomembranes, possess a key role in the uptake mechanism. These protein structures are pivotal for the membrane invagination and for the construction of the vesicles after bounding the extracellular molecule to the membrane receptor. Clathrin has a great diameter in comparison with caveolin-coated vesicles and was also considered as a selective route for the translocation of compounds into cells through specific receptors on the surface of
Many determinants influence the internalization process, such as the nature of CPP or the cell type, the cargo, and the experimental conditions (temperature
model (membrane destabilization) [23] and the pore formation model (barrelstave) [24]. Positively charged CPPs interact with negatively charged membrane components like phospholipid bilayer or heparan sulfate. Such interaction is dwelling on the first stage of all of these mechanisms, followed by destabilization of the
membrane and finished by crossing of the CPP on the lipid membrane.
especially with TAT, penetratin, or polyarginines [21].
both are involved in the cellular uptake progress [22].
**168**
the cell [27].
and pH) [22].
**Figure 1.**
*obtain a good cellular penetration.*
**Figure 2.** *CPP translocation mechanisms.*
### **3.2 Delivery of chemotherapeutic agents**
Chemotherapy used for treatment of cancer has a lot of defects because of the toxicity of the drugs to normal healthy cells and also to resistance developed by tumor cells to the anticancer drug [28]. The major inconveniences with used cancer chemotherapy are the absence of specificity target to tumor cells and thus poor antitumor effect. The challenge in cancer therapy is to know how to deliver a drug intact to the cytosol of every cancer cell, sparing healthy cells.
It was shown that polyarginines carry cargoes that exceed 500 Da by molecular electroporation across the cell membrane which may solve part of the drug delivery problem [29]. However, the use of well-chosen linkers and anions can help target cancer cells and contribute to successful conjugation process. For example, the CXC chemokine receptor 4 (CXCR4) is overexpressed in different types of cancer, including prostate, breast, colon, and small-cell lung cancer. Snyder et al. linked the CXCR4 receptor ligand, DV3, to two transducible anticancer peptides: a p53-activating peptide (DV3-TATp53C′) and a cyclin-dependent kinase 2 antagonist peptide (DV3-TAT-RxL). Treatment of tumor cells expressing the CXCR4 receptor with either the DV3-TATp53C′ or DV3-TAT-RxL targeted peptides resulted in an enhancement of tumor cell killing compared with treatment with nontargeted parental peptides [30]. Furthermore, hypoxia-inducible factor-1 (HIF), the transcription factor central to oxygen homeostasis, is regulated via the oxygendependent degradation domains (ODD) of its α isoforms (HIFα). The amino- and carboxyl-terminal sequences of ODD (NODD and CODD) were fused to TAT and injected into sponges implanted subcutaneously (s.c.) in mice by William et al. They demonstrated that this injection causes a markedly accelerated local angiogenic response and induction of glucose transporter-1 gene expression, thus opening additional therapeutic avenues for ischemic diseases [31].
In some cancer cells, such as melanoma (common eye cancers in adults), p53 seems to be inhibited by overexpression of HDM2. A transducible peptide that inhibits HDM2 and Bcl-2 for their ability to induce tumor-specific apoptosis in
these cells was tested [30]. In this study, it was demonstrated that the anti-Bcl-2 peptide induced apoptosis in tumor cells but also caused variable levels of toxicity in normal cells and tissues. On the contrary, the anti-HDM2 peptide induced apoptosis in tumor cells, with little effect on normal cells in a therapeutic dose range. This peptide also caused regression of retinoblastoma in rabbit eyes, with minimal damage to normal ocular tissues. They conclude that the inhibition of HDM2 may be a promising strategy for the treatment of uveal melanoma and retinoblastoma, and that strategy may be an effective technology for local delivery of anticancer therapy to the eye.
Most of the patients with sporadic renal cell carcinomas (RCCs) exhibit mutation of the Hippel-Lindau (VHL) tumor suppressor gene. Conjugation of the protein transduction domain of HIV-TAT protein to the amino acid sequence (104–123) in the beta-domain of the VHL gene product (pVHL) arrested and then reduced proliferation and invasion of 786-O renal cancer cells in vitro. Besides, daily i.p. injections with the conjugate put off and, in some cases, caused partial regression of renal tumors that were implanted in the dorsal flank of nude mice [32].
The tumor suppressor gene *p16INK4A*, an inhibitor of cdk3 4, is often inactivated via intragenic mutation, homozygous deletion, and methylation-associated transcriptional silencing in a large number of human cancers, mainly in pancreatic cancer. Treated animals with the p16-derived synthetic peptide coupled with the Antennapedia carrier sequence, in which we designated as Trojan p16 peptide, showed reduced AsPC-1 and BxPC-3 s.c. tumors, respectively. Thus, we conclude that Trojan p16 peptide system, a gene-oriented peptide coupled with a peptide vector, functions for experimental pancreatic cancer therapy [33].
Recently, it was shown by Sonia et al. that coupling doxorubicin (Dox) to three cell-penetrating peptides Tat, penetratin, and maurocalcine (Dox-CPPs) is a good strategy to overcome Dox resistance in MDA-MB 231 breast cancer cells and CHO cells (**Figure 3**) [3, 34]. We also reported that all conjugates are able to promote cell apoptosis in the breast cancer-resistant cells MDA-MB 231 at lesser concentration needed for Dox alone. Indeed, apoptosis death was shown to be correlated with ladder-internucleosomal degradation, chromatin contraction, caspase activation, Bad and Bax activation by oligomerization on the mitochondrial membrane, and liberation of cytochrome c. Despite the effective Bcl-2 overexpression in apoptosis induced by the Dox alone, such potency was shown to be insufficient in case of Dox-CPP-triggered cell apoptotic death. Otherwise, these results suggest that there are other apoptotic signaling pathways, independent of mitochondrial one, which are implicated in Dox-CPP apoptosis. Moreover, greater effectiveness of Dox when coupled to CPPs is not due only to its higher accumulation on the cells but also to the incitement of other signaling pathways. These pathways include death receptors and activation of the JNK pathway [4, 35].
**Figure 3.**
*Cellular internalization of Dox by MCa. MDA-MB231 cells treated with (a) RPMI, (B) Dox alone (red), and (C) Dox coupled to Dox at the same concentration (red).*
**171**
lular uptake.
**diagnosis**
CPPs [39].
*Cell-Penetrating Peptides: A Challenge for Drug Delivery DOI: http://dx.doi.org/10.5772/intechopen.91684*
nanospheres (GNSs) conjugated to four α-helical CPPs [37].
Another study led by Leslie Walker et al. showed that conjugated Dox to both ELP and SynB1 prevents tumor development in mice. In fact, conjugation of Dox to SynB1-ELP was more efficient in tumor inhibition under hyperthermic condition than Dox alone, which was twofold higher. Such conception was considered hopeful peptide candidates for drug delivery [36]. The anticancer activity of Dox was also enhanced when constructed a drug delivery system by developing 25 nm gold
A thermally sensitive quantum dot that exhibits an "on-demand" cellular uptake behavior via temperature-induced "shielding/deshielding" of CPP on the surface was synthesized. Poly(N-isopropylacrylamide) (PNIPAAm) and CPP were biotinylated at their terminal ends and co-immobilized onto the surface of streptavidincoated quantum dots (QDs-Strep) through biotin-streptavidin interaction. Namely, under a lower critical solution temperature (LCST), the hydrated PNIPAAm chains blocked CPP cellular uptake. This effect was broken down when the LCST was raised to allow CPP moieties to be exposed on the cell surface, leading to QD cel-
Additionally, the "shielding/deshielding" temperature of CPP was also used for siRNA delivery system. Biotinylated siRNA was coupled to the surface of TSQDs. Indeed, the amount of corresponding gene silencing was increased due to the surface exposure of CPP within a rising temperature above the LCST [38].
Over the last decade, a great attention has been assigned to the importance of CPP on drug transportation of bioactive molecules in various preclinical studies. In fact, novel computational basics have been made in order to develop knowledge on
Previously, different researchers have developed a few in silico algorithm approaches for CPP prediction (CPPpred) and screening to facilitate throughput CPP-based research. The in silico screening/prediction methods aimed on the use of scales of chemical characteristic, such as z-descriptors [40, 41]. It is generally followed by experimental validation to make it reliable with less cost and timeconsuming approach. Later on, other CPP prediction applied neural network (NN) strategies were developed and consist on introducing an N-to-1 NN. The network proceeds by a sequence of 5 to 30 amino acids in length, as input, and gives a prediction of how probably each peptide is to be cell penetrating [42]. This CPPpred offers an advantage since it was developed with repetition-reduced training and test sets. Over the years, the commitment therapeutic importance of CPPs motivated other teams to develop the first version of CPP database, i.e., CPPsite which supports broad information on the promising use of CPPs [43]. The CPPsite manually created database of 843 experimentally described CPPs. Each consulting gives us data of the peptide involving peptide sequence, peptide name, nature of peptide, origin, chirality, uptake efficiency, subcellular localization, etc. A deep area of userfriendly tools has been integrated in this database like analyzing and browsing tools. Moreover, they have introduced other informations concerning peptide sequences such as secondary/tertiary structure and physicochemical properties of peptides. This database version was then developed and updated as a CPPsite 2.0 and holds 1855 entries, including 1012 recent new entries [44]. The renovated version contains further data concerning chemically modified CPPs used on the in vivo model. In addition to other informations on delivered cargoes by CPPs (proteins, molecules, nanoparticles, DNA, RNA, etc.), secondary and tertiary structures of
**4. Optimization methods for CPP-mediated cancer therapy and**
*Cell-Penetrating Peptides: A Challenge for Drug Delivery DOI: http://dx.doi.org/10.5772/intechopen.91684*
*Cheminformatics and Its Applications*
to the eye.
these cells was tested [30]. In this study, it was demonstrated that the anti-Bcl-2 peptide induced apoptosis in tumor cells but also caused variable levels of toxicity in normal cells and tissues. On the contrary, the anti-HDM2 peptide induced apoptosis in tumor cells, with little effect on normal cells in a therapeutic dose range. This peptide also caused regression of retinoblastoma in rabbit eyes, with minimal damage to normal ocular tissues. They conclude that the inhibition of HDM2 may be a promising strategy for the treatment of uveal melanoma and retinoblastoma, and that strategy may be an effective technology for local delivery of anticancer therapy
Most of the patients with sporadic renal cell carcinomas (RCCs) exhibit mutation of the Hippel-Lindau (VHL) tumor suppressor gene. Conjugation of the protein transduction domain of HIV-TAT protein to the amino acid sequence (104–123) in the beta-domain of the VHL gene product (pVHL) arrested and then reduced proliferation and invasion of 786-O renal cancer cells in vitro. Besides, daily i.p. injections with the conjugate put off and, in some cases, caused partial regression of
The tumor suppressor gene *p16INK4A*, an inhibitor of cdk3 4, is often inactivated via intragenic mutation, homozygous deletion, and methylation-associated transcriptional silencing in a large number of human cancers, mainly in pancreatic cancer. Treated animals with the p16-derived synthetic peptide coupled with the Antennapedia carrier sequence, in which we designated as Trojan p16 peptide, showed reduced AsPC-1 and BxPC-3 s.c. tumors, respectively. Thus, we conclude that Trojan p16 peptide system, a gene-oriented peptide coupled with a peptide
Recently, it was shown by Sonia et al. that coupling doxorubicin (Dox) to three cell-penetrating peptides Tat, penetratin, and maurocalcine (Dox-CPPs) is a good strategy to overcome Dox resistance in MDA-MB 231 breast cancer cells and CHO cells (**Figure 3**) [3, 34]. We also reported that all conjugates are able to promote cell apoptosis in the breast cancer-resistant cells MDA-MB 231 at lesser concentration needed for Dox alone. Indeed, apoptosis death was shown to be correlated with ladder-internucleosomal degradation, chromatin contraction, caspase activation, Bad and Bax activation by oligomerization on the mitochondrial membrane, and liberation of cytochrome c. Despite the effective Bcl-2 overexpression in apoptosis induced by the Dox alone, such potency was shown to be insufficient in case of Dox-CPP-triggered cell apoptotic death. Otherwise, these results suggest that there are other apoptotic signaling pathways, independent of mitochondrial one, which are implicated in Dox-CPP apoptosis. Moreover, greater effectiveness of Dox when coupled to CPPs is not due only to its higher accumulation on the cells but also to the incitement of other signaling pathways. These pathways include death receptors
*Cellular internalization of Dox by MCa. MDA-MB231 cells treated with (a) RPMI, (B) Dox alone (red), and*
renal tumors that were implanted in the dorsal flank of nude mice [32].
vector, functions for experimental pancreatic cancer therapy [33].
and activation of the JNK pathway [4, 35].
*(C) Dox coupled to Dox at the same concentration (red).*
**170**
**Figure 3.**
Another study led by Leslie Walker et al. showed that conjugated Dox to both ELP and SynB1 prevents tumor development in mice. In fact, conjugation of Dox to SynB1-ELP was more efficient in tumor inhibition under hyperthermic condition than Dox alone, which was twofold higher. Such conception was considered hopeful peptide candidates for drug delivery [36]. The anticancer activity of Dox was also enhanced when constructed a drug delivery system by developing 25 nm gold nanospheres (GNSs) conjugated to four α-helical CPPs [37].
A thermally sensitive quantum dot that exhibits an "on-demand" cellular uptake behavior via temperature-induced "shielding/deshielding" of CPP on the surface was synthesized. Poly(N-isopropylacrylamide) (PNIPAAm) and CPP were biotinylated at their terminal ends and co-immobilized onto the surface of streptavidincoated quantum dots (QDs-Strep) through biotin-streptavidin interaction. Namely, under a lower critical solution temperature (LCST), the hydrated PNIPAAm chains blocked CPP cellular uptake. This effect was broken down when the LCST was raised to allow CPP moieties to be exposed on the cell surface, leading to QD cellular uptake.
Additionally, the "shielding/deshielding" temperature of CPP was also used for siRNA delivery system. Biotinylated siRNA was coupled to the surface of TSQDs. Indeed, the amount of corresponding gene silencing was increased due to the surface exposure of CPP within a rising temperature above the LCST [38].
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ffe82432-4883-4adc-b03b-937c1baf5090.169 | **4. Optimization methods for CPP-mediated cancer therapy and diagnosis**
Over the last decade, a great attention has been assigned to the importance of CPP on drug transportation of bioactive molecules in various preclinical studies. In fact, novel computational basics have been made in order to develop knowledge on CPPs [39].
Previously, different researchers have developed a few in silico algorithm approaches for CPP prediction (CPPpred) and screening to facilitate throughput CPP-based research. The in silico screening/prediction methods aimed on the use of scales of chemical characteristic, such as z-descriptors [40, 41]. It is generally followed by experimental validation to make it reliable with less cost and timeconsuming approach. Later on, other CPP prediction applied neural network (NN) strategies were developed and consist on introducing an N-to-1 NN. The network proceeds by a sequence of 5 to 30 amino acids in length, as input, and gives a prediction of how probably each peptide is to be cell penetrating [42]. This CPPpred offers an advantage since it was developed with repetition-reduced training and test sets.
Over the years, the commitment therapeutic importance of CPPs motivated other teams to develop the first version of CPP database, i.e., CPPsite which supports broad information on the promising use of CPPs [43]. The CPPsite manually created database of 843 experimentally described CPPs. Each consulting gives us data of the peptide involving peptide sequence, peptide name, nature of peptide, origin, chirality, uptake efficiency, subcellular localization, etc. A deep area of userfriendly tools has been integrated in this database like analyzing and browsing tools. Moreover, they have introduced other informations concerning peptide sequences such as secondary/tertiary structure and physicochemical properties of peptides.
This database version was then developed and updated as a CPPsite 2.0 and holds 1855 entries, including 1012 recent new entries [44]. The renovated version contains further data concerning chemically modified CPPs used on the in vivo model. In addition to other informations on delivered cargoes by CPPs (proteins, molecules, nanoparticles, DNA, RNA, etc.), secondary and tertiary structures of natural and chemical CPPs (including CPP with D-amino acids) were also predicted in view of their important role in the functionality of CPPs and stored in the database. Numerous tools for information browse and analysis are combined in this database and considered as a useful resource since it is compatible for all users, including smartphone and tablet.
CPP prediction sites are a promising assist to the researchers to design cell penetrating peptide, as well as making different modification and to investigate their effect on cell penetration potency [45].
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ffe82432-4883-4adc-b03b-937c1baf5090.170 | **5. Conclusion**
The progressive and continuous application of CPPs shows that they are efficient delivery vectors. Because of the need to ameliorate the drug delivery, a great number of CPP-based applications are still drawing the attention of researchers.
In this review, the current tendency in drug delivery by CPPs is summed up. Conjugation with CPP increases cell-surface affinity and eventual cellular uptake of bioactive molecules.
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ffe82432-4883-4adc-b03b-937c1baf5090.171 | **Author details**
Sonia Aroui\* and Abderraouf Kenani Unité de Recherche UR 12ES08 "Signalisation Cellulaire et Pathologies", Faculté de Médecine de Monastir, Monastir, Tunisie
\*Address all correspondence to: sonia\[email protected]
© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
**173**
*Cell-Penetrating Peptides: A Challenge for Drug Delivery DOI: http://dx.doi.org/10.5772/intechopen.91684*
> [10] Wu X, Gehring W. Cellular uptake of the *Antennapedia homeodomain* polypeptide by macropinocytosis. Biochemical and Biophysical Research
[11] Ziegler A, Nervi P, Durrenberger M, Seelig J. The cationic cell-penetrating peptide CPP TAT derived from the
Communications. 2013;**13**:1-1
HIV-1 protein tat is rapidly
Optical, biophysical and
2005;**44**:138-148
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2004;**6**:189-196
2000;**469**:179-185
peptides: From technology to physiology. Nature Cell Biology.
[15] Fajloun Z, Kharrat R, Chen L, Lecomte C, Di Luccio E, Bichet D, et al. Chemical synthesis and characterization of maurocalcine, a scorpion toxin that activates Ca(2+) release channel/ ryanodine receptors. FEBS Letters.
[16] Esteve E, Smida-Rezgui S, Sarkozi S, Szegedi C, Regaya I, Chen L, et al. Critical amino acid residues determine the binding affinity
and the Ca2+ release efficacy of maurocalcine in skeletal muscle cells. The Journal of Biological Chemistry.
2003;**278**:37822-37831
transported into living fibroblasts:
metabolic evidence. Biochemistry.
[12] Green M, Loewenstein PM. Autonomous functional domains of chemically synthesized human immunodeficiency virus tat trans-activator protein. Cell.
[13] Vives E, Brodin P, Lebleu B. A truncated HIV-1 tat protein basic domain rapidly translocates through the plasma membrane and accumulates in the cell nucleus. The Journal of Biological Chemistry.
[14] Joliot A, Prochiantz A. Transduction
[1] El-Sayed A, Futaki S, Harashima H. The AAPS Journal. 2009;**11**:13-22
**References**
[2] Moon JI, Han MJ, Yu SH, Lee EH, Kim SM, Han K, et al. Enhanced delivery of protein fused to cell penetrating peptides to mammalian cells. BMB Reports. May 2019;**52**(5):324-329
[3] Aroui S, Ram N, Appaix F, Ronjat M, Kenani A, Pirollet F, et al. Maurocalcine as a non toxic drug carrier overcomes doxorubicin resistance in the cancer cell line MDA-MB 231. Pharmaceutical
Research. 2008;**10**:9782-9801
[4] Aroui S, Brahim S, De Waard M, Bréard J, Kenani A. Efficient induction of apoptosis by doxorubicin coupled to cell-penetrating peptides compared to unconjugated doxorubicin in the human breast cancer cell line MDA-MB 231. Cancer Letters. 2009;**285**:28-38
[5] Lönn P, Dowdy SF. Cationic PTD/ CPP-mediated macromolecular delivery: Charging into the cell. Expert Opinion on Drug Delivery. 2015;**12**(10):1627-1636
[6] Schroeder JA, Bitler BG. Anti-cancer therapies that utilize cell penetrating peptides. Recent Patents on Anti-Cancer
[7] Jarver P, Langel U. Cell-penetrating
[8] Lehto T, Kurrikoff K, Langel U. Cellpenetrating peptides for the delivery of nucleic acids. Expert Opinion on Drug
Drug Discovery. 2010;**5**:1-10
peptides-a brief introduction. Biochimica et Biophysica Acta.
2006;**1758**:260-263
Delivery. 2012;**9**:823-836
2008;**29**:24274-24284
[9] Ram N, Aroui S, Jaumain E, Bichraoui H, Mabrouk K, Ronjat M, et al. Direct peptide interaction with surface glycosaminoglycans contributes to the cell penetration of maurocalcine. The Journal of Biological Chemistry.
*Cell-Penetrating Peptides: A Challenge for Drug Delivery DOI: http://dx.doi.org/10.5772/intechopen.91684*
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ffe82432-4883-4adc-b03b-937c1baf5090.173 | *Edited by Amalia Stefaniu, Azhar Rasul and Ghulam Hussain*
Cheminformatics has emerged as an applied branch of Chemistry that involves multidisciplinary knowledge, connecting related fields such as chemistry, computer science, biology, pharmacology, physics, and mathematical statistics.The book is organized in two sections, including multiple aspects related to advances in the development of informatic tools and their specific use in compound structure databases with various applications in life sciences, mainly in medicinal chemistry, for identification and development of new therapeutically active molecules. The book covers aspects related to genomic analysis, semantic similarity, chemometrics, pattern recognition techniques, chemical reactivity prediction, drug-likeness assessment, bioavailability, biological target recognition, machine-based drug discovery and design. Results from various computational tools and methods are discussed in the context of new compound design and development, sharing promising opportunities, and perspectives.
Published in London, UK © 2020 IntechOpen © monsitj / iStock
Cheminformatics and its Applications
Cheminformatics
and its Applications
*Edited by Amalia Stefaniu,*
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ffee18f3-6e2b-4d7c-b1a7-110a12a2adbc.0 | *Edited by Dan C. Dumitras*
The present book includes several contributions aiming a deeper understanding of the basic processes in the operation of CO2 lasers (lasing on non-traditional bands, frequency stabilization, photoacoustic spectroscopy) and achievement of new systems (CO2 lasers generating ultrashort pulses or high average power, lasers based on diffusion cooled V-fold geometry, transmission of IR radiation through hollow core microstructured fibers). The second part of the book is dedicated to applications in material processing (heat treatment, welding, synthesis of new materials, micro fluidics) and in medicine (clinical applications, dentistry, non-ablative therapy, acceleration of protons for cancer treatment).
ISBN 978-953-51-0351-6
CO2 Laser - Optimisation and Application
Photo by IdealPhoto30 / iStock
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ffee18f3-6e2b-4d7c-b1a7-110a12a2adbc.1 | CO2 Laser Optimisation and Application
*Edited by Dan C. Dumitras*
**CO2 LASER –**
**APPLICATION**
Edited by **Dan C. Dumitras**
**OPTIMISATION AND**
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ffee18f3-6e2b-4d7c-b1a7-110a12a2adbc.2 | **CO2 LASER – OPTIMISATION AND APPLICATION**
Edited by **Dan C. Dumitras**
http://dx.doi.org/10.5772/2496 Edited by Dan C. Dumitras
#### **Contributors**
Getulio De Vasconcelos, Mikhail Polyanskiy, Marcus Babzien, Mohammadreza Riahi, Andrey Pryamikov, Aleksey F Kosolapov, Victor G Plotnichenko, Evgeny M Dianov, Dan C. Dumitras, Nasrin Zand, Rakesh Kumar Soni, Jin-Sung Kim, Joanna Barbara Radziejewska, Keiichiro Urabe, Kunihide Tachibana, Afia Kouadri - David, Akira Endo, Vladimir Petukhov, Rui Lobo, Ram Vaderhobli, Pinal Viraparia, Turchetti
#### **© The Editor(s) and the Author(s) 2012**
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First published in Croatia, 2012 by INTECH d.o.o. eBook (PDF) Published by IN TECH d.o.o. Place and year of publication of eBook (PDF): Rijeka, 2019. IntechOpen is the global imprint of IN TECH d.o.o. Printed in Croatia
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CO2 Laser - Optimisation and Application Edited by Dan C. Dumitras p. cm. ISBN 978-953-51-0351-6 eBook (PDF) ISBN 978-953-51-4981-1
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ffee18f3-6e2b-4d7c-b1a7-110a12a2adbc.4 | **Meet the editor**
Prof Dr Eng Dan Constantin Dumitras graduated at the Faculty of Electronics, University Politehnica Bucharest in 1970. He obtained his PhD at the Institute of Atomic Physics, Bucharest in 1978. Since 1970 he has been involved in research on laser physics and applications (frequency stabilization of lasers, photoacoustic spectroscopy, laser applications in medicine and biology,
material processing and ultrashort pulse, high intensity lasers - extreme light) at the Department of Lasers, Institute of Atomic Physics and at the National Institute for Laser, Plasma and Radiation Physics, Bucharest. He works as a professor and a PhD supervisor at the Faculty of Applied Sciences, University Politehnica Bucharest. He is the author and/or editor of 15 books and has published more than 120 papers in scientific journals. He held more than 200 presentations at the international conferences (30 invited lectures).
Contents
**Preface IX**
**Part 1 Basic Processes 1**
**Part 2 New Systems 137**
Chapter 4 **Ultrashort Pulses 139**
Akira Endo
Rakesh Kumar Soni
**I. Principles 3**
Chapter 1 **CO2 Laser Photoacoustic Spectroscopy:**
Chapter 2 **CO2 Laser Photoacoustic Spectroscopy:**
Chapter 3 **CO2 Lasing on Non-Traditional Bands 103** Vladimir Petukhov and Vadim Gorobets
Chapter 5 **High Average Power Pulsed CO2 Laser**
Chapter 6 **Diffusion Cooled V-Fold CO2 Laser 179**
Chapter 7 **Heterodyne Interferometer for Measurement**
Chapter 8 **Transmission of CO2 Laser Radiation Through**
A. D. Pryamikov, A. F. Kosolapov, V. G. Plotnichenko and E. M. Dianov
Keiichiro Urabe and Kunihide Tachibana
Dan C. Dumitras, Ana Maria Bratu and Cristina Popa
**II. Instrumentation and Applications 43** Dan C. Dumitras, Ana Maria Bratu and Cristina Popa
Mikhail N. Polyanskiy and Marcus Babzien
**for Short Wavelength Light Sources 163**
**of Electron Density in High-Pressure Plasmas 209**
**Glass Hollow Core Microstructured Fibers 227**
### Contents
#### **Preface XI**
### **Part 1 Basic Processes 1** Chapter 1 **CO2 Laser Photoacoustic Spectroscopy: I. Principles 3** Dan C. Dumitras, Ana Maria Bratu and Cristina Popa Chapter 2 **CO2 Laser Photoacoustic Spectroscopy: II. Instrumentation and Applications 43** Dan C. Dumitras, Ana Maria Bratu and Cristina Popa Chapter 3 **CO2 Lasing on Non-Traditional Bands 103** Vladimir Petukhov and Vadim Gorobets **Part 2 New Systems 137** Chapter 4 **Ultrashort Pulses 139** Mikhail N. Polyanskiy and Marcus Babzien Chapter 5 **High Average Power Pulsed CO2 Laser for Short Wavelength Light Sources 163** Akira Endo Chapter 6 **Diffusion Cooled V-Fold CO2 Laser 179** Rakesh Kumar Soni Chapter 7 **Heterodyne Interferometer for Measurement of Electron Density in High-Pressure Plasmas 209** Keiichiro Urabe and Kunihide Tachibana Chapter 8 **Transmission of CO2 Laser Radiation Through Glass Hollow Core Microstructured Fibers 227** A. D. Pryamikov, A. F. Kosolapov, V. G. Plotnichenko and E. M. Dianov
#### **Part 3 Material Processing 249**
Chapter 9 **Application of Laser-Burnishing Treatment for Improvement of Surface Layer Properties 251** Joanna Radziejewska
#### Chapter 10 **Covering with Carbon Black and Thermal Treatment by CO2 Laser Surfaces of AISI 4340 Steel 275** G. Vasconcelos, D. C. Chagas and A. N. Dias
#### **Part 4 Medical Applications 355**
### Preface
The molecular carbon dioxide laser was invented in 1964 by C. K. N. Patel at Bell Labs. Immediately, it proved to be a high-power, continuous wave (CW) laser and a relatively high-efficiency gas laser (20-25% conversion of electrical energy into laser radiation), both in CW or pulsed operation. As a matter of fact, the CO2 lasers are the highest-power CW lasers (more than 100 kW) and one of the highest-energy pulsed gas laser (100 kJ) that are currently available. It demonstrated the utility in different device concepts and found a wide range of applications, from basic sciences till material processing and medicine, because it has a well established technology, it is versatile, simple to operate and relatively cheap on investment and maintenance.
The present book includes several contributions aiming a deeper understanding of the basic processes in the operation of CO2 lasers (lasing on non-traditional bands, frequency stabilization, photoacoustic spectroscopy) and achievement of new systems (CO2 lasers generating ultrashort pulses or high average power, lasers based on diffusion cooled V-fold geometry, transmission of IR radiation through hollow core microstructured fibers). The second part of the book is dedicated to applications in material processing (heat treatment, welding, synthesis of new materials, micro fluidics) and in medicine (clinical applications, dentistry, non-ablative therapy, acceleration of protons for cancer treatment).
The editor would like to thank all the chapter authors for their effort in completion of this book.
> **Dan C. Dumitras** National Institute for Laser, Plasma and Radiation Physics (INFLPR) Romania
**Part 1**
**Basic Processes**
**Part 1**
**Basic Processes**
**1**
**I. Principles**
*Romania*
**CO2 Laser Photoacoustic Spectroscopy:**
*Department of Lasers, National Institute for Laser, Plasma, and Radiation Physics, Bucharest*
Laser photoacoustic spectroscopy (LPAS) has emerged over the last decade as a very powerful investigation technique, capable of measuring trace gas concentrations at ppmV (parts per million by volume), or even sub-ppbV (parts per billion by volume) level. Recent achievements in this field have made it possible to fully characterize the method and improve the design of instrument components in view of the task they are expected to
The photoacoustic (PA) (formerly also known as optoacoustic) effect consisting in sound generation from the interaction of light and matter was discovered by Alexander Graham Bell (Bell, 1880). He noticed that focused intensity-modulated light (chopped sunlight) falling on an optically absorbing solid substance produced an audible sound. In the next year, light absorption was detected through its accompanying acoustic effect not only in solids, but also in liquids and gases by Bell (Bell, 1881), Tyndall (Tyndall, 1881), Röntgen (Röntgen, 1881), and Preece (Preece, 1881). They found the sound was stronger when the substance was placed in a sample cell (then called "photophone" and later "spectrophone"). It was Bell again that first described the resonant amplification of the PA signal (Bell, 1881). The PA effect was also investigated at different light wavelengths. Bell and Preece were among the first to notice a PA signal for an aerosol when they experimented with cigar smoke. The advances of photoacoustic spectroscopy up to the invention of the laser were
Over the last five decades, technological developments in the field of lasers and highsensitivity pressure detection systems (microphones and electronics) have contributed to the substantial progress of photoacoustic spectroscopy. The introduction of laser light sources emitting highly monochromatic and collimated intense beams have opened up new areas of research. Lasers provide the advantage of high spectral power density owing to their intrinsic narrow linewidth. This laser linewidth is usually much smaller than the molecular absorption linewidth (GHz region at atmospheric pressure), and therefore it is not an important issue in most measurements. A true revival of PA spectroscopy was due to Kerr and Atwood (Kerr & Atwood, 1968), who made the earliest experiments with a laser illuminated PA detector in 1968, and Kreuzer (Kreuzer, 1971), who first measured gas
**1. Introduction**
**1.1 Historical remarks**
reviewed by Kaiser in 1959 (Kaiser, 1959).
fulfill.
Dan C. Dumitras, Ana Maria Bratu and Cristina Popa
### **CO2 Laser Photoacoustic Spectroscopy: I. Principles**
Dan C. Dumitras, Ana Maria Bratu and Cristina Popa *Department of Lasers, National Institute for Laser, Plasma, and Radiation Physics, Bucharest Romania*
#### **1. Introduction**
Laser photoacoustic spectroscopy (LPAS) has emerged over the last decade as a very powerful investigation technique, capable of measuring trace gas concentrations at ppmV (parts per million by volume), or even sub-ppbV (parts per billion by volume) level. Recent achievements in this field have made it possible to fully characterize the method and improve the design of instrument components in view of the task they are expected to fulfill.
#### **1.1 Historical remarks**
The photoacoustic (PA) (formerly also known as optoacoustic) effect consisting in sound generation from the interaction of light and matter was discovered by Alexander Graham Bell (Bell, 1880). He noticed that focused intensity-modulated light (chopped sunlight) falling on an optically absorbing solid substance produced an audible sound. In the next year, light absorption was detected through its accompanying acoustic effect not only in solids, but also in liquids and gases by Bell (Bell, 1881), Tyndall (Tyndall, 1881), Röntgen (Röntgen, 1881), and Preece (Preece, 1881). They found the sound was stronger when the substance was placed in a sample cell (then called "photophone" and later "spectrophone"). It was Bell again that first described the resonant amplification of the PA signal (Bell, 1881). The PA effect was also investigated at different light wavelengths. Bell and Preece were among the first to notice a PA signal for an aerosol when they experimented with cigar smoke. The advances of photoacoustic spectroscopy up to the invention of the laser were reviewed by Kaiser in 1959 (Kaiser, 1959).
Over the last five decades, technological developments in the field of lasers and highsensitivity pressure detection systems (microphones and electronics) have contributed to the substantial progress of photoacoustic spectroscopy. The introduction of laser light sources emitting highly monochromatic and collimated intense beams have opened up new areas of research. Lasers provide the advantage of high spectral power density owing to their intrinsic narrow linewidth. This laser linewidth is usually much smaller than the molecular absorption linewidth (GHz region at atmospheric pressure), and therefore it is not an important issue in most measurements. A true revival of PA spectroscopy was due to Kerr and Atwood (Kerr & Atwood, 1968), who made the earliest experiments with a laser illuminated PA detector in 1968, and Kreuzer (Kreuzer, 1971), who first measured gas
CO2 Laser Photoacoustic Spectroscopy: I. Principles 5
At present many research groups are actively involved in the development of LPAS systems for various applications in different disciplines, including nondestructive evaluation of materials, environmental analysis, agricultural, biological, and medical applications, investigation of physical processes (phase transitions, heat and mass transfer, kinetic studies), and many others. Our facility, which was originally designed for ethylene (C2H4) analysis at the low ppb level, is adaptable with minor modifications to a broad range of
A gaseous molecule that absorbs electromagnetic radiation is excited to a higher electronic, vibrational or rotational quantum state. The excited state loses its energy by radiation processes, such as spontaneous (fluorescence) or stimulated emission, and/or by collisional relaxation, in which energy is converted into translational energy. Radiative emission and chemical reactions do not play an important role in the case of vibrational excitation, because the radiative lifetimes of the vibrational levels are long compared with the time needed for collisional deactivation at pressures used in photoacoustics (∼1 bar), and the photon energy is too small to induce chemical reactions. Thus, in practice the absorbed energy is completely released via either fluorescence (at low pressures) or collisions. The latter give rise to a gas temperature increase due to energy transfer to translation as heat, appearing as translational (kinetic) energy of the gas molecules. The deposited heat power density is proportional to the absorption coefficient and incident light intensity. The nonradiative relaxation process occurs when the relaxation time can compete with the radiative lifetime of the excited energy levels. Radiative decay has a characteristic lifetime of 10-7 s at visible wavelengths as compared with 10-2 s in IR at 10 μm. For nonradiative decay these values depend on pressure (the decay time
is inversely proportional to pressure) and may vary strongly at atmospheric pressures (10-3-10-
There are three techniques of linear laser spectroscopy, based on measurement of different
The most important optical process, as far as spectroscopic trace gas detection is concerned, is based on the extinction of radiation by molecular absorption. The absorption features and strengths specific of each molecule make it possible to identify trace gases and determine their concentrations. Absorption coefficients are typically on the order of 1 cm-1 (one wave number). The absorption of trace gas molecules in a gas mixture may be monitored by detecting the attenuation of the laser beam over a fixed absorption path length *L*. According to the Beer-Lambert law, the transmitted laser power in the absence of saturation is given by:
where *P*(0) and *P*(*L*) are the laser powers before and after the absorption cell, respectively;
(cm-1) is the absorption coefficient at a given pressure of the gas at a specific laser
*P L P L P cL* () () = α= α 0 exp( ) 0 exp *<sup>p</sup>* () ( ) , (1)
τ
α*p*
gases and vapors having absorption spectra in the infrared (IR).
8 s) depending on the gas nature and the involved energy level.
**2. Basic principles**
physical quantities:
**2.1 Linear laser spectroscopy methods**
concentrations using a PA detector and a laser in 1971. Later experiments by Kreuzer and collaborators (Kreuzer & Patel, 1971; Kreuzer et. al., 1972) effectively demonstrated the extremely high sensitivity attainable by this method. To improve the detection of atmospheric pollutants, Dewey et al. (Dewey et. al., 1973) have used in 1973 an acoustic resonance chamber and have reached amplification factors higher than 100. In 1977, the feasibility of *in situ* aerosol measurements, which were important for atmospheric applications, was first reported by Bruce and Pinnick (Bruce & Pinnick, 1977) and Terhune and Anderson (Terhune & Anderson, 1977). Subsequently, many experimental and theoretical works have been reported in the literature, proving the applicability of the method not only in spectroscopy, but also in various fields of physics, chemistry, biology, medicine, and engineering. The potential of laser photoacoustic spectroscopy has been discussed in several review articles (Patel & Tam, 1981; West, 1983; Hess, 1983; Tam, 1986; Sigrist, 1986, 2003; Meyer & Sigrist, 1990; Harren & Reuss, 1997; Harren et al., 2000; Miklos et al., 2001; Schmid, 2006) and books (Pao, 1977; Rosencwaig, 1980; Zharov & Letokhov, 1986; Hess, 1989; Mandelis, 1992, 1994; Bicanic, 1992; Gusev & Karabutov, 1993; Sigrist, 1994; Mandelis & Hess, 1997).
#### **1.2 Features of a gas sensor**
The most important features of a gas sensor include high sensitivity and selectivity, large dynamic range, high accuracy and precision, good temporal resolution, ease of use, versatility, reliability, robustness, and multicomponent capability. Gas chromatographs are neither sensitive nor fast enough. Although there is no ideal instrument that would fulfill all the requirements mentioned above, a spectroscopic method and particularly the simple setup of LPAS provide several unique advantages, notably the multicomponent capability, high sensitivity and selectivity, wide dynamic range, immunity to electromagnetic interferences, convenient real time data analysis, operational simplicity, relative portability, relatively low cost per unit, easy calibration, and generally no need for sample preparation. LPAS is primarily a calorimetric technique and, as such, differs completely from other previous techniques, as the absorbed energy can be determined directly, instead of via measurement of the intensity of the transmitted or backscattered radiation. In conjunction with tunable lasers, *in situ* monitoring of many substances occurring at ppbV or even pptV (parts per trillion by volume) concentrations is a routine task today. PA detection provides not only high sensitivity but also the necessary selectivity for analyzing multicomponent mixtures by the use of line-tunable IR lasers, e.g., CO lasers (Sigrist et al., 1989) or CO2 lasers (Meyer & Sigrist, 1990).
CO2 laser photoacoustic spectroscopy offers a sensitive technique for detection and monitoring of trace gases at low concentrations. The CO2 laser is of special interest, as it ensures high output power in a wavelength region (9-11 μm) where more than 250 molecular gases/vapors of environmental concern for atmospheric, industrial, medical, military, and scientific spheres exhibit strong absorption bands (Hubert, 1983). This laser, however, can be only stepwise tuned when operated in cw (continuous wave). Nevertheless, it is an ideal source to push the sensitivity of PA gas detection into the concentration range of ppbV or even lower. Instruments based on LPAS have nearly attained the theoretical noise equivalent absorption detectivity of 10-10 cm-1 in controlled laboratory conditions (Harren et al., 1990). This high sensitivity cannot be achieved in real detection conditions due to the coherent photoacoustical background signal and interfering background absorption of normal atmospheric constituents.
At present many research groups are actively involved in the development of LPAS systems for various applications in different disciplines, including nondestructive evaluation of materials, environmental analysis, agricultural, biological, and medical applications, investigation of physical processes (phase transitions, heat and mass transfer, kinetic studies), and many others. Our facility, which was originally designed for ethylene (C2H4) analysis at the low ppb level, is adaptable with minor modifications to a broad range of gases and vapors having absorption spectra in the infrared (IR).
#### **2. Basic principles**
4 CO2 Laser – Optimisation and Application
concentrations using a PA detector and a laser in 1971. Later experiments by Kreuzer and collaborators (Kreuzer & Patel, 1971; Kreuzer et. al., 1972) effectively demonstrated the extremely high sensitivity attainable by this method. To improve the detection of atmospheric pollutants, Dewey et al. (Dewey et. al., 1973) have used in 1973 an acoustic resonance chamber and have reached amplification factors higher than 100. In 1977, the feasibility of *in situ* aerosol measurements, which were important for atmospheric applications, was first reported by Bruce and Pinnick (Bruce & Pinnick, 1977) and Terhune and Anderson (Terhune & Anderson, 1977). Subsequently, many experimental and theoretical works have been reported in the literature, proving the applicability of the method not only in spectroscopy, but also in various fields of physics, chemistry, biology, medicine, and engineering. The potential of laser photoacoustic spectroscopy has been discussed in several review articles (Patel & Tam, 1981; West, 1983; Hess, 1983; Tam, 1986; Sigrist, 1986, 2003; Meyer & Sigrist, 1990; Harren & Reuss, 1997; Harren et al., 2000; Miklos et al., 2001; Schmid, 2006) and books (Pao, 1977; Rosencwaig, 1980; Zharov & Letokhov, 1986; Hess, 1989; Mandelis, 1992, 1994; Bicanic, 1992; Gusev &
The most important features of a gas sensor include high sensitivity and selectivity, large dynamic range, high accuracy and precision, good temporal resolution, ease of use, versatility, reliability, robustness, and multicomponent capability. Gas chromatographs are neither sensitive nor fast enough. Although there is no ideal instrument that would fulfill all the requirements mentioned above, a spectroscopic method and particularly the simple setup of LPAS provide several unique advantages, notably the multicomponent capability, high sensitivity and selectivity, wide dynamic range, immunity to electromagnetic interferences, convenient real time data analysis, operational simplicity, relative portability, relatively low cost per unit, easy calibration, and generally no need for sample preparation. LPAS is primarily a calorimetric technique and, as such, differs completely from other previous techniques, as the absorbed energy can be determined directly, instead of via measurement of the intensity of the transmitted or backscattered radiation. In conjunction with tunable lasers, *in situ* monitoring of many substances occurring at ppbV or even pptV (parts per trillion by volume) concentrations is a routine task today. PA detection provides not only high sensitivity but also the necessary selectivity for analyzing multicomponent mixtures by the use of line-tunable IR lasers, e.g., CO lasers (Sigrist et al., 1989) or CO2 lasers
CO2 laser photoacoustic spectroscopy offers a sensitive technique for detection and monitoring of trace gases at low concentrations. The CO2 laser is of special interest, as it ensures high output power in a wavelength region (9-11 μm) where more than 250 molecular gases/vapors of environmental concern for atmospheric, industrial, medical, military, and scientific spheres exhibit strong absorption bands (Hubert, 1983). This laser, however, can be only stepwise tuned when operated in cw (continuous wave). Nevertheless, it is an ideal source to push the sensitivity of PA gas detection into the concentration range of ppbV or even lower. Instruments based on LPAS have nearly attained the theoretical noise equivalent absorption detectivity of 10-10 cm-1 in controlled laboratory conditions (Harren et al., 1990). This high sensitivity cannot be achieved in real detection conditions due to the coherent photoacoustical background signal and interfering background
Karabutov, 1993; Sigrist, 1994; Mandelis & Hess, 1997).
**1.2 Features of a gas sensor**
(Meyer & Sigrist, 1990).
absorption of normal atmospheric constituents.
#### **2.1 Linear laser spectroscopy methods**
A gaseous molecule that absorbs electromagnetic radiation is excited to a higher electronic, vibrational or rotational quantum state. The excited state loses its energy by radiation processes, such as spontaneous (fluorescence) or stimulated emission, and/or by collisional relaxation, in which energy is converted into translational energy. Radiative emission and chemical reactions do not play an important role in the case of vibrational excitation, because the radiative lifetimes of the vibrational levels are long compared with the time needed for collisional deactivation at pressures used in photoacoustics (∼1 bar), and the photon energy is too small to induce chemical reactions. Thus, in practice the absorbed energy is completely released via either fluorescence (at low pressures) or collisions. The latter give rise to a gas temperature increase due to energy transfer to translation as heat, appearing as translational (kinetic) energy of the gas molecules. The deposited heat power density is proportional to the absorption coefficient and incident light intensity. The nonradiative relaxation process occurs when the relaxation time can compete with the radiative lifetime of the excited energy levels. Radiative decay has a characteristic lifetime of 10-7 s at visible wavelengths as compared with 10-2 s in IR at 10 μm. For nonradiative decay these values depend on pressure (the decay time τ is inversely proportional to pressure) and may vary strongly at atmospheric pressures (10-3-10- 8 s) depending on the gas nature and the involved energy level.
There are three techniques of linear laser spectroscopy, based on measurement of different physical quantities:
The most important optical process, as far as spectroscopic trace gas detection is concerned, is based on the extinction of radiation by molecular absorption. The absorption features and strengths specific of each molecule make it possible to identify trace gases and determine their concentrations. Absorption coefficients are typically on the order of 1 cm-1 (one wave number). The absorption of trace gas molecules in a gas mixture may be monitored by detecting the attenuation of the laser beam over a fixed absorption path length *L*. According to the Beer-Lambert law, the transmitted laser power in the absence of saturation is given by:
$$P(L) = P(0) \exp(\alpha\_p L) = P(0) \exp(\alpha x L) \, , \tag{1}$$
where *P*(0) and *P*(*L*) are the laser powers before and after the absorption cell, respectively; α*p* (cm-1) is the absorption coefficient at a given pressure of the gas at a specific laser
CO2 Laser Photoacoustic Spectroscopy: I. Principles 7
The fluorescence method requires that a certain part of the excitation should relax through radiative channels. This condition is fulfilled by detecting atoms and molecules in the UV, visible, and near-IR spectral regions. As a principal advantage of the fluorescence techniques, the observed signal is proportional to the concentration of the measured species and the accuracy, therefore, depends on the magnitude of the signal relative to detector noise. The sensitivity is so high, that it makes it possible to detect single atoms in the laser
The basic principle of all photothermal (PT) techniques is the absorption of light in a sample leading to a change in its thermal state. This may be a change in temperature or another thermodynamic parameter of the sample that is related to temperature. Measurement of either the temperature, pressure or density change that occurs due to optical absorption is ultimately the basis for all PT spectroscopic methods. PT analysis can be considered as an indirect absorption measurement, since the measured quantity is not an optical signal. (It should be noted here that the classical absorption measurement is not a direct measurement either. Though the measured value in this case is an optical one, namely the transmitted light, the absorbed light quantity is derived from the difference of the incident energy and the transmitted one). The sample heating which produces the PT signal is directly correlated to the absorbed electromagnetic energy. Unlike in conventional transmission spectroscopy, neither scattered nor reflected light contributes to the signal. Although a PT effect can be induced by any light source, lasers are nowadays the preferred source of excitation for two reasons: (i) To a first approximation, the PT signal is proportional to the temperature rise in the sample and thus proportional to the absorbed energy. (ii) For many applications, the selectivity of a PT analysis, as with any other absorption method, depends on the tunability
PA spectroscopy is an indirect technique in that an effect of absorption is measured rather than absorption itself. Hence the name of photoacoustic: light absorption is detected through its accompanying acoustic effect. The advantage of photoacoustics is that the absorption of light is measured on a zero background; this is in contrast with direct absorption techniques, where a decrease of the source light intensity has to be observed. The spectral dependence of absorption makes it possible to determine the nature of the trace components. The PA method is primarily a calorimetric technique, which measures the precise number of absorbent molecules by simply measuring the amplitude of an acoustic signal. In LPAS the nonradiative relaxation which generates heat is of primary importance.
In the IR spectral region, nonradiative relaxation is much faster than radiative decay.
spectroscopy methods is presented in Table 1 (Zharov & Letokhov, 1986).
PA spectroscopy relies on the PA effect for the detection of absorbing analytes. The sample gas is in a confined (resonant or nonresonant) chamber, where modulated (e.g., chopped) radiation enters via an IR-transparent window and is locally absorbed by IR-active molecular species. The temperature of the gas thereby increases, leading to a periodic expansion and contraction of the gas volume synchronous with the modulation frequency of the radiation. This generates a pressure wave that can be acoustically detected by a suitable sensor, e.g., by a microphone. The advantages of the PA method are high sensitivity and small sample volume; besides, the acoustic measurement makes optical detection unnecessary. The main drawback is caused by the sensitivity to acoustic noise, because the measurements are based on an acoustic signal. A comparison of the linear laser
beam.
of the excitation wavelength.
wavelength: α*p* = α*c*; α (cm-1 atm-1) is the gas absorption coefficient (the absorption coefficient normalized to unit concentration), and *c* (atm) is the trace gas concentration. Also, α*p* = *Ntot*σ, where σ (cm2) is the absorption cross section per molecule and *Ntot* = 2.5x1019 molecules cm-3 is the number of absorbing molecules per cubic centimeters at 1013 mbar and 20oC. It results:
$$\varepsilon = -\frac{1}{\alpha L} \ln \frac{P(L)}{P(0)} = -\frac{1}{\alpha L} \ln \left( 1 - \frac{\Delta P}{P(0)} \right) \equiv \frac{1}{\alpha L} \frac{\Delta P}{P(0)},\tag{2}$$
which is valid for Δ*P*/*P*(0) << 1 (optically thin sample), where Δ*P* = *P*(0) – *P*(*L*). For a given *L*, the detection limit is given by the smallest relative change Δ*Pmin*/*P*(0) that can be measured in the transmitted signal. For dilute mixtures and modest absorption path lengths, the desired signal is the small difference between two large values so that high quantitative accuracies in signal intensities are required. The most sensitive method employs frequency modulation and harmonic detection. The sensitivity depends on the linewidth, and for atmospherically broadened lines, Reid et al. (Reid et al., 1978) have reached Δ*Pmin*/*P*(0) ≅ 10-5 in a diode laser spectrometer (1050-1150 cm-1). With a path length of 100 m, the result is a sensitivity of 10-9 cm-1, which corresponds to concentrations of 3 ppbV of a weakly absorbing molecule such as SO2 (ν1 band), or 0.01 ppbV of a strongly absorbing molecule such as CO. Assuming the same detectable attenuation Δ*Pmin*/*P*(0) ≅ 10-5, a path length *L* = 1 m, and an absorption coefficient α = 30.4 cm-1atm-1 (typical of fundamental absorption in the mid-IR), one obtains a minimum detectable absorption coefficient α*<sup>p</sup>* = 10-7 cm-1. This number corresponds to a concentration of 3.3 ppbV at atmospheric pressure. Conventional absorption techniques, which require precise measurements of the difference between two nearly equal signals are, however, unable to realize the full potential of the higher power levels now attainable. Improvement may be obtained by: increasing the path length *L* in a multipass or intracavity arrangement, or using wavelength modulation, i.e., by modulating the wavelength of the incident intensity across a molecular absorption line. In multipass transmission absorption spectroscopy, a multipass transmission cell (White cell) filled with analyte gas with mirrors at each end is used. The beam is folded back and forth through the cell, creating an extended yet defined optical path length within a confined space.
Cavity ringdown spectroscopy confines gas in an optically reflective cavity where laser radiation is introduced. Radiation amplitude decays at a certain rate in the absence of absorption. An absorbing sample gas in the cavity increases the rate of decay, thus indicating the presence of an absorbing species. The advantages of the method are high sensitivity and a small sample volume, while indirect measurement is an important drawback: as the measured parameter is the rate of light intensity decay, decay caused by absorption by the analyte of interest has to be distinguished from the one caused by the mirrors and other cavity-dependent losses.
In linear detection, sensitivity is limited by laser power fluctuations, and a considerable improvement can be obtained by the dark background methods, in which one measures a quantity that is directly proportional to absorption, rather than that part of the laser beam which is absorbed. In the visible, this can be done by monitoring the fluorescence from the upper level of the transition. In the IR, however, the spontaneous emission rate is too low, and most of the excess vibrational energy is converted to heat through inelastic collisions.
coefficient normalized to unit concentration), and *c* (atm) is the trace gas concentration.
2.5x1019 molecules cm-3 is the number of absorbing molecules per cubic centimeters at 1013
11 1 ln ln 1
=− =− − ≅ αα α
*L P L P LP*
which is valid for Δ*P*/*P*(0) << 1 (optically thin sample), where Δ*P* = *P*(0) – *P*(*L*). For a given *L*, the detection limit is given by the smallest relative change Δ*Pmin*/*P*(0) that can be measured in the transmitted signal. For dilute mixtures and modest absorption path lengths, the desired signal is the small difference between two large values so that high quantitative accuracies in signal intensities are required. The most sensitive method employs frequency modulation and harmonic detection. The sensitivity depends on the linewidth, and for atmospherically broadened lines, Reid et al. (Reid et al., 1978) have reached Δ*Pmin*/*P*(0) ≅ 10-5 in a diode laser spectrometer (1050-1150 cm-1). With a path length of 100 m, the result is a sensitivity of 10-9 cm-1, which corresponds to concentrations of 3 ppbV of a weakly
such as CO. Assuming the same detectable attenuation Δ*Pmin*/*P*(0) ≅ 10-5, a path length *L* = 1
number corresponds to a concentration of 3.3 ppbV at atmospheric pressure. Conventional absorption techniques, which require precise measurements of the difference between two nearly equal signals are, however, unable to realize the full potential of the higher power levels now attainable. Improvement may be obtained by: increasing the path length *L* in a multipass or intracavity arrangement, or using wavelength modulation, i.e., by modulating the wavelength of the incident intensity across a molecular absorption line. In multipass transmission absorption spectroscopy, a multipass transmission cell (White cell) filled with analyte gas with mirrors at each end is used. The beam is folded back and forth through the
Cavity ringdown spectroscopy confines gas in an optically reflective cavity where laser radiation is introduced. Radiation amplitude decays at a certain rate in the absence of absorption. An absorbing sample gas in the cavity increases the rate of decay, thus indicating the presence of an absorbing species. The advantages of the method are high sensitivity and a small sample volume, while indirect measurement is an important drawback: as the measured parameter is the rate of light intensity decay, decay caused by absorption by the analyte of interest has to be distinguished from the one caused by the
In linear detection, sensitivity is limited by laser power fluctuations, and a considerable improvement can be obtained by the dark background methods, in which one measures a quantity that is directly proportional to absorption, rather than that part of the laser beam which is absorbed. In the visible, this can be done by monitoring the fluorescence from the upper level of the transition. In the IR, however, the spontaneous emission rate is too low, and most of the excess vibrational energy is converted to heat through inelastic collisions.
cell, creating an extended yet defined optical path length within a confined space.
( ) ( ) ( )
0 00 *P L P P*
( )
ν
mid-IR), one obtains a minimum detectable absorption coefficient
α
(cm-1 atm-1) is the gas absorption coefficient (the absorption
(cm2) is the absorption cross section per molecule and *Ntot* =
1 band), or 0.01 ppbV of a strongly absorbing molecule
= 30.4 cm-1atm-1 (typical of fundamental absorption in the
α
, (2)
*<sup>p</sup>* = 10-7 cm-1. This
Δ Δ
wavelength:
Also, α*p* = *Ntot*σ, where
α*p* = α*c*; α
mbar and 20oC. It results:
absorbing molecule such as SO2 (
m, and an absorption coefficient
mirrors and other cavity-dependent losses.
σ
*c*
The fluorescence method requires that a certain part of the excitation should relax through radiative channels. This condition is fulfilled by detecting atoms and molecules in the UV, visible, and near-IR spectral regions. As a principal advantage of the fluorescence techniques, the observed signal is proportional to the concentration of the measured species and the accuracy, therefore, depends on the magnitude of the signal relative to detector noise. The sensitivity is so high, that it makes it possible to detect single atoms in the laser beam.
The basic principle of all photothermal (PT) techniques is the absorption of light in a sample leading to a change in its thermal state. This may be a change in temperature or another thermodynamic parameter of the sample that is related to temperature. Measurement of either the temperature, pressure or density change that occurs due to optical absorption is ultimately the basis for all PT spectroscopic methods. PT analysis can be considered as an indirect absorption measurement, since the measured quantity is not an optical signal. (It should be noted here that the classical absorption measurement is not a direct measurement either. Though the measured value in this case is an optical one, namely the transmitted light, the absorbed light quantity is derived from the difference of the incident energy and the transmitted one). The sample heating which produces the PT signal is directly correlated to the absorbed electromagnetic energy. Unlike in conventional transmission spectroscopy, neither scattered nor reflected light contributes to the signal. Although a PT effect can be induced by any light source, lasers are nowadays the preferred source of excitation for two reasons: (i) To a first approximation, the PT signal is proportional to the temperature rise in the sample and thus proportional to the absorbed energy. (ii) For many applications, the selectivity of a PT analysis, as with any other absorption method, depends on the tunability of the excitation wavelength.
PA spectroscopy is an indirect technique in that an effect of absorption is measured rather than absorption itself. Hence the name of photoacoustic: light absorption is detected through its accompanying acoustic effect. The advantage of photoacoustics is that the absorption of light is measured on a zero background; this is in contrast with direct absorption techniques, where a decrease of the source light intensity has to be observed. The spectral dependence of absorption makes it possible to determine the nature of the trace components. The PA method is primarily a calorimetric technique, which measures the precise number of absorbent molecules by simply measuring the amplitude of an acoustic signal. In LPAS the nonradiative relaxation which generates heat is of primary importance. In the IR spectral region, nonradiative relaxation is much faster than radiative decay.
PA spectroscopy relies on the PA effect for the detection of absorbing analytes. The sample gas is in a confined (resonant or nonresonant) chamber, where modulated (e.g., chopped) radiation enters via an IR-transparent window and is locally absorbed by IR-active molecular species. The temperature of the gas thereby increases, leading to a periodic expansion and contraction of the gas volume synchronous with the modulation frequency of the radiation. This generates a pressure wave that can be acoustically detected by a suitable sensor, e.g., by a microphone. The advantages of the PA method are high sensitivity and small sample volume; besides, the acoustic measurement makes optical detection unnecessary. The main drawback is caused by the sensitivity to acoustic noise, because the measurements are based on an acoustic signal. A comparison of the linear laser spectroscopy methods is presented in Table 1 (Zharov & Letokhov, 1986).
CO2 Laser Photoacoustic Spectroscopy: I. Principles 9
τ
2. Excitation of a fraction of the ground-state molecular population of the target molecule by absorption of the incident laser radiation that is stored as vibrational-rotational energy; the amount of energy absorbed from the laser beam depends on the absorption
3. Energy exchange processes between vibrational levels (V-V: vibration to vibration transfer) and from vibrational states to rotational and translational degrees of freedom (V-R, T transfer); the energy which is absorbed by a vibrational-rotational transition is almost completely converted to the kinetic energy of the gas molecules by collisional de-excitation of the excited state; the efficiency of this conversion from deposited to translational energy depends on the pressure and internal energy level structure of the molecule; vibrational relaxation is usually so fast that it does not limit the sensitivity; however, notable anomalies occur in the case of diatomic molecules, such as CO, where vibrational relaxation is slow in the absence of a suitable collision partner, and of the dilute mixtures of CO2 in N2, where the vibrational energy is trapped in slowly relaxing vibrational states of N2; the kinetic energy is then converted into periodic local heating
4. Expansion and contraction of the gas in a closed volume that give rise to pressure variation which is an acoustic wave; the input of photon energy with correct timing
5. Monitoring the resulting acoustic waves with a microphone; the efficiency at which sound is transmitted to the microphone depends on the geometry of the cell and the
Fig. 1. Schematic of the physical processes occurring during optical excitation of molecules
From kinetic gas theory it can be estimated that a molecule performs 109-1010 collisions per second at 1 bar pressure. This means that at atmospheric pressure the photon energy is transformed into an acoustical signal in about 10-5-10-6 s. For most polyatomic molecules signal production is even faster. The time needed by the pressure wave to travel from the laser beam area to the microphone in the acoustic cell is therefore in most cases longer than the vibrational relaxation time. For a distance of a few centimeters this transit time is about 10-4 s. The time delay between excitation and detection of the pressure wave, however, is influenced not only by energy transfer processes and the transit time, but also by the response time of the gas-microphone system, being about 10-4 s or longer (Hess, 1983).
leads to the formation of a standing acoustic wave in the resonator.
the range
τ
at the modulation frequency.
in photoacoustic spectroscopy.
thermodynamic properties of the buffer gas.
*th* >> 1/*f* >>
coefficient, which is a function of pressure.
τ
*nr*, where
nonradiative lifetime of the excited energy state of the molecule.
device may also be employed, or the laser beam is modulated directly by modulation of its power supply; the extremely narrowband emission of the laser allows the specific excitation of molecular states; the laser power should be modulated with a frequency in
*th* is the thermal relaxation time, and
τ*nr* the
Table 1. Comparison of linear laser spectroscopy methods.
The favorable properties of LPAS are essentially determined by the characteristics of the laser. The kind and number of detectable substances are related to the spectral overlapping of the laser emission with the absorption bands of the trace gas molecules. Thus, the accessible wavelength range, tunability, and spectral resolution of the laser are of prime importance. With respect to minimum detectable concentrations (LPAS sensitivity), a laser with high output power *PL* is a benefit, because the PA signal is proportional to *PL*. The broad dynamic range is an inherent feature of LPAS and therefore is not affected by the choice of the radiation source. In contrast to remote-sensing methods, LPAS is a detection technique applied locally to samples enclosed in a PA cell. In order to still obtain some spatial resolution, either the samples have to be transported to the system, or the system has to be portable. The temporal resolution of LPAS is determined by the time needed for laser tuning and the gas exchange within the cell. Thus, a small volume PA cell and a fast tunable laser are a plus.
The availability of suitable laser sources plays a key role, as they control the sensitivity (laser power), selectivity (tuning range), and practicability (ease of use, size, cost, and reliability) that can be achieved with the photoacoustic technique. The CO2 laser perfectly fits the bill for a trace gas monitoring system based on LPAS. This IR laser combines simple operation and high output powers. The frequency spacing between two adjacent CO2-laser transitions range from 1 to 2 cm-1. By contrast, the typical width of a molecular absorption line is approximately 0.05 to 0.1 cm-1 for atmospheric conditions. Since this is not a continuously tunable source, coincidences between laser transitions and trace gas absorption lines are mandatory. Fortunately, this does not hamper its applicability to trace gas detection, as numerous gases exhibit characteristic absorption bands within the wavelength range of the CO2 laser which extends from 9 to 12 μm when different CO2 isotopes are used. The CO2 laser spectral output occurs in the wavelength region where a large number of compounds (including many industrial substances whose adverse health effects are a growing concern) possess strong characteristic absorption features and where absorptive interferences from water vapors, carbon dioxide, and other major atmospheric gaseous components may influence the measurements.
#### **2.2 PA effect in gases**
The PA effect in gases can be divided into five main steps (Fig. 1):
1. Modulation of the laser radiation (either in amplitude or frequency) at a wavelength that overlaps with a spectral feature of the target species; an electrooptical modulation
UV – far IR 10-5 – 10-9
1 -
Table 1. Comparison of linear laser spectroscopy methods.
**Absorption Fluorescence PA**
1 – 10-12
The favorable properties of LPAS are essentially determined by the characteristics of the laser. The kind and number of detectable substances are related to the spectral overlapping of the laser emission with the absorption bands of the trace gas molecules. Thus, the accessible wavelength range, tunability, and spectral resolution of the laser are of prime importance. With respect to minimum detectable concentrations (LPAS sensitivity), a laser with high output power *PL* is a benefit, because the PA signal is proportional to *PL*. The broad dynamic range is an inherent feature of LPAS and therefore is not affected by the choice of the radiation source. In contrast to remote-sensing methods, LPAS is a detection technique applied locally to samples enclosed in a PA cell. In order to still obtain some spatial resolution, either the samples have to be transported to the system, or the system has to be portable. The temporal resolution of LPAS is determined by the time needed for laser tuning and the gas exchange within the cell. Thus, a small volume PA cell and a fast tunable
The availability of suitable laser sources plays a key role, as they control the sensitivity (laser power), selectivity (tuning range), and practicability (ease of use, size, cost, and reliability) that can be achieved with the photoacoustic technique. The CO2 laser perfectly fits the bill for a trace gas monitoring system based on LPAS. This IR laser combines simple operation and high output powers. The frequency spacing between two adjacent CO2-laser transitions range from 1 to 2 cm-1. By contrast, the typical width of a molecular absorption line is approximately 0.05 to 0.1 cm-1 for atmospheric conditions. Since this is not a continuously tunable source, coincidences between laser transitions and trace gas absorption lines are mandatory. Fortunately, this does not hamper its applicability to trace gas detection, as numerous gases exhibit characteristic absorption bands within the wavelength range of the CO2 laser which extends from 9 to 12 μm when different CO2 isotopes are used. The CO2 laser spectral output occurs in the wavelength region where a large number of compounds (including many industrial substances whose adverse health effects are a growing concern) possess strong characteristic absorption features and where absorptive interferences from water vapors, carbon dioxide, and other major atmospheric gaseous components may
1. Modulation of the laser radiation (either in amplitude or frequency) at a wavelength that overlaps with a spectral feature of the target species; an electrooptical modulation
UV and visible Up to single atoms
Radiative channels of relaxation
UV – far IR 10-7 – 10-10 1 – 10-3 Nonradiative
channels of relaxation
**Method Characteristics**
Spectral range Sensitivity (cm-1) Time resolution (s) Necessary conditions
laser are a plus.
influence the measurements.
The PA effect in gases can be divided into five main steps (Fig. 1):
**2.2 PA effect in gases**
device may also be employed, or the laser beam is modulated directly by modulation of its power supply; the extremely narrowband emission of the laser allows the specific excitation of molecular states; the laser power should be modulated with a frequency in the range τ*th* >> 1/*f* >> τ*nr*, where τ*th* is the thermal relaxation time, and τ*nr* the nonradiative lifetime of the excited energy state of the molecule.
Fig. 1. Schematic of the physical processes occurring during optical excitation of molecules in photoacoustic spectroscopy.
From kinetic gas theory it can be estimated that a molecule performs 109-1010 collisions per second at 1 bar pressure. This means that at atmospheric pressure the photon energy is transformed into an acoustical signal in about 10-5-10-6 s. For most polyatomic molecules signal production is even faster. The time needed by the pressure wave to travel from the laser beam area to the microphone in the acoustic cell is therefore in most cases longer than the vibrational relaxation time. For a distance of a few centimeters this transit time is about 10-4 s. The time delay between excitation and detection of the pressure wave, however, is influenced not only by energy transfer processes and the transit time, but also by the response time of the gas-microphone system, being about 10-4 s or longer (Hess, 1983).
CO2 Laser Photoacoustic Spectroscopy: I. Principles 11
usual lengths of the PA cells (∼30 cm), the fractional absorption is very small (10-6-10-2), which means that in the worst case less than 1% of the incident laser power is absorbed in the sample gas inside the PA cell. It follows that the powermeter measures the real value of
Another advantage of photoacoustic spectroscopy as a tool for trace gas analysis is that very few photons are absorbed as the laser beam passes through the sample cell. As a result, notwithstanding the losses from absorption in the windows, the transmitted beam typically has sufficient power for analyzing samples in successive cells, via a multiplexing arrangement. A multiplexed photoacoustic sensor can be used to monitor many different samples simultaneously so that one instrument can be deployed to monitor up to 20 different locations within a clean room, industrial plant or other facility (Pushkarsky et al.,
Following the terminology introduced by Miklos et al. (Miklos et al., 2001), the name 'PA resonator' will be used for the cavity in which the resonant amplification of the PA signal takes place. The term PA cell (or PA detector; both terms are used in the literature to describe the device in which the PA signal is generated and monitored) is reserved for the entire acoustic unit, including the resonator, acoustic baffles and filters, windows, gas inlets and outlets, and microphone(s). Finally, PA instrument (PA sensor) stands for a complete setup, including the PA cell, light source, gas handling system, and electronics used for
It is interesting to mention that the *reverse* PA effect, called "sonoluminiscence", consists in the generation of optical radiation by acoustic waves, while the *inverse* PA effect is the generation of sound due to optical energy being lost from a sample, instead of being
A PA cell can be operated either in nonresonant mode or at an acoustic resonance frequency specific to the PA resonator. In the so-called nonresonant mode, the modulation frequency is much lower than the first acoustic resonance frequency of the PA resonator. In this case, the wavelength of the generated acoustic wave is larger than the cell dimensions. Thus, the generation of standing acoustic waves is not possible. A nonresonant PA cell lacks any means of energy accumulation in the acoustic wave, i.e., the induced pressure fluctuations are a function of the energy absorbed on that cycle alone and, in fact, any acoustic energy remaining from previous cycles tends only to produce noise on the desired signal. The main drawbacks of the nonresonant scheme are the low modulation frequency, which makes the system susceptible to 1/*f* noise, and the relatively large background signal generated by absorption in the windows of the cell and by radiation scattered to the walls. Nevertheless, the acoustically nonresonant cell has an advantage in low-pressure operation, as the signal, and hence the SNR, remains constant as pressure is decreased, whereas for the resonant cell, it drops almost linearly with decreasing pressure (Fig. 3) (Dumitras et al., 2007b). Also, the background signal, which limits the sensitivity of the nonresonant cell at atmospheric pressure, has been found to depend approximately linearly on pressure and would be less
the laser power inside the PA cell (we have "transparent" gases).
deposited in a sample as in the usual PA effect (Tam, 1986).
troublesome in low-pressure operation (Gerlach & Amer, 1978).
2002).
signal processing.
**3. Photoacoustic signal**
**3.1 Resonant cells**
#### **2.3 Typical laser photoacoustic setup**
A typical setup of a resonant LPAS, as used in the authors' laboratory for gas studies, is shown in Fig. 2. The continuous wave laser radiation is amplitude-modulated by a mechanical chopper operating at an acoustic resonance frequency of the PA cell. It is then focused by a lens and directed through the resonant PA cell. The transmitted laser power is monitored with a powermeter (signal *PL* in Fig. 2). Inside the cell the radiation produces pressure modulation recorded by microphone as an acoustical signal *V*, which is processed by a lock-in amplifier locked to the chopper frequency. The normalized absorption can then be deduced as being proportional to *V*/*PL* ratio (Cristescu et al., 1997, Dumitras et al., 2007a).
Fig. 2. Typical laser photoacoustic setup for trace gas measurements.
The power reading after beam passage through the PA cell can only be used for "transparent" gas samples. Let us evaluate if this condition is fulfilled. If the absorption is assumed to follow the Beer-Lambert law (Eq. 1), in the case of small absorption, the fractional absorption of the laser beam in the PA cell is given as Δ*P*/*P*(0) ≅ α*Lc* (Eq. 2). The quantity α*Lc* is known as the optical density of the gas in the resonator tube (this quantity is also called absorbance). Therefore, the PA signal proportional to Δ*P* depends linearly on the absorption coefficient, and its dependence on gas concentration is also linear. At α*Lc* = 0.06, a deviation of ~3% results from the linear behavior (~10% for α*Lc* = 0.07). An optical density of 0.06 (an ethylene concentration of 65 ppmV for *L* = 30 cm, the length of our cell) may thus be regarded as the upper limit of the linear range of a PA detector. Consequently, the PA signal can be modeled as a linear function of concentration in the full range from a few tens of pptV to 65 ppmV ethylene, so that the range spans over 6 orders of magnitude! Taking into account typical values for the absorption coefficients of the species to be measured (e.g., for ethylene at concentrations in the range 1 ppbV-10 ppmV, α*c* ≅ 3x10-8-3x10-4 cm-1) and
A typical setup of a resonant LPAS, as used in the authors' laboratory for gas studies, is shown in Fig. 2. The continuous wave laser radiation is amplitude-modulated by a mechanical chopper operating at an acoustic resonance frequency of the PA cell. It is then focused by a lens and directed through the resonant PA cell. The transmitted laser power is monitored with a powermeter (signal *PL* in Fig. 2). Inside the cell the radiation produces pressure modulation recorded by microphone as an acoustical signal *V*, which is processed by a lock-in amplifier locked to the chopper frequency. The normalized absorption can then be deduced as being proportional to *V*/*PL* ratio (Cristescu et al., 1997, Dumitras et al.,
Fig. 2. Typical laser photoacoustic setup for trace gas measurements.
a deviation of ~3% results from the linear behavior (~10% for
for ethylene at concentrations in the range 1 ppbV-10 ppmV,
fractional absorption of the laser beam in the PA cell is given as Δ*P*/*P*(0) ≅
absorption coefficient, and its dependence on gas concentration is also linear. At
The power reading after beam passage through the PA cell can only be used for "transparent" gas samples. Let us evaluate if this condition is fulfilled. If the absorption is assumed to follow the Beer-Lambert law (Eq. 1), in the case of small absorption, the
also called absorbance). Therefore, the PA signal proportional to Δ*P* depends linearly on the
of 0.06 (an ethylene concentration of 65 ppmV for *L* = 30 cm, the length of our cell) may thus be regarded as the upper limit of the linear range of a PA detector. Consequently, the PA signal can be modeled as a linear function of concentration in the full range from a few tens of pptV to 65 ppmV ethylene, so that the range spans over 6 orders of magnitude! Taking into account typical values for the absorption coefficients of the species to be measured (e.g.,
*Lc* is known as the optical density of the gas in the resonator tube (this quantity is
α
α
α
*Lc* = 0.07). An optical density
*c* ≅ 3x10-8-3x10-4 cm-1) and
*Lc* (Eq. 2). The
*Lc* = 0.06,
α
**2.3 Typical laser photoacoustic setup**
2007a).
quantity
α
usual lengths of the PA cells (∼30 cm), the fractional absorption is very small (10-6-10-2), which means that in the worst case less than 1% of the incident laser power is absorbed in the sample gas inside the PA cell. It follows that the powermeter measures the real value of the laser power inside the PA cell (we have "transparent" gases).
Another advantage of photoacoustic spectroscopy as a tool for trace gas analysis is that very few photons are absorbed as the laser beam passes through the sample cell. As a result, notwithstanding the losses from absorption in the windows, the transmitted beam typically has sufficient power for analyzing samples in successive cells, via a multiplexing arrangement. A multiplexed photoacoustic sensor can be used to monitor many different samples simultaneously so that one instrument can be deployed to monitor up to 20 different locations within a clean room, industrial plant or other facility (Pushkarsky et al., 2002).
Following the terminology introduced by Miklos et al. (Miklos et al., 2001), the name 'PA resonator' will be used for the cavity in which the resonant amplification of the PA signal takes place. The term PA cell (or PA detector; both terms are used in the literature to describe the device in which the PA signal is generated and monitored) is reserved for the entire acoustic unit, including the resonator, acoustic baffles and filters, windows, gas inlets and outlets, and microphone(s). Finally, PA instrument (PA sensor) stands for a complete setup, including the PA cell, light source, gas handling system, and electronics used for signal processing.
It is interesting to mention that the *reverse* PA effect, called "sonoluminiscence", consists in the generation of optical radiation by acoustic waves, while the *inverse* PA effect is the generation of sound due to optical energy being lost from a sample, instead of being deposited in a sample as in the usual PA effect (Tam, 1986).
#### **3. Photoacoustic signal**
#### **3.1 Resonant cells**
A PA cell can be operated either in nonresonant mode or at an acoustic resonance frequency specific to the PA resonator. In the so-called nonresonant mode, the modulation frequency is much lower than the first acoustic resonance frequency of the PA resonator. In this case, the wavelength of the generated acoustic wave is larger than the cell dimensions. Thus, the generation of standing acoustic waves is not possible. A nonresonant PA cell lacks any means of energy accumulation in the acoustic wave, i.e., the induced pressure fluctuations are a function of the energy absorbed on that cycle alone and, in fact, any acoustic energy remaining from previous cycles tends only to produce noise on the desired signal. The main drawbacks of the nonresonant scheme are the low modulation frequency, which makes the system susceptible to 1/*f* noise, and the relatively large background signal generated by absorption in the windows of the cell and by radiation scattered to the walls. Nevertheless, the acoustically nonresonant cell has an advantage in low-pressure operation, as the signal, and hence the SNR, remains constant as pressure is decreased, whereas for the resonant cell, it drops almost linearly with decreasing pressure (Fig. 3) (Dumitras et al., 2007b). Also, the background signal, which limits the sensitivity of the nonresonant cell at atmospheric pressure, has been found to depend approximately linearly on pressure and would be less troublesome in low-pressure operation (Gerlach & Amer, 1978).
CO2 Laser Photoacoustic Spectroscopy: I. Principles 13
govern the loss mechanisms that determine the quality factors of the resonances and also
A comparison of the microphone signals for nonresonant operation at 100 Hz and resonant operation at 564 Hz is depicted in Fig. 4 (a) and (b), respectively, together with the chopper waveforms. For nonresonant operation, the laser beam was amplitude-modulated with a duty cycle (pulse duration divided by the pulse period) of 25%, and the PA signal exhibits ringing at the resonant frequency on top of the 100-Hz square wave. For resonant operation, the laser beam was amplitude-modulated with a duty cycle of 50% and the microphone output was simply a coherent sine wave. In Fig. 4 (b), the data were recorded with a
The resonant cells can be adequatelly characterized by a model based on an acoustic
(a)
(b) Fig. 4. Microphone signals for: (a) nonresonant operation of the PA cell (Pushkarsky et al., 2002); (b) resonant operation of the PA cell (our cell), recorded with a Tektronix DPO 7104 Digital Oscilloscope, horizontal scale 1 ms/div, vertical scales 1 V/div. (rectangle wave) and
Several distinct resonances can be generated if the dimensions of a cavity are comparable with the acoustic wavelength. The standing wave patterns and resonance frequencies depend on the shape and size of the PA resonator. The most frequently used resonator is the cylinder, the symmetry of which coincides well with that of a laser beam propagating along the cylinder axis. The natural acoustic resonance frequencies of a lossless cylindrical resonator (fully reflecting walls) are determined as a solution of the wave equation in
cause small shifts in the resonant frequencies.
concentration of 1 ppmV of ethylene in the PA cell.
transmission line (Cristescu et al., 2000).
0.5 V/div. (sine wave amplified x 100).
cylindrical coordinates (Hess, 1983):
**3.2 Resonance frequencies**
Fig. 3. Dependence of the PA cell responsivity on the total gas pressure in the cell (measured with 1 ppmV ethylene in nitrogen).
Nonresonant operation can compete with enhanced resonant operation only at much lower frequencies and smaller cell volumes; however, a number of practical difficulties have been cited. At low frequencies, gas inlet-outlet ports act as pneumatic short circuits for the induced pressure (Kritchman et al., 1978). Excess acoustic energy in previous cycles of the modulated light can produce noise in the nonresonant signal, while in resonant operation this type of noise is avoided because the energy in each cycle contributes to a standing wave (Kamm, 1976). For the small, nonresonant cell, attachment of the microphone can lead to difficulties in extracting the optimum pressure response signal (Dewey, 1977).
With increasing modulation frequency, the acoustic wavelength equals the cell dimensions at a certain point, and the resonant modes of the cell can be excited, leading to an amplification of the signal. The signal can be boosted manifold by: a) designing the sample cell as an acoustic resonance chamber, so that the pressure fluctuations produced by spatially and temporally nonuniform excitation contribute to standing acoustic waves within the chamber, and b) minimizing dissipation of the acoustic energy and modulating the laser beam spatially and temporally at a frequency which coincides with one of the natural resonant acoustic frequencies of the chamber. The system becomes an acoustic amplifier in the sense that the energy existing in the standing wave is many times higher than the energy input per cycle, and the signal is amplified by a quality factor *Q*. The final signal amplification obtainable depends on the resonator losses. After an initial transient state, during which energy is accumulated in the standing acoustic wave, a steady state is reached in which the energy lost per cycle by various dissipation processes is equal to the energy gained per cycle by absorption of IR laser photons. Resonance properties mainly depend on the geometry and size of the cavity. For an acoustically resonant PA cell, important parameters will include gas characteristics such as heat capacity, thermal conductivity, viscosity, energies and relaxation times of the molecular vibrations and the sound velocity which determines the resonant frequencies of the cavity. Other parameters
Fig. 3. Dependence of the PA cell responsivity on the total gas pressure in the cell (measured
0 200 400 600 800 1000
p (mbar)
Nonresonant operation can compete with enhanced resonant operation only at much lower frequencies and smaller cell volumes; however, a number of practical difficulties have been cited. At low frequencies, gas inlet-outlet ports act as pneumatic short circuits for the induced pressure (Kritchman et al., 1978). Excess acoustic energy in previous cycles of the modulated light can produce noise in the nonresonant signal, while in resonant operation this type of noise is avoided because the energy in each cycle contributes to a standing wave (Kamm, 1976). For the small, nonresonant cell, attachment of the microphone can lead to
With increasing modulation frequency, the acoustic wavelength equals the cell dimensions at a certain point, and the resonant modes of the cell can be excited, leading to an amplification of the signal. The signal can be boosted manifold by: a) designing the sample cell as an acoustic resonance chamber, so that the pressure fluctuations produced by spatially and temporally nonuniform excitation contribute to standing acoustic waves within the chamber, and b) minimizing dissipation of the acoustic energy and modulating the laser beam spatially and temporally at a frequency which coincides with one of the natural resonant acoustic frequencies of the chamber. The system becomes an acoustic amplifier in the sense that the energy existing in the standing wave is many times higher than the energy input per cycle, and the signal is amplified by a quality factor *Q*. The final signal amplification obtainable depends on the resonator losses. After an initial transient state, during which energy is accumulated in the standing acoustic wave, a steady state is reached in which the energy lost per cycle by various dissipation processes is equal to the energy gained per cycle by absorption of IR laser photons. Resonance properties mainly depend on the geometry and size of the cavity. For an acoustically resonant PA cell, important parameters will include gas characteristics such as heat capacity, thermal conductivity, viscosity, energies and relaxation times of the molecular vibrations and the sound velocity which determines the resonant frequencies of the cavity. Other parameters
difficulties in extracting the optimum pressure response signal (Dewey, 1977).
with 1 ppmV ethylene in nitrogen).
0
50
100
150
R (cmV/W)
200
250
300
govern the loss mechanisms that determine the quality factors of the resonances and also cause small shifts in the resonant frequencies.
A comparison of the microphone signals for nonresonant operation at 100 Hz and resonant operation at 564 Hz is depicted in Fig. 4 (a) and (b), respectively, together with the chopper waveforms. For nonresonant operation, the laser beam was amplitude-modulated with a duty cycle (pulse duration divided by the pulse period) of 25%, and the PA signal exhibits ringing at the resonant frequency on top of the 100-Hz square wave. For resonant operation, the laser beam was amplitude-modulated with a duty cycle of 50% and the microphone output was simply a coherent sine wave. In Fig. 4 (b), the data were recorded with a concentration of 1 ppmV of ethylene in the PA cell.
The resonant cells can be adequatelly characterized by a model based on an acoustic transmission line (Cristescu et al., 2000).
Fig. 4. Microphone signals for: (a) nonresonant operation of the PA cell (Pushkarsky et al., 2002); (b) resonant operation of the PA cell (our cell), recorded with a Tektronix DPO 7104 Digital Oscilloscope, horizontal scale 1 ms/div, vertical scales 1 V/div. (rectangle wave) and 0.5 V/div. (sine wave amplified x 100).
#### **3.2 Resonance frequencies**
Several distinct resonances can be generated if the dimensions of a cavity are comparable with the acoustic wavelength. The standing wave patterns and resonance frequencies depend on the shape and size of the PA resonator. The most frequently used resonator is the cylinder, the symmetry of which coincides well with that of a laser beam propagating along the cylinder axis. The natural acoustic resonance frequencies of a lossless cylindrical resonator (fully reflecting walls) are determined as a solution of the wave equation in cylindrical coordinates (Hess, 1983):
CO2 Laser Photoacoustic Spectroscopy: I. Principles 15
For nonideal gases, the sound velocity can be approximately calculated by the following
Little attention has been given to the role of the buffer gas (defined as the optically nonabsorbing gaseous component in photoacoustic detectors). In principle, the molecular weight and the thermodynamic and transport properties of the buffer gas should have a significant impact on the photoacoustic signal. One would also expect the energy transfer between the absorbing species and the buffer gas to play an important role in PA detection (Thomas III et al., 1978; Gondal, 1997). In a mixture of ideal gases, the sound velocity *<sup>s</sup> v* and consequently the resonant frequencies of a PA resonator depend on the effective specific
( )1/2
( ) ( ) 1 1 *b a p p b a v v*
*xC x C xC x C* + −
+ −
Here *<sup>b</sup> Cp* , *<sup>b</sup> Cv* , *<sup>a</sup> Cp* and *<sup>a</sup> Cv* are the heat capacities of the buffer and absorbing gases, respectively; *M<sup>b</sup>* and *M<sup>a</sup>* are their molecular weights; and *x* is the fractional concentration of the buffer gas. When the molecular weight of the buffer gas is increased, the resonance frequency of the PA resonator shifts to lower values. In conclusion, the resonance frequency is a sensitive function of temperature and gas composition, both of which influence the
At a fixed temperature, *vs* also depends on the water content in the air (Rooth et al., 1990):
*p* <sup>γ</sup> =− − <sup>γ</sup>
pressures of water and air are denoted as *pw* and *pair*. The sound velocity in dry air is written
*<sup>s</sup>*<sup>0</sup> *v* . The increase of the resonance frequency of a 30-cm long longitudinal resonator at ambient temperature is 0.90 Hz for 1% of water vapors added to the gas. For all practical purposes, the variation of the resonance frequency with the CO2 concentration is negligible: -0.15 Hz per 1000 ppmV. For a given water vapor concentration, the resonance frequency provides information about the gas temperature inside the resonator. In most cases, the PA
<sup>5</sup> <sup>1</sup>
*w w*
*air air*
8
*air* are the ratios of the specific heats of water vapor and air. The partial
'
*s s*
cell resonance frequency has to be determined experimentally.
0
*<sup>p</sup> v v*
where the specific heat ratio γ and the average molecular weight *M* are:
γ =
where *B* is the second virial coefficient and *p* is the pressure*.*
heat ratio and the average mass of the mixture:
( ) 1/2 *v RT Bp M <sup>s</sup>* =γ + 2 / , (7)
*v RT M <sup>s</sup>* = γ / , (8)
( ) 1 *M b a* = +− *xM x M* . (10)
, (9)
. (11)
formula:
speed of sound.
Here γ*w* and γ
as '
$$f\_{kun} = \frac{\upsilon\_s}{2} \left[ \left( \frac{k}{L} \right)^2 + \left( \frac{\alpha\_{mu}}{\pi r} \right)^2 \right]^{1/2},\tag{3}$$
where *vs* is the sound velocity, *L* and *r* are the length and radius of the cylinder, the *k, m, n* indices (non-negative integers) refer to the values of the longitudinal, azimuthal, and radial modes, respectively, and α*mn* is the *n*-th root of the derivative of the *m*-th Bessel function:
$$\frac{\text{dJ}\_m(z)}{\text{d}z} = 0\tag{4}$$
(α<sup>00</sup> = 0, α<sup>01</sup> = 3.8317, α<sup>02</sup> = 7.0153, α<sup>10</sup> = 1.8412, α<sup>11</sup> = 5.3311, α<sup>12</sup> = 8.5360, etc.). For the first longitudinal mode, *k* = 1, *m* = 0, *n* = 0 and *f*100 = *f*0 = *vs*/2*L*.
In deducing Eq. (3), it was assumed that there was no phase shift on reflection of the pressure wave from the cavity walls caused either by wall compliance or boundary layer effects. If we depart from the assumption of complete wall rigidity, the boundary layer can be seen to cause significant frequency deviations from the above formula. To evaluate the frequency from Eq. (3), we must know the sound velocity, which may vary with frequency and pressure due to molecular relaxation effects and the nonideal behavior of the gas.
In reality, frequencies at the resonances are somewhat smaller. The corresponding resonance frequencies for PA resonators with open-open ends can be obtained from the following expression (Morse & Ingard, 1986):
$$f\_0 = \frac{\upsilon\_s}{2\left(L + \Delta L\right)} \,\,\,\tag{5}$$
where the quantity Δ*L* is the so-called end correction, which should be added to the length of the pipe for each open end. Physically, the end correction can be understood as an effect of the mismatch between the one-dimensional acoustic field inside the pipe and the threedimensional field outside that is radiated by the open end. The end correction can be approximated by the following expression: Δ*L* ≅ 0.6*r*, where *r* is the radius of the pipe (Miklos et al., 2001). More precisely, the end correction slightly decreases with frequency; therefore the resonance frequencies of an open pipe are not harmonically related but slightly stretched. In our experimental setup, the resonance frequency for 0.96 ppmV of ethylene in pure nitrogen is 564 Hz at *L* = 30 cm. By taking *vs* = 343 m/s in nitrogen at 22oC (the sound velocity in nitrogen of 330 m/s at 0oC was corrected for the room temperature), we have Δ*L* ≅ 0.2 cm for the two open ends of our PA resonator and Δ*L* ≅ 0.6*r* (*r* = 0.35 cm).
In an ideal gas, the sound velocity is given by:
$$w\_s = \left(\gamma RT \;/\; M\right)^{1/2},\tag{6}$$
where γ = *Cp*/*Cv* is the ratio of specific heats at constant pressure and volume, *R* is the idealgas constant, *T* is the absolute temperature, and *M* is the molecular weight. The sound velocity in an ideal gas only depends on temperature and remains unchanged at pressure modifications if γ is constant. In the case of ideal gases, γ = 1.4 for diatomic gases and γ = 1.33 for triatomic gases. Experimentally, the following values have been measured: 1.404 for N2, 1.401 for O2, 1.404 for CO, 1.32 for H2O, 1.31 for NH3, 1.31 for CH4, and 1.25 for C2H4.
*s mn*
where *vs* is the sound velocity, *L* and *r* are the length and radius of the cylinder, the *k, m, n* indices (non-negative integers) refer to the values of the longitudinal, azimuthal, and radial
> <sup>d</sup> ( ) <sup>0</sup> d *mJ z*
In deducing Eq. (3), it was assumed that there was no phase shift on reflection of the pressure wave from the cavity walls caused either by wall compliance or boundary layer effects. If we depart from the assumption of complete wall rigidity, the boundary layer can be seen to cause significant frequency deviations from the above formula. To evaluate the frequency from Eq. (3), we must know the sound velocity, which may vary with frequency and pressure due to molecular relaxation effects and the nonideal behavior of the gas.
In reality, frequencies at the resonances are somewhat smaller. The corresponding resonance frequencies for PA resonators with open-open ends can be obtained from the following
( ) <sup>0</sup> 2
where the quantity Δ*L* is the so-called end correction, which should be added to the length of the pipe for each open end. Physically, the end correction can be understood as an effect of the mismatch between the one-dimensional acoustic field inside the pipe and the threedimensional field outside that is radiated by the open end. The end correction can be approximated by the following expression: Δ*L* ≅ 0.6*r*, where *r* is the radius of the pipe (Miklos et al., 2001). More precisely, the end correction slightly decreases with frequency; therefore the resonance frequencies of an open pipe are not harmonically related but slightly stretched. In our experimental setup, the resonance frequency for 0.96 ppmV of ethylene in pure nitrogen is 564 Hz at *L* = 30 cm. By taking *vs* = 343 m/s in nitrogen at 22oC (the sound velocity in nitrogen of 330 m/s at 0oC was corrected for the room temperature), we have Δ*L*
( )1/2
for triatomic gases. Experimentally, the following values have been measured: 1.404 for N2,
1.401 for O2, 1.404 for CO, 1.32 for H2O, 1.31 for NH3, 1.31 for CH4, and 1.25 for C2H4.
= *Cp*/*Cv* is the ratio of specific heats at constant pressure and volume, *R* is the idealgas constant, *T* is the absolute temperature, and *M* is the molecular weight. The sound velocity in an ideal gas only depends on temperature and remains unchanged at pressure
γ
≅ 0.2 cm for the two open ends of our PA resonator and Δ*L* ≅ 0.6*r* (*r* = 0.35 cm).
is constant. In the case of ideal gases,
α
<sup>10</sup> = 1.8412,
2
*<sup>v</sup> <sup>k</sup> <sup>f</sup> L r* <sup>α</sup> = + <sup>π</sup>
*kmn*
α
α
<sup>02</sup> = 7.0153,
longitudinal mode, *k* = 1, *m* = 0, *n* = 0 and *f*100 = *f*0 = *vs*/2*L*.
α
modes, respectively, and
<sup>01</sup> = 3.8317,
expression (Morse & Ingard, 1986):
In an ideal gas, the sound velocity is given by:
(α<sup>00</sup> = 0, α
where γ
modifications if
γ
1/2 <sup>2</sup> <sup>2</sup>
*mn* is the *n*-th root of the derivative of the *m*-th Bessel function:
<sup>11</sup> = 5.3311,
α
*<sup>s</sup> <sup>v</sup> <sup>f</sup> L L* <sup>=</sup> + Δ , (5)
*v RT M <sup>s</sup>* = γ / , (6)
= 1.4 for diatomic gases and
γ= 1.33
, (3)
<sup>12</sup> = 8.5360, etc.). For the first
*<sup>z</sup>* <sup>=</sup> (4)
For nonideal gases, the sound velocity can be approximately calculated by the following formula:
$$\upsilon v\_s = \left[ \left\{ \left( RT + 2Bp \right) / M \right\}^{1/2} \right]^{1/2},\tag{7}$$
where *B* is the second virial coefficient and *p* is the pressure*.*
Little attention has been given to the role of the buffer gas (defined as the optically nonabsorbing gaseous component in photoacoustic detectors). In principle, the molecular weight and the thermodynamic and transport properties of the buffer gas should have a significant impact on the photoacoustic signal. One would also expect the energy transfer between the absorbing species and the buffer gas to play an important role in PA detection (Thomas III et al., 1978; Gondal, 1997). In a mixture of ideal gases, the sound velocity *<sup>s</sup> v* and consequently the resonant frequencies of a PA resonator depend on the effective specific heat ratio and the average mass of the mixture:
$$
\overline{\upsilon}\_s = \left(\overline{\gamma}RT \;/\,\,\overline{M}\right)^{1/2}\,\,\,\,\,\tag{8}
$$
where the specific heat ratio γ and the average molecular weight *M* are:
$$\overline{\gamma} = \frac{\boldsymbol{\infty} \mathbf{C}\_p^b + (1 - \boldsymbol{\chi}) \mathbf{C}\_p^a}{\boldsymbol{\infty} \mathbf{C}\_v^b + (1 - \boldsymbol{\chi}) \mathbf{C}\_v^a} \; \; \tag{9}$$
$$
\overline{M} = \mathfrak{x}M^b + (1 - \mathfrak{x})M^a \,. \tag{10}
$$
Here *<sup>b</sup> Cp* , *<sup>b</sup> Cv* , *<sup>a</sup> Cp* and *<sup>a</sup> Cv* are the heat capacities of the buffer and absorbing gases, respectively; *M<sup>b</sup>* and *M<sup>a</sup>* are their molecular weights; and *x* is the fractional concentration of the buffer gas. When the molecular weight of the buffer gas is increased, the resonance frequency of the PA resonator shifts to lower values. In conclusion, the resonance frequency is a sensitive function of temperature and gas composition, both of which influence the speed of sound.
At a fixed temperature, *vs* also depends on the water content in the air (Rooth et al., 1990):
$$\boldsymbol{\upsilon}\_{s} = \boldsymbol{\upsilon}\_{s0}^{\cdot} \left[ \mathbf{1} - \frac{p\_w}{p\_{air}} \left( \frac{\boldsymbol{\gamma}\_w}{\boldsymbol{\gamma}\_{air}} - \frac{\mathbf{5}}{\mathbf{8}} \right) \right]. \tag{11}$$
Here γ*w* and γ*air* are the ratios of the specific heats of water vapor and air. The partial pressures of water and air are denoted as *pw* and *pair*. The sound velocity in dry air is written as ' *<sup>s</sup>*<sup>0</sup> *v* . The increase of the resonance frequency of a 30-cm long longitudinal resonator at ambient temperature is 0.90 Hz for 1% of water vapors added to the gas. For all practical purposes, the variation of the resonance frequency with the CO2 concentration is negligible: -0.15 Hz per 1000 ppmV. For a given water vapor concentration, the resonance frequency provides information about the gas temperature inside the resonator. In most cases, the PA cell resonance frequency has to be determined experimentally.
CO2 Laser Photoacoustic Spectroscopy: I. Principles 17
In a carefully designed high quality resonator, the contribution of the first three effects can be minimized. The dominant contribution is caused by the viscous and thermal boundary layer losses. Throughout the major portion of the resonator volume the expansion and contraction of the gas occur adiabatically. We neglect heat conduction and viscous losses in the volume of the gas because the acoustic power loss from these effects is very small. However, the wall consists of a material with a thermal conduction coefficient much greater than that of the gas. Thermal dissipation occurs because expansion and contraction of the gas do not proceed adiabatically near the walls, where the process will change to isothermal. The temperature variation changes exponentially from the adiabatic propagation regime in the gas to a zero value at the wall. This leads to heat conduction within a transition region (thermal boundary layer), which is responsible for the thermal dissipation process. Outside a thin boundary layer with thickness *dh*, near the wall, the thermal losses can be neglected:
1/2
. (12)
is the density of mass, *Cp* is the molar heat
, (13)
are much smaller than the radius of the PA
= *Cp*/*Cv* = 1.4. As a result,
ν = (2μ/ρω0)1/2 =
γ
2
*<sup>K</sup> <sup>d</sup>*
*p*
*C* <sup>=</sup> ρω
ρ
The viscous dissipation can be explained by the boundary conditions imposed by the walls. At the surface, the tangential component of the acoustic velocity is zero, whereas inside the cavity, it is proportional to the acoustic pressure gradient. Thus, viscoelastic dissipation occurs in a transition region with a thickness *dv*, which is called the viscous boundary layer:
> 1/2 2 *<sup>v</sup> <sup>d</sup>* <sup>μ</sup> <sup>=</sup> ρω
> > ν
reached when the wavelength of sound is comparable to the cross-sectional dimensions of
of nitrogen at standard pressure (*p* = 1 atm) and room temperature: *K* = 2.552x10-2 W/(m K),
the values for the thermal and viscous boundary layer thicknesses are, respectively: *dh* =
2.2(*f*0)-1/2 (mm Hz1/2) ≅ 0.093 mm (at *f*0 = 564 Hz). Therefore, at atmospheric pressure and
The volumetric or bulk losses are caused by processes that tend to establish equilibrium in
Friction due to compressional motion and the transformation of organized energy into heat due to temperature gradients are responsible for the free space viscous and thermal losses.
ρ
0*Cp*)1/2 = 2.6(*f*0)-1/2 (mm Hz1/2) ≅ 0.110 mm (at *f*0 = 564 Hz) and *d*
audio frequencies, both boundary layers are only a fraction of a millimeter thick.
4. relaxational damping (dissipative relaxation processes within polyatomic gases).
<< *r*), which yields a lower frequency limit. The upper frequency limit is
= 1.142 kg/m3 and
*<sup>s</sup>* = *vs*/*f*<sup>0</sup> ≅ *r*). The magnitude of *dh* and *dv* can be calculated by using the properties
*h*
Here *K* is the thermal conductivity of the gas,
is the viscosity coefficient.
= 1.76x10-5 Pa s, *Cp* = 1.04x103 J/(kg K),
the propagating wave. These damping processes are: 1. free space viscous and thermal dissipation;
ν
Equations (12) and (13) are only valid if *dh* and *d*
= 2π*f* is the angular frequency.
capacity, and
where μ
the tube (
μ
(2*K*/ρω
resonator (*dh*, *d*
λ
2. diffusion effects; 3. radiation effects; and
ω
Since the resonance frequency is proportional to the sound velocity, the temperature dependence of the sound velocity is directly mirrored by the resonance frequency. The sound velocity in air has a temperature coefficient of about 0.18%/oC, thus a frequency shift δ*f* ≅ 0.0018*f*0Δ*T* is expected for a temperature change Δ*T* (oC) (Miklos et al., 2001). The true resonance frequency may therefore deviate from the fixed modulation frequency by δ*f*. Then the PA signal will not be excited at the peak of the resonance, but slightly to one side. Since a detuning from the resonance peak by 0.46 (Δ*f*(10%)/Δ*f*(FWHM) = 16/35) results in a 10% drop of the PA signal (see Fig. 5), the detuning should be smaller than ±0.23*f*0/*Q* for 10% signal stability. This stability can be ensured, if the condition *Q*Δ*T* ≤ 128 (δ*f* ≤ 0.23Δ*f* or 0,0018*f*0Δ*T* ≤ 0.23Δ*f* or *Q*Δ*T* ≤ 0.23/0.0018) is fulfilled (Δ*T* ≤ 7.9oC in our case for *Q* =16.1). The corresponding condition for PA signal stability of 2% can be written as *Q*Δ*T* ≤ 64 (Δ*T* ≤ 4oC in our case). These examples clearly show that low-*Q* photoacoustic resonators are not sensitive to temperature variations and consequently do not need temperature stabilization or active tracking of the resonance to adjust the modulation frequency.
Fig. 5. Resonance curve of our PA cell showing the full width at half maximum (FWHM) and the full widths for a signal drop of 2% and 10% from its maximum.
#### **3.3 Dissipation processes**
The various dissipation processes occurring in an acoustic cavity were first discussed at length by Kamm (Kamm, 1976). The energy accumulation attainable in a standing wave of a resonant cavity is many times larger than the energy loss occurring during a single period of an acoustic oscillation. This acoustical amplification effect is limited, however, by various dissipation processes. The losses can be divided into surface effects and volumetric ones (Johnson et al., 1982). The surface losses are due to the interaction of the standing wave with the internal resonator surface and may be subdivided into the following dissipation processes:
Since the resonance frequency is proportional to the sound velocity, the temperature dependence of the sound velocity is directly mirrored by the resonance frequency. The sound velocity in air has a temperature coefficient of about 0.18%/oC, thus a frequency shift
*f* ≅ 0.0018*f*0Δ*T* is expected for a temperature change Δ*T* (oC) (Miklos et al., 2001). The true
the PA signal will not be excited at the peak of the resonance, but slightly to one side. Since a detuning from the resonance peak by 0.46 (Δ*f*(10%)/Δ*f*(FWHM) = 16/35) results in a 10% drop of the PA signal (see Fig. 5), the detuning should be smaller than ±0.23*f*0/*Q* for 10% signal stability. This stability can be ensured, if the condition *Q*Δ*T* ≤ 128 (δ*f* ≤ 0.23Δ*f* or 0,0018*f*0Δ*T* ≤ 0.23Δ*f* or *Q*Δ*T* ≤ 0.23/0.0018) is fulfilled (Δ*T* ≤ 7.9oC in our case for *Q* =16.1). The corresponding condition for PA signal stability of 2% can be written as *Q*Δ*T* ≤ 64 (Δ*T* ≤ 4oC in our case). These examples clearly show that low-*Q* photoacoustic resonators are not sensitive to temperature variations and consequently do not need temperature stabilization
1.0 Cell 2
564
<sup>560</sup> <sup>568</sup> Δ*f* (2%)
Fig. 5. Resonance curve of our PA cell showing the full width at half maximum (FWHM)
540 550 560 570 580 590
<sup>547</sup> <sup>582</sup> Δ*f* (FWHM)
Δ*f* (10%) 556 572
Chopper frequency *f* (Hz)
The various dissipation processes occurring in an acoustic cavity were first discussed at length by Kamm (Kamm, 1976). The energy accumulation attainable in a standing wave of a resonant cavity is many times larger than the energy loss occurring during a single period of an acoustic oscillation. This acoustical amplification effect is limited, however, by various dissipation processes. The losses can be divided into surface effects and volumetric ones (Johnson et al., 1982). The surface losses are due to the interaction of the standing wave with the internal resonator surface and may be subdivided into the following dissipation processes:
3. losses due to wave scattering at surface obstructions such as gas inlet/outlet,
4. viscous and thermal dissipation in the boundary layer at the smooth internal surfaces.
and the full widths for a signal drop of 2% and 10% from its maximum.
**3.3 Dissipation processes**
1. compliance of the chamber walls;
microphones, and windows;
2. dissipation at the microphone diaphragm;
0.7
0.8
PA signal *V* (%)
0.9
δ*f*. Then
resonance frequency may therefore deviate from the fixed modulation frequency by
or active tracking of the resonance to adjust the modulation frequency.
δ
In a carefully designed high quality resonator, the contribution of the first three effects can be minimized. The dominant contribution is caused by the viscous and thermal boundary layer losses. Throughout the major portion of the resonator volume the expansion and contraction of the gas occur adiabatically. We neglect heat conduction and viscous losses in the volume of the gas because the acoustic power loss from these effects is very small. However, the wall consists of a material with a thermal conduction coefficient much greater than that of the gas. Thermal dissipation occurs because expansion and contraction of the gas do not proceed adiabatically near the walls, where the process will change to isothermal. The temperature variation changes exponentially from the adiabatic propagation regime in the gas to a zero value at the wall. This leads to heat conduction within a transition region (thermal boundary layer), which is responsible for the thermal dissipation process. Outside a thin boundary layer with thickness *dh*, near the wall, the thermal losses can be neglected:
$$d\_h = \left(\frac{2K}{\text{pooC}\_p}\right)^{1/2}.\tag{12}$$
Here *K* is the thermal conductivity of the gas, ρ is the density of mass, *Cp* is the molar heat capacity, and ω= 2π*f* is the angular frequency.
The viscous dissipation can be explained by the boundary conditions imposed by the walls. At the surface, the tangential component of the acoustic velocity is zero, whereas inside the cavity, it is proportional to the acoustic pressure gradient. Thus, viscoelastic dissipation occurs in a transition region with a thickness *dv*, which is called the viscous boundary layer:
$$d\_v = \left(\frac{2\mu}{\rho \alpha}\right)^{1/2} \text{ \textsuperscript{\textsuperscript{\textsuperscript{\textsuperscript{\textsuperscript{\boxminus}}}}}} \text{ \textsuperscript{\textsuperscript{\textsuperscript{\boxminus}}}} \text{ \textsuperscript{\textsuperscript{\boxminus}} \text{ \textsuperscript{\boxminus}} \text{ \textsuperscript{\boxminus}} \text{ \textsuperscript{\boxminus}} \text{ \textsuperscript{\boxminus}} \text{ \textsuperscript{\boxminus}} \text{ \textsuperscript{\boxminus}} \text{ \textsuperscript{\boxminus}} \text{ \textsuperscript{\boxminus}} \text{ \textsuperscript{\boxminus}} \text{ \textsuperscript{\boxminus}} \text{ \textsuperscript{\boxminus}} \text{ \textsuperscript{\boxminus}} \text{ \textsuperscript{\boxminus}} \text{ \textsuperscript{\boxminus}} \text{ \textsuperscript{\boxminus}} \text{ \textsuperscript{\boxminus}} \text{ \textsuperscript{\boxminus}} \text{ \textsuperscript{\boxminus}} \text{ \textsuperscript{\boxminus}} \text{ \textsuperscript{\boxminus}} \text{ \textsuperscript{\boxminus}} \text{ \textsuperscript{\boxminus}} \text{ \textsuperscript{\boxminus}} \text{ \textsuperscript{\boxminus}} \text{ \textsuperscript{\boxminus}} \text{ \textsuperscript{\boxminus}} \text{ \textsuperscript{\boxminus}} \text{ \textsuperscript{\boxminus}} \text{ \textsuperscript{\boxminus}} \text{ \textsuperscript{\boxminus}} \text{ \textsuperscript{\boxminus}} \text{ \textsuperscript{\boxminus}} \text{ \textsuperscript{\boxminus}} \text{ \textsuperscript{\boxminus}} \text{ \} \end{bmatrix} \begin{array}{c \, \boxminus} \text{ \emph{\prime}} \text{ \emph{\prime}} \text{ \emph{\prime}} \text{ \end{array} \tag{13}$$
where μis the viscosity coefficient.
Equations (12) and (13) are only valid if *dh* and *d*ν are much smaller than the radius of the PA resonator (*dh*, *d*ν << *r*), which yields a lower frequency limit. The upper frequency limit is reached when the wavelength of sound is comparable to the cross-sectional dimensions of the tube (λ*<sup>s</sup>* = *vs*/*f*<sup>0</sup> ≅ *r*). The magnitude of *dh* and *dv* can be calculated by using the properties of nitrogen at standard pressure (*p* = 1 atm) and room temperature: *K* = 2.552x10-2 W/(m K), μ = 1.76x10-5 Pa s, *Cp* = 1.04x103 J/(kg K), ρ = 1.142 kg/m3 and γ = *Cp*/*Cv* = 1.4. As a result, the values for the thermal and viscous boundary layer thicknesses are, respectively: *dh* = (2*K*/ρω0*Cp*)1/2 = 2.6(*f*0)-1/2 (mm Hz1/2) ≅ 0.110 mm (at *f*0 = 564 Hz) and *d*ν = (2μ/ρω0)1/2 = 2.2(*f*0)-1/2 (mm Hz1/2) ≅ 0.093 mm (at *f*0 = 564 Hz). Therefore, at atmospheric pressure and audio frequencies, both boundary layers are only a fraction of a millimeter thick.
The volumetric or bulk losses are caused by processes that tend to establish equilibrium in the propagating wave. These damping processes are:
Friction due to compressional motion and the transformation of organized energy into heat due to temperature gradients are responsible for the free space viscous and thermal losses.
CO2 Laser Photoacoustic Spectroscopy: I. Principles 19
In practice, we only include three contributions: viscous and thermal dissipation in the boundary layer at the smooth internal surfaces (surface loss), free space viscous and thermal
The transformation of the absorbed laser energy into heat is usually modeled by a simple relaxation process, while the well-known acoustic-wave equation is applied to calculate the sound-pressure field. The laws of fluid mechanics and thermodynamics can be used to model the acoustic and thermal wave generation in gases. The governing physical equations are the laws of conservation of energy, momentum, mass, and the thermodynamic equation of state. The physical quantities characterizing the acoustic and thermal processes are the
, and the three components of the particle velocity
**r r <sup>r</sup>** , (18)
, and **v** (and by neglecting the influence of the
≈ *d*ν
() ( ) , , *Ht It* = α*<sup>p</sup>* **r r** , (19)
, *dh*) nor for gases
ρ
derived for the acoustic pressure changes, *p* (Miklos et al., 2001):
2
equation (18) is not valid for capillary tubes with a small diameter (2*r*
with exceptionally high viscosity or heat conductivity.
amplitude of heat production rate, *H*, is given by:
ρ
2 2
thermal and viscous interactions of the gas), a linear (inhomogeneous) wave equation can be
( ) ( )( ) ( ) <sup>2</sup>
where *H*(**r**,*t*) is the heat density deposited in the gas by light absorption. The term on the right-hand side of the equation describes the heat input changes over time. When the heat input is constant, this term is zero and no pressure wave is generated. Thus, the heat input must be modulated, which requires that the laser radiation be also amplitude or frequency modulated. A modulated laser beam generates periodic sound due to the periodic localized heating of the gas. From an acoustic point of view, the PA cell is a linear acoustic system, which responds as a whole to the disturbance generated by light absorption. The differential
When the absorbing gas can be modeled by a two-level system consisting of the vibrational ground and the excited state, Meyer and Sigrist (Meyer & Sigrist, 1990) found that the
where *I*(**r**,*t*) is the intensity of the laser beam. This equation is valid only when the laser beam is slowly chopped in the kHz range or below, and in the absence of optical saturation. If the cross-sectional dimensions of a resonator are much smaller than the acoustic wavelength, the excited sound field develops a spatial variation only along the length of the resonator, i.e., a one-dimensional acoustic field is generated. A narrow pipe (or tube) can be regarded as a one-dimensional acoustic resonator. A pressure wave propagating in the pipe will be reflected by an open end with the opposite phase. Through multiple reflections a standing wave pattern with pressure nodes will be formed. Therefore, open pipes should have resonances when the pipe length is equal to an integer multiple of the half wavelength. Bernegger and Sigrist (Bernegger & Sigrist, 1987) proved that the plane acoustic wave propagation can be modeled by the one-dimensional analogue of the electrical current flow
, , , 1 *<sup>s</sup> p t H t v pt <sup>t</sup> <sup>t</sup>* ∂ ∂ − ∇ = γ− <sup>∂</sup> <sup>∂</sup>
dissipation, and relaxation losses (volumetric losses).
**3.5 Pressure amplitude**
temperature *T*, pressure *p*, density
vector **v**. By eliminating the variables *T*,
These two processes are often called Stokes-Kirchhoff losses and are small compared with surface damping. Diffusion and radiation effects are normally very small. Nevertheless, radiation losses through openings, e.g., pipes connecting the resonator to buffer volumes, cannot be neglected. The radiation losses can be reduced by increasing the acoustic input impedance of the openings. This is achieved by terminating the cavity resonator at the openings with acoustic band-stop filters, which prevent sound escape from the resonator. Relaxational effects can add a significant contribution in diatomic and polyatomic molecules. The reason for the relaxational losses is the phase difference between gas pressure and density in the dispersion region, leading to an irreversible conversion of sound energy into thermal energy.
#### **3.4 Quality factor**
The amount of signal enhancement that occurs when the laser is modulated at a resonance frequency is determined by the quality factor. At resonance, the amplitude of the PA signal is *Q* times larger than the amplitude far from the resonance frequency, i.e., the amplification is equal to the value of the *Q* factor. The quality factor of the system, *Q*, is the ratio between the energy stored in a specific mode (the acoustic wave) and the energy losses per cycle of this acoustic wave:
$$Q = \frac{2\pi \,\text{accumulated energy}}{\text{energy lost over one period}}.\tag{14}$$
For high *Q* values the quality factor can be deduced dividing the resonance frequency by its bandwidth at the 0.707 amplitude point:
$$Q = \frac{f\_0}{\Delta f} = \frac{\mathbf{o}\_0}{\Delta \mathbf{o}} \tag{15}$$
where *f*0 and Δ*f* are the resonance frequency and the full-width value of the resonance profile (ω0 = 2π*f*0 and Δω = 2πΔ*f*). The full width is measured between the points where the amplitude of the resonance profile is at 1/ 2 the peak value amplitude (half-maximum values of the intensity). Therefore, Δ*f* is also called the full width at half maximum (FWHM). *Q* is typically between 10 and 50 for longitudinal resonators, but can be as high as 1000 for spherical cavities.
Also, the quality factor can be calculated as (Kamm, 1976; Bernegger & Sigrist, 1987):
$$Q = \frac{2S}{2\pi r \left[ d\_v + \left( \gamma - 1 \right) d\_h \right]} \Big/ \tag{16}$$
where *S* stands for the cross section of the resonator tube and *r* for the radius of the tube. By introducing the radius (*r* = 3.5 mm) of the PA resonator we used and the values for the thermal and viscous boundary layer thicknesses determined in the previous section, Eq. (16) yields *Q* = 14.2, in agreement with the experimentally determined value (*Q* = 16.1).
The overall *Q* factor for a resonance may be found by summing all the losses, expressed as 1/*Qi*:
$$\frac{1}{\mathcal{Q}\_{\text{tot}}} = \sum\_{i} \frac{1}{\mathcal{Q}\_{i}}.\tag{17}$$
In practice, we only include three contributions: viscous and thermal dissipation in the boundary layer at the smooth internal surfaces (surface loss), free space viscous and thermal dissipation, and relaxation losses (volumetric losses).
#### **3.5 Pressure amplitude**
18 CO2 Laser – Optimisation and Application
These two processes are often called Stokes-Kirchhoff losses and are small compared with surface damping. Diffusion and radiation effects are normally very small. Nevertheless, radiation losses through openings, e.g., pipes connecting the resonator to buffer volumes, cannot be neglected. The radiation losses can be reduced by increasing the acoustic input impedance of the openings. This is achieved by terminating the cavity resonator at the openings with acoustic band-stop filters, which prevent sound escape from the resonator. Relaxational effects can add a significant contribution in diatomic and polyatomic molecules. The reason for the relaxational losses is the phase difference between gas pressure and density in the dispersion region, leading to an irreversible conversion of sound
The amount of signal enhancement that occurs when the laser is modulated at a resonance frequency is determined by the quality factor. At resonance, the amplitude of the PA signal is *Q* times larger than the amplitude far from the resonance frequency, i.e., the amplification is equal to the value of the *Q* factor. The quality factor of the system, *Q*, is the ratio between the energy stored in a specific mode (the acoustic wave) and the energy losses per cycle of
2 accumulated energy
For high *Q* values the quality factor can be deduced dividing the resonance frequency by its
0 0 *<sup>f</sup> <sup>Q</sup> <sup>f</sup>*
where *f*0 and Δ*f* are the resonance frequency and the full-width value of the resonance profile
of the resonance profile is at 1/ 2 the peak value amplitude (half-maximum values of the intensity). Therefore, Δ*f* is also called the full width at half maximum (FWHM). *Q* is typically between 10 and 50 for longitudinal resonators, but can be as high as 1000 for spherical cavities.
> 2 2 1 *<sup>h</sup> <sup>S</sup> <sup>Q</sup> rd d* <sup>ν</sup> <sup>=</sup> π + γ−
where *S* stands for the cross section of the resonator tube and *r* for the radius of the tube. By introducing the radius (*r* = 3.5 mm) of the PA resonator we used and the values for the thermal and viscous boundary layer thicknesses determined in the previous section, Eq. (16)
The overall *Q* factor for a resonance may be found by summing all the losses, expressed as 1/*Qi*: 1 1 *Q Q tot <sup>i</sup> <sup>i</sup>*
( )
Also, the quality factor can be calculated as (Kamm, 1976; Bernegger & Sigrist, 1987):
yields *Q* = 14.2, in agreement with the experimentally determined value (*Q* = 16.1).
= 2πΔ*f*). The full width is measured between the points where the amplitude
energy lost over one period *<sup>Q</sup>* <sup>π</sup> <sup>=</sup> . (14)
<sup>ω</sup> = = Δ Δω , (15)
, (16)
<sup>=</sup> , (17)
energy into thermal energy.
**3.4 Quality factor**
this acoustic wave:
0 = 2π*f*0 and Δ
(ω
bandwidth at the 0.707 amplitude point:
ω
The transformation of the absorbed laser energy into heat is usually modeled by a simple relaxation process, while the well-known acoustic-wave equation is applied to calculate the sound-pressure field. The laws of fluid mechanics and thermodynamics can be used to model the acoustic and thermal wave generation in gases. The governing physical equations are the laws of conservation of energy, momentum, mass, and the thermodynamic equation of state. The physical quantities characterizing the acoustic and thermal processes are the temperature *T*, pressure *p*, density ρ, and the three components of the particle velocity vector **v**. By eliminating the variables *T*, ρ, and **v** (and by neglecting the influence of the thermal and viscous interactions of the gas), a linear (inhomogeneous) wave equation can be derived for the acoustic pressure changes, *p* (Miklos et al., 2001):
$$\frac{\partial^2 p(\mathbf{r}, t)}{\partial t^2} - v\_s^2 \nabla^2 p(\mathbf{r}, t) = (\gamma - 1) \frac{\partial H(\mathbf{r}, t)}{\partial t},\tag{18}$$
where *H*(**r**,*t*) is the heat density deposited in the gas by light absorption. The term on the right-hand side of the equation describes the heat input changes over time. When the heat input is constant, this term is zero and no pressure wave is generated. Thus, the heat input must be modulated, which requires that the laser radiation be also amplitude or frequency modulated. A modulated laser beam generates periodic sound due to the periodic localized heating of the gas. From an acoustic point of view, the PA cell is a linear acoustic system, which responds as a whole to the disturbance generated by light absorption. The differential equation (18) is not valid for capillary tubes with a small diameter (2*r* ≈ *d*ν, *dh*) nor for gases with exceptionally high viscosity or heat conductivity.
When the absorbing gas can be modeled by a two-level system consisting of the vibrational ground and the excited state, Meyer and Sigrist (Meyer & Sigrist, 1990) found that the amplitude of heat production rate, *H*, is given by:
$$H\left(\mathbf{r},t\right) = \alpha\_p I\left(\mathbf{r},t\right) \,. \tag{19}$$
where *I*(**r**,*t*) is the intensity of the laser beam. This equation is valid only when the laser beam is slowly chopped in the kHz range or below, and in the absence of optical saturation.
If the cross-sectional dimensions of a resonator are much smaller than the acoustic wavelength, the excited sound field develops a spatial variation only along the length of the resonator, i.e., a one-dimensional acoustic field is generated. A narrow pipe (or tube) can be regarded as a one-dimensional acoustic resonator. A pressure wave propagating in the pipe will be reflected by an open end with the opposite phase. Through multiple reflections a standing wave pattern with pressure nodes will be formed. Therefore, open pipes should have resonances when the pipe length is equal to an integer multiple of the half wavelength.
Bernegger and Sigrist (Bernegger & Sigrist, 1987) proved that the plane acoustic wave propagation can be modeled by the one-dimensional analogue of the electrical current flow
CO2 Laser Photoacoustic Spectroscopy: I. Principles 21
Based on this formula, we can estimate the magnitude of the cell constant. By introducing in Eq. (21) or Eq. (22) the values for our medium-*Q* resonator (*r* = 3.5 mm, *L* = 30 cm, *Q* = 16.1 and *f*0 = 564 Hz; *Seff* ≅ 0.4 cm2, Δ*f* = 35 Hz), it follows *C* = 4720 Pa cm/W, which is almost twice as much as the experimentally measured value (2500 Pa cm/W). If an open resonator is built into a closed PA cell, then the pressure generated by the PA effect will be distributed over the entire closed volume. Therefore, the total volume of the PA cell must be taken into account instead of the volume of the resonator. A PA resonator optimized for high-*Q* performance (*Seff* ∼80 cm2, *Q* ∼1000 at *f*0 = 1 kHz) has a cell constant of about 800 Pa cm/W. The cell constant of a low-*Q* resonator is a complicated function of several parameters, and therefore cannot be determined with sufficient accuracy by calculation. It has to be determined experimentally by calibration measurements using certified gas mixtures.
The possibilities for improving the cell constant of acoustic resonators are limited (Miklos et al., 2001). The only parameter that can really be changed over a broader range is the effective cross section of the cell. A reduction of the cell diameter will increase the cell constant. A lower limit is set by the diameter and divergence of the laser beam employed. The cell constant for modulated measurements is inversely proportional to the FWHM value of the resonance profile. Unfortunately, the half width cannot be reduced indefinitely, because it scales approximately with the surface-to-volume ratio of the resonator. As the cross section of the cell is reduced, the surface-to-volume ratio increases. It is therefore impossible to achieve small cross sections and a small bandwidth (high *Q*) simultaneously. The smallest
diameter used in practical systems is several millimeters, the largest about 10 cm.
( )
*<sup>p</sup> <sup>V</sup>* γ− α <sup>=</sup> <sup>ω</sup>
maximum in the SNR for a certain combination of cell size and modulation frequency.
the cylindrical laser beam propagates exactly along the cylinder axis.
0
It should be noted that the amplitude of the PA pressure signal is a function of (1) the heat
relaxation times of the absorbing gas, and (5) damping effects of the buffer gas (*Q*). The first four factors contribute to the power going into the sound wave, and the last mechanism determines the *Q* of the resonances. From Eq. (23) it follows that the amplitude of the pressure wave (the PA pressure signal) is proportional to the absorption coefficient and laser power, but inversely proportional to the modulation frequency and effective cross section *V*/*L* of the PA resonator. Thus, the signal increases with decreasing resonator dimensions and modulation frequency. As the noise increases with a decrease of these parameters, there is a
For resonant operation, the modulation frequency is tuned to one of the resonance modes of
acoustic resonator are excited as a result. The resonance amplitude is proportional to *Q*, while the amplitudes of the other resonances are inversely proportional to the quantity 2 2 ω −ω*<sup>m</sup>* . Therefore, distant resonances will not be excited efficiently. Certain resonances can be suppressed for special symmetry conditions, e.g., azimuthal modes cannot be excited if
The measured PA signal also depends on the exact position of the microphone in the resonator. The signal detected by the microphone is proportional to the integral average of
*<sup>m</sup>*. Not only the *m-*th mode, but all the other modes of the
1 *p L LP QG*
), (2) laser power (*PL*), (3) modulation frequency (
. (23)
ω
0), (4) vibrational
Combining Eqs. (20) and (21), we have:
γ
ω = ω
capacity of the mixture (
the PA resonator, i.e.,
in a transmission line. According to this theory, a cell constant *C* (Pa cm/W) only dependent on the geometry of the cell (it includes the losses of the PA resonator), which relates the pressure amplitude *p* with the absorbed laser power *PL*, can be defined at resonance frequency:
$$p = \mathbb{C}\left(\alpha = \alpha\_0\right) \alpha\_p P\_{\mathbb{L}, \mathcal{I}} \tag{20}$$
where *p* (N/m2 = Pa) is the pressure response of the cell, α*<sup>p</sup>* (cm-1) is the absorption coefficient at a given pressure of the gas at the laser wavelength, and *PL* (W) is the laser power. The units of *C* are given in Pa cm/W based on the usual dimensions of *p*, α*<sup>p</sup>*, and *PL*. Here, the angular frequency is ω0 = 2π*f*0, where *f*0 is the resonance frequency; for a longitudinal resonant cell, the first resonance frequency is *f*<sup>0</sup> = *vs*/2*L* (Eq. 3), so that ω0 = π*vs*/*L*. *C* is usually determined by calibration measurements, where one single absorbing substance with known absorption spectrum is investigated.
Equation (20) implies that for a reasonably small laser power (no saturation), slow modulation frequency ω0 (ω0τ << 1, where τ is the thermal relaxation time characteristic for the cooling of the gas to equilibrium), and small absorption (α*pL* << 1), the sound pressure amplitude depends linearly on the absorption coefficient and the laser power.
#### **3.6 Cell constant**
For a given PA cell geometry ("high-*Q*" case), Kreuzer (Kreuzer, 1977) deduced that:
$$C\left(\alpha\_0\right) = \frac{\left(\gamma - 1\right)LQG}{\alpha\_0 V},\tag{21}$$
where *V* is the volume of the PA resonator and *G* a geometrical factor (depending on the transverse beam profile but not on the cell length) on the order of 1 Pa m3/W s (Bijnen et al. (Bijnen et al., 1996) found a value *G* = 1.2 Pa m3/W s for their specific experimental conditions). Since the quantities in Eq. (21) are independent of the laser power and absorption coefficient, these factors can be regarded as characteristic setup quantities for PA resonators. The quantity *C* describes the sensitivity of the PA resonator at a given resonance frequency. It is widely known as the 'cell constant'. It depends on the size of the resonator, the frequency, and the *Q* factor of the resonance selected for PA detection. It also depends on the spatial overlap of the laser beam and the standing acoustic wave pattern. Its 'cell constant' name is therefore misleading, as it characterizes the complete measurement arrangement (including the acoustic resonator with a selected resonance, microphone position, and laser beam profile with spatial location) rather than the mere PA cell. Moreover, it depends on frequency, and its value differs for different resonance modes. Therefore, it would more appropriately be called a 'PA setup constant' (Miklos et al., 2001) rather than a 'cell constant'. However, since the name 'cell constant' is already established in the literature, we will continue to use it hereinafter.
As the cell constant is inversely proportional to an effective cross section defined by *Seff* = *V*/*L* and ω0/*Q* = Δω(Eq. 15), it follows that:
$$C\left(\text{co}\_{0}\right) = \frac{\left(\gamma - 1\right)G}{\Delta \text{coS}\_{\text{eff}}}\,. \tag{22}$$
in a transmission line. According to this theory, a cell constant *C* (Pa cm/W) only dependent on the geometry of the cell (it includes the losses of the PA resonator), which relates the pressure amplitude *p* with the absorbed laser power *PL*, can be defined at resonance frequency:
coefficient at a given pressure of the gas at the laser wavelength, and *PL* (W) is the laser
*vs*/*L*. *C* is usually determined by calibration measurements, where one single absorbing
Equation (20) implies that for a reasonably small laser power (no saturation), slow
τ
For a given PA cell geometry ("high-*Q*" case), Kreuzer (Kreuzer, 1977) deduced that:
ω =
( ) ( ) <sup>0</sup>
<sup>1</sup> *LQG <sup>C</sup>*
γ −
where *V* is the volume of the PA resonator and *G* a geometrical factor (depending on the transverse beam profile but not on the cell length) on the order of 1 Pa m3/W s (Bijnen et al. (Bijnen et al., 1996) found a value *G* = 1.2 Pa m3/W s for their specific experimental conditions). Since the quantities in Eq. (21) are independent of the laser power and absorption coefficient, these factors can be regarded as characteristic setup quantities for PA resonators. The quantity *C* describes the sensitivity of the PA resonator at a given resonance frequency. It is widely known as the 'cell constant'. It depends on the size of the resonator, the frequency, and the *Q* factor of the resonance selected for PA detection. It also depends on the spatial overlap of the laser beam and the standing acoustic wave pattern. Its 'cell constant' name is therefore misleading, as it characterizes the complete measurement arrangement (including the acoustic resonator with a selected resonance, microphone position, and laser beam profile with spatial location) rather than the mere PA cell. Moreover, it depends on frequency, and its value differs for different resonance modes. Therefore, it would more appropriately be called a 'PA setup constant' (Miklos et al., 2001) rather than a 'cell constant'. However, since the name 'cell constant' is already established in
As the cell constant is inversely proportional to an effective cross section defined by *Seff* =
( ) ( ) <sup>0</sup>
ω =
*C*
1 *eff G*
*S* γ −
0
ω
*V*
power. The units of *C* are given in Pa cm/W based on the usual dimensions of *p*,
longitudinal resonant cell, the first resonance frequency is *f*<sup>0</sup> = *vs*/2*L* (Eq. 3), so that
where *p* (N/m2 = Pa) is the pressure response of the cell,
substance with known absorption spectrum is investigated.
the cooling of the gas to equilibrium), and small absorption (
ω0 (ω0τ
the literature, we will continue to use it hereinafter.
(Eq. 15), it follows that:
ω
ω0 = 2π
<< 1, where
amplitude depends linearly on the absorption coefficient and the laser power.
Here, the angular frequency is
modulation frequency
**3.6 Cell constant**
*V*/*L* and
ω0/*Q* = Δ
π
( ) <sup>0</sup> *<sup>p</sup> <sup>L</sup> p* = ω=ω α *C P* , (20)
α
*f*0, where *f*0 is the resonance frequency; for a
is the thermal relaxation time characteristic for
α
*<sup>p</sup>* (cm-1) is the absorption
*pL* << 1), the sound pressure
, (21)
Δω . (22)
α
*<sup>p</sup>*, and *PL*.
ω0 = Based on this formula, we can estimate the magnitude of the cell constant. By introducing in Eq. (21) or Eq. (22) the values for our medium-*Q* resonator (*r* = 3.5 mm, *L* = 30 cm, *Q* = 16.1 and *f*0 = 564 Hz; *Seff* ≅ 0.4 cm2, Δ*f* = 35 Hz), it follows *C* = 4720 Pa cm/W, which is almost twice as much as the experimentally measured value (2500 Pa cm/W). If an open resonator is built into a closed PA cell, then the pressure generated by the PA effect will be distributed over the entire closed volume. Therefore, the total volume of the PA cell must be taken into account instead of the volume of the resonator. A PA resonator optimized for high-*Q* performance (*Seff* ∼80 cm2, *Q* ∼1000 at *f*0 = 1 kHz) has a cell constant of about 800 Pa cm/W. The cell constant of a low-*Q* resonator is a complicated function of several parameters, and therefore cannot be determined with sufficient accuracy by calculation. It has to be determined experimentally by calibration measurements using certified gas mixtures.
The possibilities for improving the cell constant of acoustic resonators are limited (Miklos et al., 2001). The only parameter that can really be changed over a broader range is the effective cross section of the cell. A reduction of the cell diameter will increase the cell constant. A lower limit is set by the diameter and divergence of the laser beam employed. The cell constant for modulated measurements is inversely proportional to the FWHM value of the resonance profile. Unfortunately, the half width cannot be reduced indefinitely, because it scales approximately with the surface-to-volume ratio of the resonator. As the cross section of the cell is reduced, the surface-to-volume ratio increases. It is therefore impossible to achieve small cross sections and a small bandwidth (high *Q*) simultaneously. The smallest diameter used in practical systems is several millimeters, the largest about 10 cm.
Combining Eqs. (20) and (21), we have:
$$p = \frac{(\gamma - 1)\alpha\_p L P\_L Q G}{\alpha\_0 V}.\tag{23}$$
It should be noted that the amplitude of the PA pressure signal is a function of (1) the heat capacity of the mixture (γ), (2) laser power (*PL*), (3) modulation frequency (ω0), (4) vibrational relaxation times of the absorbing gas, and (5) damping effects of the buffer gas (*Q*). The first four factors contribute to the power going into the sound wave, and the last mechanism determines the *Q* of the resonances. From Eq. (23) it follows that the amplitude of the pressure wave (the PA pressure signal) is proportional to the absorption coefficient and laser power, but inversely proportional to the modulation frequency and effective cross section *V*/*L* of the PA resonator. Thus, the signal increases with decreasing resonator dimensions and modulation frequency. As the noise increases with a decrease of these parameters, there is a maximum in the SNR for a certain combination of cell size and modulation frequency.
For resonant operation, the modulation frequency is tuned to one of the resonance modes of the PA resonator, i.e., ω = ω*<sup>m</sup>*. Not only the *m-*th mode, but all the other modes of the acoustic resonator are excited as a result. The resonance amplitude is proportional to *Q*, while the amplitudes of the other resonances are inversely proportional to the quantity 2 2 ω −ω*<sup>m</sup>* . Therefore, distant resonances will not be excited efficiently. Certain resonances can be suppressed for special symmetry conditions, e.g., azimuthal modes cannot be excited if the cylindrical laser beam propagates exactly along the cylinder axis.
The measured PA signal also depends on the exact position of the microphone in the resonator. The signal detected by the microphone is proportional to the integral average of
CO2 Laser Photoacoustic Spectroscopy: I. Principles 23
In a longitudinally excited resonator, a smaller acoustic gain, as a consequence of a relatively
Fig. 6. Graphical representation of Eq. (26): dependence of the normalized cell constant on
123
According to Eqs. (24-26), to obtain a higher acoustic signal in a longitudinally excited resonator with a low *Q*-factor (a higher *C*), it is necessary to have a resonator with a large length and a small diameter. Yet, narrowing the tube diameter and increasing the tube length are restricted by the divergence of the laser beam over the length of the cell. The maximum length is limited by the minimum frequency at which the cell is to be operated or
α
minimum possible diameter is set by the beam diameter or the volume-to-surface ratio that is needed to minimize adsorption and desorption at the cell walls. A too small diameter of the PA cell gives rise to high PA background signals due to absorption of the wings of the gaussian laser beam profile. On the other hand, a high quality factor is required in order to decrease the background signal caused by window heating. In conclusion, the optimization of the PA cell geometry depends on the specific experimental conditions and the application
When the resonance contributions are included, the photoacoustic voltage signal can be obtained at a given operating frequency simply by multiplying the pressure response (Eq.
absorption coefficient at a given wavelength; *C* (Pa cm W-1), the cell constant; *SM* (V Pa-1), the microphone responsivity; *PL* (W), the cw laser power (unchopped value; 2x measured average value); and *c* (atm), the trace gas concentration (usually given in units of per cent, ppmV, ppbV or pptV). This equation reveals that the photoacoustic signal is linearly dependent on laser power. Thus, sensitive measurements benefit from using as much laser power as is reasonably available. Moreover, the signal is directly dependent on the number of molecules in the optical path (trace gas concentration), which means that this technique is
α*p* = α*c*):
*max* that is to be detected (*L* << 1/
*L* = 3*L*<sup>0</sup> *L* = 2*L*<sup>0</sup>
*r*/*r*<sup>0</sup> *L* = *L*<sup>0</sup>
*V CS P c* = α *<sup>M</sup> <sup>L</sup>* , (27)
α
α
(cm-1 atm-1), the gas
*max*). The
tube radius and resonator length.
for which it is designed.
**3.8 Voltage signal**
by the maximum absorption coefficient
20) by the microphone responsivity (*V* = *pSM* and
where: *V* (V) is the photoacoustic signal (peak-to-peak value);
low *Q* value, is compensated for by the signal gain due to the smaller diameter.
1
0
2
3
*C*/*C*<sup>0</sup>
the pressure over the microphone membrane. Since mostly miniature microphones are applied in photoacoustics, the integral can be approximated by the value of the pressure amplitude at the microphone location.
Angeli et al. (Angeli et al., 1992) reported a dependency of the cell constant on the kind of calibration gas. They concluded that the cell constant could not be determined unambiguously by a calibration measurement using a single absorbing species, indicating the "nonabsolute" character of photoacoustic spectroscopy. This result would have severe implications and would render analyses of multicomponent gas mixtures very difficult or impossible. Fortunately, Thöny and Sigrist (Thöny & Sigrist, 1995) proved that detailed investigations including a number of different gases and measurements on numerous laser transitions contradict those observations and revealed the expected independence of the cell constant within the measurement errors.
#### **3.7 Optimization of the PA cell geometry**
Since the PA signal is inversely proportional to the cell volume and modulation frequency, high PA signal levels can be obtained by taking a small cell volume (< 10 cm3) and low modulation frequencies (< 100 Hz). However, noise sources (intrinsic noise of the microphone, amplifier noise, external acoustic noise) show a characteristic 1/*f* frequency dependence, and therefore the SNR of such a gas-microphone cell is usually quite small. The SNR of a PA cell can be increased by applying higher modulation frequencies (in the kHz region) and acoustic amplification of the PA signal. For this reason, resonant PA cells operating on longitudinal, azimuthal, radial, or Helmholtz resonances have been developed. Furthermore, resonant cells can be designed for multipass or intracavity operation.
A qualitative behavior for *Q* and ω0 can be derived from simple geometrical considerations. So, for *Q*, the energy stored in a specific mode is proportional to the cell volume (∝ *r*2*L*), while the energy losses per cycle of the acoustic wave are proportional to the cell surface (2π*rL*) and to the thicknesses *dh* ≈ *d*ν = *d* ∝ ω<sup>0</sup>-1/2 ∝ *L*1/2. Therefore:
$$
\partial \mathfrak{h} \lnot L^{\mathfrak{i}} \, \tag{24}
$$
$$\mathcal{Q}(a\mathfrak{b}) \lnot \left(r\text{\textquotedblleft}L/r\text{\textquotedblleft}L^{1/2}\right) \lnot rL^{-1/2},\tag{25}$$
and
$$\mathbf{C}(\alpha\_0) \approx \text{(L)} (r\mathbf{L}^{\cdot 1/2}) / \text{(}\mathbf{L}^{\cdot 1}\text{)} (r\mathbf{2}\mathbf{L}) \approx r\mathbf{^{1}L^{1/2}}.\tag{26}$$
which is represented graphically in Fig. 6. These equations show that the product *Q*(ω0)*C*(ω0) is nearly independent of the cell dimensions for any kind of resonant PA cell. The operation of the cell in a longitudinal mode is more advantageous because it makes it possible to optimize the resonance frequency and the *Q*-factor independently, which cannot be achieved in the case of radial resonance.
Cell geometries with large diameter-to-length ratios designed to excite the resonance in the radial or azimuthal acoustic modes possess high *Q* values and high resonance frequencies, but have low cell constants. PA cells with high *Q* values are sensitive to long-term drifts (e.g., due to thermal expansion if the temperature is not carefully controlled), so that they require an active locking of the modulation frequency on the resonance frequency of the cell.
the pressure over the microphone membrane. Since mostly miniature microphones are applied in photoacoustics, the integral can be approximated by the value of the pressure
Angeli et al. (Angeli et al., 1992) reported a dependency of the cell constant on the kind of calibration gas. They concluded that the cell constant could not be determined unambiguously by a calibration measurement using a single absorbing species, indicating the "nonabsolute" character of photoacoustic spectroscopy. This result would have severe implications and would render analyses of multicomponent gas mixtures very difficult or impossible. Fortunately, Thöny and Sigrist (Thöny & Sigrist, 1995) proved that detailed investigations including a number of different gases and measurements on numerous laser transitions contradict those observations and revealed the expected independence of the cell
Since the PA signal is inversely proportional to the cell volume and modulation frequency, high PA signal levels can be obtained by taking a small cell volume (< 10 cm3) and low modulation frequencies (< 100 Hz). However, noise sources (intrinsic noise of the microphone, amplifier noise, external acoustic noise) show a characteristic 1/*f* frequency dependence, and therefore the SNR of such a gas-microphone cell is usually quite small. The SNR of a PA cell can be increased by applying higher modulation frequencies (in the kHz region) and acoustic amplification of the PA signal. For this reason, resonant PA cells operating on longitudinal, azimuthal, radial, or Helmholtz resonances have been developed.
So, for *Q*, the energy stored in a specific mode is proportional to the cell volume (∝ *r*2*L*), while the energy losses per cycle of the acoustic wave are proportional to the cell surface
which is represented graphically in Fig. 6. These equations show that the product
The operation of the cell in a longitudinal mode is more advantageous because it makes it possible to optimize the resonance frequency and the *Q*-factor independently, which cannot
Cell geometries with large diameter-to-length ratios designed to excite the resonance in the radial or azimuthal acoustic modes possess high *Q* values and high resonance frequencies, but have low cell constants. PA cells with high *Q* values are sensitive to long-term drifts (e.g., due to thermal expansion if the temperature is not carefully controlled), so that they require an active locking of the modulation frequency on the resonance frequency of the cell.
0) is nearly independent of the cell dimensions for any kind of resonant PA cell.
<sup>0</sup>-1/2 ∝ *L*1/2. Therefore:
0 can be derived from simple geometrical considerations.
0) ∝ (*r*2*L*/*rLL*1/2) ∝ *rL*-1/2, (25)
0) ∝ (*L*)(*rL*-1/2)/( *L*-1)( *r*2*L*) ∝ *r*-1L1/2. (26)
<sup>0</sup> ∝ *L*-1, (24)
Furthermore, resonant cells can be designed for multipass or intracavity operation.
ω
ω
ν = *d* ∝ ω
*Q*(ω
*C*(ω
be achieved in the case of radial resonance.
amplitude at the microphone location.
constant within the measurement errors.
A qualitative behavior for *Q* and
(2π*rL*) and to the thicknesses *dh* ≈ *d*
and
*Q*(ω0)*C*(ω
**3.7 Optimization of the PA cell geometry**
In a longitudinally excited resonator, a smaller acoustic gain, as a consequence of a relatively low *Q* value, is compensated for by the signal gain due to the smaller diameter.
Fig. 6. Graphical representation of Eq. (26): dependence of the normalized cell constant on tube radius and resonator length.
According to Eqs. (24-26), to obtain a higher acoustic signal in a longitudinally excited resonator with a low *Q*-factor (a higher *C*), it is necessary to have a resonator with a large length and a small diameter. Yet, narrowing the tube diameter and increasing the tube length are restricted by the divergence of the laser beam over the length of the cell. The maximum length is limited by the minimum frequency at which the cell is to be operated or by the maximum absorption coefficient α*max* that is to be detected (*L* << 1/α*max*). The minimum possible diameter is set by the beam diameter or the volume-to-surface ratio that is needed to minimize adsorption and desorption at the cell walls. A too small diameter of the PA cell gives rise to high PA background signals due to absorption of the wings of the gaussian laser beam profile. On the other hand, a high quality factor is required in order to decrease the background signal caused by window heating. In conclusion, the optimization of the PA cell geometry depends on the specific experimental conditions and the application for which it is designed.
#### **3.8 Voltage signal**
When the resonance contributions are included, the photoacoustic voltage signal can be obtained at a given operating frequency simply by multiplying the pressure response (Eq. 20) by the microphone responsivity (*V* = *pSM* and α*p* = α*c*):
$$V = \mathfrak{\alpha} \mathbb{C} S\_M P\_L \mathfrak{c} \text{ \(\mathfrak{\alpha}\)}\tag{27}$$
where: *V* (V) is the photoacoustic signal (peak-to-peak value); α (cm-1 atm-1), the gas absorption coefficient at a given wavelength; *C* (Pa cm W-1), the cell constant; *SM* (V Pa-1), the microphone responsivity; *PL* (W), the cw laser power (unchopped value; 2x measured average value); and *c* (atm), the trace gas concentration (usually given in units of per cent, ppmV, ppbV or pptV). This equation reveals that the photoacoustic signal is linearly dependent on laser power. Thus, sensitive measurements benefit from using as much laser power as is reasonably available. Moreover, the signal is directly dependent on the number of molecules in the optical path (trace gas concentration), which means that this technique is
CO2 Laser Photoacoustic Spectroscopy: I. Principles 25
been predicted (Sigrist, 1986) and experimentally proved (Harren et al., 1990). Such sensitivity makes it possible to detect many trace constituents in the sub-ppbV range. Theoretical calculations (see Section 2) predict the linearity of the signal response over a concentration range as broad as 7 orders of magnitude. This wide dynamic range, characteristic of LPAS, is important for air pollution monitoring, as it helps conduct measurements in polluted areas at the source (emission) as well as in rural areas (immission)
A PA signal may become saturated due to either a large concentration of the measured analyte or high laser power levels. We showed in Section 2 that, in the case of ethylene, the signal starts to saturate at a concentration of 65 ppmV. As a matter of fact, Thöny and Sigrist (Thöny & Sigrist, 1995) observed weak saturation effects on 10P(14) CO2 laser transition for a concentration of 100 ppmV of ethylene. The degree of saturation is gas dependent. We found (Section 2.3) that a deviation of ~3% from linear behavior resulted in an optical
By increasing laser intensity, the excitation pumping rate of the molecules grows higher, and a molecule is more likely to absorb a nearby photon before it relaxes to the ground state. So, as the molecules in the excited state increase in numbers, the number of molecules which can absorb laser radiation is reduced. The gas actually becomes as though more transparent to laser radiation, and the effective absorption coefficient per unit laser power is lowered; this is called laser power saturation. Saturation due to nonlinear absorption of the laser power only occurs in focused high-power laser beams or when the PA cell is placed intracavity in a laser, so that the laser power can be on the order of tens of watts or even higher than 100 W. The pumping rate to a higher vibrational-rotational level is proportional to the laser light intensity; in the case of saturation it exceeds the collisional de-excitation rates.
Harren et al. (Harren et al., 1990) studied the saturation effects by placing the PA cell intracavity of a waveguide CO2 laser. Extracavity, the ratio between 10P(14) and 10P(16) lines is 5.96 ± 0.2. Intracavity, this ratio becomes 2.8 ± 0.3 (47% from its extracavity value) at an intracavity laser power of 130 W (for a laser beam waist of 0.282 mm, that is at a laser intensity higher than 200 kW/cm2). By lowering the intracavity laser power, this ratio increases to its extracavity value. This effect is caused by saturation of the transitions in
rate due to the high intracavity power. When the laser beam waist is increased to 1.02 mm (laser intensity is decreased to 15.9 kW/cm2), the ratio of the absorption coefficients of C2H4 on the 10P(14) and 10P(16) CO2 laser lines increased to 4.7 ± 0.5 (78% of its extracavity value). To compensate for the saturation effect, these authors used an absorption coefficient of 23.7 cm-1atm-1 (78% of 30.4 cm-1atm-1 at an intracavity power of 100 W) for C2H4 at the
By using an intracavity arrangement where the CO2 laser power was varied between 10 and 70 W, Groot (Groot, 2002) measured the saturation parameter of ethylene for several laser
10) through collisions becomes slow in comparison with the pump
α
/(1 + *P*/*Ps*), where *Ps* (W) is the laser power saturation
C2H4 at the 10P(14) CO2 laser line. Depletion from the vibrational excited level (
*cmin* ≅10-10 cm-1 for 1 W incident laser power have
ν
*<sup>e</sup>* to the intrinsic absorption
7) via other
α*min* = α
detection limits on the order of
(Sigrist et al., 1989).
density
α
vibrational levels (e.g.,
10P(14) CO2 laser line.
α
is given by
coefficient
ν
lines. The relation of the effective absorption coefficient
α*e* = α
*Lc* = 0.06.
**3.9 Saturation effects**
truly a "zero-baseline" approach, since no signal will be generated if the target molecules are not present.
Equation (27) is valid as long as absorption is small (α*pL* << 1), and the modulation frequency is higher than the inverse of the molecular diffusion time but lower than the inverse of the molecular relaxation time. The PA signal is linearly dependent on the absorption coefficient, cell constant, microphone responsivity, incident laser power, and absorbent trace gas concentration. Thus, by doubling *Q* (and consequently *C*), or the microphone responsivity, or the laser power, or the number of absorbing molecules in the optical path, the voltage will also double. The peak-to-peak value of the signal is obtained by multiplying by 2 2 the rms voltage amplitude measured by the lock-in amplifier. As a rule, another parameter is used to characterize the PA cell, namely:
$$R = \mathbb{C}S\_{M'} \tag{28}$$
where *R* (V cm/W) is the (voltage) responsivity of the PA cell or the calibration constant. The cell constant *C* is multiplied by the responsivity of the microphone given in V/Pa units. A comparison of different PA cells can be made independently of the application in terms of this figure of merit. However, the cell characterization can be used only if a calibrated microphone is available. In this way, Eq. (27) becomes:
$$V = \mathfrak{\alpha} RP\_L \mathfrak{c} \text{ .} \tag{29}$$
To increase the detection sensitivity, we have to ensure: a) a cell constant as large as possible (optimization of the PA resonator); b) a large microphone responsivity; c) a laser power as high as possible, provided that saturation does not become a limiting factor; d) a narrow bandwidth of the lock-in amplifier, and e) a high absorption coefficient of the trace gas to be measured at the laser wavelength. It is also useful to increase the number *n* of microphones (connected in series), but this number is limited by the dimensions of the PA cell. The summation of the signals from the single microphones results in an *n*-times higher effective PA signal, because the total responsivity *SM to*t is increased *n*-fold, i.e.:
$$S\_{Mut} = nS\_M \,. \tag{30}$$
On the other hand, the incoherent noise only increases by *n* . One thus obtains:
$$\text{SNR}\_{\text{tot}} = \sqrt{n} \,\text{SNR} \,\,. \tag{31}$$
The minimum measurable voltage signal *V* = *Vmin* is obtained at SNR = 1, where the minimum detectable concentration *c* = *cmin* can be recorded:
$$
\sigma\_{\text{min}} = \frac{V\_{\text{min}}}{\alpha P\_L R} \,. \tag{32}
$$
The sensitivity of PA instruments increases with the laser power, as *V* ∝ α*PL*. However, the voltage signal does not depend on the length of the absorption path. Furthermore, in contrast to other techniques based on absorption spectroscopy, the response of the acoustic detector is independent of the electromagnetic radiation wavelength as long as the absorption coefficient is fixed. According to theoretical considerations, extremely low detection limits on the order of α*min* = α*cmin* ≅10-10 cm-1 for 1 W incident laser power have been predicted (Sigrist, 1986) and experimentally proved (Harren et al., 1990). Such sensitivity makes it possible to detect many trace constituents in the sub-ppbV range. Theoretical calculations (see Section 2) predict the linearity of the signal response over a concentration range as broad as 7 orders of magnitude. This wide dynamic range, characteristic of LPAS, is important for air pollution monitoring, as it helps conduct measurements in polluted areas at the source (emission) as well as in rural areas (immission) (Sigrist et al., 1989).
#### **3.9 Saturation effects**
24 CO2 Laser – Optimisation and Application
truly a "zero-baseline" approach, since no signal will be generated if the target molecules
frequency is higher than the inverse of the molecular diffusion time but lower than the inverse of the molecular relaxation time. The PA signal is linearly dependent on the absorption coefficient, cell constant, microphone responsivity, incident laser power, and absorbent trace gas concentration. Thus, by doubling *Q* (and consequently *C*), or the microphone responsivity, or the laser power, or the number of absorbing molecules in the optical path, the voltage will also double. The peak-to-peak value of the signal is obtained by multiplying by 2 2 the rms voltage amplitude measured by the lock-in amplifier. As a
where *R* (V cm/W) is the (voltage) responsivity of the PA cell or the calibration constant. The cell constant *C* is multiplied by the responsivity of the microphone given in V/Pa units. A comparison of different PA cells can be made independently of the application in terms of this figure of merit. However, the cell characterization can be used only if a calibrated
To increase the detection sensitivity, we have to ensure: a) a cell constant as large as possible (optimization of the PA resonator); b) a large microphone responsivity; c) a laser power as high as possible, provided that saturation does not become a limiting factor; d) a narrow bandwidth of the lock-in amplifier, and e) a high absorption coefficient of the trace gas to be measured at the laser wavelength. It is also useful to increase the number *n* of microphones (connected in series), but this number is limited by the dimensions of the PA cell. The summation of the signals from the single microphones results in an *n*-times higher effective
The minimum measurable voltage signal *V* = *Vmin* is obtained at SNR = 1, where the
min
*c*
The sensitivity of PA instruments increases with the laser power, as *V* ∝
min
*L V*
*P R* <sup>=</sup> <sup>α</sup>
voltage signal does not depend on the length of the absorption path. Furthermore, in contrast to other techniques based on absorption spectroscopy, the response of the acoustic detector is independent of the electromagnetic radiation wavelength as long as the absorption coefficient is fixed. According to theoretical considerations, extremely low
α
*R CS* = *<sup>M</sup>* , (28)
*V RP c* = α *<sup>L</sup>* . (29)
*S nS Mtot M* = . (30)
SNR SNR tot = *n* . (31)
. (32)
α
*PL*. However, the
*pL* << 1), and the modulation
Equation (27) is valid as long as absorption is small (
rule, another parameter is used to characterize the PA cell, namely:
PA signal, because the total responsivity *SM to*t is increased *n*-fold, i.e.:
minimum detectable concentration *c* = *cmin* can be recorded:
On the other hand, the incoherent noise only increases by *n* . One thus obtains:
microphone is available. In this way, Eq. (27) becomes:
are not present.
A PA signal may become saturated due to either a large concentration of the measured analyte or high laser power levels. We showed in Section 2 that, in the case of ethylene, the signal starts to saturate at a concentration of 65 ppmV. As a matter of fact, Thöny and Sigrist (Thöny & Sigrist, 1995) observed weak saturation effects on 10P(14) CO2 laser transition for a concentration of 100 ppmV of ethylene. The degree of saturation is gas dependent. We found (Section 2.3) that a deviation of ~3% from linear behavior resulted in an optical density α*Lc* = 0.06.
By increasing laser intensity, the excitation pumping rate of the molecules grows higher, and a molecule is more likely to absorb a nearby photon before it relaxes to the ground state. So, as the molecules in the excited state increase in numbers, the number of molecules which can absorb laser radiation is reduced. The gas actually becomes as though more transparent to laser radiation, and the effective absorption coefficient per unit laser power is lowered; this is called laser power saturation. Saturation due to nonlinear absorption of the laser power only occurs in focused high-power laser beams or when the PA cell is placed intracavity in a laser, so that the laser power can be on the order of tens of watts or even higher than 100 W. The pumping rate to a higher vibrational-rotational level is proportional to the laser light intensity; in the case of saturation it exceeds the collisional de-excitation rates.
Harren et al. (Harren et al., 1990) studied the saturation effects by placing the PA cell intracavity of a waveguide CO2 laser. Extracavity, the ratio between 10P(14) and 10P(16) lines is 5.96 ± 0.2. Intracavity, this ratio becomes 2.8 ± 0.3 (47% from its extracavity value) at an intracavity laser power of 130 W (for a laser beam waist of 0.282 mm, that is at a laser intensity higher than 200 kW/cm2). By lowering the intracavity laser power, this ratio increases to its extracavity value. This effect is caused by saturation of the transitions in C2H4 at the 10P(14) CO2 laser line. Depletion from the vibrational excited level (ν7) via other vibrational levels (e.g., ν10) through collisions becomes slow in comparison with the pump rate due to the high intracavity power. When the laser beam waist is increased to 1.02 mm (laser intensity is decreased to 15.9 kW/cm2), the ratio of the absorption coefficients of C2H4 on the 10P(14) and 10P(16) CO2 laser lines increased to 4.7 ± 0.5 (78% of its extracavity value). To compensate for the saturation effect, these authors used an absorption coefficient of 23.7 cm-1atm-1 (78% of 30.4 cm-1atm-1 at an intracavity power of 100 W) for C2H4 at the 10P(14) CO2 laser line.
By using an intracavity arrangement where the CO2 laser power was varied between 10 and 70 W, Groot (Groot, 2002) measured the saturation parameter of ethylene for several laser lines. The relation of the effective absorption coefficient α*<sup>e</sup>* to the intrinsic absorption coefficient α is given by α*e* = α/(1 + *P*/*Ps*), where *Ps* (W) is the laser power saturation
CO2 Laser Photoacoustic Spectroscopy: I. Principles 27
c. Coherent photoacoustic background signal. This signal, which is always present in the PA detector, is caused by the laser beam, yet not by light absorption in the bulk of the gas. Rather it is due to laser beam heating of the windows and of the absorbates at their surfaces, and heating of the PA resonator walls by the reflected or scattered light owing to imperfections of the focusing lens, windows and inner walls of the PA resonator. This signal is in phase with, and at the same frequency as, the laser intensity modulation. Therefore, it is not filtered out by the lock-in amplifier connected to the microphone. Thus, a background signal proportional to the laser power becomes the
The background signal in the PA cells may arise from several sources, some of which are
1. Window surface absorption: the molecules absorbed on the window surface and/or the window surface itself absorb the modulated laser radiation, and the resulting gas
2. Window bulk absorption: even the highest quality ZnSe window substrates exhibit a
3. Off-axis radiation within the cell: light scattered from the windows and at the edge of the chopper blade may strike the inside walls of the PA resonator, where it may be
5. Small amounts of contamination that may outgas from the cell materials, seals, and so forth. The detection limit of the PA cell is determined by the combined effect of the intrinsic stochastic noise of the microphone, acoustic background noise, and photoacoustic background signal. Background signals are deterministic, and to the extent that they can be quantified and minimized, do not reduce the performance of the cell significantly. The detection limit is defined either at a signal-to-noise ratio of unity (SNR = 1) or at a signal-to-
The amplifier input noise and microphone noise are gaussian in nature, that is, the amount of noise is proportional to the square root of the bandwidth in which the noise is measured. All of these noise sources are incoherent. The input noise of the SR830 lock-in amplifier used in our experiments is about 6 nV (rms)/ Hz . Microphone noise, which is manifested as a noise voltage present at the microphone output terminals, can be expressed as a product between the normalized noise pressure value owing to both thermal agitation of the diaphragm and cartridge responsivity at the corresponding frequency and the square root of the measurement bandwidth. The electrical noise of Knowles EK models electret microphones is 40 nV (rms)/ Hz . The overall random noise of multiple sources is determined by taking the square root of the sum of the squares of all the individual incoherent noise figures. For gaussian noise, the peak-to-peak value is about 5 times the rms noise value, while for the two other types of noises, the rms value must be multiplied by a factor of 2 2 ≅ 2.8 to obtain the peak-to-peak amplitude. Electrical noise usually has a broadband frequency spectrum and can be reduced efficiently by narrowband filtering of the signal, as is done in the phase sensitive detection. A detection bandwidth of 0.25 Hz was set (a time constant of 1 second) in all of our measurements. Electrical noise can be reduced by using state-of-the-art (and therefore very expensive) lock-in amplifiers and/or by using longer time averaging (the noise decreases with
the square root of the averaging time) at the cost of longer measurement times.
main factor that limits sensitivity.
heating in the cell generates a pressure pulse.
4. Light scattering or absorption due to microaerosols.
residual window absorption of ∼10-3 cm-1.
listed below (Gerlach & Amer, 1980):
absorbed and produce a signal.
background ratio of unity (SBR = 1).
parameter and represents a measure for the relaxation rate. At *P* = *Ps*, the absorption coefficient decreases to half its initial value. The following values were obtained for *Ps*: 178 W at 10P(8) line; 102 W at 10P(10); 112 W at 10P(12); 51.8 W at 10P(14); 101 W at 10P(16); 128 W at 10P(18), and 112 W at 10P(20). The strongest saturation effect was observed at 10P(14) line, where the absorption coefficient is the largest. The saturation for this line at a laser power of 130 W corresponds to the equivalent absorption coefficient α*e* = 0.285α. The stronger saturation in this case compared with the results of Harren et al. (Harren et al., 1990) could be accounted for by a tighter focusing of the laser beam (smaller beam waist). As a matter of fact, saturation is determined by the laser beam intensity (irradiance) rather than the laser power. Power saturation does not depend on the gas concentration in the PA cell (if the absorbing gas concentration is not too high).
#### **4. Noises and limiting factors**
#### **4.1 Noises**
In order to obtain an optimum SNR, noise control and interfering signals have to be taken into account (Dutu et al., 1994a; Dutu et al., 1994b). These limiting factors are discussed in the following two sections.
Noise plays an important role in all photoacoustic measurements and is of particular importance in the detection of ultralow gas concentrations, because the noise level limits the ultimate sensitivity. In the photoacoustic literature, the detection level is usually defined by the SNR, where the noise is given by the microphone signal measured with the laser light blocked. However, when light hits the PA cell, an additional background signal is generated which exists even when the absorbing species are not present in the detector. The background signal is often larger than the noise signal, and therefore the detection limit or sensitivity has to be defined by the signal-to-background ratio (SBR) in most experiments. Unfortunately, it is common practice to consider only the SNR. This procedure yields an extrapolated detection limit that may be far too small. The background signal is usually determined with a nonabsorbing gas, such as nitrogen, in the PA detector. It is influenced by many system properties, such as the pointing stability, the beam divergence, and the diameter of the laser beam.
For photoacoustic spectroscopy, "noise" often has a structure that is coherent with the signal from the target species, and therefore should more appropriately be treated as a background signal, not as noise. The background signal can be determined by measuring the acoustic signal in the absence of absorbers (i.e., with pure nitrogen), but with the same flow and in the same pressure conditions as those used for the sample gases.
The sensitivity-limiting factors which are encountered in LPAS can be classified into three categories:
parameter and represents a measure for the relaxation rate. At *P* = *Ps*, the absorption coefficient decreases to half its initial value. The following values were obtained for *Ps*: 178 W at 10P(8) line; 102 W at 10P(10); 112 W at 10P(12); 51.8 W at 10P(14); 101 W at 10P(16); 128 W at 10P(18), and 112 W at 10P(20). The strongest saturation effect was observed at 10P(14) line, where the absorption coefficient is the largest. The saturation for this line at a laser
stronger saturation in this case compared with the results of Harren et al. (Harren et al., 1990) could be accounted for by a tighter focusing of the laser beam (smaller beam waist). As a matter of fact, saturation is determined by the laser beam intensity (irradiance) rather than the laser power. Power saturation does not depend on the gas concentration in the PA
In order to obtain an optimum SNR, noise control and interfering signals have to be taken into account (Dutu et al., 1994a; Dutu et al., 1994b). These limiting factors are discussed in
Noise plays an important role in all photoacoustic measurements and is of particular importance in the detection of ultralow gas concentrations, because the noise level limits the ultimate sensitivity. In the photoacoustic literature, the detection level is usually defined by the SNR, where the noise is given by the microphone signal measured with the laser light blocked. However, when light hits the PA cell, an additional background signal is generated which exists even when the absorbing species are not present in the detector. The background signal is often larger than the noise signal, and therefore the detection limit or sensitivity has to be defined by the signal-to-background ratio (SBR) in most experiments. Unfortunately, it is common practice to consider only the SNR. This procedure yields an extrapolated detection limit that may be far too small. The background signal is usually determined with a nonabsorbing gas, such as nitrogen, in the PA detector. It is influenced by many system properties, such as the pointing stability, the beam divergence, and the
For photoacoustic spectroscopy, "noise" often has a structure that is coherent with the signal from the target species, and therefore should more appropriately be treated as a background signal, not as noise. The background signal can be determined by measuring the acoustic signal in the absence of absorbers (i.e., with pure nitrogen), but with the same flow and in
The sensitivity-limiting factors which are encountered in LPAS can be classified into three
a. Electrical noise, by which we mean any random fluctuation, whether electronic or acoustic, which does not have a fixed phase relation with the modulation of the laser
b. Coherent acoustic background noise, meaning a signal caused by the modulation process, but not attributable to the presence of the light beam in the PA cell. This signal is at the same frequency as, and locked in phase with respect to, the laser intensity
the same pressure conditions as those used for the sample gases.
intensity. It determines the ultimate detector sensitivity.
α
*e* = 0.285
α. The
power of 130 W corresponds to the equivalent absorption coefficient
cell (if the absorbing gas concentration is not too high).
**4. Noises and limiting factors**
the following two sections.
diameter of the laser beam.
categories:
modulation.
**4.1 Noises**
c. Coherent photoacoustic background signal. This signal, which is always present in the PA detector, is caused by the laser beam, yet not by light absorption in the bulk of the gas. Rather it is due to laser beam heating of the windows and of the absorbates at their surfaces, and heating of the PA resonator walls by the reflected or scattered light owing to imperfections of the focusing lens, windows and inner walls of the PA resonator. This signal is in phase with, and at the same frequency as, the laser intensity modulation. Therefore, it is not filtered out by the lock-in amplifier connected to the microphone. Thus, a background signal proportional to the laser power becomes the main factor that limits sensitivity.
The background signal in the PA cells may arise from several sources, some of which are listed below (Gerlach & Amer, 1980):
The detection limit of the PA cell is determined by the combined effect of the intrinsic stochastic noise of the microphone, acoustic background noise, and photoacoustic background signal. Background signals are deterministic, and to the extent that they can be quantified and minimized, do not reduce the performance of the cell significantly. The detection limit is defined either at a signal-to-noise ratio of unity (SNR = 1) or at a signal-tobackground ratio of unity (SBR = 1).
The amplifier input noise and microphone noise are gaussian in nature, that is, the amount of noise is proportional to the square root of the bandwidth in which the noise is measured. All of these noise sources are incoherent. The input noise of the SR830 lock-in amplifier used in our experiments is about 6 nV (rms)/ Hz . Microphone noise, which is manifested as a noise voltage present at the microphone output terminals, can be expressed as a product between the normalized noise pressure value owing to both thermal agitation of the diaphragm and cartridge responsivity at the corresponding frequency and the square root of the measurement bandwidth. The electrical noise of Knowles EK models electret microphones is 40 nV (rms)/ Hz . The overall random noise of multiple sources is determined by taking the square root of the sum of the squares of all the individual incoherent noise figures. For gaussian noise, the peak-to-peak value is about 5 times the rms noise value, while for the two other types of noises, the rms value must be multiplied by a factor of 2 2 ≅ 2.8 to obtain the peak-to-peak amplitude. Electrical noise usually has a broadband frequency spectrum and can be reduced efficiently by narrowband filtering of the signal, as is done in the phase sensitive detection. A detection bandwidth of 0.25 Hz was set (a time constant of 1 second) in all of our measurements. Electrical noise can be reduced by using state-of-the-art (and therefore very expensive) lock-in amplifiers and/or by using longer time averaging (the noise decreases with the square root of the averaging time) at the cost of longer measurement times.
CO2 Laser Photoacoustic Spectroscopy: I. Principles 29
the noise-equivalent absorption one must multiply 1.5x10-9 W cm-1/ *Hz* by *B*1/2*P*-1, where *B* is the bandwidth and *P* the laser power. In order to get an idea of the sensitivity that can be achieved for a representative trace gas, the equivalent ethylene concentration that would give the same signal level is also tabulated. To get the noise-equivalent ethylene
Our coherent acoustical background was 2.6 μV or 9.2x10-5 Pa, equivalent to an absorption of 2.6x10-8 W cm-1. To get the equivalent absorption coefficient divide the latter number by *PL* (5.9x10-9 cm-1). This background signal is dependent on the location of the PA cell in
The coherent photoacoustic background was 2.7 μV/W, or 9.6x10-5 Pa/W, assuming the beam was optimally aligned. This is equivalent to an absorption coefficient of 2.7x10-8 cm-1, or an ethylene concentration of about 0.89 ppbV, independent of the laser power. Since the noise and coherent acoustical background can be made negligible by using high laser power, as is done in intracavity operation, the coherent photoacoustic background will be
In order to obtain a maximized SNR, a resonance frequency under 1 kHz is necessary. Under 1 kHz, the noise level is determined by the 1/*f* amplifier noise, showing a frequency
Below 1 kHz, the 1/*f* amplifier noise is the main source. Above 1 kHz, the frequency independent Brownian noise takes over. Since the pressure amplitude is inversely proportional to the square root of the resonance frequency (*p* ∝ *C*, Eq. 20), a convenient resonance can be found between 500 and 1500 Hz. This limits the choice to a cell length of 100-300 mm. If optimal signal enhancement were the only argument, one would rather choose a large (300 mm) resonator (*C* ∝ *L*1/2, Eq. 26). Shorter resonator lengths are necessary in the case of an intracavity setup due to the limited space inside the cavity. Also, a fast time
When the sample gas is flown continuously through the detector, acoustical noise can be produced, if the gas flow is turbulent, if acoustical noise from the surroundings is coupled directly into the detector sample space or into the tubes connected to the detector and then propagated into the detector, or if acoustic disturbances from the pump running the sample gas through the detector are propagated through the tubes. Thick detector and tube walls, small flow rates, mounting of the cell and chopper in separate sound insulating boxes, etc.
The background signal can be minimized by placing the windows at nodes of the mode being excited and by introducing buffer volumes at both ends of the cell. The ratio of buffer to resonator diameters must be large enough, and the buffer length has to be equal to one-
Interference of other absorbing substances may impair the theoretical detection limit in a multicomponent analysis of the real atmosphere. Such interference may be caused by other
<sup>0</sup>-1/2 (because *C* ∝ *Q*/
ω
ω<sup>0</sup>-1/2.
ω
0, *Q* ∝ (*dv*, *dh*)-1 and (*dv*, *dh*) ∝
<sup>0</sup>1/2. Above 1 kHz, where the
ω0-
concentration, multiply 4.9x10-11 W / *Hz* by *B*1/2*P*-1.
. Together with *C* ∝
response of the cell requires a short cell length.
must be chosen to suppress these noise contributions.
1/2, see Sections 3.6, 3.4 and 3.3), we get a SNR proportional to
1/*f* amplifier noise is negligible, the SNR is proportional to
the ultimate limit of sensitivity.
ω
fourth of resonator length.
**4.2 Gas interference**
behavior of 1/
relation to the sound sources associated with the modulation process.
ω
The two types of coherent background, however, are extremely narrowband signals at the same frequency as the modulation and hence cannot be filtered out. In addition, since the signal and the coherent photoacoustic background signal are both proportional to laser power, no improvement will be achieved as the laser power is increased.
Table 2 shows the magnitudes of these limiting factors in the case Brewster windows are used. We expressed each factor in several different sets of units (Dutu et al., 1994b): voltage, pressure amplitude, equivalent absorption coefficient that would give the same pressure amplitude, and the concentration of ethylene that would be required to give that much absorption.
a The equivalent peak-to-peak pressure was obtained by dividing the peak-to-peak noise level to microphone sensitivity: 2 2 *<sup>i</sup> VN* /*SM*, where *<sup>i</sup> VN* is either *<sup>e</sup> VN* , *ac VN* or *<sup>b</sup> VN* (in our case, *SM* = 8x10-2 V/Pa) b The equivalent absorption was obtained by dividing the peak-to-peak noise level to cell responsivity: 2 2 *<sup>i</sup> VN* /*R*, where *<sup>i</sup> VN* is either *<sup>e</sup> VN* , *ac VN* or *<sup>b</sup> VN* (in our case, *R* = 280 V cm/W)
c The equivalent C2H4 concentration was obtained by dividing the equivalent absorption to the C2H4 absorption coefficient at one atmosphere pressure of the gas at the 10P(14) laser wavelength (α*<sup>p</sup>* = 30.4 cm-1) d We expressed the coherent acoustical background noise in V independent of the bandwidth, as did Gerlach and Amer (Gerlach & Amer, 1980) and Beck (Beck, 1985) (and not in V/ *Hz* as used by Harren et al. (Harren et al., 1990)); our measurements show that the acoustical background noise was independent of the lock-in bandwidth when the equivalent noise bandwidth (ENBW, the effective bandwidth for gaussian noise) of the low pass filter was varied between 0.08 Hz and 8 Hz (the lock-in time constant *T* was changed between 0.3 and 30 s, where ENBW = 1/(4*<sup>T</sup>*) for a slope of 6 dB/oct) e The coherent photoacoustical background signal was measured in pure nitrogen at atmospheric pressure (1011 mbar) and at a temperature of 22oC: 12 μV at a laser power of 4.4 W for 10P(14) line of the CO2 laser; this signal was the same both in a static gas or at a flow rate of 50 sccm (standard cubic centimeters per minute)
f The same as the limiting sensitivity of the cell, *Scell*
g With a laser power *PL* = 4.4 W, the minimum measurable concentration of ethylene was 0.2 ppbV, the same as the limiting measurable concentration of ethylene, *clim* (Table 2, Part II)
h The same as the minimum measurable absorption coefficient, α*min* (Table 2, Part II)
i The same as the minimum detectable concentration, *cmin* (Table 2, Part II)
Table 2. Noises measured in our PA system.
The limiting electrical noise measured at resonance frequency was *<sup>e</sup> VN* = 0.15 μV / Hz . At atmospheric pressure, the acoustic background noise was *ac VN* = 2.6 μV (at resonance frequency) under normal working conditions. A photoacoustic background signal of *<sup>b</sup> VN* (nitrogen) = 2.7 μV/W was observed, in phase and at resonance frequency, as the cell was filled with pure N2 at atmospheric pressure.
To determine the actual levels of noises that would be observed in practice, the random electrical noise level of 0.15 μV/ *Hz* , for example, must be multiplied by *B*1/2. Also, to get
The two types of coherent background, however, are extremely narrowband signals at the same frequency as the modulation and hence cannot be filtered out. In addition, since the signal and the coherent photoacoustic background signal are both proportional to laser
Table 2 shows the magnitudes of these limiting factors in the case Brewster windows are used. We expressed each factor in several different sets of units (Dutu et al., 1994b): voltage, pressure amplitude, equivalent absorption coefficient that would give the same pressure amplitude,
> **Equivalent pressurea**
5.3x10-6 Pa/ Hz
9.2x10-5 Pa
9.5x10-5 Pa/W
**Equivalent absorptionb**
1.5x10-9 W cm-1/ Hz
2.6x10-8 W cm-1 f
2.7x10-8
cm-1 h 8.9x10-10 i
**Equivalent C2H4 concentrationc**
4.9x10-11 W / Hz
8.6x10-10 W g
α
and the concentration of ethylene that would be required to give that much absorption.
a The equivalent peak-to-peak pressure was obtained by dividing the peak-to-peak noise level to
absorption coefficient at one atmosphere pressure of the gas at the 10P(14) laser wavelength (
2 *<sup>i</sup> VN* /*R*, where *<sup>i</sup> VN* is either *<sup>e</sup> VN* , *ac VN* or *<sup>b</sup> VN* (in our case, *R* = 280 V cm/W)
same as the limiting measurable concentration of ethylene, *clim* (Table 2, Part II)
The same as the minimum detectable concentration, *cmin* (Table 2, Part II)
microphone sensitivity: 2 2 *<sup>i</sup> VN* /*SM*, where *<sup>i</sup> VN* is either *<sup>e</sup> VN* , *ac VN* or *<sup>b</sup> VN* (in our case, *SM* = 8x10-2 V/Pa) b The equivalent absorption was obtained by dividing the peak-to-peak noise level to cell responsivity: 2
*<sup>p</sup>* = 30.4 cm-1) d We expressed the coherent acoustical background noise in V independent of the bandwidth, as did
g With a laser power *PL* = 4.4 W, the minimum measurable concentration of ethylene was 0.2 ppbV, the
The limiting electrical noise measured at resonance frequency was *<sup>e</sup> VN* = 0.15 μV / Hz . At atmospheric pressure, the acoustic background noise was *ac VN* = 2.6 μV (at resonance frequency) under normal working conditions. A photoacoustic background signal of *<sup>b</sup> VN* (nitrogen) = 2.7 μV/W was observed, in phase and at resonance frequency, as the cell was
To determine the actual levels of noises that would be observed in practice, the random electrical noise level of 0.15 μV/ *Hz* , for example, must be multiplied by *B*1/2. Also, to get
α
*min* (Table 2, Part II)
c The equivalent C2H4 concentration was obtained by dividing the equivalent absorption to the C2H4
Gerlach and Amer (Gerlach & Amer, 1980) and Beck (Beck, 1985) (and not in V/ *Hz* as used by Harren et al. (Harren et al., 1990)); our measurements show that the acoustical background noise was independent of the lock-in bandwidth when the equivalent noise bandwidth (ENBW, the effective bandwidth for gaussian noise) of the low pass filter was varied between 0.08 Hz and 8 Hz (the lock-in time constant *T* was changed between 0.3 and 30 s, where ENBW = 1/(4*<sup>T</sup>*) for a slope of 6 dB/oct) e The coherent photoacoustical background signal was measured in pure nitrogen at atmospheric pressure (1011 mbar) and at a temperature of 22oC: 12 μV at a laser power of 4.4 W for 10P(14) line of the CO2 laser; this signal was the same both in a static gas or at a flow rate of 50 sccm (standard cubic
power, no improvement will be achieved as the laser power is increased.
**(rms) value**
0.15 μV/ Hz
2.6 μV
2.7 μV/W
**Noise type Root-mean-square**
Electrical noise, *<sup>e</sup> VN*
Coherent acoustic background noised, *ac VN*
Coherent photoacoustic background signale, *<sup>b</sup> VN*
centimeters per minute)
The same as the limiting sensitivity of the cell, *Scell*
Table 2. Noises measured in our PA system.
filled with pure N2 at atmospheric pressure.
h The same as the minimum measurable absorption coefficient,
f
i
the noise-equivalent absorption one must multiply 1.5x10-9 W cm-1/ *Hz* by *B*1/2*P*-1, where *B* is the bandwidth and *P* the laser power. In order to get an idea of the sensitivity that can be achieved for a representative trace gas, the equivalent ethylene concentration that would give the same signal level is also tabulated. To get the noise-equivalent ethylene concentration, multiply 4.9x10-11 W / *Hz* by *B*1/2*P*-1.
Our coherent acoustical background was 2.6 μV or 9.2x10-5 Pa, equivalent to an absorption of 2.6x10-8 W cm-1. To get the equivalent absorption coefficient divide the latter number by *PL* (5.9x10-9 cm-1). This background signal is dependent on the location of the PA cell in relation to the sound sources associated with the modulation process.
The coherent photoacoustic background was 2.7 μV/W, or 9.6x10-5 Pa/W, assuming the beam was optimally aligned. This is equivalent to an absorption coefficient of 2.7x10-8 cm-1, or an ethylene concentration of about 0.89 ppbV, independent of the laser power. Since the noise and coherent acoustical background can be made negligible by using high laser power, as is done in intracavity operation, the coherent photoacoustic background will be the ultimate limit of sensitivity.
In order to obtain a maximized SNR, a resonance frequency under 1 kHz is necessary. Under 1 kHz, the noise level is determined by the 1/*f* amplifier noise, showing a frequency behavior of 1/ω. Together with *C* ∝ ω<sup>0</sup>-1/2 (because *C* ∝ *Q*/ω0, *Q* ∝ (*dv*, *dh*)-1 and (*dv*, *dh*) ∝ ω0- 1/2, see Sections 3.6, 3.4 and 3.3), we get a SNR proportional to ω<sup>0</sup>1/2. Above 1 kHz, where the 1/*f* amplifier noise is negligible, the SNR is proportional to ω<sup>0</sup>-1/2.
Below 1 kHz, the 1/*f* amplifier noise is the main source. Above 1 kHz, the frequency independent Brownian noise takes over. Since the pressure amplitude is inversely proportional to the square root of the resonance frequency (*p* ∝ *C*, Eq. 20), a convenient resonance can be found between 500 and 1500 Hz. This limits the choice to a cell length of 100-300 mm. If optimal signal enhancement were the only argument, one would rather choose a large (300 mm) resonator (*C* ∝ *L*1/2, Eq. 26). Shorter resonator lengths are necessary in the case of an intracavity setup due to the limited space inside the cavity. Also, a fast time response of the cell requires a short cell length.
When the sample gas is flown continuously through the detector, acoustical noise can be produced, if the gas flow is turbulent, if acoustical noise from the surroundings is coupled directly into the detector sample space or into the tubes connected to the detector and then propagated into the detector, or if acoustic disturbances from the pump running the sample gas through the detector are propagated through the tubes. Thick detector and tube walls, small flow rates, mounting of the cell and chopper in separate sound insulating boxes, etc. must be chosen to suppress these noise contributions.
The background signal can be minimized by placing the windows at nodes of the mode being excited and by introducing buffer volumes at both ends of the cell. The ratio of buffer to resonator diameters must be large enough, and the buffer length has to be equal to onefourth of resonator length.
#### **4.2 Gas interference**
Interference of other absorbing substances may impair the theoretical detection limit in a multicomponent analysis of the real atmosphere. Such interference may be caused by other
CO2 Laser Photoacoustic Spectroscopy: I. Principles 31
Another method is a multicomponent analysis approach. The PA spectrum of an arbitrary gas mixture is represented by a linear combination of the absorption spectra of all constituents. Hence, the absorption spectra of all expected constituents that contribute to the total absorption have to be determined prior to the analysis of a multicomponent gas mixture. Let us assume a nitrogen atmosphere including a mixture of *n* absorbing gases at unknown concentration levels *c*1, *c*2, ... , *cn*, low enough to assure linearity. The PA signal
) of the *n* absorbing compounds *j* (*j* = 1, 2, ... , *n*) with their concentration *cj* and their
α*j*(λ
each compound. In some cases, the calculation becomes more complicated due to different phases of photoacoustic signals generated by the individual constituents of the mixture. For some components, e.g. CO2, a temporal delay in the production of the PA signal may occur. This effect is known as kinetic cooling and results in a phase shift of the PA signal. So, the PA signal has to be considered as the vector sum of the individual signals from each compound:
> () () () () 1 1 *n n*
where *R* (V cm/W) is the cell responsivity. By sequentially tuning the laser to *m* different
photoacoustic signals *Vi* from which we derive a set of *m* linear equations for the unknown
power), and *m* ≥ *n* (it should be noted that the system of linear equations is only well defined if the number *m* of laser transitions is higher than the number of gas components *j*);
laser power at that wavelength. The measurements result in PA signals *Vi* from all components *j* in the gas mixture which absorb on the wavelength of the laser transition,
The minimum number of measurements at different laser wavelengths must be equal to at
11 1 11 1
*n nn n nn*
*n n*
... ... :::: ... ...
*bb aa*
= =
1 1
= = *n*
*i Vbc ijij* <sup>1</sup>
for *j* = 1, 2, ... , *n*. The coefficients *bji* have units of atm V-1. If the number *m* of CO2 laser lines used to carry out the analysis of a gas mixture is equal to or higher than the number *n* of absorbing components in the sample (whose CO2 laser absorption coefficients are known), the unknown concentrations of each *n* component can be determined with the proper
*bb aa*
() ()
*ij* is the absorption coefficient of the *j-*th trace absorbant gas at wavelength
least the number of unknown trace gases, *m* = *n*. In this case we can define:
*B*
λ
*j j V V RP c* = =
*j L jj*
1 1 *n n i i i ij i j ij j j j V RP c a c*
= =
λ= λ= λ α λ , (33)
λ= αλ = , (34)
*ij* (a constant for a given gas, a given wavelength and a given laser
1
−
) is the sum of the individual signals from
*<sup>i</sup>*, *i* = 1, 2, ... , *m*, we obtain *m* measured
λ
. (35)
. (36)
*<sup>i</sup>*, while *Pi* is the
λ*i*.
*V*(λ
wavelength-dependent absorption coefficients
wavelengths (discrete CO2-laser transitions)
concentration levels *cj*:
λ
*i*), *aij* = *RPi*
α
where *Pi* = *PL*(
Therefore,
α
molecular systems present in the environment or substances that are entrained by the carrier flux. If an interfering species is present in the environment, its effect can be minimized by either the introduction of scrubbers and cryogenic traps or the use of dual beam techniques using two PA cells. Sample-entrained interfering species present a more serious problem, since they will be present only near the source and therefore cannot be eliminated by dual beam spectroscopy.
In ambient air, one finds CO2 concentrations of 330-365 ppmV (0.033%-0.0365%) (Sigrist, 1986; Harren et al., 1990; Rooth et al., 1990). This level may rise to about 1% in the practical conditions of an agricultural application. This poses a serious practical problem. The CO2 molecule possesses absorption vibrational band transitions *v*<sup>1</sup> → *v*3 (1000 → 0001) and 2*v*<sup>2</sup> → *v*3 (0200 → 0001) which are weak, while the lower levels are barely populated at room temperature (∼1%). However, due to the exact coincidence of these vibrational-rotational transitions with the CO2 laser lines and the relatively high concentration of CO2 in comparison with trace gases like C2H4, carbon dioxide is inevitably excited by CO2 laser radiation, and the related photoacoustic signal may exceed the trace signal by many orders of magnitude. The absorption coefficient increases strongly with temperature, but is independent of the CO2 concentration over a wide range. A 1.5% concentration of CO2 has an absorption strength comparable to 1 ppmV of C2H4 (for CO2 at the 10P(14) laser line, α(CO2) = 2.1x10-3 atm-1cm-1 and *c*(C2H4) = *c*(CO2)α(CO2)/α(C2H4) = 10-6 atm = 1 ppmV). At the 10P(14) line of CO2, 360 ppmV of CO2 has an absorption coefficient equal to that of 24.8 ppbV of C2H4. Similarly, at the 9R(30) line of CO2 at 21oC, the same concentration of CO2 has an absorption coefficient equal to that of 13.5 ppbV of NH3. Water vapor exhibits a broad continuum with occasional weak lines in the frequency range of the CO2 laser (for H2O at the 10P(14) laser line, α(H2O) = 2.85x10-5 atm-1cm-1). The two dominant peaks are the absorption lines on 10R(20) and the most favorable for ambient air measurement, the 10P(40) laser transition. The absorption of 1% of water vapor in air (50% relative humidity at 18oC) is about the same as that of 9.4 ppbV of C2H4 at the 10P(14) line or 5 ppbV of NH3 at the 9R(30) line of CO2 (Rooth et al., 1990). However, at a constant temperature the absorption coefficient α(H2O) depends on the water vapor concentration *x* and appears to obey the relation: α(H2O) = α0*x*, where α0 is a constant. The natural unpolluted atmosphere contains H2O at a concentration level of ~1.5%.
Ammonia (a colorless, poisonous gas with a characteristic smell and well solvable in water) is vibrationally excited to the ν2 state, usually by means of the *saR*(5, *K*) transitions at λ = 9.22 μm. These levels can be excited by the 9R(30) line of the CO2 laser, where the absorption coefficient α(NH3) has a value of 56 cm-1 atm-1 (Rooth et al., 1990). Ammonia is present in the atmosphere in concentrations ranging from below 0.1 ppbV over open water up to several tens of ppbV in areas with intensive livestock breeding.
Due to the additive character of the photoacoustic signal under normal atmospheric conditions, the presence of a large amount of water vapor and carbon dioxide impedes C2H4 detection in the low-concentration range (ppbV). Consequently, some means of selective spectral discrimination is required if ethylene is to be detected interference free in the matrix of absorbing gases. There are several ways to overcome this problem. The first is to remove CO2 from the flowing sample by absorption on a KOH (potassium hydroxide)-based scrubber inserted between the sampling cell and the PA cell (a specific chemical reaction results: KOH → K2CO3 and water). In this way, concentrations below 1 ppmV CO2 (equivalent to a concentration of 0.07 ppbV of C2H4) can be achieved without influencing the C2H4 concentration.
molecular systems present in the environment or substances that are entrained by the carrier flux. If an interfering species is present in the environment, its effect can be minimized by either the introduction of scrubbers and cryogenic traps or the use of dual beam techniques using two PA cells. Sample-entrained interfering species present a more serious problem, since they will be present only near the source and therefore cannot be eliminated by dual
In ambient air, one finds CO2 concentrations of 330-365 ppmV (0.033%-0.0365%) (Sigrist, 1986; Harren et al., 1990; Rooth et al., 1990). This level may rise to about 1% in the practical conditions of an agricultural application. This poses a serious practical problem. The CO2 molecule possesses absorption vibrational band transitions *v*<sup>1</sup> → *v*3 (1000 → 0001) and 2*v*<sup>2</sup> → *v*3 (0200 → 0001) which are weak, while the lower levels are barely populated at room temperature (∼1%). However, due to the exact coincidence of these vibrational-rotational transitions with the CO2 laser lines and the relatively high concentration of CO2 in comparison with trace gases like C2H4, carbon dioxide is inevitably excited by CO2 laser radiation, and the related photoacoustic signal may exceed the trace signal by many orders of magnitude. The absorption coefficient increases strongly with temperature, but is independent of the CO2 concentration over a wide range. A 1.5% concentration of CO2 has an absorption strength comparable to 1 ppmV of C2H4 (for CO2 at the 10P(14) laser line,
> α(CO2)/α
(H2O) = 2.85x10-5 atm-1cm-1). The two dominant peaks are the
(H2O) depends on the water vapor concentration *x* and appears to
2 state, usually by means of the *saR*(5, *K*) transitions at
0 is a constant. The natural unpolluted atmosphere
the 10P(14) line of CO2, 360 ppmV of CO2 has an absorption coefficient equal to that of 24.8 ppbV of C2H4. Similarly, at the 9R(30) line of CO2 at 21oC, the same concentration of CO2 has an absorption coefficient equal to that of 13.5 ppbV of NH3. Water vapor exhibits a broad continuum with occasional weak lines in the frequency range of the CO2 laser (for H2O at
absorption lines on 10R(20) and the most favorable for ambient air measurement, the 10P(40) laser transition. The absorption of 1% of water vapor in air (50% relative humidity at 18oC) is about the same as that of 9.4 ppbV of C2H4 at the 10P(14) line or 5 ppbV of NH3 at the 9R(30) line of CO2 (Rooth et al., 1990). However, at a constant temperature the
Ammonia (a colorless, poisonous gas with a characteristic smell and well solvable in water)
9.22 μm. These levels can be excited by the 9R(30) line of the CO2 laser, where the absorption
the atmosphere in concentrations ranging from below 0.1 ppbV over open water up to
Due to the additive character of the photoacoustic signal under normal atmospheric conditions, the presence of a large amount of water vapor and carbon dioxide impedes C2H4 detection in the low-concentration range (ppbV). Consequently, some means of selective spectral discrimination is required if ethylene is to be detected interference free in the matrix of absorbing gases. There are several ways to overcome this problem. The first is to remove CO2 from the flowing sample by absorption on a KOH (potassium hydroxide)-based scrubber inserted between the sampling cell and the PA cell (a specific chemical reaction results: KOH → K2CO3 and water). In this way, concentrations below 1 ppmV CO2 (equivalent to a concentration of 0.07 ppbV of C2H4) can be achieved without influencing the C2H4 concentration.
(NH3) has a value of 56 cm-1 atm-1 (Rooth et al., 1990). Ammonia is present in
(C2H4) = 10-6 atm = 1 ppmV). At
λ=
beam spectroscopy.
the 10P(14) laser line,
absorption coefficient
α
obey the relation:
coefficient
(CO2) = 2.1x10-3 atm-1cm-1 and *c*(C2H4) = *c*(CO2)
α
α
(H2O) =
contains H2O at a concentration level of ~1.5%.
α
ν
several tens of ppbV in areas with intensive livestock breeding.
0*x*, where
α
α
is vibrationally excited to the
α
Another method is a multicomponent analysis approach. The PA spectrum of an arbitrary gas mixture is represented by a linear combination of the absorption spectra of all constituents. Hence, the absorption spectra of all expected constituents that contribute to the total absorption have to be determined prior to the analysis of a multicomponent gas mixture. Let us assume a nitrogen atmosphere including a mixture of *n* absorbing gases at unknown concentration levels *c*1, *c*2, ... , *cn*, low enough to assure linearity. The PA signal *V*(λ) of the *n* absorbing compounds *j* (*j* = 1, 2, ... , *n*) with their concentration *cj* and their wavelength-dependent absorption coefficients α*j*(λ) is the sum of the individual signals from each compound. In some cases, the calculation becomes more complicated due to different phases of photoacoustic signals generated by the individual constituents of the mixture. For some components, e.g. CO2, a temporal delay in the production of the PA signal may occur. This effect is known as kinetic cooling and results in a phase shift of the PA signal. So, the PA signal has to be considered as the vector sum of the individual signals from each compound:
$$V\left(\lambda\right) = \sum\_{j=1}^{n} V\_{j}\left(\lambda\right) = RP\_{L}\left(\lambda\right)\sum\_{j=1}^{n} c\_{j}\alpha\_{j}\left(\lambda\right),\tag{33}$$
where *R* (V cm/W) is the cell responsivity. By sequentially tuning the laser to *m* different wavelengths (discrete CO2-laser transitions) λ*<sup>i</sup>*, *i* = 1, 2, ... , *m*, we obtain *m* measured photoacoustic signals *Vi* from which we derive a set of *m* linear equations for the unknown concentration levels *cj*:
$$V\_i\left(\lambda\_i\right) = RP\_i \sum\_{j=1}^n \alpha\_{ij}\left(\lambda\_i\right)c\_j = \sum\_{j=1}^n a\_{ij}c\_j \tag{34}$$
where *Pi* = *PL*(λ*i*), *aij* = *RPi*α*ij* (a constant for a given gas, a given wavelength and a given laser power), and *m* ≥ *n* (it should be noted that the system of linear equations is only well defined if the number *m* of laser transitions is higher than the number of gas components *j*); α*ij* is the absorption coefficient of the *j-*th trace absorbant gas at wavelength λ*<sup>i</sup>*, while *Pi* is the laser power at that wavelength. The measurements result in PA signals *Vi* from all components *j* in the gas mixture which absorb on the wavelength of the laser transition, λ*i*. The minimum number of measurements at different laser wavelengths must be equal to at least the number of unknown trace gases, *m* = *n*. In this case we can define:
$$B = \begin{pmatrix} b\_{11} \dots b\_{1u} \\ \vdots & \vdots \\ b\_{n1} \dots b\_{nu} \end{pmatrix} = \begin{pmatrix} a\_{11} \dots a\_{1u} \\ \vdots & \vdots \\ a\_{n1} \dots a\_{nn} \end{pmatrix}^{-1} \tag{35}$$
Therefore,
$$c\_j = \sum\_{i=1}^{m} b\_{ji} V\_i \, . \tag{36}$$
for *j* = 1, 2, ... , *n*. The coefficients *bji* have units of atm V-1. If the number *m* of CO2 laser lines used to carry out the analysis of a gas mixture is equal to or higher than the number *n* of absorbing components in the sample (whose CO2 laser absorption coefficients are known), the unknown concentrations of each *n* component can be determined with the proper
CO2 Laser Photoacoustic Spectroscopy: I. Principles 33
θ
Hz and an excitation at 9R(28) CO2 line (9.23 µm) (Rooth et al., 1990).
vapor concentration. The corresponding experimental data are plotted for a frequency of 560
In Fig. 8, the phase of the calculated heat production rate for a CO2 – N2 – O2 – H2O mixture is plotted as a function of the concentrations *CO*<sup>2</sup> *c* and *H O*<sup>2</sup> *c* [19]. The data used for this plot were those for 10R(20) CO2 laser transition, i.e., *I*0 = 20 W/cm2, *H O*<sup>2</sup> σ = 3.5x10-23 cm2 and <sup>σ</sup> <sup>2</sup>*OH* = 1.0x10-22 cm2, and a chopper frequency *f* = 2650 Hz. As demonstrated in Fig. 8, the phase reversal only occurs within rather narrow concentration ranges. Thus, a heat-rate phase different from 0o or 180o is rarely expected for low H2O and CO2 concentrations.
Fig. 8. Calculated phase of heat production rate for a CO2 – N2 – O2 – H2O mixture as function of the concentrations *CO*<sup>2</sup> *c* and *H O*<sup>2</sup> *c* and for *<sup>N</sup>*<sup>2</sup> *c* = 0.8 and *<sup>O</sup>*<sup>2</sup> *c* = 0.2 (Meyer & Sigrist, 1990).
The multicomponent analysis can utilize the phase information of the photoacoustic response to suppress the CO2 signal. A high concentration of CO2 yields a phase shift of the signal with respect to the acoustic signal of ethylene. A combined signal for a CO2-C2H4 mixture is less than the sum of both individual amplitudes (vectorially added). The zero phase of the two-phase vector lock-in amplifier is adjusted to pure C2H4 absorption, and thus a mixture of CO2 and C2H4 in air is measured on two CO2 laser transitions. One obtains four pieces of information, i.e. the CO2-C2H4 mixture phase shift and absorption coefficients
for air with 340 ppmV CO2 as a function of water
Fig. 7. Predicted amplitude R and phase
selection of laser lines. The solution of sets of simultaneous equations is generally required to estimate the concentrations of each species in a multicomponent mixture and select the optimal wavelengths for a fixed number of laser lines. The selection of CO2 laser wavelengths for the optimum detection of a single species in the presence of interferences can usually be carried out by comparing the corresponding laser absorption profiles.
This method was used by Perlmutter et al. (Perlmutter et al., 1979), who observed a minus sign of the calculated CO2 concentration level. The minus sign stems from the fact that the absorption coefficients of CO2 were taken to be positive in the numerical analysis. Actually the absorption coefficients of CO2 present in nitrogen at low concentration levels (up to ~0.5%), at CO2 laser transitions, are of negative sign. The absolute values are unchanged. This minus sign is associated with the kinetic cooling effect. They found experimentally that in a longitudinal resonant PA cell (chopping frequency = 1 kHz) the CO2 gives a 180o ± 10o out-of-phase PA signal relative to operation with normal gases like ethylene. This is true when CO2 is present at concentration levels up to ~0.5% in nitrogen. At concentration levels higher than ~0.5%, the kinetic cooling phase deviation does not exceed ~180o and highly depends on concentration, thus leading to an increasing PA signal level.
A crucial feature of photoacoustics on gas mixtures is the molecular dynamics involved in the conversion of internal molecular energy to heat (Olaffson et al., 1989; Henningsen et al., 1990). This is particularly important when dealing with mixtures involving CO2 and N2. The near degeneracy between the fundamental asymmetric stretch of CO2 (2349 cm-1) and the N2 *v* = 1 vibration (2331 cm-1) leads to a large cross section for resonant energy transfer. In a CO2 laser, this mechanism is used to advantage by adding N2 to the gas mixture in order to increase the pump rate by energy transfer from the vibrationally excited N2 to CO2 in the ground state. In our case, the situation is reversed. Thus, following absorption of CO2 laser radiation, an excited CO2 molecule transfers its excitation energy to N2, where it resides for a long time owing to the metastable character of the excited N2 levels (the lifetime of the vibrational level *v* = 1 is ≅ 1 ms at 1 atm; 1 atm = 101.325 kPa). Since the CO2 molecule was initially taken out of an excited state, and the transition was a hot band transition, there now is a non-equilibrium situation among the CO2 vibrational levels, and equilibrium is eventually restored at the expense of translational energy. Thus, following radiation absorption, a transient cooling of the CO2 gas takes place, and the effect is therefore referred to as kinetic cooling. In trace gas detection, this means that the photoacoustic phase of the CO2 signal will be significantly different from the phase of the trace gas signal, where no kinetic cooling is involved. The situation is further complicated if water is present in the gas mixture, since water molecules are effective in deexciting the metastable N2 levels and hence in reducing the phase contrast. The presence of 1% water vapors speeds up the relaxation of vibrationally excited N2, and this effect reduces the phase contrast to about 135o down from 180o. This phase contrast is a very important aid in the analysis of mixtures where one of the components is strongly dominant, since a quantitative analysis of the phase contrast may provide information about the H2O concentration.
The presence of H2O and CO2 will always influence the measurement of C2H4 and NH3 concentrations. These background gases absorb CO2 laser radiation and produce simultaneously occurring photoacoustic signals. A comparison of the predicted amplitude and phase of the photoacoustic signal with experimental data is given in Fig. 7 (Rooth et al., 1990).
selection of laser lines. The solution of sets of simultaneous equations is generally required to estimate the concentrations of each species in a multicomponent mixture and select the optimal wavelengths for a fixed number of laser lines. The selection of CO2 laser wavelengths for the optimum detection of a single species in the presence of interferences
This method was used by Perlmutter et al. (Perlmutter et al., 1979), who observed a minus sign of the calculated CO2 concentration level. The minus sign stems from the fact that the absorption coefficients of CO2 were taken to be positive in the numerical analysis. Actually the absorption coefficients of CO2 present in nitrogen at low concentration levels (up to ~0.5%), at CO2 laser transitions, are of negative sign. The absolute values are unchanged. This minus sign is associated with the kinetic cooling effect. They found experimentally that in a longitudinal resonant PA cell (chopping frequency = 1 kHz) the CO2 gives a 180o ± 10o out-of-phase PA signal relative to operation with normal gases like ethylene. This is true when CO2 is present at concentration levels up to ~0.5% in nitrogen. At concentration levels higher than ~0.5%, the kinetic cooling phase deviation does not exceed ~180o and highly
A crucial feature of photoacoustics on gas mixtures is the molecular dynamics involved in the conversion of internal molecular energy to heat (Olaffson et al., 1989; Henningsen et al., 1990). This is particularly important when dealing with mixtures involving CO2 and N2. The near degeneracy between the fundamental asymmetric stretch of CO2 (2349 cm-1) and the N2 *v* = 1 vibration (2331 cm-1) leads to a large cross section for resonant energy transfer. In a CO2 laser, this mechanism is used to advantage by adding N2 to the gas mixture in order to increase the pump rate by energy transfer from the vibrationally excited N2 to CO2 in the ground state. In our case, the situation is reversed. Thus, following absorption of CO2 laser radiation, an excited CO2 molecule transfers its excitation energy to N2, where it resides for a long time owing to the metastable character of the excited N2 levels (the lifetime of the vibrational level *v* = 1 is ≅ 1 ms at 1 atm; 1 atm = 101.325 kPa). Since the CO2 molecule was initially taken out of an excited state, and the transition was a hot band transition, there now is a non-equilibrium situation among the CO2 vibrational levels, and equilibrium is eventually restored at the expense of translational energy. Thus, following radiation absorption, a transient cooling of the CO2 gas takes place, and the effect is therefore referred to as kinetic cooling. In trace gas detection, this means that the photoacoustic phase of the CO2 signal will be significantly different from the phase of the trace gas signal, where no kinetic cooling is involved. The situation is further complicated if water is present in the gas mixture, since water molecules are effective in deexciting the metastable N2 levels and hence in reducing the phase contrast. The presence of 1% water vapors speeds up the relaxation of vibrationally excited N2, and this effect reduces the phase contrast to about 135o down from 180o. This phase contrast is a very important aid in the analysis of mixtures where one of the components is strongly dominant, since a quantitative analysis of the phase contrast may
The presence of H2O and CO2 will always influence the measurement of C2H4 and NH3 concentrations. These background gases absorb CO2 laser radiation and produce simultaneously occurring photoacoustic signals. A comparison of the predicted amplitude and phase of the photoacoustic signal with experimental data is given in Fig. 7 (Rooth et al.,
can usually be carried out by comparing the corresponding laser absorption profiles.
depends on concentration, thus leading to an increasing PA signal level.
provide information about the H2O concentration.
1990).
Fig. 7. Predicted amplitude R and phase θ for air with 340 ppmV CO2 as a function of water vapor concentration. The corresponding experimental data are plotted for a frequency of 560 Hz and an excitation at 9R(28) CO2 line (9.23 µm) (Rooth et al., 1990).
In Fig. 8, the phase of the calculated heat production rate for a CO2 – N2 – O2 – H2O mixture is plotted as a function of the concentrations *CO*<sup>2</sup> *c* and *H O*<sup>2</sup> *c* [19]. The data used for this plot were those for 10R(20) CO2 laser transition, i.e., *I*0 = 20 W/cm2, *H O*<sup>2</sup> σ = 3.5x10-23 cm2 and <sup>σ</sup> <sup>2</sup>*OH* = 1.0x10-22 cm2, and a chopper frequency *f* = 2650 Hz. As demonstrated in Fig. 8, the phase reversal only occurs within rather narrow concentration ranges. Thus, a heat-rate phase different from 0o or 180o is rarely expected for low H2O and CO2 concentrations.
Fig. 8. Calculated phase of heat production rate for a CO2 – N2 – O2 – H2O mixture as function of the concentrations *CO*<sup>2</sup> *c* and *H O*<sup>2</sup> *c* and for *<sup>N</sup>*<sup>2</sup> *c* = 0.8 and *<sup>O</sup>*<sup>2</sup> *c* = 0.2 (Meyer & Sigrist, 1990).
The multicomponent analysis can utilize the phase information of the photoacoustic response to suppress the CO2 signal. A high concentration of CO2 yields a phase shift of the signal with respect to the acoustic signal of ethylene. A combined signal for a CO2-C2H4 mixture is less than the sum of both individual amplitudes (vectorially added). The zero phase of the two-phase vector lock-in amplifier is adjusted to pure C2H4 absorption, and thus a mixture of CO2 and C2H4 in air is measured on two CO2 laser transitions. One obtains four pieces of information, i.e. the CO2-C2H4 mixture phase shift and absorption coefficients
CO2 Laser Photoacoustic Spectroscopy: I. Principles 35
the difference in the two signals yields the CO2 concentration with the help of Eq. (37); the fourth line, 9R(30) together with 9R(28) provides the NH3 concentration). Nägele and Sigrist (Nägele & Sigrist, 2000) recorded the PA signal on two transitions for each compound, carefully selected for maximum absorption, minimum absorption interference, and good laser performance. In addition, they measured the PA signal on two laser transitions (10P(12), 10P(40)), for which all of the investigated gases exhibit negligible absorption, to verify the constant background signal. Therefore, the spectra to monitor ethylene (10P(14), 10P(16)), ethanol (9P(8), 9P(32)), methanol (9P(34), 9P(36)), and CO2 (10P(20), 9R(20)) comprise ten different transitions. Thus cross references are possible and the background signal, which is the same for these lines, can be subtracted. This extension to several laser lines yields better detection limits and selectivity, although the time for one full
Based on previous discussion, PA spectroscopy, performed with tunable CO2 lasers as radiation
i. Its high sensitivity makes it possible to measure absorption coefficients on the order of 10-8 cm-1, corresponding to densities of μg/m3 or concentrations of ppbV (10-9 atm) for
iv. It has high selectivity, meaning that it can clearly distinguish among various compounds; v. The experimental setup is rather simple, immune to interference, and, for example, does
vi. Relative portability for *in situ* measurements (carried on mobile trailers in the
vii. Operational simplicity and real time data analysis make it capable of performing quasi
x. Specially designed PA cells can perform continuous measurements on flowing gas mixtures, i.e., a much better temporal resolution can be achieved than the one provided
The outstanding features of the PA cell, most importantly its small size, simplicity, and robustness, cannot be fully utilized unless it is combined with a suitable laser source. The recent commercial availability of sealed-off, medium-power (50 W), grating-tunable CO2 lasers has paved the way to the development of instrumentation with excellent sensitivity and compact footprints that can be readily deployed in industrial or medical settings. Further improvements are possible by using resonant PA cells with high *Q* factors (limited, though, by
Other laser sources were successfully applied in photoacoustic spectroscopy. Recent developments in compact near-infrared (NIR) and IR all-solid-state tunable lasers, such as the tunable semiconductor lasers, quantum-cascade lasers, and devices utilizing non-linear
fluctuations of the modulation frequency), multipass, or intracavity arrangements.
viii. The calibration with certified gases and gas mixtures is straightforward and reliable; ix. Detection linearity and a wide dynamic range of at least 6 orders of magnitude are offered (from several fractions of ppbV to tens of ppmV), i.e., the same apparatus can be
sources, offers the following main characteristics relevant to *in situ* trace gas monitoring:
iii. A large number of gases and vapors are measurable with the same instrument;
ii. The PA cell responsivity is independent of radiation wavelength;
troposphere or on balloon-borne systems to the stratosphere);
used for low (immission) and high (emission) concentrations.
not require cryogenic cooling of the IR detectors, etc.;
measurement increases with the number of lines.
**5. Conclusions**
most substances;
continuous measurements;
by, *inter alia*, gas chromatography.
for both lines. From this, with known absorption coefficients for both lines, the CO2 and C2H4 concentrations can be extracted (Rooth et al., 1990). A good estimate is obtained from the difference between the two measured signals *Va* and *Vb*. Putting *Va* = *Ra*exp(*i*θ*<sup>a</sup>*) and *Vb* = *Rb*exp(*i*θ*<sup>b</sup>*) the magnitude of the difference is found with the cosine rule:
$$\mathbb{E}\left|\boldsymbol{V}\_{a} - \boldsymbol{V}\_{b}\right| = \left[\boldsymbol{R}\_{a}^{2} + \boldsymbol{R}\_{b}^{2} - 2\boldsymbol{R}\_{a}\boldsymbol{R}\_{b}\cos\left(\boldsymbol{\Theta}\_{a} - \boldsymbol{\Theta}\_{b}\right)\right]^{1/2}.\tag{37}$$
Here it is only the difference between the two phase angles that is required, so absolute calibrations can be avoided. This approach has the advantage that a high laser power can be used, and no partial failure of the scrubber can falsify the C2H4 concentration.
In a multicomponent mixture, this effect can be taken into account by measuring the amplitudes *Vi* of the PA signal at the laser transitions *i* as well as its phases θ*<sup>i</sup>*, where the number *i* = 1... *m* stands for the discrete CO2 laser transitions with the powers *P*(λ*<sup>i</sup>*) = *Pi*. Thus, similar to Eq. (34) we have the following equation for the PA signal amplitude:
$$\mathbf{V}\_i \mathbf{\dot{\mathbf{\bar{v}}}} \cos \Theta\_i = \mathbf{R} P\_i \sum\_{j=1}^n \mathbf{c}\_j \mathbf{\alpha}\_{ij} \left(\lambda\_i \right) \cos \Theta\_{ij} \tag{38}$$
with *i* =1... *m*, *j* = 1... *n*, *n* ≤ *m*, where *cj* is the concentration of the gas component *j* and α*ij* is the absorption coefficient of the gas compound *j* at the laser transition *i*. The phase θ*ij* is a mathematical aid for easy calculation. It is nearly independent of the laser transition *i* for a certain gas component and can thus be written as θ*<sup>j</sup>*. In our wavelength range it is only CO2 which shows a phase θ*j* = π, whereas all the other gases studied so far show a phase θ*<sup>j</sup>* = 0. In real measurements small deviations of the phases from the predicted ones occur due to measurement errors. Nevertheless, the approximation θ*ij* = θ*<sup>j</sup>* = 0 for all the other air compounds is well justified. It should be noted that the system of linear equations is only well defined if the number of gas components *j* is smaller than the number *m* of laser transitions, i.e., for *n* ≤ *m*. Based on measurements of the signals *Vi*, phases θ*<sup>j</sup>*, and laser powers *Pi*, and knowing the absorption coefficients α*ij* from literature data or calibration measurements, the unknown concentrations *cj* can be derived by solving the above equation system. The algorithm of the data analysis has been described by Meyer and Sigrist (Meyer & Sigrist, 1990). For multicomponent mixtures an algorithm, e.g., a nonlinear Levenberg-Marquardt fit (Moeckli et al., 1998) is employed to fit the measured spectrum on the basis of calibration spectra of the individual compounds.
The concentrations of C2H4, CO2, and H2O in nitrogen at atmospheric pressure can be determined by measuring the PA signals using three CO2 laser transitions, e.g., 10P(14), 10P(20) and 10R(20) (Sigrist et al., 1989). The 10P(14) and 10R(20) transitions coincide with sharp peaks of the IR spectra of C2H4 (α = 30.4 cm-1atm-1) and H2O (α = 8.36x10-4 cm-1atm-1), respectively. The 10P(20) line that is used to measure CO2 concentration (α = 2.2x10-3 cm-1atm-1) could be replaced by many other transitions without much change in sensitivity, because CO2 is relatively spectrally flat.
Rooth et al. (Rooth et al., 1990) determined the H2O, CO2, and NH3 contents in ambient air by using four laser transitions (10R(20) is used to compute water vapor concentration using the absorption coefficient at the actual gas temperature; the influence of the CO2 absorption on the measurement of H2O is also taken into account by using the 9R(18) and 9R(28) lines; the difference in the two signals yields the CO2 concentration with the help of Eq. (37); the fourth line, 9R(30) together with 9R(28) provides the NH3 concentration). Nägele and Sigrist (Nägele & Sigrist, 2000) recorded the PA signal on two transitions for each compound, carefully selected for maximum absorption, minimum absorption interference, and good laser performance. In addition, they measured the PA signal on two laser transitions (10P(12), 10P(40)), for which all of the investigated gases exhibit negligible absorption, to verify the constant background signal. Therefore, the spectra to monitor ethylene (10P(14), 10P(16)), ethanol (9P(8), 9P(32)), methanol (9P(34), 9P(36)), and CO2 (10P(20), 9R(20)) comprise ten different transitions. Thus cross references are possible and the background signal, which is the same for these lines, can be subtracted. This extension to several laser lines yields better detection limits and selectivity, although the time for one full measurement increases with the number of lines.
#### **5. Conclusions**
34 CO2 Laser – Optimisation and Application
for both lines. From this, with known absorption coefficients for both lines, the CO2 and C2H4 concentrations can be extracted (Rooth et al., 1990). A good estimate is obtained from
Here it is only the difference between the two phase angles that is required, so absolute calibrations can be avoided. This approach has the advantage that a high laser power can be
In a multicomponent mixture, this effect can be taken into account by measuring the
1 cos cos *n i i i j ij i ij j*
mathematical aid for easy calculation. It is nearly independent of the laser transition *i* for a
real measurements small deviations of the phases from the predicted ones occur due to
compounds is well justified. It should be noted that the system of linear equations is only well defined if the number of gas components *j* is smaller than the number *m* of laser
measurements, the unknown concentrations *cj* can be derived by solving the above equation system. The algorithm of the data analysis has been described by Meyer and Sigrist (Meyer & Sigrist, 1990). For multicomponent mixtures an algorithm, e.g., a nonlinear Levenberg-Marquardt fit (Moeckli et al., 1998) is employed to fit the measured spectrum on the basis of
The concentrations of C2H4, CO2, and H2O in nitrogen at atmospheric pressure can be determined by measuring the PA signals using three CO2 laser transitions, e.g., 10P(14), 10P(20) and 10R(20) (Sigrist et al., 1989). The 10P(14) and 10R(20) transitions coincide with
1atm-1) could be replaced by many other transitions without much change in sensitivity,
Rooth et al. (Rooth et al., 1990) determined the H2O, CO2, and NH3 contents in ambient air by using four laser transitions (10R(20) is used to compute water vapor concentration using the absorption coefficient at the actual gas temperature; the influence of the CO2 absorption on the measurement of H2O is also taken into account by using the 9R(18) and 9R(28) lines;
=
with *i* =1... *m*, *j* = 1... *n*, *n* ≤ *m*, where *cj* is the concentration of the gas component *j* and
the absorption coefficient of the gas compound *j* at the laser transition *i*. The phase
( )
θ
, whereas all the other gases studied so far show a phase
α
= 30.4 cm-1atm-1) and H2O (
θ*ij* = θ
θ= α λ θ , (38)
*<sup>j</sup>*. In our wavelength range it is only CO2
*ij* from literature data or calibration
α
( ) 1/2 2 2 | | 2 cos *V V R R RR a b a b ab a b* − = + − θ −θ . (37)
θ
θ
*<sup>a</sup>*) and *Vb* =
*<sup>i</sup>*, where the
α*ij* is
θ*ij* is a
*<sup>j</sup>*, and laser
θ*<sup>j</sup>* = 0. In
*<sup>j</sup>* = 0 for all the other air
θ
= 8.36x10-4 cm-1atm-1),
= 2.2x10-3 cm-
α
λ*<sup>i</sup>*) = *Pi*.
the difference between the two measured signals *Va* and *Vb*. Putting *Va* = *Ra*exp(*i*
*<sup>b</sup>*) the magnitude of the difference is found with the cosine rule:
used, and no partial failure of the scrubber can falsify the C2H4 concentration.
amplitudes *Vi* of the PA signal at the laser transitions *i* as well as its phases
*V RP c*
transitions, i.e., for *n* ≤ *m*. Based on measurements of the signals *Vi*, phases
α
respectively. The 10P(20) line that is used to measure CO2 concentration (
certain gas component and can thus be written as
measurement errors. Nevertheless, the approximation
powers *Pi*, and knowing the absorption coefficients
calibration spectra of the individual compounds.
sharp peaks of the IR spectra of C2H4 (
because CO2 is relatively spectrally flat.
θ*j* = π
which shows a phase
number *i* = 1... *m* stands for the discrete CO2 laser transitions with the powers *P*(
Thus, similar to Eq. (34) we have the following equation for the PA signal amplitude:
*Rb*exp(*i*θ
> Based on previous discussion, PA spectroscopy, performed with tunable CO2 lasers as radiation sources, offers the following main characteristics relevant to *in situ* trace gas monitoring:
The outstanding features of the PA cell, most importantly its small size, simplicity, and robustness, cannot be fully utilized unless it is combined with a suitable laser source. The recent commercial availability of sealed-off, medium-power (50 W), grating-tunable CO2 lasers has paved the way to the development of instrumentation with excellent sensitivity and compact footprints that can be readily deployed in industrial or medical settings. Further improvements are possible by using resonant PA cells with high *Q* factors (limited, though, by fluctuations of the modulation frequency), multipass, or intracavity arrangements.
Other laser sources were successfully applied in photoacoustic spectroscopy. Recent developments in compact near-infrared (NIR) and IR all-solid-state tunable lasers, such as the tunable semiconductor lasers, quantum-cascade lasers, and devices utilizing non-linear
CO2 Laser Photoacoustic Spectroscopy: I. Principles 37
harmonic of a Nd:YAG laser and their IR tuning range is limited to approximately 2 µm. Difference frequency generation (DFG) is certainly the most promising technique for the extension of the tuning range of an existing tunable laser to the mid IR (2.5-4.5 µm) (Fischer & Sigrist, 2002). Spectrometers using DFG were applied to monitoring, for example, formaldehyde in ambient air at 3.53 µm (Rehle et al., 2001) and volcanic gases (CH4, CO2,
Angeli, G.Z.; Solyom, A.M.; Miklos, A. & Bicanic, D.D. (1992). Calibration of a Windowless
Beck, S.M. (1985). Cell Coatings to Minimize Sample (NH3 and N2H4) Adsorption for Low-
Beenen, A. & Niessner, R. (1998). Development of Photoacoustic Gas Sensor for In-Situ and
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**6. References**
optical mixing in non-linear crystals (OPOs, difference frequency generators (DFGs), optical parametric amplifiers (OPAs)), have significantly advanced the application of photoacoustic techniques in sensitive trace gas analysis.
NIR diode lasers are becoming more and more popular due to recent development of cheap high-quality, compact sources having a spectral emission which falls in the absorption range of many molecules of great practical interest. The range of the available NIR diode lasers spans from about 0.8 to 2.1 µm. Many gases like methane, acetylene, CO, and CO2 exhibit overtone absorptions in the 1.55-1.65 µm wavelength region which can be covered by a conventional external cavity diode laser (ECDL). Detection limits are lower compared with measurements in the fundamental absorption region but still sufficient for many applications. A combination of diode lasers with PA detection was used by various authors to detect ammonia (Fehér et al., 1994; Schmohl et al., 2002), methane (Schäfer et al., 1998), elemental carbon (Petzold et al., 1995), toluene (Beenen et al., 1998), and water vapor (Bozóki et al., 1999). Different experimental arrangements such as external cavity diode lasers (Sneider et al., 1997) or intracavity PA cells (Bozóki et al., 1996) were tested. At present, 1.6 µm diode lasers, coinciding with the first vibrational overtones and combination bands of molecules containing a CH bond, are those that – in the NIR range – provide the best tradeoff between cost and molecular detection efficiency (Boschetti et al., 2002).
Recent progress with quantum-cascade lasers makes them attractive sources in the important 3- to 5-µm spectral range. This area is important not only because the characteristic absorption bands of, among others, CO, N2O, HCl and CH2O, lie herein, but also because there is an atmospheric transparent window in this range. Soon after the first appearance of these lasers, gas monitoring applications using various detection schemes were reported (Sharpe et al., 1998; Kosterev et al., 2008). Quantum-cascade lasers were used to detect ammonia and water vapor at 8.5 µm (Paldus et al., 1999), NO at 5.2 µm (Menzel et al., 2001), 12CH4, 13CH4 and N2O isotopomers at 8.1 µm (Gagliardi et al., 2002), trace gases (CH4, N2O, H2O) in laboratory air at 7.9 µm (Kosterev & Tittel, 2002), carbon dioxide, methanol and ammonia at 10.1/10.3 µm (Hofstetter et al., 2001), CH4 and NO at 7.9 µm and 5.3 µm (Grossel et al, 2006; Grossel et al., 2007) and simultaneously CO and SO2 at 4.56 µm and 7.38 µm (Liu et al., 2011). In contrast to semiconductor (diode) lasers, quantum-cascade lasers are unipolar light sources based on only one type of carrier, usually electrons, making intraband transitions between confined energy levels within the conduction band. The term "cascade" comes from the fact that the confined energy levels are arranged the way of a waterfall, so that electrons undergoing lasing transitions travel from one stage to the next, just like water does in a multiple-step water cascade. Therefore, one electron can emit sequentially up to *n* photons when *n* steps are present. The emission wavelength of a quantum-cascade laser is determined not by the semiconductor bandgap but by the quantum confinement in the quantum wells created by the quantum-well material and the barrier material. Therefore, quantum-cascade lasers can span a wide wavelength range using the same material system. Quantum-cascade lasers with wavelengths from 3.5 to 13 µm have been fabricated by use of the same material system (InGaAs wells and InAlAs barriers).
Widely tunable, narrowband optical parametric oscillator (OPO)-based laser sources were used for trace gas spectroscopy in the fundamental C-H stretch vibration region (3-5 µm) (Bohren et al., 1997), to detect ethane at 3.34 µm (Kühnemann et al., 1998; van Herpen et al., 2002), N2O at 2.86 µm (Costopoulos et al., 2002), or methane at 3.39 µm (Miklós et al., 2002). Most of today's commercial OPOs are based on BaB2O4 crystals (BBO) pumped by the third harmonic of a Nd:YAG laser and their IR tuning range is limited to approximately 2 µm. Difference frequency generation (DFG) is certainly the most promising technique for the extension of the tuning range of an existing tunable laser to the mid IR (2.5-4.5 µm) (Fischer & Sigrist, 2002). Spectrometers using DFG were applied to monitoring, for example, formaldehyde in ambient air at 3.53 µm (Rehle et al., 2001) and volcanic gases (CH4, CO2, HCl, SO2, H 2O vapor) at 3.3-4.4 µm (Richter et al., 2002).
#### **6. References**
36 CO2 Laser – Optimisation and Application
optical mixing in non-linear crystals (OPOs, difference frequency generators (DFGs), optical parametric amplifiers (OPAs)), have significantly advanced the application of photoacoustic
NIR diode lasers are becoming more and more popular due to recent development of cheap high-quality, compact sources having a spectral emission which falls in the absorption range of many molecules of great practical interest. The range of the available NIR diode lasers spans from about 0.8 to 2.1 µm. Many gases like methane, acetylene, CO, and CO2 exhibit overtone absorptions in the 1.55-1.65 µm wavelength region which can be covered by a conventional external cavity diode laser (ECDL). Detection limits are lower compared with measurements in the fundamental absorption region but still sufficient for many applications. A combination of diode lasers with PA detection was used by various authors to detect ammonia (Fehér et al., 1994; Schmohl et al., 2002), methane (Schäfer et al., 1998), elemental carbon (Petzold et al., 1995), toluene (Beenen et al., 1998), and water vapor (Bozóki et al., 1999). Different experimental arrangements such as external cavity diode lasers (Sneider et al., 1997) or intracavity PA cells (Bozóki et al., 1996) were tested. At present, 1.6 µm diode lasers, coinciding with the first vibrational overtones and combination bands of molecules containing a CH bond, are those that – in the NIR range – provide the best trade-
Recent progress with quantum-cascade lasers makes them attractive sources in the important 3- to 5-µm spectral range. This area is important not only because the characteristic absorption bands of, among others, CO, N2O, HCl and CH2O, lie herein, but also because there is an atmospheric transparent window in this range. Soon after the first appearance of these lasers, gas monitoring applications using various detection schemes were reported (Sharpe et al., 1998; Kosterev et al., 2008). Quantum-cascade lasers were used to detect ammonia and water vapor at 8.5 µm (Paldus et al., 1999), NO at 5.2 µm (Menzel et al., 2001), 12CH4, 13CH4 and N2O isotopomers at 8.1 µm (Gagliardi et al., 2002), trace gases (CH4, N2O, H2O) in laboratory air at 7.9 µm (Kosterev & Tittel, 2002), carbon dioxide, methanol and ammonia at 10.1/10.3 µm (Hofstetter et al., 2001), CH4 and NO at 7.9 µm and 5.3 µm (Grossel et al, 2006; Grossel et al., 2007) and simultaneously CO and SO2 at 4.56 µm and 7.38 µm (Liu et al., 2011). In contrast to semiconductor (diode) lasers, quantum-cascade lasers are unipolar light sources based on only one type of carrier, usually electrons, making intraband transitions between confined energy levels within the conduction band. The term "cascade" comes from the fact that the confined energy levels are arranged the way of a waterfall, so that electrons undergoing lasing transitions travel from one stage to the next, just like water does in a multiple-step water cascade. Therefore, one electron can emit sequentially up to *n* photons when *n* steps are present. The emission wavelength of a quantum-cascade laser is determined not by the semiconductor bandgap but by the quantum confinement in the quantum wells created by the quantum-well material and the barrier material. Therefore, quantum-cascade lasers can span a wide wavelength range using the same material system. Quantum-cascade lasers with wavelengths from 3.5 to 13 µm have been fabricated by use of the same material system (InGaAs wells and InAlAs barriers).
Widely tunable, narrowband optical parametric oscillator (OPO)-based laser sources were used for trace gas spectroscopy in the fundamental C-H stretch vibration region (3-5 µm) (Bohren et al., 1997), to detect ethane at 3.34 µm (Kühnemann et al., 1998; van Herpen et al., 2002), N2O at 2.86 µm (Costopoulos et al., 2002), or methane at 3.39 µm (Miklós et al., 2002). Most of today's commercial OPOs are based on BaB2O4 crystals (BBO) pumped by the third
off between cost and molecular detection efficiency (Boschetti et al., 2002).
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**2**
*Romania*
**CO2 Laser Photoacoustic Spectroscopy:**
*Department of Lasers, National Institute for Laser, Plasma, and Radiation Physics, Bucharest*
In this chapter, the main components of an instrument based on laser photoacoustic spectroscopy (LPAS) principles are described in detail. Special emphasis is laid on the home-built, frequency-stabilized, line-tunable CO2-laser source and the resonant photoacoustic cell. All of the parameters that are characteristic to the photoacoustic cell, including the limiting sensitivity of the system, are measured and compared with the best results reported by other authors. Approaches to improve current sensor performance are also discussed. Other aspects of a functional photoacoustic instrument, such as the gas
Two experimental set-ups were designed and characterized with the photoacoustic (PA) cell in an external configuration: the first one with a low power CO2 laser where the saturation effects are negligible, and a second one with a high power CO2 laser where the saturation effects are important. In the first case, the minimum detectable concentration was 0.9 ppbV (parts per billion by volume), while in the second case this parameter was improved to 0.21 ppbV. Comparing with the best results published previously in the literature, our minimum detectable concentration is better by a factor of 4.2 in the first case and by a factor of 18 in the
The next section is dealing with several applications developed in our laboratory. We present a precise measurement of the absorption coefficients of ethylene and ammonia at CO2 laser wavelengths. For ethylene, the values obtained at 10-µm band excellently agree with other measurements reported in the literature, while important differences were found for the absorption coefficients at 9-µm band. Other applications in plant physiology and medicine (lipid peroxidation and measurement of human biomarkers) are briefly reviewed. Exhaled breath air analysis represents an attractive and promising novel approach for noninvasive detection of human biomarkers associated with different diseases. Due to extremely low level of the substances of interest in exhaled breath air and the interference of many components at a given laser wavelength, we investigated several measures to increase the accuracy for a single trace gas measurement: a) We studied the efficiency of absorptive trapping and cryogenic trapping to remove carbon dioxide and water vapors from exhaled breath samples. As a result, we found the minimum volume for the KOH trap and the optimum flow rate for transferring gas samples from collecting bags to the photoacoustic (PA) cell. b) We refined breath sample collection procedures from patients under medical
handling system and data acquisition and processing, are outlined.
**1. Introduction**
second case.
**II. Instrumentation and Applications**
Dan C. Dumitras, Ana Maria Bratu and Cristina Popa
Photoacoustic and Absorption Spectroscopy. *Appl. Phys. B,* Vol.66, No.4, (April 1998), pp. 511-516, ISSN 0946-2171
### **CO2 Laser Photoacoustic Spectroscopy: II. Instrumentation and Applications**
Dan C. Dumitras, Ana Maria Bratu and Cristina Popa *Department of Lasers, National Institute for Laser, Plasma, and Radiation Physics, Bucharest Romania*
#### **1. Introduction**
42 CO2 Laser – Optimisation and Application
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In this chapter, the main components of an instrument based on laser photoacoustic spectroscopy (LPAS) principles are described in detail. Special emphasis is laid on the home-built, frequency-stabilized, line-tunable CO2-laser source and the resonant photoacoustic cell. All of the parameters that are characteristic to the photoacoustic cell, including the limiting sensitivity of the system, are measured and compared with the best results reported by other authors. Approaches to improve current sensor performance are also discussed. Other aspects of a functional photoacoustic instrument, such as the gas handling system and data acquisition and processing, are outlined.
Two experimental set-ups were designed and characterized with the photoacoustic (PA) cell in an external configuration: the first one with a low power CO2 laser where the saturation effects are negligible, and a second one with a high power CO2 laser where the saturation effects are important. In the first case, the minimum detectable concentration was 0.9 ppbV (parts per billion by volume), while in the second case this parameter was improved to 0.21 ppbV. Comparing with the best results published previously in the literature, our minimum detectable concentration is better by a factor of 4.2 in the first case and by a factor of 18 in the second case.
The next section is dealing with several applications developed in our laboratory. We present a precise measurement of the absorption coefficients of ethylene and ammonia at CO2 laser wavelengths. For ethylene, the values obtained at 10-µm band excellently agree with other measurements reported in the literature, while important differences were found for the absorption coefficients at 9-µm band. Other applications in plant physiology and medicine (lipid peroxidation and measurement of human biomarkers) are briefly reviewed.
Exhaled breath air analysis represents an attractive and promising novel approach for noninvasive detection of human biomarkers associated with different diseases. Due to extremely low level of the substances of interest in exhaled breath air and the interference of many components at a given laser wavelength, we investigated several measures to increase the accuracy for a single trace gas measurement: a) We studied the efficiency of absorptive trapping and cryogenic trapping to remove carbon dioxide and water vapors from exhaled breath samples. As a result, we found the minimum volume for the KOH trap and the optimum flow rate for transferring gas samples from collecting bags to the photoacoustic (PA) cell. b) We refined breath sample collection procedures from patients under medical
CO2 Laser Photoacoustic Spectroscopy: II. Instrumentation and Applications 45
photoacoustic background signal makes it possible to increase the overall sensitivity of the instrument. This was proved by comparing our results with those obtained with intracavity arrangements (see Table 3). Also, the dynamic range of the PA method is considerably reduced by intracavity operation. Optical saturation may occur for molecules with high absorption cross section while uncontrollable signal changes may be obtained at higher overall absorption in the PA cell, because the loss of light intensity influences the gain of the laser. This effect may cause erroneous results when the sample concentration changes are large. Therefore, high-sensitivity single- and multipass extracavity PA detectors offer a
Various modulation methods are applied in PA spectroscopy. It is necessary to distinguish between the modulation of incident radiation and that of sample absorption (Sigrist et al., 1989). The first schemes include the widely used amplitude modulation of the incident radiation by mechanical choppers, electro-optic and acousto-optic modulators as well as the modulation of the laser emission itself by pulsed excitation, Q-switching, and mode-locking. On the other hand, frequency or wavelength modulation of the incident radiation provides the advantage of eliminating the continuum background PA signal caused by wavelengthindependent absorption, e.g., by the cell window. The absorption characteristics of the sample can be modulated based on the Zeeman or Stark effect, i.e., by applying modulated magnetic or electric fields to the sample. Consequently, the absorption wavelength of the sample is varied, which corresponds to a wavelength modulation method. The continuum background is suppressed as a result. For example, a reduction of the background by a factor of 500 was achieved by Stark modulation compared with the one obtained in the same PA cell with conventional amplitude modulation by a chopper (Kavaya et al., 1979). However, it should be noted that the application of the Stark modulation scheme in trace gas detection is restricted to molecules with a permanent electric dipole moment like ammonia (NH3), nitric oxide (NO), etc. Nevertheless, a considerable increase in sensitivity and, even more important, in selectivity in multicomponent mixtures can be achieved.
The light beam was modulated with a high quality, low vibration noise and variable speed (4-4000 Hz) mechanical chopper model DigiRad C-980 or C-995 (30 slot aperture) operated at the appropriate resonant frequency of the cell (564 Hz). The laser beam diameter is typically 5 mm at the point of insertion of the chopper blade and is nearly equal to the width of the chopper aperture. An approximately square waveform was produced with a modulation depth of 100% and a duty cycle of 50% so that the average power measured by the powermeter at the exit of the PA cell is half the cw value. By enclosing the chopper wheel in a housing with a small hole (10 mm) allowing the laser beam to pass, chopperinduced sound vibrations in air that can be transmitted to the microphone detector as noise interference are reduced. A phase reference signal is provided for use with a lock-in amplifier. The generated acoustic waves are detected by microphones mounted in the cell wall, whose signal is fed to a lock-in amplifier locked to the modulation frequency. The lock-in amplifier is a highly flexible signal recovery and analysis instrument, as it is able to measure accurately a single-frequency signal obscured by noise sources many thousands of times larger than itself. It rejects random noise, transients, incoherent discrete frequency interference and harmonics of measurement frequency. A lock-in measures an ac signal and produces a dc output proportional to the ac signal. Because the dc output level is usually greater than the ac input, a lock-in is termed an amplifier. The lock-in can also gauge the phase relationship of two signals at the same frequency. A demodulator, or phase-sensitive
simpler alternative to intracavity devices.
treatment: alveolar collection vs. mixed expiratory collection; collecting bags; preparation of patients (antiseptic mouthwash, avoiding food for at least 12 hours); clean transfer of the gas samples from disposable bags to the PA cell in less than 6 hours. c) We measured the photoacoustic signal at different CO2 laser wavelengths to distinguish the influence of various absorbent gas components in the total content.
One study involved assessment of breath ethylene and ammonia levels in patients with renal failure receiving hemodialysis (HD) treatment. Our measurements demonstrated that HD determines simultaneously a large increase of ethylene concentration in the exhaled breath (due to the oxidative stress) and a reduction of the ammonia concentration, correlated to the blood urea nitrogen level. Analysis of ethylene and ammonia traces from breath may provide insight into severity of oxidative stress and metabolic disturbances and give information for determining efficacy and endpoint of HD.
#### **2. Experimental arrangement**
#### **2.1 General schematic**
The block diagram of the laser photoacoustic spectrometer was presented in the previous chapter (Fig. 2, Part I). The cw, tunable CO2-laser beam is chopped, focused by a ZnSe lens, and introduced in the PA cell. After passing through the PA cell, the power of the laser beam is measured by a laser radiometer Rk-5700 from Laser Probe Inc. with a measuring head RkT-30. Its digital output is introduced in the data acquisition interface module together with the output from the lock-in amplifier. All experimental data are processed and stored by a computer (Dumitras et al., 2007). The frequency stabilized, line tunable CO2 lasers (low power and high power, respectively) will be described in the next section. Both CO2 lasers are used in two parallel measuring lines, where two independent experiments can be conducted simultaneously. A view of the two parallel measurement lines with laser photoacoustic sensors is shown in Fig. 1.
Fig. 1. General view of the PA sensors (two parallel measurement lines).
We decided to use an extracavity arrangement because it has several advantages. In spite of a lower laser power available to excite the absorbing gas in the PA cell, a smaller coherent
treatment: alveolar collection vs. mixed expiratory collection; collecting bags; preparation of patients (antiseptic mouthwash, avoiding food for at least 12 hours); clean transfer of the gas samples from disposable bags to the PA cell in less than 6 hours. c) We measured the photoacoustic signal at different CO2 laser wavelengths to distinguish the influence of
One study involved assessment of breath ethylene and ammonia levels in patients with renal failure receiving hemodialysis (HD) treatment. Our measurements demonstrated that HD determines simultaneously a large increase of ethylene concentration in the exhaled breath (due to the oxidative stress) and a reduction of the ammonia concentration, correlated to the blood urea nitrogen level. Analysis of ethylene and ammonia traces from breath may provide insight into severity of oxidative stress and metabolic disturbances and
The block diagram of the laser photoacoustic spectrometer was presented in the previous chapter (Fig. 2, Part I). The cw, tunable CO2-laser beam is chopped, focused by a ZnSe lens, and introduced in the PA cell. After passing through the PA cell, the power of the laser beam is measured by a laser radiometer Rk-5700 from Laser Probe Inc. with a measuring head RkT-30. Its digital output is introduced in the data acquisition interface module together with the output from the lock-in amplifier. All experimental data are processed and stored by a computer (Dumitras et al., 2007). The frequency stabilized, line tunable CO2 lasers (low power and high power, respectively) will be described in the next section. Both CO2 lasers are used in two parallel measuring lines, where two independent experiments can be conducted simultaneously. A view of the two parallel measurement lines with laser
various absorbent gas components in the total content.
give information for determining efficacy and endpoint of HD.
Fig. 1. General view of the PA sensors (two parallel measurement lines).
We decided to use an extracavity arrangement because it has several advantages. In spite of a lower laser power available to excite the absorbing gas in the PA cell, a smaller coherent
**2. Experimental arrangement**
photoacoustic sensors is shown in Fig. 1.
**2.1 General schematic**
photoacoustic background signal makes it possible to increase the overall sensitivity of the instrument. This was proved by comparing our results with those obtained with intracavity arrangements (see Table 3). Also, the dynamic range of the PA method is considerably reduced by intracavity operation. Optical saturation may occur for molecules with high absorption cross section while uncontrollable signal changes may be obtained at higher overall absorption in the PA cell, because the loss of light intensity influences the gain of the laser. This effect may cause erroneous results when the sample concentration changes are large. Therefore, high-sensitivity single- and multipass extracavity PA detectors offer a simpler alternative to intracavity devices.
Various modulation methods are applied in PA spectroscopy. It is necessary to distinguish between the modulation of incident radiation and that of sample absorption (Sigrist et al., 1989). The first schemes include the widely used amplitude modulation of the incident radiation by mechanical choppers, electro-optic and acousto-optic modulators as well as the modulation of the laser emission itself by pulsed excitation, Q-switching, and mode-locking. On the other hand, frequency or wavelength modulation of the incident radiation provides the advantage of eliminating the continuum background PA signal caused by wavelengthindependent absorption, e.g., by the cell window. The absorption characteristics of the sample can be modulated based on the Zeeman or Stark effect, i.e., by applying modulated magnetic or electric fields to the sample. Consequently, the absorption wavelength of the sample is varied, which corresponds to a wavelength modulation method. The continuum background is suppressed as a result. For example, a reduction of the background by a factor of 500 was achieved by Stark modulation compared with the one obtained in the same PA cell with conventional amplitude modulation by a chopper (Kavaya et al., 1979). However, it should be noted that the application of the Stark modulation scheme in trace gas detection is restricted to molecules with a permanent electric dipole moment like ammonia (NH3), nitric oxide (NO), etc. Nevertheless, a considerable increase in sensitivity and, even more important, in selectivity in multicomponent mixtures can be achieved.
The light beam was modulated with a high quality, low vibration noise and variable speed (4-4000 Hz) mechanical chopper model DigiRad C-980 or C-995 (30 slot aperture) operated at the appropriate resonant frequency of the cell (564 Hz). The laser beam diameter is typically 5 mm at the point of insertion of the chopper blade and is nearly equal to the width of the chopper aperture. An approximately square waveform was produced with a modulation depth of 100% and a duty cycle of 50% so that the average power measured by the powermeter at the exit of the PA cell is half the cw value. By enclosing the chopper wheel in a housing with a small hole (10 mm) allowing the laser beam to pass, chopperinduced sound vibrations in air that can be transmitted to the microphone detector as noise interference are reduced. A phase reference signal is provided for use with a lock-in amplifier.
The generated acoustic waves are detected by microphones mounted in the cell wall, whose signal is fed to a lock-in amplifier locked to the modulation frequency. The lock-in amplifier is a highly flexible signal recovery and analysis instrument, as it is able to measure accurately a single-frequency signal obscured by noise sources many thousands of times larger than itself. It rejects random noise, transients, incoherent discrete frequency interference and harmonics of measurement frequency. A lock-in measures an ac signal and produces a dc output proportional to the ac signal. Because the dc output level is usually greater than the ac input, a lock-in is termed an amplifier. The lock-in can also gauge the phase relationship of two signals at the same frequency. A demodulator, or phase-sensitive
CO2 Laser Photoacoustic Spectroscopy: II. Instrumentation and Applications 47
The variation of the background and calibration with transversal or longitudinal translation of the PA cell against the propagation beam direction is very small as long as the laser beam does not strike the walls of the cell. The optimum position of the PA cell against the focusing lens (the center of the resonant tube, i.e. the position of the microphones) is 450 mm (for a focusing length of 400 mm). On extending this optimum distance by 30 mm, the signal is decreased by 0.14%, while when shortening the distance by 60 mm, the signal decreases by 0.61%. The dependence of both signal and background signal on the transversal position of the resonant tube relative to the laser beam is similarly low. We found that the cell calibration and background were virtually invariable for reasonably small longitudinal or
ZnSe, where *n* is the refraction index of the material), the window reflectivity becomes nonzero, and the reflected beam can heat the walls of the cell, making a further contribution to the background signal. Nevertheless, for a ± 10% variation of the angle of incidence relative to the Brewster angle, the reflectance increases from zero to only 2.7% and 1.7%, respectively, meaning that a small deviation from the Brewster angle will not change
Another variable we investigated was the polarization angle of the beam. The cell response in terms of background signal displays a rather broad flat minimum, provided the incidence angle on the window ensures a minimum reflection loss. The data indicate that a deviation of several degrees from vertical polarization can be tolerated. In conclusion, we found that the calibration and background signal were not extremely sensitive to slight misalignments of the beam.
Several authors employed an iris diaphragm in close proximity of the PA cell entrance window to provide spatial filtering in order to reduce the background noise signal caused by off-axis radiation impinging on the internal wall of the chamber. With such a diaphragm, we found that the background signal increased significantly owing to laser beam diffraction at the edges of the aperture. Using high quality optical components (diffraction grating, coupling mirror, lens and windows) together with a well controlled laser beam makes the
We have designed, constructed and optimized a rugged sealed-off CO2 laser (named LIR-25 SF), step-tunable on more than 60 vibrational-rotational lines and frequency stabilized by the use of plasma tube impedance variations detected as voltage fluctuations (the optovoltaic method) (Dumitras et al., 1981; Dumitras et al., 1985; Dutu et al., 1985). The glass tube has an inner diameter of 7 mm and a discharge length of 53 cm. At both ends of the tube we attached ZnSe windows at Brewster angle. The laser is water cooled around the discharge tube. The dc discharge is driven by a high-voltage power supply. The end reflectors of the laser cavity are a piezoelectrically driven, partially (85%) reflecting ZnSe mirror at one end and a line-selecting grating (135 lines/mm, blazed at 10.6 µm) at the other.
A free-running or unstabilized laser is subject to many perturbations of its frequency. First
keeping a constant length is the prime objective in frequency stabilization schemes. Possible perturbations of the cavity length can be divided into two groups: external effects (thermal
θ*B* (θ
*<sup>B</sup>* = arctan*n* = 67.38o for
ν/ν
= Δ*L*/*L*), so
transversal movements of the cell.
**2.2 CO2 lasers**
As the angle of incidence deviates from the Brewster angle
insertion of an iris diaphragm in the PA instrument unnecessary.
Piezoelectric ceramics such as lead zirconate titanate (PZT) can be used.
of all, changes in cavity length affect the frequency of an oscillating mode (Δ
dramatically the reflectance or the angle of refraction.
detector (PSD), is the basis for a lock-in amplifier. This circuit rectifies the signals coming in at the desired frequency. The PSD output is also a function of the phase angle between the input signal and the amplifier's internal reference signal generated by a phased-locked loop locked to an external reference (chopper). We used a dual-phase, digital lock-in amplifier Stanford Research Systems model SR 830 with the following characteristics: full scale sensitivity, 2 nV - 1 V; input noise, 6 nV (rms)/ Hz at 1 kHz; dynamic reserve, greater than 100 dB; frequency range, 1 mHz – 102 kHz; time constants, 10 μs – 30 s (reference > 200 Hz), or up to 30000 s (reference < 200 Hz).
The diverging IR laser beam is converged by a ZnSe focusing lens (*f* = 400 mm). In this way, a slightly focused laser beam is passed through the photoacoustic cell without wall interactions. The laser beam diameter *D* = 2*w* (or its radius, *w*), which is a very important issue in LPAS, was calculated at different locations on the beam's propagation path (Fig. 2). A too large beam compared to the inner diameter of the resonant tube could increase the coherent photoacoustical background signal to impracticable values. The calculation was made in three steps: a) inside the laser cavity; b) between the laser coupling mirror and the focusing lens, and c) after the focusing lens (including at the center of the PA cell and at the Brewster windows of the cell).
Fig. 2. Geometry of the laser beam from its waist to the exit of the PA cell.
The laser resonator has a stable-type configuration, being made of a diffraction grating equivalent to a totally reflecting flat mirror and a coupling concave spherical mirror with the radius of curvature *R*1 = 10 m. We have calculated the parameters of the ideal gaussian beam inside and outside the laser resonator, i.e., for *M*2 = 1, and we have obtained the following values: *w*0 = 2.91 mm (or a beam waist diameter 2*w*0 = 5.83 mm); *w*1 = 3.02 mm (2*w*1 = 6.04 mm); *w*2 = 3.16 (2*w*2 = 6.32 mm); *w*3 = 1.75 mm (2*w*3 = 3.51 mm); *w*4 = 0.51 mm (2*w*4 = 1.03 mm) and *w*5 = 2.26 mm (2*w*5 = 4.53 mm) (for *Lc* = 690 mm, *L*1 = 1060 mm and *L*2 = 450 mm). It follows that as the laser beam travels along the PA cell, its diameter is small enough compared to the diameter of the resonant tube (7 mm) to avoid wall absorptions, which ensures that the chosen geometry minimizes the coherent photoacoustical background signal, as intended.
The pertinent questions we set out to answer in assessing the performance of our PA cell were whether the chosen geometry minimized the coherent background signal, and how sensitive the background and calibration were to slight beam deviations from the intended path. Obviously, if the calibration would vary significantly with small movements of the beam, the accuracy of measurements made with the PA cell would be adversely affected unless the cell alignment was very carefully adjusted and rigidly fixed.
The variation of the background and calibration with transversal or longitudinal translation of the PA cell against the propagation beam direction is very small as long as the laser beam does not strike the walls of the cell. The optimum position of the PA cell against the focusing lens (the center of the resonant tube, i.e. the position of the microphones) is 450 mm (for a focusing length of 400 mm). On extending this optimum distance by 30 mm, the signal is decreased by 0.14%, while when shortening the distance by 60 mm, the signal decreases by 0.61%. The dependence of both signal and background signal on the transversal position of the resonant tube relative to the laser beam is similarly low. We found that the cell calibration and background were virtually invariable for reasonably small longitudinal or transversal movements of the cell.
As the angle of incidence deviates from the Brewster angle θ*B* (θ*<sup>B</sup>* = arctan*n* = 67.38o for ZnSe, where *n* is the refraction index of the material), the window reflectivity becomes nonzero, and the reflected beam can heat the walls of the cell, making a further contribution to the background signal. Nevertheless, for a ± 10% variation of the angle of incidence relative to the Brewster angle, the reflectance increases from zero to only 2.7% and 1.7%, respectively, meaning that a small deviation from the Brewster angle will not change dramatically the reflectance or the angle of refraction.
Another variable we investigated was the polarization angle of the beam. The cell response in terms of background signal displays a rather broad flat minimum, provided the incidence angle on the window ensures a minimum reflection loss. The data indicate that a deviation of several degrees from vertical polarization can be tolerated. In conclusion, we found that the calibration and background signal were not extremely sensitive to slight misalignments of the beam.
Several authors employed an iris diaphragm in close proximity of the PA cell entrance window to provide spatial filtering in order to reduce the background noise signal caused by off-axis radiation impinging on the internal wall of the chamber. With such a diaphragm, we found that the background signal increased significantly owing to laser beam diffraction at the edges of the aperture. Using high quality optical components (diffraction grating, coupling mirror, lens and windows) together with a well controlled laser beam makes the insertion of an iris diaphragm in the PA instrument unnecessary.
#### **2.2 CO2 lasers**
46 CO2 Laser – Optimisation and Application
detector (PSD), is the basis for a lock-in amplifier. This circuit rectifies the signals coming in at the desired frequency. The PSD output is also a function of the phase angle between the input signal and the amplifier's internal reference signal generated by a phased-locked loop locked to an external reference (chopper). We used a dual-phase, digital lock-in amplifier Stanford Research Systems model SR 830 with the following characteristics: full scale sensitivity, 2 nV - 1 V; input noise, 6 nV (rms)/ Hz at 1 kHz; dynamic reserve, greater than 100 dB; frequency range, 1 mHz – 102 kHz; time constants, 10 μs – 30 s (reference > 200 Hz),
The diverging IR laser beam is converged by a ZnSe focusing lens (*f* = 400 mm). In this way, a slightly focused laser beam is passed through the photoacoustic cell without wall interactions. The laser beam diameter *D* = 2*w* (or its radius, *w*), which is a very important issue in LPAS, was calculated at different locations on the beam's propagation path (Fig. 2). A too large beam compared to the inner diameter of the resonant tube could increase the coherent photoacoustical background signal to impracticable values. The calculation was made in three steps: a) inside the laser cavity; b) between the laser coupling mirror and the focusing lens, and c) after the focusing lens (including at the center of the PA cell and at the
Fig. 2. Geometry of the laser beam from its waist to the exit of the PA cell.
unless the cell alignment was very carefully adjusted and rigidly fixed.
The laser resonator has a stable-type configuration, being made of a diffraction grating equivalent to a totally reflecting flat mirror and a coupling concave spherical mirror with the radius of curvature *R*1 = 10 m. We have calculated the parameters of the ideal gaussian beam inside and outside the laser resonator, i.e., for *M*2 = 1, and we have obtained the following values: *w*0 = 2.91 mm (or a beam waist diameter 2*w*0 = 5.83 mm); *w*1 = 3.02 mm (2*w*1 = 6.04 mm); *w*2 = 3.16 (2*w*2 = 6.32 mm); *w*3 = 1.75 mm (2*w*3 = 3.51 mm); *w*4 = 0.51 mm (2*w*4 = 1.03 mm) and *w*5 = 2.26 mm (2*w*5 = 4.53 mm) (for *Lc* = 690 mm, *L*1 = 1060 mm and *L*2 = 450 mm). It follows that as the laser beam travels along the PA cell, its diameter is small enough compared to the diameter of the resonant tube (7 mm) to avoid wall absorptions, which ensures that the chosen geometry minimizes the coherent photoacoustical
The pertinent questions we set out to answer in assessing the performance of our PA cell were whether the chosen geometry minimized the coherent background signal, and how sensitive the background and calibration were to slight beam deviations from the intended path. Obviously, if the calibration would vary significantly with small movements of the beam, the accuracy of measurements made with the PA cell would be adversely affected
or up to 30000 s (reference < 200 Hz).
Brewster windows of the cell).
background signal, as intended.
We have designed, constructed and optimized a rugged sealed-off CO2 laser (named LIR-25 SF), step-tunable on more than 60 vibrational-rotational lines and frequency stabilized by the use of plasma tube impedance variations detected as voltage fluctuations (the optovoltaic method) (Dumitras et al., 1981; Dumitras et al., 1985; Dutu et al., 1985). The glass tube has an inner diameter of 7 mm and a discharge length of 53 cm. At both ends of the tube we attached ZnSe windows at Brewster angle. The laser is water cooled around the discharge tube. The dc discharge is driven by a high-voltage power supply. The end reflectors of the laser cavity are a piezoelectrically driven, partially (85%) reflecting ZnSe mirror at one end and a line-selecting grating (135 lines/mm, blazed at 10.6 µm) at the other. Piezoelectric ceramics such as lead zirconate titanate (PZT) can be used.
A free-running or unstabilized laser is subject to many perturbations of its frequency. First of all, changes in cavity length affect the frequency of an oscillating mode (Δν/ν = Δ*L*/*L*), so keeping a constant length is the prime objective in frequency stabilization schemes. Possible perturbations of the cavity length can be divided into two groups: external effects (thermal
CO2 Laser Photoacoustic Spectroscopy: II. Instrumentation and Applications 49
The error signal generated by the phase detector can serve to drive the cavity resonance to the center of the laser gain curve. In this way, the electronic feedback loop seeks the center of the lasing gain profile, the lock-in point being the zero crossing of the phase detector response. If the mean mode frequency is lower than the line center frequency, the phase of the observed laser intensity variation is opposite to the one we have where the mode frequency is higher than the line center frequency. The amplitude of the jitter output
In lieu of using an IR detector to sense the laser intensity variation, the cavity length is adjusted using the optovoltaic effect. As the internal laser radiation field intensity is altered by changing the resonant cavity alignment, the discharge impedance, which is proportional to the slope of the curve of laser output power *versus* frequency, is also modulated. The impedance variation is determined by exciting the plasma tube with a high-speed currentregulated power supply and measuring the resulting variation in the voltage drop across the plasma tube (the optovoltaic effect). An intensity variation of 1% is sufficient to change the discharge impedance significantly (~ 0.1%). By using a current regulated power supply, the voltage impedance fluctuation is detected as an ac component of the voltage drop across
Before any attempt is made to stabilize the frequency of a laser, a single frequency output must be ensured. For this purpose, a laser operating in the lowest transversal mode (TEM00) must be designed (Dumitras et al., 1976). The single line operation of the CO2 laser is achieved with a dispersive element (diffraction grating). The cavity length of *Lc* = 690 mm
Calculating the collisional broadening in a mixture of CO2, N2, He, Xe, and H2 at a total
MHz. We therefore conclude that a single frequency operation is obtained when a
To increase the number of oscillating lines, especially those with a smaller gain, and obtain reliable long term operation at a single specific wavelength, some form of wavelength selection introduced in the optical cavity is generally required. As optical dispersion is incorporated by using a diffraction grating or Brewster-angle prisms within the laser cavity, the laser can be made to oscillate on only one vibrational-rotational line, otherwise the particular transition on which the CO2 laser operates depends on the length of the resonator. That is why the total reflecting mirror must be replaced by a diffraction grating, which is tilted about its groove axis to the blaze angle and acts as a frequency selective reflector. Light diffracted into the first order maximum is returned along the optical axis and taken as laser output, while light in other orders as well as any other wavelength is returned off-axis and gets lost. Another advantage of a laser resonator with a grating is that the laser can be
We used a flat diffraction grating with 135 grooves/mm, blazed at 10.6 μm and having a peak efficiency of 96%, mounted in a Littrow configuration. With such a grating, the vibrational-rotational lines emitted by the CO2 laser in the range P(50) – 10.9329 μm and R(44) – 9.1549 μm can be selected by controlling the grating angle in the range 47o33'32'' to 38o10'02'', which can be set to the desired laser transition with a micrometric screw. This grating presents a good dispersion, as the P(18) and P(20) lines (10.4 μm band) are separated by a 6'38'' angular
difference (as compared with 2'49'' for a diffraction grating with 75 grooves/mm).
ν >Δν*<sup>c</sup>*/2).
pressure of 34 mbar gives a collisional full linewidth at half maximum (FWHM) Δ
ν
= *c*/2*Lc* = 217 MHz.
ν*<sup>c</sup>* = 119
corresponds to a separation between two longitudinal modes of Δ
longitudinal mode is tuned on the top of the gain curve (Δ
tuned over the entire oscillating linewidth from the line center.
increases with the frequency offset from the line center.
the plasma tube.
variations of the spacer material, changes in atmospheric conditions, mechanical vibrations, variations in the position of optical components and in magnetic fields) and internal effects, which are generally related to the discharge noise.
Active power stabilization based on a piezo-driven out-coupling mirror is used in our laser. The principle of the stabilization schemes is based on a comparison between the frequency of a single frequency laser (single-mode, single-line) and some stable point of reference. If the laser frequency is different from that of the benchmark, an error-sensing discriminant is used to derive a signal proportional to the deviation. This error signal is used to control the laser oscillating frequency and retune it to the reference one. Such a servo-loop (closed loop feedback) locks the laser frequency to that of the reference. For moderate stability, the CO2 laser line profile can be used as the discriminating curve (Dumitras et al., 1981). This method is more appropriate for the CO2 laser than for other lasers because the CO2 vibrationalrotational line profile is narrow and has much steeper slopes than, for example, that of the neon line in a He-Ne laser. The error signal is produced by allowing the laser resonance cavity to "ride" around the steep part of the line profile slope, and its amplitude is dependent on the change in cavity mirror separation. This scheme requires internal frequency modulation (jittering) of the laser in order to sense the sign of the derived error signal. Stabilization is then obtained by re-establishing the required separation with a servo-system.
The CO2 laser is frequency stabilized to the center of the curve representing its output power *versus* frequency (the molecular resonance) upon the variation of plasma tube impedance, when the optical power extracted from the medium is modulated (Dutu et al., 1985). In this closed-loop active stabilization, the cavity length is controlled by a piezoelectrically driven mirror along the cavity axis, which responds to the sum of a dc control voltage, plus a small jitter signal at some convenient frequency (~500 Hz). As can be seen in Fig. 3, where a curve of laser line gain *versus* frequency is drawn, the small cavity jitter induces a sinusoidal variation in laser output as the cavity mode scans across the transition gain profile, which is compared in phase to the jitter voltage.
Fig. 3. Generation of the error-frequency signal.
variations of the spacer material, changes in atmospheric conditions, mechanical vibrations, variations in the position of optical components and in magnetic fields) and internal effects,
Active power stabilization based on a piezo-driven out-coupling mirror is used in our laser. The principle of the stabilization schemes is based on a comparison between the frequency of a single frequency laser (single-mode, single-line) and some stable point of reference. If the laser frequency is different from that of the benchmark, an error-sensing discriminant is used to derive a signal proportional to the deviation. This error signal is used to control the laser oscillating frequency and retune it to the reference one. Such a servo-loop (closed loop feedback) locks the laser frequency to that of the reference. For moderate stability, the CO2 laser line profile can be used as the discriminating curve (Dumitras et al., 1981). This method is more appropriate for the CO2 laser than for other lasers because the CO2 vibrationalrotational line profile is narrow and has much steeper slopes than, for example, that of the neon line in a He-Ne laser. The error signal is produced by allowing the laser resonance cavity to "ride" around the steep part of the line profile slope, and its amplitude is dependent on the change in cavity mirror separation. This scheme requires internal frequency modulation (jittering) of the laser in order to sense the sign of the derived error signal. Stabilization is then
The CO2 laser is frequency stabilized to the center of the curve representing its output power *versus* frequency (the molecular resonance) upon the variation of plasma tube impedance, when the optical power extracted from the medium is modulated (Dutu et al., 1985). In this closed-loop active stabilization, the cavity length is controlled by a piezoelectrically driven mirror along the cavity axis, which responds to the sum of a dc control voltage, plus a small jitter signal at some convenient frequency (~500 Hz). As can be seen in Fig. 3, where a curve of laser line gain *versus* frequency is drawn, the small cavity jitter induces a sinusoidal variation in laser output as the cavity mode scans across the
which are generally related to the discharge noise.
obtained by re-establishing the required separation with a servo-system.
transition gain profile, which is compared in phase to the jitter voltage.
Fig. 3. Generation of the error-frequency signal.
The error signal generated by the phase detector can serve to drive the cavity resonance to the center of the laser gain curve. In this way, the electronic feedback loop seeks the center of the lasing gain profile, the lock-in point being the zero crossing of the phase detector response. If the mean mode frequency is lower than the line center frequency, the phase of the observed laser intensity variation is opposite to the one we have where the mode frequency is higher than the line center frequency. The amplitude of the jitter output increases with the frequency offset from the line center.
In lieu of using an IR detector to sense the laser intensity variation, the cavity length is adjusted using the optovoltaic effect. As the internal laser radiation field intensity is altered by changing the resonant cavity alignment, the discharge impedance, which is proportional to the slope of the curve of laser output power *versus* frequency, is also modulated. The impedance variation is determined by exciting the plasma tube with a high-speed currentregulated power supply and measuring the resulting variation in the voltage drop across the plasma tube (the optovoltaic effect). An intensity variation of 1% is sufficient to change the discharge impedance significantly (~ 0.1%). By using a current regulated power supply, the voltage impedance fluctuation is detected as an ac component of the voltage drop across the plasma tube.
Before any attempt is made to stabilize the frequency of a laser, a single frequency output must be ensured. For this purpose, a laser operating in the lowest transversal mode (TEM00) must be designed (Dumitras et al., 1976). The single line operation of the CO2 laser is achieved with a dispersive element (diffraction grating). The cavity length of *Lc* = 690 mm corresponds to a separation between two longitudinal modes of Δν = *c*/2*Lc* = 217 MHz. Calculating the collisional broadening in a mixture of CO2, N2, He, Xe, and H2 at a total pressure of 34 mbar gives a collisional full linewidth at half maximum (FWHM) Δν*<sup>c</sup>* = 119 MHz. We therefore conclude that a single frequency operation is obtained when a longitudinal mode is tuned on the top of the gain curve (Δν >Δν*<sup>c</sup>*/2).
To increase the number of oscillating lines, especially those with a smaller gain, and obtain reliable long term operation at a single specific wavelength, some form of wavelength selection introduced in the optical cavity is generally required. As optical dispersion is incorporated by using a diffraction grating or Brewster-angle prisms within the laser cavity, the laser can be made to oscillate on only one vibrational-rotational line, otherwise the particular transition on which the CO2 laser operates depends on the length of the resonator. That is why the total reflecting mirror must be replaced by a diffraction grating, which is tilted about its groove axis to the blaze angle and acts as a frequency selective reflector. Light diffracted into the first order maximum is returned along the optical axis and taken as laser output, while light in other orders as well as any other wavelength is returned off-axis and gets lost. Another advantage of a laser resonator with a grating is that the laser can be tuned over the entire oscillating linewidth from the line center.
We used a flat diffraction grating with 135 grooves/mm, blazed at 10.6 μm and having a peak efficiency of 96%, mounted in a Littrow configuration. With such a grating, the vibrational-rotational lines emitted by the CO2 laser in the range P(50) – 10.9329 μm and R(44) – 9.1549 μm can be selected by controlling the grating angle in the range 47o33'32'' to 38o10'02'', which can be set to the desired laser transition with a micrometric screw. This grating presents a good dispersion, as the P(18) and P(20) lines (10.4 μm band) are separated by a 6'38'' angular difference (as compared with 2'49'' for a diffraction grating with 75 grooves/mm).
CO2 Laser Photoacoustic Spectroscopy: II. Instrumentation and Applications 51
ruggedly. The mirrors can be adjusted in angle by sliding two stainless steel spherical joints with respect to one another. As the mirrors are adjusted into their final position, the adjustment
The diffraction grating is mounted in a similar holder and is rotated by a micrometric screw (0 – 25 mm), as shown in Fig. 6. The laser tube was rigidly mounted in the cavity with two concentric rings, the inner one being adjusted by three screws. This system allows the laser tube axis to fall into line with the mirror centers and provides good transverse stability.
To achieve active stabilization, an automatic frequency control circuit (lock-in stabilizer) is used to maintain an axial mode on the top of the gain curve. The block diagram of the frequency stabilization system based on plasma tube impedance variations is shown in Fig. 7. A sinusoidal signal with a rough frequency of 500 Hz derived from the pilot oscillator is applied to the piezoelectric transducer, resulting in a simultaneous frequency and amplitude
The ac voltage drop across the tube is passed through a tuned amplifier and then synchronously detected. The demodulation of the ac signal is performed in the phasesensitive detector, with phase (continuously adjustable) determined by the phase shift
modulation of the laser output, dependent on the amplitude of the sinusoidal signal.
screws clamp the mirror holder tightly so that no inadvertent movement is possible.
Fig. 5. Homebuilt frequency stabilized CO2 laser model LIR-25 SF.
Fig. 6. Diffraction grating-drive mechanism.
To meet the frequency stability requirements, the laser cavity must be so constructed as to reduce the effects of ambient vibration and thermal variations on the output frequency of the laser. This places less stringent demands on the performance of the servo-system controlling the laser frequency. To minimize the thermal length changes (Δ*L*/*L* = αΔ*T*) in the mirror support structure, a material with a low expansion coefficient has to be used for the spacers between the endplates of the cavity which carry the mirrors. Such a material is Invar, which has an expansion coefficient α = 1.26x10-6/oC. To obtain a passive instability of 3x10-8 for the laser frequency, the temperature variation must not exceed 0.024oC. Such constant temperature is hard to maintain, especially in longer lasers, where high power inputs and high heat dissipation cause large temperature instabilities.
Stiffness is a most desirable attribute for minimizing fractional changes in the cavity length. Special measures were taken against mechanical vibrations (by eliminating high frequency vibrations), variations in the position of optical components (by supporting rigidly any intracavity element) as well as to prevent magnetic fields and acoustically borne vibrations, which can be reduced by shielding the laser with some form of enclosure. The design of the remaining structure was chosen so as to avoid the lowering of the basic first resonance. The joints between the elements of the structure, especially the joints perpendicular to the laser axis, were so designed that they did not have any low frequency resonances. To have a spring constant of the joint high enough and to avoid joints using only the spring force of a few screws to connect significant masses, we used large contact areas under compressional stress. We chose a cylindrical shape for the mechanical structure, including the housing, because of its high resistance to bending deformation. A section through the laser cavity assembly is given in Fig. 4 and a photo of the mechanical structure, laser tube, and control panel is presented in Fig. 5.
Fig. 4. Longitudinal section through the laser cavity assembly.
To keep the weight unchanged and still maintain the required thermal characteristics, the endplates of the cavity which carry the mirror and the diffraction grating were primarily constructed of aluminum. The three invar rods are potted into the aluminum frame so as to ensure intimate contact between the invar rods and the rigid aluminum structure. To remove the problems associated with weak spring-type controls, the mirror holders were designed
To meet the frequency stability requirements, the laser cavity must be so constructed as to reduce the effects of ambient vibration and thermal variations on the output frequency of the laser. This places less stringent demands on the performance of the servo-system
mirror support structure, a material with a low expansion coefficient has to be used for the spacers between the endplates of the cavity which carry the mirrors. Such a material is
3x10-8 for the laser frequency, the temperature variation must not exceed 0.024oC. Such constant temperature is hard to maintain, especially in longer lasers, where high power
Stiffness is a most desirable attribute for minimizing fractional changes in the cavity length. Special measures were taken against mechanical vibrations (by eliminating high frequency vibrations), variations in the position of optical components (by supporting rigidly any intracavity element) as well as to prevent magnetic fields and acoustically borne vibrations, which can be reduced by shielding the laser with some form of enclosure. The design of the remaining structure was chosen so as to avoid the lowering of the basic first resonance. The joints between the elements of the structure, especially the joints perpendicular to the laser axis, were so designed that they did not have any low frequency resonances. To have a spring constant of the joint high enough and to avoid joints using only the spring force of a few screws to connect significant masses, we used large contact areas under compressional stress. We chose a cylindrical shape for the mechanical structure, including the housing, because of its high resistance to bending deformation. A section through the laser cavity assembly is given in Fig. 4 and a photo of the mechanical structure, laser tube, and control
α
= 1.26x10-6/oC. To obtain a passive instability of
Δ*T*) in the
controlling the laser frequency. To minimize the thermal length changes (Δ*L*/*L* =
inputs and high heat dissipation cause large temperature instabilities.
Fig. 4. Longitudinal section through the laser cavity assembly.
To keep the weight unchanged and still maintain the required thermal characteristics, the endplates of the cavity which carry the mirror and the diffraction grating were primarily constructed of aluminum. The three invar rods are potted into the aluminum frame so as to ensure intimate contact between the invar rods and the rigid aluminum structure. To remove the problems associated with weak spring-type controls, the mirror holders were designed
α
Invar, which has an expansion coefficient
panel is presented in Fig. 5.
ruggedly. The mirrors can be adjusted in angle by sliding two stainless steel spherical joints with respect to one another. As the mirrors are adjusted into their final position, the adjustment screws clamp the mirror holder tightly so that no inadvertent movement is possible.
Fig. 5. Homebuilt frequency stabilized CO2 laser model LIR-25 SF.
The diffraction grating is mounted in a similar holder and is rotated by a micrometric screw (0 – 25 mm), as shown in Fig. 6. The laser tube was rigidly mounted in the cavity with two concentric rings, the inner one being adjusted by three screws. This system allows the laser tube axis to fall into line with the mirror centers and provides good transverse stability.
Fig. 6. Diffraction grating-drive mechanism.
To achieve active stabilization, an automatic frequency control circuit (lock-in stabilizer) is used to maintain an axial mode on the top of the gain curve. The block diagram of the frequency stabilization system based on plasma tube impedance variations is shown in Fig. 7. A sinusoidal signal with a rough frequency of 500 Hz derived from the pilot oscillator is applied to the piezoelectric transducer, resulting in a simultaneous frequency and amplitude modulation of the laser output, dependent on the amplitude of the sinusoidal signal.
The ac voltage drop across the tube is passed through a tuned amplifier and then synchronously detected. The demodulation of the ac signal is performed in the phasesensitive detector, with phase (continuously adjustable) determined by the phase shift
CO2 Laser Photoacoustic Spectroscopy: II. Instrumentation and Applications 53
signal is introduced in the phase sensitive detector (PSD module) together with a phase
The error signal is obtained in the output of the electronic dc amplifier module DCA, by processing the output signal of the phase sensitive detector. The DCA module has two modes of operation, namely: (a) gain of ten amplifier with integrating time constant of 1 second; (b) high gain integrator with output slewing rate of about 6 V/s per volt of input. The first mode allows the observation of smoothed output of the demodulator (stabilization discriminator), while the second one is used in closed loop stabilization to observe the error signal. A high voltage amplifier (HVA) that works on the principle of a switching high voltage
The logic electronic system includes four specialized modules, namely: a low voltage stabilized power supply (SPS), a logic drive circuit (LDC), a power meter circuit for laser radiation (PMC), and a beam controller (BCS). Every parameter and state that are monitored or selected in the operation of the frequency stabilized CO2 laser LIR-25 SF are displayed on the rear panel of the laser (Fig. 5) by means of nine light emitting diodes (LED) driven by the panel display circuit (PDC). The output beam of the laser can be sent to either the powermeter head or the exit by a two-position reflecting mirror, controlled by a beam
power supply drives the piezoelectric transducer under the control of the error signal.
ν/ν
POWERMETER CO2 LASER
A supplementary power stabilization is possible by using an additional feedback loop (Dumitras et al., 2006). The principle used for power stabilization took into account the dependence of the output power on discharge current. The new feedback loop (Fig. 8) modifies the discharge current so to maintain the output power at a constant value imposed
The performance of the two feedback loops used for frequency and power stabilization of the CO2 laser were evaluated by using computer data acquisition. It was monitored simultaneously the output power, the discharge current and the temperature inside laser cavity, while the power instability was calculated for a period of 2 minutes. The importance of the two feedback loops to reduce the output power instability can be remarked from these measurements. Thus, when both the frequency stabilization loop and the power
COMPARATOR INTEGRATING AMPLIFIER
= 3x10-8 or Δ
ν
= 1 MHz (3x10-5 cm-1).
CURRENT CONTROLLED HV POWER SUPPLY
shutter circuit and a beam selector switch placed on the rear panel.
Fig. 8. Supplementary feedback loop used for laser power stabilization.
The long-term frequency instability was Δ
LASER
by the reference.
adjustable component of the reference signal from the modulation oscillator.
circuit. The demodulator output is processed by the operational integrator and a high voltage dc amplifier. The dc bias together with this error correction signal is applied to the piezoelectric transducer, thus closing the feedback loop. The error signal generated by the phase detector serves to drive the cavity resonance to the center of the laser gain curve and compensates for the effects of slow drift.
Fig. 7. Block-diagram of the automatic frequency control electronics.
There are fifteen functional modules grouped into three main blocks in accordance with the specific function they play in the operation of the frequency stabilized CO2 laser:
The high-speed current regulated power supply includes three modules namely: the power supply converter – PSC, the power supply feedback – PSF, and the power supply rectifier PSR. The high frequency of the converter makes it possible to minimize the electronic components that are used in making the high voltage transformer and the low pass filter of the high voltage rectifier.
The modulation signal that is needed to scan the laser line profile is generated by a pilot oscillator and applied to the piezoelectric transducer through a modulation amplifier. Both circuits are placed on the same functional module OMA (Oscillator & Modulation Amplifier). For cw CO2 lasers excited by current regulated power supplies, the modulation signal which appears in the emitted radiation can be measured according to the optovoltaic effect using a simple band pass filter like that noted with OVP (Optovoltaic Probe). The detected optovoltaic signal is amplified through a two-stage tuned ac amplifier – ACA. The amplified optovoltaic
circuit. The demodulator output is processed by the operational integrator and a high voltage dc amplifier. The dc bias together with this error correction signal is applied to the piezoelectric transducer, thus closing the feedback loop. The error signal generated by the phase detector serves to drive the cavity resonance to the center of the laser gain curve and
compensates for the effects of slow drift.
Fig. 7. Block-diagram of the automatic frequency control electronics.
the frequency stabilized CO2 operation.
the high voltage rectifier.
There are fifteen functional modules grouped into three main blocks in accordance with the
3. a logic electronic system which allows an efficient operative control and monitoring of
The high-speed current regulated power supply includes three modules namely: the power supply converter – PSC, the power supply feedback – PSF, and the power supply rectifier PSR. The high frequency of the converter makes it possible to minimize the electronic components that are used in making the high voltage transformer and the low pass filter of
The modulation signal that is needed to scan the laser line profile is generated by a pilot oscillator and applied to the piezoelectric transducer through a modulation amplifier. Both circuits are placed on the same functional module OMA (Oscillator & Modulation Amplifier). For cw CO2 lasers excited by current regulated power supplies, the modulation signal which appears in the emitted radiation can be measured according to the optovoltaic effect using a simple band pass filter like that noted with OVP (Optovoltaic Probe). The detected optovoltaic signal is amplified through a two-stage tuned ac amplifier – ACA. The amplified optovoltaic
specific function they play in the operation of the frequency stabilized CO2 laser:
2. a servo control system, which controls the length of the resonant cavity;
1. a high speed current regulated power supply, which excites the cw CO2 laser tube;
signal is introduced in the phase sensitive detector (PSD module) together with a phase adjustable component of the reference signal from the modulation oscillator.
The error signal is obtained in the output of the electronic dc amplifier module DCA, by processing the output signal of the phase sensitive detector. The DCA module has two modes of operation, namely: (a) gain of ten amplifier with integrating time constant of 1 second; (b) high gain integrator with output slewing rate of about 6 V/s per volt of input. The first mode allows the observation of smoothed output of the demodulator (stabilization discriminator), while the second one is used in closed loop stabilization to observe the error signal. A high voltage amplifier (HVA) that works on the principle of a switching high voltage power supply drives the piezoelectric transducer under the control of the error signal.
The logic electronic system includes four specialized modules, namely: a low voltage stabilized power supply (SPS), a logic drive circuit (LDC), a power meter circuit for laser radiation (PMC), and a beam controller (BCS). Every parameter and state that are monitored or selected in the operation of the frequency stabilized CO2 laser LIR-25 SF are displayed on the rear panel of the laser (Fig. 5) by means of nine light emitting diodes (LED) driven by the panel display circuit (PDC). The output beam of the laser can be sent to either the powermeter head or the exit by a two-position reflecting mirror, controlled by a beam shutter circuit and a beam selector switch placed on the rear panel.
The long-term frequency instability was Δν/ν = 3x10-8 or Δν= 1 MHz (3x10-5 cm-1).
A supplementary power stabilization is possible by using an additional feedback loop (Dumitras et al., 2006). The principle used for power stabilization took into account the dependence of the output power on discharge current. The new feedback loop (Fig. 8) modifies the discharge current so to maintain the output power at a constant value imposed by the reference.
Fig. 8. Supplementary feedback loop used for laser power stabilization.
The performance of the two feedback loops used for frequency and power stabilization of the CO2 laser were evaluated by using computer data acquisition. It was monitored simultaneously the output power, the discharge current and the temperature inside laser cavity, while the power instability was calculated for a period of 2 minutes. The importance of the two feedback loops to reduce the output power instability can be remarked from these measurements. Thus, when both the frequency stabilization loop and the power
CO2 Laser Photoacoustic Spectroscopy: II. Instrumentation and Applications 55
To investigate the possibility of using a high power laser in an extracavity configuration, we introduced in the experimental set-up a commercial CO2 laser (Coherent GEM SELECT 50TM laser) (Fig. 10) with output power till 50 W and tunable on 73 different lines (Fig. 11). When this laser is tuned on 10P(14) line, the maximum power delivered after chopper and
Fig. 11. Tunability of the high power CO2 laser with diffraction grating: P(max) = 50 W; Tunability: 9R(8) – 9R(40); 9P(4) – 9P(46); 10R(6) – 10R(36); 10P(6) – 10P(40); No. of lines: 73
9.2 9.4 9.6 9.8 10.0 10.2 10.4 10.6 10.8 11.0
10R(24) 10R(30) 10R(28) 10R(34) 10R(36)
10R(18) 10R(16) 10R(12)
10R(10)
10R(8)
10R(6)
10P(6)
10P(8) 10P(10) 10P(12)
10P(14) 10P(16) 10P(18)
10P(22)
10P(26)
10P(28) 10P(30) 10P(32)
10P(34)
10P(36) 10P(38)
10P(40)
Wavelength (μm)
In the literature the PA cells are often characterized as "nonresonant" or "resonant". This terminology is misleading, because any PA cell can be operated at an acoustic resonance or far from its resonance. Thus, it is preferable to label the system in terms of its nonresonant or
To design an optimum acoustically resonant PA cell to be used in CO2 laser photoacoustic
i. the fraction of laser energy absorbed by the gas must be maximized by increasing either the incident laser power (but maintaining a large SNR) or the optical density of the gas
ii. cell responsivity needs to be as high as possible, because the voltage response is
iii. the microphone responsivity has to be as high as possible, and the use of many
iv. the design must make it possible to operate the cell at an acoustic resonance, and the resonance frequency must lie between 400 and 1000 Hz, where the microphone noise is
v. the quality factor *Q* of the acoustic resonance must not exceed 50 in order to decrease
focusing lens is 14.5 W.
**COHERENT**
9P(22)
9P(12)
9P(10)
9P(6)
9P(4)
9R(12) 9R(10)
9R(14) 9R(18)
9R(20)9R(24)
9R(30) 9R(32) 9R(36)
9R(40)
9R(8)
9P(16)
9P(26)
9P(28)
9P(34)
9P(36) 9P(38) 9P(40) 9P(42) 9P(44)
(10P(14):
0
10
20
30
Power (W)
40
50
λ
**2.3 Photoacoustic cell**
resonant mode of operation.
(Eq. 2, Part I);
minimal;
= 10.53 µm; 9R(30):
proportional to it (Eq. 29, Part I);
λ
spectroscopy, the following requirements have to be met:
microphones is advisable (Eqs. 30 and 31, Part I);
the influence of small deviations from the resonance frequency;
= 9.22 µm).
9P(46)
stabilization loop are opened and forced hot air perturbation is introduced, the power instability is 10.2%, a value too large for many applications; when the frequency stabilization loop is closed and the power stabilization loop is opened (no perturbation), the power instability is reduced to 1.06%; when the frequency stabilization loop is closed and the power stabilization loop is opened and forced hot air perturbation is introduced into laser cavity, the power instability increases only a little, to 1.57%; and when both the frequency stabilization loop and the power stabilization loop are closed and forced hot air perturbation is introduced, the power instability is reduced drastically, to 0.28%. If there is no perturbation and the two feedback loops are closed, the power instability is only 0.23%. In conclusion, the power instability was reduced by four times with this supplementary feedback loop, from 1% to as low as 0.23% for a period of 2 minutes.
The tunability of our CO2 laser is presented in Fig. 9. We observed the oscillation of 62 different vibrational-rotational lines in both the 10.4 μm and 9.4 μm bands. In this way, the laser was line tunable between 9.2 μm and 10.8 μm with powers varying between 1 and 6.5 W depending on the emitted laser transition. More than 20 lines had output powers in excess of 5 W.
Fig. 9. Tunability of the low power CO2 laser with diffraction grating: P(max) = 6.5 W; Tunability: 9R(8) – 9R(34); 9P(8) – 9P(36); 10R(6) – 10R(36); 10P(6) – 10P(40); No. of lines: 62.
Fig. 10. Coherent GEM SELECT 50TM CO2 laser in the experimental setup.
To investigate the possibility of using a high power laser in an extracavity configuration, we introduced in the experimental set-up a commercial CO2 laser (Coherent GEM SELECT 50TM laser) (Fig. 10) with output power till 50 W and tunable on 73 different lines (Fig. 11). When this laser is tuned on 10P(14) line, the maximum power delivered after chopper and focusing lens is 14.5 W.
Fig. 11. Tunability of the high power CO2 laser with diffraction grating: P(max) = 50 W; Tunability: 9R(8) – 9R(40); 9P(4) – 9P(46); 10R(6) – 10R(36); 10P(6) – 10P(40); No. of lines: 73 (10P(14): λ = 10.53 µm; 9R(30): λ= 9.22 µm).
#### **2.3 Photoacoustic cell**
54 CO2 Laser – Optimisation and Application
stabilization loop are opened and forced hot air perturbation is introduced, the power instability is 10.2%, a value too large for many applications; when the frequency stabilization loop is closed and the power stabilization loop is opened (no perturbation), the power instability is reduced to 1.06%; when the frequency stabilization loop is closed and the power stabilization loop is opened and forced hot air perturbation is introduced into laser cavity, the power instability increases only a little, to 1.57%; and when both the frequency stabilization loop and the power stabilization loop are closed and forced hot air perturbation is introduced, the power instability is reduced drastically, to 0.28%. If there is no perturbation and the two feedback loops are closed, the power instability is only 0.23%. In conclusion, the power instability was reduced by four times with this supplementary
The tunability of our CO2 laser is presented in Fig. 9. We observed the oscillation of 62 different vibrational-rotational lines in both the 10.4 μm and 9.4 μm bands. In this way, the laser was line tunable between 9.2 μm and 10.8 μm with powers varying between 1 and 6.5 W depending on
the emitted laser transition. More than 20 lines had output powers in excess of 5 W.
Fig. 9. Tunability of the low power CO2 laser with diffraction grating: P(max) = 6.5 W; Tunability: 9R(8) – 9R(34); 9P(8) – 9P(36); 10R(6) – 10R(36); 10P(6) – 10P(40); No. of lines: 62.
9.2 9.4 9.6 9.8 10.0 10.2 10.4 10.6 10.8
Wavelength (μm)
10P(30)
10P(26)
10P(20)
10P(14)
10P(10)
10P(8)
10P(6)
10R(6)
10R(8) 10R(10)
10R(14)
10R(18)
10R(24)
10R(28)
10R(32)
10R(34)
10R(36)
10P(38)
10P(36)
10P(34) 10P(32)
Fig. 10. Coherent GEM SELECT 50TM CO2 laser in the experimental setup.
feedback loop, from 1% to as low as 0.23% for a period of 2 minutes.
9P(36) 9P(34)
9P(32)
9P(28)
9P(22)
9P(16)
9P(12)
No. 2 **LIR 25 SF**
9R(12)
9R(16)
9R(24)
9R(26)
9R(28)
9R(10)
9R(8)
9R(30)
9R(34)
9R(32)
9P(8)
Power (W)
In the literature the PA cells are often characterized as "nonresonant" or "resonant". This terminology is misleading, because any PA cell can be operated at an acoustic resonance or far from its resonance. Thus, it is preferable to label the system in terms of its nonresonant or resonant mode of operation.
To design an optimum acoustically resonant PA cell to be used in CO2 laser photoacoustic spectroscopy, the following requirements have to be met:
CO2 Laser Photoacoustic Spectroscopy: II. Instrumentation and Applications 57
Following these guidelines, a PA cell was designed, constructed, and tested. An H-type cylindrical cell designed for resonant photoacoustic spectroscopy in gases is shown in Fig. 12. The longitudinal resonant cell is a cylinder with microphones located at the loop position of the first longitudinal mode (the maximum pressure amplitude). Some general considerations imply that the coherent photoacoustic background signal caused by window heating is decreased if the beam enters the cell at the pressure nodes of the resonance. The advantage of mounting the windows at the pressure nodes is well demonstrated, and the window heating signal is decreased by the *Q* factor. The laser beam enters and exits the cell at the Brewster angle. It is more advantageous to have the beam pass through the windows
Fig. 12. Schematic of the PA cell designed for the first longitudinal resonance mode.
reflection, but also to obtain a noncorrosive surface to withstand aggressive gases.
The influence of scattered light onto the PA background signal can be minimized by using a highly reflecting polished material, with a good thermally conducting substrate. Bijnen et al. (Bijnen et al., 1996) investigated different materials for the resonant tube and found that the background signal decreased for polished stainless steel, polished brass, and polished, goldcoated copper in a ratio of 6:2:1, respectively. In the case of the CO2 laser, the best performance was obtained by employing a copper tube with a polished gold coating as resonator material. Because of the excellent heat-conducting properties, the absorbed heat can be quickly dispersed in the copper tube. The gold coating was used not only to optimize laser radiation
Many polar compounds (e.g. ammonia) are highly adsorptive and produce an error in real time concentration measurements by adhering to the detector surfaces. These molecules interact strongly with most metals and many insulating materials. Ammonia is a good model compound for these molecules as it shows the characteristic adsorptive behavior that is not a health hazard at low concentrations. The rate of ammonia adsorption on the gas handling surfaces depends on the surface material and temperature, and on the mixture concentration, flow rate, and pressure. Comparing the ammonia results with those for ethylene, which interacts weakly with most surfaces, provides a measure of the cell-sample interaction. Beck (Beck, 1985) evaluated the suitability of several surface materials for minimizing sample adsorption loss. Four materials–304 stainless steel, gold, paraffin wax, and Teflon–were tested using ammonia as a sample. The results show that both metals interact strongly with the sample. Teflon coating (thickness <25 μm) was found to provide accurate real time response for ammonia sample flows. Also, no signal decay is observed following flow termination. Additionally, the coatings must not degrade the acoustic response of the cell. The Teflon coating actually increases the cell *Q* by a small amount (1 percent). This is attributed to the smooth slick surface obtained by Teflon coating which
*<sup>B</sup>* is nearly constant over a wide range of wavelengths, and
*<sup>B</sup>* with wavelength can be tolerated since reflectivity increases very slowly for
at the Brewster angle (
small deviations from
θ
variations of
θ*B*), as θ
θ*B*.
Various ways to design (cylindrical geometry, H geometry, T geometry, or using a Helmholtz resonator) and operate (longitudinal, azimuthal, radial, or Helmholtz resonances) resonant PA cells have been studied (Zharov & Letokhov, 1986). Furthermore, PA cells for multipass (Koch & Lahmann, 1978; Nägele & Sigrist, 2000) or intracavity operation (Fung & Lin, 1986; Harren et al., 1990a) were designed. The effect of window heating in the amplitude modulation schemes has been minimized by introducing acoustic baffles (Dewey, 1977), developing windowless cells (Gerlach & Amer, 1980; Miklos & Lörincz, 1989; Angeli et al., 1992), or using tunable air columns (Bijnen et al., 1996). In many cases the window-heating signal can be markedly reduced by positioning the entrance and exit of the light beam at nodes of the mode being excited.
A cylindrical cell operated at a radial resonance and having Brewster windows mounted at the pressure nodes of the first radial mode, as presented by Gerlach and Amer (Gerlach & Amer, 1980), does not fulfill all these requirements. Therefore, an open resonant cell excited in its first longitudinal acoustic mode was developed to fulfill most of these requirements.
The H-type longitudinally resonant cell was chosen to form the core of our measuring instrument. Dividing the PA cell into a central chamber and two buffer chambers adjacent to the Brewster windows, a design which lowered significantly the coherent photoacoustic background noise, was first proposed by Tonelli et al. (Tonelli et al., 1983). The characteristics of this type of PA cell have been discussed by Nodov (Nodov, 1978), Kritchman et al. (Kritchman et al., 1978), and Harren et al. (Harren et al., 1990a). Its main advantages are: (a) stable operation at a relatively low frequency; a quality factor of about 20, i.e., much lower than that of a radial resonator, which makes it less sensitive to environmental changes; the efficient conversion of radial to longitudinal modes and the relatively long wavelength guarantee a sufficiently high photoacoustic amplitude; (b) a longitudinal resonator is not noticeably influenced by the gas flow at the desired flow rate of several L/h; noise by gas flow phenomena is negligible for properly positioned inlet and outlet ports; (c) window noise is minimal if the windows are located at a quarter wavelength from the ends of the resonator tube; (d) the construction is rugged and simple and can be achieved with low adhesion materials.
vi. the electrical noise and the coherent acoustic background noise must be as low as possible; this can be done by using low noise microphones, good acoustic and vibration isolation, low noise electronics, and good electronic isolation (no ground loops, proper shielding); vii. the coherent photoacoustic background signal due to the heating of the walls and windows must be minimized by using optical components of very high quality and
viii. the cell must enable continuous gas flow operation, and consequently not only the cell windows, but also the gas inlets and outlets have to be positioned at pressure nodes of
ix. the cell must have low gas consumption and fast response, and the cell volume has to be sufficiently small to prevent prohibitive dilution when the produced trace gas is
x. the adsorption and desorption rates on the surfaces in direct contact with the sample gas that can influence particularly measurements on sealed-off samples must be minimized by using special cell materials and reducing the surface-to-volume ratio; xi. the effect of the loss mechanisms which we can control must be minimized by an
Various ways to design (cylindrical geometry, H geometry, T geometry, or using a Helmholtz resonator) and operate (longitudinal, azimuthal, radial, or Helmholtz resonances) resonant PA cells have been studied (Zharov & Letokhov, 1986). Furthermore, PA cells for multipass (Koch & Lahmann, 1978; Nägele & Sigrist, 2000) or intracavity operation (Fung & Lin, 1986; Harren et al., 1990a) were designed. The effect of window heating in the amplitude modulation schemes has been minimized by introducing acoustic baffles (Dewey, 1977), developing windowless cells (Gerlach & Amer, 1980; Miklos & Lörincz, 1989; Angeli et al., 1992), or using tunable air columns (Bijnen et al., 1996). In many cases the window-heating signal can be markedly reduced by positioning the entrance and
A cylindrical cell operated at a radial resonance and having Brewster windows mounted at the pressure nodes of the first radial mode, as presented by Gerlach and Amer (Gerlach & Amer, 1980), does not fulfill all these requirements. Therefore, an open resonant cell excited in its first longitudinal acoustic mode was developed to fulfill most of these requirements. The H-type longitudinally resonant cell was chosen to form the core of our measuring instrument. Dividing the PA cell into a central chamber and two buffer chambers adjacent to the Brewster windows, a design which lowered significantly the coherent photoacoustic background noise, was first proposed by Tonelli et al. (Tonelli et al., 1983). The characteristics of this type of PA cell have been discussed by Nodov (Nodov, 1978), Kritchman et al. (Kritchman et al., 1978), and Harren et al. (Harren et al., 1990a). Its main advantages are: (a) stable operation at a relatively low frequency; a quality factor of about 20, i.e., much lower than that of a radial resonator, which makes it less sensitive to environmental changes; the efficient conversion of radial to longitudinal modes and the relatively long wavelength guarantee a sufficiently high photoacoustic amplitude; (b) a longitudinal resonator is not noticeably influenced by the gas flow at the desired flow rate of several L/h; noise by gas flow phenomena is negligible for properly positioned inlet and outlet ports; (c) window noise is minimal if the windows are located at a quarter wavelength from the ends of the resonator tube; (d) the construction is rugged and simple and can be
flowed through the cell volume by a continuous gas stream;
exit of the light beam at nodes of the mode being excited.
introducing acoustic baffles;
appropriate system design.
achieved with low adhesion materials.
the resonance;
Following these guidelines, a PA cell was designed, constructed, and tested. An H-type cylindrical cell designed for resonant photoacoustic spectroscopy in gases is shown in Fig. 12. The longitudinal resonant cell is a cylinder with microphones located at the loop position of the first longitudinal mode (the maximum pressure amplitude). Some general considerations imply that the coherent photoacoustic background signal caused by window heating is decreased if the beam enters the cell at the pressure nodes of the resonance. The advantage of mounting the windows at the pressure nodes is well demonstrated, and the window heating signal is decreased by the *Q* factor. The laser beam enters and exits the cell at the Brewster angle. It is more advantageous to have the beam pass through the windows at the Brewster angle (θ*B*), as θ*<sup>B</sup>* is nearly constant over a wide range of wavelengths, and variations of θ*<sup>B</sup>* with wavelength can be tolerated since reflectivity increases very slowly for small deviations from θ*B*.
Fig. 12. Schematic of the PA cell designed for the first longitudinal resonance mode.
The influence of scattered light onto the PA background signal can be minimized by using a highly reflecting polished material, with a good thermally conducting substrate. Bijnen et al. (Bijnen et al., 1996) investigated different materials for the resonant tube and found that the background signal decreased for polished stainless steel, polished brass, and polished, goldcoated copper in a ratio of 6:2:1, respectively. In the case of the CO2 laser, the best performance was obtained by employing a copper tube with a polished gold coating as resonator material. Because of the excellent heat-conducting properties, the absorbed heat can be quickly dispersed in the copper tube. The gold coating was used not only to optimize laser radiation reflection, but also to obtain a noncorrosive surface to withstand aggressive gases.
Many polar compounds (e.g. ammonia) are highly adsorptive and produce an error in real time concentration measurements by adhering to the detector surfaces. These molecules interact strongly with most metals and many insulating materials. Ammonia is a good model compound for these molecules as it shows the characteristic adsorptive behavior that is not a health hazard at low concentrations. The rate of ammonia adsorption on the gas handling surfaces depends on the surface material and temperature, and on the mixture concentration, flow rate, and pressure. Comparing the ammonia results with those for ethylene, which interacts weakly with most surfaces, provides a measure of the cell-sample interaction. Beck (Beck, 1985) evaluated the suitability of several surface materials for minimizing sample adsorption loss. Four materials–304 stainless steel, gold, paraffin wax, and Teflon–were tested using ammonia as a sample. The results show that both metals interact strongly with the sample. Teflon coating (thickness <25 μm) was found to provide accurate real time response for ammonia sample flows. Also, no signal decay is observed following flow termination. Additionally, the coatings must not degrade the acoustic response of the cell. The Teflon coating actually increases the cell *Q* by a small amount (1 percent). This is attributed to the smooth slick surface obtained by Teflon coating which
CO2 Laser Photoacoustic Spectroscopy: II. Instrumentation and Applications 59
large for optimal operation. A practical radius can be deduced to be *rbuf* ≈ 3*r*. The gas absorption amplitude is barely influenced above this value. A small radius for the resonator of the resonant cell is advantageous as it enhances gas absorption. The limit for the resonator radius is mainly determined by the wings of the gaussian laser beam profile hitting the wall; *r* should roughly be at least three times the radius of the laser beam to
Our PA cell design, based on the above considerations, is shown in Fig. 13 in cross sectional longitudinal view. The PA cell is made of stainless steel and Teflon to reduce the outgassing problems and consists of an acoustic resonator (pipe), windows, gas inlets and outlets, and microphones. It also contains an acoustic filter to suppress the flow and window noise. ZnSe windows at the Brewster angle are glued with epoxy (Torr-Seal) to their respective mounts. The resonant conditions are obtained as longitudinal standing waves in an open tube (resonator) are placed coaxially inside a larger chamber. We use an open end tube type of resonator, excited in its first longitudinal mode. To achieve a larger signal (eqs. 26 and 27, Part I), we chose a long absorption path length (*L* = 300 mm) and an inner diameter of the pipe of 2*r* = 7 mm (*r* ≅ 1.5*w*5, see Fig. 2). The fundamental longitudinal wave, therefore, has a
wavelength is ∼1% longer than the nominal value, in accordance with predictions of Eq. (5, Part I). The two buffer volumes placed near the Brewster windows have a length *Lbuf* = 75
The total cell volume is approximately 1.0 dm3 (total length 450 mm, and inner diameter 57 mm, minus inner mechanical parts). For flowing conditions, however, it is advantageous to reduce the active volume of the cell. Especially if the flow rate is smaller than 1 L/h (16.6
/8) and a diameter 2*rbuf* = 57 mm (*rbuf* ≅ 8*r*). The inner wall of the stainless steel resonator tube is polished. It is centered inside the outer stainless steel tube with Teflon spacers. A massive spacer is positioned at one end to prevent bypassing of gas in the flow system; the other is partially open to avoid the formation of closed volumes. Gas is admitted and exhausted through two ports located near the ends of the resonator tube. The perturbation of the acoustic resonator amplitude by the gas flow noise is thus minimized.
*<sup>s</sup>* = 2*L* = 600 mm (and a resonance frequency *f*0 = 564 Hz). The effective
reduce wall absorption to an acceptable level.
λ
Fig. 13. A resonant photoacoustic cell with buffer volumes.
nominal wavelength
mm (λ
would decrease any surface frictional or scattering loss of acoustic energy. Rooth et al. (Rooth et al., 1990) tested the following wall materials – stainless steel 304, gold (on Nicoated stainless steel), Teflon PTFE, and Teflon PFA – in contact with the gas. Stainless steel proved to be an almost unsaturable reservoir for ammonia at pptV levels. The number of stored molecules exceeded by a factor of 10 or more the number of potential locations on the total geometric surface. Despite its inferior properties in terms of adsorption, Olafsson et al. (Olafsson et al., 1989) used a stainless steel cell for detecting NH3 and found that an operating temperature of 100oC combined with water vapors led to a very significant reduction of NH3 adsorption. Apparently, the water molecules stick to the walls even more efficiently than NH3, and the cell walls are effectively coated with water. Later on, the sample cell was constructed with Teflon as wall material (Olafsson et al., 1992).
Since an open pipe efficiently picks up and amplifies noise from the environment, it should be surrounded by an enclosure. In order to ensure high acoustic reflections at the pipe ends, a sudden change of the cross section is necessary. Therefore, the resonator pipe should open up into a larger volume or to buffers with a much larger cross section. The buffers can be optimized to minimize flow noise and/or window signals. The length of the two buffers accounting for half the resonator length is chosen such as to minimize the acoustic background signal originating from absorption by the ZnSe windows. Open pipes were introduced for PA detection as early as 1977 (Zharov & Letokhov, 1986), and the most sensitive PA detectors currently used are based on open resonant pipes. In resonant cells, window signals can be diminished by using λ/4 buffers next to the windows. These buffers, placed perpendicular to the resonator axis near the windows, are tuned to the resonator frequency and act as interference filters for the window signals (the coupling of the window signals into the resonator is reduced by large buffer volumes that act as interference dampers).
It was found both theoretically and experimentally that the signal amplitude decreases drastically when buffer length *Lbuf* < λ/8 (Bijnen et al., 1996); the resonance frequency and the quality factor, for both the window and gas signals, are not much affected by changing the buffer length. The length of the buffer is optimal for window signal suppression when *L* = 2*Lbuf* (*L* >> *r*) (*L* and *r* stand for resonator length and radius). The dependence of the gas absorption *pg* and window absorption pressure *pw* on the ratio of the buffer and resonator radii is:
$$p\_s \approx \frac{\sqrt{L}}{r} \left(1 - \frac{r^2}{r\_{buf}^2}\right) \tag{1}$$
$$p\_w \approx \frac{r}{r\_{bw}\sqrt{L}} \,\tag{2}$$
The ratio between the gas absorption signal and window signal then becomes:
$$\frac{p\_g}{p\_w} \approx \left(\frac{r\_{bw}}{r}\right)^2 L. \tag{3}$$
The optimal buffer length, resulting in an optimal suppression of the photoacoustic background signal, is λ/4. Choosing a buffer length of λ/8 has the advantage of a shorter cell, and a good, though not optimal, suppression of the window signal is still possible in this case. If the volume and overall size of the buffers pose no problem, their radii have to be
would decrease any surface frictional or scattering loss of acoustic energy. Rooth et al. (Rooth et al., 1990) tested the following wall materials – stainless steel 304, gold (on Nicoated stainless steel), Teflon PTFE, and Teflon PFA – in contact with the gas. Stainless steel proved to be an almost unsaturable reservoir for ammonia at pptV levels. The number of stored molecules exceeded by a factor of 10 or more the number of potential locations on the total geometric surface. Despite its inferior properties in terms of adsorption, Olafsson et al. (Olafsson et al., 1989) used a stainless steel cell for detecting NH3 and found that an operating temperature of 100oC combined with water vapors led to a very significant reduction of NH3 adsorption. Apparently, the water molecules stick to the walls even more efficiently than NH3, and the cell walls are effectively coated with water. Later on, the
Since an open pipe efficiently picks up and amplifies noise from the environment, it should be surrounded by an enclosure. In order to ensure high acoustic reflections at the pipe ends, a sudden change of the cross section is necessary. Therefore, the resonator pipe should open up into a larger volume or to buffers with a much larger cross section. The buffers can be optimized to minimize flow noise and/or window signals. The length of the two buffers accounting for half the resonator length is chosen such as to minimize the acoustic background signal originating from absorption by the ZnSe windows. Open pipes were introduced for PA detection as early as 1977 (Zharov & Letokhov, 1986), and the most sensitive PA detectors currently used are based on open resonant pipes. In resonant cells,
λ
placed perpendicular to the resonator axis near the windows, are tuned to the resonator frequency and act as interference filters for the window signals (the coupling of the window signals into the resonator is reduced by large buffer volumes that act as interference dampers). It was found both theoretically and experimentally that the signal amplitude decreases
quality factor, for both the window and gas signals, are not much affected by changing the buffer length. The length of the buffer is optimal for window signal suppression when *L* = 2*Lbuf* (*L* >> *r*) (*L* and *r* stand for resonator length and radius). The dependence of the gas absorption
<sup>2</sup> 1 *<sup>g</sup>*
*L r*
*r r* ∝ −
> *buf r*
*g buf*
*p r <sup>L</sup> p r* <sup>∝</sup>
The optimal buffer length, resulting in an optimal suppression of the photoacoustic
cell, and a good, though not optimal, suppression of the window signal is still possible in this case. If the volume and overall size of the buffers pose no problem, their radii have to be
2
*buf*
2
λ
*pg* and window absorption pressure *pw* on the ratio of the buffer and resonator radii is:
*w*
The ratio between the gas absorption signal and window signal then becomes:
*w*
/4. Choosing a buffer length of
/4 buffers next to the windows. These buffers,
, (1)
. (3)
/8 has the advantage of a shorter
/8 (Bijnen et al., 1996); the resonance frequency and the
*<sup>p</sup> r L* <sup>∝</sup> . (2)
sample cell was constructed with Teflon as wall material (Olafsson et al., 1992).
λ
*p*
window signals can be diminished by using
drastically when buffer length *Lbuf* <
background signal, is
λ
large for optimal operation. A practical radius can be deduced to be *rbuf* ≈ 3*r*. The gas absorption amplitude is barely influenced above this value. A small radius for the resonator of the resonant cell is advantageous as it enhances gas absorption. The limit for the resonator radius is mainly determined by the wings of the gaussian laser beam profile hitting the wall; *r* should roughly be at least three times the radius of the laser beam to reduce wall absorption to an acceptable level.
Our PA cell design, based on the above considerations, is shown in Fig. 13 in cross sectional longitudinal view. The PA cell is made of stainless steel and Teflon to reduce the outgassing problems and consists of an acoustic resonator (pipe), windows, gas inlets and outlets, and microphones. It also contains an acoustic filter to suppress the flow and window noise. ZnSe windows at the Brewster angle are glued with epoxy (Torr-Seal) to their respective mounts. The resonant conditions are obtained as longitudinal standing waves in an open tube (resonator) are placed coaxially inside a larger chamber. We use an open end tube type of resonator, excited in its first longitudinal mode. To achieve a larger signal (eqs. 26 and 27, Part I), we chose a long absorption path length (*L* = 300 mm) and an inner diameter of the pipe of 2*r* = 7 mm (*r* ≅ 1.5*w*5, see Fig. 2). The fundamental longitudinal wave, therefore, has a nominal wavelength λ*<sup>s</sup>* = 2*L* = 600 mm (and a resonance frequency *f*0 = 564 Hz). The effective wavelength is ∼1% longer than the nominal value, in accordance with predictions of Eq. (5, Part I). The two buffer volumes placed near the Brewster windows have a length *Lbuf* = 75 mm (λ/8) and a diameter 2*rbuf* = 57 mm (*rbuf* ≅ 8*r*). The inner wall of the stainless steel resonator tube is polished. It is centered inside the outer stainless steel tube with Teflon spacers. A massive spacer is positioned at one end to prevent bypassing of gas in the flow system; the other is partially open to avoid the formation of closed volumes. Gas is admitted and exhausted through two ports located near the ends of the resonator tube. The perturbation of the acoustic resonator amplitude by the gas flow noise is thus minimized.
Fig. 13. A resonant photoacoustic cell with buffer volumes.
The total cell volume is approximately 1.0 dm3 (total length 450 mm, and inner diameter 57 mm, minus inner mechanical parts). For flowing conditions, however, it is advantageous to reduce the active volume of the cell. Especially if the flow rate is smaller than 1 L/h (16.6
CO2 Laser Photoacoustic Spectroscopy: II. Instrumentation and Applications 61
frequency of the laser modulation or the cell resonance itself were to shift by a few hertz. PA cells exhibiting narrow resonances (as in the case of excitation of the radial modes) require tight control of both temperature and laser modulation frequency to avoid responsivity
9 Cell 2
547 582
564
The acoustic resonator is characterized by the quality factor *Q*, which is defined as the ratio of the resonance frequency to the frequency bandwidth between half power points. The amplitude of the microphone signal is 1/ 2 of the maximum amplitude at these points, because the energy of the standing wave is proportional to the square of the induced pressure. The quality factor was measured by filling the PA cell with 1 ppmV of ethylene buffered in nitrogen at a total pressure of 1 atm and by tuning the modulation frequency in 10 Hz increments (2 Hz increments near the top of the curve) across the resonance profile to estimate the half width, as described above. For this PA cell, the profile width at half intensity was 35 Hz, yielding a quality factor *Q* = 16.1 (Eq. 15, Part I) at a resonance frequency *f*0 = 564 Hz. The experimentally determined resonance is not completely symmetric, as the curve rises steeply on one side and becomes less steep on the other side of the maximum. This asymmetry is caused by a coherent superposition of the standing
500 520 540 560 580 600 620 640 660
Chopper frequency *f* (Hz)
acoustic waves in the detection region of the microphones (Karbach & Hess, 1985).
ν
The calibration of the PA system is usually performed with a reference gas. We calibrated our PA cell with the widely used reference gas ethylene, whose absorption coefficients are accurately known at CO2-laser wavelengths. Ethylene is well suited for this purpose, since it interacts only weakly with common cell surface materials. Ethylene is chemically inert, has the same molecular weight as nitrogen and possesses no permanent dipole moment which means negligible adsorption on the cell walls. Furthermore, its spectrum within the CO2 laser wavelength range is highly structured. In particular it exhibits a characteristic absorption peak at the 10P(14) laser transition at 949.49 cm-1 which is caused by the
commercially prepared, certified mixture containing 0.96 ppmV C2H4 in pure nitrogen throughout our investigations. For calibration we examined this reference mixture at a total pressure *p* of approximately 1013 mbar and a temperature *T* ≅ 23oC and using the commonly accepted value of the absorption coefficient of 30.4 cm-1atm-1 at the 10P(14) line of the 12C16O2 laser. For shorter time intervals (by changing the calibration gas mixture after a careful vacuum cleaning of the PA cell), the variation of the cell constant was smaller than
7 vibration of C2H4 centered at 948.7715 cm-1. We used a
Fig. 15. Resonance curve for the first longitudinal mode of the PA cell.
PA signal *V* (mV)
losses during the experiment.
proximity of the Q branch of the
sccm - standard cubic centimeters per minute), the replenish time for the 1.0 dm3 cell becomes impractical. The buffer volume at the entrance port of the cell affects the renewal time τ considerably. The buffer volume is approximately 200 cm3, yielding a time constant of 12 min (τ = *V*/*Rflow* = (200 cm3)/(16.6 cm3/min) = 12 min). By reducing the buffer volume to 24 cm3, with *rbuf* ≅ 3*r* (diameter 20 mm, length 75 mm), a τ of 1.5 min is obtained. However, an increased acoustical noise level was observed, due to the gas flow.
In photoacoustic measurements in the gas phase, microphones are usually employed as sensing elements of the acoustic waves generated by the heat deposition of the absorbing molecules. Although high-quality condenser microphones offer the best noise performance, they are rarely used in photoacoustic gas detection because of their large size, lower robustness, and relatively high price. The most common microphones employed are miniature electret devices originally developed as hearing aids. The choice of a miniature microphone is particularly advantageous since it can be readily incorporated in the resonant cavity without significantly degrading the *Q* of the resonance. The frequency response of electret microphones extends beyond 10 kHz, and the response to incident pressure waves is linear over many orders of magnitude.
In our PA cells there are four Knowles electret EK-3033 or EK-23024 miniature microphones in series (sensitivity 20 mV/Pa each at 564 Hz) mounted flush with the wall. They are situated at the loops of the standing wave pattern, at an angle of 90o to one another. The microphones are coupled to the resonator by holes (1 mm diameter) positioned on the central perimeter of the resonator. The battery-powered microphones are mounted in a Teflon ring pulled over the resonator tube (Fig. 14). It is of significant importance to prevent gas leakage from inside the resonator tube along the Teflon microphone holder, since minute spacing between the holder and resonator tube produces a dramatic decrease of the microphone signal and the *Q* value. The electrical output from these microphones is summed and the signal is selectively amplified by a two-phase lock-in amplifier tuned to the chopper frequency.
Fig. 14. Teflon rings used to mount the microphones flush with the tube wall.
The resonance curve of our PA cell (cell response in rms volts) was recorded as a function of laser beam chopping frequency and the results are plotted in Fig. 15. An accurate method is to construct the resonance curve point by point. In this case, the acoustic signal is measured at different fixed frequencies thus avoiding potential problems arising from the slow formation of a steady state standing wave in the resonator and the finite time resolution of the lock-in amplifier. It is evident from these data that the cell resonance curve is fairly broad, implying that the absorption measurements would not be considerably affected if the
sccm - standard cubic centimeters per minute), the replenish time for the 1.0 dm3 cell becomes impractical. The buffer volume at the entrance port of the cell affects the renewal
In photoacoustic measurements in the gas phase, microphones are usually employed as sensing elements of the acoustic waves generated by the heat deposition of the absorbing molecules. Although high-quality condenser microphones offer the best noise performance, they are rarely used in photoacoustic gas detection because of their large size, lower robustness, and relatively high price. The most common microphones employed are miniature electret devices originally developed as hearing aids. The choice of a miniature microphone is particularly advantageous since it can be readily incorporated in the resonant cavity without significantly degrading the *Q* of the resonance. The frequency response of electret microphones extends beyond 10 kHz, and the response to incident pressure waves is
In our PA cells there are four Knowles electret EK-3033 or EK-23024 miniature microphones in series (sensitivity 20 mV/Pa each at 564 Hz) mounted flush with the wall. They are situated at the loops of the standing wave pattern, at an angle of 90o to one another. The microphones are coupled to the resonator by holes (1 mm diameter) positioned on the central perimeter of the resonator. The battery-powered microphones are mounted in a Teflon ring pulled over the resonator tube (Fig. 14). It is of significant importance to prevent gas leakage from inside the resonator tube along the Teflon microphone holder, since minute spacing between the holder and resonator tube produces a dramatic decrease of the microphone signal and the *Q* value. The electrical output from these microphones is summed and the signal is selectively amplified by a two-phase lock-in amplifier tuned to the
Fig. 14. Teflon rings used to mount the microphones flush with the tube wall.
The resonance curve of our PA cell (cell response in rms volts) was recorded as a function of laser beam chopping frequency and the results are plotted in Fig. 15. An accurate method is to construct the resonance curve point by point. In this case, the acoustic signal is measured at different fixed frequencies thus avoiding potential problems arising from the slow formation of a steady state standing wave in the resonator and the finite time resolution of the lock-in amplifier. It is evident from these data that the cell resonance curve is fairly broad, implying that the absorption measurements would not be considerably affected if the
24 cm3, with *rbuf* ≅ 3*r* (diameter 20 mm, length 75 mm), a
linear over many orders of magnitude.
an increased acoustical noise level was observed, due to the gas flow.
considerably. The buffer volume is approximately 200 cm3, yielding a time constant of
= *V*/*Rflow* = (200 cm3)/(16.6 cm3/min) = 12 min). By reducing the buffer volume to
τ
of 1.5 min is obtained. However,
time τ
12 min (
τ
chopper frequency.
frequency of the laser modulation or the cell resonance itself were to shift by a few hertz. PA cells exhibiting narrow resonances (as in the case of excitation of the radial modes) require tight control of both temperature and laser modulation frequency to avoid responsivity losses during the experiment.
Fig. 15. Resonance curve for the first longitudinal mode of the PA cell.
The acoustic resonator is characterized by the quality factor *Q*, which is defined as the ratio of the resonance frequency to the frequency bandwidth between half power points. The amplitude of the microphone signal is 1/ 2 of the maximum amplitude at these points, because the energy of the standing wave is proportional to the square of the induced pressure. The quality factor was measured by filling the PA cell with 1 ppmV of ethylene buffered in nitrogen at a total pressure of 1 atm and by tuning the modulation frequency in 10 Hz increments (2 Hz increments near the top of the curve) across the resonance profile to estimate the half width, as described above. For this PA cell, the profile width at half intensity was 35 Hz, yielding a quality factor *Q* = 16.1 (Eq. 15, Part I) at a resonance frequency *f*0 = 564 Hz. The experimentally determined resonance is not completely symmetric, as the curve rises steeply on one side and becomes less steep on the other side of the maximum. This asymmetry is caused by a coherent superposition of the standing acoustic waves in the detection region of the microphones (Karbach & Hess, 1985).
The calibration of the PA system is usually performed with a reference gas. We calibrated our PA cell with the widely used reference gas ethylene, whose absorption coefficients are accurately known at CO2-laser wavelengths. Ethylene is well suited for this purpose, since it interacts only weakly with common cell surface materials. Ethylene is chemically inert, has the same molecular weight as nitrogen and possesses no permanent dipole moment which means negligible adsorption on the cell walls. Furthermore, its spectrum within the CO2 laser wavelength range is highly structured. In particular it exhibits a characteristic absorption peak at the 10P(14) laser transition at 949.49 cm-1 which is caused by the proximity of the Q branch of the ν7 vibration of C2H4 centered at 948.7715 cm-1. We used a commercially prepared, certified mixture containing 0.96 ppmV C2H4 in pure nitrogen throughout our investigations. For calibration we examined this reference mixture at a total pressure *p* of approximately 1013 mbar and a temperature *T* ≅ 23oC and using the commonly accepted value of the absorption coefficient of 30.4 cm-1atm-1 at the 10P(14) line of the 12C16O2 laser. For shorter time intervals (by changing the calibration gas mixture after a careful vacuum cleaning of the PA cell), the variation of the cell constant was smaller than
CO2 Laser Photoacoustic Spectroscopy: II. Instrumentation and Applications 63
mV/Pa, *Q* = 340
mV/Pa, *Q* = 40
mV/Pa, *Q* = 43
mV/Pa, *Q* = 32
mV/Pa, *Q* = 70
a The value of 50 V cm/W reported for a rms signal of 162 mV was corrected for a peak-to-peak signal
b In brackets are the values we deduced using the authors' specifications for microphone responsivity. c These authors calculated the cell constant by using a PA signal 2 rms instead of a 2 2 x rms signal; as a result, their cell constant (and responsivity) value is half the one
Based on the measured noises, background signals, and cell responsivity, all parameters characterizing the PA instrument can be evaluated (see Table 2). Some of them depend on the CO2 laser and the PA cell, while others are determined by either the coherent acoustic
Several factors influence the lowest levels at which selected compounds can be detected by CO2 laser spectroscopic techniques, within prescribed confidence limits, in a multicomponent mixture. These factors, some of which are interdependent, include (1) the sensitivity or
*f*<sup>0</sup> = 1986 Hz,
4.18x16 cm2, *f*<sup>0</sup> = 2.65 kHz, 1 microphone, *SM* = 12.5
4.18x16 cm2, *f*<sup>0</sup> = 2.86 kHz, 2 microphones, *SM* = 25 mV/Pa, *Q* = 168
*L* = 15 cm, *f*<sup>0</sup> = 1030 Hz, 1 microphone, *SM* = 1000
*L* = 45 cm, *f*<sup>0</sup> = 555 Hz, 1 microphone, *SM* = 1000
*L* = 10 cm, *f*<sup>0</sup> = 1600 Hz, 3 microphones, *SM* = 60
Cell volume = 1 cm3,
5x12 cm2, *f*<sup>0</sup> = 1250 Hz, 16 microphones, *SM* = 160
*f*<sup>0</sup> = 1915 Hz, 1 microphone,
*<sup>f</sup>*= 1600 Hz 1.45
1 microphone, *SM* = 26 mV/Pa (52) 2000
*SM* = 22 mV/Pa, *Q* = 49.7 (71.5) 3250c
*min* (cm-1) of the particular CO2 laser detection technique
**(V cm/W)**
3.5 (280)
1.72 (69)
(2000) 2000
(1.64) 1640
(270) 4500
260 (1625)
*C*
**(Pa cm/W)**
**Authors Excitation mode PA cell characteristics** *<sup>R</sup>*
(Meyer & Sigrist,
(Thöny & Sigrist,
(Fink et al., 1996)
(Harren et al.,
(Harren et al.,
(Henningsen & Melander, 1997)
(Nägele & Sigrist,
(Pushkarsky et al., 2002)
1997)
1997)
2000)
of 402 mV.
(Bijnen *et al*., 1996)
1995)
1990) First radial mode
Resonance in vicinity of the first radial mode
mode
mode (intracavity operation)
Second
mode
PA cell
mode
we obtained by our methodology of calculus.
minimum detectable absorptivity
First longitudinal
First longitudinal
longitudinal mode
First longitudinal
First longitudinal mode in a
multipass resonant
First longitudinal
Table 1. Comparison of our results with different PA cells.
background noise or the coherent photoacoustic background signal.
α
Nonresonant operation, pulsed excitation
2%. The calibration also depends to some extent on the modulation waveform, since only the fundamental Fourier component of that waveform is resonant with, and hence significantly excites, the first longitudinal mode.
Using this PA cell and an optimized experimental arrangement we measured a cell responsivity *R* = 280 V cm/W. With the total responsivity of the four microphones *SM tot* = 80 mV/Pa (20 mV/Pa each) (Eq. 30, Part I), a cell constant *C* = 3500 Pa cm/W can be calculated (Eq. 28, Part I). A comparison of these PA cell parameters with other results reported in the literature is presented in Table 1. Different photoacoustic resonator designs such as longitudinal organ pipe resonators excited in the first longitudinal mode, closed longitudinal resonators excited in the second longitudinal mode, and cylindrical resonators excited in the first radial mode (Fig. 16) were used by various authors. As can be noticed from the table, the cell responsivity we obtained is one of the best values that have been reported up to now.
2%. The calibration also depends to some extent on the modulation waveform, since only the fundamental Fourier component of that waveform is resonant with, and hence
Using this PA cell and an optimized experimental arrangement we measured a cell responsivity *R* = 280 V cm/W. With the total responsivity of the four microphones *SM tot* = 80 mV/Pa (20 mV/Pa each) (Eq. 30, Part I), a cell constant *C* = 3500 Pa cm/W can be calculated (Eq. 28, Part I). A comparison of these PA cell parameters with other results reported in the literature is presented in Table 1. Different photoacoustic resonator designs such as longitudinal organ pipe resonators excited in the first longitudinal mode, closed longitudinal resonators excited in the second longitudinal mode, and cylindrical resonators excited in the first radial mode (Fig. 16) were used by various authors. As can be noticed from the table, the cell responsivity we obtained is one of the best values that have been
> *L* = 30 cm, *f*<sup>0</sup> = 564 Hz, 4 microphones, *SM* = 80 mV/Pa, *Q* = 16.1
*L* = 20 cm, *f* = 33.3 Hz,
*L* = 20 cm, *f* = 695 Hz,
*L* = 30 cm, *f*<sup>0</sup> = 695 Hz,
*L* = 15 cm, 2*r* = 1.5 cm,
*L* = 60 cm, *f*<sup>0</sup> = 555 Hz,
*L* = 10 cm, *f*<sup>0</sup> = 1608 Hz,
*L* = 30 cm, *f*<sup>0</sup> = 556 Hz,
*L* = 30 cm, *f*<sup>0</sup> = 560 Hz, 4 microphones, *SM* = 40 mV/Pa, *Q* = 16.4
*L* = 30 cm, *f*<sup>0</sup> = 560 Hz, 4 microphones, *SM* = 40
*L* = 10 cm, *f*<sup>0</sup> = 1653 Hz,
1 microphone, *SM* = 10 mV/Pa,
mV/Pa, *Q* = 20
*Q* = 31.8
6.54x15.56 cm2, *f*<sup>0</sup> = 2.7 kHz, 1 microphone, *SM* = 11mV/Pa + preamplif.x10, *Q* = 560
1 microphone 16.3
1 microphone 121a
1 microphone, *Q* = 17.4 <sup>56</sup>
4 microphones <sup>114</sup>
1 microphone, *SM* = 50 mV/Pa 1-10 (20-200)b
1 microphone, *Q* = 52 <sup>1990</sup>
1 microphone, *SM* = 10 mV/Pa (39) <sup>3900</sup>
**(V cm/W)**
280 3500
26.5 241
(200) 5000
(160) 4000
(37) 3700
*C*
**(Pa cm/W)**
**Authors Excitation mode PA cell characteristics** *<sup>R</sup>*
First longitudinal
mode
operation
operation
Nonresonant operation
longitudinal mode
First longitudinal
First longitudinal
First longitudinal
First longitudinal
First longitudinal
Second
mode
mode
mode
mode
mode (intracavity operation)
(Crane, 1978) Nonresonant
(Hubert, 1983) Nonresonant
1980) First radial mode
(Ryan et al., 1983) First longitudinal mode
significantly excites, the first longitudinal mode.
reported up to now.
(Dumitras et al., 2007) - Our results
(Gerlach & Amer,
(Gandurin et al.,
(Bernegger & Sigrist, 1987)
(Sauren et al.,
(Rooth et al., 1990)
(Harren et al., 1990b)
(Harren et al., 1990a)
(Harren et al., 1990a)
1986)
1989)
a The value of 50 V cm/W reported for a rms signal of 162 mV was corrected for a peak-to-peak signal of 402 mV.
b In brackets are the values we deduced using the authors' specifications for microphone responsivity. c These authors calculated the cell constant by using a PA signal 2
rms instead of a 2 2 x rms signal; as a result, their cell constant (and responsivity) value is half the one we obtained by our methodology of calculus.
Table 1. Comparison of our results with different PA cells.
Based on the measured noises, background signals, and cell responsivity, all parameters characterizing the PA instrument can be evaluated (see Table 2). Some of them depend on the CO2 laser and the PA cell, while others are determined by either the coherent acoustic background noise or the coherent photoacoustic background signal.
Several factors influence the lowest levels at which selected compounds can be detected by CO2 laser spectroscopic techniques, within prescribed confidence limits, in a multicomponent mixture. These factors, some of which are interdependent, include (1) the sensitivity or minimum detectable absorptivity α*min* (cm-1) of the particular CO2 laser detection technique
CO2 Laser Photoacoustic Spectroscopy: II. Instrumentation and Applications 65
the coherent acoustical background noise-equivalent signal *ac VN* , *PL* is the laser excitation power, and Δ*<sup>f</sup>*
g The limiting sensitivity of the system or the microphone noise-limited minimum detectable absorption coefficient (the equivalent bulk absorption coefficient in the gas) is *Ssys* = *Scell*/*PL* (the minimum detectable absorption strength is defined as the strength that gives a SNR at the transducer output equal to one). h The limiting sensitivity of the system gives a limiting measurable concentration of ethylene of *clim* = *Ssys*/
α
The minimum detectable absorption coefficient or the equivalent absorption coefficient of C2H4 for a minimum detectable concentration (the absorption coefficient corresponding to the synchronous
mThe minimum detectable absorption cross-section per molecule of ethylenedetermined by the synchronous background signal is the ratio between the equivalent absorption coefficient of C2H4 for minimum detectable
n Knowing the cell responsivity, we can determine the sensitivity of the photoacoustic cell (the rms voltage amplitude measured by the lock-in amplifier) to measure 1 ppb of a given gas at a given laser frequency
(a)
(b)
(c) Fig. 16. Different photoacoustic resonator designs: (a) longitudinal organ pipe resonator, excited in the first longitudinal mode; (b) closed longitudinal resonator excited in the second
longitudinal mode; (c) cylindrical resonator excited in the first radial mode.
number of absorbing molecules per cubic centimeter (*Ntot* = 2.5x1019 cm-3 at 1013 mbar and 20oC).
α*PLRc* (α
i The minimum measurable signal in nitrogen, as determined by the coherent photoacoustical
j The minimum detectable peak-to-peak pressure amplitude can be determined by dividing the minimum measurable signal in nitrogen by the responsivity of the microphone: *pmin* = 2 2 *Vmin*/*SM*. k The minimum measurable signal in nitrogen, limited by the synchronous background signal, gives a
*el* α is the absorption coefficient corresponding to
2*PmeasR,* where *Pmeas* is the measured laser power
*min*/*Ntot*, where *Ntot* is the
= 30.4 cm-1 atm-1, *PL* = 1 W, *R* = 280 V cm/W, and
= 2 2 *<sup>b</sup> VN* /*R* (or the ratio between the peak-to-peak value of the
σ*min* = α α.
defined as min / / *el D PL cell* =α Δ = Δ *f S f* , where min
background signal, is *Vmin* = *<sup>b</sup> VN* (nitrogen) x *PL*.
minimum detectable concentration *cmin* = 2 2 *Vmin*/
synchronous background signal and the cell responsivity).
concentration and the number of absorbing molecules per unit volume:
α*min* = *cmin*α
with 1 W of unchopped laser power: 2 2 *Vppb* =
Table 2. PA cell parameters.
is the detection bandwidth).
after chopper: *PL* = 2*Pmeas*.
background signal) is
*c* = 10-9 atm).
l
employed; (2) the absorption coefficients and spectral uniqueness of both the compounds of interest and interferences; (3) the total number of compounds that absorb within the wavelength regions of the CO2 laser output; (4) the wavelengths and number of CO2 laser lines used for monitoring; and (5) the output power of the laser at each of these lines when the photoacoustic technique is employed. The minimum concentrations of various vapors that can be detected under interference-free conditions by the CO2 laser photoacoustic technique are given by the relationship *cmin* = α*min*/α(λ). Here α*min* is the minimum detectable absorptivity value for the photoacoustic detection system in units of cm-1, and α(λ) is the absorption coefficient in units of cm-1atm-1 of the vapor of interest at the CO2 laser monitoring wavelength. In order to determine the concentrations of the various gas mixture components, it is necessary to know the absolute absorption coefficients for every gas component at the laser wavelengths. Our CO2 laser photoacoustic system with a α*min* value of 2.7x10-8 cm-1 should provide interference-free minimum detectable concentrations between 0.9 ppbV and 270 ppbV for vapors with usual absorption coefficients of 30-0.1 cm-1atm-1.
a This quality factor value corresponds to a full bandwidth at the 0.707 amplitude points of Δ*f* = *f*0/*Q* ≅ 35 Hz b The cell responsivity is the signal per unit power per unit absorption coefficient; in our case, the signal per unit power is 11.6 mV/4.0 W = 2.9x10-3 V/W (rms value) or 8.2x10-3 V/W (peak-to-peak value) for 0.96 ppmV of C2H4 (the absorption coefficient α*\** = 30.4 cm-1 atm-1 x 0.96x10-6 atm = 2.92x10-5 cm-1, where α = 30.4 cm-1 atm-1 is the absorption coefficient of C2H4 at 10P(14) line of the CO2 laser), so that *R* = 8.2x10-3 V/W/2.92x10-5 cm-1 ≅ 280 V cm/W; the same responsivity was obtained with the etalon mixture of 10 ppmV of C2H4 in N2: *R* = 8.4x10-2 V/W/3x10-4 cm-1 ≅ 280 V cm/W
c The microphone responsivity is determined from the Knowles data-sheet for the microphone type 3033: 54 dB (≅500) attenuation at 564 Hz from 1 V/0.1 Pa, leading to *SM* ≅ 20 mV/Pa
d The cell constant is the pressure amplitude per unit absorption coefficient per unit power: *C* = *R*/*SM* e The pressure amplitude response per unit incident power for 1 ppmV of C2H4 is *p*/*PL* = *C*α*\**
f The limiting sensitivity of the cell is *Scell* = 2 2 / *ac V R <sup>N</sup>* (several authors, e.g., (Harren et al., 1990a) used the rms value of the voltage instead of its peak-to-peak value, resulting in a limiting sensitivity of the cell and of the system and the limiting measurable concentration of ethylene lower by a factor of 2.84; other authors, e.g., (Kosterev et al., 2005), used a parameter named "sensitivity to absorption",
defined as min / / *el D PL cell* =α Δ = Δ *f S f* , where min *el* α is the absorption coefficient corresponding to the coherent acoustical background noise-equivalent signal *ac VN* , *PL* is the laser excitation power, and Δ*<sup>f</sup>* is the detection bandwidth).
g The limiting sensitivity of the system or the microphone noise-limited minimum detectable absorption coefficient (the equivalent bulk absorption coefficient in the gas) is *Ssys* = *Scell*/*PL* (the minimum detectable absorption strength is defined as the strength that gives a SNR at the transducer output equal to one). h The limiting sensitivity of the system gives a limiting measurable concentration of ethylene of *clim* = *Ssys*/α. i The minimum measurable signal in nitrogen, as determined by the coherent photoacoustical background signal, is *Vmin* = *<sup>b</sup> VN* (nitrogen) x *PL*.
j The minimum detectable peak-to-peak pressure amplitude can be determined by dividing the minimum measurable signal in nitrogen by the responsivity of the microphone: *pmin* = 2 2 *Vmin*/*SM*. k The minimum measurable signal in nitrogen, limited by the synchronous background signal, gives a minimum detectable concentration *cmin* = 2 2 *Vmin*/α2*PmeasR,* where *Pmeas* is the measured laser power after chopper: *PL* = 2*Pmeas*.
l The minimum detectable absorption coefficient or the equivalent absorption coefficient of C2H4 for a minimum detectable concentration (the absorption coefficient corresponding to the synchronous background signal) is α*min* = *cmin*α = 2 2 *<sup>b</sup> VN* /*R* (or the ratio between the peak-to-peak value of the synchronous background signal and the cell responsivity).
mThe minimum detectable absorption cross-section per molecule of ethylenedetermined by the synchronous background signal is the ratio between the equivalent absorption coefficient of C2H4 for minimum detectable concentration and the number of absorbing molecules per unit volume: σ*min* = α*min*/*Ntot*, where *Ntot* is the number of absorbing molecules per cubic centimeter (*Ntot* = 2.5x1019 cm-3 at 1013 mbar and 20oC). n Knowing the cell responsivity, we can determine the sensitivity of the photoacoustic cell (the rms voltage
amplitude measured by the lock-in amplifier) to measure 1 ppb of a given gas at a given laser frequency with 1 W of unchopped laser power: 2 2 *Vppb* = α*PLRc* (α = 30.4 cm-1 atm-1, *PL* = 1 W, *R* = 280 V cm/W, and *c* = 10-9 atm).
Table 2. PA cell parameters.
64 CO2 Laser – Optimisation and Application
employed; (2) the absorption coefficients and spectral uniqueness of both the compounds of interest and interferences; (3) the total number of compounds that absorb within the wavelength regions of the CO2 laser output; (4) the wavelengths and number of CO2 laser lines used for monitoring; and (5) the output power of the laser at each of these lines when the photoacoustic technique is employed. The minimum concentrations of various vapors that can be detected under interference-free conditions by the CO2 laser photoacoustic
> α*min*/α(λ). Here α
absorption coefficient in units of cm-1atm-1 of the vapor of interest at the CO2 laser monitoring wavelength. In order to determine the concentrations of the various gas mixture components, it is necessary to know the absolute absorption coefficients for every gas component at the
should provide interference-free minimum detectable concentrations between 0.9 ppbV and
Microphone responsivityc, *SM* (V/Pa) 4 x20x10-3 = 8x10-2
a This quality factor value corresponds to a full bandwidth at the 0.707 amplitude points of Δ*f* = *f*0/*Q* ≅ 35 Hz b The cell responsivity is the signal per unit power per unit absorption coefficient; in our case, the signal per unit power is 11.6 mV/4.0 W = 2.9x10-3 V/W (rms value) or 8.2x10-3 V/W (peak-to-peak value) for
> α*\**
= 30.4 cm-1 atm-1 is the absorption coefficient of C2H4 at 10P(14) line of the CO2 laser), so that *R* = 8.2x10-3 V/W/2.92x10-5 cm-1 ≅ 280 V cm/W; the same responsivity was obtained with the etalon mixture
c The microphone responsivity is determined from the Knowles data-sheet for the microphone type
d The cell constant is the pressure amplitude per unit absorption coefficient per unit power: *C* = *R*/*SM*
f The limiting sensitivity of the cell is *Scell* = 2 2 / *ac V R <sup>N</sup>* (several authors, e.g., (Harren et al., 1990a) used the rms value of the voltage instead of its peak-to-peak value, resulting in a limiting sensitivity of the cell and of the system and the limiting measurable concentration of ethylene lower by a factor of 2.84; other authors, e.g., (Kosterev et al., 2005), used a parameter named "sensitivity to absorption",
, *Scell* (W cm-1) 2.6x10-8
, *Vmin* (μV) (root mean square) 12
*min* (cm-1) 2.7x10-8
, *pmin* (Pa) 4.2x10-4
σ
= 30.4 cm-1 atm-1 x 0.96x10-6 atm = 2.92x10-5 cm-1, where
*min* (cm2) 1.1x10-27
3.0
α*\**
absorptivity value for the photoacoustic detection system in units of cm-1, and
**Parameter/units Value**
Cell responsivityb, *R* (V cm/W) 280
Cell constantd, *C* (Pa cm/W) 3.5x103 Pressure amplitude responsee, *p*/*PL* (Pa/W) 10-1
Limiting sensitivity of the systemg, *Ssys* (cm-1) (at 4.4 W laser power) 5.9x10-9 Limiting measurable concentration of ethyleneh, *clim* (ppbV) 0.2
Minimum detectable concentrationk, *cmin* (ppbV) 0.89
, α
Cell sensitivity for 1 ppbV of C2H4 at 1 W of unchopped laser powern,
3033: 54 dB (≅500) attenuation at 564 Hz from 1 V/0.1 Pa, leading to *SM* ≅ 20 mV/Pa
e The pressure amplitude response per unit incident power for 1 ppmV of C2H4 is *p*/*PL* = *C*
Minimum detectable absorption cross-section per moleculem,
of 10 ppmV of C2H4 in N2: *R* = 8.4x10-2 V/W/3x10-4 cm-1 ≅ 280 V cm/W
laser wavelengths. Our CO2 laser photoacoustic system with a
270 ppbV for vapors with usual absorption coefficients of 30-0.1 cm-1atm-1.
*min* is the minimum detectable
*min* value of 2.7x10-8 cm-1
564 16.1
α
α(λ) is the
technique are given by the relationship *cmin* =
Resonance frequency, *f*0 (Hz)
Limiting sensitivity of the cellf
Minimum measurable signal in nitrogeni
Minimum detectable pressure amplitudej
0.96 ppmV of C2H4 (the absorption coefficient
Minimum detectable absorptivityl
*Vppb* (μV at 1 ppbV)
α
Quality factora, *Q*
Fig. 16. Different photoacoustic resonator designs: (a) longitudinal organ pipe resonator, excited in the first longitudinal mode; (b) closed longitudinal resonator excited in the second longitudinal mode; (c) cylindrical resonator excited in the first radial mode.
CO2 Laser Photoacoustic Spectroscopy: II. Instrumentation and Applications 67
To keep the flow noise at a sufficiently low level, the flow must be in the laminar regime. Another practical requirement is the time response, which is determined by the gas sample exchange rate in the resonant cell. In addition, a delay occurs because the gas flows from the inlet to the resonant cell first through the acoustic filter. A response time of < 1 s and a delay of < 10 s may occur in the continuous flow mode. Taking into account the largest dimension and the limiting value of the Reynolds number (*Re* ≤ 2300 for laminar flow), the flow velocity should not exceed 1.7 m/s. This value is far too large, since it would give a flow rate of 4.8 L/min (large gas consumption). As an operating value a flow rate of up to 0.5 L/min was used. With this value, the maximum flow velocity was about 15 cm/s, the Reynolds
from the rise time of the PA signal is longer due to gas mixing in the buffer volumes of the cell. Nevertheless, the measured response times are below 10 s for nonadsorbing gases. Adsorption on the PA cell may increase the response time significantly. Note that the adsorption effect can be effectively reduced by using appropriate wall materials and higher wall temperatures. The adsorption effects can prevent accurate determination of the vapor pressure, since even with an *in situ* sample the vapor pressure will equilibrate between the
Let us consider the system to be a cell with constant volume. For an ideal gas, d*T*/d*E* = (*nCv*)-1, so that d*p*/d*E* = *R*/(*CvV*). Here *T* is the temperature, *p* is the pressure, *E* is the absorbed energy, *n* is the number of moles of the gas, *Cv* is the molar heat capacity at constant volume of the gas mixture, *R* is the gas constant, and *V* is the volume of the cell. For a real photoacoustic system the contents of the cell do not completely equilibrate at the modulation frequency; thus the above equations are a simplification. However, they do emphasize the basic constraints for maximizing the photoacoustic signal; (1) the gas mixture in the cell should have the lowest possible molar heat capacity, and (2) the effective volume of the cell should be as small as possible. The latter is also advantageous when the sample amount is limited. The ideal conditions for PA spectroscopy are a gas mixture consisting of a small amount of sample buffered in a large amount of a nonabsorbing gas with a low *Cv*, such as rare gases, inside a cell with the smallest possible volume. Since the laser beam has cylindrical symmetry, the best way to minimize the volume is to use a cylindrically symmetric cell with an internal diameter barely larger than the diameter of the laser beam. Cylindrical cells, without additions, such as baffles, gas valves, sample fingers, or microphone tubes, and having a diameter of less than about 1/4 of wavelength, behave as simple pipes. Since the speed of sound is inversely proportional to the mass density of the gas (see Eq. 6, Part I), the frequency is lower for a more massive gas. The speed of sound is effectively independent of the total buffer gas pressure. Some works suggest that using the heavier noble gases as buffers increases the signal-to-noise level in acoustically resonant PA
Davidson et al. (Davidson et al., 1990) investigated the importance of window noise and the role of acoustic baffles in photoacoustic spectroscopy. Small amounts of dirt or imperfections can cause heating at the windows and thus the production of a photoacoustic background signal. Window absorption is a major problem for intracavity photoacoustic spectroscopy because of the high light intensity inside the cavity. The signature of window noise is that it is laser frequency independent; its intensity tracks the intensity of the exciting radiation. This signal can actually mask the signal of interest (see spectrum D in Fig. 17). It was shown that the window noise should decrease with increasing modulation frequency
τ
< 0.7 s. The response time determined
number *Re* < 200, and the calculated response time
rate of vapor emission and rate of plating out.
spectroscopy (Thomas III et al., 1978).
A comparison of the results we obtained by using an extracavity PA cell and the experimental parameters measured using intracavity PA cells is given in Table 3. If we take into consideration only the parameters determined by the coherent acoustical background noise (SNR = 1), then the minimum detectable absorptivity *Ssys* (cm-1) is much lower (1-2 orders of magnitude) in the intracavity arrangements due to the increased laser power. Unfortunately, only SNR is often considered in the literature, which yields an extrapolated detection limit that may be considerably too small. In reality, other background signals such as window absorption limit the ultimate sensitivity. These background signals must always be taken into consideration, as they can only be reduced, but not eliminated. In real PA instruments, the minimum measurable signal *Vmin* is higher in extracavity PA cells (3 times in our case) and much higher in intracavity PA cells (hundreds or even thousands of times) than the coherent acoustic background noise *ac VN* . From this table it clearly follows that the best sensitivity is obtained with our extracavity PA instrument, with α*min* (cm-1) being better by one or two orders of magnitude than in intracavity arrangements.
a For a bandwidth of 1 Hz.
b The authors claim that they determined a coherent acoustical background noise of 0.1 μV, but their measured *Ssys* and *clim* correspond to *ac VN* = 0.81 μV.
c The value of *Scell* = 1.4x10-8 cm-1, as cited by the authors, was corrected for a peak-to-peak value of the coherent acoustical background noise.
d A factor of 0.78 was introduced either in the absorption coefficient of ethylene or in the intracavity laser power to compensate for the influence of saturation; the value of *Ssys* = 1.8x10-10 cm-1 claimed by the authors was corrected for a peak-to-peak value of the coherent acoustical background noise.
e The value of 6 pptV claimed by authors was corrected for a peak-to-peak value of the coherent acoustical background noise
Table 3. Comparison of our results (extracavity PA cell) with the experimental parameters determined with intracavity PA cells.
A comparison of the results we obtained by using an extracavity PA cell and the experimental parameters measured using intracavity PA cells is given in Table 3. If we take into consideration only the parameters determined by the coherent acoustical background noise (SNR = 1), then the minimum detectable absorptivity *Ssys* (cm-1) is much lower (1-2 orders of magnitude) in the intracavity arrangements due to the increased laser power. Unfortunately, only SNR is often considered in the literature, which yields an extrapolated detection limit that may be considerably too small. In reality, other background signals such as window absorption limit the ultimate sensitivity. These background signals must always be taken into consideration, as they can only be reduced, but not eliminated. In real PA instruments, the minimum measurable signal *Vmin* is higher in extracavity PA cells (3 times in our case) and much higher in intracavity PA cells (hundreds or even thousands of times) than the coherent acoustic background noise *ac VN* . From this table it clearly follows that the
α
**(Dumitras et al., 2007) (Harren et al., 1990a) (Fink et al., 1996)**
*min* (cm-1) being better
best sensitivity is obtained with our extracavity PA instrument, with
by one or two orders of magnitude than in intracavity arrangements.
*R* (V cm/W) 280 37 52 *SM tot* (mV/Pa) 80 10 26 *C* (Pa cm/W) 3500 3700 2000 *PL* (W) 4.4 100 40 *p*/*PL* (Pa/W) 10-1 1.1x10-1 6x10-2 Parameters determined by the coherent acoustic background noise (SNR = 1) *ac VN* (rms)a (μV) 2.6 0.5 0.81b *Scell* (W cm-1) 2.6x10-8 4.0x10-8 c 4.4x10-8 *Ssys* (cm-1) 5.9x10-9 5.1x10-10 d 1.1x10-9 *clim* (pptV) 200 17e 34 Parameters determined by the coherent photoacoustic background signal (SBR = 1)
*<sup>b</sup> VN* (rms) (μV/W) 2.7 1.5 26 *Vmin* (rms) (μV) 12 117d 1040 *pmin* (Pa) 4.2x10-4 3.3x10-2 1.1x10-1 *cmin* (ppbV) 0.9 3.8 46
*min* (cm-1) 2.7x10-8 1.2x10-7 1.4x10-6
b The authors claim that they determined a coherent acoustical background noise of 0.1 μV, but their
c The value of *Scell* = 1.4x10-8 cm-1, as cited by the authors, was corrected for a peak-to-peak value of the
d A factor of 0.78 was introduced either in the absorption coefficient of ethylene or in the intracavity laser power to compensate for the influence of saturation; the value of *Ssys* = 1.8x10-10 cm-1 claimed by the
Table 3. Comparison of our results (extracavity PA cell) with the experimental parameters
authors was corrected for a peak-to-peak value of the coherent acoustical background noise. e The value of 6 pptV claimed by authors was corrected for a peak-to-peak value of the coherent
**Parameter Our results**
PA cell and CO2 laser
a For a bandwidth of 1 Hz.
acoustical background noise
measured *Ssys* and *clim* correspond to *ac VN* = 0.81 μV.
coherent acoustical background noise.
determined with intracavity PA cells.
α
To keep the flow noise at a sufficiently low level, the flow must be in the laminar regime. Another practical requirement is the time response, which is determined by the gas sample exchange rate in the resonant cell. In addition, a delay occurs because the gas flows from the inlet to the resonant cell first through the acoustic filter. A response time of < 1 s and a delay of < 10 s may occur in the continuous flow mode. Taking into account the largest dimension and the limiting value of the Reynolds number (*Re* ≤ 2300 for laminar flow), the flow velocity should not exceed 1.7 m/s. This value is far too large, since it would give a flow rate of 4.8 L/min (large gas consumption). As an operating value a flow rate of up to 0.5 L/min was used. With this value, the maximum flow velocity was about 15 cm/s, the Reynolds number *Re* < 200, and the calculated response time τ < 0.7 s. The response time determined from the rise time of the PA signal is longer due to gas mixing in the buffer volumes of the cell. Nevertheless, the measured response times are below 10 s for nonadsorbing gases. Adsorption on the PA cell may increase the response time significantly. Note that the adsorption effect can be effectively reduced by using appropriate wall materials and higher wall temperatures. The adsorption effects can prevent accurate determination of the vapor pressure, since even with an *in situ* sample the vapor pressure will equilibrate between the rate of vapor emission and rate of plating out.
Let us consider the system to be a cell with constant volume. For an ideal gas, d*T*/d*E* = (*nCv*)-1, so that d*p*/d*E* = *R*/(*CvV*). Here *T* is the temperature, *p* is the pressure, *E* is the absorbed energy, *n* is the number of moles of the gas, *Cv* is the molar heat capacity at constant volume of the gas mixture, *R* is the gas constant, and *V* is the volume of the cell. For a real photoacoustic system the contents of the cell do not completely equilibrate at the modulation frequency; thus the above equations are a simplification. However, they do emphasize the basic constraints for maximizing the photoacoustic signal; (1) the gas mixture in the cell should have the lowest possible molar heat capacity, and (2) the effective volume of the cell should be as small as possible. The latter is also advantageous when the sample amount is limited. The ideal conditions for PA spectroscopy are a gas mixture consisting of a small amount of sample buffered in a large amount of a nonabsorbing gas with a low *Cv*, such as rare gases, inside a cell with the smallest possible volume. Since the laser beam has cylindrical symmetry, the best way to minimize the volume is to use a cylindrically symmetric cell with an internal diameter barely larger than the diameter of the laser beam.
Cylindrical cells, without additions, such as baffles, gas valves, sample fingers, or microphone tubes, and having a diameter of less than about 1/4 of wavelength, behave as simple pipes. Since the speed of sound is inversely proportional to the mass density of the gas (see Eq. 6, Part I), the frequency is lower for a more massive gas. The speed of sound is effectively independent of the total buffer gas pressure. Some works suggest that using the heavier noble gases as buffers increases the signal-to-noise level in acoustically resonant PA spectroscopy (Thomas III et al., 1978).
Davidson et al. (Davidson et al., 1990) investigated the importance of window noise and the role of acoustic baffles in photoacoustic spectroscopy. Small amounts of dirt or imperfections can cause heating at the windows and thus the production of a photoacoustic background signal. Window absorption is a major problem for intracavity photoacoustic spectroscopy because of the high light intensity inside the cavity. The signature of window noise is that it is laser frequency independent; its intensity tracks the intensity of the exciting radiation. This signal can actually mask the signal of interest (see spectrum D in Fig. 17). It was shown that the window noise should decrease with increasing modulation frequency
CO2 Laser Photoacoustic Spectroscopy: II. Instrumentation and Applications 69
In an attempt to reduce the effects of microphone noise, the output of two microphones was summed in the low-noise preamplifier. Two microphones seem better than one when observing the noise level, but the difference is not as obvious in the spectra that were taken. However, the absolute signal level does increase with an increasing number of microphones;
High isolation against window absorption may be obtained by introducing acoustic baffles between the windows and the resonator. However, the resonator also introduces additional noise. Pressure fluctuations from turbulence in the gas and acoustic flux reaching the resonator from the surroundings will be amplified along with the signal. Furthermore the amplification provided by a resonator is accompanied by an increased sensitivity to parameters which affect
Besides cleaning the cell windows, a very careful rinsing of the inner walls of the cell is also very important. If the inner of the cell is not properly cleaned before a measurement, a considerable drift of the photoacoustic background signal is observed if the gas flow is interrupted. This fact demonstrates that the desorption of IR-absorbing gases and vapors from the cell walls can make a large contribution to the background signal. That is why no measurement started until after the PA cell had been rinsed with pure nitrogen till the coherent photoacoustical background noise reached the minimum value of 2.7 μV/W.
To distinguish the gas absorption signal from other signals (e.g., from the walls, windows, or interfering gases), one has to switch the CO2 laser to other laser lines. However, repositioning the laser beam to its original wavelength can change the configuration of the laser cavity (deviation in grating position, thermal drift) and result in irreproducible absorption signals if the operation is not carefully conducted. Using a CO2 laser stabilized on the top of the gain curve ensures that both the laser frequency and output power are reestablished with high accuracy when the laser operation is changed from one line to another.
Whenever monitoring is performed by flowing the gas mixture through a cell, a crucial question is whether the measured signal, which represents the trace gas concentration in the interaction region, also reflects the concentration at the source. On their way to the cell, the different components of a gas mixture may react with one another, form clusters or aerosols, and react with or be adsorbed on particles present, or on the sampling line and cell walls. Adsorption problems are particularly severe for polar molecules with large dipole moments, such as water and ammonia, but they can be reduced by a proper choice of materials.
The vacuum/gas handling system is an important element in these measurements owing to its role in ensuring PA cell and gas purity. The Teflon/stainless steel system can perform several functions without necessitating any disconnections. It can be used to pump out the cell, mix gases in the desired proportions, and monitor the total pressure of gases. Whenever possible, the PA cell was employed in the gas flow mode of operation to minimize any tendency for the vapor to stick to the cell walls and the effects of the subsequent outgassing of contaminants, which would otherwise lead to increasing background signals during an
To design an efficient vacuum/gas handling system to be used in LPAS, one must make
when the sample is weakly absorbing, more microphones are an advantage.
the resonance frequency, such as gas composition, temperature, and pressure.
**2.4 Gas handling system**
experimental run.
sure that the following operations can be carried out:
(Rosengren, 1975), which suggests that a relatively high modulation frequency is advantageous. The approach to this problem is to keep the windows as clean as possible and place acoustic baffles between the windows and the body of the sample cell where the microphones are mounted.
Fig. 17. Photoacoustic spectra: (A) clean windows on baffled cell; (B) clean windows on unbaffled cell; (C) dirty windows on baffled cell; (D) dirty windows on unbaffled cell (Davidson et al., 1990).
The acoustic baffles hinder the propagation of the window noise into the central region of the cell near the microphones. In an attempt to quantify the usefulness of the baffles, four spectra are shown in Fig. 17. These spectra illustrate the microphone signal versus laser frequency. For both the baffled and unbaffled cells, a longitudinal resonance frequency was used. From this figure it is immediately obvious how important it is to have clean windows. If the sample makes it difficult to keep the windows clean, a cell with baffles will perform much better than one without them. Even with clean windows, the baffles give a flatter base line.
The random noise level visible on the base lines of spectra A and B in Fig. 17 is probably due to a combination of the ambient lab noise, noise from the microphones, associated electronics, and the fluctuations of the laser itself. The ultimate limit of a microphone's sensitivity is set by the random thermal fluctuations in the sample and of the microphone diaphragm. In practice, the random fluctuations of the laser do not seem to be critical. Also, the combined electronic noise of the lock-in, preamplifier, and FET amplifier in the microphones totaled about 3 μV. This is at least a factor of 10 less than the noise level observed in the spectra. The noise that is visible in the spectra stems almost exclusively from thermal fluctuations and ambient lab noise. One of the major ambient noise sources is the mechanical chopper. In practice, this seems to set the detection limit. Acoustically isolating the chopper improves the noise, but replacing it with a nonmechanical modulator is better still, as it also speeds up modulation.
In an attempt to reduce the effects of microphone noise, the output of two microphones was summed in the low-noise preamplifier. Two microphones seem better than one when observing the noise level, but the difference is not as obvious in the spectra that were taken. However, the absolute signal level does increase with an increasing number of microphones; when the sample is weakly absorbing, more microphones are an advantage.
High isolation against window absorption may be obtained by introducing acoustic baffles between the windows and the resonator. However, the resonator also introduces additional noise. Pressure fluctuations from turbulence in the gas and acoustic flux reaching the resonator from the surroundings will be amplified along with the signal. Furthermore the amplification provided by a resonator is accompanied by an increased sensitivity to parameters which affect the resonance frequency, such as gas composition, temperature, and pressure.
Besides cleaning the cell windows, a very careful rinsing of the inner walls of the cell is also very important. If the inner of the cell is not properly cleaned before a measurement, a considerable drift of the photoacoustic background signal is observed if the gas flow is interrupted. This fact demonstrates that the desorption of IR-absorbing gases and vapors from the cell walls can make a large contribution to the background signal. That is why no measurement started until after the PA cell had been rinsed with pure nitrogen till the coherent photoacoustical background noise reached the minimum value of 2.7 μV/W.
To distinguish the gas absorption signal from other signals (e.g., from the walls, windows, or interfering gases), one has to switch the CO2 laser to other laser lines. However, repositioning the laser beam to its original wavelength can change the configuration of the laser cavity (deviation in grating position, thermal drift) and result in irreproducible absorption signals if the operation is not carefully conducted. Using a CO2 laser stabilized on the top of the gain curve ensures that both the laser frequency and output power are reestablished with high accuracy when the laser operation is changed from one line to another.
#### **2.4 Gas handling system**
68 CO2 Laser – Optimisation and Application
(Rosengren, 1975), which suggests that a relatively high modulation frequency is advantageous. The approach to this problem is to keep the windows as clean as possible and place acoustic baffles between the windows and the body of the sample cell where the
Fig. 17. Photoacoustic spectra: (A) clean windows on baffled cell; (B) clean windows on unbaffled cell; (C) dirty windows on baffled cell; (D) dirty windows on unbaffled cell
The acoustic baffles hinder the propagation of the window noise into the central region of the cell near the microphones. In an attempt to quantify the usefulness of the baffles, four spectra are shown in Fig. 17. These spectra illustrate the microphone signal versus laser frequency. For both the baffled and unbaffled cells, a longitudinal resonance frequency was used. From this figure it is immediately obvious how important it is to have clean windows. If the sample makes it difficult to keep the windows clean, a cell with baffles will perform much better than one without them. Even with clean windows, the baffles give a flatter base line.
The random noise level visible on the base lines of spectra A and B in Fig. 17 is probably due to a combination of the ambient lab noise, noise from the microphones, associated electronics, and the fluctuations of the laser itself. The ultimate limit of a microphone's sensitivity is set by the random thermal fluctuations in the sample and of the microphone diaphragm. In practice, the random fluctuations of the laser do not seem to be critical. Also, the combined electronic noise of the lock-in, preamplifier, and FET amplifier in the microphones totaled about 3 μV. This is at least a factor of 10 less than the noise level observed in the spectra. The noise that is visible in the spectra stems almost exclusively from thermal fluctuations and ambient lab noise. One of the major ambient noise sources is the mechanical chopper. In practice, this seems to set the detection limit. Acoustically isolating the chopper improves the noise, but replacing it with a nonmechanical modulator is better
microphones are mounted.
(Davidson et al., 1990).
still, as it also speeds up modulation.
Whenever monitoring is performed by flowing the gas mixture through a cell, a crucial question is whether the measured signal, which represents the trace gas concentration in the interaction region, also reflects the concentration at the source. On their way to the cell, the different components of a gas mixture may react with one another, form clusters or aerosols, and react with or be adsorbed on particles present, or on the sampling line and cell walls. Adsorption problems are particularly severe for polar molecules with large dipole moments, such as water and ammonia, but they can be reduced by a proper choice of materials.
The vacuum/gas handling system is an important element in these measurements owing to its role in ensuring PA cell and gas purity. The Teflon/stainless steel system can perform several functions without necessitating any disconnections. It can be used to pump out the cell, mix gases in the desired proportions, and monitor the total pressure of gases. Whenever possible, the PA cell was employed in the gas flow mode of operation to minimize any tendency for the vapor to stick to the cell walls and the effects of the subsequent outgassing of contaminants, which would otherwise lead to increasing background signals during an experimental run.
To design an efficient vacuum/gas handling system to be used in LPAS, one must make sure that the following operations can be carried out:
CO2 Laser Photoacoustic Spectroscopy: II. Instrumentation and Applications 71
The pressure of the gases added to the PA cell was determined by means of three Baratron pressure gauges (MKS Instruments, Inc.): 622A (0-1000 mbar), 122A (0-1000 mbar), and 122A
We use thermal mass flowmeters, or mass flow controllers (MFC), to deliver stable and known gas flows to the PA cell. The most critical processes will require flow measurement accuracies of 1% or better in the range 1000 to 10 sccm (7x10-4 to 7x10-6 mol/sec; 1 sccm (at 0oC) = 7.436x10-7 mol/sec). The digital MFCs sense the mass flow from the temperature difference between two temperature sensors in thermal contact with the gas stream and then process the information digitally with a microcontroller. The analog sensor output is amplified and digitized before it is sent to a microprocessor to compute the final control valve position. The gas flow in our gas handling system is adjusted by two gas flow controllers, MKS 1179A (0 - 1000 sccm) and MKS 2259CC (0 - 200 sccm), which are
By using an adequate scrubber for CO2 filtration, the CO2 interference problem can be resolved. The CO2 trap must neither alter the ethylene concentration level, nor introduce new interfering gases. By using a CO2 trap with a volume of 120 cm3 filled with fresh KOH pellets, we succeeded to reduce the CO2 content in the exhaled breath (mixt expiratory air collection) of a healthy nonsmoking young person from 3.4% (equivalent to an ethylene concentration of 2.35 ppmV) to 156 ppmV (equivalent to an ethylene concentration of 10.8
The water vapors could be additionally filtered by a cryogenic trap filled with liquid nitrogen. Using a simple and small cryogenic trap, we demonstrated the negative influence in our experiments (Dumitras et al., 2008). The liquid nitrogen temperature -196oC (77 K) is bellow the frozen point of the ethylene gas -169.2oC (104 K), so the practical effects is just to frozen both water vapors and ethylene. Introducing in the flow a calibrated mixture of 1 ppm C2H4 in N2, we observed after filling the cell at 1 atm pressure that the maximum ethylene concentration is only 51 ppb, diminished by a factor of 20 from the initial ethylene concentration. The level starts to increase suddenly at the point where we stopped the liquid nitrogen admission in the trap. In conclusion, a simple nitrogen trap is not suited for our experiments involving ethylene, but a special thermocontrolled trap can do the job, setting the working temperature below -78.5oC (194.5K), the freezing point of CO2, but above -
Fig. 19. General view of the gas mixing station.
(0-100 mbar), connected to a digital two-channel unit PDR-C-2C.
connected to a digital four-channel instrument MKS 247C.
ppbV), that is a reduction factor of 218 (see Section 2.7).
169.2oC (104 K), the freezing point of ethylene.
A vacuum/gas handling system to be used in PA experiments was designed and implemented based on these guidelines. The schematic of the gas handling chain is shown in Fig. 18, while a general view of the valves and distribution system is given in Fig. 19.
#### Fig. 18. Gas handling system.
Gas transport lines throughout the gas mixing station were made of Teflon (Swagelok PFA-T6M-1M-30M, 6 mm inner diameter and 1 mm wall thickness) to minimize adsorption and contamination. The toggle valves V1-V17 (Swagelok SS-1GS6mm) and union tees T1-T11 (Swagelok SS-6MO-3) were made of stainless steel. No valve grease was used. The PA cell gas inlet and outlet were connected to the gas handling system with Swagelok fittings (male connectors SS-6MO-1-2RT). Connections to the inlet and outlet valves of the PA cell were made via flexible Teflon tubing so as to minimize the coupling of mechanical vibrations to the PA cell. The flexible lines also make it possible to position the PA cell during optical alignment.
Fig. 19. General view of the gas mixing station.
i. evacuation by the vacuum system of the entire gas handling system, including the PA
ii. controlled introduction of a gas or gas mixture either for rinsing the PA cell and the gas handling system with pure nitrogen or for calibrating the PA spectrometer with a
iv. safe insertion in the gas handling system of a sample cuvette (usually made of Pyrex
v. filtration of certain gases (carbon dioxide and water vapors), which interfere with the
vi. controlled introduction of the trace gas to be measured from the sample cuvette or bag into the PA cell by a nonabsorbing gas (nitrogen or synthetic air) acting as carrier; vii. controlled change of the sample and carrier gas flow rates within a broad range (10-
ix. quick monitoring of the trace gas concentration in the sample gas by ensuring a
A vacuum/gas handling system to be used in PA experiments was designed and implemented based on these guidelines. The schematic of the gas handling chain is shown in Fig. 18, while a general view of the valves and distribution system is given in Fig. 19.
Gas transport lines throughout the gas mixing station were made of Teflon (Swagelok PFA-T6M-1M-30M, 6 mm inner diameter and 1 mm wall thickness) to minimize adsorption and contamination. The toggle valves V1-V17 (Swagelok SS-1GS6mm) and union tees T1-T11 (Swagelok SS-6MO-3) were made of stainless steel. No valve grease was used. The PA cell gas inlet and outlet were connected to the gas handling system with Swagelok fittings (male connectors SS-6MO-1-2RT). Connections to the inlet and outlet valves of the PA cell were made via flexible Teflon tubing so as to minimize the coupling of mechanical vibrations to the PA cell. The flexible lines also make it possible to position the PA cell during optical alignment.
iii. pressure measurement in the PA cell and in different sections of the system;
viii. simultaneous measurement of two sample gases (e.g., ethylene and ammonia);
glass) or aluminum-coated plastic bag with the trace gas sample;
response time on the order of minutes or even seconds.
cell, either totally or in different sections;
certified gas mixture;
trace gas to be measured;
1000 sccm);
Fig. 18. Gas handling system.
The pressure of the gases added to the PA cell was determined by means of three Baratron pressure gauges (MKS Instruments, Inc.): 622A (0-1000 mbar), 122A (0-1000 mbar), and 122A (0-100 mbar), connected to a digital two-channel unit PDR-C-2C.
We use thermal mass flowmeters, or mass flow controllers (MFC), to deliver stable and known gas flows to the PA cell. The most critical processes will require flow measurement accuracies of 1% or better in the range 1000 to 10 sccm (7x10-4 to 7x10-6 mol/sec; 1 sccm (at 0oC) = 7.436x10-7 mol/sec). The digital MFCs sense the mass flow from the temperature difference between two temperature sensors in thermal contact with the gas stream and then process the information digitally with a microcontroller. The analog sensor output is amplified and digitized before it is sent to a microprocessor to compute the final control valve position. The gas flow in our gas handling system is adjusted by two gas flow controllers, MKS 1179A (0 - 1000 sccm) and MKS 2259CC (0 - 200 sccm), which are connected to a digital four-channel instrument MKS 247C.
By using an adequate scrubber for CO2 filtration, the CO2 interference problem can be resolved. The CO2 trap must neither alter the ethylene concentration level, nor introduce new interfering gases. By using a CO2 trap with a volume of 120 cm3 filled with fresh KOH pellets, we succeeded to reduce the CO2 content in the exhaled breath (mixt expiratory air collection) of a healthy nonsmoking young person from 3.4% (equivalent to an ethylene concentration of 2.35 ppmV) to 156 ppmV (equivalent to an ethylene concentration of 10.8 ppbV), that is a reduction factor of 218 (see Section 2.7).
The water vapors could be additionally filtered by a cryogenic trap filled with liquid nitrogen. Using a simple and small cryogenic trap, we demonstrated the negative influence in our experiments (Dumitras et al., 2008). The liquid nitrogen temperature -196oC (77 K) is bellow the frozen point of the ethylene gas -169.2oC (104 K), so the practical effects is just to frozen both water vapors and ethylene. Introducing in the flow a calibrated mixture of 1 ppm C2H4 in N2, we observed after filling the cell at 1 atm pressure that the maximum ethylene concentration is only 51 ppb, diminished by a factor of 20 from the initial ethylene concentration. The level starts to increase suddenly at the point where we stopped the liquid nitrogen admission in the trap. In conclusion, a simple nitrogen trap is not suited for our experiments involving ethylene, but a special thermocontrolled trap can do the job, setting the working temperature below -78.5oC (194.5K), the freezing point of CO2, but above - 169.2oC (104 K), the freezing point of ethylene.
CO2 Laser Photoacoustic Spectroscopy: II. Instrumentation and Applications 73
mouth and trachea), where air does not come into contact with the alveoli of the lungs. The following part of a breath, about 350 mL, is "alveolar" breath, which comes from the lungs, where gaseous exchange between the blood and breath air takes place. Dead space air can be interpreted as essential for the detection, and depends on the type of molecule detected from the breath test. For example, the dead-space is used to quantify the amount of the NO molecules. In the case of an asthmatic patient, if the airways are inflamed, a high-level of NO is released into the airways and into the dead-space air. But for volatile organic compounds (VOCs) exchanged between blood and alveolar air, the dead-space air is a "contaminant" diluting the concentrations of VOCs when breath air is collected. In terms of the origin of the collected breath gases, there are three basic collection approaches: 1. *upper airway collection* for NO test; this means that only dead-space gas is collected (it is only for the NO test); 2. *alveolar collection*; this means that pure alveolar gas is collected (for tests of other inorganic gases and VOCs); 3. *mixed expiratory collection*; this means that total breath air, including dead-space air and alveolar gas is collected (appropriate for tests of special gases and VOCs). Because the mixed expiratory collection method is easy to perform in spontaneously breathing subjects requiring no additional equipment, it has been most frequently used in practical applications. However, concentrations of endogenous substances in alveolar air are two to three times higher than those found in mixed expiratory samples, because there is no dilution by dead-space gas. Collection of breath air can be performed for a single breath or for collection of individual breathes over a certain period of time. If the sample is collected through a single breath, one has to be sure that this single
The rate of change in concentration of a species *i* in a flowing cell is given by:
*t tC*
Fig. 21. Time response of the PA spectrometer for a gas flow rate of 100 sccm.
= *V*/*Rflow* is the renewal time constant.
( ) [ ] ( )*tCF V R*
where *Rflow* is the gas flow rate (liter/min or sccm), *Fi* is the feed concentration of species *i*, and *V* is the cell volume. This equation assumes that the adsorption rate of *i* is zero, and that gas mixing inside the cell is instantaneous. Integrating Eq. (4) with the initial condition that
*<sup>i</sup> flow* <sup>=</sup> <sup>−</sup> <sup>d</sup>
*ii*
<sup>d</sup> , (4)
( ) [ ( ) *V/tRexpFtC* ] *ii* 1 −−= *flow* , (5)
breath is representative.
*Ci*(0) = 0 gives
and τ
The following gases were used throughout the experiments:
The flow rate was usually set at a low value of 30-100 sccm in all experiments in order to eliminate the acoustic noise of the gas flow, and all measurements were carried out with the PA cell at atmospheric pressure. Flow noise increases upwards of 10 L/h (167 sccm) were found to limit the minimum response time of the detector. The flow velocity minimizes the accumulation of the produced gases in the sampling cell. The carrier gas we used was either nitrogen or synthetic air, and its flow rate through the system was monitored by the calibrated flowmeter. Provision is made for bypassing the flowmeter with the gas mixture flow prior to a measurement to equilibrate the feedline surfaces. This ensures that the measured rise times are an exclusive function of the cell characteristics. A measurement is initiated by diverting the gas flow from the bypass through the flowmeter and PA cell and monitoring the photoacoustic signal rise that follows.
As far as the sampling procedure is concerned, we use an extractive method, based on the collection of trace gas samples by some type of container or collecting medium and subsequent analysis in the laboratory. A problem may arise at this point due to some alterations of the gas composition caused by adsorption and desorption processes on the inner surface of the collecting container. The breath samples we analyzed were obtained from volunteers who agreed to provide such samples at certain time intervals. The volunteers were asked to exhale into a sample bag with a normal exhalation flow rate. The breath samples were collected in 0.75-liter aluminum-coated bags (QuinTron, Milwaukee, Wisconsin, USA) equipped with valves that sealed them after filling (Fig. 20). The bags were inserted into the gas handling system, which ensured a better control by means of two independently adjusted flow controllers of the upstream pressure and the flow rate through the sample bag.
Fig. 20. Aluminum-coated plastic bag with sample gas.
The exhaled air is a heterogeneous gas. For a healthy individual, the first part of a exhaled breath, roughly 150 mL, consists of "dead-space" air from the upper airways (such as the
The flow rate was usually set at a low value of 30-100 sccm in all experiments in order to eliminate the acoustic noise of the gas flow, and all measurements were carried out with the PA cell at atmospheric pressure. Flow noise increases upwards of 10 L/h (167 sccm) were found to limit the minimum response time of the detector. The flow velocity minimizes the accumulation of the produced gases in the sampling cell. The carrier gas we used was either nitrogen or synthetic air, and its flow rate through the system was monitored by the calibrated flowmeter. Provision is made for bypassing the flowmeter with the gas mixture flow prior to a measurement to equilibrate the feedline surfaces. This ensures that the measured rise times are an exclusive function of the cell characteristics. A measurement is initiated by diverting the gas flow from the bypass through the flowmeter and PA cell and
As far as the sampling procedure is concerned, we use an extractive method, based on the collection of trace gas samples by some type of container or collecting medium and subsequent analysis in the laboratory. A problem may arise at this point due to some alterations of the gas composition caused by adsorption and desorption processes on the inner surface of the collecting container. The breath samples we analyzed were obtained from volunteers who agreed to provide such samples at certain time intervals. The volunteers were asked to exhale into a sample bag with a normal exhalation flow rate. The breath samples were collected in 0.75-liter aluminum-coated bags (QuinTron, Milwaukee, Wisconsin, USA) equipped with valves that sealed them after filling (Fig. 20). The bags were inserted into the gas handling system, which ensured a better control by means of two independently adjusted flow controllers of the upstream pressure and the flow rate through
The exhaled air is a heterogeneous gas. For a healthy individual, the first part of a exhaled breath, roughly 150 mL, consists of "dead-space" air from the upper airways (such as the
The following gases were used throughout the experiments:
nitrogen 6.0 (purity 99.9999%).
the sample bag.
monitoring the photoacoustic signal rise that follows.
Fig. 20. Aluminum-coated plastic bag with sample gas.
hydrocarbons max. 0.1 ppmV, nitrogen oxides max. 0.1 ppmV);
mouth and trachea), where air does not come into contact with the alveoli of the lungs. The following part of a breath, about 350 mL, is "alveolar" breath, which comes from the lungs, where gaseous exchange between the blood and breath air takes place. Dead space air can be interpreted as essential for the detection, and depends on the type of molecule detected from the breath test. For example, the dead-space is used to quantify the amount of the NO molecules. In the case of an asthmatic patient, if the airways are inflamed, a high-level of NO is released into the airways and into the dead-space air. But for volatile organic compounds (VOCs) exchanged between blood and alveolar air, the dead-space air is a "contaminant" diluting the concentrations of VOCs when breath air is collected. In terms of the origin of the collected breath gases, there are three basic collection approaches: 1. *upper airway collection* for NO test; this means that only dead-space gas is collected (it is only for the NO test); 2. *alveolar collection*; this means that pure alveolar gas is collected (for tests of other inorganic gases and VOCs); 3. *mixed expiratory collection*; this means that total breath air, including dead-space air and alveolar gas is collected (appropriate for tests of special gases and VOCs). Because the mixed expiratory collection method is easy to perform in spontaneously breathing subjects requiring no additional equipment, it has been most frequently used in practical applications. However, concentrations of endogenous substances in alveolar air are two to three times higher than those found in mixed expiratory samples, because there is no dilution by dead-space gas. Collection of breath air can be performed for a single breath or for collection of individual breathes over a certain period of time. If the sample is collected through a single breath, one has to be sure that this single breath is representative.
The rate of change in concentration of a species *i* in a flowing cell is given by:
$$\frac{\text{d}C\_i(t)}{\text{d}t} = \frac{R\_{flow}}{V} [F\_i - C\_i(t)],\tag{4}$$
where *Rflow* is the gas flow rate (liter/min or sccm), *Fi* is the feed concentration of species *i*, and *V* is the cell volume. This equation assumes that the adsorption rate of *i* is zero, and that gas mixing inside the cell is instantaneous. Integrating Eq. (4) with the initial condition that *Ci*(0) = 0 gives
$$C\_{\hat{1}}(t) = F\_{\hat{1}} \left[ 1 - \exp\left(-\mathcal{R}\_{flow} t \;/\; V\right) \right] \tag{5}$$
and τ= *V*/*Rflow* is the renewal time constant.
Fig. 21. Time response of the PA spectrometer for a gas flow rate of 100 sccm.
CO2 Laser Photoacoustic Spectroscopy: II. Instrumentation and Applications 75
The software user interface contains three panels which display in real time the following parameters: CO2 laser power level; PA signal; and trace gas concentration. Another window
All settings and properties are stored to disk from session to session. In addition, a file may be automatically generated when running an experiment, including: a) **Laser power** stores powermeter readings of power incident on the sample as a function of time; b) **PA signal** stores the instantaneous values of the PA signal measured by the lock-in amplifier as a function of time; c) **Trace gas concentration** stores the time evolution of the trace gas
We designed and characterized two experimental set-ups with the PA cell in an external configuration: the first one with a low power CO2 laser where the saturation effects are negligible, and a second one with a high power CO2 laser where the saturation effects are important and have to be taken into consideration. We measured all relevant features and determined all quantities used in literature to compare our findings with the best results reported in the previous published papers. All measurements were done in nitrogen and ethylene with the 10P(14) line of a cw CO2 laser. We succeeded to obtain a minimum detectable concentration better by more than a factor of 10 compared to the best results
To investigate the possibility of using a high power laser in an extracavity configuration, we introduced in the experimental set-up a commercial CO2 laser (Coherent GEM SELECT 50TM laser) with output power till 50 W and tunable on 73 different lines (Dumitras et al., 2010). When this laser is tuned on 10P(14) line, the maximum power delivered after chopper and
To change the laser power inside the PA cell we tried either to modify the input power in the laser (RF power supply) or to introduce a beam splitter in the path of the laser beam. Unfortunately, both methods change significantly the beam path inside the PA cell, thus perturbing unacceptably the results of the measurements. The waveguide laser has a poor beam pointing because it has a short optical resonator and the variation of the transverse RF
The saturation effects at high laser power were investigated by using the method of truncation of a gaussian laser beam. This approach was possible because the laser beam is very close to a gaussian beam (M2 < 1.1). The method consists in passing the beam through an aperture with known diameter (Fig. 23). To avoid deformations owing to heating, we used water cooled metallic diaphragms with diameters between 1.42 mm and 5.03 mm. All
When a gaussian beam of radius *w* is truncated by an aperture of radius *a* (Fig. 24), the power transmitted through the aperture is *T* = *P*(*a*)/*P* = 1 – exp(-2*a*2/*w*2). When 2*a* = 2*w*, *T* ≅ 86%, that is 86% of the laser power is transmitted through the aperture (this is known as 86% criterion). When 2*a* = π*w*, *T* ≅ 99%, that is 99% of the laser power is transmitted through the aperture and we have the 99% criterion. This formula offers a possibility to measure precisely the diameter of the laser beam at the position of the diaphragm. By knowing the radius of the aperture (*a*) and by measuring the laser power before and after the aperture (*P*
diaphragms were placed at a distance of 450 mm from the beam waist of the laser.
(countdown) indicates the number of remaining measurement points.
concentration for a given laser wavelength.
**2.6 Low power vs. high power lasers**
previously reported in the literature.
excitation modifies the laser gain profile.
focusing lens is 14.5 W.
The solid line in Fig. 21 shows the rise time curve predicted by Eq. (5) for the experimental parameter values: *V* = 1200 cm3 and *Rflow* = 100 sccm. The total renewal time of the gas content in the system (sampling cell, scrubber, and photoacoustic cell) is τ = 12 min (1/*e* time). This τ value is small compared to the time response of certain biological samples (e.g., the C2H4 production of a single flower, 0.02-0.3 nL/g/h) (Harren et al., 1990b).
#### **2.5 Data acquisition and processing**
The acquisition and processing of the recorded data was done with Keithley TestPoint software. TestPoint data acquisition software provides a development environment in which data acquisition applications can be generated. A graphical editor is provided for creating a user interface, or "panel", which the user sees and interacts with as the application executes. A user panel is made of pictorial elements that represent such things as switches, variable controls, numerical, text and selection boxes, bar displays, graphs, and strip charts. In addition, an application editor is provided, which ensures some interactive means of specifying how the visual elements on the user panel interact with the data sources and processing functions to achieve application goals. TestPoint uses an automated textual description of the operations carried out by each user panel element.
We developed a modular software architecture aimed at controlling the experiments, collecting data, and preprocessing information. It helps automate the process of collecting and processing experimental results. The software controls the chopper frequency, transfers powermeter readings, normalizes data, and automatically stores files. It allows the user to set parameters such as the PA cell responsivity (a constant used to normalize raw data), gas absorption coefficient, number of averaged samples at every measurement point, sample acquisition rate, and total number of measurement points. This software interfaces the following instruments:
The software user interface allows the user to set or read input data and instantaneous values for the PA voltage (rms), average laser power after chopper, and trace gas concentration. Users may set experimental parameters for the PA cell responsivity and gas absorption coefficient. They are also provided with a text input to write a description of the experiments or take other notes. The user interface also provides data visualization (Fig. 22).
Fig. 22. Software user interface used to record trace gas concentrations.
The software user interface contains three panels which display in real time the following parameters: CO2 laser power level; PA signal; and trace gas concentration. Another window (countdown) indicates the number of remaining measurement points.
All settings and properties are stored to disk from session to session. In addition, a file may be automatically generated when running an experiment, including: a) **Laser power** stores powermeter readings of power incident on the sample as a function of time; b) **PA signal** stores the instantaneous values of the PA signal measured by the lock-in amplifier as a function of time; c) **Trace gas concentration** stores the time evolution of the trace gas concentration for a given laser wavelength.
#### **2.6 Low power vs. high power lasers**
74 CO2 Laser – Optimisation and Application
The solid line in Fig. 21 shows the rise time curve predicted by Eq. (5) for the experimental parameter values: *V* = 1200 cm3 and *Rflow* = 100 sccm. The total renewal time of the gas
The acquisition and processing of the recorded data was done with Keithley TestPoint software. TestPoint data acquisition software provides a development environment in which data acquisition applications can be generated. A graphical editor is provided for creating a user interface, or "panel", which the user sees and interacts with as the application executes. A user panel is made of pictorial elements that represent such things as switches, variable controls, numerical, text and selection boxes, bar displays, graphs, and strip charts. In addition, an application editor is provided, which ensures some interactive means of specifying how the visual elements on the user panel interact with the data sources and processing functions to achieve application goals. TestPoint uses an automated textual
We developed a modular software architecture aimed at controlling the experiments, collecting data, and preprocessing information. It helps automate the process of collecting and processing experimental results. The software controls the chopper frequency, transfers powermeter readings, normalizes data, and automatically stores files. It allows the user to set parameters such as the PA cell responsivity (a constant used to normalize raw data), gas absorption coefficient, number of averaged samples at every measurement point, sample acquisition rate, and total number of measurement points. This software interfaces the
The software user interface allows the user to set or read input data and instantaneous values for the PA voltage (rms), average laser power after chopper, and trace gas concentration. Users may set experimental parameters for the PA cell responsivity and gas absorption coefficient. They are also provided with a text input to write a description of the experiments or take other notes. The user interface also provides data visualization (Fig. 22).
value is small compared to the time response of certain biological samples (e.g.,
τ
= 12 min (1/*e*
content in the system (sampling cell, scrubber, and photoacoustic cell) is
description of the operations carried out by each user panel element.
Fig. 22. Software user interface used to record trace gas concentrations.
the C2H4 production of a single flower, 0.02-0.3 nL/g/h) (Harren et al., 1990b).
time). This
τ
following instruments: • lock-in amplifier;
• laser powermeter; • gas flowmeter.
• chopper;
**2.5 Data acquisition and processing**
We designed and characterized two experimental set-ups with the PA cell in an external configuration: the first one with a low power CO2 laser where the saturation effects are negligible, and a second one with a high power CO2 laser where the saturation effects are important and have to be taken into consideration. We measured all relevant features and determined all quantities used in literature to compare our findings with the best results reported in the previous published papers. All measurements were done in nitrogen and ethylene with the 10P(14) line of a cw CO2 laser. We succeeded to obtain a minimum detectable concentration better by more than a factor of 10 compared to the best results previously reported in the literature.
To investigate the possibility of using a high power laser in an extracavity configuration, we introduced in the experimental set-up a commercial CO2 laser (Coherent GEM SELECT 50TM laser) with output power till 50 W and tunable on 73 different lines (Dumitras et al., 2010). When this laser is tuned on 10P(14) line, the maximum power delivered after chopper and focusing lens is 14.5 W.
To change the laser power inside the PA cell we tried either to modify the input power in the laser (RF power supply) or to introduce a beam splitter in the path of the laser beam. Unfortunately, both methods change significantly the beam path inside the PA cell, thus perturbing unacceptably the results of the measurements. The waveguide laser has a poor beam pointing because it has a short optical resonator and the variation of the transverse RF excitation modifies the laser gain profile.
The saturation effects at high laser power were investigated by using the method of truncation of a gaussian laser beam. This approach was possible because the laser beam is very close to a gaussian beam (M2 < 1.1). The method consists in passing the beam through an aperture with known diameter (Fig. 23). To avoid deformations owing to heating, we used water cooled metallic diaphragms with diameters between 1.42 mm and 5.03 mm. All diaphragms were placed at a distance of 450 mm from the beam waist of the laser.
When a gaussian beam of radius *w* is truncated by an aperture of radius *a* (Fig. 24), the power transmitted through the aperture is *T* = *P*(*a*)/*P* = 1 – exp(-2*a*2/*w*2). When 2*a* = 2*w*, *T* ≅ 86%, that is 86% of the laser power is transmitted through the aperture (this is known as 86% criterion). When 2*a* = π*w*, *T* ≅ 99%, that is 99% of the laser power is transmitted through the aperture and we have the 99% criterion. This formula offers a possibility to measure precisely the diameter of the laser beam at the position of the diaphragm. By knowing the radius of the aperture (*a*) and by measuring the laser power before and after the aperture (*P*
CO2 Laser Photoacoustic Spectroscopy: II. Instrumentation and Applications 77
a factor of 6. We can observe that the saturation effects manifest immediately as the laser power is increased more than 2 W. The cause of the saturation is that the collisional relaxation to de-excite the molecules cannot keep up with the excitation rate by the laser beam intensity. By increasing laser intensity, the excitation pumping rate of the molecules grows higher and a molecule is more likely to absorb a nearby photon before it relaxes to the ground state. So, as the number of molecules in the excited state increases, the number of molecules which can absorb laser radiation is reduced. That is why we introduced a supplementary scale in Fig. 27, representing the cell responsivity function on the intensity of the focused beam inside the PA cell. A thorough analysis of the laser beam propagation through the focusing lens and in the PA cell was done (Dumitras et al., 2007). For a lens with a focal length of 400 mm, we got in this case a beam diameter at beam waist of 0.89 mm. It can be remarked that saturation starts at laser intensities greater than 0.5 kW/cm2. This result shows that saturation effects manifest even at low laser intensities. Previously, Harren et al. (Harren et al., 1990a) observed a strong saturation at a much higher intensity (200 kW/cm2; the laser power was ten times higher and the beam area was ten times smaller than in our case at 13 W), when the PA cell was placed intracavity of a waveguide CO2 laser. Our conclusion is that high power lasers could be used in PA systems, but saturation effects should be taken into consideration (by making a correlation between the PA cell
responsivity and the working laser intensity, as in Fig. 27).
Fig. 26. Variation of laser power function on diaphragm aperture size.
on laser power and on laser intensity in the focal spot.
Fig. 27. Saturation effects measured as the dependence of the PA cell responsivity function
An important question is: what happens with the system noises when a diaphragm is introduced in the laser beam path? We proceeded to record the coherent PA background
and *P*(*a*), respectively), we can determine immediately the radius of the laser beam (*w*). As it can be seen in Fig. 25, by using five different diaphragms, the resulting average diameter is 2*w* = (7.09 ± 0.2) mm, with an error of less than 3%.
Fig. 23. Attenuation of a laser beam by a diaphragm.
Fig. 24. Truncation of a gaussian laser beam.
Fig. 25. Measurement of laser beam diameter by the method of truncation.
Figure 26 shows the attenuation of the laser beam when different diaphragms were placed in its path. The solid line is the theoretical curve given by the above equation. By introducing these five diaphragms, the laser power was varied between less than 2 W and near 10 W. In this way, we were able to investigate the laser power range from low power where saturation effects have no significance till high power where saturation effects manifest strongly.
We investigated the influence of saturation by measuring the dependence of the PA cell responsivity function on laser power (Fig. 27). From low laser power regime (under 2 W) where the saturation effects are not important till high power regime (14.5 W, no diaphragm), the PA cell responsivity decreases from 312 V cm/W till 52 V cm/W, that is by
and *P*(*a*), respectively), we can determine immediately the radius of the laser beam (*w*). As it can be seen in Fig. 25, by using five different diaphragms, the resulting average diameter is
2*w* = (7.09 ± 0.2) mm, with an error of less than 3%.
Fig. 23. Attenuation of a laser beam by a diaphragm.
Fig. 24. Truncation of a gaussian laser beam.
manifest strongly.
Fig. 25. Measurement of laser beam diameter by the method of truncation.
Figure 26 shows the attenuation of the laser beam when different diaphragms were placed in its path. The solid line is the theoretical curve given by the above equation. By introducing these five diaphragms, the laser power was varied between less than 2 W and near 10 W. In this way, we were able to investigate the laser power range from low power where saturation effects have no significance till high power where saturation effects
We investigated the influence of saturation by measuring the dependence of the PA cell responsivity function on laser power (Fig. 27). From low laser power regime (under 2 W) where the saturation effects are not important till high power regime (14.5 W, no diaphragm), the PA cell responsivity decreases from 312 V cm/W till 52 V cm/W, that is by a factor of 6. We can observe that the saturation effects manifest immediately as the laser power is increased more than 2 W. The cause of the saturation is that the collisional relaxation to de-excite the molecules cannot keep up with the excitation rate by the laser beam intensity. By increasing laser intensity, the excitation pumping rate of the molecules grows higher and a molecule is more likely to absorb a nearby photon before it relaxes to the ground state. So, as the number of molecules in the excited state increases, the number of molecules which can absorb laser radiation is reduced. That is why we introduced a supplementary scale in Fig. 27, representing the cell responsivity function on the intensity of the focused beam inside the PA cell. A thorough analysis of the laser beam propagation through the focusing lens and in the PA cell was done (Dumitras et al., 2007). For a lens with a focal length of 400 mm, we got in this case a beam diameter at beam waist of 0.89 mm. It can be remarked that saturation starts at laser intensities greater than 0.5 kW/cm2. This result shows that saturation effects manifest even at low laser intensities. Previously, Harren et al. (Harren et al., 1990a) observed a strong saturation at a much higher intensity (200 kW/cm2; the laser power was ten times higher and the beam area was ten times smaller than in our case at 13 W), when the PA cell was placed intracavity of a waveguide CO2 laser. Our conclusion is that high power lasers could be used in PA systems, but saturation effects should be taken into consideration (by making a correlation between the PA cell responsivity and the working laser intensity, as in Fig. 27).
Fig. 26. Variation of laser power function on diaphragm aperture size.
Fig. 27. Saturation effects measured as the dependence of the PA cell responsivity function on laser power and on laser intensity in the focal spot.
An important question is: what happens with the system noises when a diaphragm is introduced in the laser beam path? We proceeded to record the coherent PA background
CO2 Laser Photoacoustic Spectroscopy: II. Instrumentation and Applications 79
of 18 in the second case. In a molecular gas with a high absorption coefficient (e.g. SF6), the
Interference of other absorbing substances may impair the theoretical detection limit in a multicomponent analysis of the real samples. Such interference may be caused by other molecular systems present in the environment or substances that are entrained by the carrier flux. If an interfering species is present in the environment, its effect can be minimized by either the introduction of scrubbers and cryogenic traps or the use of dual beam techniques
The CO2 laser spectral outputs occur in the wavelength region where a large number of compounds possess strong absorption features and where absorptive interferences from water vapors, carbon dioxide, and other major atmospheric gaseous components may
The breath air is a mixture of nitrogen, oxygen, carbon dioxide, water, inert gases, and traces of VOCs (Table 5). The matrix elements in breath air vary widely from person to person, both qualitatively and quantitatively, particularly for VOCs. More than 1000 trace VOCs have been distinguished in human breath air, at concentrations from ppmV to pptV levels. Only a small number of VOCs are common to everyone, including isoprene, acetone, ethane, and methanol, which are products of core metabolic processes. In addition to these VOCs, exhaled NO, H2, NH3, and CO are related to health condition and can reflect a potential disease of the individual or a recent exposure to a drug or an environmental pollutant. **Component Inhaled air (%) Exhaled air (%)**
> Nitrogen 78.0 78.0 Oxygen 21.0 16.0 Carbon dioxide 0.04 3.0-5.0 Argon 0.93 1.0 Water 2.0 5.0-6.0
> > 0.01
Table 5. Concentration of different components in inhaled and exhaled air.
scrubber filled with a chemical active agent, KOH in our case (Bratu et al., 2011).
A healthy adult human has a respiratory rate of 12-15 breaths/min at rest, inspiring and expiring 6-8 L of air per minute. O2 enters the blood and CO2 is eliminated through the alveoli. When the end-tidal concentration of CO2 in healthy persons is measured, a large change of CO2 concentration is observed between the inhaled air (~ 0.04%) and the exhaled air (~ 4%). The exact amount of exhaled CO2 varies according to the fitness, energy expenditure and diet of a particular person, with regular values of 3-5%. Due to this high concentration of carbon dioxide in the breath and because CO2 laser lines are absorbed by this gas, it is necessary to remove most of the carbon dioxide from the exhaled air by introducing a
Due to the exact coincidence of the CO2 vibrational-rotational transitions with the CO2 laser lines, carbon dioxide at high concentration in comparison with trace gases like C2H4 is
250x10-9 (250 ppb) 6x10-9 (6 ppb)
minimum detectable concentration could be as low as 9 pptV.
**2.7 Removal of interfering gases**
using two photoacoustic (PA) cells.
Other ammonia ethylene
influence the measurements.
signal on laser power (with and without a diaphragm) and the results are given in Fig. 28. The background signal is huge when a diaphragm is inserted into the system, being of more than 50 times higher than in the case that no diaphragm limits the laser beam. Truncation distorts the intensity pattern of the transmitted beam in both the near-field (Fresnel) and farfield (Fraunhofer) regions. The diffraction effects on an ideal gaussian beam of a sharpedged circular aperture even as large as 2*a* = 2*w* (99% criterion) will cause near-field diffraction ripples with an intensity variation Δ*I*/*I* ≅ ± 17% in the near field, along with a peak intensity reduction of ≅ 17% on axis in the far field (Siegman, 1986). In conclusion, the method of diaphragms used to measure the saturation effects is applicable, but in a laser PA system used in practice an aperture has never to be introduced.
Fig. 28. Dependence of coherent PA background signal on laser power (with and without a diaphragm).
Table 4. Comparison of low vs. high laser power configurations: 10P(14) laser line in N2\* (α *=* 0 cm-1atm-1), C2H4\*\* (α *=* 30.4 cm-1atm-1), and SF6\*\*\* (α *=* 686 cm-1atm-1).
To this date, the minimum detectable concentrations obtained by us in ethylene (0.9 ppbV with a low power laser and 0.21 ppbV with a high power laser) are the best values reported in the literature, improving this parameter by a factor of 4.2 in the first case and by a factor of 18 in the second case. In a molecular gas with a high absorption coefficient (e.g. SF6), the minimum detectable concentration could be as low as 9 pptV.
#### **2.7 Removal of interfering gases**
78 CO2 Laser – Optimisation and Application
signal on laser power (with and without a diaphragm) and the results are given in Fig. 28. The background signal is huge when a diaphragm is inserted into the system, being of more than 50 times higher than in the case that no diaphragm limits the laser beam. Truncation distorts the intensity pattern of the transmitted beam in both the near-field (Fresnel) and farfield (Fraunhofer) regions. The diffraction effects on an ideal gaussian beam of a sharpedged circular aperture even as large as 2*a* = 2*w* (99% criterion) will cause near-field diffraction ripples with an intensity variation Δ*I*/*I* ≅ ± 17% in the near field, along with a peak intensity reduction of ≅ 17% on axis in the far field (Siegman, 1986). In conclusion, the method of diaphragms used to measure the saturation effects is applicable, but in a laser PA
Fig. 28. Dependence of coherent PA background signal on laser power (with and without a
A comparison of low laser power vs. high laser power configurations is presented in Table 4. It seems resonable that high power lasers could be used in PA instruments provided that
**Parameter Low power High power Factor** Output laser power (W) 5.5 33 > 6.0 Average laser power (at cell exit) (W) 2.2 14.5 > 6.6
Cell responsivity *R* (V cm/W)\*\* 280 312 > 1.1 Signal saturation\*\* Small Very high > 6.0
*min* (ppbV)\*\*
0.9 0.04
α
*min* (cm-1)\*\* 2.7x10-8 0.64x10-8 < 4.3
Better 4.2 x Better 18 x
*=* 686 cm-1atm-1).
*min* (ppbV)\*\*\*
Table 4. Comparison of low vs. high laser power configurations: 10P(14) laser line in N2\* (
To this date, the minimum detectable concentrations obtained by us in ethylene (0.9 ppbV with a low power laser and 0.21 ppbV with a high power laser) are the best values reported in the literature, improving this parameter by a factor of 4.2 in the first case and by a factor
*=* 30.4 cm-1atm-1), and SF6\*\*\* (
α
2.7 0.7 < 4.0
0.21 0.009 < 4.3
α
system used in practice an aperture has never to be introduced.
the saturation is considered and compensated.
Coherent photoacoustic background signal
Best value previously reported (Harren et al., 1990)
α
Minimum detectable concentration *c*
*c*
Minimum detectable absorptivity
*min* = 3.8 ppbV)\*\*
*=* 0 cm-1atm-1), C2H4\*\* (
diaphragm).
(µV/W)\*
(*c*
Interference of other absorbing substances may impair the theoretical detection limit in a multicomponent analysis of the real samples. Such interference may be caused by other molecular systems present in the environment or substances that are entrained by the carrier flux. If an interfering species is present in the environment, its effect can be minimized by either the introduction of scrubbers and cryogenic traps or the use of dual beam techniques using two photoacoustic (PA) cells.
The CO2 laser spectral outputs occur in the wavelength region where a large number of compounds possess strong absorption features and where absorptive interferences from water vapors, carbon dioxide, and other major atmospheric gaseous components may influence the measurements.
The breath air is a mixture of nitrogen, oxygen, carbon dioxide, water, inert gases, and traces of VOCs (Table 5). The matrix elements in breath air vary widely from person to person, both qualitatively and quantitatively, particularly for VOCs. More than 1000 trace VOCs have been distinguished in human breath air, at concentrations from ppmV to pptV levels. Only a small number of VOCs are common to everyone, including isoprene, acetone, ethane, and methanol, which are products of core metabolic processes. In addition to these VOCs, exhaled NO, H2, NH3, and CO are related to health condition and can reflect a potential disease of the individual or a recent exposure to a drug or an environmental pollutant.
Table 5. Concentration of different components in inhaled and exhaled air.
A healthy adult human has a respiratory rate of 12-15 breaths/min at rest, inspiring and expiring 6-8 L of air per minute. O2 enters the blood and CO2 is eliminated through the alveoli. When the end-tidal concentration of CO2 in healthy persons is measured, a large change of CO2 concentration is observed between the inhaled air (~ 0.04%) and the exhaled air (~ 4%). The exact amount of exhaled CO2 varies according to the fitness, energy expenditure and diet of a particular person, with regular values of 3-5%. Due to this high concentration of carbon dioxide in the breath and because CO2 laser lines are absorbed by this gas, it is necessary to remove most of the carbon dioxide from the exhaled air by introducing a scrubber filled with a chemical active agent, KOH in our case (Bratu et al., 2011).
Due to the exact coincidence of the CO2 vibrational-rotational transitions with the CO2 laser lines, carbon dioxide at high concentration in comparison with trace gases like C2H4 is
CO2 Laser Photoacoustic Spectroscopy: II. Instrumentation and Applications 81
solution with very low chloride content. It reacts violently with acid and it is corrosive in moist air toward metals such as zinc, aluminum, tin and lead, forming a combustible, explosive gas. It absorbs rapidly carbon dioxide and water from air. Cautions must be taken when used because the inhaled dust is caustic and irritant, and touching skin or clothes
We have investigated the efficiency of the KOH scrubber using four recipients with different volumes (13 cm3, 45 cm3, 120 cm3, and 213 cm3, respectively), and we found out what type has to be used in order to reduce efficiently the amount of CO2 from the exhaled air sample (Bratu et al., 2011). The KOH scrubber must neither change the ethylene concentration level, nor introduce new interfering gases. The measurements were made each time on the same person (healthy female, 30 years old) and with a new filling of KOH pellets. The gas from the sample bag was transferred into the PA cell at a controlled flow rate of 300 sccm (only for the 13 cm3 trap) or 600 sccm, in order to ensure a sufficient time of flow in the scrubber column and to minimize any tendency for the vapor to stick to the cell walls or any other effects of internal outgassing of contaminants, which would otherwise lead to increase background signals during an experimental run. The typical resulting final pressure inside the PA cell was around 700 mbar and the corresponding responsivity was 170 cmV/W (see
The experimental results without the KOH scrubber showed an equivalent ethylene absorption concentration of 2750 ppbV (with alveolar air collection) and 2350 ppbV (with mixt expiratory air collection), representing mainly the contribution of ethylene, carbon dioxide, water vapors and ammonia to the absorption of 10P(14) CO2 laser line (Fig. 29). We tested the efficiency of traps filled with KOH and having different volumes (between 13 cm3
For the first measurement we used a trap with a small volume of 13 cm3 of KOH scrubber, and we obtained a decrease of the PA signal down to 1-3 mV. The equivalent ethylene concentration was 435 ppb and 240 ppb, respectively (alveolar air collection vs. mixt expiratory air collection), indicating that the CO2 concentration was reduced by factors of 6.3
could lead to less or more severe chemical burnings.
and 213 cm3) in removing CO2 from exhaled air.
Fig. 29. Efficiency of KOH traps for CO2 removal from exhaled air.
Fig. 3, Part I).
inevitably excited by CO2 laser radiation and the related photoacoustic signal may exceed the trace signal by many orders of magnitude. The absorption coefficient increases strongly with temperature, but it is independent of the CO2 concentration over a wide range. Ethylene can be excited by the 10P(14) line of the CO2 laser, where the maximum absorption coefficient α(C2H4) has a value of 30.4 cm-1 atm-1 and ammonia by the 9R(30) line where α(NH3) = 56 cm-1 atm-1 (Dumitras et al., 2011). A 4% concentration of CO2 has an absorption strength comparable to 2760 ppbV of C2H4 (at the 10P(14) laser line, α(CO2) = 2.1x10-3 atm-1cm-1 and *c*(C2H4) = *c*(CO2)α(CO2)/α(C2H4). This equivalent ethylene concentration was found also experimentally (see Fig. 29, measurement without trap). So, the photoacoustic signal is 100 times higher owing to exhaled carbon dioxide in comparison with the usual concentration of ethylene in exhaled air. Similarly, at the 9R(30) line of CO2 laser, the same concentration of CO2 has an absorption coefficient equal to that of 1500 ppbV of NH3. This value is also considerably higher (6 times) compared to the real range of breath concentration which is situated approximately at 250 ppb for ammonia.
Water vapor exhibits a broad continuum with occasional weak lines in the frequency range of the CO2 laser (for H2O at the 10P (14) laser line, α(H2O) = 2.85x10-5 atm-1cm-1). The two dominant peaks are the absorption lines on 10R(20) and the most favorable one for ambient air measurement, the 10P(40) laser transition. A 5% concentration of H2O has an absorption strength comparable to 46.9 ppbV of C2H4, that is the normal concentration of water in exhaled air has approximately the same influence in the photoacoustic signal as the normal concentration of ethylene.
Due to the additive character of the photoacoustic signal under normal pressure conditions, the presence of a large amount of water vapor and carbon dioxide impedes C2H4 detection in the low-concentration range (ppbV). Consequently, some means of selective spectral discrimination is required if ethylene is to be detected interference free in the matrix of absorbing gases. There are several ways to overcome this problem. One way is to remove CO2 from the flowing sample by absorption on a KOH-based scrubber inserted between the sampling cell and the PA cell. Taking into account the nature of the specific chemical reactions involved in the CO2 removal by KOH, a certain amount of water is also absorbed from the sample passing the scrubber. In this way, concentrations below 1 ppmV CO2 (equivalent to a concentration of 0.07 ppbV of C2H4) can be achieved without influencing the C2H4 or NH3 concentration.
Before entering the photoacoustic cell, the gas mixture passes through a KOH scrubber (Fig. 18), which retains most of the interfering carbon dioxide. The removal of CO2 is limited to the absorbent surface of the pellets. Hence, the larger the surface area or the more porous the granular solid, the larger the capacity of the system to absorb CO2. At the same time, the flow resistance varies inversely proportional to the particle size. Large particles offer less resistance, but have the disadvantage of providing a smaller total area for reaction. The granules of KOH that we used were typically Merck KOH pellets GR for analysis, with approximate dimensions of 10x7x2 mm. When residence time (time of contact between CO2 and absorbent) is less than 1 second, CO2 absorption capacity is greatly reduced, so we introduced flow controllers in order to ensure this pre-requisite.
Potassiun hydroxide is a caustic compound of strong alkaline chemical, dissolving readily in water, giving off much heat and forming a caustic solution. It is a white deliquescent solid in the form of pellets obtained by concentration of purified electrolytic potassium hydroxide
inevitably excited by CO2 laser radiation and the related photoacoustic signal may exceed the trace signal by many orders of magnitude. The absorption coefficient increases strongly with temperature, but it is independent of the CO2 concentration over a wide range. Ethylene can be excited by the 10P(14) line of the CO2 laser, where the maximum absorption
(NH3) = 56 cm-1 atm-1 (Dumitras et al., 2011). A 4% concentration of CO2 has an absorption
found also experimentally (see Fig. 29, measurement without trap). So, the photoacoustic signal is 100 times higher owing to exhaled carbon dioxide in comparison with the usual concentration of ethylene in exhaled air. Similarly, at the 9R(30) line of CO2 laser, the same concentration of CO2 has an absorption coefficient equal to that of 1500 ppbV of NH3. This value is also considerably higher (6 times) compared to the real range of breath
Water vapor exhibits a broad continuum with occasional weak lines in the frequency range
dominant peaks are the absorption lines on 10R(20) and the most favorable one for ambient air measurement, the 10P(40) laser transition. A 5% concentration of H2O has an absorption strength comparable to 46.9 ppbV of C2H4, that is the normal concentration of water in exhaled air has approximately the same influence in the photoacoustic signal as the normal
Due to the additive character of the photoacoustic signal under normal pressure conditions, the presence of a large amount of water vapor and carbon dioxide impedes C2H4 detection in the low-concentration range (ppbV). Consequently, some means of selective spectral discrimination is required if ethylene is to be detected interference free in the matrix of absorbing gases. There are several ways to overcome this problem. One way is to remove CO2 from the flowing sample by absorption on a KOH-based scrubber inserted between the sampling cell and the PA cell. Taking into account the nature of the specific chemical reactions involved in the CO2 removal by KOH, a certain amount of water is also absorbed from the sample passing the scrubber. In this way, concentrations below 1 ppmV CO2 (equivalent to a concentration of 0.07 ppbV of C2H4) can be achieved without influencing the
Before entering the photoacoustic cell, the gas mixture passes through a KOH scrubber (Fig. 18), which retains most of the interfering carbon dioxide. The removal of CO2 is limited to the absorbent surface of the pellets. Hence, the larger the surface area or the more porous the granular solid, the larger the capacity of the system to absorb CO2. At the same time, the flow resistance varies inversely proportional to the particle size. Large particles offer less resistance, but have the disadvantage of providing a smaller total area for reaction. The granules of KOH that we used were typically Merck KOH pellets GR for analysis, with approximate dimensions of 10x7x2 mm. When residence time (time of contact between CO2 and absorbent) is less than 1 second, CO2 absorption capacity is greatly reduced, so we
Potassiun hydroxide is a caustic compound of strong alkaline chemical, dissolving readily in water, giving off much heat and forming a caustic solution. It is a white deliquescent solid in the form of pellets obtained by concentration of purified electrolytic potassium hydroxide
introduced flow controllers in order to ensure this pre-requisite.
strength comparable to 2760 ppbV of C2H4 (at the 10P(14) laser line,
concentration which is situated approximately at 250 ppb for ammonia.
α(CO2)/α
of the CO2 laser (for H2O at the 10P (14) laser line,
(C2H4) has a value of 30.4 cm-1 atm-1 and ammonia by the 9R(30) line where
α
α
(H2O) = 2.85x10-5 atm-1cm-1). The two
(C2H4). This equivalent ethylene concentration was
(CO2) = 2.1x10-3 atm-
coefficient
α
α
1cm-1 and *c*(C2H4) = *c*(CO2)
concentration of ethylene.
C2H4 or NH3 concentration.
solution with very low chloride content. It reacts violently with acid and it is corrosive in moist air toward metals such as zinc, aluminum, tin and lead, forming a combustible, explosive gas. It absorbs rapidly carbon dioxide and water from air. Cautions must be taken when used because the inhaled dust is caustic and irritant, and touching skin or clothes could lead to less or more severe chemical burnings.
We have investigated the efficiency of the KOH scrubber using four recipients with different volumes (13 cm3, 45 cm3, 120 cm3, and 213 cm3, respectively), and we found out what type has to be used in order to reduce efficiently the amount of CO2 from the exhaled air sample (Bratu et al., 2011). The KOH scrubber must neither change the ethylene concentration level, nor introduce new interfering gases. The measurements were made each time on the same person (healthy female, 30 years old) and with a new filling of KOH pellets. The gas from the sample bag was transferred into the PA cell at a controlled flow rate of 300 sccm (only for the 13 cm3 trap) or 600 sccm, in order to ensure a sufficient time of flow in the scrubber column and to minimize any tendency for the vapor to stick to the cell walls or any other effects of internal outgassing of contaminants, which would otherwise lead to increase background signals during an experimental run. The typical resulting final pressure inside the PA cell was around 700 mbar and the corresponding responsivity was 170 cmV/W (see Fig. 3, Part I).
The experimental results without the KOH scrubber showed an equivalent ethylene absorption concentration of 2750 ppbV (with alveolar air collection) and 2350 ppbV (with mixt expiratory air collection), representing mainly the contribution of ethylene, carbon dioxide, water vapors and ammonia to the absorption of 10P(14) CO2 laser line (Fig. 29). We tested the efficiency of traps filled with KOH and having different volumes (between 13 cm3 and 213 cm3) in removing CO2 from exhaled air.
Fig. 29. Efficiency of KOH traps for CO2 removal from exhaled air.
For the first measurement we used a trap with a small volume of 13 cm3 of KOH scrubber, and we obtained a decrease of the PA signal down to 1-3 mV. The equivalent ethylene concentration was 435 ppb and 240 ppb, respectively (alveolar air collection vs. mixt expiratory air collection), indicating that the CO2 concentration was reduced by factors of 6.3
CO2 Laser Photoacoustic Spectroscopy: II. Instrumentation and Applications 83
Fig. 30. Decrease of KOH trap efficiency when the same fill was used for multiple
The nonlinearity of the CO2 removal could be explained by the mechanism of the chemical reactions. First, the CO2 combines with the water vapors present in the exhaled air in the
CO2 + H2O H2CO3. (6) Further, the last one combines with the KOH, creating potassium carbonate and water, and
H2CO3 + 2KOH K2CO3 + 2H2O + Energy. (7) In the same time, K2CO3 is a highly hygroscopic compound with a retaining capacity of 0.2 g
The water is of high importance in limiting the rate of CO2 absorption. High CO2 concentrations entering the KOH absorber generates large quantities of water, because the reaction (7) is producing water. We know that the absorption rate is greater thanks to the film of moisture coating the pellets, but the same film impedes the access to the active potassium hydroxide pellet volume. More dedicated studies should be made in order to
In conclusion, we determined experimentally that in the process of CO2 removal from the breath air samples, a quantity of minimum 120 cm3 KOH pellets should be used for a sampling bag of 750 mL in order to keep the detection of ethylene and ammonia traces free of CO2 interference. It should be mentioned that this volume of 120 cm3 must be reconsidered for sample bags with a greater volume (> 750 mL) or when the gas transfer rate
Based on our experience, we summarize a list of actions to obtain a minimum detectable
H2O/1 g K2CO3, so the generated water will be only partially returned in the circuit.
establish the moisture content for an optimum rate of absorption.
from the bag to the PA cell is larger (> 600 sccm).
concentration (*cmin*) as low as possible (Dumitras et al., 2010):
**2.8 Recipe for an optimum PA system**
measurements.
form of carbonic acid:
releasing a small amount of heat:
and 9.8, respectively. Only in the case of this trap we observed a peculiar behaviour. Even if the laser power is constant, the PA signal and consequently the equivalent ethylene concentration increases in time after transfering the gas sample in PA cell. The increase of concentration starts from 50 ppbV and continues until it stabilizes at a level of 435/240 ppbV (after 10-15 minutes). It is known that C2H4 (28.05 g/mol molar mass) is lighter than CO2 (44.0099 g/mol molar mass). Because of that, we can say that after passing the KOH scrubber, first C2H4 enters in the PA cell and then CO2 when the trap is no longer effective. So, at the beginning, we measured only the C2H4 concentration and then CO2 starts to strongly interfere in absorption. It is possible that due to the geometry of the cell, a longer time is required in order to attain the total homogeneity of the molecules inside the resonant tube of the cell, but this is not advantageous for repeated measurements.
Larger KOH traps proved to be more efficient in removal of CO2 from the exhaled air. For the traps with volumes of 45 cm3, 120 cm3, and 213 cm3, respectively, the measured equivalent ethylene concentrations were 41.5/23.6 ppbV, 30/10.8 ppbV, and 26.8/9.1 ppbV, respectively. For larger traps (120 cm3 and 213 cm3), approximately same results were obtained, indicating that most of the CO2 was removed. By using larger traps, a higher transfer rate of the gas mixture in the PA cell is possible, doubling the flow rate to 600 sccm.
For the two largest volumes, we succeeded to reduce the CO2 content from the exhaled air at a level influencing no more the C2H4 and NH3 concentration values, fact proved by the constant evolution in time of all parameters. Therefore, the trap is effective only for a enough large amount of KOH pellets. We found that a minimum volume of 120 cm3 of KOH scrubber and a transfer rate of 600 sccm were optimum to insure the required efficiency.
Analyzing the four cases when we inserted the scrubber, the dependence between the removed content of CO2 and the used KOH quantity proved to be nonlinear, as one could expect. If we consider the content of the sample totally free of CO2 after passing through the 213 cm3 and 120 cm3 KOH traps, we calculated a residual content of CO2 in alveolar collection of 0.58% (5800 ppm) for the 13 cm3 trap and of 0.016% (160 ppm) for the 45 cm3 trap (less than half of the CO2 concentration in the inhaled air).
We measured also the efficiency of the KOH scrubber when it is used for multiple measurements (Fig. 30). A clear saturation effect is evident: the KOH scrubber is not anymore efficient when the same fill is used for multiple runs (it cannot absorb completely the CO2 from the gas mixture). In the case of alveolar collection, the equivalent ethylene concentration increases by 2.3 times for the second run, by 2.6 times for the third run and by 3.4 times for the fourth run. When we measured the mixed expiratory collection, this saturation effect is even larger: the equivalent ethylene concentration increases by 2.4 times for the second run, by 8.5 times for the third run and by 20.2 times for the fourth run. The conclusion is that a new fill of KOH scrubber must be introduced after each measurement.
The lungs and airways are always moist, and inspired gas is rapidly saturated with water vapor in the upper segments of the respiratory system. The temperature in the airways and lungs is most identical with deep body temperature (approximately 37oC); at this temperature water vapor has a partial pressure of 47 torr (~6.2%). The increased saturation found at the third and fourth run for mixed expiratory collection is explained by a higher quantity of water vapors in exhaled breath (originating both from lungs and from upper segments of the respiratory system).
and 9.8, respectively. Only in the case of this trap we observed a peculiar behaviour. Even if the laser power is constant, the PA signal and consequently the equivalent ethylene concentration increases in time after transfering the gas sample in PA cell. The increase of concentration starts from 50 ppbV and continues until it stabilizes at a level of 435/240 ppbV (after 10-15 minutes). It is known that C2H4 (28.05 g/mol molar mass) is lighter than CO2 (44.0099 g/mol molar mass). Because of that, we can say that after passing the KOH scrubber, first C2H4 enters in the PA cell and then CO2 when the trap is no longer effective. So, at the beginning, we measured only the C2H4 concentration and then CO2 starts to strongly interfere in absorption. It is possible that due to the geometry of the cell, a longer time is required in order to attain the total homogeneity of the molecules inside the resonant
Larger KOH traps proved to be more efficient in removal of CO2 from the exhaled air. For the traps with volumes of 45 cm3, 120 cm3, and 213 cm3, respectively, the measured equivalent ethylene concentrations were 41.5/23.6 ppbV, 30/10.8 ppbV, and 26.8/9.1 ppbV, respectively. For larger traps (120 cm3 and 213 cm3), approximately same results were obtained, indicating that most of the CO2 was removed. By using larger traps, a higher transfer rate of the gas mixture in the PA cell is possible, doubling the flow rate to 600 sccm. For the two largest volumes, we succeeded to reduce the CO2 content from the exhaled air at a level influencing no more the C2H4 and NH3 concentration values, fact proved by the constant evolution in time of all parameters. Therefore, the trap is effective only for a enough large amount of KOH pellets. We found that a minimum volume of 120 cm3 of KOH scrubber and a transfer rate of 600 sccm were optimum to insure the required efficiency.
Analyzing the four cases when we inserted the scrubber, the dependence between the removed content of CO2 and the used KOH quantity proved to be nonlinear, as one could expect. If we consider the content of the sample totally free of CO2 after passing through the 213 cm3 and 120 cm3 KOH traps, we calculated a residual content of CO2 in alveolar collection of 0.58% (5800 ppm) for the 13 cm3 trap and of 0.016% (160 ppm) for the 45 cm3
We measured also the efficiency of the KOH scrubber when it is used for multiple measurements (Fig. 30). A clear saturation effect is evident: the KOH scrubber is not anymore efficient when the same fill is used for multiple runs (it cannot absorb completely the CO2 from the gas mixture). In the case of alveolar collection, the equivalent ethylene concentration increases by 2.3 times for the second run, by 2.6 times for the third run and by 3.4 times for the fourth run. When we measured the mixed expiratory collection, this saturation effect is even larger: the equivalent ethylene concentration increases by 2.4 times for the second run, by 8.5 times for the third run and by 20.2 times for the fourth run. The conclusion is that a new fill of KOH scrubber must be introduced after each measurement. The lungs and airways are always moist, and inspired gas is rapidly saturated with water vapor in the upper segments of the respiratory system. The temperature in the airways and lungs is most identical with deep body temperature (approximately 37oC); at this temperature water vapor has a partial pressure of 47 torr (~6.2%). The increased saturation found at the third and fourth run for mixed expiratory collection is explained by a higher quantity of water vapors in exhaled breath (originating both from lungs and from upper
tube of the cell, but this is not advantageous for repeated measurements.
trap (less than half of the CO2 concentration in the inhaled air).
segments of the respiratory system).
Fig. 30. Decrease of KOH trap efficiency when the same fill was used for multiple measurements.
The nonlinearity of the CO2 removal could be explained by the mechanism of the chemical reactions. First, the CO2 combines with the water vapors present in the exhaled air in the form of carbonic acid:
$$\text{CO} \star \text{H}\_2\text{O} \Leftrightarrow \text{H}\_2\text{CO} \tag{6}$$
Further, the last one combines with the KOH, creating potassium carbonate and water, and releasing a small amount of heat:
$$\text{H}\_2\text{CO}\_3 + 2\text{KOH} \Leftrightarrow \text{K}\_2\text{CO}\_3 + 2\text{H}\_2\text{O} + \text{Energy.} \tag{7}$$
In the same time, K2CO3 is a highly hygroscopic compound with a retaining capacity of 0.2 g H2O/1 g K2CO3, so the generated water will be only partially returned in the circuit.
The water is of high importance in limiting the rate of CO2 absorption. High CO2 concentrations entering the KOH absorber generates large quantities of water, because the reaction (7) is producing water. We know that the absorption rate is greater thanks to the film of moisture coating the pellets, but the same film impedes the access to the active potassium hydroxide pellet volume. More dedicated studies should be made in order to establish the moisture content for an optimum rate of absorption.
In conclusion, we determined experimentally that in the process of CO2 removal from the breath air samples, a quantity of minimum 120 cm3 KOH pellets should be used for a sampling bag of 750 mL in order to keep the detection of ethylene and ammonia traces free of CO2 interference. It should be mentioned that this volume of 120 cm3 must be reconsidered for sample bags with a greater volume (> 750 mL) or when the gas transfer rate from the bag to the PA cell is larger (> 600 sccm).
#### **2.8 Recipe for an optimum PA system**
Based on our experience, we summarize a list of actions to obtain a minimum detectable concentration (*cmin*) as low as possible (Dumitras et al., 2010):
CO2 Laser Photoacoustic Spectroscopy: II. Instrumentation and Applications 85
The recipe presented above applied on an extracavity laser PA configuration allowed us to achieve one of the most sensitive PA system with a detection limit of 2.7x10-8 cm-1 for a low power laser or even 0.64x10-8 cm-1 for a high power laser. In this way, the method based on laser photoacoustic spectroscopy became a powerful tool for measurement of trace gases
We have measured precisely the absorption coefficients of ethylene (Dumitras et al., 2007)
The ethylene absorption coefficients for various CO2 laser transitions have been measured in various experiments. Discrepancies as high as ~15% have been found in the absolute IR values observed at many laser transitions. Such discrepancies are typical of many other gases and are partially associated with the difficulty of producing proper gas samples with known concentration levels. Unfortunately, large discrepancies are also found between measurements of the relative spectral signatures (the ratio between absorption coefficients at different wavelengths). Knowledge of the relative spectral signatures rather than absolute ones is sufficient for trace gas identification. We also note that it is rather problematic to obtain highly accurate measurements of the absolute values of the absorption coefficients of gases using the photoacoustic effect. The reason for this is the need of an absolute calibration of the cell. The calibrations cited in the literature are all based on *a priori* knowledge of the absorption coefficient of a gas at some selected wavelength. However, in
Photoacoustics is emerging as a standard technique for measuring extremely low absorptions independent of the path length and offers a degree of parameter control that cannot be attained by other methods. Radiation absorption by the gas creates a pressure signal which is sensed by the microphone. The resulting signal, processed by a phase sensitive detector, is directly proportional to the absorption coefficient and laser power (or laser power absorbed per unit volume). The sensitivity of the technique is such that absorptions of <10-7 cm-1 can be measured over path lengths of a few tens of centimeters. The small volume of the chamber makes it possible to accurately control the gas parameters,
α
gas or vapor and at a common concentration is called the optoacoustic absorption spectrum or signature and is unique to a combination of vapor and laser. These signatures or "fingerprints" are absolute entities, unique only to the laser frequency and species, which provide the specifics of instrument performance in terms of detection limit and interference
To improve the measurement of ethylene absorption coefficients, a special procedure was followed. Prior to each run, the gas mixture was flowed at 100 sccm for several minutes to stabilize the boundary layer on the cell walls, since a certain amount of adsorption would occur and possibly influence background signals; after this conditioning period, the cell was closed off and used in measurement. For every gas fill with 0.96 ppmV ethylene buffered in pure nitrogen, the responsivity of the cell was determined supposing an absorption
, for all laser wavelengths, for a particular
all cases the absorption coefficient was actually known only to a few percent.
and the system can be operated with static fills or in continuous gas flow mode.
(Dumitras et al., 1996a).
**3.1 Measurement of absorption coefficients**
The set of values of the absorption coefficients
rejection (Cristescu et al., 2000b).
and ammonia (Dumitras et al., 2011) at CO2 laser wavelengths.
**3. Applications**
The recipe presented above applied on an extracavity laser PA configuration allowed us to achieve one of the most sensitive PA system with a detection limit of 2.7x10-8 cm-1 for a low power laser or even 0.64x10-8 cm-1 for a high power laser. In this way, the method based on laser photoacoustic spectroscopy became a powerful tool for measurement of trace gases (Dumitras et al., 1996a).
### **3. Applications**
84 CO2 Laser – Optimisation and Application
d. Minimize the coherent acoustic background noise, *ac VN* (caused by the modulation process):
cryogenic trap, respectively, between the sampling cell and the PA cell;
g. Increase the laser power while maintaining the noises at lower values:
intracavity configuration (smaller cavity transmission coefficient);
τ--1/2).
a. Increase the cell constant, *C*:
small diameter (*C* ∝ *r* -1).
amplified by the quality factor, *Q*;
b. Increase the microphone responsivity:
microphones connected in series. c. Minimize the electrical noise, *<sup>e</sup> VN* : - use state-of-the-art lock-in amplifiers; - use longer time averaging ( *<sup>e</sup> VN* ∝
possible polished surfaces);
of the laser beam;
unknown trace gases);
of ethylene).
acoustic band-stop filters (buffer volumes);
reduction less than 2% is obtained if Δ*T* ≤ 4oC);
e. Minimize the coherent PA background signal, *<sup>b</sup> VN* :
#### **3.1 Measurement of absorption coefficients**
We have measured precisely the absorption coefficients of ethylene (Dumitras et al., 2007) and ammonia (Dumitras et al., 2011) at CO2 laser wavelengths.
The ethylene absorption coefficients for various CO2 laser transitions have been measured in various experiments. Discrepancies as high as ~15% have been found in the absolute IR values observed at many laser transitions. Such discrepancies are typical of many other gases and are partially associated with the difficulty of producing proper gas samples with known concentration levels. Unfortunately, large discrepancies are also found between measurements of the relative spectral signatures (the ratio between absorption coefficients at different wavelengths). Knowledge of the relative spectral signatures rather than absolute ones is sufficient for trace gas identification. We also note that it is rather problematic to obtain highly accurate measurements of the absolute values of the absorption coefficients of gases using the photoacoustic effect. The reason for this is the need of an absolute calibration of the cell. The calibrations cited in the literature are all based on *a priori* knowledge of the absorption coefficient of a gas at some selected wavelength. However, in all cases the absorption coefficient was actually known only to a few percent.
Photoacoustics is emerging as a standard technique for measuring extremely low absorptions independent of the path length and offers a degree of parameter control that cannot be attained by other methods. Radiation absorption by the gas creates a pressure signal which is sensed by the microphone. The resulting signal, processed by a phase sensitive detector, is directly proportional to the absorption coefficient and laser power (or laser power absorbed per unit volume). The sensitivity of the technique is such that absorptions of <10-7 cm-1 can be measured over path lengths of a few tens of centimeters. The small volume of the chamber makes it possible to accurately control the gas parameters, and the system can be operated with static fills or in continuous gas flow mode.
The set of values of the absorption coefficients α, for all laser wavelengths, for a particular gas or vapor and at a common concentration is called the optoacoustic absorption spectrum or signature and is unique to a combination of vapor and laser. These signatures or "fingerprints" are absolute entities, unique only to the laser frequency and species, which provide the specifics of instrument performance in terms of detection limit and interference rejection (Cristescu et al., 2000b).
To improve the measurement of ethylene absorption coefficients, a special procedure was followed. Prior to each run, the gas mixture was flowed at 100 sccm for several minutes to stabilize the boundary layer on the cell walls, since a certain amount of adsorption would occur and possibly influence background signals; after this conditioning period, the cell was closed off and used in measurement. For every gas fill with 0.96 ppmV ethylene buffered in pure nitrogen, the responsivity of the cell was determined supposing an absorption
CO2 Laser Photoacoustic Spectroscopy: II. Instrumentation and Applications 87
Fig. 31. Absorption coefficients of ethylene at CO2 laser wavelengths. The inset shows an
largest discrepancies are recorded for the 9R(28), 9R(30), and 9R(22) laser lines).
5 from other reported data in the case of the CO2 laser 9R lines.
absorption coefficients can also be displayed.
There is general agreement with the results of Brewer et al. (Brewer et al., 1982) for the 0001- 1000 band. The difference between our results and those obtained by the above mentioned authors is less than 10% for the majority of the investigated lines while only for five lines the discrepancy is higher, between 10% and 20%. By contrast, the values determined in the present work are consistently higher in the 0001-0200 band. The difference is larger by 10- 50% for the P branch, while our values in the R branch are higher by a factor of 1.5-5.5 (the
λ(μm)
9.2 9.4 9.6 9.8 10.0 10.2 10.4 10.6 10.8
10R(34)
9P(30) 9P(34)
10R(24)
10P(36)
10P(24)
10R(6)
10P(6)
10P(14)
C2H4
9.2 9.4 9.6 9.8
9P(10)
9P(18)
9R(10)
9R(18)
9R(30)
The present work was carried out using a methodology which gave the best possible control over the ethylene partial pressure and background signals. The background levels and calibration of the PA cell were checked before and after every experimental run. The present study is considered reliable, particularly in view of the careful attention that was paid to controlling the gas composition and noise signals. No apparent fault could be found with either the apparatus or methodology that would account for the discrepancy by factors of 2-
For the measurement of the absorption coefficients of ammonia (Dumitras et al., 2011), the software user interface allows to record the laser power, the PA signal and the calculated absorption coefficients on different panels**.** The evolution in time of the measurement of the
The gas mixture was flowed at 100 sccm for several minutes to stabilize the boundary layer on the cell walls, since a certain amount of adsorption would occur and possibly influence background signals; after this conditioning period, the cell was closed off and used in measurement. For every gas fill with 10 ppm ammonia buffered in pure nitrogen, the responsivity of the cell was determined supposing an absorption coefficient of 57.12 cm-1atm-1 at 9R(30) laser transition. This is in accordance both to the measurements reported by Brewer & Bruce (Brewer & Bruce, 1978) and by our tests, when the responsivity of the PA system was checked by measuring the well known absorption coefficient of ethylene at 10P(14) line of the CO2 laser. After measurements at all laser lines, the cell responsivity was checked again, to eliminate any possibility of gas desorption during the measurement. The
values at each laser line were obtained by using the measured PA signal and laser power and by knowing precisely the ammonia concentration (10.6 ppm) and the responsivity of the
α
enlarged view of the measurements for the 9-μm band.
α
(cm-1atm-1)
0
1
2
coefficient of 30.4 cm-1atm-1 at 10P(14) laser transition. After measurements at all laser lines, the cell responsivity was checked again, to eliminate any possibility of gas desorption during the measurement. The partial pressure of ethylene was enough to have significant PA signals for all laser lines and low enough to be far away from the saturation regime (observations were only made at a C2H4 concentration of 100 ppmV). The α values at each laser line were obtained from Eq. (29, Part I) using the measured PA signal and laser power (the cell responsivity and ethylene concentration were known). An average over several independent measurements at each line was used to improve the overall accuracy of the results. The values to be presented are thought to be the best published to date.
The absolute magnitudes of the absorption coefficients were calculated as mean values of several independent measurements. An absorption coefficient corresponding to each CO2 laser transition was determined from two sets of 50 different measurements. Every set of measurements was initiated by the frequency stabilization of a given line of the CO2 laser. From one set of measurements to another, the closed loop of the frequency stabilization circuit was interrupted, the laser was tuned again to the top of the gain curve, and then the frequency stabilization was set and checked by watching the long term stability. Inside one set, 50 independent measurements were made at a rate of one per second to assess reproducibility. From one measurement to the next, the error measurement of the absorption coefficient was calculated as the ratio between the maximum difference (maximum value minus minimum value) and the average value. The final value of the ethylene absorption coefficient is given by the arithmetic mean of the two sets of measurements, while the absorption coefficient error is chosen as the larger value of the two sets. The same procedure was applied for every absorption coefficient of ethylene.
To measure the absorption coefficients of ethylene, the software user interface was modified to allow that the laser power, PA signal, and calculated absorption coefficients function on time (or number of measurements) be recorded on different panels.
The results of our measurements for ethylene are given in Fig. 31. Because of the large spacing between laser transitions (1.2-2 cm-1 apart), strong differences of absorption occur. Our results are compared to those of Brewer *et al*. (Brewer et al., 1982) that were also obtained by a photoacoustic method. The difference between the two spectral patterns suggests problems in the measurement techniques (for example, frequency deviation from the laser line center, gas calibration, system purity, linearity, precision) and/or data analysis. The different temperatures and atmospheric pressures at which the measurements were made cannot account for the discrepancies, because Persson *et al*. (Persson et al., 1980) measured a change in absorption coefficient of only 5% at the 10P(14) line for a temperature change of 30oC (negative temperature coefficient), while the changes caused by a pressure difference of 40 Torr are <5% for all CO2 laser wavelengths.
The random coincidence between the emission and absorption lines will be such that some laser lines will lie close to the centers of the absorbing lines and others will be far away in the wings. The result is a spectral representation unique to that molecule. As a consequence of the superposition of different pressure-broadened C2H4 transitions (ν7 vibration), a strong absorption is obtained at the 10P(14) laser line (absorption coefficient of 30.4 cm-1atm-1 at 949.479 cm-1). C2H4 has weaker absorption coefficients at the 10P(12) and 10P(16) CO2 laser transitions (4.36 cm-1atm-1 at 951.192 cm-1 and 5.10 cm-1atm-1 at 947.742 cm-1, respectively). Also, in Fig. 31 ethylene is seen to possess moderately strong absorption profiles within the 9.4-μm band.
coefficient of 30.4 cm-1atm-1 at 10P(14) laser transition. After measurements at all laser lines, the cell responsivity was checked again, to eliminate any possibility of gas desorption during the measurement. The partial pressure of ethylene was enough to have significant PA signals for all laser lines and low enough to be far away from the saturation regime
laser line were obtained from Eq. (29, Part I) using the measured PA signal and laser power (the cell responsivity and ethylene concentration were known). An average over several independent measurements at each line was used to improve the overall accuracy of the
The absolute magnitudes of the absorption coefficients were calculated as mean values of several independent measurements. An absorption coefficient corresponding to each CO2 laser transition was determined from two sets of 50 different measurements. Every set of measurements was initiated by the frequency stabilization of a given line of the CO2 laser. From one set of measurements to another, the closed loop of the frequency stabilization circuit was interrupted, the laser was tuned again to the top of the gain curve, and then the frequency stabilization was set and checked by watching the long term stability. Inside one set, 50 independent measurements were made at a rate of one per second to assess reproducibility. From one measurement to the next, the error measurement of the absorption coefficient was calculated as the ratio between the maximum difference (maximum value minus minimum value) and the average value. The final value of the ethylene absorption coefficient is given by the arithmetic mean of the two sets of measurements, while the absorption coefficient error is chosen as the larger value of the two
α
values at each
(observations were only made at a C2H4 concentration of 100 ppmV). The
results. The values to be presented are thought to be the best published to date.
sets. The same procedure was applied for every absorption coefficient of ethylene.
time (or number of measurements) be recorded on different panels.
difference of 40 Torr are <5% for all CO2 laser wavelengths.
of the superposition of different pressure-broadened C2H4 transitions (
To measure the absorption coefficients of ethylene, the software user interface was modified to allow that the laser power, PA signal, and calculated absorption coefficients function on
The results of our measurements for ethylene are given in Fig. 31. Because of the large spacing between laser transitions (1.2-2 cm-1 apart), strong differences of absorption occur. Our results are compared to those of Brewer *et al*. (Brewer et al., 1982) that were also obtained by a photoacoustic method. The difference between the two spectral patterns suggests problems in the measurement techniques (for example, frequency deviation from the laser line center, gas calibration, system purity, linearity, precision) and/or data analysis. The different temperatures and atmospheric pressures at which the measurements were made cannot account for the discrepancies, because Persson *et al*. (Persson et al., 1980) measured a change in absorption coefficient of only 5% at the 10P(14) line for a temperature change of 30oC (negative temperature coefficient), while the changes caused by a pressure
The random coincidence between the emission and absorption lines will be such that some laser lines will lie close to the centers of the absorbing lines and others will be far away in the wings. The result is a spectral representation unique to that molecule. As a consequence
absorption is obtained at the 10P(14) laser line (absorption coefficient of 30.4 cm-1atm-1 at 949.479 cm-1). C2H4 has weaker absorption coefficients at the 10P(12) and 10P(16) CO2 laser transitions (4.36 cm-1atm-1 at 951.192 cm-1 and 5.10 cm-1atm-1 at 947.742 cm-1, respectively). Also, in Fig. 31 ethylene is seen to possess moderately strong absorption profiles within the 9.4-μm band.
ν
7 vibration), a strong
Fig. 31. Absorption coefficients of ethylene at CO2 laser wavelengths. The inset shows an enlarged view of the measurements for the 9-μm band.
There is general agreement with the results of Brewer et al. (Brewer et al., 1982) for the 0001- 1000 band. The difference between our results and those obtained by the above mentioned authors is less than 10% for the majority of the investigated lines while only for five lines the discrepancy is higher, between 10% and 20%. By contrast, the values determined in the present work are consistently higher in the 0001-0200 band. The difference is larger by 10- 50% for the P branch, while our values in the R branch are higher by a factor of 1.5-5.5 (the largest discrepancies are recorded for the 9R(28), 9R(30), and 9R(22) laser lines).
The present work was carried out using a methodology which gave the best possible control over the ethylene partial pressure and background signals. The background levels and calibration of the PA cell were checked before and after every experimental run. The present study is considered reliable, particularly in view of the careful attention that was paid to controlling the gas composition and noise signals. No apparent fault could be found with either the apparatus or methodology that would account for the discrepancy by factors of 2- 5 from other reported data in the case of the CO2 laser 9R lines.
For the measurement of the absorption coefficients of ammonia (Dumitras et al., 2011), the software user interface allows to record the laser power, the PA signal and the calculated absorption coefficients on different panels**.** The evolution in time of the measurement of the absorption coefficients can also be displayed.
The gas mixture was flowed at 100 sccm for several minutes to stabilize the boundary layer on the cell walls, since a certain amount of adsorption would occur and possibly influence background signals; after this conditioning period, the cell was closed off and used in measurement. For every gas fill with 10 ppm ammonia buffered in pure nitrogen, the responsivity of the cell was determined supposing an absorption coefficient of 57.12 cm-1atm-1 at 9R(30) laser transition. This is in accordance both to the measurements reported by Brewer & Bruce (Brewer & Bruce, 1978) and by our tests, when the responsivity of the PA system was checked by measuring the well known absorption coefficient of ethylene at 10P(14) line of the CO2 laser. After measurements at all laser lines, the cell responsivity was checked again, to eliminate any possibility of gas desorption during the measurement. The α values at each laser line were obtained by using the measured PA signal and laser power and by knowing precisely the ammonia concentration (10.6 ppm) and the responsivity of the
CO2 Laser Photoacoustic Spectroscopy: II. Instrumentation and Applications 89
texture of the fruit), aging and senescence of leaves and flowers and finally, the abscission of
The ethylene biosynthesis process in plants follows the MSAE pathway: L-methionine (amino acid) – SAM (S-adenosyl methionine) – ACC (aminocyclopropane-1-carboxylic acid) – C2H4. Ethylene, or its precursor ACC, stimulates seed germination of many species at concentrations as low as 0.2 ppb. During germination, a complex cross-talking between
Tomato is an useful model plant for studying ethylene action. Three tomato mutants altered in ripening process affects different steps in ethylene synthesis and perception, resulting in a delay of fruit maturation and pigmentation: Never ripe (*Nr*) is mutated in an ethylene receptor and exhibits delayed and incomplete fruit maturation; ripening inhibitor (*rin*) is a delayed gene that causes the block of ripening before the respiratory burst; and non ripening (*nor*) shows pleiotropic effects analogue to *rin.* The aim of our study was to investigate the ethylene emission during seed germination of these 3 mutants, correlation with their germination ability and analysis of ethylene role on the loss of germinability during seed senescence.
The ethylene production per seed measured during seed germination and seedling elongation is presented in Fig. 33. In these genes, ethylene influences not only fruit ripening, but also the seed germination. The germination index and the percentage of germination of the 5-years-old seeds of the mutants are higher in respect to the control (Ny – New Yorker), in spite of the lower ethylene production of germinating seeds. Conversely to other species, in 5-years-old tomato seeds an inverse correlation between ethylene production and percentage of germination exists. During seed senescence, ethylene accumulation occurs and some processes, triggered during germination, result altered. Further analysis is required to clarify
the interaction between ethylene and other hormones like auxin, ABA and cytochinin.
Fig. 33. Ethylene production (ppt/seed) measured during seed germination and seedling
Climacteric fruits show a respiratory rise during ripening (tomato, pear, fig, mango, banana), while others belong to nonclimacteric fruits (cherry, strawberry, lemon). Fruit ripening (yellowing, softening, respiration, autocatalytic ethylene production) and abscission are regulated by ethylene. During ripening, tomatoes show a strong increase in ethylene production coinciding with the climacteric rise in respiration (CO2 production). Ethylene is also involved in the postmaturation processes, playing an important role in fruit ripening. Storage and shipping of fruits in terms of wounding effects, temperature, composition of atmospheric gases, postharvest pathogens or seal-packing conditions are
leaves and fruits (Cristescu et al., 1998; Cristescu et al., 1999; Dumitras et al., 2004).
several plant hormones exists.
elongation.
PA cell (312 V cm/W for high power laser). An average over several independent measurements at each line was used to improve the overall accuracy of the results.
The results of our measurements for ammonia are given in Fig. 32. The experimental results show a spectral representation unique to the ammonia molecule. As it can be seen from Fig. 32, ammonia has weaker absorption coefficients at other CO2 laser transitions; some other significant values for the absorption coefficient were found for 9R and 9P bands: 9R(16) α = 11.29 cm-1atm-1 (error ± 1.4%), 9P(20) α = 2.10 cm-1atm-1 (error ± 2%) and 9P(34) α = 3.99 cm-1atm-1 (error ± 0.62%). In the 10R band the measurements gave: 10R(14) α = 6.17 cm-1atm-1 (error ± 1.5%), 10R(8) α = 20.08 cm-1atm-1 (error ± 1.3%), 10R(6) α = 26.2 cm-1atm-1 (error ± 1.7%), and for the 10P band: 10P(32) α = 12.45 cm-1atm-1 (error ± 2.9%), 10P(34) α = 14.07 cm-1atm-1 (error ± 0.48%) and 10P(36) α = 7.39 cm-1atm-1 (error ± 0.83%). Compared to the other values reported previously in the literature (Brewer & Bruce, 1978), our measurements indicate a general good agreement.
Fig. 32. Absorption coefficients of ammonia at CO2 laser wavelengths.
#### **3.2 Applications in plant physiology**
Ethylene acts as a vegetal hormone produced by all plant tissues. It is transported by diffusion through plant tissues and increases the plasmatic membrane permeability. It has multiple effects on the cell metabolism: increases the oxidative processes, the transport inside the cells and the biodegradation of the organic acids and chlorophyll. Ethylene plays a major role in many metabolic processes: seed and bud dormancy, seed germination promotion, roots induction, development of plantlets (inhibitor of elongation and promotion of lateral shoots), grown promotion, leaf expansion, epinasty (downward curvature of leaves due to the growth of cells on the upper side of the petiole), flowering, wilting of flowers, fruit ripening (ethylene induces some biochemical modifications which produce polyalcohols, hydrocarbons and different oxygenated combinations responsible for the taste, aroma and
PA cell (312 V cm/W for high power laser). An average over several independent
The results of our measurements for ammonia are given in Fig. 32. The experimental results show a spectral representation unique to the ammonia molecule. As it can be seen from Fig. 32, ammonia has weaker absorption coefficients at other CO2 laser transitions; some other significant values for the absorption coefficient were found for 9R and 9P bands: 9R(16) -
α
= 3.99 cm-1atm-1 (error ± 0.62%). In the 10R band the measurements gave: 10R(14) -
Compared to the other values reported previously in the literature (Brewer & Bruce, 1978),
α
= 2.10 cm-1atm-1 (error ± 2%) and 9P(34) -
= 12.45 cm-1atm-1 (error ± 2.9%),
= 7.39 cm-1atm-1 (error ± 0.83%).
α= 26.2
= 20.08 cm-1atm-1 (error ± 1.3%), 10R(6) -
α
α
measurements at each line was used to improve the overall accuracy of the results.
= 11.29 cm-1atm-1 (error ± 1.4%), 9P(20) -
cm-1atm-1 (error ± 1.7%), and for the 10P band: 10P(32) -
our measurements indicate a general good agreement.
= 14.07 cm-1atm-1 (error ± 0.48%) and 10P(36) -
Fig. 32. Absorption coefficients of ammonia at CO2 laser wavelengths.
Ethylene acts as a vegetal hormone produced by all plant tissues. It is transported by diffusion through plant tissues and increases the plasmatic membrane permeability. It has multiple effects on the cell metabolism: increases the oxidative processes, the transport inside the cells and the biodegradation of the organic acids and chlorophyll. Ethylene plays a major role in many metabolic processes: seed and bud dormancy, seed germination promotion, roots induction, development of plantlets (inhibitor of elongation and promotion of lateral shoots), grown promotion, leaf expansion, epinasty (downward curvature of leaves due to the growth of cells on the upper side of the petiole), flowering, wilting of flowers, fruit ripening (ethylene induces some biochemical modifications which produce polyalcohols, hydrocarbons and different oxygenated combinations responsible for the taste, aroma and
**3.2 Applications in plant physiology**
= 6.17 cm-1atm-1 (error ± 1.5%), 10R(8) -
α
α
α
10P(34) -
α
texture of the fruit), aging and senescence of leaves and flowers and finally, the abscission of leaves and fruits (Cristescu et al., 1998; Cristescu et al., 1999; Dumitras et al., 2004).
The ethylene biosynthesis process in plants follows the MSAE pathway: L-methionine (amino acid) – SAM (S-adenosyl methionine) – ACC (aminocyclopropane-1-carboxylic acid) – C2H4. Ethylene, or its precursor ACC, stimulates seed germination of many species at concentrations as low as 0.2 ppb. During germination, a complex cross-talking between several plant hormones exists.
Tomato is an useful model plant for studying ethylene action. Three tomato mutants altered in ripening process affects different steps in ethylene synthesis and perception, resulting in a delay of fruit maturation and pigmentation: Never ripe (*Nr*) is mutated in an ethylene receptor and exhibits delayed and incomplete fruit maturation; ripening inhibitor (*rin*) is a delayed gene that causes the block of ripening before the respiratory burst; and non ripening (*nor*) shows pleiotropic effects analogue to *rin.* The aim of our study was to investigate the ethylene emission during seed germination of these 3 mutants, correlation with their germination ability and analysis of ethylene role on the loss of germinability during seed senescence.
The ethylene production per seed measured during seed germination and seedling elongation is presented in Fig. 33. In these genes, ethylene influences not only fruit ripening, but also the seed germination. The germination index and the percentage of germination of the 5-years-old seeds of the mutants are higher in respect to the control (Ny – New Yorker), in spite of the lower ethylene production of germinating seeds. Conversely to other species, in 5-years-old tomato seeds an inverse correlation between ethylene production and percentage of germination exists. During seed senescence, ethylene accumulation occurs and some processes, triggered during germination, result altered. Further analysis is required to clarify the interaction between ethylene and other hormones like auxin, ABA and cytochinin.
Fig. 33. Ethylene production (ppt/seed) measured during seed germination and seedling elongation.
Climacteric fruits show a respiratory rise during ripening (tomato, pear, fig, mango, banana), while others belong to nonclimacteric fruits (cherry, strawberry, lemon). Fruit ripening (yellowing, softening, respiration, autocatalytic ethylene production) and abscission are regulated by ethylene. During ripening, tomatoes show a strong increase in ethylene production coinciding with the climacteric rise in respiration (CO2 production). Ethylene is also involved in the postmaturation processes, playing an important role in fruit ripening. Storage and shipping of fruits in terms of wounding effects, temperature, composition of atmospheric gases, postharvest pathogens or seal-packing conditions are
CO2 Laser Photoacoustic Spectroscopy: II. Instrumentation and Applications 91
1.2-1.4 ppb for dead cells, both after 5 hours and 24 hours from the treatment. The increase of ethylene content clearly demonstrates that lipid peroxidation took place owing to the toxic effect of heavy metals (Fig. 34). The measurements were performed on 10P(14) line of the CO2 laser by means of a nitrogen flow-through system (25 sccm). The CO2 laser, tuned off resonance on the 10P(20) line from time to time, causes a clear drop in the observed signal.
The radiation damage in living matter develops along complex chains of events that follow the absorption of energy: a) physical stage: the energy transfer from the radiation to the matter leads mainly to molecular excitations and ionization; b) chemical stage: the primary reactive species (free atoms and radicals, that are usually extremely unstable), undergo secondary single reactions or a succession of reactions among each other and with their environment, causing damage to molecules of biological importance; c) biological stage: molecular changes occurring in a living organism may cause alterations in the system organization, which become macroscopically observable as biological effects. A substantial part of the total ionising radiation effect concerns water radiolysis, water being a major component of living tissues present in all biological systems. Many water ions and radicals are generated inside tissues as primary reactive species. Aqueous free radicals are very reactive and induce oxidative degradation of phospholipids in cell membranes (lipid peroxidation). The aim of our investigation was to measure the X-ray induced ethylene emission in mice breath and to analyse breath exhaled from patients under external X-ray beam therapy for cancer treatment. For the purpose of verifying the radioinduced effect, living mice (B6C3F1 and C57B1/6J male mice, between 3 and 6 months old) have been exposed to the total body action of a 250 kV X-ray apparatus GILARDONI model CHF-320-G. At 250 kV voltage and 15 mA current by using a 0.5 mm Cu filter, the measured dose rate was 90.1 cGy/min at 68.4 cm from the source. The value of the X-ray dose given to the treated mice (9 Gy per total body) is comparable, as order of magnitude, to the therapeutic doses given to a human patient in the course of cancer treatment by radiotherapy. The mice were divided in treated and control groups. Each treated mouse received a substantial amount of X-rays in the whole body, while the control mouse received a zero dose. Samples of the breathing air have been collected before and after irradiation. The breathing air has been concentrated on active coal absorbing pellets for a time as long as 1.5 hours, successively expanded into 0.5 liters sample bags, and then transferred into the photoacoustic cell in order to perform the analysis of ethylene content. The PA analysis of ethylene content, by using the above described
Fig. 34. Toxic effect of Cd on Jurkat cells.
important factors to establish the optimal environment necessary for their long term conservation. The aim of our experiments was to monitor the ethylene emission in plants and fruits at low temperature, together with the effect of the temperature at different ripening stages (important for optimization of different stages in agricultural procedures) and to study the effects of mechanical wounding of fruits.
Ethylene emission was monitored from plantlets at different temperatures and it saturates after about 20 min (Lai et al., 2003). The temperature effect is evident in the emission intensity, which increases almost a factor four from 15o to 25oC. There is a remarkable decrease in the lag time for the gas emission at the optimum temperature for biosynthesis of ethylene (25oC). At temperatures lower than 22oC, this lag time is about 6 min, while drops to less than 2 min at 25oC. Temperature does not influence the ethylene emission of immature fruits (0.004 ppb/g and 0.005 ppb/g for 15oC and 25oC, respectively), while it becomes important when the ripening process is triggered in a maturated fruit (0.012 ppb/g and 1.56 ppb/g for 15oC and 25oC, respectively). The same result is obtained for plantlets. Temperature is important for ACC oxidase activity (decreased at low temperatures). Mechanical wounding exerts its effect at the step where SAM is converted to ACC, the direct precursor of ethylene; this step, regulated by the enzyme ACC synthase is rate limiting in the cascade of events leading to an increase of ethylene production.
#### **3.3 Investigation of lipid peroxidation**
The oxidative modification of biological molecules is an essential part of the normal biological activity in the human organism. An excess in some oxidant activities does cause injury to cells and tissues. Particular attention is devoted to the oxidant activity of free radicals. An increased free radical formation in the organism is involved in the pathophysiology of several diseases. One of the events generated by free radicals interaction with biomolecules is the oxidative degradation of fatty acids. Oxidative stress is the origin or cause of lipid peroxidation and, as a consequence, of a wide variety of pathophysiological processes. Lipid peroxidation is the free-radical-induced oxidative degradation of polyunsaturated fatty acids. Biomembranes and cells are thereby disrupted, causing cell damage and cell death. As a marker of free-radical-mediated damage in the human body, the measurement of the exhaled volatile hydrocarbons, such as ethylene, is a good noninvasive method to monitor lipid peroxidation.
We have studied lipid peroxidation as a consequence of ionising radiation and heavy metals in living cells (Dumitras et al., 2004). Most heavy metals have a toxic action on human cells and may induce lipid peroxidation. Cadmium is a toxic agent which is supposed to affect the transport of ion through the cell membrane. Cadmium and calcium ionic radii are similar, so Cd can be picked up through the Ca transport mechanism. On the other hand, the Cd permeability through the calcium channel is very poor, so Cd can be considered as a blocker of the calcium channel as well. We tried to determine the extent of the toxic action of Cd *in vitro* by monitoring the ethylene concentration in the breathing air of human cells cultured in a liquid medium to which cadmium chloride was added. Cells of the human leukemic T cell line (Jurkat) were kept in a culture in RPMI 1640 medium containing 10% FBS, 1% L-glutamine and 1% penicillin streptomycin at 37oC in a humidified incubator with 5% CO2 and 95% air.
The measurement of ethylene before and after treatment of the culture of human cells with CdCl2 has shown that the concentration has increased from 0.5 ppb for control (live cells) to 1.2-1.4 ppb for dead cells, both after 5 hours and 24 hours from the treatment. The increase of ethylene content clearly demonstrates that lipid peroxidation took place owing to the toxic effect of heavy metals (Fig. 34). The measurements were performed on 10P(14) line of the CO2 laser by means of a nitrogen flow-through system (25 sccm). The CO2 laser, tuned off resonance on the 10P(20) line from time to time, causes a clear drop in the observed signal.
Fig. 34. Toxic effect of Cd on Jurkat cells.
90 CO2 Laser – Optimisation and Application
important factors to establish the optimal environment necessary for their long term conservation. The aim of our experiments was to monitor the ethylene emission in plants and fruits at low temperature, together with the effect of the temperature at different ripening stages (important for optimization of different stages in agricultural procedures)
Ethylene emission was monitored from plantlets at different temperatures and it saturates after about 20 min (Lai et al., 2003). The temperature effect is evident in the emission intensity, which increases almost a factor four from 15o to 25oC. There is a remarkable decrease in the lag time for the gas emission at the optimum temperature for biosynthesis of ethylene (25oC). At temperatures lower than 22oC, this lag time is about 6 min, while drops to less than 2 min at 25oC. Temperature does not influence the ethylene emission of immature fruits (0.004 ppb/g and 0.005 ppb/g for 15oC and 25oC, respectively), while it becomes important when the ripening process is triggered in a maturated fruit (0.012 ppb/g and 1.56 ppb/g for 15oC and 25oC, respectively). The same result is obtained for plantlets. Temperature is important for ACC oxidase activity (decreased at low temperatures). Mechanical wounding exerts its effect at the step where SAM is converted to ACC, the direct precursor of ethylene; this step, regulated by the enzyme ACC synthase is rate limiting in
The oxidative modification of biological molecules is an essential part of the normal biological activity in the human organism. An excess in some oxidant activities does cause injury to cells and tissues. Particular attention is devoted to the oxidant activity of free radicals. An increased free radical formation in the organism is involved in the pathophysiology of several diseases. One of the events generated by free radicals interaction with biomolecules is the oxidative degradation of fatty acids. Oxidative stress is the origin or cause of lipid peroxidation and, as a consequence, of a wide variety of pathophysiological processes. Lipid peroxidation is the free-radical-induced oxidative degradation of polyunsaturated fatty acids. Biomembranes and cells are thereby disrupted, causing cell damage and cell death. As a marker of free-radical-mediated damage in the human body, the measurement of the exhaled volatile hydrocarbons, such as ethylene, is a good
We have studied lipid peroxidation as a consequence of ionising radiation and heavy metals in living cells (Dumitras et al., 2004). Most heavy metals have a toxic action on human cells and may induce lipid peroxidation. Cadmium is a toxic agent which is supposed to affect the transport of ion through the cell membrane. Cadmium and calcium ionic radii are similar, so Cd can be picked up through the Ca transport mechanism. On the other hand, the Cd permeability through the calcium channel is very poor, so Cd can be considered as a blocker of the calcium channel as well. We tried to determine the extent of the toxic action of Cd *in vitro* by monitoring the ethylene concentration in the breathing air of human cells cultured in a liquid medium to which cadmium chloride was added. Cells of the human leukemic T cell line (Jurkat) were kept in a culture in RPMI 1640 medium containing 10% FBS, 1% L-glutamine and 1% penicillin streptomycin at 37oC in a humidified
The measurement of ethylene before and after treatment of the culture of human cells with CdCl2 has shown that the concentration has increased from 0.5 ppb for control (live cells) to
and to study the effects of mechanical wounding of fruits.
the cascade of events leading to an increase of ethylene production.
**3.3 Investigation of lipid peroxidation**
noninvasive method to monitor lipid peroxidation.
incubator with 5% CO2 and 95% air.
The radiation damage in living matter develops along complex chains of events that follow the absorption of energy: a) physical stage: the energy transfer from the radiation to the matter leads mainly to molecular excitations and ionization; b) chemical stage: the primary reactive species (free atoms and radicals, that are usually extremely unstable), undergo secondary single reactions or a succession of reactions among each other and with their environment, causing damage to molecules of biological importance; c) biological stage: molecular changes occurring in a living organism may cause alterations in the system organization, which become macroscopically observable as biological effects. A substantial part of the total ionising radiation effect concerns water radiolysis, water being a major component of living tissues present in all biological systems. Many water ions and radicals are generated inside tissues as primary reactive species. Aqueous free radicals are very reactive and induce oxidative degradation of phospholipids in cell membranes (lipid peroxidation). The aim of our investigation was to measure the X-ray induced ethylene emission in mice breath and to analyse breath exhaled from patients under external X-ray beam therapy for cancer treatment.
For the purpose of verifying the radioinduced effect, living mice (B6C3F1 and C57B1/6J male mice, between 3 and 6 months old) have been exposed to the total body action of a 250 kV X-ray apparatus GILARDONI model CHF-320-G. At 250 kV voltage and 15 mA current by using a 0.5 mm Cu filter, the measured dose rate was 90.1 cGy/min at 68.4 cm from the source. The value of the X-ray dose given to the treated mice (9 Gy per total body) is comparable, as order of magnitude, to the therapeutic doses given to a human patient in the course of cancer treatment by radiotherapy. The mice were divided in treated and control groups. Each treated mouse received a substantial amount of X-rays in the whole body, while the control mouse received a zero dose. Samples of the breathing air have been collected before and after irradiation. The breathing air has been concentrated on active coal absorbing pellets for a time as long as 1.5 hours, successively expanded into 0.5 liters sample bags, and then transferred into the photoacoustic cell in order to perform the analysis of ethylene content. The PA analysis of ethylene content, by using the above described
CO2 Laser Photoacoustic Spectroscopy: II. Instrumentation and Applications 93
Free radicals come from two major sources: endogenous and exogenous. Endogenous free radicals are produced in the body by different mechanisms whereas exogenous sources of free radicals include air pollution, ionizing radiation, cigarette smoke, inflammation etc. The ultimate step in the peroxidative chain reaction is the formation of different hydrocarbons molecules, depending on the molecular arrangement of the fatty acid involved. In the human body, the fatty acids inside the membrane lipids are mainly linoleic acid and arachidonic acid. The peroxidation of these fatty acids produces two volatile alkanes: ethylene and pentane, respectively. Both of them are considered in literature to be good biomarkers of free radical induced lipid peroxidation in humans. The fact that ethylene is highly volatile, not significantly metabolized by the body and not soluble in body fat, means that this diffuses rapidly into bloodstream after generation and it is transported to the lungs. In
Generally speaking, exhaled breath analysis (called breath test) can be represented as follows: production of the biomarker during a particular biochemical reaction or a complex metabolic process; diffusion of biomarker through tissues and input into haematic flow; possible intermediate accumulation (buffering); possible trapping of biomarker by utilization and assimilation systems or natural chemical transformation; transport to the lungs; transmembrane diffusion to the air space of lungs; diffusion of biomarker and their mixing with inhaled air in the alveolar space of lungs; release of biomarker in the breathing air; collection of a breath sample and assessment of the biomarker in the breath sample.
To get an efficient breath air sample, we used aluminized multi-patient collection bags (750 mL aluminum-coated bags-QuinTron), composed of a disposable mouthpiece and a teemouthpiece assembly (it includes a plastic tee and a removable one-way flutter valve). Multi-patient collection bags (Fig. 35) are designed to collect multiple samples from patients
Mouthpiece and tee-connector 0.75 L aluminum-coated bag 0.40 L discard bag
How is properly collected a breath sample? After an approximately normal inspiration (avoiding filling the lungs at maximum), the subject places the mouthpiece in his/her mouth, forming a tight seal around it with the lips. A normal expiration is then made through the mouth, in order to empty the lungs of as much air as required to provide the alveolar sample. The first portion of the expired air goes out, after which the valve is opened the tee-piece, the remaining expired air being redirected into the collection bag. When a
After the alveolar air sample is collected, the sample gas is transferred into the PA cell and can be analyzed immediately or later. In either case, it is recommendable to seal the large port with the collection bag port cap furnished with the collection bag. The use of the port
suitable sample is collected, the patient stops exhaling and removes the mouthpiece.
the lungs, the gas is excreted in the expired breath and then is collected.
and hold a sample for maximum 6 hours.
Fig. 35. Breath sample collection system.
procedure, takes only few minutes and, after calibration, allows for immediate data release. The radioinduced production of ethylene in the animal appears to be at clearly detectable levels, since the exhaled ethylene increases more than 4 times in the mouse breath after the irradiation (12.4 ppb for control mice before exposure, compared to 55.9 ppb for irradiated mice, after exposure) (Giubileo et al., 2003).
The cigarette smoke contains many toxic components (heavy metals, free radicals, chemicals) that may induce ethylene formation by lipid peroxidation in the lung epithelium (Dumitras et al., 2008; Giubileo et al., 2004). In order to monitor the damages caused by the inhaled smoke, we performed a breath test which gives us information about the volatile compounds under normal and stress circumstances. The exhaled air from the subject being tested was collected inside aluminized bags and then the sample gas was transferred into the measurement PA cell. In all experiments, a high value of ethylene concentration was found immediately after smoking, followed by a slower decrease. A total concentration of 4040 ppb of ethylene was measured in cigarette smoke. In the exhaled breath of a smoker, we found an ethylene concentration of 39 ppb immediately after smoking and even 1.4 ppb at half an hour from smoking a single cigarette, compared to 0.6 ppb as base (before smoking). Ethylene is dangerous for smokers because ethylene oxide is a chemical product that induces cancer in the lungs. For the moment, it is difficult to separate the exogenous and endogenous origin of the ethylene in the smoker's breath.
#### **3.4 Measurement of human biomarkers**
The application of laser photoacoustic spectroscopy for fast and precise measurements of breath biomarkers has opened up new promises for monitoring and diagnostics in recent years, especially because breath test is a non-invasive method, safe, rapid and acceptable to patients.
The detection of biomarkers in the human breath for the purpose of diagnosis has a long history. Ancient Greek physicians already knew that the aroma of human breath could provide clues to diagnosis. The perceptive clinician was alert for the sweet, fruity odor of acetone in patients with uncontrolled diabetes; the musty, fishy reek of advanced liver disease; the urine-like smell that accompanies failing kidneys; and the putrid stench of a lung abscess. Modern breath analysis is a noninvasive medical diagnostic method that distinguishes among more than 1000 compounds in exhaled breath.
Human breath includes hundreds of VOCs in low concentrations even though fewer than fifty of these are found in the majority of normal human's breath. Some of these VOCs (ethane, n-pentane, butane, ethanol, acetone) have been identified as biomarkers to some specific pathologies, including lipid peroxidation, heart failure, asthma, cystic fibrosis, diabetic ketoacidosis, alcohol intoxication, renal failure, and others. However, due to the low concentrations and presence of a large number of chemical species in exhaled air, breath analysis requires high sensitive and selective instrumentation to detect and identify the atypical concentrations of specific biomarkers (Cernat et al., 2010; Popa et al., 2011a). In order to assess the physiological meaning and the diagnostic potential of these substances, the biochemical pathways of generation have to be known.
Ethylene from the human breath is a marker of oxidant stress (in patients on hemodialisys, in acute myocardial infarction, in inflammatory diseases and ultraviolet radiation damage of human skin) and can be directly attributed to biochemical events surrounding lipid peroxidation (Dumitras et al., 2005).
procedure, takes only few minutes and, after calibration, allows for immediate data release. The radioinduced production of ethylene in the animal appears to be at clearly detectable levels, since the exhaled ethylene increases more than 4 times in the mouse breath after the irradiation (12.4 ppb for control mice before exposure, compared to 55.9 ppb for irradiated
The cigarette smoke contains many toxic components (heavy metals, free radicals, chemicals) that may induce ethylene formation by lipid peroxidation in the lung epithelium (Dumitras et al., 2008; Giubileo et al., 2004). In order to monitor the damages caused by the inhaled smoke, we performed a breath test which gives us information about the volatile compounds under normal and stress circumstances. The exhaled air from the subject being tested was collected inside aluminized bags and then the sample gas was transferred into the measurement PA cell. In all experiments, a high value of ethylene concentration was found immediately after smoking, followed by a slower decrease. A total concentration of 4040 ppb of ethylene was measured in cigarette smoke. In the exhaled breath of a smoker, we found an ethylene concentration of 39 ppb immediately after smoking and even 1.4 ppb at half an hour from smoking a single cigarette, compared to 0.6 ppb as base (before smoking). Ethylene is dangerous for smokers because ethylene oxide is a chemical product that induces cancer in the lungs. For the moment, it is difficult to separate the exogenous
The application of laser photoacoustic spectroscopy for fast and precise measurements of breath biomarkers has opened up new promises for monitoring and diagnostics in recent years, especially because breath test is a non-invasive method, safe, rapid and acceptable to patients. The detection of biomarkers in the human breath for the purpose of diagnosis has a long history. Ancient Greek physicians already knew that the aroma of human breath could provide clues to diagnosis. The perceptive clinician was alert for the sweet, fruity odor of acetone in patients with uncontrolled diabetes; the musty, fishy reek of advanced liver disease; the urine-like smell that accompanies failing kidneys; and the putrid stench of a lung abscess. Modern breath analysis is a noninvasive medical diagnostic method that
Human breath includes hundreds of VOCs in low concentrations even though fewer than fifty of these are found in the majority of normal human's breath. Some of these VOCs (ethane, n-pentane, butane, ethanol, acetone) have been identified as biomarkers to some specific pathologies, including lipid peroxidation, heart failure, asthma, cystic fibrosis, diabetic ketoacidosis, alcohol intoxication, renal failure, and others. However, due to the low concentrations and presence of a large number of chemical species in exhaled air, breath analysis requires high sensitive and selective instrumentation to detect and identify the atypical concentrations of specific biomarkers (Cernat et al., 2010; Popa et al., 2011a). In order to assess the physiological meaning and the diagnostic potential of these substances,
Ethylene from the human breath is a marker of oxidant stress (in patients on hemodialisys, in acute myocardial infarction, in inflammatory diseases and ultraviolet radiation damage of human skin) and can be directly attributed to biochemical events surrounding lipid
mice, after exposure) (Giubileo et al., 2003).
**3.4 Measurement of human biomarkers**
and endogenous origin of the ethylene in the smoker's breath.
distinguishes among more than 1000 compounds in exhaled breath.
the biochemical pathways of generation have to be known.
peroxidation (Dumitras et al., 2005).
Free radicals come from two major sources: endogenous and exogenous. Endogenous free radicals are produced in the body by different mechanisms whereas exogenous sources of free radicals include air pollution, ionizing radiation, cigarette smoke, inflammation etc. The ultimate step in the peroxidative chain reaction is the formation of different hydrocarbons molecules, depending on the molecular arrangement of the fatty acid involved. In the human body, the fatty acids inside the membrane lipids are mainly linoleic acid and arachidonic acid. The peroxidation of these fatty acids produces two volatile alkanes: ethylene and pentane, respectively. Both of them are considered in literature to be good biomarkers of free radical induced lipid peroxidation in humans. The fact that ethylene is highly volatile, not significantly metabolized by the body and not soluble in body fat, means that this diffuses rapidly into bloodstream after generation and it is transported to the lungs. In the lungs, the gas is excreted in the expired breath and then is collected.
Generally speaking, exhaled breath analysis (called breath test) can be represented as follows: production of the biomarker during a particular biochemical reaction or a complex metabolic process; diffusion of biomarker through tissues and input into haematic flow; possible intermediate accumulation (buffering); possible trapping of biomarker by utilization and assimilation systems or natural chemical transformation; transport to the lungs; transmembrane diffusion to the air space of lungs; diffusion of biomarker and their mixing with inhaled air in the alveolar space of lungs; release of biomarker in the breathing air; collection of a breath sample and assessment of the biomarker in the breath sample.
To get an efficient breath air sample, we used aluminized multi-patient collection bags (750 mL aluminum-coated bags-QuinTron), composed of a disposable mouthpiece and a teemouthpiece assembly (it includes a plastic tee and a removable one-way flutter valve). Multi-patient collection bags (Fig. 35) are designed to collect multiple samples from patients and hold a sample for maximum 6 hours.
Mouthpiece and tee-connector 0.75 L aluminum-coated bag 0.40 L discard bag
Fig. 35. Breath sample collection system.
How is properly collected a breath sample? After an approximately normal inspiration (avoiding filling the lungs at maximum), the subject places the mouthpiece in his/her mouth, forming a tight seal around it with the lips. A normal expiration is then made through the mouth, in order to empty the lungs of as much air as required to provide the alveolar sample. The first portion of the expired air goes out, after which the valve is opened the tee-piece, the remaining expired air being redirected into the collection bag. When a suitable sample is collected, the patient stops exhaling and removes the mouthpiece.
After the alveolar air sample is collected, the sample gas is transferred into the PA cell and can be analyzed immediately or later. In either case, it is recommendable to seal the large port with the collection bag port cap furnished with the collection bag. The use of the port
CO2 Laser Photoacoustic Spectroscopy: II. Instrumentation and Applications 95
for the maintenance of the normal pH balance necessary to sustain life. Ammonia is processed in the liver, kidneys and skeletal muscles. Typically, ammonia and ammonium ions (in a healthy individual) are converted into urea in the liver through the urea cycle (Krebs-Henseileit cycle). The urea is then transported through the blood-stream to be excreted into urine by the kidneys. The reversibility of the process requires an equilibrium concentration of ammonia related to the blood urea nitrogen (BUN) loading of the blood. As small molecules, ammonia and ammonium ions can penetrate the blood-lung barrier, and appear in exhaled breath. In the case of kidney dysfunction, urea is unable to be excreted, causing an excessive build up of ammonia in the blood. People with kidney failure have a marked odor of ammonia ("fishy") on their breath, which can be an indicator of this disease. Volunteer participants (Table 6) were recruited from patients receiving HD treatment at the renal dialysis clinics at the IHS Fundeni (International Healthcare Systems), Bucharest. Subjects were dialyzed 3 times per week, with a 4 h dialysis session. They were instructed to use antiseptic mouthwash before each breath sampling, to avoid oral bacteria (over 700 species of bacteria live in our mouths and can interfere with our molecules of interest). HD was accomplished with BAXTER dialysis machines using DICEA (and XENIUM) high performance cellulose diacetate hollow fibre dialyser-gamma series (DICEA 170G) with following characteristics: surface area of 1.7 m2, ultrafiltration rate 12.5 mL/hr/mmHg,
inner diameter of 200 microns and membrane thickness of 15 microns.
**UpostHD (mg/dl)**
Table 6. The particular data of patients and the experimental measurements of breath
A special mention should be made: NH3 is a highly adsorbing compound and the results of successive measurements are often altered by the molecules previously adsorbed on the pathway and cell walls. To ensure the quality of each measurement, an intensive cycle of N2 washing was performed between samples, in order to have a maximum increase of 10% for the background photoacoustic signal. It has to be underlined that the measured photoacoustic signal is due mainly to the absorption of ammonia and ethylene, respectively, but some traces of CO2, H2O, ethanol, etc., influence the measurements (overall contribution is less than 10%). Experimental measurements in order to detect traces of ethylene and ammonia were performed for a healthy volunteer (C. A. male, 26 years old) and for 6 patients with renal failure. Particular data of patients are summarized in Table 6. The exhaled air samples were collected before, during (about 1 hour after the start of HD) and immediately after the HD procedure. Analysis of pre-dialysis urea level and post-dialysis urea level (normal limit in the range of 19 - 43 mg/dL) was made at MedCenter, Bucharest (VITROS 51). The results are
before HD
P1 Male 67 2005 147 37 0.03 0.13 0.52 4.63 3.58 2.39 P2 Male 80 2004 131 39 0.23 0.51 0.93 4.28 2.82 1.53 P3 Male 79 2008 136 22 0.17 0.31 0.91 2.89 2.06 0.67 P4 Male 22 2010 135 21 0.14 0.19 0.84 5.71 4.08 3.24 P5 Male 54 2010 174 48 0.18 0.43 0.89 4.79 3.07 1.5 P6 Male 66 2005 147 66 - - - 2.8 2.01 1.66
during HD
**C2H4 (ppm) NH3 (ppm)**
before HD
during HD
after HD
after HD
**UpreHD (mg/dl)**
**since**
ethylene and ammonia concentrations (± 10% data error).
**Patient Gender Age HD**
also presented in Table 6.
cap assures that the sample volume will not be lost due to a leak. Its use also avoids the contamination of the sample by gas diffusion through the one-way valve in the large port, if the sample is stored for a long period of time prior to its analysis.
Radiation therapy (also called radiotherapy, X-ray therapy, or irradiation) is the use of certain type of energy (called ionizing radiation) to kill cancer cells and shrink tumors. Radiation therapy injures or destroys cells in the area being treated by damaging their genetic material, making it impossible for these cells to continue grow and divide. Although radiation damages both cancer cells and normal cells, most normal cells can recover from the effects of radiation and function properly. The goal of radiotherapy is to damage as many cancer cells as possible, while limiting harm to nearby healthy tissue.
The effect of ionizing radiation on living cells is supposed to modify the oxidative stress status in the human body through an increase in the peroxidation processes started by the free water radicals generated by indirect radiation effect in living tissue. Important events of the peroxidation take place in the cell membranes determining the release of small linear hydrocarbon molecules through the lipid peroxidation pathways. A fraction of the hydrocarbon molecules generated in the tissue (one among them is the ethylene) will be transported to the lungs by the blood and release in the exhaled breath.
We have analyzed exhaled air from 6 patients between 32 and 77 years old receiving radiation treatment based on X-ray external beam after malign tumor surgery (Dumitras et al., 2008). Breath samples were taken from volunteers at certain time intervals (before, immediately after and at 15 minutes after the X-ray therapy). The patients received fractional doses as high as 2 to 8 Gy depending on the type of cancer. For this experiment patients were asked to exhale into sample bags at a normal exhalation flow rate.
The exhaled air sample was transferred in the PA cell and analyzed in the continuous nitrogen flux. The KOH trap inserted in the gas circuit is used to remove as much as possible the high quantity of CO2 from the exhaled air. To subtract from the final results any influence of the interfering gases (CO2, H2O vapors) we applied the changing lines method, using 3 lines: 10P(14), 10P(16), and 10P(26).
We have measured the following levels of ethylene for a patient (female, 77 years old) with mammary cancer treated by X-ray therapy with a dose of 8 Gy: a) before X-ray therapy: c = 18.6 ppbV; b) Immediately after X-ray therapy: c = 23.17 ppbV; c) 15 min after the X-ray therapy: c = 10.83 ppbV. As a first observation of our measurement we see that, indeed, after the X-ray irradiation the ethylene concentration rises, showing that lipid peroxidation took place. So, it is possible to detect the process in the very first minute after irradiation. The effect of lipid peroxidation is more powerful on the cancer cells, while the healthy cells even affected have higher recovery ability. A surprising decrease in the level of ethylene concentration was observed in the exhaled air after 15 minutes, the level being even lower than the normal level of the patient (e.g. the level measured before any irradiation). This could be explained as a body reaction to the increased level of peroxidic attack: higher the rate of damage, higher the self-defense response of the human organism. Further work is required in order to verify this hypothesis.
In separate studies (Popa et al., 2011b; Popa et al., 2011c), we investigated the breath ethylene and the breath ammonia levels in patients with renal failure receiving haemodialysis (HD) treatment. Human bodies use ammonia in a number of ways, including
cap assures that the sample volume will not be lost due to a leak. Its use also avoids the contamination of the sample by gas diffusion through the one-way valve in the large port, if
Radiation therapy (also called radiotherapy, X-ray therapy, or irradiation) is the use of certain type of energy (called ionizing radiation) to kill cancer cells and shrink tumors. Radiation therapy injures or destroys cells in the area being treated by damaging their genetic material, making it impossible for these cells to continue grow and divide. Although radiation damages both cancer cells and normal cells, most normal cells can recover from the effects of radiation and function properly. The goal of radiotherapy is to damage as
The effect of ionizing radiation on living cells is supposed to modify the oxidative stress status in the human body through an increase in the peroxidation processes started by the free water radicals generated by indirect radiation effect in living tissue. Important events of the peroxidation take place in the cell membranes determining the release of small linear hydrocarbon molecules through the lipid peroxidation pathways. A fraction of the hydrocarbon molecules generated in the tissue (one among them is the ethylene) will be
We have analyzed exhaled air from 6 patients between 32 and 77 years old receiving radiation treatment based on X-ray external beam after malign tumor surgery (Dumitras et al., 2008). Breath samples were taken from volunteers at certain time intervals (before, immediately after and at 15 minutes after the X-ray therapy). The patients received fractional doses as high as 2 to 8 Gy depending on the type of cancer. For this experiment
The exhaled air sample was transferred in the PA cell and analyzed in the continuous nitrogen flux. The KOH trap inserted in the gas circuit is used to remove as much as possible the high quantity of CO2 from the exhaled air. To subtract from the final results any influence of the interfering gases (CO2, H2O vapors) we applied the changing lines method,
We have measured the following levels of ethylene for a patient (female, 77 years old) with mammary cancer treated by X-ray therapy with a dose of 8 Gy: a) before X-ray therapy: c = 18.6 ppbV; b) Immediately after X-ray therapy: c = 23.17 ppbV; c) 15 min after the X-ray therapy: c = 10.83 ppbV. As a first observation of our measurement we see that, indeed, after the X-ray irradiation the ethylene concentration rises, showing that lipid peroxidation took place. So, it is possible to detect the process in the very first minute after irradiation. The effect of lipid peroxidation is more powerful on the cancer cells, while the healthy cells even affected have higher recovery ability. A surprising decrease in the level of ethylene concentration was observed in the exhaled air after 15 minutes, the level being even lower than the normal level of the patient (e.g. the level measured before any irradiation). This could be explained as a body reaction to the increased level of peroxidic attack: higher the rate of damage, higher the self-defense response of the human organism. Further work is
In separate studies (Popa et al., 2011b; Popa et al., 2011c), we investigated the breath ethylene and the breath ammonia levels in patients with renal failure receiving haemodialysis (HD) treatment. Human bodies use ammonia in a number of ways, including
the sample is stored for a long period of time prior to its analysis.
many cancer cells as possible, while limiting harm to nearby healthy tissue.
transported to the lungs by the blood and release in the exhaled breath.
using 3 lines: 10P(14), 10P(16), and 10P(26).
required in order to verify this hypothesis.
patients were asked to exhale into sample bags at a normal exhalation flow rate.
for the maintenance of the normal pH balance necessary to sustain life. Ammonia is processed in the liver, kidneys and skeletal muscles. Typically, ammonia and ammonium ions (in a healthy individual) are converted into urea in the liver through the urea cycle (Krebs-Henseileit cycle). The urea is then transported through the blood-stream to be excreted into urine by the kidneys. The reversibility of the process requires an equilibrium concentration of ammonia related to the blood urea nitrogen (BUN) loading of the blood. As small molecules, ammonia and ammonium ions can penetrate the blood-lung barrier, and appear in exhaled breath. In the case of kidney dysfunction, urea is unable to be excreted, causing an excessive build up of ammonia in the blood. People with kidney failure have a marked odor of ammonia ("fishy") on their breath, which can be an indicator of this disease.
Volunteer participants (Table 6) were recruited from patients receiving HD treatment at the renal dialysis clinics at the IHS Fundeni (International Healthcare Systems), Bucharest. Subjects were dialyzed 3 times per week, with a 4 h dialysis session. They were instructed to use antiseptic mouthwash before each breath sampling, to avoid oral bacteria (over 700 species of bacteria live in our mouths and can interfere with our molecules of interest). HD was accomplished with BAXTER dialysis machines using DICEA (and XENIUM) high performance cellulose diacetate hollow fibre dialyser-gamma series (DICEA 170G) with following characteristics: surface area of 1.7 m2, ultrafiltration rate 12.5 mL/hr/mmHg, inner diameter of 200 microns and membrane thickness of 15 microns.
Table 6. The particular data of patients and the experimental measurements of breath ethylene and ammonia concentrations (± 10% data error).
A special mention should be made: NH3 is a highly adsorbing compound and the results of successive measurements are often altered by the molecules previously adsorbed on the pathway and cell walls. To ensure the quality of each measurement, an intensive cycle of N2 washing was performed between samples, in order to have a maximum increase of 10% for the background photoacoustic signal. It has to be underlined that the measured photoacoustic signal is due mainly to the absorption of ammonia and ethylene, respectively, but some traces of CO2, H2O, ethanol, etc., influence the measurements (overall contribution is less than 10%).
Experimental measurements in order to detect traces of ethylene and ammonia were performed for a healthy volunteer (C. A. male, 26 years old) and for 6 patients with renal failure. Particular data of patients are summarized in Table 6. The exhaled air samples were collected before, during (about 1 hour after the start of HD) and immediately after the HD procedure. Analysis of pre-dialysis urea level and post-dialysis urea level (normal limit in the range of 19 - 43 mg/dL) was made at MedCenter, Bucharest (VITROS 51). The results are also presented in Table 6.
CO2 Laser Photoacoustic Spectroscopy: II. Instrumentation and Applications 97
The most important result is the correlation found between urea data (measured by blood analysis) and the individual breath ammonia and ethylene concentrations (measured by
This analysis demonstrates that HD determines simultaneously a large increase of the ethylene concentration in the exhaled breath (owing to the oxidative stress) and a reduction
Laser photoacoustic spectroscopy technique demonstrated that it will play an important role in the future of exhaled breath analysis. The key attributes of this technique is sensitivity,
The applications of resonant PA spectroscopy include concentration measurements and trace gas analysis, accurate determinations of thermophysical properties, detections of dynamic processes such as gas mixing or chemical reactions, relaxation processes (determinations of collisional lifetimes of specified quantum states and routes of energy exchange in polyatomic molecules), spectroscopic experiments, studies of aerosols, etc. Trace-gas sensing is a rapidly developing field, in demand for applications such as process and air-quality measurements, atmospheric monitoring, breath diagnostics, biology and
More than 250 different volatile organic compounds including air pollutants originating from the burning of fossil fuels, traffic, or natural sources can be identified and measured with a CO2 laser based PA instrument. Such studies are prompted by the growing public concern about serious environmental problems such as acid rain, photochemical smog,
Breath analysis is a noninvasive medical diagnostic method that distinguishes among more than 1000 compounds in exhaled breath. Many of these compounds, if measured accurately at very low concentration levels, typically in the range of few ppbV, can be used to identify particular medical conditions. Measuring human biomarkers in exhaled breath is expected to revolutionize diagnosis and management of many diseases and may soon lead to rapid, improved, lower-cost diagnosis, which will in turn ensure expanded life spans and an improved quality of life. For example, ammonia levels in the breath can be used to determine the exact time necessary for an optimal degree of dialysis for a patient with end-
Trace-gas detection techniques based on PA spectroscopy make it possible to discover and control plant physiology mechanisms such as those responsible for germination, blossoming, senescence, ripening, wounding effects, post anaerobic injury, etc. Many agriculturally interesting gases (ethylene, methane, water vapor concentration, carbon dioxide, ammonia, ozone) can be measured *in situ* and in real time with CO2 and CO laser
In chemistry, PA spectroscopy is useful in the monitoring of chemical processes (reaction rates, equilibrium constants, enthalpies), identification of different compounds (even
The techniques of PA spectroscopy can be extended to the detection of a wide variety of industrial gases, including benzene, hydrogen cyanide, acetylene, carbon monoxide, and carbon
photoacoustic technique), shown in Figs. 36 (b) and 36 (d) for six patients.
of the ammonia concentration, correlated to the level of blood urea nitrogen.
selectivity, fast and real time response and ease to use.
agriculture, chemistry, and security and workplace surveillance.
stratospheric ozone depletion, and global climatic changes.
isomers and radicals), and dimerization of fatty acid vapors**.**
stage renal disease at every session.
based photoacoustic spectrometers.
**4. Conclusions**
Experimental measurements of breath ethylene and ammonia concentrations for the patients (P1-P6) with renal failure and for the healthy subject (P0) were performed and the results are presented in Fig. 36. The control P0 values are 0.006 ppm ethylene and 0.25 ppm ammonia. All measurements were made at 10P(14) CO2 laser line (10.53 μm), where the ethylene absorption coefficient has the largest value (30.4 cm-1atm-1), and at 9R(30) CO2 laser line (9.22 μm), where the ammonia absorption coefficient has the maximum value of 57 cm-1atm-1.
Fig. 36. Ethylene and ammonia concentrations in exhaled breath of patients under HD treatment: (a) ethylene concentrations; (b) breath ethylene concentration correlation with urea level; (c) ammonia concentrations; (d) breath ethylene concentration correlation with urea level.
As a first observation of our measurements (shown in Fig. 36a), we see that, immediately after HD treatment, the ethylene concentration increases, proving the presence of lipid peroxidation. Oxidative stress is a persistent manifestation at patients with renal failure, showing an imbalance between oxidant and antioxidant systems. HD is associated with increased oxidative stress and all treated patients are exposed to this stress. This situation appears to be due to an increased production of free radicals during HD and immediately after HD and a net reduction of many antioxidants.
In Fig. 36 (c) we can observe, as expected, a reduction of ammonia concentration in exhaled breath at patients under HD treatment, which means that ammonia detection in human breath using LPAS system can be used for determining the exact time necessary at every session for the desired state of HD for a patient with end stage renal disease and, in the same time, could serve as a broad noninvasive screen for incipient renal disease.
The most important result is the correlation found between urea data (measured by blood analysis) and the individual breath ammonia and ethylene concentrations (measured by photoacoustic technique), shown in Figs. 36 (b) and 36 (d) for six patients.
This analysis demonstrates that HD determines simultaneously a large increase of the ethylene concentration in the exhaled breath (owing to the oxidative stress) and a reduction of the ammonia concentration, correlated to the level of blood urea nitrogen.
Laser photoacoustic spectroscopy technique demonstrated that it will play an important role in the future of exhaled breath analysis. The key attributes of this technique is sensitivity, selectivity, fast and real time response and ease to use.
#### **4. Conclusions**
96 CO2 Laser – Optimisation and Application
Experimental measurements of breath ethylene and ammonia concentrations for the patients (P1-P6) with renal failure and for the healthy subject (P0) were performed and the results are presented in Fig. 36. The control P0 values are 0.006 ppm ethylene and 0.25 ppm ammonia. All measurements were made at 10P(14) CO2 laser line (10.53 μm), where the ethylene absorption coefficient has the largest value (30.4 cm-1atm-1), and at 9R(30) CO2 laser line (9.22 μm), where the ammonia absorption coefficient has the maximum value of 57 cm-1atm-1.
(a) (b)
(c) (d)
Fig. 36. Ethylene and ammonia concentrations in exhaled breath of patients under HD treatment: (a) ethylene concentrations; (b) breath ethylene concentration correlation with urea level; (c) ammonia concentrations; (d) breath ethylene concentration correlation with urea level.
As a first observation of our measurements (shown in Fig. 36a), we see that, immediately after HD treatment, the ethylene concentration increases, proving the presence of lipid peroxidation. Oxidative stress is a persistent manifestation at patients with renal failure, showing an imbalance between oxidant and antioxidant systems. HD is associated with increased oxidative stress and all treated patients are exposed to this stress. This situation appears to be due to an increased production of free radicals during HD and immediately
In Fig. 36 (c) we can observe, as expected, a reduction of ammonia concentration in exhaled breath at patients under HD treatment, which means that ammonia detection in human breath using LPAS system can be used for determining the exact time necessary at every session for the desired state of HD for a patient with end stage renal disease and, in the
same time, could serve as a broad noninvasive screen for incipient renal disease.
after HD and a net reduction of many antioxidants.
The applications of resonant PA spectroscopy include concentration measurements and trace gas analysis, accurate determinations of thermophysical properties, detections of dynamic processes such as gas mixing or chemical reactions, relaxation processes (determinations of collisional lifetimes of specified quantum states and routes of energy exchange in polyatomic molecules), spectroscopic experiments, studies of aerosols, etc. Trace-gas sensing is a rapidly developing field, in demand for applications such as process and air-quality measurements, atmospheric monitoring, breath diagnostics, biology and agriculture, chemistry, and security and workplace surveillance.
More than 250 different volatile organic compounds including air pollutants originating from the burning of fossil fuels, traffic, or natural sources can be identified and measured with a CO2 laser based PA instrument. Such studies are prompted by the growing public concern about serious environmental problems such as acid rain, photochemical smog, stratospheric ozone depletion, and global climatic changes.
Breath analysis is a noninvasive medical diagnostic method that distinguishes among more than 1000 compounds in exhaled breath. Many of these compounds, if measured accurately at very low concentration levels, typically in the range of few ppbV, can be used to identify particular medical conditions. Measuring human biomarkers in exhaled breath is expected to revolutionize diagnosis and management of many diseases and may soon lead to rapid, improved, lower-cost diagnosis, which will in turn ensure expanded life spans and an improved quality of life. For example, ammonia levels in the breath can be used to determine the exact time necessary for an optimal degree of dialysis for a patient with endstage renal disease at every session.
Trace-gas detection techniques based on PA spectroscopy make it possible to discover and control plant physiology mechanisms such as those responsible for germination, blossoming, senescence, ripening, wounding effects, post anaerobic injury, etc. Many agriculturally interesting gases (ethylene, methane, water vapor concentration, carbon dioxide, ammonia, ozone) can be measured *in situ* and in real time with CO2 and CO laser based photoacoustic spectrometers.
In chemistry, PA spectroscopy is useful in the monitoring of chemical processes (reaction rates, equilibrium constants, enthalpies), identification of different compounds (even isomers and radicals), and dimerization of fatty acid vapors**.**
The techniques of PA spectroscopy can be extended to the detection of a wide variety of industrial gases, including benzene, hydrogen cyanide, acetylene, carbon monoxide, and carbon
CO2 Laser Photoacoustic Spectroscopy: II. Instrumentation and Applications 99
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dioxide, as well as a broad range of chemical warfare agents, including nerve gases (Sarin, Soman, Tabun), blistering agents (phosgene, mustard gas), and poisonous gases (hydrogen cyanide), explosives (TNT, PETN), and harmful drugs (heroin, morphine, narcotine).
Our previous research on LPAS (Dumitras et al., 1993; Dutu et al., 1994a; Dutu et al., 1994b; Dumitras et al., 1996a; Dumitras et al., 1996b; Cristescu et al., 1997; Cristescu et al., 2000a) has led to the development of new applications in plant physiology (seed germination, ripening of climacteric fruits, plant response to pathogen infection) (Cristescu et al., 1998; Cristescu et al., 1999; Lai et al., 2003), measurement of gas absorption coefficients (ethylene, ammonia) (Cristescu et al., 2000b; Dumitras et al., 2007; Dumitras et al., 2011), and medicine (cultures of human cells doped with heavy metals, ionizing radiation damage in living organisms, lipid peroxidation in lung epithelium following the inhalation of cigarette smoke, exhaled breath from patients treated by anti-tumor radioisotope therapy and patients under HD treatment) (Giubileo et al., 2003; Dumitras et al., 2004; Giubileo et al., 2004; Dumitras et al., 2005; Cernat et al., 2010; Popa et al., 2011a; Popa et al., 2011b; Popa et al., 2011c; Popa & Matei, 2011).
Extensions of various aspects of this work are currently being pursued in our laboratory.
#### **5. References**
dioxide, as well as a broad range of chemical warfare agents, including nerve gases (Sarin, Soman, Tabun), blistering agents (phosgene, mustard gas), and poisonous gases (hydrogen
Our previous research on LPAS (Dumitras et al., 1993; Dutu et al., 1994a; Dutu et al., 1994b; Dumitras et al., 1996a; Dumitras et al., 1996b; Cristescu et al., 1997; Cristescu et al., 2000a) has led to the development of new applications in plant physiology (seed germination, ripening of climacteric fruits, plant response to pathogen infection) (Cristescu et al., 1998; Cristescu et al., 1999; Lai et al., 2003), measurement of gas absorption coefficients (ethylene, ammonia) (Cristescu et al., 2000b; Dumitras et al., 2007; Dumitras et al., 2011), and medicine (cultures of human cells doped with heavy metals, ionizing radiation damage in living organisms, lipid peroxidation in lung epithelium following the inhalation of cigarette smoke, exhaled breath from patients treated by anti-tumor radioisotope therapy and patients under HD treatment) (Giubileo et al., 2003; Dumitras et al., 2004; Giubileo et al., 2004; Dumitras et al., 2005; Cernat et
cyanide), explosives (TNT, PETN), and harmful drugs (heroin, morphine, narcotine).
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**3**
*Belarus*
**CO2 Lasing on Non-Traditional Bands**
Construction of powerful and efficient laser sources, lasing in various IR ranges, is of importance for further development of a number of trends, e.g., spectroscopy, laser chemistry, sounding of the atmosphere, and metrology. The most natural way to solve this problem is to use unconventional (nontraditional) transitions to produce lasing in commonly used CO2 lasers. The spectral range of CO2 lasers is greatly increased in lasing on transitions of the so-called "hot" band 0111-1110, whose P-branch is in the range of 10.9-11.3 µm*.* Thorough investigations of gain, vibrational temperatures (*T1, T2, T3*), and output parameters on lines of the hot band made it possible to achieve efficient lasing both for pulse
In studying the lasing spectrum of hot transitions in TEA CO2 lasers some lines not belonging to the 0111-1110 band. We suggested, that these lasing lines belong to higher level transitions, e.g., 1001-2000 (0400), which were called "doubly hot," i.e., transitions in which compared to hot transitions two deformation quanta or one symmetric quantum rather than one
In the present work lasing in both a TEA laser and a low-pressure laser with longitudinal discharge on some transitions of the CO2 molecule in the range of 11.0-11.6 µm is reported. The rather high resolution of the spectral equipment used and calculation of transition frequencies on the basis of recent spectroscopic constants made it possible to identify definitively the lasing lines obtained as belonging to the doubly hot bands 0221-1220 and 100l-2000 and the sequence hot band 0112-1111. To find optimum conditions for lasing on the aforementioned bands experimental studies of vibrational temperatures in active media of a
Earlier the lasing on the 0200(1000)-0110 band of the CO2 molecule has been obtained in the specific systems at cryogenic temperatures under the lowest efficiency. The optimization of the active medium and its electrical discharge pumping conditions based on the original technique of the temperature model allowed to obtain in the simple TE CO2 laser with UV preionization the powerful lasing on the 0200-0110 band at room temperature. The dependencies of the output and spectral performances of the 16 (14) micrometers lasing vs. a content of the active
To increase the power performances of the 16 (14) microns CO2 laser the possibility of lasing on the 0201(1001)-0111 band have been experimentally and theoretically investigated under the combined (electrical + optical) excitation of the active medium. The conditions for
deformation quantum is added both to the upper and to the lower energy level.
TEA CO2 laser and a low-pressure laser with longitudinal discharge were carried out.
medium, pumping parameters and cavity characteristics have been carried out.
**1. Introduction**
TEA and for cw longitudinal-discharge CO2 lasers.
*Institute of Physics of National Academy of Sciences of Belarus*
Vladimir Petukhov and Vadim Gorobets
## **CO2 Lasing on Non-Traditional Bands**
Vladimir Petukhov and Vadim Gorobets
*Institute of Physics of National Academy of Sciences of Belarus Belarus*
#### **1. Introduction**
102 CO2 Laser – Optimisation and Application
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Ryan, J.S.; Hubert, M.H. & Crane, R.A. (1983). Water Vapor Absorption at Isotopic CO2 Laser Wavelengths. *Appl. Opt.,* Vol.22, No.5, (March 1975), pp. 711-717, ISSN 0003-6935 Sauren, H.; Bicanic, D.; Jelink, H. & Reuss, J. (1989). High-Sensitivity, Interference-Free,
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with a Tunable CO2 Laser. *Appl. Opt.,* Vol.31, No.15, (May 1992), pp. 2657-2668,
Dependence of NH3 and C2H4 Absorption Cross Sections at CO2 Laser Wavelengths. *Appl. Opt.,* Vol.19, No.10, (May 1980), pp.1711-1715, ISSN 0003-6935 Popa, C. & Matei, C. (2011). Photoacoustic Assessment of Oxidative Stress in Dialysis and
Radiotherapy by LPAS System. *Optoelectron. Adv. Mater. – Rapid Commun.*, Vol. 5,
Studies of Ethylene and Ammonia as Biomarkers at Patients with Different Medical Disorders. *U. P. B. Sci. Bull., Series A,* Vol.73, No.2, pp. 167-174, ISSN 1223-7027 Popa, C.; Bratu, A.M.; Matei, C.; Cernat, R.; Popescu, A. & Dumitras, D.C. (2011b).
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Construction of powerful and efficient laser sources, lasing in various IR ranges, is of importance for further development of a number of trends, e.g., spectroscopy, laser chemistry, sounding of the atmosphere, and metrology. The most natural way to solve this problem is to use unconventional (nontraditional) transitions to produce lasing in commonly used CO2 lasers. The spectral range of CO2 lasers is greatly increased in lasing on transitions of the so-called "hot" band 0111-1110, whose P-branch is in the range of 10.9-11.3 µm*.* Thorough investigations of gain, vibrational temperatures (*T1, T2, T3*), and output parameters on lines of the hot band made it possible to achieve efficient lasing both for pulse TEA and for cw longitudinal-discharge CO2 lasers.
In studying the lasing spectrum of hot transitions in TEA CO2 lasers some lines not belonging to the 0111-1110 band. We suggested, that these lasing lines belong to higher level transitions, e.g., 1001-2000 (0400), which were called "doubly hot," i.e., transitions in which compared to hot transitions two deformation quanta or one symmetric quantum rather than one deformation quantum is added both to the upper and to the lower energy level.
In the present work lasing in both a TEA laser and a low-pressure laser with longitudinal discharge on some transitions of the CO2 molecule in the range of 11.0-11.6 µm is reported. The rather high resolution of the spectral equipment used and calculation of transition frequencies on the basis of recent spectroscopic constants made it possible to identify definitively the lasing lines obtained as belonging to the doubly hot bands 0221-1220 and 100l-2000 and the sequence hot band 0112-1111. To find optimum conditions for lasing on the aforementioned bands experimental studies of vibrational temperatures in active media of a TEA CO2 laser and a low-pressure laser with longitudinal discharge were carried out.
Earlier the lasing on the 0200(1000)-0110 band of the CO2 molecule has been obtained in the specific systems at cryogenic temperatures under the lowest efficiency. The optimization of the active medium and its electrical discharge pumping conditions based on the original technique of the temperature model allowed to obtain in the simple TE CO2 laser with UV preionization the powerful lasing on the 0200-0110 band at room temperature. The dependencies of the output and spectral performances of the 16 (14) micrometers lasing vs. a content of the active medium, pumping parameters and cavity characteristics have been carried out.
To increase the power performances of the 16 (14) microns CO2 laser the possibility of lasing on the 0201(1001)-0111 band have been experimentally and theoretically investigated under the combined (electrical + optical) excitation of the active medium. The conditions for
CO2 Lasing on Non-Traditional Bands 105
where Kr *,* Ks*,* Kh are measured small signal gains of the corresponding regular, sequence and hot band lines; T is the translational temperature determined from the gain distribution over the regular band lines (Petukhov et. al.., 1985). The lock-in amplifier and box-car integrator used in the recording system allowed us to achieve a better than 2% measurement
> *10.4* µ*m*
*m 16.4*
*m 9.3*
Fig. 1. Simplified diagram of lower vibrational levels of the CO2 molecule.
µ*m*
*m 14*
addition, an increased specific energy input is also required.
The first step was optimization of active medium composition and pressure *P* and of discharge current. The measurement of the small signal gain Kh has shown that the optimum mixture for the hot band is CO2:N2:He = 1:1.4:3.5 at a total pressure *P* = 11 Torr *(I* = 15 mA) in which on the strong lines *Kh* = 0.08m-1. It is important that this mixture contains less He and has a large partial content of CO2 as compared with the mixture 1:1.6:6.5 (*P* = 15 Torr, *I =* 10 mA) optimum for the regular band 0001-1000(0200). The theoretical and experimental investigations of vibrational temperatures of a CO2 molecule has shown that to obtain considerable hot band gain *Kh* of a CO2 molecule it is necessary to heat up the *v2(v1)* mode characterized by vibrational temperature *T2* along with the excitation of the *v3* mode (temperature *T3).* For the conventional values of *T3* ≈1600 – 2200 K realized in an electric discharge the Kh gain is shown to achieve its maximum if T2~l/3*T3* (Bertel et. al.., 1983). Such a relationship between *T2* and *T3* can be reached in gas mixtures with greater CO2 and lesser He contents, as compared to those optimal for the regular band oscillation. In
02<sup>0</sup> 0
031 0
µ*m*
> *10.4* µ*m*
01<sup>1</sup> 1
µ*m*
020 1 *9.4* µ*m*
> *9.4* µ*m*
011 0
*16* µ 00<sup>0</sup> 1
000 0
00<sup>0</sup> 2
gain accuracy for all bands.
E, cm-1
1000
2000
3000
4000
100 0
111 0
10<sup>0</sup> 1
> *14.1* µ
*10.8* µ
obtaining effective lasing at the rotational-vibrational transitions of the 0201-0111 (λ = 16.4 µm) and 1001-0111 (λ = 14.1 µm) bands of the CO2 molecule are examined. To obtain population inversion in the indicated channels one should initially populate the 0002 vibrational level, considerable population of which can be accomplished comparatively simply, for example, in an electric discharge. Then a powerful two-frequency radiation resonant with the 0002-0201 (1001) and 0111-1110(0310) transitions acts on the medium excited in such a way. We will discuss by what means such a scheme of lasing in one active medium can be accomplished.
The lidar complex of equipment based on CO2 laser specially designed for atmospheric sensing, with tuning on generation lines in the spectral ranges 9-11.3 and 4.5-5.6 µm will be described. Considerable extension of the spectral range to the short-wave region is attained due to effective CO2 laser second harmonic generation in nonlinear crystals. Taking into the real potentialities of the lidar complex in hand, using a package of spectroscopic data HITRAN, computer simulation of atmospheric transmission has been made. On this basis, by the method of differential absorption a method has been elaborated for measuring of small concentrations of a number of gases.
#### **2. Effective oscillation of a cw CO2 laser in the range of 11** μ**m (01<sup>1</sup> 1-11<sup>1</sup> 0 band)**
The CO2 laser oscillation spectrum expansion to the long-wave region is of interest for various scientific and practical applications, for example, for spectroscopy, atmosphere monitoring, etc. From this point of view, the use of the P-branch of the hot 0111- 1110 band (10.9—11.4 µm) ( see Fig. 1) has considerable promise. Weak hot band lasing was registered in the middle of the 60s in specific long tube (2—4 m) laser systems. The problem of obtaining the hot band oscillation in commercially available cw CO2 lasers is associated with the low gain realized under conventional conditions. Therefore, the effective hot band cw CO2 laser oscillation demands, first of all, as in the pulsed TEA CO2 system, comprehensive study of excitation and active medium composition effects on the hot band gain.
In this work we present experimental results of searching for optimal conditions of the hot band line lasing in a cw CO2 laser with a commercial 1.2 m sealed-off tube. The hot band gain and output optimization was carried out depending on the active medium composition as well as on the discharge current.
The gain in the active medium was measured by small signal probing using the compensation method. We used as a probing laser a specially developed cw stabilized CO2 laser tunable over many of the hot band, sequence 0002-1001(0201) band and regular 0001- 1000(0200) band lines.
Analysis of the results obtained was carried out on the basis of the universally accepted CO2-molecule vibrational temperature model (Petukhov et. al.., 1985). The vibrational temperatures of the asymmetric (ν3) and bound symmetric-bend (2ν2≈ν1) mode, *T3* and *T2,* respectively, have been determined from the following expressions (Petukhov et. al.., 1985):
$$T\_3 = -\frac{3380}{\ln \frac{K\_s}{2.1 \cdot K\_t} - \frac{36}{T}} , T\_2 = -\frac{960}{\ln \frac{K\_h}{K\_t} - \frac{18}{T}} ,\tag{1}$$
obtaining effective lasing at the rotational-vibrational transitions of the 0201-0111 (λ = 16.4 µm) and 1001-0111 (λ = 14.1 µm) bands of the CO2 molecule are examined. To obtain population inversion in the indicated channels one should initially populate the 0002 vibrational level, considerable population of which can be accomplished comparatively simply, for example, in an electric discharge. Then a powerful two-frequency radiation resonant with the 0002-0201 (1001) and 0111-1110(0310) transitions acts on the medium excited in such a way. We will discuss by what means such a scheme of lasing in one active medium
The lidar complex of equipment based on CO2 laser specially designed for atmospheric sensing, with tuning on generation lines in the spectral ranges 9-11.3 and 4.5-5.6 µm will be described. Considerable extension of the spectral range to the short-wave region is attained due to effective CO2 laser second harmonic generation in nonlinear crystals. Taking into the real potentialities of the lidar complex in hand, using a package of spectroscopic data HITRAN, computer simulation of atmospheric transmission has been made. On this basis, by the method of differential absorption a method has been elaborated for measuring of
The CO2 laser oscillation spectrum expansion to the long-wave region is of interest for various scientific and practical applications, for example, for spectroscopy, atmosphere monitoring, etc. From this point of view, the use of the P-branch of the hot 0111- 1110 band (10.9—11.4 µm) ( see Fig. 1) has considerable promise. Weak hot band lasing was registered in the middle of the 60s in specific long tube (2—4 m) laser systems. The problem of obtaining the hot band oscillation in commercially available cw CO2 lasers is associated with the low gain realized under conventional conditions. Therefore, the effective hot band cw CO2 laser oscillation demands, first of all, as in the pulsed TEA CO2 system, comprehensive
In this work we present experimental results of searching for optimal conditions of the hot band line lasing in a cw CO2 laser with a commercial 1.2 m sealed-off tube. The hot band gain and output optimization was carried out depending on the active medium composition
The gain in the active medium was measured by small signal probing using the compensation method. We used as a probing laser a specially developed cw stabilized CO2 laser tunable over many of the hot band, sequence 0002-1001(0201) band and regular 0001-
Analysis of the results obtained was carried out on the basis of the universally accepted CO2-molecule vibrational temperature model (Petukhov et. al.., 1985). The vibrational temperatures of the asymmetric (ν3) and bound symmetric-bend (2ν2≈ν1) mode, *T3* and *T2,* respectively, have been determined from the following expressions (Petukhov et. al.., 1985):
<sup>3380</sup> <sup>960</sup> , , <sup>36</sup> <sup>18</sup> ln ln
*s h t t*
− − <sup>⋅</sup>
*KT KT*
(1)
**2. Effective oscillation of a cw CO2 laser in the range of 11** μ**m**
study of excitation and active medium composition effects on the hot band gain.
3 2
*T T K K*
= − = −
2.1
can be accomplished.
**(01<sup>1</sup>**
**1-11<sup>1</sup>**
**0 band)**
as well as on the discharge current.
1000(0200) band lines.
small concentrations of a number of gases.
where Kr *,* Ks*,* Kh are measured small signal gains of the corresponding regular, sequence and hot band lines; T is the translational temperature determined from the gain distribution over the regular band lines (Petukhov et. al.., 1985). The lock-in amplifier and box-car integrator used in the recording system allowed us to achieve a better than 2% measurement gain accuracy for all bands.
Fig. 1. Simplified diagram of lower vibrational levels of the CO2 molecule.
The first step was optimization of active medium composition and pressure *P* and of discharge current. The measurement of the small signal gain Kh has shown that the optimum mixture for the hot band is CO2:N2:He = 1:1.4:3.5 at a total pressure *P* = 11 Torr *(I* = 15 mA) in which on the strong lines *Kh* = 0.08m-1. It is important that this mixture contains less He and has a large partial content of CO2 as compared with the mixture 1:1.6:6.5 (*P* = 15 Torr, *I =* 10 mA) optimum for the regular band 0001-1000(0200). The theoretical and experimental investigations of vibrational temperatures of a CO2 molecule has shown that to obtain considerable hot band gain *Kh* of a CO2 molecule it is necessary to heat up the *v2(v1)* mode characterized by vibrational temperature *T2* along with the excitation of the *v3* mode (temperature *T3).* For the conventional values of *T3* ≈1600 – 2200 K realized in an electric discharge the Kh gain is shown to achieve its maximum if T2~l/3*T3* (Bertel et. al.., 1983). Such a relationship between *T2* and *T3* can be reached in gas mixtures with greater CO2 and lesser He contents, as compared to those optimal for the regular band oscillation. In addition, an increased specific energy input is also required.
CO2 Lasing on Non-Traditional Bands 107
where gain is fairly high *(Kr*≈0.6 m-1) has shown that in this case an output power increase is
After optimization of the gas content, pressure and discharge current we optimized the laser resonator. In the optimal case, the laser resonator was formed by a flat 100 lines/mm-1 grating and a totally reflecting concave mirror *(R =* 3m). The resonator length was 1.5 m.
Fig. 3. Vibrational T3, T2 and transitional T temperatures (a) and CO2 molecules dissociation degree D (b) vs the discharge current I for the CO2:N2:He – 1:1.4:3.5 (*P* – 11 Torr) mixture
without Xe (•) and with optimal Xe content (0.3 Torr) (×).
About 6% of the radiation was extracted through the grating zeroth order.
not large (~15%).
Fig. 2. A typical pattern of the hot band gain *Kh* (a) and output power *Wh* (b) as functions of the disharge current *I* for the CO2:N2:He – 1:1.4:3.5 (*P* – 11 Torr) mixture without Xe (•) and with optimal Xe content (0.3 Torr) (×).
It is known for the CO2 laser regular band that addition of Xe to the active medium sometimes results in an output power increase (Gorobets et al., 1990). We have also investigated the influence of Xe on the characteristics of the active medium and the lasing parameters (Fig. 2). It has been found that small additions of Xe to the mixture (~ 30% of the CO2 content) increase *Kh* by 25% and the lasing power in the hot band by a factor of 1.5. The analysis of vibrational temperatures shows that this is due to the increase in excitation efficiency of vibrations of N2 and the *v3* asymmetric mode of CO2 in electric discharge [Fig. 3(a)]. Besides, using the reconstruction method of *Kh , Ks* and *Kr* gains we have found from the experimental values of *T3, T2* and *T* that addition of Xe reduces the CO2 molecule dissociation in the discharge [Fig. 3(b)]. This also results in an output increase. It is noteworthy that addition of Xe considerably improves the output parameters only for the low gain transitions. The study of the Xe effect on the laser output for the regular band
Fig. 2. A typical pattern of the hot band gain *Kh* (a) and output power *Wh* (b) as functions of the disharge current *I* for the CO2:N2:He – 1:1.4:3.5 (*P* – 11 Torr) mixture without Xe (•) and
It is known for the CO2 laser regular band that addition of Xe to the active medium sometimes results in an output power increase (Gorobets et al., 1990). We have also investigated the influence of Xe on the characteristics of the active medium and the lasing parameters (Fig. 2). It has been found that small additions of Xe to the mixture (~ 30% of the CO2 content) increase *Kh* by 25% and the lasing power in the hot band by a factor of 1.5. The analysis of vibrational temperatures shows that this is due to the increase in excitation efficiency of vibrations of N2 and the *v3* asymmetric mode of CO2 in electric discharge [Fig. 3(a)]. Besides, using the reconstruction method of *Kh , Ks* and *Kr* gains we have found from the experimental values of *T3, T2* and *T* that addition of Xe reduces the CO2 molecule dissociation in the discharge [Fig. 3(b)]. This also results in an output increase. It is noteworthy that addition of Xe considerably improves the output parameters only for the low gain transitions. The study of the Xe effect on the laser output for the regular band
with optimal Xe content (0.3 Torr) (×).
where gain is fairly high *(Kr*≈0.6 m-1) has shown that in this case an output power increase is not large (~15%).
After optimization of the gas content, pressure and discharge current we optimized the laser resonator. In the optimal case, the laser resonator was formed by a flat 100 lines/mm-1 grating and a totally reflecting concave mirror *(R =* 3m). The resonator length was 1.5 m. About 6% of the radiation was extracted through the grating zeroth order.
Fig. 3. Vibrational T3, T2 and transitional T temperatures (a) and CO2 molecules dissociation degree D (b) vs the discharge current I for the CO2:N2:He – 1:1.4:3.5 (*P* – 11 Torr) mixture without Xe (•) and with optimal Xe content (0.3 Torr) (×).
CO2 Lasing on Non-Traditional Bands 109
1983, Gorobets et al., 1990). According to the measurements, mixtures of the composition CO2:N2:Ne = 1:1:1, in which with an increased specific energy contribution the gain coefficients for a weak signal are — 0.2 m-1 for doubly hot transitions and —0.3 m—1 for sequence hot ones, are optimum for TEA CO2 lasers. The vibrational temperatures *T3* and *T2* must have values of —2000 K and —650 K, respectively. For low-pressure lasers with longitudinal discharge mixtures of the compositions CO2:N2:He:Xe = 1:1.2:2.5:0.4 (doubly hot bands) and 1:1.5:2.5:0.4 (sequence hot) are optimum. The gain coefficient in such mixtures for the aforementioned transitions can reach —0.04 m-1 (*T3*—1800 K, *T2*—600 K). It should be noted that at these transitions the gain is considerably lower than that at the ordinary (~20 times) and hot (~4 times) bands, and consequently a high-Q cavity, lack of harmful losses, and careful selection of the active medium and the conditions of its
The lasing mode on the new transitions was studied first on a TEA CO2 laser with UV preionization. The distance between electrodes that were 4 cm wide was 2 cm. The length of the discharge gap was 70 cm. The main charge and the UV preionization were energized from a battery of low-inductance capacitors with a total capacitance of 0.25 µF, charged to a voltage of 30 kV. The design and of the laser and its performance are described in detail in (Gorobets et al., 1995). A two-transmission three-mirror resonator was used to increase the length of the active medium to 140 cm. A planar grating with 150 lines/mm working in the first order according to an autocollimation scheme with a reflection coefficient not less than —90%, was one of the end mirrors of the resonator. Radiation from the resonator (~5%) was extracted through the zero order of the grating. The other two mirrors were spherical (R = 10 m) with a highly reflective coating. The active medium of the laser was a mixture of gases of the composition CO2:N2:He = 0.8:1.0:1.2 with a total pressure of 200 Torr, which is close to the optimum found from experimental investigations of vibrational temperatures. For this mixture lasing was achieved at more than 10 lines in new bands of the 11.3-11.6 µm range. At the strongest lines the energy in the pulse exceeded 150 mJ. The peak power with a pulse length at half-height of ~0.5 µs attained ~0.3 MW. More thorough investigations of the lasing spectrum of the new transitions were done in the present work for a low-pressure CO2 laser. Experiments were performed with a GL-501 production-type gas-discharge tube of an LG-22 commercial laser (FSUE RPC "Istok", Fryasino, Russia). The inner diameter of the tube— 15 mm, length of the discharge gap —1.2 m. The tube, which worked in the sealed-off mode, was filled with a gas mixture of the composition CO2:N2:He:Xe = 1:1.2:2.5:0.4 under the total pressure of 13.5 Torr. The total reflection spherical mirror (*R* = 3 m) of the commercial laser was not replaced, and a diffraction grating, which worked according to an autocollimation scheme in the first order, was used instead of the output mirror. The emission was extracted through the zero order. The cavity base was 1.5 m. Most of the new lines were obtained with a grating with 100 lines/mm (reflection coefficient – 95%, extraction of emission – 3%). A number of lines in the range of 11.0-11.4 µm, where comparatively strong hot transition are located, were successfully obtained with a more selective grating with 150 lines/mm (93 and 3%, respectively). In addition, to increase the Q-factor of the cavity the germanium etalon was placed before the grating (perpendicular to the output radiation), which not only increased the Q-factor of the grating, returning 75% of the radiation back to the cavity, but also increased its selectivity substantially. As a whole, this device, consisting of a grating and an etalon, was a highly selective output mirror with a reflection coefficient of 97.5% for a grating with 100 lines/mm and 95.5% for a grating with 150 lines/mm. It should be noted,
excitation are necessary to obtain lasing on transitions with such a low gain.
Thus, after the above improvements the commercially available sealed-off laser (LG-22) (FSUE RPC "Istok", Fryasino, Russia) oscillates on more than 30 lines of the P-branch of the 0111- 1110 band in the 10.9—11.3 µm range with output power no less than 0.5 W. On strong lines (P(16)—P(26)) output power was ~ 6W at efficiency ~3% which makes up ~40% of analogous laser parameters in the case of oscillation on the lines of regular bands 0001-1000 (0200) under optimum conditions.
#### **3. New laser transitions of the CO2 molecule in the wavelength range of 11.0-11.6** μ**m**
Construction of powerful and efficient laser sources, generating in various IR ranges, is of importance for further development of a number of trends, e.g., spectroscopy, laser chemistry, isotope separation, sounding of the atmosphere, and metrology. The easiest and most natural way to solve this problem is to use unconventional transitions to produce lasing in commonly used CO2 lasers (Churakov et al., 1987). The spectral range of CO2 lasers is greatly increased in lasing on transitions of the so-called "hot" band 0111-1110, whose Pbranch is in the range of 10.9-11.3 µm. Thorough investigations of gain, vibrational temperatures, and output parameters on lines of the hot band made it possible to achieve efficient lasing both for pulse TEA and for cw longitudinal-discharge CO2 lasers.
In studying the lasing spectrum of hot transitions in TEA CO2 lasers (Bertel at al., 1983). some lines not belonging to the 0111-1110 band occurred in the spectral range of 875-882 cm1 that were not identified due to the poor resolution of the monochromator used and the lack of reliable spectroscopic data in the literature at that time. It was suggested, that these lasing lines belong to higher level transitions, e.g., 10°l-20°0(0400), which were called 'doubly hot," i.e., transitions in which compared to hot transitions two deformation quanta or one symmetric quantum rather than one deformation quantum is added both to the upper and to the lower energy level.
In the present work lasing in both a TEA laser and a low-pressure laser with longitudinal discharge on some transitions of the CO2 molecule in the range of 11.0-11.6 µm is reported for the first time. The rather high resolution of the spectral equipment used and calculation of transition frequencies on the basis of recent spectroscopic constants made it possible to identify definitively the lasing lines obtained as belonging to the doubly hot bands 022l-1220 and 10°1-2000 and the sequence hot band 0112-1111 (Fig. 1).
Within the scope of a commonly used model of vibrational temperatures (Gordiets at al. 1980, Smith and Thompson, 1981) let us analyze what gain coefficients for a weak signal can be realized for the aforementioned bands in electrical-discharge lasers. Estimates showed that to achieve a suitable gain at the new transitions it is necessary, together with the heating up of the asymmetric type of oscillations, characterized by the vibrational temperature *T3,* to strongly excite the connected deformation and symmetric modes *(T2).* Moreover for the sequence hot band 0112-1111 there is one additional condition – the excitation of the asymmetric mode must be at the same high level as for the sequence bands 0002-1001 (02°1) (Petukhov et. al.., 1985).
To find optimum conditions for lasing on the aforementioned bands experimental studies of vibrational temperatures in active media of a TEA CO2 laser and a low-pressure laser with longitudinal discharge were carried out using to the techniques described in (Bertel at al.,
Thus, after the above improvements the commercially available sealed-off laser (LG-22) (FSUE RPC "Istok", Fryasino, Russia) oscillates on more than 30 lines of the P-branch of the 0111- 1110 band in the 10.9—11.3 µm range with output power no less than 0.5 W. On strong lines (P(16)—P(26)) output power was ~ 6W at efficiency ~3% which makes up ~40% of analogous laser parameters in the case of oscillation on the lines of regular bands 0001-1000
**3. New laser transitions of the CO2 molecule in the wavelength range of**
efficient lasing both for pulse TEA and for cw longitudinal-discharge CO2 lasers.
and 10°1-2000 and the sequence hot band 0112-1111 (Fig. 1).
Construction of powerful and efficient laser sources, generating in various IR ranges, is of importance for further development of a number of trends, e.g., spectroscopy, laser chemistry, isotope separation, sounding of the atmosphere, and metrology. The easiest and most natural way to solve this problem is to use unconventional transitions to produce lasing in commonly used CO2 lasers (Churakov et al., 1987). The spectral range of CO2 lasers is greatly increased in lasing on transitions of the so-called "hot" band 0111-1110, whose Pbranch is in the range of 10.9-11.3 µm. Thorough investigations of gain, vibrational temperatures, and output parameters on lines of the hot band made it possible to achieve
In studying the lasing spectrum of hot transitions in TEA CO2 lasers (Bertel at al., 1983). some lines not belonging to the 0111-1110 band occurred in the spectral range of 875-882 cm1 that were not identified due to the poor resolution of the monochromator used and the lack of reliable spectroscopic data in the literature at that time. It was suggested, that these lasing lines belong to higher level transitions, e.g., 10°l-20°0(0400), which were called 'doubly hot," i.e., transitions in which compared to hot transitions two deformation quanta or one symmetric quantum rather than one deformation quantum is added both to the upper and
In the present work lasing in both a TEA laser and a low-pressure laser with longitudinal discharge on some transitions of the CO2 molecule in the range of 11.0-11.6 µm is reported for the first time. The rather high resolution of the spectral equipment used and calculation of transition frequencies on the basis of recent spectroscopic constants made it possible to identify definitively the lasing lines obtained as belonging to the doubly hot bands 022l-1220
Within the scope of a commonly used model of vibrational temperatures (Gordiets at al. 1980, Smith and Thompson, 1981) let us analyze what gain coefficients for a weak signal can be realized for the aforementioned bands in electrical-discharge lasers. Estimates showed that to achieve a suitable gain at the new transitions it is necessary, together with the heating up of the asymmetric type of oscillations, characterized by the vibrational temperature *T3,* to strongly excite the connected deformation and symmetric modes *(T2).* Moreover for the sequence hot band 0112-1111 there is one additional condition – the excitation of the asymmetric mode must be at the same high level as for the sequence bands 0002-1001 (02°1)
To find optimum conditions for lasing on the aforementioned bands experimental studies of vibrational temperatures in active media of a TEA CO2 laser and a low-pressure laser with longitudinal discharge were carried out using to the techniques described in (Bertel at al.,
(0200) under optimum conditions.
**11.0-11.6** μ**m**
to the lower energy level.
(Petukhov et. al.., 1985).
1983, Gorobets et al., 1990). According to the measurements, mixtures of the composition CO2:N2:Ne = 1:1:1, in which with an increased specific energy contribution the gain coefficients for a weak signal are — 0.2 m-1 for doubly hot transitions and —0.3 m—1 for sequence hot ones, are optimum for TEA CO2 lasers. The vibrational temperatures *T3* and *T2* must have values of —2000 K and —650 K, respectively. For low-pressure lasers with longitudinal discharge mixtures of the compositions CO2:N2:He:Xe = 1:1.2:2.5:0.4 (doubly hot bands) and 1:1.5:2.5:0.4 (sequence hot) are optimum. The gain coefficient in such mixtures for the aforementioned transitions can reach —0.04 m-1 (*T3*—1800 K, *T2*—600 K). It should be noted that at these transitions the gain is considerably lower than that at the ordinary (~20 times) and hot (~4 times) bands, and consequently a high-Q cavity, lack of harmful losses, and careful selection of the active medium and the conditions of its excitation are necessary to obtain lasing on transitions with such a low gain.
The lasing mode on the new transitions was studied first on a TEA CO2 laser with UV preionization. The distance between electrodes that were 4 cm wide was 2 cm. The length of the discharge gap was 70 cm. The main charge and the UV preionization were energized from a battery of low-inductance capacitors with a total capacitance of 0.25 µF, charged to a voltage of 30 kV. The design and of the laser and its performance are described in detail in (Gorobets et al., 1995). A two-transmission three-mirror resonator was used to increase the length of the active medium to 140 cm. A planar grating with 150 lines/mm working in the first order according to an autocollimation scheme with a reflection coefficient not less than —90%, was one of the end mirrors of the resonator. Radiation from the resonator (~5%) was extracted through the zero order of the grating. The other two mirrors were spherical (R = 10 m) with a highly reflective coating. The active medium of the laser was a mixture of gases of the composition CO2:N2:He = 0.8:1.0:1.2 with a total pressure of 200 Torr, which is close to the optimum found from experimental investigations of vibrational temperatures. For this mixture lasing was achieved at more than 10 lines in new bands of the 11.3-11.6 µm range. At the strongest lines the energy in the pulse exceeded 150 mJ. The peak power with a pulse length at half-height of ~0.5 µs attained ~0.3 MW. More thorough investigations of the lasing spectrum of the new transitions were done in the present work for a low-pressure CO2 laser.
Experiments were performed with a GL-501 production-type gas-discharge tube of an LG-22 commercial laser (FSUE RPC "Istok", Fryasino, Russia). The inner diameter of the tube— 15 mm, length of the discharge gap —1.2 m. The tube, which worked in the sealed-off mode, was filled with a gas mixture of the composition CO2:N2:He:Xe = 1:1.2:2.5:0.4 under the total pressure of 13.5 Torr. The total reflection spherical mirror (*R* = 3 m) of the commercial laser was not replaced, and a diffraction grating, which worked according to an autocollimation scheme in the first order, was used instead of the output mirror. The emission was extracted through the zero order. The cavity base was 1.5 m. Most of the new lines were obtained with a grating with 100 lines/mm (reflection coefficient – 95%, extraction of emission – 3%). A number of lines in the range of 11.0-11.4 µm, where comparatively strong hot transition are located, were successfully obtained with a more selective grating with 150 lines/mm (93 and 3%, respectively). In addition, to increase the Q-factor of the cavity the germanium etalon was placed before the grating (perpendicular to the output radiation), which not only increased the Q-factor of the grating, returning 75% of the radiation back to the cavity, but also increased its selectivity substantially. As a whole, this device, consisting of a grating and an etalon, was a highly selective output mirror with a reflection coefficient of 97.5% for a grating with 100 lines/mm and 95.5% for a grating with 150 lines/mm. It should be noted,
CO2 Lasing on Non-Traditional Bands 111
375 Hz, a length of the excitation pulse of ∼5O μs, an average current of 8.5 mA. Under these conditions with careful adjustment of the diffraction grating and the etalon we managed to obtain more than *50* new lasing lines (see Table 1). Lasing wavelengths were measured with an SPM-2 monochromator (Carl Zeiss Jena, Germany) with a highly selective diffraction grating, whose resolution was not worse than 0.0005 μm. Absolute calibration of the monochromator was done using the technique described in (Gorobets et al., 1992), which is based on a search for a line with an anomalously high gain, e.g., the line *P(23)* of the hot band. In addition, correction calibration against known wavelengths of hot transitions was done on virtually the entire investigated spectrum. New lines were identified by comparing measured and calculated values of transition wavelengths. Calculations were done using standard methods. Values of the constants G, B, D, H were well known (Witteman, 1987). The peak power (intensity) on the strongest lines of the new bands with a lasing pulse length at half-height of *—50* ns was — 30 W. The average output power reached —0.2 W. Lasing was achieved at a number of new transitions and in the continuous mode with the discharge tube being energized from a dc power supply. More than 25 new lasing lines with λ = 11.1-11.4 μm, belonging to all the aforementioned bands, were observed in this mode in
the spectral range studied. The output power on strong lines attained 0.25 W.
broadens the potentialities of simple laser systems on CO2 for various applications.
search new and perfecting of known methods of optimization CO2 lasers.
on the information about the temperatures of the medium.
**4. Optimisation of a cw CO2 laser output**
**4.1 Optimisation technique**
The characteristics of the output radiation given in the present work are not the best attainable. Optimization of the active medium composition, the conditions of its excitation, and the cavity parameters will make it possible to increase the efficiency of lasing at the new transitions. However at present the large number of new lasing lines obtained substantially
To the present time the number of optimization methods of CO2 laser power parameters is developed. However, the known methods are either complex, since they are based on the calculations calling for a knowledge of a great number of parameters or by virtue of sufficiently rough approximations, not always provide the necessary accuracy, as in the development of laser systems generating on the nonregular transitions – 0002-1001, 0201 (sequence bands); 0111- 1110 (hot band); 0221-1220, 0201-1200 (double hot bands). The gain on these transitions is much weaker than on the regular transitions 0001-1000, 0200 and hence a careful optimization of the active medium composition and of the resonator and pumping parameters is required to provide the lasing on them. Therefore until now remains to actual
We have developed and experimentally tested the method of optimizations of the cw CO2 lasers energy parameters. To realize it, it is necessary to known the vibrational temperatures of the symmetrical (*T1*), bending (*T2*) and asymmetrical (*T3*) modes of the CO2 molecule vibrations. At the present time, the generally recognized fact is that the knowledge of these temperatures as well of the gas temperature (*T*) of the gas mixture makes it possible to determine all the most important characteristics of the active medium (population of the energy levels, the energy accumulated in different modes of CO2, the efficiency of excitation, and so on). Next, the main energy characteristics of the laser system can be calculated based
that this original technique made it possible to separate weak lines of the new transitions from closely positioned ones in some regions of the spectrum of stronger hot lines.
\*The lines are not identified ambiguously (they may belong to the both lines).
Table 1. Measured and calculated values of wavelengths and experimental values of intensities for new transitions
The lasing spectrum in the range of 11.0-11.6 μm was studied in detail with the gasdischarge tube being energized from a pulsed source. It was found experimentally that the following pumping parameters are optimum for lasing at the new transitions: a pulse rate of
that this original technique made it possible to separate weak lines of the new transitions
0221-1220 1001-2000
Line λmeas, µm λcal, µm Intensity,
*P(46)\** 11.3173 11.317088 35
W
from closely positioned ones in some regions of the spectrum of stronger hot lines.
W
*P(25)* 11.3991 11.399538 25 0112-1111 *P(26)* 11.4117 11.411523 18
\*The lines are not identified ambiguously (they may belong to the both lines).
Table 1. Measured and calculated values of wavelengths and experimental values of
The lasing spectrum in the range of 11.0-11.6 μm was studied in detail with the gasdischarge tube being energized from a pulsed source. It was found experimentally that the following pumping parameters are optimum for lasing at the new transitions: a pulse rate of
*P(27)* 11.4241 11.423790 16 *P(23)* 11.0408 11.041722 25 *P(28)* 11.4357 11.435948 15 *P(24)* 11.0512 11.050460 18 *P(29)* 11.4494 11.448451 15 *P(25)* 11.0650 11.064261 25 *P(30)* 11.4596 11.460775 15 *P(26)* 11.0733 11.072789 15 *P(31)* 11.4730 11.473525 9 *P(28)* 11.0953 11.095487 30 *P(32)* 11.4855 11.486009 10 *P(29)* 11.1096 11.110543 30 *P(33)* 11.4981 11.499017 8 *P(30)* 11.1176 11.118556 25 *P(34)* 11.5112 11.511655 12 *P(32)* 11.1425 11.142003 28 *P(35) 11.5245* 11.524934 4 *P(33)* 11.1591 11.158465 20 *P(36)* 11.5375 11.537715 4 *P(34)* 11.1667 11.165831 30 *P(36)* 11.1908 11.190045 30
*P(12)* 11.2513 11.251536 10 *P(14)* 11.0358 11.036728 10 *P(14)\** 11.2729 11.273239 25 *P(16)* 11.0590 11.058304 10 *P(15)* 11.2834 11.284243 15 *P(18)* 11.0794 11.080312 15 *P(16)* 11.2944 11.295321 16 *P(20)* 11.1027 11.102758 20 *P(17)* 11.3064 11.306522 24 *P(22)* 11.1260 11.125648 20 *P(18)\** 11.3173 11.317785 35 *P(24)* 11.1488 11.148987 16 *P(19)* 11.3300 11.329186 30 *P(28)* 11.1961 11.197036 20 *P(20)* 11.3410 11.340633 28 *P(30)* 11.2223 11.221757 17 *P(21)* 11.3517 11.352241 25 *P(32)* 11.2467 11.246953 20 *P(22)* 11.3634 11.363870 20 *P(34)* 11.2729 11.272628 25 *P(23)* 11.3754 11.375690 20 *P(36)* 11.2978 11.298791 20 *P(24)* 11.3872 11.387499 30 *P(38)* 11.3251 11.325449 10
Line λmeas, µm λcal, µm Intensity,
0111-1110
*P(47)* 11.3075 11.307393 40 *P(49)* 11.3352 11.334278 35 *P(50)* 11.3601 11.359850 40 *P(51)* 11.3610 11.361589 18 *P(53)* 11.3907 11.389331 20
intensities for new transitions
375 Hz, a length of the excitation pulse of ∼5O μs, an average current of 8.5 mA. Under these conditions with careful adjustment of the diffraction grating and the etalon we managed to obtain more than *50* new lasing lines (see Table 1). Lasing wavelengths were measured with an SPM-2 monochromator (Carl Zeiss Jena, Germany) with a highly selective diffraction grating, whose resolution was not worse than 0.0005 μm. Absolute calibration of the monochromator was done using the technique described in (Gorobets et al., 1992), which is based on a search for a line with an anomalously high gain, e.g., the line *P(23)* of the hot band. In addition, correction calibration against known wavelengths of hot transitions was done on virtually the entire investigated spectrum. New lines were identified by comparing measured and calculated values of transition wavelengths. Calculations were done using standard methods. Values of the constants G, B, D, H were well known (Witteman, 1987).
The peak power (intensity) on the strongest lines of the new bands with a lasing pulse length at half-height of *—50* ns was — 30 W. The average output power reached —0.2 W. Lasing was achieved at a number of new transitions and in the continuous mode with the discharge tube being energized from a dc power supply. More than 25 new lasing lines with λ = 11.1-11.4 μm, belonging to all the aforementioned bands, were observed in this mode in the spectral range studied. The output power on strong lines attained 0.25 W.
The characteristics of the output radiation given in the present work are not the best attainable. Optimization of the active medium composition, the conditions of its excitation, and the cavity parameters will make it possible to increase the efficiency of lasing at the new transitions. However at present the large number of new lasing lines obtained substantially broadens the potentialities of simple laser systems on CO2 for various applications.
#### **4. Optimisation of a cw CO2 laser output**
#### **4.1 Optimisation technique**
To the present time the number of optimization methods of CO2 laser power parameters is developed. However, the known methods are either complex, since they are based on the calculations calling for a knowledge of a great number of parameters or by virtue of sufficiently rough approximations, not always provide the necessary accuracy, as in the development of laser systems generating on the nonregular transitions – 0002-1001, 0201 (sequence bands); 0111- 1110 (hot band); 0221-1220, 0201-1200 (double hot bands). The gain on these transitions is much weaker than on the regular transitions 0001-1000, 0200 and hence a careful optimization of the active medium composition and of the resonator and pumping parameters is required to provide the lasing on them. Therefore until now remains to actual search new and perfecting of known methods of optimization CO2 lasers.
We have developed and experimentally tested the method of optimizations of the cw CO2 lasers energy parameters. To realize it, it is necessary to known the vibrational temperatures of the symmetrical (*T1*), bending (*T2*) and asymmetrical (*T3*) modes of the CO2 molecule vibrations. At the present time, the generally recognized fact is that the knowledge of these temperatures as well of the gas temperature (*T*) of the gas mixture makes it possible to determine all the most important characteristics of the active medium (population of the energy levels, the energy accumulated in different modes of CO2, the efficiency of excitation, and so on). Next, the main energy characteristics of the laser system can be calculated based on the information about the temperatures of the medium.
CO2 Lasing on Non-Traditional Bands 113
obtain expressions for determining *T3* for the above-indicated bands. For example, for the
3 3 3 \* \* \* 3 3 1 3
(5)
<sup>1</sup> 1 exp exp exp exp ,
6 2 3
6 2 3
1
15
3 3 3 \* \* \* 3 3 1 3
*K K hv hv hv hv K kT kT k kT T*
<sup>−</sup> = ⋅− − ⋅ − − − −
Similar expressions are also true for other bands. Thus, if the all temperatures in the regime of amplification and the loss factors are known, it is an easy matter to calculate the output power for different bands. The temperatures *T3, T2, T1* and *T* can be determined from the measurement of the gain factor of a weak signal on the lines of different bands by the
Figure 4 shows a block diagram of the experimental setup for CO2 laser optimization. A sealed off a cw CO2 laser was the source of probe radiation. It could be tuned over the vibrational-rotational lines of the regular (0001-1000*,* 0200) bands, the sequence (0002-1001,
0201) bands, the hot (0111- 1110) band or the new (0221-1220, 0201-1200….) bands.
1 – probing laser; 2 – discharge tube; 3 – 100% reflection mirror; 4 – grating; 5 – additional mirror; 6 – iris diaphragm; 7 – chopper; 8 – ZnSe plane-parallel plate; 9 – mirror; 10 – interference filter; 11 – photo detector; 12 – polarizer; 13 – spectrum analyzer; 14 – ADC; 15 – computer; 16 gas valve.
14
14
Fig. 4. Experimental setup for CO2 laser optimization
*hv hv hv hv kT kT kT kT*
−− ⋅ −−− − =
<sup>1</sup> 1 exp exp exp exp
0002-1001 band it has the form
**4.2 Experimental setup**
5
7
12 10
12
13
11
11
1
4
*h us loss loss s g*
method described in (Petukhov et. al.., 1985).
In our works we used the method of determination of vibrational temperatures, which is based on the measurements of the gain on separate vibrational lines of regular and nonregular CO2 bands (Petukhov et. al.., 1985). The advantages of this method are the possibility of determination of all vibrational and translational temperatures at once, a relative simplicity and sufficiently high accuracy as compared with other known methods. Besides, a knowledge of the absolute values of the gain factors and of the active medium composition is not needed here, which is sometimes very important.
It would appear reasonable that within the limits of the model of vibrational temperatures the output power (*P*) for every above-indicated bands is dependent only on temperatures. The experimental investigations performed by us show that for a typical low-pressure CO2 laser with a longitudinal continuous discharge the active medium in the lasing regime differs significantly from that in the absence of lasing in only the value of the asymmetric vibration temperatures *T3*, while for the other temperatures *T2, T1* and *T* the difference is insignificant (less than 10 %). Such a temperature approximation is predominantly due to fact that the energy capacitance for vibrations of symmetric and bending modes of CO2 is much greater than that for vibrations of the asymmetric mode as well as due to the constant effective heat abstraction from the low laser levels. This approximation may be thought of as by true for laser system with on efficiency of transformation of the energy contributed to the discharge to the lasing energy of —10 % or less percent, which is characteristic of all real continuous CO2 lasers. In this case, using the ratio between the temperature of the asymmetric mode and average number of vibrational quanta accumulated in this mode we can write the following simple expression for the output power in every above-indicated band:
$$\mathbf{e}\_3 = \exp\left(-\frac{hv\_3}{kT\_3}\right) \Big/ \left[1 - \exp\left(\frac{hv\_3}{kT\_3}\right)\right] \tag{2}$$
$$P = A \cdot \frac{K\_{\rm loss}^{\rm ns}}{K\_{\rm loss}^{h} - K\_{\rm loss}^{\rm ns}} \cdot \left( \frac{\exp\left(-\frac{h\upsilon\_{3}}{kT\_{3}}\right)}{\left[1 - \exp\left(-\frac{h\upsilon\_{3}}{kT\_{3}}\right)\right]} - \frac{\exp\left(-\frac{h\upsilon\_{3}}{kT\_{3}}\right)}{\left[1 - \exp\left(-\frac{h\upsilon\_{3}}{kT\_{3}}\right)\right]} \right) \tag{3}$$
where *A* is the proportionality factor dependent on the CO2 content and independent on the lasing band; *Klossus* is the useful loss factor; *Klossh* is the harmful loss factor; *T3* and *T3\** are the vibrational temperatures of the asymmetric mode of the CO2 molecule in the regime of amplification and lasing, respectively.
The temperature *T3* as well as *T2, T1* and *T* can be found if the gain factors of the weak signal in different bands are known (Petukhov et. al.., 1985). To determine the temperature *T3\** we will draw on the fact that in the regime of lasing (continuous) the gain factor is equal to the total loss factor:
$$K\_{\mathcal{g}}^{\*}\left(T\_3^{\*}, T\_2, T\_1, T\right) = K\_{\text{loss}}^{h} - K\_{\text{loss}}^{us} \tag{4}$$
Then, using the dependence of the gain factor on the difference in the population of the upper and low laser levels, expressed through vibrational temperatures. We can easily
In our works we used the method of determination of vibrational temperatures, which is based on the measurements of the gain on separate vibrational lines of regular and nonregular CO2 bands (Petukhov et. al.., 1985). The advantages of this method are the possibility of determination of all vibrational and translational temperatures at once, a relative simplicity and sufficiently high accuracy as compared with other known methods. Besides, a knowledge of the absolute values of the gain factors and of the active medium
It would appear reasonable that within the limits of the model of vibrational temperatures the output power (*P*) for every above-indicated bands is dependent only on temperatures. The experimental investigations performed by us show that for a typical low-pressure CO2 laser with a longitudinal continuous discharge the active medium in the lasing regime differs significantly from that in the absence of lasing in only the value of the asymmetric vibration temperatures *T3*, while for the other temperatures *T2, T1* and *T* the difference is insignificant (less than 10 %). Such a temperature approximation is predominantly due to fact that the energy capacitance for vibrations of symmetric and bending modes of CO2 is much greater than that for vibrations of the asymmetric mode as well as due to the constant effective heat abstraction from the low laser levels. This approximation may be thought of as by true for laser system with on efficiency of transformation of the energy contributed to the discharge to the lasing energy of —10 % or less percent, which is characteristic of all real continuous CO2 lasers. In this case, using the ratio between the temperature of the asymmetric mode and average number of vibrational quanta accumulated in this mode we can write the
composition is not needed here, which is sometimes very important.
following simple expression for the output power in every above-indicated band:
3
*us loss h us loss loss*
*P A*
amplification and lasing, respectively.
total loss factor:
3 3
(2)
(3)
3 3 exp 1 exp , *hv hv kT kT*
exp exp
*K kT kT*
*K K hv hv*
=⋅ ⋅ <sup>−</sup> <sup>−</sup> − − − −
where *A* is the proportionality factor dependent on the CO2 content and independent on the lasing band; *Klossus* is the useful loss factor; *Klossh* is the harmful loss factor; *T3* and *T3\** are the vibrational temperatures of the asymmetric mode of the CO2 molecule in the regime of
The temperature *T3* as well as *T2, T1* and *T* can be found if the gain factors of the weak signal in different bands are known (Petukhov et. al.., 1985). To determine the temperature *T3\** we will draw on the fact that in the regime of lasing (continuous) the gain factor is equal to the
Then, using the dependence of the gain factor on the difference in the population of the upper and low laser levels, expressed through vibrational temperatures. We can easily
( ) \* \*
1 exp 1 exp
3 3
*hv hv*
<sup>−</sup> <sup>−</sup>
3 3 3 3
*kT kT*
3 3
3 21 ,,, , *h us K T TTT K K <sup>g</sup>* = − *loss loss* (4)
\*
\*
,
ε= − −
obtain expressions for determining *T3* for the above-indicated bands. For example, for the 0002-1001 band it has the form
$$\begin{aligned} &\left[1-\exp\left(-\frac{h\upsilon\_{3}}{kT\_{3}}\right)\right]\cdot\left\{\exp\left(-\frac{h\upsilon\_{3}}{kT\_{3}}\right)-\exp\left(-\frac{h\upsilon\mathbf{1}}{kT\_{1}}\right)\exp\left(-\frac{h\upsilon\_{3}}{kT\_{3}}\right)\right\}=\\ &=\frac{K\_{\text{loss}}^{h}-K\_{\text{loss}}^{us}}{K\_{\text{s}}^{s}}\cdot\left[1-\exp\left(-\frac{h\upsilon\_{3}}{kT\_{3}}\right)\right]\cdot\left\{\exp\left(-\frac{h\upsilon\_{3}}{kT\_{3}}\right)-\exp\left(-\frac{h\upsilon\mathbf{1}}{kT\_{1}}\right)\exp\left(-\frac{h\upsilon\mathbf{1}}{kT\_{3}}\right)\right\}\,,\end{aligned} \tag{5}$$
Similar expressions are also true for other bands. Thus, if the all temperatures in the regime of amplification and the loss factors are known, it is an easy matter to calculate the output power for different bands. The temperatures *T3, T2, T1* and *T* can be determined from the measurement of the gain factor of a weak signal on the lines of different bands by the method described in (Petukhov et. al.., 1985).
#### **4.2 Experimental setup**
Figure 4 shows a block diagram of the experimental setup for CO2 laser optimization. A sealed off a cw CO2 laser was the source of probe radiation. It could be tuned over the vibrational-rotational lines of the regular (0001-1000*,* 0200) bands, the sequence (0002-1001, 0201) bands, the hot (0111- 1110) band or the new (0221-1220, 0201-1200….) bands.
1 – probing laser; 2 – discharge tube; 3 – 100% reflection mirror; 4 – grating; 5 – additional mirror; 6 – iris diaphragm; 7 – chopper; 8 – ZnSe plane-parallel plate; 9 – mirror; 10 – interference filter; 11 – photo detector; 12 – polarizer; 13 – spectrum analyzer; 14 – ADC; 15 – computer; 16 gas valve.
Fig. 4. Experimental setup for CO2 laser optimization
CO2 Lasing on Non-Traditional Bands 115
Fig. 5. Dependencies of temperatures *T3* (∇), *T2* (O), and *T* (×) (a) and output power *P* (b) from a discharge current (× - for P(18) 0001-1000, O - for P(19) 0002-1001, ∇ - for P(19) 0111-
Figure 5b shows the calculated curves and experimental values of the output power for the considered bands. At first we calculated the value of *P/A* for every band in accordance with discharge current. Then, we determined the proportionality coefficient A from the experimental data for the 0001-1001 band at *I*=25 mA. As this takes place, the coefficient A has a common value for all lasing bands. Next, the dependence P on I was constructed.
The method of output optimization of cw CO2 lasers has been developed. The method is based on vibrational and translational temperatures determination by gain measurements on the ro-vibrational lines of regular (0001-1000, 0200) and nonregular (0002-1001,0201; 0111- 1110; (0221-1220, 0201-1200…) bands of CO2 molecule. To test the validity of the method, the experiment realization has been done for a low pressure CO2 laser with the cw longitudinal discharge, that can oscillate on the lines of regular and nonregular lines. The good agreement between calculation and experiment data has been observed. Thus, a good agreement between the calculated and experimental data, which is observed, as a whole, for all the investigated bands, is demonstration that this method can be applied to the optimization of the output power of cw CO2 lasers. This method can be also successfully used for the optimization of the output parameters depending on the pumping and Q-factor of the resonator of the lasers generating only on the regular transitions 0001-1000 and 0001-0200.
Earlier the lasing on the 0200(1000)-0110 band of the CO2 molecule (see Fig. 1) has been obtained in the specific systems at cryogenic temperatures under the lowest efficiency (Wexler, 1987). The optimization of the active medium and its electrical discharge pumping conditions based on the original technique of the temperature model (gain measurements on the several bands: 0001-1000, 0002-1001, 0111- 1110 of CO2 molecule) allowed to obtain in the simple TE CO2 laser with UV preionization (the active media length is 65 cm the width of electrodes is 2.5 cm, the interval between electrodes is 1.8 cm, (the voltage on the 0.2 micro Farad capacitor is 6.5 kV) the powerful lasing on the 0200(1000)-0110 bands at the room temperature. The output pulse energy of 57 mJ and the peak power of some tens kWatt have
**0(10<sup>0</sup>**
**0)-01<sup>1</sup>**
**0 and 02<sup>0</sup>**
**1(10<sup>0</sup> 1)-**
**5. 16(14) microns TE CO2 laser working on the 02<sup>0</sup>**
1110, - for P(19) 0221-1220)
**01<sup>1</sup>**
**1 bands**
A production-type water-cooled sealed-off gas-discharge tube of GL-501 type (FSUE RPC "Istok", Fryasino, Russia) was used as an active element of the probe laser. It has dischargegap length of ~1 .2 m, and the inside diameter is of 15 mm. The tube was filled with a CO : N2 : He :Xe mixture in a proportion of 1.0:1.6:4.0:0.6 at a total pressure of *13.5* Torr. The laser cavity was formed by ~100% reflecting mirror with a curvature radius of 3 m built in the tube, a plane diffraction grating and an additional mirror with a large curvature radius.
We used a nonconventional scheme of the laser cavity. The diffraction grating operated in the first diffraction order in the nonLittrow scheme. Laser radiation was extracted from the cavity through the zero order. Our studies have shown that the diffraction grating with 150 lines/mm and a reflectance of >95%, combined with the additional mirror with a curvature radius of 10 m, are optimal for obtaining the necessary high spectral resolution with a sufficiently high output power. A more detailed description of the construction of the probe laser is given in the next part.
A signal from the probe laser passed a two times through the active medium under study in the discharge tube and was recorded by a liquid nitrogen cooled photo detector. This discharge tube was similar to one used as the active element of the probe laser. In addition to the measuring signal, we used a reference signal that does not pass along the investigated active medium and appears as a result of reflection of a portion of radiation from the ZnSe plane-parallel plate (see Fig. 4). This portion of radiation was directed to the another liquid nitrogen cooled Ge:Au photodetector of the reference channel, which makes automatically possible to account for the possible instability of the output laser radiation by way of normalization of the measuring signal to reference one.
The electric signals from two photodetectors were fed into an two-channel digital registration system on the base of PC. Lasing wavelengths were measured with SPM-2 spectrum analyzer (Carl Zeiss Jena, Germany) with a highly selective diffraction grating, whose resolution was not worse than 0.0005 µm.
#### **4.3 Results and discussions**
To test the method proposed we have performed experimental investigations and calculations of the output power (*P*) dependence on the discharge current (*I*) for a cw CO2 laser, operated on four different bands (0001-1000, 0002-1001, 111- 1110 and 0221-1220). The cw CO2 laser was similar to one used as the probe laser. The only distinction is the using of the appropriate diffraction grating with optimum Q-factor for each band.
The temperatures T3, T2 and T (see Fig. 5a), used in the calculations, were determined from the measurement of the gain factor of a weak signal by the method described in (Petukhov et. al.., 1985). For our experiments *T1* is approximately equal *T2*. According to our calculations the loss factors for different bands have the following values:
$$\begin{aligned} \text{for } P \text{ (18) } \ 00^0 \mathbf{1} - 10^0 \mathbf{0} - K\_{\text{loss}}^h &= \mathbf{4}.8 \times 10^{-4} \, cm^{-1} \, \text{;} \, K\_{\text{loss}}^{us} = \mathbf{5}.3 \times 10^{-4} \, cm^{-1}, \\\\ \text{for } P \text{ (19) } \ 00^0 \mathbf{2} - 10^0 \mathbf{1} - K\_{\text{loss}}^h &= \mathbf{4}.8 \times 10^{-4} \, cm^{-1} \, \text{;} \, K\_{\text{loss}}^{us} = \mathbf{5}.3 \times 10^{-4} \, cm^{-1}, \\\\ \text{for } P \text{ (19) } \ 01^1 \mathbf{1} - 11^1 \mathbf{0} - K\_{\text{loss}}^h &= \mathbf{2}.3 \times 10^{-4} \, cm^{-1} \, \text{;} \, K\_{\text{loss}}^{us} = \mathbf{1}.9 \times 10^{-4} \, cm^{-1}, \\\\ \text{for } P \text{ (19) } \ 02^2 \mathbf{1} - \mathbf{1}2^2 \mathbf{0} - K\_{\text{loss}}^h &= \mathbf{2}.1 \times 10^{-4} \, cm^{-1} \, \text{;} \, K\_{\text{loss}}^{us} = \mathbf{1}.1 \times 10^{-4} \, cm^{-1}. \end{aligned}$$
A production-type water-cooled sealed-off gas-discharge tube of GL-501 type (FSUE RPC "Istok", Fryasino, Russia) was used as an active element of the probe laser. It has dischargegap length of ~1 .2 m, and the inside diameter is of 15 mm. The tube was filled with a CO : N2 : He :Xe mixture in a proportion of 1.0:1.6:4.0:0.6 at a total pressure of *13.5* Torr. The laser cavity was formed by ~100% reflecting mirror with a curvature radius of 3 m built in the tube, a plane diffraction grating and an additional mirror with a large curvature radius.
We used a nonconventional scheme of the laser cavity. The diffraction grating operated in the first diffraction order in the nonLittrow scheme. Laser radiation was extracted from the cavity through the zero order. Our studies have shown that the diffraction grating with 150 lines/mm and a reflectance of >95%, combined with the additional mirror with a curvature radius of 10 m, are optimal for obtaining the necessary high spectral resolution with a sufficiently high output power. A more detailed description of the construction of the probe
A signal from the probe laser passed a two times through the active medium under study in the discharge tube and was recorded by a liquid nitrogen cooled photo detector. This discharge tube was similar to one used as the active element of the probe laser. In addition to the measuring signal, we used a reference signal that does not pass along the investigated active medium and appears as a result of reflection of a portion of radiation from the ZnSe plane-parallel plate (see Fig. 4). This portion of radiation was directed to the another liquid nitrogen cooled Ge:Au photodetector of the reference channel, which makes automatically possible to account for the possible instability of the output laser radiation by way of
The electric signals from two photodetectors were fed into an two-channel digital registration system on the base of PC. Lasing wavelengths were measured with SPM-2 spectrum analyzer (Carl Zeiss Jena, Germany) with a highly selective diffraction grating,
To test the method proposed we have performed experimental investigations and calculations of the output power (*P*) dependence on the discharge current (*I*) for a cw CO2 laser, operated on four different bands (0001-1000, 0002-1001, 111- 1110 and 0221-1220). The cw CO2 laser was similar to one used as the probe laser. The only distinction is the using of the
The temperatures T3, T2 and T (see Fig. 5a), used in the calculations, were determined from the measurement of the gain factor of a weak signal by the method described in (Petukhov et. al.., 1985). For our experiments *T1* is approximately equal *T2*. According to our
> ( ) 0 0 4 1 4 1 18 00 1 10 0 4.8 10 ; 5.3 10 , *h us loss loss for P K cm K cm* − − − − − − =× =×
> ( ) 0 0 4 1 4 1 19 00 2 10 1 4.8 10 ; 5.3 10 , *h us loss loss for P K cm K cm* − − − − − − =× =×
( ) 1 1 4 1 4 1 19 01 1 11 0 2.3 10 ; 1.9 10 , *h us loss loss for P K cm K cm* − − − − − − =× =×
( ) 2 2 4 1 4 1 19 02 1 12 0 2.1 10 ; 1.1 10 , *h us loss loss for P K cm K cm* − − − − − − =× =×
appropriate diffraction grating with optimum Q-factor for each band.
calculations the loss factors for different bands have the following values:
laser is given in the next part.
**4.3 Results and discussions**
normalization of the measuring signal to reference one.
whose resolution was not worse than 0.0005 µm.
Fig. 5. Dependencies of temperatures *T3* (∇), *T2* (O), and *T* (×) (a) and output power *P* (b) from a discharge current (× - for P(18) 0001-1000, O - for P(19) 0002-1001, ∇ - for P(19) 0111- 1110, - for P(19) 0221-1220)
Figure 5b shows the calculated curves and experimental values of the output power for the considered bands. At first we calculated the value of *P/A* for every band in accordance with discharge current. Then, we determined the proportionality coefficient A from the experimental data for the 0001-1001 band at *I*=25 mA. As this takes place, the coefficient A has a common value for all lasing bands. Next, the dependence P on I was constructed.
The method of output optimization of cw CO2 lasers has been developed. The method is based on vibrational and translational temperatures determination by gain measurements on the ro-vibrational lines of regular (0001-1000, 0200) and nonregular (0002-1001,0201; 0111- 1110; (0221-1220, 0201-1200…) bands of CO2 molecule. To test the validity of the method, the experiment realization has been done for a low pressure CO2 laser with the cw longitudinal discharge, that can oscillate on the lines of regular and nonregular lines. The good agreement between calculation and experiment data has been observed. Thus, a good agreement between the calculated and experimental data, which is observed, as a whole, for all the investigated bands, is demonstration that this method can be applied to the optimization of the output power of cw CO2 lasers. This method can be also successfully used for the optimization of the output parameters depending on the pumping and Q-factor of the resonator of the lasers generating only on the regular transitions 0001-1000 and 0001-0200.
#### **5. 16(14) microns TE CO2 laser working on the 02<sup>0</sup> 0(10<sup>0</sup> 0)-01<sup>1</sup> 0 and 02<sup>0</sup> 1(10<sup>0</sup> 1)- 01<sup>1</sup> 1 bands**
Earlier the lasing on the 0200(1000)-0110 band of the CO2 molecule (see Fig. 1) has been obtained in the specific systems at cryogenic temperatures under the lowest efficiency (Wexler, 1987). The optimization of the active medium and its electrical discharge pumping conditions based on the original technique of the temperature model (gain measurements on the several bands: 0001-1000, 0002-1001, 0111- 1110 of CO2 molecule) allowed to obtain in the simple TE CO2 laser with UV preionization (the active media length is 65 cm the width of electrodes is 2.5 cm, the interval between electrodes is 1.8 cm, (the voltage on the 0.2 micro Farad capacitor is 6.5 kV) the powerful lasing on the 0200(1000)-0110 bands at the room temperature. The output pulse energy of 57 mJ and the peak power of some tens kWatt have
CO2 Lasing on Non-Traditional Bands 117
present time is widely used to describe processes occurring in the active media of CO2 lasers and amplifiers. According to this model, population of the vibrational levels is unambiguously connected with the vibrational temperature of the symmetric (*T1*), bending (*T2*). and asymmetric (*T3*) modes of the CO2 molecule. We performed experimental investigations of the vibrational temperatures in the active medium of the TEA CO2 laser, directed toward a search for the optimum conditions for lasing in the 16(14) µm channel. The vibrational temperatures *T3* and *T2* (*T1=T2* for conditions examined) were determined from the ratios of the measured amplification coefficients of a weak signal at the individual rotational-vibrational
Let us examine what kind of the small gain and the output energy can be attained in the TEA CO2 laser on the 0201(1001)-0111 transitions. On the basis of the experimentally determined vibrational temperatures *T3* and *T2* (see Fig. 6) using the well-known expression (Gordiets et al., 1980) we calculated the small gain. The calculations shown that the small gain in the 0201(1001)- 0111 band can attain a significant value (>1m-1). The necessary conditions for the effective lasing have been determined. It is shown that in optimum conditions the output energy can reach *1.3* J/l at the peak power 5 MW and at the full
**6. A stabilized cw CO2 laser automatically switched between generations**
This part describes a cw CO2 (CO) laser with stabilized output parameters that can be automatically switched from line to line. The laser generates 115 vibration-rotation CO2 lines between 9.15 and 11.3 μm and 100 CO lines between 5.3 and 6.4 μm. The laser is switched from CO2 operation to CO operation by replacing a sealed laser tube. Then computerized
Although there are many publications on tunable lasers (Gorobets et al., 1992) it is premature to think that all design and operation problems of tunable CO2, and especially of CO lasers, have been resolved. Computer control over the tuning of the generation wavelength is required (Gorobets et al., 1992). Fully computerized CO2, and CO lasers could be extensively used to monitor active media to improve lidar systems, in the spectroscopy
We have described the design of a laser head with a sealed tube and separate units (a highvoltage power supply, unit for tuning the lasing wavelength, an AFT unit, and a modulator) of an actively stabilized a cw CO2 (CO laser) that can he automatically switched between generation lines. The laser is switched from CO2 to CO operation by replacing the discharge
Further improvements include an electro-mechanical drive for the diffraction grating, an electronic control unit compatible with various computers, interfaces, and a control
The laser structure is shown in Fig. 7. The GL 501 (CO2) or GL-509 (CO) (FSUE RPC "Istok", Fryasino, Russia) commercial discharge tubes 1 are used because they have similar
lines of the 0001-1000, 0002-1001 and 111-1110 bands by the procedure described early.
efficiency of 2 *%.*
and analysis of gases.
**6.1 Laser structure**
control of the laser spectrum is described.
algorithm to link the laser to the computer.
**lines**
tube.
been reached. The dependencies of the output and spectral performances of the 16 (14) micrometers lasing vs. a content of the active medium, pumping parameters and cavity characteristics have been carried out.
To increase the power performances of the 16 (14) microns CO2 laser the possibility of lasing on the 0201(1001)-0111 band have been experimentally and theoretically investigated under the combined (electrical + optical) excitation of the active medium. The conditions for obtaining effective lasing at the rotational-vibrational transitions of the 0201-0111 (λ = 16.4 µm) and 1001-0111 (λ= 14.1 µm) bands of the CO2 molecule are examined. To obtain population inversion in the indicated channels one should initially populate the 0002 vibrational level, considerable population of which can he accomplished comparatively simply, for example. in an electric discharge (Petukhov et. al.., 1985)*.* Then a powerful twofrequency radiation resonant with the 0002-0201(1001) and 0111-1110 (0310) transitions, saturating an individual rotational-vibrational transition, acts on the medium excited in such a way. As a result of this the first electromagnetic field, resonant with the 0002- 0201(1001) transition, populates the upper laser level 0201(1001), while simultaneously the second field, resonant with the 0111-1110 (0310) transitions, depopulates the lower level 0111 which also leads to inversion of the populations in the 0201 (1001)-0111 16(14) µm channel. In (Churakov et al., 1987). we have discussed by what means such a scheme of lasing in one active medium can be accomplished.
Fig. 6. Vibrational temperatures *T2* and *T3* vs N2 content at fixed values of *PCO2* 5(1), 10 (2), 20 (3), and 30% (4) in the mixture CO2:N2:He (*P* = 200 Torr, *U*= 30 kV, *C* = 0.25 μF)
Let us examine formation of inversion on the 0201(1001)-0111 transition using the example of an ordinary pulsed TEA CO2 laser. For this we employ a temperature model which at the
been reached. The dependencies of the output and spectral performances of the 16 (14) micrometers lasing vs. a content of the active medium, pumping parameters and cavity
To increase the power performances of the 16 (14) microns CO2 laser the possibility of lasing on the 0201(1001)-0111 band have been experimentally and theoretically investigated under the combined (electrical + optical) excitation of the active medium. The conditions for obtaining effective lasing at the rotational-vibrational transitions of the 0201-0111 (λ = 16.4 µm) and 1001-0111 (λ= 14.1 µm) bands of the CO2 molecule are examined. To obtain population inversion in the indicated channels one should initially populate the 0002 vibrational level, considerable population of which can he accomplished comparatively simply, for example. in an electric discharge (Petukhov et. al.., 1985)*.* Then a powerful twofrequency radiation resonant with the 0002-0201(1001) and 0111-1110 (0310) transitions, saturating an individual rotational-vibrational transition, acts on the medium excited in such a way. As a result of this the first electromagnetic field, resonant with the 0002- 0201(1001) transition, populates the upper laser level 0201(1001), while simultaneously the second field, resonant with the 0111-1110 (0310) transitions, depopulates the lower level 0111 which also leads to inversion of the populations in the 0201 (1001)-0111 16(14) µm channel. In (Churakov et al., 1987). we have discussed by what means such a scheme of lasing in one
Fig. 6. Vibrational temperatures *T2* and *T3* vs N2 content at fixed values of *PCO2* 5(1), 10 (2), 20
Let us examine formation of inversion on the 0201(1001)-0111 transition using the example of an ordinary pulsed TEA CO2 laser. For this we employ a temperature model which at the
(3), and 30% (4) in the mixture CO2:N2:He (*P* = 200 Torr, *U*= 30 kV, *C* = 0.25 μF)
characteristics have been carried out.
active medium can be accomplished.
present time is widely used to describe processes occurring in the active media of CO2 lasers and amplifiers. According to this model, population of the vibrational levels is unambiguously connected with the vibrational temperature of the symmetric (*T1*), bending (*T2*). and asymmetric (*T3*) modes of the CO2 molecule. We performed experimental investigations of the vibrational temperatures in the active medium of the TEA CO2 laser, directed toward a search for the optimum conditions for lasing in the 16(14) µm channel. The vibrational temperatures *T3* and *T2* (*T1=T2* for conditions examined) were determined from the ratios of the measured amplification coefficients of a weak signal at the individual rotational-vibrational lines of the 0001-1000, 0002-1001 and 111-1110 bands by the procedure described early.
Let us examine what kind of the small gain and the output energy can be attained in the TEA CO2 laser on the 0201(1001)-0111 transitions. On the basis of the experimentally determined vibrational temperatures *T3* and *T2* (see Fig. 6) using the well-known expression (Gordiets et al., 1980) we calculated the small gain. The calculations shown that the small gain in the 0201(1001)- 0111 band can attain a significant value (>1m-1). The necessary conditions for the effective lasing have been determined. It is shown that in optimum conditions the output energy can reach *1.3* J/l at the peak power 5 MW and at the full efficiency of 2 *%.*
#### **6. A stabilized cw CO2 laser automatically switched between generations lines**
This part describes a cw CO2 (CO) laser with stabilized output parameters that can be automatically switched from line to line. The laser generates 115 vibration-rotation CO2 lines between 9.15 and 11.3 μm and 100 CO lines between 5.3 and 6.4 μm. The laser is switched from CO2 operation to CO operation by replacing a sealed laser tube. Then computerized control of the laser spectrum is described.
Although there are many publications on tunable lasers (Gorobets et al., 1992) it is premature to think that all design and operation problems of tunable CO2, and especially of CO lasers, have been resolved. Computer control over the tuning of the generation wavelength is required (Gorobets et al., 1992). Fully computerized CO2, and CO lasers could be extensively used to monitor active media to improve lidar systems, in the spectroscopy and analysis of gases.
We have described the design of a laser head with a sealed tube and separate units (a highvoltage power supply, unit for tuning the lasing wavelength, an AFT unit, and a modulator) of an actively stabilized a cw CO2 (CO laser) that can he automatically switched between generation lines. The laser is switched from CO2 to CO operation by replacing the discharge tube.
Further improvements include an electro-mechanical drive for the diffraction grating, an electronic control unit compatible with various computers, interfaces, and a control algorithm to link the laser to the computer.
#### **6.1 Laser structure**
The laser structure is shown in Fig. 7. The GL 501 (CO2) or GL-509 (CO) (FSUE RPC "Istok", Fryasino, Russia) commercial discharge tubes 1 are used because they have similar
CO2 Lasing on Non-Traditional Bands 119
The rotating arm *4* comprises the grating holder *12* which aligns the grating in vertical and horizontal planes, a two-stage reduction gear-box *13* with a worm-wheel (the transfer ratio is 1/1620), and a small stepper motor *14*. The precise reduction box, which has split gears, can rotate the grating and the corner mirror 6 through 40° and can set the grating angle to within 10". The laser spectrum can be also tuned manually with a calibrated wheel 15. When active, the control unit feeds pulses to the stepper motor, and the counter displays the number of motor steps on a front panel. The turning rate can be set to between 50—500 step/sec (up to 5 lines/sec). It can be also made to go in single steps. The motion of the arm
An initial-state indicator with a low-voltage spark discharger acts as one limit switch and was designed to set the rotation arm in the starting position. At a constant voltage of several volts, the spark discharge in air occurs at a very small gap width (~1.5 µm*)* and, hence, the discharger generates a signal. The uncertainty the gap width at which the discharger generates a signal is less than 1 µm, with corresponds to a grating turn of ±4". Thus the reproducibi1ity and accuracy of the initial position is good and corresponds to one step of the system. Note that an error of one step is not significant when switching the laser to a particular line since the distance between neighboring lines in CO2 and CO lasers in terms of
The laser tuning system can be linked to computers through appropriate interfaces and software. The turning angles of the diffraction grating with respect to the cavity axis corresponding to each laser line are leaded into the computer. The turning angles in terms of motor steps are derived from the grating pitch. When the laser is operating, the calculated angles may differ from the real values by some constant. This difference may be due to composition variations of the gas in the tube as a result of the electric discharge. Even so the intervals between neighboring spectral lines, and hence the distance between the grating positions in terms of the turning angle remain unchanged. The correction to the calculated positions should been found experimentally for all lines in terms of motor steps. This experimental correction also takes into account the uncertainty of the position of the zero-
The correction can be determined in two ways using the laser tuning software. The first method is semiautomatic and requires an external spectral device (a monochromator or a gas cell with a known absorption spectrum, e.g., NH3). The position of the grating for the selected reference line is determined, and the difference between this value and the calculated posit ion is fed into computer as the correction. In the second method, the correction is determined automatically by finding a reference line without an external spectral reference (Gorobets et al., 1992). The correction is determined using bright lines which can be easily identified in the output spectrum. In the case of the CO2 laser a good line is P(56) in the 0001-1000 band, which coincides with the P(23) line of the 0111-1110 band. The algorithm for finding this line was described in (Gorobets et al., 1992) and it reliably
Once the experimental correction is found, the laser is tuned to the selected line. The correction is added to the angle corresponding to the selected line, and the computer moves the grating to the correct angle with respect to the previous position, then it activates the AFC system.
The temperature of the liquid cooling the laser tube should be kept constant. This is particularly important for the CO laser since the number of lasing lines, especially in the
determines the correction. A similar method is possible for the CO laser.
the grating turning angle is ~ 240", which is equivalent to 60 steps of the motor.
is limited in both directions by limit switches.
angle discharger.
structures and the same discharge distance – 1.2 m. The laser cavity is formed by a 100% end mirror *2* with a curvature radius of ~3 m that is integrated in the tube and a flat diffraction grating *3* with 100 lines/mm set on a rotating arm 4 and a PZT drive *5*. The grating reflects in the first diffraction order when operating with CO2 and in the second order with CO (the reflectivity is about 90% in both cases). There is an additional mirror 6 on the rotating arm which forms a corner reflector (Gorobets et al., 1992) to keep the direction of the output beam unchanged when the laser is switched from line to line. This optic layout is particularly suited to the laser tubes we were employing. Besides, when the output beam has zero-order reflection at the grating, the optic losses are much lower, and more laser lines can he generated, which is essential for CO tubes whose gains are relatively small.
The diffraction-grating rotating arm and the tube braces 7 are fixed on three invar rods *8*. The end plate *9* is rigidly fixed to the laser frame, and unit *4* is attached to the lower rod via two bearings. As a result, thermal expansion in the rods does not lead to any misalignment of the grating. The use of invar rods and the high rigidity of the structure leads to a good passive stability of the cavity length.
1 – laser tube; 2 – end mirror; 3 - diffraction grating; 4 – rotating arm stage; 5 – piezo-electric drive; 6 – turning mirror; 7 – laser tube braces; 8 – invar rods; 9 – end plate; 10 – pyro-electric detector; 11 – iris diaphragm; 12 – holder of the diffraction grating; 13 – gear-box with a worm wheel; 14 – stepper motor; 15 – graduated wheel; 16 – turning mirror
Fig. 7. Diagram of the laser structure
The radiation frequency is stabilized by coupling the output power to the wavelength and an appropriate curve is used in the stabilization system. The laser is in fact stabilized by automatically tuning the cavity length with the PZT drive *5* to which the diffraction grating *3* is fixed. The AFC circuit is similar to that of the Edinburgh instruments lasers. The signal for feedback loop is taken from a pyro-electric detector 10 which is exposed to the radiation reflected from the GaAs Brewster window of the discharge tube (see Fig. 7). The laser only generates the fundamental transverse mode because of the iris diaphragm 11 and the AFC system.
The key element in the laser is the system for tuning the lasing wavelength. Structurally, it is the rotating arm *4* of the diffraction grating and driven by the electronics driving the AFC system. The laser is switched between the lines by turning the grating with respect to the cavity axis.
structures and the same discharge distance – 1.2 m. The laser cavity is formed by a 100% end mirror *2* with a curvature radius of ~3 m that is integrated in the tube and a flat diffraction grating *3* with 100 lines/mm set on a rotating arm 4 and a PZT drive *5*. The grating reflects in the first diffraction order when operating with CO2 and in the second order with CO (the reflectivity is about 90% in both cases). There is an additional mirror 6 on the rotating arm which forms a corner reflector (Gorobets et al., 1992) to keep the direction of the output beam unchanged when the laser is switched from line to line. This optic layout is particularly suited to the laser tubes we were employing. Besides, when the output beam has zero-order reflection at the grating, the optic losses are much lower, and more laser lines
can he generated, which is essential for CO tubes whose gains are relatively small.
passive stability of the cavity length.
15 – graduated wheel; 16 – turning mirror Fig. 7. Diagram of the laser structure
system.
The diffraction-grating rotating arm and the tube braces 7 are fixed on three invar rods *8*. The end plate *9* is rigidly fixed to the laser frame, and unit *4* is attached to the lower rod via two bearings. As a result, thermal expansion in the rods does not lead to any misalignment of the grating. The use of invar rods and the high rigidity of the structure leads to a good
1 – laser tube; 2 – end mirror; 3 - diffraction grating; 4 – rotating arm stage; 5 – piezo-electric drive; 6 – turning mirror; 7 – laser tube braces; 8 – invar rods; 9 – end plate; 10 – pyro-electric detector; 11 – iris diaphragm; 12 – holder of the diffraction grating; 13 – gear-box with a worm wheel; 14 – stepper motor;
The radiation frequency is stabilized by coupling the output power to the wavelength and an appropriate curve is used in the stabilization system. The laser is in fact stabilized by automatically tuning the cavity length with the PZT drive *5* to which the diffraction grating *3* is fixed. The AFC circuit is similar to that of the Edinburgh instruments lasers. The signal for feedback loop is taken from a pyro-electric detector 10 which is exposed to the radiation reflected from the GaAs Brewster window of the discharge tube (see Fig. 7). The laser only generates the fundamental transverse mode because of the iris diaphragm 11 and the AFC
The key element in the laser is the system for tuning the lasing wavelength. Structurally, it is the rotating arm *4* of the diffraction grating and driven by the electronics driving the AFC system. The laser is switched between the lines by turning the grating with respect to the cavity axis.
The rotating arm *4* comprises the grating holder *12* which aligns the grating in vertical and horizontal planes, a two-stage reduction gear-box *13* with a worm-wheel (the transfer ratio is 1/1620), and a small stepper motor *14*. The precise reduction box, which has split gears, can rotate the grating and the corner mirror 6 through 40° and can set the grating angle to within 10". The laser spectrum can be also tuned manually with a calibrated wheel 15. When active, the control unit feeds pulses to the stepper motor, and the counter displays the number of motor steps on a front panel. The turning rate can be set to between 50—500 step/sec (up to 5 lines/sec). It can be also made to go in single steps. The motion of the arm is limited in both directions by limit switches.
An initial-state indicator with a low-voltage spark discharger acts as one limit switch and was designed to set the rotation arm in the starting position. At a constant voltage of several volts, the spark discharge in air occurs at a very small gap width (~1.5 µm*)* and, hence, the discharger generates a signal. The uncertainty the gap width at which the discharger generates a signal is less than 1 µm, with corresponds to a grating turn of ±4". Thus the reproducibi1ity and accuracy of the initial position is good and corresponds to one step of the system. Note that an error of one step is not significant when switching the laser to a particular line since the distance between neighboring lines in CO2 and CO lasers in terms of the grating turning angle is ~ 240", which is equivalent to 60 steps of the motor.
The laser tuning system can be linked to computers through appropriate interfaces and software. The turning angles of the diffraction grating with respect to the cavity axis corresponding to each laser line are leaded into the computer. The turning angles in terms of motor steps are derived from the grating pitch. When the laser is operating, the calculated angles may differ from the real values by some constant. This difference may be due to composition variations of the gas in the tube as a result of the electric discharge. Even so the intervals between neighboring spectral lines, and hence the distance between the grating positions in terms of the turning angle remain unchanged. The correction to the calculated positions should been found experimentally for all lines in terms of motor steps. This experimental correction also takes into account the uncertainty of the position of the zeroangle discharger.
The correction can be determined in two ways using the laser tuning software. The first method is semiautomatic and requires an external spectral device (a monochromator or a gas cell with a known absorption spectrum, e.g., NH3). The position of the grating for the selected reference line is determined, and the difference between this value and the calculated posit ion is fed into computer as the correction. In the second method, the correction is determined automatically by finding a reference line without an external spectral reference (Gorobets et al., 1992). The correction is determined using bright lines which can be easily identified in the output spectrum. In the case of the CO2 laser a good line is P(56) in the 0001-1000 band, which coincides with the P(23) line of the 0111-1110 band. The algorithm for finding this line was described in (Gorobets et al., 1992) and it reliably determines the correction. A similar method is possible for the CO laser.
Once the experimental correction is found, the laser is tuned to the selected line. The correction is added to the angle corresponding to the selected line, and the computer moves the grating to the correct angle with respect to the previous position, then it activates the AFC system.
The temperature of the liquid cooling the laser tube should be kept constant. This is particularly important for the CO laser since the number of lasing lines, especially in the
CO2 Lasing on Non-Traditional Bands 121
be monitored. The instability over the measurement time needs to be known in many applications. For example, when determining contaminants by the differential method (at the absorption line and off the line), the measurement may last from several seconds to a
The laser beam was modulated by the chapter and fed to a light detector cooled by liquid nitrogen (Gorobets et al., 1995). The detector's electric signal was processed by a lock-in amplifier, digitized and sent to a computer. One measurement, including the signal processing in the ADC took about 0.7 s. Measurements lasting over 70 s demonstrated that the output power of the laser generating at the P(24) line of the 00°1-1000 band of the CO2 molecule varied by ±1.6% around the mean with the AFC system on and by 12.5% with the AFC system off. The larger instability measured by the second method may be due to longer time constant of the calorimeter. The short-term power instability was also measured using an digital oscilloscope (band-width of about 1MHz) connected to the Ge:Au detector. The instability over times of the order of microseconds was estimated to be several times smaller than the long-term instability quoted above. Several lasers have been used to monitor the atmosphere and to obtain spectroscopic measurements over a long time. They have proven
Fig. 8. Trace of the output power over 1 h at the P(16) line of the CO2 molecule's 0001-0200
**7. Detection of small N2O concentrations using a frequency doubling 12C18O2**
The destruction of the protective ozone layer of the Earth (so called "ozone holes") can result in a global environmental and climatic catastrophe showing for many years a continuous unflagging. It is well known that the products of human activity such as freons and nitrous oxide (N2O) are responsible for "ozone holes". Freons appear as a result of manufacturing some kinds of plastics and using refrigerators. Nitrous oxides penetrate into the atmosphere primarily due to microbiological changes in soil caused by agricultural human activity. Moreover, (Crutzen, 1996) determined that there is a direct coupling
between the life of microorganisms in soil and the ozone layer thickness.
minute. The output power instability in this time interval was measured as follows.
to the reliable devices with a long service life.
band
**laser**
short-wave band, depends on the gas temperature in the tube (Aleinikov and Masychev, 1990). We used a standard water cooler with a closed cycle to remove the heat from the laser tube. It cools the tube with distilled water at a temperature between 2 and 10 °C and keeps it constant to within 1 °C.
The stabilized power unit is standard for CO2 lasers and an additional current stabilizer built around a vacuum tube. The current stabilizer suppresses current oscillations by several orders of magnitude, especially those at the mains frequency of 50 Hz. The current through the tube can be tuned between 10 and 40 mA.
To modulate the laser power, we used a electromechanical chopper. It was built around a electric motor. A thin precisely made disk with sixteen slits made from titanium foil 0.1 mm thick was mounted on the motor axis. The signal for the feedback of the active frequency control was taken from an optic couple. The electronics drive the modulator at 125, 250, 500, and 1000 Hz, and it can be detuned by ±5.9% from these frequencies. With a crystal oscillator and automatic control of the modulation frequency its very stable (the frequency usually differs from the preset value by less than 0.01%).
#### **6.2 Output laser parameters**
The laser characteristics have been measured on an optic bench using traditional techniques (Gorobets et al., 1995). We first consider the spectral and energy parameters.
Since the diffraction grating has a reflectivity of 90% the laser with a CO2 tube, generates about 90 lines between 9.15 and 10. 95 µm (0001-1000, 02°0 bands) The output power in the fundamental mode reaches 10 W for strong lines and is over 1 W side lines. Using the same grating, the laser generates 25 lines in the P-branch of he hot 0111–1110 band of the CO2 molecule. In this case the spectrum is shifted to the red to 10.94 –11.25 µm. The output power at strong lines was 3-4 W and 0.5 W at the band edge. However the conditions needed for the hot baud, where the gain is considerably smaller, are not optimum. When the laser tube is filled with CO2, N2, He and Xe, output was considerably higher and, the number of lines was larger (see 1-3 parts).
When the CL-509 tube is inserted, the laser efficiently generates about 100 vibration-rotation lines of the CO molecule between 5.28 and 6.43 µm. Output powers at the strongest lines were ~1 W in the fundamental mode at the optimum discharge current. The lines were identified using the data in (Aleinikov and Masychev, 1990).
Note that the parameters of the grating are better when generating CO and hot of CO2 lines, where the gain is smaller than in the more conventional 0001-1000,0200 bands. The lines in the conventional bands will clearly be stronger in a cavity with a lower Q factor.
#### **6.3 Instability of the output laser power**
The long-term instability of the output power was checked using a laser calorimeter whose signal was fed to a chart-recorder. Figure 8 shows a typical plot of the laser output power over one hour. The instability in the laser output power on the P(16) line of the 00°1-02°0 band over one hour was ±1.1%. Similar measurements with other lines of CO and CO2 molecules demonstrated that the long-term instability of the laser power is less than ±1.25%. However the time constant of the calorimeter is long and the short-term instability could not
short-wave band, depends on the gas temperature in the tube (Aleinikov and Masychev, 1990). We used a standard water cooler with a closed cycle to remove the heat from the laser tube. It cools the tube with distilled water at a temperature between 2 and 10 °C and keeps it
The stabilized power unit is standard for CO2 lasers and an additional current stabilizer built around a vacuum tube. The current stabilizer suppresses current oscillations by several orders of magnitude, especially those at the mains frequency of 50 Hz. The current through
To modulate the laser power, we used a electromechanical chopper. It was built around a electric motor. A thin precisely made disk with sixteen slits made from titanium foil 0.1 mm thick was mounted on the motor axis. The signal for the feedback of the active frequency control was taken from an optic couple. The electronics drive the modulator at 125, 250, 500, and 1000 Hz, and it can be detuned by ±5.9% from these frequencies. With a crystal oscillator and automatic control of the modulation frequency its very stable (the frequency
The laser characteristics have been measured on an optic bench using traditional techniques
Since the diffraction grating has a reflectivity of 90% the laser with a CO2 tube, generates about 90 lines between 9.15 and 10. 95 µm (0001-1000, 02°0 bands) The output power in the fundamental mode reaches 10 W for strong lines and is over 1 W side lines. Using the same grating, the laser generates 25 lines in the P-branch of he hot 0111–1110 band of the CO2 molecule. In this case the spectrum is shifted to the red to 10.94 –11.25 µm. The output power at strong lines was 3-4 W and 0.5 W at the band edge. However the conditions needed for the hot baud, where the gain is considerably smaller, are not optimum. When the laser tube is filled with CO2, N2, He and Xe, output was considerably higher and, the
When the CL-509 tube is inserted, the laser efficiently generates about 100 vibration-rotation lines of the CO molecule between 5.28 and 6.43 µm. Output powers at the strongest lines were ~1 W in the fundamental mode at the optimum discharge current. The lines were
Note that the parameters of the grating are better when generating CO and hot of CO2 lines, where the gain is smaller than in the more conventional 0001-1000,0200 bands. The lines in
The long-term instability of the output power was checked using a laser calorimeter whose signal was fed to a chart-recorder. Figure 8 shows a typical plot of the laser output power over one hour. The instability in the laser output power on the P(16) line of the 00°1-02°0 band over one hour was ±1.1%. Similar measurements with other lines of CO and CO2 molecules demonstrated that the long-term instability of the laser power is less than ±1.25%. However the time constant of the calorimeter is long and the short-term instability could not
the conventional bands will clearly be stronger in a cavity with a lower Q factor.
(Gorobets et al., 1995). We first consider the spectral and energy parameters.
constant to within 1 °C.
**6.2 Output laser parameters**
number of lines was larger (see 1-3 parts).
**6.3 Instability of the output laser power**
identified using the data in (Aleinikov and Masychev, 1990).
the tube can be tuned between 10 and 40 mA.
usually differs from the preset value by less than 0.01%).
be monitored. The instability over the measurement time needs to be known in many applications. For example, when determining contaminants by the differential method (at the absorption line and off the line), the measurement may last from several seconds to a minute. The output power instability in this time interval was measured as follows.
The laser beam was modulated by the chapter and fed to a light detector cooled by liquid nitrogen (Gorobets et al., 1995). The detector's electric signal was processed by a lock-in amplifier, digitized and sent to a computer. One measurement, including the signal processing in the ADC took about 0.7 s. Measurements lasting over 70 s demonstrated that the output power of the laser generating at the P(24) line of the 00°1-1000 band of the CO2 molecule varied by ±1.6% around the mean with the AFC system on and by 12.5% with the AFC system off. The larger instability measured by the second method may be due to longer time constant of the calorimeter. The short-term power instability was also measured using an digital oscilloscope (band-width of about 1MHz) connected to the Ge:Au detector. The instability over times of the order of microseconds was estimated to be several times smaller than the long-term instability quoted above. Several lasers have been used to monitor the atmosphere and to obtain spectroscopic measurements over a long time. They have proven to the reliable devices with a long service life.
Fig. 8. Trace of the output power over 1 h at the P(16) line of the CO2 molecule's 0001-0200 band
#### **7. Detection of small N2O concentrations using a frequency doubling 12C18O2 laser**
The destruction of the protective ozone layer of the Earth (so called "ozone holes") can result in a global environmental and climatic catastrophe showing for many years a continuous unflagging. It is well known that the products of human activity such as freons and nitrous oxide (N2O) are responsible for "ozone holes". Freons appear as a result of manufacturing some kinds of plastics and using refrigerators. Nitrous oxides penetrate into the atmosphere primarily due to microbiological changes in soil caused by agricultural human activity. Moreover, (Crutzen, 1996) determined that there is a direct coupling between the life of microorganisms in soil and the ozone layer thickness.
CO2 Lasing on Non-Traditional Bands 123
lasers the discharge chambers of which are much more difficult to pump out as compared to low-pressure sealed-off lasers. Another problem is the necessity of proper choice of materials which accumulate less ordinary oxygen. Therefore, we have performed detailed investigations aimed at active medium optimization for high-energy parameters with a simultaneous decrease in the price of the active medium based on isotopically substituted molecules 12C18O2 at the expense of its dilution with inexpensive carbon dioxide 12C16O2 .
This section gives the results of our spectral investigations of the gain and the lasing for the TEA CO2 laser operating both on 12C18O2 and, just for comparison, on ordinary 12C16O2 . The analysis of the conditions required for efficient lasing in the range of 9 µm is given too.
Experiments were performed with a UV-preionized TEA module specialty developed for lidar systems (Gorobets et al., 1995). The module had a working volume of 70 × 2.5 × 2 cm. The distance between electrodes was 2 cm. Both main and auxiliary discharges were fed from low-inductance capacitors having a total capacity of 0.2 µF charged up to the 25 kV
The isotopically substituted form of carbon dioxide 12C18O2 with an 18O enrichment factor of 80% obtained as a gas mixture containing 4% 12C16O2 , 32% 12C16O18O , and 64% 12C1802 was used in the experiments. It is much more expensive to prepare a mixture with a higher factor of enrichment for 12C18O2 . The first measurements and calculations concerned the gain. For example, Fig. 9 shows respective gain for some lines of the P-branch of 00°1-10°0 band of the 12C18O2 molecule (λ~9.4 µm) and of the 12C1602 molecule (λ~10.6 µm) for the mixture 12C16O2 : 12C16O18O : 12C18O2 : N2 : He = 10:3:7:20:60 (the manufactured mixture was diluted with 12C16O2) with a total pressure of 500 Torr. Measurements carried out at t = 4 µs after the start of the discharge when the highest gains were realized. Gain measurements were performed by probing the active medium with a cw laser, lasing on the corresponding lines.
Fig. 9. Measured ( Ο ) and calculated ( ⏐ ) values of the gain at the lines of the 10P(12C16O2) (a) and 9P(12C18O2 ) (b). Dashed lines – the gain calculated without lines overlapping.
voltage. The discharge duration was ~500 ns.
The conservation of the ozone envelope depends on many factors. However it is beyond doubt that modern reliable techniques monitoring the atmosphere for the presence of freons and nitrous oxide would assist greatly in a solution of this serious global problem. Laser gas-analysis methods are well suited to this task. They are capable of working with high speed, i.e. practically in real time mode. The ability to determine extremely low gas concentrations (for laser photoacoustics on the level of 0.1 ppb) and to cover extensive areas of the earth from a single point of observation (for lidars – about 10 km) give them unquestionable advantages as compared to other known diagnostic methods. There is reliable and effective laser procedure based on CO2 laser for the detection of prevailing freons, the strong absorption bands of which overlap with emission lines of the laser.
Spectral analysis of N2O has shown that the characteristic feature of this molecule consists of the absorption now low in the ranges where known effective lasers can operate. There are only the complex and (or) inefficient multitasked systems with nonlinear frequency conversation (generation of harmonics with the subsequent frequency summation), parametric oscillators, and tunable diode lasers. Therefore, the development of reliable and efficient laser methods for N2O sensing remains a topical problem. An additional difficulty arising with the development of such methods applies to the necessity to detect low concentrations of nitrous oxide (background content of this gas in the atmosphere is 0.2—0.4 ppm).
The main goal of the present investigation is the development of a reliable and highefficiency laser method for detecting low concentrations of N2O. The other goal of the work is the test of this method as a remote gas analyzer. The procedure is based on the idea of using a nonlinear frequency-doubled CO2 laser operating on the isotopic carbon dioxide modification 12*C*18O2. Such a powerful laser system would emit neighboring lines both coinciding well and adjacent to N2O absorption lines. This fact allows one to apply the highly accurate technique which uses corresponding on/off line pairs for the differential absorption. The high efficiency of the system and strong absorption of N2O molecules (we use the strongest band in the range of λ ~ 4.5 µm) would give a possibility to measure low gas concentrations both in short and long (~10 km) measurement paths.
#### **7.1 Active medium optimization of the 12C18O2 laser**
It is known that the use isotopically substituted carbon dioxide molecules make it possible to increase substantially the number of lines and to extend the spectral range of CO2 lasers. That is important for different applications, in particular for atmospheric gas detection. The use of 12C18O2 as molecules of the active medium of lasers is of special interest, since for this molecule the maximum gain lies at wavelength ~9.4 µm and not ~10.6 µm, as for 12C16O2, and, consequently. there is a possibility for efficient lasing in a shorter wavelength range down to 8.9 µm. The doubled emission frequencies of 12C18O2 laser well coincide with absorption lines of many molecules including nitrous oxide.
However, for a number of reasons, and particularly because of the much higher price, CO2 lasers based on isotopically substituted carbon dioxide molecules are not in wide use. For 12C18O2, molecules there is also the problem of the isotoporeplacement of 18O2 with 16O2 as these molecules are active in discharge plasma. The electrode surface, discharge chamber and tubes walls accumulate with time ordinary oxygen 16O2. Then, under the discharge conditions, 16O2 replaces (isotopically) 18O2 *,* in the active medium. This results in rapid degradation of the 12C18O2 active medium. This fact is especially important for TEA CO2-
The conservation of the ozone envelope depends on many factors. However it is beyond doubt that modern reliable techniques monitoring the atmosphere for the presence of freons and nitrous oxide would assist greatly in a solution of this serious global problem. Laser gas-analysis methods are well suited to this task. They are capable of working with high speed, i.e. practically in real time mode. The ability to determine extremely low gas concentrations (for laser photoacoustics on the level of 0.1 ppb) and to cover extensive areas of the earth from a single point of observation (for lidars – about 10 km) give them unquestionable advantages as compared to other known diagnostic methods. There is reliable and effective laser procedure based on CO2 laser for the detection of prevailing
freons, the strong absorption bands of which overlap with emission lines of the laser.
oxide (background content of this gas in the atmosphere is 0.2—0.4 ppm).
gas concentrations both in short and long (~10 km) measurement paths.
**7.1 Active medium optimization of the 12C18O2 laser**
absorption lines of many molecules including nitrous oxide.
Spectral analysis of N2O has shown that the characteristic feature of this molecule consists of the absorption now low in the ranges where known effective lasers can operate. There are only the complex and (or) inefficient multitasked systems with nonlinear frequency conversation (generation of harmonics with the subsequent frequency summation), parametric oscillators, and tunable diode lasers. Therefore, the development of reliable and efficient laser methods for N2O sensing remains a topical problem. An additional difficulty arising with the development of such methods applies to the necessity to detect low concentrations of nitrous
The main goal of the present investigation is the development of a reliable and highefficiency laser method for detecting low concentrations of N2O. The other goal of the work is the test of this method as a remote gas analyzer. The procedure is based on the idea of using a nonlinear frequency-doubled CO2 laser operating on the isotopic carbon dioxide modification 12*C*18O2. Such a powerful laser system would emit neighboring lines both coinciding well and adjacent to N2O absorption lines. This fact allows one to apply the highly accurate technique which uses corresponding on/off line pairs for the differential absorption. The high efficiency of the system and strong absorption of N2O molecules (we use the strongest band in the range of λ ~ 4.5 µm) would give a possibility to measure low
It is known that the use isotopically substituted carbon dioxide molecules make it possible to increase substantially the number of lines and to extend the spectral range of CO2 lasers. That is important for different applications, in particular for atmospheric gas detection. The use of 12C18O2 as molecules of the active medium of lasers is of special interest, since for this molecule the maximum gain lies at wavelength ~9.4 µm and not ~10.6 µm, as for 12C16O2, and, consequently. there is a possibility for efficient lasing in a shorter wavelength range down to 8.9 µm. The doubled emission frequencies of 12C18O2 laser well coincide with
However, for a number of reasons, and particularly because of the much higher price, CO2 lasers based on isotopically substituted carbon dioxide molecules are not in wide use. For 12C18O2, molecules there is also the problem of the isotoporeplacement of 18O2 with 16O2 as these molecules are active in discharge plasma. The electrode surface, discharge chamber and tubes walls accumulate with time ordinary oxygen 16O2. Then, under the discharge conditions, 16O2 replaces (isotopically) 18O2 *,* in the active medium. This results in rapid degradation of the 12C18O2 active medium. This fact is especially important for TEA CO2lasers the discharge chambers of which are much more difficult to pump out as compared to low-pressure sealed-off lasers. Another problem is the necessity of proper choice of materials which accumulate less ordinary oxygen. Therefore, we have performed detailed investigations aimed at active medium optimization for high-energy parameters with a simultaneous decrease in the price of the active medium based on isotopically substituted molecules 12C18O2 at the expense of its dilution with inexpensive carbon dioxide 12C16O2 .
This section gives the results of our spectral investigations of the gain and the lasing for the TEA CO2 laser operating both on 12C18O2 and, just for comparison, on ordinary 12C16O2 . The analysis of the conditions required for efficient lasing in the range of 9 µm is given too.
Experiments were performed with a UV-preionized TEA module specialty developed for lidar systems (Gorobets et al., 1995). The module had a working volume of 70 × 2.5 × 2 cm. The distance between electrodes was 2 cm. Both main and auxiliary discharges were fed from low-inductance capacitors having a total capacity of 0.2 µF charged up to the 25 kV voltage. The discharge duration was ~500 ns.
The isotopically substituted form of carbon dioxide 12C18O2 with an 18O enrichment factor of 80% obtained as a gas mixture containing 4% 12C16O2 , 32% 12C16O18O , and 64% 12C1802 was used in the experiments. It is much more expensive to prepare a mixture with a higher factor of enrichment for 12C18O2 . The first measurements and calculations concerned the gain. For example, Fig. 9 shows respective gain for some lines of the P-branch of 00°1-10°0 band of the 12C18O2 molecule (λ~9.4 µm) and of the 12C1602 molecule (λ~10.6 µm) for the mixture 12C16O2 : 12C16O18O : 12C18O2 : N2 : He = 10:3:7:20:60 (the manufactured mixture was diluted with 12C16O2) with a total pressure of 500 Torr. Measurements carried out at t = 4 µs after the start of the discharge when the highest gains were realized. Gain measurements were performed by probing the active medium with a cw laser, lasing on the corresponding lines.
Fig. 9. Measured ( Ο ) and calculated ( ⏐ ) values of the gain at the lines of the 10P(12C16O2) (a) and 9P(12C18O2 ) (b). Dashed lines – the gain calculated without lines overlapping.
CO2 Lasing on Non-Traditional Bands 125
Fig. 10. Lasing energy measured in the ranges of 9.4 μm for 12C18O2 (Ο) and 10.4 μm for
Thus, the results of the investigations demonstrate that efficient lasing of the CO2 laser even with non-selective cavity is possible in the range of 9.4 µm when the 12C18O2, content is 30% of total carbon dioxide amount. When the laser operates with selective cavity such as that based on a diffraction grating, the percentage of 12C18O2, can go down to ~20%. This fact is
An experimental series performed as well by us was oriented toward providing longduration autonomous laser operation without active mixture replacement and noticeable degradation of its composition. To this end, the discharge chamber was well evacuated (no more than 0.2 Torr/day inleakage) and the material it was made of was properly selected. The best results were achieved with a glass-epoxy cylinder when the operation was virtually quasi-sealed-off, i.e. without replacement of the active medium on 12C18O2 during 1—2 months (more than 105 pulses) without noticeable decrease of the laser energy. In addition, after long operation and before working gases replacement. the old mixture was pumped
**7.2 Using a nonlinear crystal etalon for second harmonic generation from CO2 lasers** The poor efficiency of the frequency conversion attributable on the whole to mid IR lasers can be compensated to a large degree by application of non-traditional optical schemes. Therefore, the problem remaining topical in the mid IR range along with creation of higherquality crystals applies to development of novel nonlinear conversion schemes and search for the laser operation modes which are optimal for frequency conversion. To this end and with the aim of reaching high conversion efficiency, we have performed some investigations on the basis of which it was possible to realize original optical frequency conversion
through liquid nitrogen traps to recover carbon dioxide for repeated use.
12C16O2 (∆) versus 12C18O2 content
key for reduction of the price of the active medium.
schemes including intracavity versions.
After passing the medium in question, the probing emission came to a photodetector producing a signal to a digital oscilloscope, connected to a personal computer where the measurements were stored and averaged. The calculations were performed based on a vibrational temperature model (Petukhov et. al.., 1985) that is in wide use for simulations of the active medium of CO2, lasers. The measurements have shown that under the above conditions: *T=* 350 ± 10 K, *T1* = *T2* = 420±20 K, and *T3=*1475, 1510 and 1550, ±15 for molecules 12C16O2 *,* 12C16O18O, and 12C18O2, accordingly. T designates the gas temperature. T1 and T2 are the temperatures of symmetric and deformation vibrations that are virtually the same for the whole scope of carbon dioxide variations in question. T3 is the temperature of asymmetric vibrations. The technique used and apparatus to measure gains and to determine vibrational temperatures were described early. The gain spectrum was calculated using the scheme described in (Gordiets et al., 1980). Additionally the overlap of individual vibration— rotation lines in bands 0001-1000, 0001—0200, 0002-1001, 0002-0201, 0111-1110, 0111- 0310 for molecules 12C16O2 and 12C18O2 and bands 0001-1000 , 0001—0200 for molecule 12C16O18O and spectroscopic data given in (Witteman,.1987) were allowed for.
As is evident in Fig. 9, the gain maximum in the P-branch of the band 0001-1000 of the 12C16O2 molecule in the mixture of isotopic forms of carbon dioxide 12C16O2, 12C16O18O and 12C18O2, occurred at lines P(18) and P(20). These lines are especially affected by the overlap with lines from other bands. The P(18) line of the 00°1-10°0 band for the 12C18O2, molecule is shifted from lines P(3) 00°1-02°0 12C16O18O, R(8) 00°1-02°0 12C16O2 and P(26) 0111-1110 12C18O2, at 0.046, 0.045 and 0.021 cm-1, and line P(20) 0001-1000 12C18O2-from lines P(5) 00°1- 02°0 12C16O18O,R(6) 00°1-02°0 12C16O2, and P(28) 0111- 1110 12C18O2 at 0.007, 0.072, 0.085 cm-1, respectively. The homogeneous width of the line at half-height under our conditions is ~0.05 cm-1. We note that the calculation and experimental data are in a good agreement. For instance, the gain calculated for line P(18) 0001-1000 12C18O2 is 1.83×10-2 cm-1 while the measured value is (1.81 ± 0.06) x 10-2 cm-1
It's also worth noting that higher gains are realized for the 12C18O2 molecule than for the 12C16O2 molecule. As seen from Fig. 9, the gains on the strongest lines of the P-branches for both isotopes are approximately equal, though this mixture contains 1.8 times less 12C18O2 than 12C16O2. There are three reasons for this. First, the limiting gain for the lines of the Pbranch of the band 0001-1000 of 12C18O2 is approximately 1.5 times higher than that for the Pbranch of the band 0001-1000 of 12C16O2 (Witteman,.1987); second, for the lines of the Pbranch of this band of the 12C18O2 molecule in the 12C16O2+12C16O18O mixture the effect of the overlap with other bands is essential; and, finally, under the conditions of dynamic equilibrium between the v3 modes of both isotopic forms the temperature T3 for 12C18O2, is higher than for 12C16O2 .
We also measured the effect of the dilution of molecules 12C18O2 by the usual form of carbon dioxide 12C16O2, on the output energy. The lasing pulse energies measured in the ranges of 9.4 µm (12C18O2) (1) and 10.4 µm (12C16O2) (2) are given in Fig. 10. These data were acquired for the TEA-module described above using a non-selective cavity composed of a nontransmitting and germanium output (R=~50%) mirrors. The energies emitted in the ranges 10.4 and 9.4 µm are approximately equal to each other at the 30 Torr content of 12C18O2, approaching to the content of 12C18O2 at which the corresponding gains get equal. Also note that the total emission energy (E9.4 + E10.6) is independent in this case of isotoporeplacement of carbon dioxide and reaches the value of 5 J.
After passing the medium in question, the probing emission came to a photodetector producing a signal to a digital oscilloscope, connected to a personal computer where the measurements were stored and averaged. The calculations were performed based on a vibrational temperature model (Petukhov et. al.., 1985) that is in wide use for simulations of the active medium of CO2, lasers. The measurements have shown that under the above conditions: *T=* 350 ± 10 K, *T1* = *T2* = 420±20 K, and *T3=*1475, 1510 and 1550, ±15 for molecules 12C16O2 *,* 12C16O18O, and 12C18O2, accordingly. T designates the gas temperature. T1 and T2 are the temperatures of symmetric and deformation vibrations that are virtually the same for the whole scope of carbon dioxide variations in question. T3 is the temperature of asymmetric vibrations. The technique used and apparatus to measure gains and to determine vibrational temperatures were described early. The gain spectrum was calculated using the scheme described in (Gordiets et al., 1980). Additionally the overlap of individual vibration— rotation lines in bands 0001-1000, 0001—0200, 0002-1001, 0002-0201, 0111-1110, 0111- 0310 for molecules 12C16O2 and 12C18O2 and bands 0001-1000 , 0001—0200 for molecule
12C16O18O and spectroscopic data given in (Witteman,.1987) were allowed for.
measured value is (1.81 ± 0.06) x 10-2 cm-1
of carbon dioxide and reaches the value of 5 J.
higher than for 12C16O2 .
As is evident in Fig. 9, the gain maximum in the P-branch of the band 0001-1000 of the 12C16O2 molecule in the mixture of isotopic forms of carbon dioxide 12C16O2, 12C16O18O and 12C18O2, occurred at lines P(18) and P(20). These lines are especially affected by the overlap with lines from other bands. The P(18) line of the 00°1-10°0 band for the 12C18O2, molecule is shifted from lines P(3) 00°1-02°0 12C16O18O, R(8) 00°1-02°0 12C16O2 and P(26) 0111-1110 12C18O2, at 0.046, 0.045 and 0.021 cm-1, and line P(20) 0001-1000 12C18O2-from lines P(5) 00°1- 02°0 12C16O18O,R(6) 00°1-02°0 12C16O2, and P(28) 0111- 1110 12C18O2 at 0.007, 0.072, 0.085 cm-1, respectively. The homogeneous width of the line at half-height under our conditions is ~0.05 cm-1. We note that the calculation and experimental data are in a good agreement. For instance, the gain calculated for line P(18) 0001-1000 12C18O2 is 1.83×10-2 cm-1 while the
It's also worth noting that higher gains are realized for the 12C18O2 molecule than for the 12C16O2 molecule. As seen from Fig. 9, the gains on the strongest lines of the P-branches for both isotopes are approximately equal, though this mixture contains 1.8 times less 12C18O2 than 12C16O2. There are three reasons for this. First, the limiting gain for the lines of the Pbranch of the band 0001-1000 of 12C18O2 is approximately 1.5 times higher than that for the Pbranch of the band 0001-1000 of 12C16O2 (Witteman,.1987); second, for the lines of the Pbranch of this band of the 12C18O2 molecule in the 12C16O2+12C16O18O mixture the effect of the overlap with other bands is essential; and, finally, under the conditions of dynamic equilibrium between the v3 modes of both isotopic forms the temperature T3 for 12C18O2, is
We also measured the effect of the dilution of molecules 12C18O2 by the usual form of carbon dioxide 12C16O2, on the output energy. The lasing pulse energies measured in the ranges of 9.4 µm (12C18O2) (1) and 10.4 µm (12C16O2) (2) are given in Fig. 10. These data were acquired for the TEA-module described above using a non-selective cavity composed of a nontransmitting and germanium output (R=~50%) mirrors. The energies emitted in the ranges 10.4 and 9.4 µm are approximately equal to each other at the 30 Torr content of 12C18O2, approaching to the content of 12C18O2 at which the corresponding gains get equal. Also note that the total emission energy (E9.4 + E10.6) is independent in this case of isotoporeplacement
Fig. 10. Lasing energy measured in the ranges of 9.4 μm for 12C18O2 (Ο) and 10.4 μm for 12C16O2 (∆) versus 12C18O2 content
Thus, the results of the investigations demonstrate that efficient lasing of the CO2 laser even with non-selective cavity is possible in the range of 9.4 µm when the 12C18O2, content is 30% of total carbon dioxide amount. When the laser operates with selective cavity such as that based on a diffraction grating, the percentage of 12C18O2, can go down to ~20%. This fact is key for reduction of the price of the active medium.
An experimental series performed as well by us was oriented toward providing longduration autonomous laser operation without active mixture replacement and noticeable degradation of its composition. To this end, the discharge chamber was well evacuated (no more than 0.2 Torr/day inleakage) and the material it was made of was properly selected. The best results were achieved with a glass-epoxy cylinder when the operation was virtually quasi-sealed-off, i.e. without replacement of the active medium on 12C18O2 during 1—2 months (more than 105 pulses) without noticeable decrease of the laser energy. In addition, after long operation and before working gases replacement. the old mixture was pumped through liquid nitrogen traps to recover carbon dioxide for repeated use.
#### **7.2 Using a nonlinear crystal etalon for second harmonic generation from CO2 lasers**
The poor efficiency of the frequency conversion attributable on the whole to mid IR lasers can be compensated to a large degree by application of non-traditional optical schemes. Therefore, the problem remaining topical in the mid IR range along with creation of higherquality crystals applies to development of novel nonlinear conversion schemes and search for the laser operation modes which are optimal for frequency conversion. To this end and with the aim of reaching high conversion efficiency, we have performed some investigations on the basis of which it was possible to realize original optical frequency conversion schemes including intracavity versions.
CO2 Lasing on Non-Traditional Bands 127
the fixed temperature the etalon practically does not influence on oscillations of both lasers, as its bandwidth is much wider than longitudinal modes and comparable to the pressure broadened line width for the TEA laser. Therefore, it is easy to achieve a concurrence between maxima of the line shape function (for TEA and low-pressure longitudinaldischarge lasers) and transmittance bandwidth of the Fabry-Perot etalon by a small angular
However in real conditions a nonlinear crystal has a temperature drift due to the d*(nl)/*d*t* thermal expansion. For the used AgGaSe2 crystal according with the dates of Clevelend Crystals Inc. the factor of linear expansion (α) is 15x106 °C-1 and the thermo-optical factors (d*n0/*d*t* and d*ne/*d*t*) are ~50x10-6 °C-1. Our calculations (see Fig.11c) demonstrate the rather strong temperature drift of the crystal. It is especially important in the case of crystal operating for a low-pressure longitudinal-discharge CO2 laser. In our experiments the crystal was supported by a massive metal holder heated by a thermoelement and it was stabilized with accuracy <0.1°C. Besides it should be noted, that in our experiments the crystals having, the very small absorption (~0.01 cm-1) were used. This fact essentially simplifies process of temperature stabilization. Possible changes of the cavity losses because of the crystal temperature drift for a low-pressure longitudinal-discharge laser did not influence strongly on its output from the fact that we used a pulse-periodic regime of the
The optical crystal was used as a nonlinear output mirror of the TEA 12C18O2, laser. The cavity of the laser was formed by a 150 line/mm grating and an AgGaSe2 nonlinear crystal. A plate made of LiF was used to select second harmonic emission generated in the nonlinear crystal. A high-quality monocrystal sample made of AgGaSe2, with a 12 x 10-mm section *(L* = 19 mm) was used as a nonlinear output mirror of the laser. The working faces of the crystal were mechanically polished and were not coated. The highly parallel faces (better than 10") caused the sample to operate as a Fabri-Perot etalon. The angle *Ө* (phase matching angle) was adjusted near 46°, and *φ* = 45°. The angle *Ө* is such that the highest efficiency of the second harmonic oscillation is observed at a normal incidence at line 9P(32) *(*λ = 9.06 µm) and at the neighboring line 9P(34) (λ = 9.05 µm) of the isotopic modifications 12C18O2 of
The cavity length was 1.1 m. Before the output mirror (AgGaSe2 crystal) there was in the cavity an iris diaphragm (diameter ~8 mm). In the case when the TEA module was filled with the mixture 12C16O2 : 12C16O18O : 12C18O2 : N2 : He = 104 : 32 : 64 : 200: 600 at a total pressure 500 Torr and a nonlinear crystal was used as an output mirror (~60% reflection), the output energy at lines 9P(32) and 9P(34) of carbon dioxide isotope 12C18O2, was ~0.8 J while the peak power was ~4MW. The diameter of the output beam was ~7 mm. The lasing spot had a good spatial distribution. The energy density of the output emission was ~2
Under above conditions the second harmonic generation energy *(E2ω)* was 52 mJ and the peak power *(P2ω)* was ~2 MW. The conversion efficiency reached almost 15%, and at the peak power it was ~50%. The efficiency was calculated by the standard method (*η= E2ω / E<sup>ω</sup> — for* the per pulse energy and *η= P2ω / Pω —* for peak power). *Eω,, Pω* and *E2<sup>ω</sup> , P2<sup>ω</sup> —* are
energy parameters of the laser emission in the ranges 9 and 4.5 µm, respectively.
**7.3 Second harmonic generation from a TEA 12C18O2 laser**
tuning of the crystal.
lasing.
carbon dioxide.
J/cm2 (~10 MW/cm2).
In this work we used an original high-efficiency optical nonlinear conversion scheme without focusing optics developed by ourselves. An AgGaSe2 nonlinear crystal was acting as an Fabry-Perot etalon. In this case it could be placed in a laser cavity without reflectionreducing coating. However sonic problems connected with the spacing of its Fabry-Perot transmittance bandwidths and laser cavity modes can arise from it. To clarify these problems some calculations have been made (see Fig. 11).
The line shape function in Figs. 11a and 11b were calculated using the Foight and Lorentz expression, correspondingly. The intervals between the longitudinal modes for the lasers were calculated according with the condition *c/2L*, where *L* is the cavity length and c is the velocity of light. The curves in Fig. 11c are the Fabry-Perot etalon transmittance bandwidths calculated for different temperatures according with Airy's Formula for the etalon made from the AgGaSe2, crystal with the length of 17 mm.
Fig. 11. Line shape function for a low-pressure longitudinal-discharge СО2 laser (a) and for a ТЕА СО2 laser (b) and a transmittance bandwidth of the AgGaSe2 crystal acting as an Fabri-Perot etalon for different temperature variations (c).
In spite of the fact that the pressure broadened line width strongly differs for the TEA CO2 laser and the low-pressure longitudinal-discharge CO2 laser. For the first the full width at half maximum of the line shape function is about 3 GHz, and for the second ~0.l GHz. At
In this work we used an original high-efficiency optical nonlinear conversion scheme without focusing optics developed by ourselves. An AgGaSe2 nonlinear crystal was acting as an Fabry-Perot etalon. In this case it could be placed in a laser cavity without reflectionreducing coating. However sonic problems connected with the spacing of its Fabry-Perot transmittance bandwidths and laser cavity modes can arise from it. To clarify these
The line shape function in Figs. 11a and 11b were calculated using the Foight and Lorentz expression, correspondingly. The intervals between the longitudinal modes for the lasers were calculated according with the condition *c/2L*, where *L* is the cavity length and c is the velocity of light. The curves in Fig. 11c are the Fabry-Perot etalon transmittance bandwidths calculated for different temperatures according with Airy's Formula for the etalon made
Fig. 11. Line shape function for a low-pressure longitudinal-discharge СО2 laser (a) and for a ТЕА СО2 laser (b) and a transmittance bandwidth of the AgGaSe2 crystal acting as an Fabri-
In spite of the fact that the pressure broadened line width strongly differs for the TEA CO2 laser and the low-pressure longitudinal-discharge CO2 laser. For the first the full width at half maximum of the line shape function is about 3 GHz, and for the second ~0.l GHz. At
problems some calculations have been made (see Fig. 11).
from the AgGaSe2, crystal with the length of 17 mm.
Perot etalon for different temperature variations (c).
the fixed temperature the etalon practically does not influence on oscillations of both lasers, as its bandwidth is much wider than longitudinal modes and comparable to the pressure broadened line width for the TEA laser. Therefore, it is easy to achieve a concurrence between maxima of the line shape function (for TEA and low-pressure longitudinaldischarge lasers) and transmittance bandwidth of the Fabry-Perot etalon by a small angular tuning of the crystal.
However in real conditions a nonlinear crystal has a temperature drift due to the d*(nl)/*d*t* thermal expansion. For the used AgGaSe2 crystal according with the dates of Clevelend Crystals Inc. the factor of linear expansion (α) is 15x106 °C-1 and the thermo-optical factors (d*n0/*d*t* and d*ne/*d*t*) are ~50x10-6 °C-1. Our calculations (see Fig.11c) demonstrate the rather strong temperature drift of the crystal. It is especially important in the case of crystal operating for a low-pressure longitudinal-discharge CO2 laser. In our experiments the crystal was supported by a massive metal holder heated by a thermoelement and it was stabilized with accuracy <0.1°C. Besides it should be noted, that in our experiments the crystals having, the very small absorption (~0.01 cm-1) were used. This fact essentially simplifies process of temperature stabilization. Possible changes of the cavity losses because of the crystal temperature drift for a low-pressure longitudinal-discharge laser did not influence strongly on its output from the fact that we used a pulse-periodic regime of the lasing.
#### **7.3 Second harmonic generation from a TEA 12C18O2 laser**
The optical crystal was used as a nonlinear output mirror of the TEA 12C18O2, laser. The cavity of the laser was formed by a 150 line/mm grating and an AgGaSe2 nonlinear crystal. A plate made of LiF was used to select second harmonic emission generated in the nonlinear crystal. A high-quality monocrystal sample made of AgGaSe2, with a 12 x 10-mm section *(L* = 19 mm) was used as a nonlinear output mirror of the laser. The working faces of the crystal were mechanically polished and were not coated. The highly parallel faces (better than 10") caused the sample to operate as a Fabri-Perot etalon. The angle *Ө* (phase matching angle) was adjusted near 46°, and *φ* = 45°. The angle *Ө* is such that the highest efficiency of the second harmonic oscillation is observed at a normal incidence at line 9P(32) *(*λ = 9.06 µm) and at the neighboring line 9P(34) (λ = 9.05 µm) of the isotopic modifications 12C18O2 of carbon dioxide.
The cavity length was 1.1 m. Before the output mirror (AgGaSe2 crystal) there was in the cavity an iris diaphragm (diameter ~8 mm). In the case when the TEA module was filled with the mixture 12C16O2 : 12C16O18O : 12C18O2 : N2 : He = 104 : 32 : 64 : 200: 600 at a total pressure 500 Torr and a nonlinear crystal was used as an output mirror (~60% reflection), the output energy at lines 9P(32) and 9P(34) of carbon dioxide isotope 12C18O2, was ~0.8 J while the peak power was ~4MW. The diameter of the output beam was ~7 mm. The lasing spot had a good spatial distribution. The energy density of the output emission was ~2 J/cm2 (~10 MW/cm2).
Under above conditions the second harmonic generation energy *(E2ω)* was 52 mJ and the peak power *(P2ω)* was ~2 MW. The conversion efficiency reached almost 15%, and at the peak power it was ~50%. The efficiency was calculated by the standard method (*η= E2ω / E<sup>ω</sup> — for* the per pulse energy and *η= P2ω / Pω —* for peak power). *Eω,, Pω* and *E2<sup>ω</sup> , P2<sup>ω</sup> —* are energy parameters of the laser emission in the ranges 9 and 4.5 µm, respectively.
CO2 Lasing on Non-Traditional Bands 129
more attractive for such lasers than for powerful TEA systems to place a nonlinear crystal
In our experiments a monocrystal sample made of AgGaSe2 and having a high optical quality (absorption factor ~0.0l cm-1) was used. The crystal was uncoated and had the rectangular 3.5 x 8.5-mm2 section. The length of the crystal *(l)* was 17mm. Highly parallel (~.4") working faces provided for a possibility to use the crystal as a Fabri-Perot etalon. The phase-matching angle was ~46°. When the incident pumping emission was normal to the crystal, the highest efficiency of second harmonic generation occurred at lines 9P(32) and
With the optimal radius of the spherical mirror and the focus of the coated lens it was possible to decrease the diameter of the laser beam passing through the crystal by more than one order and, therefore, to increase considerably the pumping density. Along with this we have provided for the pumping beam to be quasi-parallel in the nonlinear crystal. To couple out second harmonic (~75%), we used a Brewster window (GaAs). As the second harmonic polarization was orthogonal to pumping emission, its output was much more than that
A characteristic feature of the proposed system consists of application of the absolutely reflecting (with no out coupling) cavity for pumping emission, which even more increases its intensity. Our experiments showed the highest second harmonic peak power output of the cavity had been equal to 2 W. This is more than one order higher than the analogous parameters reached with the same laser operating with typical optical systems. It is important here to use high quality lens coating with a high damage threshold. This
The lidar apparatus complex is shown in Fig. 13. The CO2 laser was either TEA or low pressure as described previously. All optical elements CO2 laser, receiving telescope with the objective, photo-detectors, beam-splitting plates, etc. were fixed on a massive metal base to provide good repeatability of the experimental results. The load carrying base of the lidar complex was placed on a construction equipped with mechanisms rotating the system in horizontal (360°) and vertical (45°) directions for immediate and reliable targeting. The target is made visible using an optical sight (12 x 50). Comparatively low dimensions (1.4 x 0.7 x 1.2 m) and mass (~200 kg) give a possibility for development of a mobile version.
A schematic drawing of the receiver/transmitter is given in Fig. 14. The CO2 laser pulses are output through the beam splitter and are directed into the atmosphere using a transmitting telescope. The emission passed through the atmosphere and trapped by the telescope of the Cassegrain type and with a 250 mm aperture. It consists of two mirrors: front and rear made of sittal with a reflecting aluminum coating protected by corrosion-preventive film. The system uses additionally a ZnSe coated multi-lens objective. The optical configuration of the objective telescope provides for focusing in the focal plane of the detected emission with a 250-mm cross-section down to a diameter ~0.5 mm. After that the detected emission gets on the photodetector which has the sensitive area with a diameter ~1 mm. Optical signals are measured in two channels. Besides the measurement signal, there is a reference one not
intracavity optical system is simply adjusted and provides with high stable output.
into the cavity.
9P(34) which were emitted by the laser.
attainable in a typical conversion system.
**7.5 Laser detection of N2O**
transmitted through the atmosphere.
#### **7.4 Low-pressure longitudinal-discharge 12C18O2 laser with frequency doubling in AgGaSe2 crystal**
There are two types of electric-discharge CO2, lasers which are promising to detect N2O content along a measurement path. First of them — low-pressure longitudinal-discharge excitation — is more efficient for small and average paths *(L* = 0.1 – 2.0 km). As a rule, it must operate with the laser beam reflection by a so-called corner reflector . The second— TEA—is suitable for long paths *(L >* 2 km) when the lidar operates either using the backscattering signal or pulses reflected by a topographic target]. This section considers the CO2 laser intended for gas analysis in small and average paths.
The laser the optical system of which is shown in Fig. 12 is automatic tunable and output stabilized as described early. The active element was a sealed-off gas-discharge tube like the industrial GL-50l (see Fig. 7) with the discharge gap of 1.2 m. Our experiments were performed with the 12C18O2 isotopic forms of carbon dioxide with the low enrichment factor with respect to 18O2 described earlier. It is known that when a gas-discharge tube is fed by a pulsed power supplier, the peak power in optimal regime may go up more than by one order as compared to cw electric pumping. This is of especial importance for lasers used in lidar systems. First, the length of the probing path increases up; secondly, pulse-periodic lasing at an optimal repetition rate (~1 kHz) are suitable for receiving and processing of optical and electrical signals and do not require additional devices for modulation.
Fig. 12. Optical scheme of the 12C18O2 laser with intracavity frequency doubling by nonlinear crystal.
Application of the pulse-periodic regime is of especial importance for second harmonic generation in nonlinear crystals. In this case the benefit in the conversion efficiency considerable at the peak power. Our experiments showed the output peak power of the laser to go up to ~l00 W (almost by one order as compared with cw lasing) at lines 9P(32) and 9P(34) of the 12C18O2 molecule (at each individual line in single-mode operation) when a pulsed power supply is applied.
It is very difficult for longitudinally excited CO2 lasers to obtain efficient frequency conversion in nonlinear crystals, as their output power is several orders lower than the peak value attainable in pulsed TEA CO2-systems. Thus, for the AgGaSe2 crystal, for instance, with a mean length *(l~* 2 cm) the second harmonic conversion efficiency attained by us with cw discharge was a little more than a tenth of a percent (in case of pulse-periodic discharge it was ~1%), which was a record-breaking value for such laser sources. Therefore, it is even
There are two types of electric-discharge CO2, lasers which are promising to detect N2O content along a measurement path. First of them — low-pressure longitudinal-discharge excitation — is more efficient for small and average paths *(L* = 0.1 – 2.0 km). As a rule, it must operate with the laser beam reflection by a so-called corner reflector . The second— TEA—is suitable for long paths *(L >* 2 km) when the lidar operates either using the backscattering signal or pulses reflected by a topographic target]. This section considers the
The laser the optical system of which is shown in Fig. 12 is automatic tunable and output stabilized as described early. The active element was a sealed-off gas-discharge tube like the industrial GL-50l (see Fig. 7) with the discharge gap of 1.2 m. Our experiments were performed with the 12C18O2 isotopic forms of carbon dioxide with the low enrichment factor with respect to 18O2 described earlier. It is known that when a gas-discharge tube is fed by a pulsed power supplier, the peak power in optimal regime may go up more than by one order as compared to cw electric pumping. This is of especial importance for lasers used in lidar systems. First, the length of the probing path increases up; secondly, pulse-periodic lasing at an optimal repetition rate (~1 kHz) are suitable for receiving and processing of
optical and electrical signals and do not require additional devices for modulation.
Fig. 12. Optical scheme of the 12C18O2 laser with intracavity frequency doubling by non-
Application of the pulse-periodic regime is of especial importance for second harmonic generation in nonlinear crystals. In this case the benefit in the conversion efficiency considerable at the peak power. Our experiments showed the output peak power of the laser to go up to ~l00 W (almost by one order as compared with cw lasing) at lines 9P(32) and 9P(34) of the 12C18O2 molecule (at each individual line in single-mode operation) when a
It is very difficult for longitudinally excited CO2 lasers to obtain efficient frequency conversion in nonlinear crystals, as their output power is several orders lower than the peak value attainable in pulsed TEA CO2-systems. Thus, for the AgGaSe2 crystal, for instance, with a mean length *(l~* 2 cm) the second harmonic conversion efficiency attained by us with cw discharge was a little more than a tenth of a percent (in case of pulse-periodic discharge it was ~1%), which was a record-breaking value for such laser sources. Therefore, it is even
**7.4 Low-pressure longitudinal-discharge 12C18O2 laser with frequency doubling in**
CO2 laser intended for gas analysis in small and average paths.
**AgGaSe2 crystal**
linear crystal.
pulsed power supply is applied.
more attractive for such lasers than for powerful TEA systems to place a nonlinear crystal into the cavity.
In our experiments a monocrystal sample made of AgGaSe2 and having a high optical quality (absorption factor ~0.0l cm-1) was used. The crystal was uncoated and had the rectangular 3.5 x 8.5-mm2 section. The length of the crystal *(l)* was 17mm. Highly parallel (~.4") working faces provided for a possibility to use the crystal as a Fabri-Perot etalon. The phase-matching angle was ~46°. When the incident pumping emission was normal to the crystal, the highest efficiency of second harmonic generation occurred at lines 9P(32) and 9P(34) which were emitted by the laser.
With the optimal radius of the spherical mirror and the focus of the coated lens it was possible to decrease the diameter of the laser beam passing through the crystal by more than one order and, therefore, to increase considerably the pumping density. Along with this we have provided for the pumping beam to be quasi-parallel in the nonlinear crystal. To couple out second harmonic (~75%), we used a Brewster window (GaAs). As the second harmonic polarization was orthogonal to pumping emission, its output was much more than that attainable in a typical conversion system.
A characteristic feature of the proposed system consists of application of the absolutely reflecting (with no out coupling) cavity for pumping emission, which even more increases its intensity. Our experiments showed the highest second harmonic peak power output of the cavity had been equal to 2 W. This is more than one order higher than the analogous parameters reached with the same laser operating with typical optical systems. It is important here to use high quality lens coating with a high damage threshold. This intracavity optical system is simply adjusted and provides with high stable output.
#### **7.5 Laser detection of N2O**
The lidar apparatus complex is shown in Fig. 13. The CO2 laser was either TEA or low pressure as described previously. All optical elements CO2 laser, receiving telescope with the objective, photo-detectors, beam-splitting plates, etc. were fixed on a massive metal base to provide good repeatability of the experimental results. The load carrying base of the lidar complex was placed on a construction equipped with mechanisms rotating the system in horizontal (360°) and vertical (45°) directions for immediate and reliable targeting. The target is made visible using an optical sight (12 x 50). Comparatively low dimensions (1.4 x 0.7 x 1.2 m) and mass (~200 kg) give a possibility for development of a mobile version.
A schematic drawing of the receiver/transmitter is given in Fig. 14. The CO2 laser pulses are output through the beam splitter and are directed into the atmosphere using a transmitting telescope. The emission passed through the atmosphere and trapped by the telescope of the Cassegrain type and with a 250 mm aperture. It consists of two mirrors: front and rear made of sittal with a reflecting aluminum coating protected by corrosion-preventive film. The system uses additionally a ZnSe coated multi-lens objective. The optical configuration of the objective telescope provides for focusing in the focal plane of the detected emission with a 250-mm cross-section down to a diameter ~0.5 mm. After that the detected emission gets on the photodetector which has the sensitive area with a diameter ~1 mm. Optical signals are measured in two channels. Besides the measurement signal, there is a reference one not transmitted through the atmosphere.
CO2 Lasing on Non-Traditional Bands 131
1 — output mirror of TEA CO2 laser; 2 — BaF2 beam-splitter; 3 — transmitting telescope; 4 — receiving telescope; 5, 6 — mirrors; 7 — receiving objective (ZnSe); 8 — photodetector (CdHgTe) of measurement channel; 9 — lens (ZnSe) of reference channel; 10 — photodetector (Ge:Au) of reference channel; 11 —
<sup>12</sup><sup>13</sup>
8 6 5
11 7
For monochromatic radiation propagating in homogeneous medium containing several
×
where *I0* and *Iλ,* are the intensities of the emission with the wavelength λ before and after its passage through a gas layer with length *L, τλ* = *L∑ik <sup>λ</sup>ic i* is the optical thickness, *k <sup>λ</sup>i* and *c i* are accordingly the absorption coefficient at the wavelength *λi* and the concentration of the *i*th
The analysis of the optical characteristics of the detected gases was performed using the differential absorption technique that is in wide use now for laser atmospheric probing. The probing is made at an on/off pair of laser emission lines. "On" line has the maximally possible resonance absorption, and "off"—minimal. The two-frequency differential absorption technique takes useful information only in the resonance absorption by a gas in question. The effects of such factors as water vapour continuum, non-resonance molecular and aerosol absorption, dust, smog, etc. scattering, atmospheric turbulence will be virtually absent due to the comparatively weak monotonic spectral dependence when "on" and "oil"
Prior to measuring N2O, the lidar complex was tested and calibrated for CO and H2O measurements using CO2 lasers both TEA and low pressure operating on the ordinary
*exp(-τλ)* , (6)
4
3
Fig. 14. Schematic drawing of the receiver/transmitter of the lidar complex.
absorbing gases, the transmission *T<sup>λ</sup>* is described by the Buger law :
2
10
9
iris; 12 — ADC; 13 — personal computer
1
lines are located near each other.
absorbing gas.
*Tλ =Iλ / I0*
1 — output mirror of CO2-laser; 2 — CO2-laser; 3 — horizontal rotation unit; 4 — vertical rotation unit; 5 — metallic base; 6 — receiver/transmitter
Fig. 13. Lidar complex.
The reference signal is produced using a 1-mm-thickness beam-splitter made of BaF2. Such a thin plate located at an angle ~50° introduces minimum loss for polarized radiation of the TEA CO2 laser. The loss in the reflection by both faces was no more than 2%. At the same time, only small portion of the emission would be reflected to provide the reference channel. Passing the focusing lens, the reflected portion comes to the sensitive area of the photodetector of the reference channel so organized that this allows automatic account of possible instability of the laser output by normalizing the measurement signal on the reference one, which essentially increases the measurement accuracy and reliability.
As photodetectors nitrogen-cooled photoresistors based on CdHgTe, InSb or germanium doped with gold (Ge:Au) were used. To increase the sensitivity in the measurement channel we used an amplifier. The photodetectors were placed on alignment units. This provided accurate adjustment of the sensitive area of the detector with respect to incident emission. Photodetector signals either arrive to a two-channel ADC or digital oscilloscope and then, via an interface unit, into a computer.
2
3
5
1 — output mirror of CO2-laser; 2 — CO2-laser; 3 — horizontal rotation unit; 4 — vertical rotation unit; 5
The reference signal is produced using a 1-mm-thickness beam-splitter made of BaF2. Such a thin plate located at an angle ~50° introduces minimum loss for polarized radiation of the TEA CO2 laser. The loss in the reflection by both faces was no more than 2%. At the same time, only small portion of the emission would be reflected to provide the reference channel. Passing the focusing lens, the reflected portion comes to the sensitive area of the photodetector of the reference channel so organized that this allows automatic account of possible instability of the laser output by normalizing the measurement signal on the
As photodetectors nitrogen-cooled photoresistors based on CdHgTe, InSb or germanium doped with gold (Ge:Au) were used. To increase the sensitivity in the measurement channel we used an amplifier. The photodetectors were placed on alignment units. This provided accurate adjustment of the sensitive area of the detector with respect to incident emission. Photodetector signals either arrive to a two-channel ADC or digital oscilloscope and then,
reference one, which essentially increases the measurement accuracy and reliability.
— metallic base; 6 — receiver/transmitter
1
4
via an interface unit, into a computer.
Fig. 13. Lidar complex.
1 — output mirror of TEA CO2 laser; 2 — BaF2 beam-splitter; 3 — transmitting telescope; 4 — receiving telescope; 5, 6 — mirrors; 7 — receiving objective (ZnSe); 8 — photodetector (CdHgTe) of measurement channel; 9 — lens (ZnSe) of reference channel; 10 — photodetector (Ge:Au) of reference channel; 11 iris; 12 — ADC; 13 — personal computer
Fig. 14. Schematic drawing of the receiver/transmitter of the lidar complex.
For monochromatic radiation propagating in homogeneous medium containing several absorbing gases, the transmission *T<sup>λ</sup>* is described by the Buger law :
$$\mathbf{T}\_{\lambda} = \mathbf{I}\_{\lambda} / I\_0 \times \exp(\mathbf{-}\mathbf{r}\lambda) \,. \tag{6}$$
where *I0* and *Iλ,* are the intensities of the emission with the wavelength λ before and after its passage through a gas layer with length *L, τλ* = *L∑ik <sup>λ</sup>ic i* is the optical thickness, *k <sup>λ</sup>i* and *c i* are accordingly the absorption coefficient at the wavelength *λi* and the concentration of the *i*th absorbing gas.
The analysis of the optical characteristics of the detected gases was performed using the differential absorption technique that is in wide use now for laser atmospheric probing. The probing is made at an on/off pair of laser emission lines. "On" line has the maximally possible resonance absorption, and "off"—minimal. The two-frequency differential absorption technique takes useful information only in the resonance absorption by a gas in question. The effects of such factors as water vapour continuum, non-resonance molecular and aerosol absorption, dust, smog, etc. scattering, atmospheric turbulence will be virtually absent due to the comparatively weak monotonic spectral dependence when "on" and "oil" lines are located near each other.
Prior to measuring N2O, the lidar complex was tested and calibrated for CO and H2O measurements using CO2 lasers both TEA and low pressure operating on the ordinary
CO2 Lasing on Non-Traditional Bands 133
Then, from the known concentration of H2O determined independently (for instance, using psychometric devices), it will be possible to calibrate the technique, i.e. obtain the evidence that the results of the laser atmospheric probing are reliable. We used the line 9P(22) that was almost fully absorbed by H2O as a reference one to check laser tuning at the selected lines. Based on such an original technique we have measured carbon dioxide and water vapour near a highway at a height about 10 m over the cart!, surface. The laser emission was reflected by a
The carbon monoxide concentration measured in autumn (500-600 p.m.) has varied from 0.8 to 1.2 ppm. The measured mean concentration of CO was ~1 ppm. The measurement
N2O measurements were performed with the same path *(2L =* 0.2 km) using tile lowpressure 12C18O2 laser with frequency doubling by a nonlinear crystal. As in the previous case, the emission was reflected by a metallized plywood sheet. Fig. 16 shows a calculated spectrum of the atmospheric gases absorption in the range of 4.5—4.55 μm. We select this spectral range as there are some doubled frequencies of efficient tines of the 12C18O2 laser. It is reasonable to select frequency doubled R(32) or R(40) as "on' line, and doubled frequencies of the neighboring R(34) or R(38)—as "off" line. It is important that the indicated lines do not coincide with the absorption lines of background gases H2O and CO which are always present in the atmosphere. In this way we carried out a number of measurements of N2O concentration along a researched path at various seasons and times of day. The analysis of the received data has shown that N2O content in the atmosphere varied considerably, and it is mainly caused by intensity of the transport movement. For example, our experiments performed in autumn in different times during a few days have shown that the N2O concentration in the path was from 0.35 to 0.5 ppm. The measurement accuracy is
We also have measured N2O for a longer path *(2L =* 1.4 km) using the frequency-doubling TEA 12C18O2 laser described earlier. In this case, the laser beam was reflected by a building wall. The averaged content of N2O was in a good agreement with the value obtained for the
The experimental investigations and the calculations carried out have proved conclusively the promising character of the technique developed for the determination of low nitrous oxide concentrations. The technique is based on the use of 12C18O2, lasers with effective
The research carried out has given a reliable technique for laser atmospheric probing of nitrous oxide and effective laser systems to implement this procedure. It is of importance that the path probing is made with a powerful molecular gas laser. Such lasers have narrow emission lines and high stability of spectral and energy output. These characteristics are achieved, as distinct from semiconductor and solid state lasers, naturally without any additional devices. Thus the laser system is simplified and the measurement accuracy increases. The 12C18O2 laser system with effective nonlinear frequency-doubling is much
A reliable procedure or remote high-accuracy laser detection of N2O as one of the principal destroyers of the protective ozone layer of the Earth has been developed. The procedure is based on using a CO2 laser system emitting efficiently in the ~4.5 μm range. In this case
promising for global network of lidar stations for atmosphere monitoring.
plywood sheet painted with a metallic color. The length of probing was 2L *=* 0.2 km.
accuracy determined from H2O calibrations was ~5%.
estimated to be better than 15%.
frequency doubling in nonlinear crystals.
shorter path.
12C16O2 with frequency doubling by a nonlinear AgGaSe2 crystal. Measurements of the CO and H2O concentrations also allowed us to account these gases as background for the N2O measurements.
The theoretical analysis of the absorption lines of CO and background gases (particularly, H2O) for a path with 2L = 200 m has shown that it is reasonable to select "on" line among the doubled frequencies of the 12C16O2, laser such as 9R(30) (at λ = 4.6099 µm the absorption is 50%/ppm). 9R(18) (λ= 4.6412 µm—45%/ppm) and 9P(24) (λ*=* 4.7931 µm—37%/ppm). Accordingly. the most suitable "off" line belongs to the same laser and are 9R(28) (λ*=* 4.6148 µm). 9R(20) (λ *=* 4.6357 µm) and 9P(26) (λ = 4.8018 µm) at which the absorption by carbon dioxide and background gases is virtually absent.
Fig. 15. Absorption spectrum of CO and background gases (H2O and CO2) in the range of 4.78 - 4.82 μm. The conditions: path length (2L)=200 m, P=1 atm, Т=287 К, gas contents: CO – 1 ppm, H2O – 10000 ppm, CO2 – 330 ppm.
Fig. 15 shows a calculated absorption spectrum of the atmospheric gases in the range of our investigations. We select this spectral range due to the following advantages. One of the four selected laser lines (9P(22)— 9P(28)), namely 9P(24) ("on line"), coincides well with the absorption peak of CO. Two of them (9P(22) and 9P(28)) coincides sufficiently well with the absorption lines of H2O, while 9P(26) demonstrating no absorption of both CO and H2O is quite suitable as the "off" line. In addition, there is no noticeable absorption by other atmospheric gases (CO2, for instance) at these lines. Then, carrying out consecutive measurements at these lines, it will be possible to measure concentrations of CO and H2O.
12C16O2 with frequency doubling by a nonlinear AgGaSe2 crystal. Measurements of the CO and H2O concentrations also allowed us to account these gases as background for the N2O
The theoretical analysis of the absorption lines of CO and background gases (particularly, H2O) for a path with 2L = 200 m has shown that it is reasonable to select "on" line among the doubled frequencies of the 12C16O2, laser such as 9R(30) (at λ = 4.6099 µm the absorption is 50%/ppm). 9R(18) (λ= 4.6412 µm—45%/ppm) and 9P(24) (λ*=* 4.7931 µm—37%/ppm). Accordingly. the most suitable "off" line belongs to the same laser and are 9R(28) (λ*=* 4.6148 µm). 9R(20) (λ *=* 4.6357 µm) and 9P(26) (λ = 4.8018 µm) at which the absorption by carbon
Fig. 15. Absorption spectrum of CO and background gases (H2O and CO2) in the range of 4.78 - 4.82 μm. The conditions: path length (2L)=200 m, P=1 atm, Т=287 К, gas contents:
Fig. 15 shows a calculated absorption spectrum of the atmospheric gases in the range of our investigations. We select this spectral range due to the following advantages. One of the four selected laser lines (9P(22)— 9P(28)), namely 9P(24) ("on line"), coincides well with the absorption peak of CO. Two of them (9P(22) and 9P(28)) coincides sufficiently well with the absorption lines of H2O, while 9P(26) demonstrating no absorption of both CO and H2O is quite suitable as the "off" line. In addition, there is no noticeable absorption by other atmospheric gases (CO2, for instance) at these lines. Then, carrying out consecutive measurements at these lines, it will be possible to measure concentrations of CO and H2O.
measurements.
dioxide and background gases is virtually absent.
CO – 1 ppm, H2O – 10000 ppm, CO2 – 330 ppm.
Then, from the known concentration of H2O determined independently (for instance, using psychometric devices), it will be possible to calibrate the technique, i.e. obtain the evidence that the results of the laser atmospheric probing are reliable. We used the line 9P(22) that was almost fully absorbed by H2O as a reference one to check laser tuning at the selected lines. Based on such an original technique we have measured carbon dioxide and water vapour near a highway at a height about 10 m over the cart!, surface. The laser emission was reflected by a plywood sheet painted with a metallic color. The length of probing was 2L *=* 0.2 km.
The carbon monoxide concentration measured in autumn (500-600 p.m.) has varied from 0.8 to 1.2 ppm. The measured mean concentration of CO was ~1 ppm. The measurement accuracy determined from H2O calibrations was ~5%.
N2O measurements were performed with the same path *(2L =* 0.2 km) using tile lowpressure 12C18O2 laser with frequency doubling by a nonlinear crystal. As in the previous case, the emission was reflected by a metallized plywood sheet. Fig. 16 shows a calculated spectrum of the atmospheric gases absorption in the range of 4.5—4.55 μm. We select this spectral range as there are some doubled frequencies of efficient tines of the 12C18O2 laser. It is reasonable to select frequency doubled R(32) or R(40) as "on' line, and doubled frequencies of the neighboring R(34) or R(38)—as "off" line. It is important that the indicated lines do not coincide with the absorption lines of background gases H2O and CO which are always present in the atmosphere. In this way we carried out a number of measurements of N2O concentration along a researched path at various seasons and times of day. The analysis of the received data has shown that N2O content in the atmosphere varied considerably, and it is mainly caused by intensity of the transport movement. For example, our experiments performed in autumn in different times during a few days have shown that the N2O concentration in the path was from 0.35 to 0.5 ppm. The measurement accuracy is estimated to be better than 15%.
We also have measured N2O for a longer path *(2L =* 1.4 km) using the frequency-doubling TEA 12C18O2 laser described earlier. In this case, the laser beam was reflected by a building wall. The averaged content of N2O was in a good agreement with the value obtained for the shorter path.
The experimental investigations and the calculations carried out have proved conclusively the promising character of the technique developed for the determination of low nitrous oxide concentrations. The technique is based on the use of 12C18O2, lasers with effective frequency doubling in nonlinear crystals.
The research carried out has given a reliable technique for laser atmospheric probing of nitrous oxide and effective laser systems to implement this procedure. It is of importance that the path probing is made with a powerful molecular gas laser. Such lasers have narrow emission lines and high stability of spectral and energy output. These characteristics are achieved, as distinct from semiconductor and solid state lasers, naturally without any additional devices. Thus the laser system is simplified and the measurement accuracy increases. The 12C18O2 laser system with effective nonlinear frequency-doubling is much promising for global network of lidar stations for atmosphere monitoring.
A reliable procedure or remote high-accuracy laser detection of N2O as one of the principal destroyers of the protective ozone layer of the Earth has been developed. The procedure is based on using a CO2 laser system emitting efficiently in the ~4.5 μm range. In this case
CO2 Lasing on Non-Traditional Bands 135
1110; (0221-1220, 0201-1200…) bands of CO2 molecule. To test the validity of the method, the experiment realization has been done for a low pressure CO2 laser with cw longitudinal discharge, that can oscillate on the lines of regular and nonregular lines. The good
We examined what kind of the small gain and the output energy can be attained in the TEA CO2 laser on the 16(14) µm 0201(1001)-0111 transitions. On tile basis of the experimentally determined vibrational temperatures *T3* and *T2* we calculated the small gain. The calculations shown that the small gain in the 0201(1001)-0111 band can attain a significant value (>1 m-1). The necessary conditions for the effective lasing have been determined. It is shown that in optimum conditions the output energy can reach *1.3* J/l at the peak power 5
The experimental investigations and the calculations carried out have proved conclusively the promising character of the technique developed for the determination of low nitrous oxide concentrations. The technique is based on the use of 12C18O2, lasers with effective frequency doubling in nonlinear crystals. The research carried out has given a reliable technique for laser atmospheric probing of nitrous oxide and effective laser systems to implement this procedure. It is of importance that the path probing is made with a powerful molecular gas laser. Such lasers have narrow emission lines and high stability of spectral and energy output. They were much promising for global network of lidar stations for
Aleinikov V.S. and Masychev V.I. (1990). *Carbon monoxide Lasers*, Radio Svyaz, Moscow, (in
Bertel I.M., Petukhov V.O., Stepanov B.I., Trushin S.A., Churakov V.V. (1982). Investigation
Bertel, I.M.; Petukhov V.V. et al. (1983) Diagnostics of active mediums of CO2 lasers with the
Bertel, I.M.; Petukhov, V.O.; Trushin, S.A. and Churakov, V.V. (1983). Study of the Gain and
Churakov, V.V.; Petukhov, V.O. and Tochitsky S.Ya. (1987). Two-color TEA CO2 laser oscillation on the lines of regular and hot hands. *Appl. Phis. B,* v .42, pp. 245-249. Crutzen, P.J. (1996). My life with O3, N2O, and other compounds, *Nobel Lecture,* Angew.
Gordiets B.F., Osipov A.I., and Shelepin L.A., (1980) Kinetic *Processes in Gases and Molecular*
Gorobets, V.A,; Petukhov, V.O.; Tochitsky S.Ya. and V. V. Churakov. (1992) Method of
Gorobets, V.A.; Petikhov, V.O.; Tochitsky S.Ya. and Churakov V.V. (1995). Transversely
Tuning a CO2 laser on a Chosen Lasing Line, *Author's Certificate 1771367 SSSR*; MKI
excited CO2 lidar laser tunable over lines of regular and nontraditional bands.
*Preprint of the IF AN BSSR* No. 289*,* (in Russian), Minsk, Belarus
of the vibrational temperature kinetics in a TEA CO2 laser*. Sov. J Quantum Electron.*,
use of nontraditional transitions of molecule CO2, *Nonequilibrium Processes in Gas*
the Conditions of Efficient Lasing on Lines of the Hot Band in a TEA CO2 laser,
agreement between calculation and experiment data has been observed
MW and at the full efficiency of 2 *%.*
atmosphere monitoring.
Russian)
H 01 S 3/22
v. 12, No 8. pp. 1044-1049
*Dynamics*, (in Russian), Minsk, Belarus
Chem. Int. Ed. Engl. 35 1758—1777.
*Gas Lasers,* (in Russian), Moscow, Russia
*Quantum Electronics,* v. 25, No *5,* pp. 489-493.
**9. References**
lasing from isotopic modification *12C18*O2 of carbon dioxide with its subsequent frequency doubling by a nonlinear crystal is used. With the object of reducing the price the composition of the active medium (both for TEA laser and low-pressure longitudinaldischarge excitation laser) has been optimized. New high-efficiency intracavity frequency doubling schemes based on nonlinear AgGaSe2 crystals have been developed for CO2 lasers of both types. Low concentrations of N2O and concentrations of the principal background gases CO and N2O have been measured under real atmosphere conditions with the aid of the lidar complex built around these lasers.
Fig. 16. Absorption spectrum of N2O and background gases (CO and H2O). The conditions: path length (2L)=0.2 km, P=1 atm, T=287 K, gas contens: N2O — 0.4 ppm, H2O — 10 000 ppm, CO — 1 ppm
#### **8. Conclusion**
Optimization of the gas content, pressure, discharge current and the cavity of a low-pressure laser with longitudinal discharge were carried out. Thus, after the above improvements the commercially available sealed-off laser oscillates on more than 30 lines of the P-branch of the 0111- 1110 band in the 10.9—11.3 µm range with output power no less than 0.5 W. On strong lines [P(16)—P(26)] output power was ~6W at efficiency ~3% which makes up ~ 40% of analogous laser parameters in the case of oscillation on the lines of regular bands 0001-1000 (0200) under optimum conditions.
The peak power on the strongest lines of the new bands (10°l-20°0(0400) with a lasing pulse was 30 W. The average output power reached 0.2 W. Lasing was achieved at a number of new transitions. More than 25 new lasing lines with λ = 11.1–11.4 μm, belonging to all the aforementioned bands, were observed in the spectral range studied.
The method of output optimization of cw CO2 lasers has been developed. The method is based on vibrational and translational temperatures determination by gain measurements on the ro-vibrational lines of regular (0001-1000, 0200) and nonregular (0002-1001, 0201; 0111-
lasing from isotopic modification *12C18*O2 of carbon dioxide with its subsequent frequency doubling by a nonlinear crystal is used. With the object of reducing the price the composition of the active medium (both for TEA laser and low-pressure longitudinaldischarge excitation laser) has been optimized. New high-efficiency intracavity frequency doubling schemes based on nonlinear AgGaSe2 crystals have been developed for CO2 lasers of both types. Low concentrations of N2O and concentrations of the principal background gases CO and N2O have been measured under real atmosphere conditions with the aid of
Fig. 16. Absorption spectrum of N2O and background gases (CO and H2O). The conditions:
Optimization of the gas content, pressure, discharge current and the cavity of a low-pressure laser with longitudinal discharge were carried out. Thus, after the above improvements the commercially available sealed-off laser oscillates on more than 30 lines of the P-branch of the 0111- 1110 band in the 10.9—11.3 µm range with output power no less than 0.5 W. On strong lines [P(16)—P(26)] output power was ~6W at efficiency ~3% which makes up ~ 40% of analogous laser parameters in the case of oscillation on the lines of regular bands 0001-1000
The peak power on the strongest lines of the new bands (10°l-20°0(0400) with a lasing pulse was 30 W. The average output power reached 0.2 W. Lasing was achieved at a number of new transitions. More than 25 new lasing lines with λ = 11.1–11.4 μm, belonging to all the
The method of output optimization of cw CO2 lasers has been developed. The method is based on vibrational and translational temperatures determination by gain measurements on the ro-vibrational lines of regular (0001-1000, 0200) and nonregular (0002-1001, 0201; 0111-
aforementioned bands, were observed in the spectral range studied.
path length (2L)=0.2 km, P=1 atm, T=287 K, gas contens: N2O — 0.4 ppm, H2O —
the lidar complex built around these lasers.
10 000 ppm, CO — 1 ppm
(0200) under optimum conditions.
**8. Conclusion**
1110; (0221-1220, 0201-1200…) bands of CO2 molecule. To test the validity of the method, the experiment realization has been done for a low pressure CO2 laser with cw longitudinal discharge, that can oscillate on the lines of regular and nonregular lines. The good agreement between calculation and experiment data has been observed
We examined what kind of the small gain and the output energy can be attained in the TEA CO2 laser on the 16(14) µm 0201(1001)-0111 transitions. On tile basis of the experimentally determined vibrational temperatures *T3* and *T2* we calculated the small gain. The calculations shown that the small gain in the 0201(1001)-0111 band can attain a significant value (>1 m-1). The necessary conditions for the effective lasing have been determined. It is shown that in optimum conditions the output energy can reach *1.3* J/l at the peak power 5 MW and at the full efficiency of 2 *%.*
The experimental investigations and the calculations carried out have proved conclusively the promising character of the technique developed for the determination of low nitrous oxide concentrations. The technique is based on the use of 12C18O2, lasers with effective frequency doubling in nonlinear crystals. The research carried out has given a reliable technique for laser atmospheric probing of nitrous oxide and effective laser systems to implement this procedure. It is of importance that the path probing is made with a powerful molecular gas laser. Such lasers have narrow emission lines and high stability of spectral and energy output. They were much promising for global network of lidar stations for atmosphere monitoring.
#### **9. References**
**Part 2**
**New Systems**
## **Part 2**
**New Systems**
CO2 136 Laser – Optimisation and Application
Gorobets, V.A.; Petukhov, V.O. and Churakov V.V. (1990) Optimization of the Output
Petukhov, V.O. et al. (1985). Fizicheskaya Gazodinamika: Eksperimental 'noe
Smith, K. and Thompson R. (1981). *Numerical Modeling of Gas Lasers,* (in Russian), Moscow,
Wexler, B.E.; Manuccia T.J. and Waynant R. (1987). CW and improved pulsed operation of the 14 µm and 16 µm CO2 lasers, *Appl. Phys. Lett,* v.31, No 11, pp. 730-732. Witteman, W. (1987). The CO2 Laser, *Springer-Verlag*, Berlin, Heidelberg, New York, London,
*AN BSSR No. 608*, (in Russian), Minsk, Belarus
Russia
Paris, Tokyo
*Diagnostic).* IHMT AS BSSR, (in Russian), Minsk, Belarus
Power of a Continuous CO2 Lasing on Unconventional Transitions, *Preprint of the IF*
Modelirovanie, *Diagnostika (Physical Gas Dynamics: Experimental Modeling and*
**4**
*USA*
**Ultrashort Pulses**
*Brookhaven National Laboratory*
Mikhail N. Polyanskiy and Marcus Babzien
Ultrashort pulses usually are defined as those lasting less than a nanosecond. Low-energetic ultrashort pulses can be used, for instance, as a probe for studying the dynamics of ultrafast processes. Amplifying these pulses can deliver extremely high peak power, allowing applications such as laser particle acceleration, or γ-ray generation via Compton scattering on relativistic electrons. Existing laser systems can provide pulses as brief as hundreds of attoseconds (10-18 s) and as powerful as tens of petawatts (1015 W); more advanced ones are being constructed or are planned (Corkum & Krausz, 2007,
Virtually all modern ultrashort-pulse lasers are based on solid-state technology and usually operate at ~1 µm wavelength. Nevertheless, these applications relying not only on the ultrashort duration or the extreme power of the laser pulse, but also on its wavelength, leave a niche for laser systems utilizing different types of active medium. The 10-micron
To demonstrate the potential of ultrashort, mid-IR pulses, we consider their employment for laser ion acceleration, presently one of the main drivers for developing ultrashort-pulse CO2 lasers (Palmer et al., 2011, Norreys, 2011). The motivation underlying the search for alternatives to conventional accelerators, wherein a system of electrodes and magnets creates the accelerating field, is the need to reduce the size and the cost of these devices. Acceleration in the electromagnetic field of a laser beam is a promising alternative for traditional acceleration schemes. In laser ion acceleration, an intense laser pulse focused on a target first ionizes it and then accelerates the charged particles from the resulting plasma. Usually this target is a metal foil, and a ~1 µm, multi-TW peak power, solid-state laser provides the ionizing/accelerating field. A very efficient acceleration regime called *Radiation Pressure Acceleration* (RPA) featuring a narrow energy-spectrum of the accelerated ions is reached when laser pulse interacts with plasma having a near-critical density (Esirkepov et al., 2004). The following formula defines critical plasma density, *Nc* (the density at which
> -3 <sup>21</sup> [cm ] 1.115 10 [μm] *<sup>c</sup> <sup>n</sup> <sup>N</sup>*
≈ ×
2
, (1)
λ
wavelength of the CO2 laser particularly is beneficial for some usages.
**1. Introduction**
Korzhimanov et al., 2011).
plasma becomes opaque):
**1.1 Niche for ultrashort-pulse CO2 lasers**
### **Ultrashort Pulses**
Mikhail N. Polyanskiy and Marcus Babzien *Brookhaven National Laboratory USA*
#### **1. Introduction**
#### **1.1 Niche for ultrashort-pulse CO2 lasers**
Ultrashort pulses usually are defined as those lasting less than a nanosecond. Low-energetic ultrashort pulses can be used, for instance, as a probe for studying the dynamics of ultrafast processes. Amplifying these pulses can deliver extremely high peak power, allowing applications such as laser particle acceleration, or γ-ray generation via Compton scattering on relativistic electrons. Existing laser systems can provide pulses as brief as hundreds of attoseconds (10-18 s) and as powerful as tens of petawatts (1015 W); more advanced ones are being constructed or are planned (Corkum & Krausz, 2007, Korzhimanov et al., 2011).
Virtually all modern ultrashort-pulse lasers are based on solid-state technology and usually operate at ~1 µm wavelength. Nevertheless, these applications relying not only on the ultrashort duration or the extreme power of the laser pulse, but also on its wavelength, leave a niche for laser systems utilizing different types of active medium. The 10-micron wavelength of the CO2 laser particularly is beneficial for some usages.
To demonstrate the potential of ultrashort, mid-IR pulses, we consider their employment for laser ion acceleration, presently one of the main drivers for developing ultrashort-pulse CO2 lasers (Palmer et al., 2011, Norreys, 2011). The motivation underlying the search for alternatives to conventional accelerators, wherein a system of electrodes and magnets creates the accelerating field, is the need to reduce the size and the cost of these devices. Acceleration in the electromagnetic field of a laser beam is a promising alternative for traditional acceleration schemes. In laser ion acceleration, an intense laser pulse focused on a target first ionizes it and then accelerates the charged particles from the resulting plasma. Usually this target is a metal foil, and a ~1 µm, multi-TW peak power, solid-state laser provides the ionizing/accelerating field. A very efficient acceleration regime called *Radiation Pressure Acceleration* (RPA) featuring a narrow energy-spectrum of the accelerated ions is reached when laser pulse interacts with plasma having a near-critical density (Esirkepov et al., 2004). The following formula defines critical plasma density, *Nc* (the density at which plasma becomes opaque):
$$N\_c \text{[cm}^{-3}] = 1.115 \times 10^{21} \left(\frac{n}{\mathcal{A} \text{[\mu m]}}\right)^2,\tag{1}$$
Ultrashort Pulses 141
where *P* is the total gas pressure, and *Ψx* is the relative concentration of the component *x*. Eq. (3) yields *Δνpressure*≈3.8 GHz at 1 bar of a typical mixture [CO2]:[N2]:[He]=1:1:8. Although direct substituting this bandwidth in Eq. (2) gives *τp*≈120 ps, in reality the pulse's spectrum is narrower than the gain bandwidth, and, correspondingly, the pulse's duration is longer. The latter fact is comprehensible by realizing that amplification is the strongest in the center of the laser line, and thus, upon amplification, the ratio between the intensity at the central frequency and that at the wing of the gain-spectrum line increases, thus narrowing the pulse's spectrum. Consequently, the minimum achievable duration of the pulse for an atmospheric-pressure CO2 laser is about 1 ns (Abrams & Wood, 1971). Pressure can be increased to broaden the gain bandwidth, and thus somewhat reduce the pulse's duration. However, the complete overlap of rotational lines separated by 55 GHz (P-) or 40 GHz (Rbranches) that would allow using an entire rotational branch, does not occur below 20- 25 bar; this is not feasible in discharge-pumped lasers having a ~10 bar practical pressure
Fig. 2. Simulated normalized gain spectra at different gas pressures. 10P, 10R, 9P, and 9R: Rotational P- and R-branches of the two vibrational transitions supporting lasing at ~10 µm
Another important consequence of the rotational structure of the CO2 gain spectrum relates to the amplification of the ultrashort pulses. The spectrum of a few-picosecond-long pulse, according to Eq. (2), overlaps several rotational lines of the gain spectrum. Hence, upon amplification, the pulse's spectrum acquires the corresponding periodic structure. In the
limit.
and ~9 µm respectively.
[ ] [ ] ( ) Δ *pressure* GHz bar 5.79 4.25 3.55 *CO*2 2 *<sup>N</sup> He ν* ≈ ⋅ Ψ + Ψ+ Ψ *P* , (3)
where *n* is the refractive index, and *λ* the laser wavelength. According to the Eq. (1), *Nc* is ~1021 cm-3 for the 1-µm lasers, and ~1019 cm-3 for the 10-µm lasers. These densities are much lower than that of solid materials. The critical density at 10 µm is comparable with the density of gases (~2.7×1019 cm-3 at 1 bar); thus, it readily is achievable for the CO2 laser's wavelength when the gas jet is used as a target. On the other hand, realizing the criticaldensity jet for 1-µm solid-state lasers is challenging. Another advantage of the longer wavelength for ion acceleration in the RPA regime is the λ2 scaling of the ponderomotive force, implying that a 100x lower intensity of the CO2 laser field will suffice for reaching a given ion energy compared with a solid-state laser.
Yet another simple consideration favors CO2 lasers for certain applications: A 10-µm laser pulse carries 10x more photons than a 1-µm pulse of the same energy. Photon density is important for applications such as γ-ray generation by Compton scattering on relativistic electrons (Yakimenko & Pogorelsky, 2006).
#### **1.2 Challenge of high peak power**
The minimum achievable duration of the pulse is defined by the bandwidth of the gain spectrum. In the simplest case of the absence of chirping (frequency variation with time), the spectrum of a pulse is the Fourier transform of its temporal profile. The pulse then is called transform-limited, and its duration, *τp*, is inversely proportional to its spectral width, *Δνp*. The following expression is valid for a transform-limited Gaussian pulse (Paschotta, 2008):
$$
\sigma\_p = \frac{0.44}{\Delta \nu\_p} \,\prime\tag{2}
$$
where *τp* and *Δνp* are defined as full-width at half-maximum (FWHM). The duration of a chirped pulse is longer than that of a transform-limited pulse of the same shape and spectral width. Fig. 1 shows the spectra of several femto- and pico-second transform-limited Gaussian pulses. Fig. 2 compares them with the gain spectrum of a typical CO2 laser mixture, clearly revealing the challenge of generating and amplifying ultrashort pulses in a CO2 laser.
Fig. 1. Spectra of transform-limited Gaussian pulses. Durations are full-width at halfmaximum (FWHM).
At low pressures (less than a few bars), the gain spectrum consists of individual rotational lines whose bandwidth (FWHM) is given by the following expression (Brimacombe & Reid, 1983):
where *n* is the refractive index, and *λ* the laser wavelength. According to the Eq. (1), *Nc* is ~1021 cm-3 for the 1-µm lasers, and ~1019 cm-3 for the 10-µm lasers. These densities are much lower than that of solid materials. The critical density at 10 µm is comparable with the density of gases (~2.7×1019 cm-3 at 1 bar); thus, it readily is achievable for the CO2 laser's wavelength when the gas jet is used as a target. On the other hand, realizing the criticaldensity jet for 1-µm solid-state lasers is challenging. Another advantage of the longer wavelength for ion acceleration in the RPA regime is the λ2 scaling of the ponderomotive force, implying that a 100x lower intensity of the CO2 laser field will suffice for reaching a
Yet another simple consideration favors CO2 lasers for certain applications: A 10-µm laser pulse carries 10x more photons than a 1-µm pulse of the same energy. Photon density is important for applications such as γ-ray generation by Compton scattering on relativistic
The minimum achievable duration of the pulse is defined by the bandwidth of the gain spectrum. In the simplest case of the absence of chirping (frequency variation with time), the spectrum of a pulse is the Fourier transform of its temporal profile. The pulse then is called transform-limited, and its duration, *τp*, is inversely proportional to its spectral width, *Δνp*. The following expression is valid for a transform-limited Gaussian pulse (Paschotta, 2008):
0.44
ν
where *τp* and *Δνp* are defined as full-width at half-maximum (FWHM). The duration of a chirped pulse is longer than that of a transform-limited pulse of the same shape and spectral width. Fig. 1 shows the spectra of several femto- and pico-second transform-limited Gaussian pulses. Fig. 2 compares them with the gain spectrum of a typical CO2 laser mixture, clearly revealing the challenge of generating and amplifying ultrashort pulses in a
*p*
<sup>≈</sup> <sup>Δ</sup> , (2)
*p*
Fig. 1. Spectra of transform-limited Gaussian pulses. Durations are full-width at half-
At low pressures (less than a few bars), the gain spectrum consists of individual rotational lines whose bandwidth (FWHM) is given by the following expression (Brimacombe & Reid,
τ
given ion energy compared with a solid-state laser.
electrons (Yakimenko & Pogorelsky, 2006).
**1.2 Challenge of high peak power**
CO2 laser.
maximum (FWHM).
1983):
$$
\Delta\nu\_{\text{pressure}}\,\text{[GHz]} = P\,\text{[bar]} \cdot \left( \text{5.79}\,\text{\textdegree C}\_{\text{CO2}} + \text{4.25}\,\text{\textdegree N}\_{N2} + \text{3.55}\,\text{\textdegree N}\_{\text{H}} \right), \tag{3}
$$
where *P* is the total gas pressure, and *Ψx* is the relative concentration of the component *x*. Eq. (3) yields *Δνpressure*≈3.8 GHz at 1 bar of a typical mixture [CO2]:[N2]:[He]=1:1:8. Although direct substituting this bandwidth in Eq. (2) gives *τp*≈120 ps, in reality the pulse's spectrum is narrower than the gain bandwidth, and, correspondingly, the pulse's duration is longer. The latter fact is comprehensible by realizing that amplification is the strongest in the center of the laser line, and thus, upon amplification, the ratio between the intensity at the central frequency and that at the wing of the gain-spectrum line increases, thus narrowing the pulse's spectrum. Consequently, the minimum achievable duration of the pulse for an atmospheric-pressure CO2 laser is about 1 ns (Abrams & Wood, 1971). Pressure can be increased to broaden the gain bandwidth, and thus somewhat reduce the pulse's duration. However, the complete overlap of rotational lines separated by 55 GHz (P-) or 40 GHz (Rbranches) that would allow using an entire rotational branch, does not occur below 20- 25 bar; this is not feasible in discharge-pumped lasers having a ~10 bar practical pressure limit.
Fig. 2. Simulated normalized gain spectra at different gas pressures. 10P, 10R, 9P, and 9R: Rotational P- and R-branches of the two vibrational transitions supporting lasing at ~10 µm and ~9 µm respectively.
Another important consequence of the rotational structure of the CO2 gain spectrum relates to the amplification of the ultrashort pulses. The spectrum of a few-picosecond-long pulse, according to Eq. (2), overlaps several rotational lines of the gain spectrum. Hence, upon amplification, the pulse's spectrum acquires the corresponding periodic structure. In the
Ultrashort Pulses 143
frequency (i.e., a multiple of the inter-mode distance), and individual pulse's duration is
The minimum pulse duration achievable via mode-locking is related to the bandwidth, *Δν,* (FWHM) of the gain spectrum according the following expression (Siegman & Kuizenga,
> 1/4 1/2 0.45 *<sup>p</sup> <sup>g</sup> <sup>f</sup> <sup>M</sup>*
where *g≡2αL* is the saturated round-trip excess gain (*α* – excess gain per unit length, *L* – resonator length), *M* is the modulation, and *f=c/2L* is its frequency. For a typical transversely excited atmospheric pressure (TEA) CO2 laser (*g*=1; *M*=1; *f*=150 MHz; *Δν*=3.8 GHz) the duration is *τp*≈600 ps. Wood et al. describe their technical realization of the mode-locked TEA CO2 laser (Wood et al., 1970, Abrams & Wood, 1971). A 10-bar mode-locked system was reported by Houtman and Meyer (Houtman & Meyer, 1987). Application of modelocking is limited in 10-micron systems by relatively long (hundreds of picoseconds) pulse
Passive self-mode-locking often occurs in TEA CO2 lasers producing a comb pulse-structure unless special countermeasures are taken. This effect is believed to be due to gain saturation (Kovalev, 1996). When a smooth pulse is required, the laser must be forced to operate at a single cavity mode that usually is achieved by using a low-pressure intra-cavity CW discharge cell ("smoothing tube"). Such a laser, combining two gain sections in a single resonator; viz., main atmospheric-pressure and a low-pressure one for spectrum narrowing, is called *hybrid* laser. Fig. 4 is an example of a hybrid TEA CO2 lasers' pulse profile with and without discharge in the smoothing tube. As Fig. 4a shows, self-mode-locking occurring when the smoothing tube not activated results in modulation at a frequency that is a multiple (here, double) of the inter-mode spacing. Activating the smoothing tube eliminates
Fig. 4. Temporal structure of the output of a hybrid TEA CO2 laser with smoothing tube discharge OFF (a), and ON (b). Resonator round-trip time is 12 ns; self-mode-locking occurs
<sup>−</sup>
( )
ν
≈ ⋅ ⋅Δ , (4)
inversely proportional to the total extent of the spectrum.
duration achievable via this technique.
the modulation, Fig. 4b.
at the doubled round-trip frequency.
τ
1969):
time-domain (inverse Fourier-transform of the spectrum), this is equivalent to a pulse train with pulse-to-pulse distance equal to the inverse separation of the spectral lines (18 ps in the P-, and 25 ps in the R-branches). Fig. 3 demonstrates this effect for a 5-ps pulse amplified in a 10-bar active medium.
Fig. *3*. Simulated dynamics of a 5-ps (FWHM) pulse spectrum (a), and its temporal structure normalized by the total energy in the train (b) amplified in a 10-bar CO2 amplifier. Dashed line: normalized gain spectrum.
We discuss below different approaches to overcome these difficulties.
#### **2. Generating the seed pulse**
In all modern schemes for producing ultrahigh-power laser pulses there are at least two major stages: (1) Generation of a low-power, ultrashort seed pulse, and, (2) amplification of the pulse. In this section, we consider different methods of creating ultrashort 10-µm pulses. Section 3, following, discusses the amplification stage.
#### **2.1 Mode-locking**
Mode-locking is the primary technique of generating ultrashort pulses in solid-state systems. In this approach, cavity modes present in the spectrum of the laser field are synchronized (locked) by modulating the cavity's Q-factor at a frequency that is a multiple of the inverse of the resonator's round-trip time, either actively (e.g., with an intra-cavity acousto-optical modulator), or passively (e.g., with a saturable absorber). Without synchronization, each mode is independent and defines its own pulse; thus, the pulse's duration is only limited by the relatively narrow bandwidth of an individual mode. However, after mode-locking, the time structure of the laser field is defined by the entire spectrum comprising *all* active modes. For the periodic spectrum of longitudinal modes, this structure is a periodic train of pulses; the pulse's repetition rate is equal to the lock-in
time-domain (inverse Fourier-transform of the spectrum), this is equivalent to a pulse train with pulse-to-pulse distance equal to the inverse separation of the spectral lines (18 ps in the P-, and 25 ps in the R-branches). Fig. 3 demonstrates this effect for a 5-ps pulse amplified in
Fig. *3*. Simulated dynamics of a 5-ps (FWHM) pulse spectrum (a), and its temporal structure normalized by the total energy in the train (b) amplified in a 10-bar CO2 amplifier. Dashed
In all modern schemes for producing ultrahigh-power laser pulses there are at least two major stages: (1) Generation of a low-power, ultrashort seed pulse, and, (2) amplification of the pulse. In this section, we consider different methods of creating ultrashort 10-µm pulses.
Mode-locking is the primary technique of generating ultrashort pulses in solid-state systems. In this approach, cavity modes present in the spectrum of the laser field are synchronized (locked) by modulating the cavity's Q-factor at a frequency that is a multiple of the inverse of the resonator's round-trip time, either actively (e.g., with an intra-cavity acousto-optical modulator), or passively (e.g., with a saturable absorber). Without synchronization, each mode is independent and defines its own pulse; thus, the pulse's duration is only limited by the relatively narrow bandwidth of an individual mode. However, after mode-locking, the time structure of the laser field is defined by the entire spectrum comprising *all* active modes. For the periodic spectrum of longitudinal modes, this structure is a periodic train of pulses; the pulse's repetition rate is equal to the lock-in
We discuss below different approaches to overcome these difficulties.
Section 3, following, discusses the amplification stage.
a 10-bar active medium.
line: normalized gain spectrum.
**2. Generating the seed pulse**
**2.1 Mode-locking**
frequency (i.e., a multiple of the inter-mode distance), and individual pulse's duration is inversely proportional to the total extent of the spectrum.
The minimum pulse duration achievable via mode-locking is related to the bandwidth, *Δν,* (FWHM) of the gain spectrum according the following expression (Siegman & Kuizenga, 1969):
$$
\sigma\_p = 0.45 \cdot \left(\frac{\text{g}}{\text{M}}\right)^{1/4} \left(f \cdot \Delta \nu\right)^{-1/2} \text{ }\tag{4}
$$
where *g≡2αL* is the saturated round-trip excess gain (*α* – excess gain per unit length, *L* – resonator length), *M* is the modulation, and *f=c/2L* is its frequency. For a typical transversely excited atmospheric pressure (TEA) CO2 laser (*g*=1; *M*=1; *f*=150 MHz; *Δν*=3.8 GHz) the duration is *τp*≈600 ps. Wood et al. describe their technical realization of the mode-locked TEA CO2 laser (Wood et al., 1970, Abrams & Wood, 1971). A 10-bar mode-locked system was reported by Houtman and Meyer (Houtman & Meyer, 1987). Application of modelocking is limited in 10-micron systems by relatively long (hundreds of picoseconds) pulse duration achievable via this technique.
Passive self-mode-locking often occurs in TEA CO2 lasers producing a comb pulse-structure unless special countermeasures are taken. This effect is believed to be due to gain saturation (Kovalev, 1996). When a smooth pulse is required, the laser must be forced to operate at a single cavity mode that usually is achieved by using a low-pressure intra-cavity CW discharge cell ("smoothing tube"). Such a laser, combining two gain sections in a single resonator; viz., main atmospheric-pressure and a low-pressure one for spectrum narrowing, is called *hybrid* laser. Fig. 4 is an example of a hybrid TEA CO2 lasers' pulse profile with and without discharge in the smoothing tube. As Fig. 4a shows, self-mode-locking occurring when the smoothing tube not activated results in modulation at a frequency that is a multiple (here, double) of the inter-mode spacing. Activating the smoothing tube eliminates the modulation, Fig. 4b.
Fig. 4. Temporal structure of the output of a hybrid TEA CO2 laser with smoothing tube discharge OFF (a), and ON (b). Resonator round-trip time is 12 ns; self-mode-locking occurs at the doubled round-trip frequency.
Ultrashort Pulses 145
inverse-quadratic relationship between *Nc* and the laser's wavelength (Eq. (1)) allows our realization of a scheme wherein a relatively low-energy pulse from a solid-state laser controls a high-power CO2-laser pulse. Combining two semiconductor switches, the first of which operates in a reflection- and the second in the transmission- configuration (Fig. 6) enables us to slice a CO2 pulse on both edges, thus producing one whose duration is limited
Fig. 6. Combination of reflection- and transmission- semiconductor switches to generate an
Reportedly, Rolland & Corkum, (1986) achieved a 130-fs pulse via this technique. Germanium is the commonest material used in semiconductor switches; silicon and
The laser-stimulated birefringence employed in the Kerr-cell technique has the advantage of low inertness, so supporting the production of an ultrashort pulse in a single step. Its minimum achievable duration is limited by that of the control pulse, and the relaxation speed of the induced birefringence. The Kerr cell depicted in Fig. 5b rotates the polarization of the CO2 laser-beam while it is being irradiated by the control pulse. A polarization filter on the cell output separates the part of the pulse with rotated polarization from the main pulse. Liquid carbon disulphide (CS2) featuring ~2 ps relaxation time usually serves as the active medium in the optical Kerr cell for slicing the CO2 laser pulses. For optimum switching efficiency, the phase angle between the control- and the CO2 laser- pulses must be
Currently, pulse-slicing is the main method of producing low-energy ultrashort (few-
Solid-state ultrafast oscillators producing several-cycle and longer optical pulses in the nearinfrared spectral region are a well-established technology. Using frequency conversion via *optical parametric amplification* (OPA) one can generate an ultrashort mid-IR pulse. One of the earliest theoretical treatments of parametric amplification was by Armstrong, Bloembergen, Ducuing, and Pershan (Armstrong et al., 1962). Using a resonant cavity to enhance output, Giordmaine and Miller demonstrated the principle (Giordmaine and Miller, 1965). Okorogu et al. demonstrated efficient, single-stage difference frequency downconversion from near-
only by the rise-time of the control pulse.
cadmium telluride also proved usable in this application.
ultrashort pulse.
equal to *π/4*.
picosecond) 10-micron seed pulses.
**2.4 All-solid-state systems**
#### **2.2 Plasma shutter and optical free-induction decay**
Highly intense laser pulses focused in gas media can initiate avalanche ionization (laser breakdown). If gas density is high enough, overcritical plasma forms, blocking the trailing part of the laser pulse, partially absorbing and partially reflecting it. This effect itself, usually termed *plasma shutter*, can be used to cut the tail of the pulse thus reducing its overall duration. However, it is not sufficient for producing an ultrashort pulse because the front part of the initial pulse passes the plasma shutter unchanged. The possibility of generating a pulse as short as a few optical cycles lies in the fact that the fast switching-out of the laser field by plasma entails a very rapid variation in the field spectra. Essentially, we can approximate the spectrum of the transmitted pulse by the Fourier-transform of a step function. Frequencies different from those present in the original pulse briefly appear in the spectrum. If we now filter-out the frequency of the initial pulse from the resulting spectrum, we end up with a very short pulse (Yablonovich, 1973, 1974a, 1974b). Free-induction decay technique is relatively simple to realize and can allow achieving ~20 ps pulse duration at the expense of large losses and strong alteration of the spectrum.
#### **2.3 Pulse-slicing techniques**
Another possibility for producing a low-energy ultrashort pulse is to slice a small fraction out of a longer one (for instance, a hundred-nanosecond output of a hybrid TEA CO2 laser similar to that in Fig. 4b) using a fast optical switch. Here, the switching speed limits the duration of the resulting pulse. Two techniques often employed for this purpose are a semiconductor optical switch (Alcock & Corkum, 1979), or a Kerr cell (Filip et al., 2002). Both rely on an ultrashort pulse of another laser (usually a solid-state one) to trigger the switch by inducing a short-living "plasma mirror" in the case of a semiconductor switch, or birefringence in that of the Kerr cell. Fig. 5 illustrates the principles of these two techniques.
Fig. 5. Simplified schematics of the techniques for pulse slicing by (a) the semiconductor switch, and, (b) the Kerr cell.
A powerful laser pulse irradiating the surface of a semiconductor partially ionizes it creating a "plasma" of free charge carriers. If the plasma's density exceeds the critical one (*Nc*), the semiconductor surface turns into a mirror, reflecting the entering laser beam. At belowcritical densities, the beam is attenuated mostly by free-carrier absorption. The duration of the reflection is determined mainly by that of the control pulse, and the speed of free carrier diffusion (typically hundreds of picoseconds). Absorption generally lasts longer (hundreds of nanoseconds), and its persistence is defined by the free carrier's recombination time. The
Highly intense laser pulses focused in gas media can initiate avalanche ionization (laser breakdown). If gas density is high enough, overcritical plasma forms, blocking the trailing part of the laser pulse, partially absorbing and partially reflecting it. This effect itself, usually termed *plasma shutter*, can be used to cut the tail of the pulse thus reducing its overall duration. However, it is not sufficient for producing an ultrashort pulse because the front part of the initial pulse passes the plasma shutter unchanged. The possibility of generating a pulse as short as a few optical cycles lies in the fact that the fast switching-out of the laser field by plasma entails a very rapid variation in the field spectra. Essentially, we can approximate the spectrum of the transmitted pulse by the Fourier-transform of a step function. Frequencies different from those present in the original pulse briefly appear in the spectrum. If we now filter-out the frequency of the initial pulse from the resulting spectrum, we end up with a very short pulse (Yablonovich, 1973, 1974a, 1974b). Free-induction decay technique is relatively simple to realize and can allow achieving ~20 ps pulse duration at the
Another possibility for producing a low-energy ultrashort pulse is to slice a small fraction out of a longer one (for instance, a hundred-nanosecond output of a hybrid TEA CO2 laser similar to that in Fig. 4b) using a fast optical switch. Here, the switching speed limits the duration of the resulting pulse. Two techniques often employed for this purpose are a semiconductor optical switch (Alcock & Corkum, 1979), or a Kerr cell (Filip et al., 2002). Both rely on an ultrashort pulse of another laser (usually a solid-state one) to trigger the switch by inducing a short-living "plasma mirror" in the case of a semiconductor switch, or birefringence in that of the Kerr cell. Fig. 5 illustrates the principles of these two techniques.
Fig. 5. Simplified schematics of the techniques for pulse slicing by (a) the semiconductor
A powerful laser pulse irradiating the surface of a semiconductor partially ionizes it creating a "plasma" of free charge carriers. If the plasma's density exceeds the critical one (*Nc*), the semiconductor surface turns into a mirror, reflecting the entering laser beam. At belowcritical densities, the beam is attenuated mostly by free-carrier absorption. The duration of the reflection is determined mainly by that of the control pulse, and the speed of free carrier diffusion (typically hundreds of picoseconds). Absorption generally lasts longer (hundreds of nanoseconds), and its persistence is defined by the free carrier's recombination time. The
**2.2 Plasma shutter and optical free-induction decay**
expense of large losses and strong alteration of the spectrum.
**2.3 Pulse-slicing techniques**
switch, and, (b) the Kerr cell.
inverse-quadratic relationship between *Nc* and the laser's wavelength (Eq. (1)) allows our realization of a scheme wherein a relatively low-energy pulse from a solid-state laser controls a high-power CO2-laser pulse. Combining two semiconductor switches, the first of which operates in a reflection- and the second in the transmission- configuration (Fig. 6) enables us to slice a CO2 pulse on both edges, thus producing one whose duration is limited only by the rise-time of the control pulse.
Fig. 6. Combination of reflection- and transmission- semiconductor switches to generate an ultrashort pulse.
Reportedly, Rolland & Corkum, (1986) achieved a 130-fs pulse via this technique. Germanium is the commonest material used in semiconductor switches; silicon and cadmium telluride also proved usable in this application.
The laser-stimulated birefringence employed in the Kerr-cell technique has the advantage of low inertness, so supporting the production of an ultrashort pulse in a single step. Its minimum achievable duration is limited by that of the control pulse, and the relaxation speed of the induced birefringence. The Kerr cell depicted in Fig. 5b rotates the polarization of the CO2 laser-beam while it is being irradiated by the control pulse. A polarization filter on the cell output separates the part of the pulse with rotated polarization from the main pulse. Liquid carbon disulphide (CS2) featuring ~2 ps relaxation time usually serves as the active medium in the optical Kerr cell for slicing the CO2 laser pulses. For optimum switching efficiency, the phase angle between the control- and the CO2 laser- pulses must be equal to *π/4*.
Currently, pulse-slicing is the main method of producing low-energy ultrashort (fewpicosecond) 10-micron seed pulses.
#### **2.4 All-solid-state systems**
Solid-state ultrafast oscillators producing several-cycle and longer optical pulses in the nearinfrared spectral region are a well-established technology. Using frequency conversion via *optical parametric amplification* (OPA) one can generate an ultrashort mid-IR pulse. One of the earliest theoretical treatments of parametric amplification was by Armstrong, Bloembergen, Ducuing, and Pershan (Armstrong et al., 1962). Using a resonant cavity to enhance output, Giordmaine and Miller demonstrated the principle (Giordmaine and Miller, 1965). Okorogu et al. demonstrated efficient, single-stage difference frequency downconversion from near-
Ultrashort Pulses 147
These cascaded nonlinear processes allow stable, repeatable conversion of the ultrafast pump pulses from the near- to the mid-IR region, while providing broad bandwidth, wavelength tunability, and ultrashort duration. The theoretical maximum energy conversion efficiency from 0.8 to 10 micron via this cascaded three-photon mixing is near 3.5%, however when real beams which are non-uniform in time and space are considered, as well as losses on the large number of optical elements required, the realized efficiency is approximately an order of magnitude lower. Pulsewidths under 500 fs are easily achievable,
All-solid-state systems provide good control of pulse synchronization and shape, but are much more elaborate than the other techniques used for ultrashort mid-IR pulse generation.
A major problem in seamlessly amplifying picosecond pulses is the discrete rotational-line structure of the gain spectrum, causing modulation of the pulse spectrum and pulse splitting. The gain spectrum's modulation can be smeared either by broadening individual rotational lines, thus assuring their better overlap, or by increasing their density. In the first case, we can use pressure- (collision-) and/or field- (power-) broadening effects. Several approaches increase line density: (1) Using an R-branch of a laser transition having a 1.4 times denser line structure than a conventional P-branch; (2) isotopic enrichment of the CO2 molecules, wherein the superposition of the slightly shifted spectra of different isotopic species (*isotopologues*) generates a denser effective spectrum; and, (3) combining the *sequence* bands of laser transition with regular ones. We briefly overview these approaches next.
*Pressure broadening.* As discussed in the Section 1.2, increasing the pressure lowers modulation in the gain spectrum. Complete suppression occurs when collisionally broadened bandwidths of the rotational lines become about twice the interline distance. However, the 20-25 bar pressure required for this, according to the Eq. (3), is impractical due to difficulties in arranging the uniform electric-discharge pumping the large volume of active gas required for building a high-power CO2 laser amplifier. Replacing discharge- with optical- pumping may afford using a pure-CO2 active medium and increased working pressure, thus considerably extending the pressure broadening effect. Rapid progress in the solid-state laser technology might well lead to the availability of a reliable source for optical excitation of CO2 active medium in the near future. Gordienko & Platonenko, (2010)
*Field broadening.* We can approximate the magnitude of line broadening (FWHM) appearing in the intense laser field due to the Autler-Townes (or dynamic Stark) effect (Autler &
transition dipole momentum, *E* is the laser field, and *h* is Plank's constant. Substituting the laser field with its expression through the intensity, *I*, and using the numerical values of the
*d E <sup>=</sup> <sup>h</sup>*
<sup>2</sup> Δ [GHz] 0.02764 [D] [W/cm ] *field ν* ≈ *d I* (5)
= Ω <sup>⋅</sup> , where *d* is the
consider that here the ErCr:YSGG (2.79 µm) laser is a promising candidate.
Townes, 1955) by the doubled Rabi frequency *Ω*: Δ 2 2 *field ν*
involved constants, we get the following equation:
as well as bandwidth covering any single branch of the CO2 gain spectrum.
**3. Amplification**
**3.1 "Smoothing" the gain spectrum**
to mid-IR (Okorogu et al., 1998). The combination of compact size, various free-space or fiber-based configurations, and efficient pump sources provide an advantageous starting point for CO2 laser seed sources. One method used for frequency conversion is covered below with attention toward stable and reliable operation as a sub-component of a larger CO2 laser system.
Titanium-doped sapphire is now the dominant laser system in many industrial and research fields such as physical chemistry, materials science and processing, and strong-field physics. Therefore, a large commercial infrastructure exists for producing reliable amplifiers delivering high energy pulses suitable for nonlinear conversion. Diode-pumped Neodymium lasers which are frequency-doubled for pumping the broadband Ti:Al2O3 gain medium are turn-key and have lifetimes on the order of 10 000 hours. Many amplifier systems are available producing pulse energies above 5 mJ near 1 kHz repetition rates. In such a configuration, the pump pulses have energy stability better than 1% because the energy is removed on a time scale comparable to the upper-state lifetime, and a quasisteady-state exists between pumping and energy extraction.
After amplification to high energy, the use of OPA provides a path for the generation of significant seed energies at 10 µm with the full bandwidth and tunability to cover the entire gain spectrum of CO2. One such commercial approach is shown in Fig. 7.
Fig. 7. Commercially-available frequency conversion system from 0.8 to 10 microns
In this configuration, the pulses from the Ti:Al2O3 amplifier are used in three separate nonlinear processes. The first is white light continuum generation in sapphire. This process creates an ultra-broadband spectrum spanning the entire visible and near-IR region while preserving the phase and temporal structure of the original pulses. This broadband radiation makes an ideal seed source for the following sections as it allows free tuning to the desired wavelengths. Because the next section uses three-wave mixing, this seed power also eliminates instabilities that would be encountered from quantum fluctuations in a pure parametric generator.
The next two stages rely on standard nonlinear crystals and act as simple parametric amplifiers that are angle-tuned to achieve gain at the desired signal and idler wavelengths. For conversion from 0.8 to 10 micron, these are approximately 1.5 and 1.7 micron, respectively. By utilizing two stages, the gain of each section can be optimized while preserving bandwidth that would be limited by a longer single crystal. The second parametric amplification stage therefore utilizes most of the pulse energy delivered from the Ti:Al2O3 amplifier.
In the final section, the signal and idler from the previous stages generate a difference frequency pulse in another nonlinear material that has transparency in the mid-IR region.
to mid-IR (Okorogu et al., 1998). The combination of compact size, various free-space or fiber-based configurations, and efficient pump sources provide an advantageous starting point for CO2 laser seed sources. One method used for frequency conversion is covered below with attention toward stable and reliable operation as a sub-component of a larger
Titanium-doped sapphire is now the dominant laser system in many industrial and research fields such as physical chemistry, materials science and processing, and strong-field physics. Therefore, a large commercial infrastructure exists for producing reliable amplifiers delivering high energy pulses suitable for nonlinear conversion. Diode-pumped Neodymium lasers which are frequency-doubled for pumping the broadband Ti:Al2O3 gain medium are turn-key and have lifetimes on the order of 10 000 hours. Many amplifier systems are available producing pulse energies above 5 mJ near 1 kHz repetition rates. In such a configuration, the pump pulses have energy stability better than 1% because the energy is removed on a time scale comparable to the upper-state lifetime, and a quasi-
After amplification to high energy, the use of OPA provides a path for the generation of significant seed energies at 10 µm with the full bandwidth and tunability to cover the entire
steady-state exists between pumping and energy extraction.
gain spectrum of CO2. One such commercial approach is shown in Fig. 7.
Fig. 7. Commercially-available frequency conversion system from 0.8 to 10 microns
In this configuration, the pulses from the Ti:Al2O3 amplifier are used in three separate nonlinear processes. The first is white light continuum generation in sapphire. This process creates an ultra-broadband spectrum spanning the entire visible and near-IR region while preserving the phase and temporal structure of the original pulses. This broadband radiation makes an ideal seed source for the following sections as it allows free tuning to the desired wavelengths. Because the next section uses three-wave mixing, this seed power also eliminates instabilities that would be encountered from quantum fluctuations in a pure
The next two stages rely on standard nonlinear crystals and act as simple parametric amplifiers that are angle-tuned to achieve gain at the desired signal and idler wavelengths. For conversion from 0.8 to 10 micron, these are approximately 1.5 and 1.7 micron, respectively. By utilizing two stages, the gain of each section can be optimized while preserving bandwidth that would be limited by a longer single crystal. The second parametric amplification stage therefore utilizes most of the pulse energy delivered from the
In the final section, the signal and idler from the previous stages generate a difference frequency pulse in another nonlinear material that has transparency in the mid-IR region.
CO2 laser system.
parametric generator.
Ti:Al2O3 amplifier.
These cascaded nonlinear processes allow stable, repeatable conversion of the ultrafast pump pulses from the near- to the mid-IR region, while providing broad bandwidth, wavelength tunability, and ultrashort duration. The theoretical maximum energy conversion efficiency from 0.8 to 10 micron via this cascaded three-photon mixing is near 3.5%, however when real beams which are non-uniform in time and space are considered, as well as losses on the large number of optical elements required, the realized efficiency is approximately an order of magnitude lower. Pulsewidths under 500 fs are easily achievable, as well as bandwidth covering any single branch of the CO2 gain spectrum.
All-solid-state systems provide good control of pulse synchronization and shape, but are much more elaborate than the other techniques used for ultrashort mid-IR pulse generation.
#### **3. Amplification**
#### **3.1 "Smoothing" the gain spectrum**
A major problem in seamlessly amplifying picosecond pulses is the discrete rotational-line structure of the gain spectrum, causing modulation of the pulse spectrum and pulse splitting. The gain spectrum's modulation can be smeared either by broadening individual rotational lines, thus assuring their better overlap, or by increasing their density. In the first case, we can use pressure- (collision-) and/or field- (power-) broadening effects. Several approaches increase line density: (1) Using an R-branch of a laser transition having a 1.4 times denser line structure than a conventional P-branch; (2) isotopic enrichment of the CO2 molecules, wherein the superposition of the slightly shifted spectra of different isotopic species (*isotopologues*) generates a denser effective spectrum; and, (3) combining the *sequence* bands of laser transition with regular ones. We briefly overview these approaches next.
*Pressure broadening.* As discussed in the Section 1.2, increasing the pressure lowers modulation in the gain spectrum. Complete suppression occurs when collisionally broadened bandwidths of the rotational lines become about twice the interline distance. However, the 20-25 bar pressure required for this, according to the Eq. (3), is impractical due to difficulties in arranging the uniform electric-discharge pumping the large volume of active gas required for building a high-power CO2 laser amplifier. Replacing discharge- with optical- pumping may afford using a pure-CO2 active medium and increased working pressure, thus considerably extending the pressure broadening effect. Rapid progress in the solid-state laser technology might well lead to the availability of a reliable source for optical excitation of CO2 active medium in the near future. Gordienko & Platonenko, (2010) consider that here the ErCr:YSGG (2.79 µm) laser is a promising candidate.
*Field broadening.* We can approximate the magnitude of line broadening (FWHM) appearing in the intense laser field due to the Autler-Townes (or dynamic Stark) effect (Autler & Townes, 1955) by the doubled Rabi frequency *Ω*: Δ 2 2 *field ν d E <sup>=</sup> <sup>h</sup>* = Ω <sup>⋅</sup> , where *d* is the transition dipole momentum, *E* is the laser field, and *h* is Plank's constant. Substituting the laser field with its expression through the intensity, *I*, and using the numerical values of the involved constants, we get the following equation:
$$
\Delta\nu\_{\text{field}}[\text{GHz}] = 0.02764 \, d[\text{D}] \, \sqrt{l[\text{W}/\text{cm}^2]} \tag{5}
$$
Ultrashort Pulses 149
For a given proportion [16O]:[18O], independent of the initial distribution of 16O and 18O between the CO2 molecules, statistical equilibration via intermolecular isotope-exchange leads to [626]:[628]:[828]= [16O]2:2[16O] [18O]:[18O]2. We note that due to the broken symmetry of the 628 molecule, it has twice as many rotational lines in each rotational branch compared to more symmetric 626- and 828- isotopologues. The combination of three CO2 isotopologues, as depicted in Fig. 8, results in a smooth spectrum already apparent at 10 bar. The gain of the isotopic mixture in the 10-micron branches at this pressure is 1.4 times lower than that of the regular gas, mainly reflecting the relatively low gain of the 828 CO2 isotopologue. Thus, a longer path through an active medium or a higher CO2 concentration is needed to maintain the same net amplification. The isotope-based approach is practically implemented in the CO2 laser of Accelerator Test Facility at Brookhaven National
*Sequence bands.* Transitions between high-lying vibrational overtones of the CO2 molecule can contribute considerably to the high-pressure amplifier gain. In this case, the rotational spectra of the regular- and sequence- bands overlap, so providing a denser effective spectrum. Exploiting the sequence band for smoothing the gain spectrum seems especially promising for the 10R branch wherein the rotational lines belonging to the sequence band 0002-[1001,0201]I fall very close to the centers of the gaps between the lines of the regular band 0001-[1000,0200]I. Simple estimation of the ratio of gains of the sequence- and the regular- band, assuming the Boltzmann energy distribution within a vibrational mode (Reid
*G G seq reg* / 2 exp( / ) 3 3 = −*h kT*
where *T3* and *ν3* are the vibrational temperature and frequency of the asymmetric stretchmode of the CO2 molecule, and *h* and *k* respectively are the Plank's and Boltzmann's constants, show that sequence band's gain reaches 50% of the regular band's gain at
To assure highly intense laser fields, special attention must be given to properly selecting and utilizing the optical elements, and to accounting for their influence on the laser field. This especially is challenging in the 10-μm spectral region because of the dearth of optical materials compared to the visible or near-IR diapasons, and lack of data on the materials' behavior under ultrashort mid-IR pulses. Below, we summarize the properties of optical materials most important for using in the high-peak-power 10-μm laser field. Table 1 gives numerical data on the refractive indices and dispersion of some popular IR materials used in
*Chromatic dispersion* plays an important role due to the hundreds of GHz-wide spectrum of (sub-) picosecond 10-μm pulses that may entail considerable pulse stretching. For example, a pulse of 1 ps (FWHM) spreads to 1.27 ps after a single pass through a 10-cm NaCl window. Accordingly, the amount and thickness of optical elements should be minimized,
*Nonlinear index, B-integral*. A high-power laser pulse propagating through a medium
*T3*=2500 K, viz., comparable to the conditions of high-pressure CO2 amplifiers.
and/or a grating compensator added for recompressing the pulse.
changes its refractive index *n* (the *Kerr effect*):
ν
, (6)
Laboratory (Section 6.1).
& Siemsen, 1976):
CO2 lasers.
**3.2 Effects in optical materials**
With Eq. (5), and the known value of the laser's transition dipole momentum *d*=0.0275 D (the value for the 10P(20) line from HITRAN database (Rothman et al., 2009)), we find that field broadening is sufficient to completely suppress modulation at a laser intensity 15-20 GW/cm2; this is reachable in the modern high-power picosecond CO2 laser systems. Capitalizing on this approach supported the attainment of 15 TW peak power in the CO2 laser system of Neptune Laboratory of the University of California, Loss Angeles (Section 6.2).
*R-branch.* The R-branches of the CO2 laser transitions have a rotational structure 1.4 times denser than the more often used P-branches (Witteman, 1987); thus, they offer better overlap between collisionally broadened lines, and, hence, yield a smoother gain spectrum (Fig. 2). Interestingly, under high-pressure conditions, such as 10 bar or higher, the overlap between rotational lines increases the peak intensity of the R-branch compared to that of the P-branch that otherwise prevails in conventional low-pressure lasers.
*Isotopic CO2.* By partially substituting the 16O atoms in CO2 gas with another stable 18O isotope, we obtain almost perfectly smooth combined spectrum after superimposing the spectra of three CO2 isotopologues (molecules with different isotopic composition): 16O-12C-16O, 16O-12C-18O, and 18O-12C-18O (Fig. 8). They often are denoted as 626, 628, and 828 wherein 2, 6, and 8, respectively, represent 12C, 16O and 18O.
Fig. 8. Simulated gain spectra of three CO2 isotopologues with different combinations of oxygen-16 and oxygen-18 atoms (no enrichment in carbon isotopes), and the effective spectrum of their mixture in the proportion [626]:[628]:[828]=0.16:0.48:0.36 (statistical equilibrium in the case of [16O]:[18O]=0.4:0.6). Total gas pressure is 10 bar.
With Eq. (5), and the known value of the laser's transition dipole momentum *d*=0.0275 D (the value for the 10P(20) line from HITRAN database (Rothman et al., 2009)), we find that field broadening is sufficient to completely suppress modulation at a laser intensity 15-20 GW/cm2; this is reachable in the modern high-power picosecond CO2 laser systems. Capitalizing on this approach supported the attainment of 15 TW peak power in the CO2 laser system of Neptune Laboratory of the University of California, Loss Angeles (Section 6.2).
*R-branch.* The R-branches of the CO2 laser transitions have a rotational structure 1.4 times denser than the more often used P-branches (Witteman, 1987); thus, they offer better overlap between collisionally broadened lines, and, hence, yield a smoother gain spectrum (Fig. 2). Interestingly, under high-pressure conditions, such as 10 bar or higher, the overlap between rotational lines increases the peak intensity of the R-branch compared to that of the P-branch
*Isotopic CO2.* By partially substituting the 16O atoms in CO2 gas with another stable 18O isotope, we obtain almost perfectly smooth combined spectrum after superimposing the spectra of three CO2 isotopologues (molecules with different isotopic composition): 16O-12C-16O, 16O-12C-18O, and 18O-12C-18O (Fig. 8). They often are denoted as 626, 628, and 828
Fig. 8. Simulated gain spectra of three CO2 isotopologues with different combinations of oxygen-16 and oxygen-18 atoms (no enrichment in carbon isotopes), and the effective spectrum of their mixture in the proportion [626]:[628]:[828]=0.16:0.48:0.36 (statistical
equilibrium in the case of [16O]:[18O]=0.4:0.6). Total gas pressure is 10 bar.
that otherwise prevails in conventional low-pressure lasers.
wherein 2, 6, and 8, respectively, represent 12C, 16O and 18O.
For a given proportion [16O]:[18O], independent of the initial distribution of 16O and 18O between the CO2 molecules, statistical equilibration via intermolecular isotope-exchange leads to [626]:[628]:[828]= [16O]2:2[16O] [18O]:[18O]2. We note that due to the broken symmetry of the 628 molecule, it has twice as many rotational lines in each rotational branch compared to more symmetric 626- and 828- isotopologues. The combination of three CO2 isotopologues, as depicted in Fig. 8, results in a smooth spectrum already apparent at 10 bar. The gain of the isotopic mixture in the 10-micron branches at this pressure is 1.4 times lower than that of the regular gas, mainly reflecting the relatively low gain of the 828 CO2 isotopologue. Thus, a longer path through an active medium or a higher CO2 concentration is needed to maintain the same net amplification. The isotope-based approach is practically implemented in the CO2 laser of Accelerator Test Facility at Brookhaven National Laboratory (Section 6.1).
*Sequence bands.* Transitions between high-lying vibrational overtones of the CO2 molecule can contribute considerably to the high-pressure amplifier gain. In this case, the rotational spectra of the regular- and sequence- bands overlap, so providing a denser effective spectrum. Exploiting the sequence band for smoothing the gain spectrum seems especially promising for the 10R branch wherein the rotational lines belonging to the sequence band 0002-[1001,0201]I fall very close to the centers of the gaps between the lines of the regular band 0001-[1000,0200]I. Simple estimation of the ratio of gains of the sequence- and the regular- band, assuming the Boltzmann energy distribution within a vibrational mode (Reid & Siemsen, 1976):
$$\mathcal{G}\_{\text{sq}} \;/ \; G\_{\text{reg}} = 2 \exp(-h\nu\_{\text{s}} \;/ \; kT\_{\text{s}}) \; \prime \tag{6}$$
where *T3* and *ν3* are the vibrational temperature and frequency of the asymmetric stretchmode of the CO2 molecule, and *h* and *k* respectively are the Plank's and Boltzmann's constants, show that sequence band's gain reaches 50% of the regular band's gain at *T3*=2500 K, viz., comparable to the conditions of high-pressure CO2 amplifiers.
#### **3.2 Effects in optical materials**
To assure highly intense laser fields, special attention must be given to properly selecting and utilizing the optical elements, and to accounting for their influence on the laser field. This especially is challenging in the 10-μm spectral region because of the dearth of optical materials compared to the visible or near-IR diapasons, and lack of data on the materials' behavior under ultrashort mid-IR pulses. Below, we summarize the properties of optical materials most important for using in the high-peak-power 10-μm laser field. Table 1 gives numerical data on the refractive indices and dispersion of some popular IR materials used in CO2 lasers.
*Chromatic dispersion* plays an important role due to the hundreds of GHz-wide spectrum of (sub-) picosecond 10-μm pulses that may entail considerable pulse stretching. For example, a pulse of 1 ps (FWHM) spreads to 1.27 ps after a single pass through a 10-cm NaCl window. Accordingly, the amount and thickness of optical elements should be minimized, and/or a grating compensator added for recompressing the pulse.
*Nonlinear index, B-integral*. A high-power laser pulse propagating through a medium changes its refractive index *n* (the *Kerr effect*):
Ultrashort Pulses 151
formation. Assuming a similar behavior in mid-IR, we conclude there is relatively small variation of the breakdown threshold fluence as a function of pulse duration for pulses of a few picoseconds or shorter; thus, as a guideline in system design, in most cases we can
We can minimize the high-power effects in optical materials on the pulse by employing the method of chirped-pulse amplification (CPA), the principle of which is illustrated in Fig. 9.
Fig. 9. Chirped-pulse amplification schematic (Perry et al., 1995). Reproduced with
Before being amplified, an ultrashort pulse is *chirped*: A wavelength-dependent delay is introduced in the pulse using a *stretcher* consisting of a couple of gratings and lenses, as shown in the Fig. 9. Because ultrashort pulses have broad spectral bandwidth, inversely proportional to their duration (Eq. (2)), it is relatively straightforward to stretch them by a few orders-of-magnitude. The chirped pulse carries virtually the same energy as the original one, but its peak power is reduced in inverse proportion to the stretching factor. The chirped pulse can be amplified to energies much higher than that achievable by directly amplifying ultrashort pulses wherein high-power effects in optical materials and active medium appear earlier. After amplification, the pulse is re-compressed in another two-grating device (the
The invention of CPA for reducing light intensity during ultrashort pulse amplification was a dramatic breakthrough in solid-state laser technology. Although implementing the CPA technique in a (sub-)picosecond CO2 laser remains to be done (the corresponding project is
permission, courtesy of the Lawrence Livermore National Laboratory.
adopt 0.5 J/cm2 for transparent optics, and 1 J/cm2 for mirrors.
**3.3 Chirped-pulse amplification**
*compressor*).
$$m = n\_0 + n\_2 I \,, \tag{7}$$
where *n0* is the linear (low-intensity) refractive index, *n2* the nonlinear index, and *I* the optical intensity. Variation of refractive index across the beam's cross-section degrades its quality. For instance, lensing (*Kerr lensing*) occurs when phase-shift in the center of the beam is considerably larger than on its edge. The additional phase-retardation introduced to the beam after propagating through an optical element due to the nonlinear index (*B-integral*) is the accepted parameter for quantifying this effect (Paschotta):
2 <sup>2</sup> *B n I z dz* ( ) π λ<sup>=</sup> , (8)
where *λ* is the wavelength, *I(z)* the optical intensity along the beam's axis, and, *z* the position in the beam's direction. Usually, a noticeable self-focusing occurs if the B-integral exceeds 3- 5. Strong wave-front distortion and eventually catastrophic filamentation may occur at larger values of the B-integral. An estimate of the B-integral using the *n2* index from the Table 1 yields *B* = 13.4 for a 1 ps (FWHM), 500 mJ/cm2 pulse passing through a 10 cm of NaCl. Expectedly, the effect will be much stronger for other materials, implying that special care must be taken in selecting materials and controlling the fluence through the optical elements. The Kerr effect also is responsible for self-chirping due to temporal variation of the phase shift defined by the pulse's temporal structure (see Section 4.1).
Table 1. Linear refractive index *n0*, chromatic dispersion *dn0/dν*, and nonlinear index *n2* of some IR materials. (a) RefractiveIndex.INFO; (b) Sheik-Bahae et al., 1991; (c) Bristow et al., 2007.
*Optical breakdown threshold*. We know far less about the ultrashort-pulse breakdown thresholds for mid-IR wavelengths and the materials used at these wavelengths than we do for visible light and near-IR. Reportedly, the values are ~0.5 J/cm2 for NaCl, and 1-2 J/cm2 for gold-coated stainless-steel mirrors for 2-ps, 10-μm pulse (Corkum, 1983). Variations in the breakdown threshold with pulse duration are best studied for fused silica at a wavelength of ~800 nm (Du et al., 1994, Stuart et al., 1996, Tien et al., 1999). Jia et al. (2006) also explored the wavelength dependence of the damage threshold over 250-2000 nm for 150 fs pulses; they concluded that there was a relatively small variation in the threshold at wavelengths above 800 nm. At pulses <10 ps, the damage threshold decreases slow with declining duration of the pulse, rather than displaying the *τ<sup>p</sup>* 1/2 dependence valid for longer pulses; this difference is explained by the gradual transition from a thermally dominated damage regime to one dominated by collisional- and multi-photon- ionization and plasma
where *n0* is the linear (low-intensity) refractive index, *n2* the nonlinear index, and *I* the optical intensity. Variation of refractive index across the beam's cross-section degrades its quality. For instance, lensing (*Kerr lensing*) occurs when phase-shift in the center of the beam is considerably larger than on its edge. The additional phase-retardation introduced to the beam after propagating through an optical element due to the nonlinear index (*B-integral*) is
> 2 <sup>2</sup> *B n I z dz* ( ) π
where *λ* is the wavelength, *I(z)* the optical intensity along the beam's axis, and, *z* the position in the beam's direction. Usually, a noticeable self-focusing occurs if the B-integral exceeds 3- 5. Strong wave-front distortion and eventually catastrophic filamentation may occur at larger values of the B-integral. An estimate of the B-integral using the *n2* index from the Table 1 yields *B* = 13.4 for a 1 ps (FWHM), 500 mJ/cm2 pulse passing through a 10 cm of NaCl. Expectedly, the effect will be much stronger for other materials, implying that special care must be taken in selecting materials and controlling the fluence through the optical elements. The Kerr effect also is responsible for self-chirping due to temporal variation of
*n0* **@ 10.6 µm (a)** *dn0/dν* **@10.6 µm**
Table 1. Linear refractive index *n0*, chromatic dispersion *dn0/dν*, and nonlinear index *n2* of some IR materials. (a) RefractiveIndex.INFO; (b) Sheik-Bahae et al., 1991; (c) Bristow et al., 2007.
*Optical breakdown threshold*. We know far less about the ultrashort-pulse breakdown thresholds for mid-IR wavelengths and the materials used at these wavelengths than we do for visible light and near-IR. Reportedly, the values are ~0.5 J/cm2 for NaCl, and 1-2 J/cm2 for gold-coated stainless-steel mirrors for 2-ps, 10-μm pulse (Corkum, 1983). Variations in the breakdown threshold with pulse duration are best studied for fused silica at a wavelength of ~800 nm (Du et al., 1994, Stuart et al., 1996, Tien et al., 1999). Jia et al. (2006) also explored the wavelength dependence of the damage threshold over 250-2000 nm for 150 fs pulses; they concluded that there was a relatively small variation in the threshold at wavelengths above 800 nm. At pulses <10 ps, the damage threshold decreases slow with declining duration of the pulse, rather than displaying the *τp*1/2 dependence valid for longer pulses; this difference is explained by the gradual transition from a thermally dominated damage regime to one dominated by collisional- and multi-photon- ionization and plasma
KCl 1.45 1.51 5.7 @ 1.06 µm NaCl 1.49 2.64 4.4 @ 1.06 µm ZnSe 2.40 2.44 290 @ 1.06 µm CdTe 2.67 1.04 -3000 @ 1.06 µm Si 3.42 0.0914 1000 @ 2.2 µm(c) Ge 4.00 0.293 2800 @ 10.6 µm
λ
the phase shift defined by the pulse's temporal structure (see Section 4.1).
the accepted parameter for quantifying this effect (Paschotta):
*n n nI* = +0 2 , (7)
<sup>=</sup> , (8)
**(10-3 THz-1) (a)** *n2* **(10-16 cm2/W) (b)**
formation. Assuming a similar behavior in mid-IR, we conclude there is relatively small variation of the breakdown threshold fluence as a function of pulse duration for pulses of a few picoseconds or shorter; thus, as a guideline in system design, in most cases we can adopt 0.5 J/cm2 for transparent optics, and 1 J/cm2 for mirrors.
#### **3.3 Chirped-pulse amplification**
We can minimize the high-power effects in optical materials on the pulse by employing the method of chirped-pulse amplification (CPA), the principle of which is illustrated in Fig. 9.
Fig. 9. Chirped-pulse amplification schematic (Perry et al., 1995). Reproduced with permission, courtesy of the Lawrence Livermore National Laboratory.
Before being amplified, an ultrashort pulse is *chirped*: A wavelength-dependent delay is introduced in the pulse using a *stretcher* consisting of a couple of gratings and lenses, as shown in the Fig. 9. Because ultrashort pulses have broad spectral bandwidth, inversely proportional to their duration (Eq. (2)), it is relatively straightforward to stretch them by a few orders-of-magnitude. The chirped pulse carries virtually the same energy as the original one, but its peak power is reduced in inverse proportion to the stretching factor. The chirped pulse can be amplified to energies much higher than that achievable by directly amplifying ultrashort pulses wherein high-power effects in optical materials and active medium appear earlier. After amplification, the pulse is re-compressed in another two-grating device (the *compressor*).
The invention of CPA for reducing light intensity during ultrashort pulse amplification was a dramatic breakthrough in solid-state laser technology. Although implementing the CPA technique in a (sub-)picosecond CO2 laser remains to be done (the corresponding project is
Ultrashort Pulses 153
rotational lines to the ~0.5 THz necessary for this regime. (3) The spectrum of the pulse covers the entire rotational branch, as happens when the pulse duration is ≤1 ps (Eq. (2)).
Despite the usual attempts researchers take in trying to avoid the undesirable effects of the nonlinear response on the high-power pulses, it might be possible to employ these phenomena to further compress ultrashort pulses. When an optical pulse propagates through a media, the refractive index of the latter changes according to Eq. (7), following the intensity of the optical field (the Kerr effect). Phase velocity, *vp=c/n,* varies accordingly. The field frequency of the pulse, whose phase velocity continuously changes, shifts proportionally to its derivative *dvp/dt*, so generating pulse chirping. Fig. 10 illustrates this
Fig. 10. Self-chirping of a pulse propagating through a nonlinear media. Top: Pulse intensity profile (leading edge is on the left); bottom dashed: Phase velocity derivative; bottom solid:
It is important to realize the difference between the chirping, as in the CPA, and chirping, as in self-chirping. In the first case, the pulse's spectrum is unchanged whereas its duration increases; in the second, the spectrum broadens while pulse duration does not change. A dispersive compressor similar to that utilized in the CPA can be arranged to shrink the self-
Self-chirping in plasma essentially is a variation of the self-chirping in nonlinear media. The distinctive feature of this case is that the variation in refractive index is caused by laser-
The last regime is preferable if the aim is to keep pulse's span as brief as possible.
**4. Pulse compression**
process.
**4.1 Self-chirping in nonlinear media**
Wave-packet of the chirped pulse.
**4.2 Self-chirping in plasma**
chirped pulse well below its original duration.
planned at the Accelerator Test Facility of the Brookhaven National Laboratory), it is reasonable to expect that the outcome will comparably be valuable.
#### **3.4 Energy extraction efficiency**
Optimal use of the energy stored in the active medium is a key characteristic of a mature laser design. The difficulty in efficiently extracting excitation energy from gaseous active media via an ultrashort pulse arises from the fact that the pulse's duration is either shorter than, or comparable to the characteristic times of the involved excitation- and relaxationprocesses. Below, we briefly consider these processes and their influence on the efficiency of extracting energy.
*Excitation and vibrational relaxation*. In a typical CO2 laser, an electric discharge is used to create population inversion. Fast electrons accelerated by electric field collide with CO2 and N2 molecules, so exciting their upper vibrational states. Nitrogen serves as reservoir for storing the excitation energy which is then transferred to CO2 via vibrational relaxation. The typical duration of discharge in a pulsed CO2 laser is about a microsecond, and vibrational relaxation times range from 1-10 ns·bar. This implies that a (sub-)picosecond laser pulse only extracts energy already available at the upper vibrational laser-level, with negligible energy deposition from the discharge or from redistribution from other vibrational levels during pulse propagation. To maximize extraction, wherever possible a regenerative amplification scheme is used, wherein the pulse passes the amplifier medium many times allowing repopulation of the upper vibrational level of the laser transition between the passes. However, realizing this scheme practically is problematic for high-energy pulses when beam must be wide to increase the active volume and avoid damaging optical elements. Thus, two-stage amplification usually is adopted (see Section 6); a regenerative amplifier providing amplification up to millijoules level is followed by a final high-energy amplifier arranged either in a single-pass configuration, or several passes are only partially overlapped. Slow pumping and vibrational relaxation limit energy extraction in the final amplification stage. A possible solution is replacing pumping by electric discharge, with optical pumping by a short laser pulse that quickly and directly excites the upper laser level, and eliminates the need for redistributing vibrational energy.
*Rotational relaxation*. Rotational relaxation processes limit the fraction of energy extractable from the upper vibrational laser level in a single pass through the active medium. The laser pulse interacts only with a limited number of rotational transitions that is defined by the overlap between the pulse's spectrum and the amplification band. At high pressure, when collisionally broadened rotational lines overlap, or for a very short pulse, when pulse's spectrum covers several rotational lines, energy is extracted from several rotational sublevels. Otherwise, energy is extracted only from a single sub-level containing about 1/15th of the entire energy stored in that vibrational level. Three scenarios support the complete emptying of the vibrational level: (1) The pulse is long enough to provide time for repopulation of the active rotational levels. Typical rotational relaxation times are ~100 ps·bar; about 15 collisions are required effectively to empty all rotational sub-levels through a single active rotational transition. Thus, the minimum required pulse duration is ~1.5 ns·bar (e.g., 150 ps at 10 bar). (2) Pressure broadening allows all rotational transitions to interact with the laser field. Using Eq. (3) we find that ~100 bar is needed to broaden the
planned at the Accelerator Test Facility of the Brookhaven National Laboratory), it is
Optimal use of the energy stored in the active medium is a key characteristic of a mature laser design. The difficulty in efficiently extracting excitation energy from gaseous active media via an ultrashort pulse arises from the fact that the pulse's duration is either shorter than, or comparable to the characteristic times of the involved excitation- and relaxationprocesses. Below, we briefly consider these processes and their influence on the efficiency of
*Excitation and vibrational relaxation*. In a typical CO2 laser, an electric discharge is used to create population inversion. Fast electrons accelerated by electric field collide with CO2 and N2 molecules, so exciting their upper vibrational states. Nitrogen serves as reservoir for storing the excitation energy which is then transferred to CO2 via vibrational relaxation. The typical duration of discharge in a pulsed CO2 laser is about a microsecond, and vibrational relaxation times range from 1-10 ns·bar. This implies that a (sub-)picosecond laser pulse only extracts energy already available at the upper vibrational laser-level, with negligible energy deposition from the discharge or from redistribution from other vibrational levels during pulse propagation. To maximize extraction, wherever possible a regenerative amplification scheme is used, wherein the pulse passes the amplifier medium many times allowing repopulation of the upper vibrational level of the laser transition between the passes. However, realizing this scheme practically is problematic for high-energy pulses when beam must be wide to increase the active volume and avoid damaging optical elements. Thus, two-stage amplification usually is adopted (see Section 6); a regenerative amplifier providing amplification up to millijoules level is followed by a final high-energy amplifier arranged either in a single-pass configuration, or several passes are only partially overlapped. Slow pumping and vibrational relaxation limit energy extraction in the final amplification stage. A possible solution is replacing pumping by electric discharge, with optical pumping by a short laser pulse that quickly and directly excites the upper laser level,
*Rotational relaxation*. Rotational relaxation processes limit the fraction of energy extractable from the upper vibrational laser level in a single pass through the active medium. The laser pulse interacts only with a limited number of rotational transitions that is defined by the overlap between the pulse's spectrum and the amplification band. At high pressure, when collisionally broadened rotational lines overlap, or for a very short pulse, when pulse's spectrum covers several rotational lines, energy is extracted from several rotational sublevels. Otherwise, energy is extracted only from a single sub-level containing about 1/15th of the entire energy stored in that vibrational level. Three scenarios support the complete emptying of the vibrational level: (1) The pulse is long enough to provide time for repopulation of the active rotational levels. Typical rotational relaxation times are ~100 ps·bar; about 15 collisions are required effectively to empty all rotational sub-levels through a single active rotational transition. Thus, the minimum required pulse duration is ~1.5 ns·bar (e.g., 150 ps at 10 bar). (2) Pressure broadening allows all rotational transitions to interact with the laser field. Using Eq. (3) we find that ~100 bar is needed to broaden the
reasonable to expect that the outcome will comparably be valuable.
and eliminates the need for redistributing vibrational energy.
**3.4 Energy extraction efficiency**
extracting energy.
rotational lines to the ~0.5 THz necessary for this regime. (3) The spectrum of the pulse covers the entire rotational branch, as happens when the pulse duration is ≤1 ps (Eq. (2)). The last regime is preferable if the aim is to keep pulse's span as brief as possible.
#### **4. Pulse compression**
#### **4.1 Self-chirping in nonlinear media**
Despite the usual attempts researchers take in trying to avoid the undesirable effects of the nonlinear response on the high-power pulses, it might be possible to employ these phenomena to further compress ultrashort pulses. When an optical pulse propagates through a media, the refractive index of the latter changes according to Eq. (7), following the intensity of the optical field (the Kerr effect). Phase velocity, *vp=c/n,* varies accordingly. The field frequency of the pulse, whose phase velocity continuously changes, shifts proportionally to its derivative *dvp/dt*, so generating pulse chirping. Fig. 10 illustrates this process.
Fig. 10. Self-chirping of a pulse propagating through a nonlinear media. Top: Pulse intensity profile (leading edge is on the left); bottom dashed: Phase velocity derivative; bottom solid: Wave-packet of the chirped pulse.
It is important to realize the difference between the chirping, as in the CPA, and chirping, as in self-chirping. In the first case, the pulse's spectrum is unchanged whereas its duration increases; in the second, the spectrum broadens while pulse duration does not change. A dispersive compressor similar to that utilized in the CPA can be arranged to shrink the selfchirped pulse well below its original duration.
#### **4.2 Self-chirping in plasma**
Self-chirping in plasma essentially is a variation of the self-chirping in nonlinear media. The distinctive feature of this case is that the variation in refractive index is caused by laser-
Ultrashort Pulses 155
chirping in the partially ionized active media, and compression in the material of the cavity windows. Controlled shortening was realized using an external gas cell by Tochitsky et al.
Measuring the temporal structure of ultrashort 10-µm pulses is not fundamentally different from measuring visible or near-IR pulses. However, due to low demand, there are very few commercial diagnostic instruments (e.g., Frequency Resolved Optical Gating, FROG) suitable for direct use in the mid-IR region. Several techniques used for diagnosing CO2
Fig. 12. Apparatuses for ultrashort mid-IR pulse diagnostics. (a) Streak camera;
band not included in the simulations); and, (c) Autocorrelator.
spectrometer can be arranged similar to that shown in Fig. 12b.
(b) Spectrometer (additional peaks in the measured spectrum are attributed to a sequence
A *streak camera* is a convenient tool for monitoring the pulse structure, providing a resolution of 1-2 ps. Because photocathodes used in streak cameras are insensitive to the mid-IR wavelengths, a frequency conversion technique must be used to shift the pulse wavelength to the visible- or near-IR- diapason. For this purpose, either a differencefrequency mixing in a nonlinear crystal (Fig. 12a), or a Kerr-cell-based optical switch where the CO2 laser's pulse controls a visible or mid-IR beam is suitable. The resolution of the streak camera is sufficient for measuring pulse splitting due to modulation on the rotational gain spectrum. However, the accuracy of measuring the duration of individual pulses is
A *spectrometer* can be used for indirect measurement of the pulse's duration. In the absence of chirping, the total bandwidth of the spectrum is inversely proportional to the duration of the individual pulses (Eq. (2)). If the beam's quality is good enough, a simple grating
(Tochitsky et al., 2001).
**5. Pulse diagnostics**
limited by 1-2 ps.
laser pulses are schematically represented in Fig. 12.
induced gas-ionization rather than the Kerr effect. Refractive index of the ionized gas is determined by the linear refractive index of the media, *n0*, and the plasma density, *Ne*:
$$n = n\_0 \left(1 - \frac{N\_c}{N\_c}\right)^{1/2} \tag{9}$$
where *Nc* is the critical density (Eq. (1)). Unlike the Kerr effect, where refractive index usually *increases* with laser intensity, strong ionization in high-intensity fields *decreases* the refractive index. Therefore, the direction of chirping is reversed: The field frequency of the leading edge of the pulse is higher than that of the trailing one (*blue chirp*), allowing the compression of the chirped pulse via the linear dispersion in an optical material. By sending the pulse through a window of a properly selected thickness, made of a material with negative group velocity dispersion (e.g., NaCl), we can delay the blue-shifted leading edge of the pulse more than the trailing edge, thus shrinking the pulse.
Inert gases (e.g., xenon) are considered promising candidates for the role of nonlinear media for CO2 laser pulse self-chirping (Gordienko et al., 2009). A complication arises because the field intensity is not constant across the beam; thus, self-chirping that is pronounced in the center of the beam becomes negligible at its edges. The beam can be homogenized using a hollow waveguide (Nisoli et al., 1997, Voronin et al., 2010), or a filamentation regime (Couairon et al., 2006, Gordienko et al., 2009). Fig. 11 shows the results of simulations of an 1.2-ps pulse compression via self-chirping in xenon in filamentation regime followed by a NaCl compressor.
Fig. 11. Simulated 1.2-ps (FWHM) pulse before (solid) and after (dashed) compression via self-chirping in xenon plasma and dispersive compression in NaCl (Gordienko et al., 2009). Time is measured in optical cycles (1 o.c.≈35 fs); P is power and Pcr is critical self-focusing power in xenon: Pcr≈λ2/4πn0n2. Reprinted by permission of Turpion Ltd.
Corcum observed unintentional pulse shortening due to plasma chirping in a picosecond CO2 amplifier (Corcum, 1985). The suggested explanation capitalized on the pulse's self-
induced gas-ionization rather than the Kerr effect. Refractive index of the ionized gas is determined by the linear refractive index of the media, *n0*, and the plasma density, *Ne*:
> <sup>0</sup> 1 *<sup>e</sup> c N*
where *Nc* is the critical density (Eq. (1)). Unlike the Kerr effect, where refractive index usually *increases* with laser intensity, strong ionization in high-intensity fields *decreases* the refractive index. Therefore, the direction of chirping is reversed: The field frequency of the leading edge of the pulse is higher than that of the trailing one (*blue chirp*), allowing the compression of the chirped pulse via the linear dispersion in an optical material. By sending the pulse through a window of a properly selected thickness, made of a material with negative group velocity dispersion (e.g., NaCl), we can delay the blue-shifted leading edge
Inert gases (e.g., xenon) are considered promising candidates for the role of nonlinear media for CO2 laser pulse self-chirping (Gordienko et al., 2009). A complication arises because the field intensity is not constant across the beam; thus, self-chirping that is pronounced in the center of the beam becomes negligible at its edges. The beam can be homogenized using a hollow waveguide (Nisoli et al., 1997, Voronin et al., 2010), or a filamentation regime (Couairon et al., 2006, Gordienko et al., 2009). Fig. 11 shows the results of simulations of an 1.2-ps pulse compression via self-chirping in xenon in filamentation regime followed by a
Fig. 11. Simulated 1.2-ps (FWHM) pulse before (solid) and after (dashed) compression via self-chirping in xenon plasma and dispersive compression in NaCl (Gordienko et al., 2009). Time is measured in optical cycles (1 o.c.≈35 fs); P is power and Pcr is critical self-focusing
Corcum observed unintentional pulse shortening due to plasma chirping in a picosecond CO2 amplifier (Corcum, 1985). The suggested explanation capitalized on the pulse's self-
power in xenon: Pcr≈λ2/4πn0n2. Reprinted by permission of Turpion Ltd.
*N* = −
*n n*
of the pulse more than the trailing edge, thus shrinking the pulse.
NaCl compressor.
1/2
, (9)
chirping in the partially ionized active media, and compression in the material of the cavity windows. Controlled shortening was realized using an external gas cell by Tochitsky et al. (Tochitsky et al., 2001).
#### **5. Pulse diagnostics**
Measuring the temporal structure of ultrashort 10-µm pulses is not fundamentally different from measuring visible or near-IR pulses. However, due to low demand, there are very few commercial diagnostic instruments (e.g., Frequency Resolved Optical Gating, FROG) suitable for direct use in the mid-IR region. Several techniques used for diagnosing CO2 laser pulses are schematically represented in Fig. 12.
Fig. 12. Apparatuses for ultrashort mid-IR pulse diagnostics. (a) Streak camera; (b) Spectrometer (additional peaks in the measured spectrum are attributed to a sequence band not included in the simulations); and, (c) Autocorrelator.
A *streak camera* is a convenient tool for monitoring the pulse structure, providing a resolution of 1-2 ps. Because photocathodes used in streak cameras are insensitive to the mid-IR wavelengths, a frequency conversion technique must be used to shift the pulse wavelength to the visible- or near-IR- diapason. For this purpose, either a differencefrequency mixing in a nonlinear crystal (Fig. 12a), or a Kerr-cell-based optical switch where the CO2 laser's pulse controls a visible or mid-IR beam is suitable. The resolution of the streak camera is sufficient for measuring pulse splitting due to modulation on the rotational gain spectrum. However, the accuracy of measuring the duration of individual pulses is limited by 1-2 ps.
A *spectrometer* can be used for indirect measurement of the pulse's duration. In the absence of chirping, the total bandwidth of the spectrum is inversely proportional to the duration of the individual pulses (Eq. (2)). If the beam's quality is good enough, a simple grating spectrometer can be arranged similar to that shown in Fig. 12b.
Ultrashort Pulses 157
Fig. 13. Layout and pulse dynamics in the BNL-ATF CO2 laser system; PS: Polarizing
Working pressure 10 bar 8 bar Gas mixture: [CO2]:[N2]:[He] 1:1:18 (isotopic CO2) 2:1:28 Active volume 1×1×80 cm3 8×10×100 cm3 Small-signal gain 1-2 %/cm 1.5-2 %/cm Number of passes 8-12 round-trips 6 passes Net amplification 105 103
Fig. 14 is a scheme of the CO2 laser system of the UCLA's Neptune Laboratory operating at 10.6 µm wavelength (10P branch). The 3-ps injection pulse is produced by slicing a portion of the output of a hybrid TEA CO2 laser (comprising a low-pressure smoothing tube to suppress energy modulation caused by self-mode-locking). The slicing is realized as a single-step process, using a CS2-filled Kerr cell controlled by a 3-ps pulse of a solid-state laser. The nanojoule injection pulse first is amplified in an 8-bar regenerative amplifier,
Table 2. Parameters of BNL-ATF laser amplifiers.
**6.2 15-TW system at UCLA's Neptune Laboratory**
**Regenerative amplifier Final amplifier**
splitter.
An *autocorrelator* technique must be established to attain the most reliable results. For instance, it can be used to periodically validate the measurements from the streak camera and the spectrometer. An autocorrelator splits the measured beam into two, and recombines it on an active element where the pulses interact, providing a measurable signal that is a function of the temporal overlap. By recording the interaction signal as a function of the time-delay between pulses, we obtain information about the temporal profile of the pulse. Usually, a nonlinear crystal is used as the active element, generating a harmonic frequency when the two pulses overlap in time. In a simpler design, the pulses' temporal overlap is evaluated by measuring the modulation of the interference pattern resulting from the interaction of the two beams. In the autocorrelator design shown in the Fig. 12c an intentionally induced slight misalignment of the interferometer's arms generates an interference pattern on the pyroelectric camera's sensor. The interference contrast serves as a measure of the temporal overlap between the pulses; the maximum modulation corresponds to zero delay, whereas the complete separation of the pulses in time entails the disappearance of interference. With this technique, we can study both the duration of individual pulses and the train's structure, which results in the periodic appearance of the interference pattern with gradually reduced modulation at delays that are multiples of the pulse-splitting period.
#### **6. Existing terawatt CO2 lasers**
Two systems worldwide now can generate terawatt peak-power, 10-µm pulses: One at the Accelerator Test Facility of the Brookhaven National Laboratory (BNL) (Polyanskiy et al., 2011); and, the other at the Neptune Laboratory of the University of California, Loss Angeles (UCLA) (Haberberger et al., 2010). These systems are described briefly below.
#### **6.1 1-TW system at BNL's Accelerator Test Facility (BNL-ATF)**
The CO2 laser system depicted in the Fig. 13 consists of a picosecond pulse-generator that produces a linear-polarized 0.1-μJ, 5-ps pulse, along with two high-pressure amplifiers that ultimately boost the laser pulses' energy to the ~5 J level.
In this system, a 5-ps pulse is sliced by the sequence of optical switches from the 200-ns, 20 mJ output of a hybrid TEA CO2 laser tuned to the 10R(14) line (10.3 μm). First, a 10-ns pulse is cut off the initial pulse with a Pockels cell, and intensified in a 3-bar UV-pre-ionized electric-discharge pre-amplifier. Then, a semiconductor optical switch, which is controlled by a 14-ps YAG laser, slices off a ~200 ps part of the pulse. Finally, the 200-ps pulse is sent through a CS2 Kerr cell controlled by a co-propagating, 5-ps, frequency-doubled YAG laserpulse. A polarization filter placed after the Kerr cell selects a 0.1-μJ, 5-ps seed pulse that then is raised to 10 mJ in multiple round-trip passes through a regenerative amplifier filled with a gas mixture featuring *isotopically enriched* carbon dioxide to prevent pulse splitting upon amplification. The amplifier is energized with UV-pre-ionized transverse electric discharge. Further amplification to 5 J is attained in 6 passes through a large-aperture (8x10 cm2), x-ray pre-ionized final amplifier. Field broadening prevents pulse splitting in the final amplifier despite using regular CO2 gas therein. The output is a single 5-ps pulse implying ~1 TW peak power. Table 2 summarizes the technical details of these two amplification stages.
An *autocorrelator* technique must be established to attain the most reliable results. For instance, it can be used to periodically validate the measurements from the streak camera and the spectrometer. An autocorrelator splits the measured beam into two, and recombines it on an active element where the pulses interact, providing a measurable signal that is a function of the temporal overlap. By recording the interaction signal as a function of the time-delay between pulses, we obtain information about the temporal profile of the pulse. Usually, a nonlinear crystal is used as the active element, generating a harmonic frequency when the two pulses overlap in time. In a simpler design, the pulses' temporal overlap is evaluated by measuring the modulation of the interference pattern resulting from the interaction of the two beams. In the autocorrelator design shown in the Fig. 12c an intentionally induced slight misalignment of the interferometer's arms generates an interference pattern on the pyroelectric camera's sensor. The interference contrast serves as a measure of the temporal overlap between the pulses; the maximum modulation corresponds to zero delay, whereas the complete separation of the pulses in time entails the disappearance of interference. With this technique, we can study both the duration of individual pulses and the train's structure, which results in the periodic appearance of the interference pattern with gradually reduced modulation at delays that are multiples of the
Two systems worldwide now can generate terawatt peak-power, 10-µm pulses: One at the Accelerator Test Facility of the Brookhaven National Laboratory (BNL) (Polyanskiy et al., 2011); and, the other at the Neptune Laboratory of the University of California, Loss Angeles
The CO2 laser system depicted in the Fig. 13 consists of a picosecond pulse-generator that produces a linear-polarized 0.1-μJ, 5-ps pulse, along with two high-pressure amplifiers that
In this system, a 5-ps pulse is sliced by the sequence of optical switches from the 200-ns, 20 mJ output of a hybrid TEA CO2 laser tuned to the 10R(14) line (10.3 μm). First, a 10-ns pulse is cut off the initial pulse with a Pockels cell, and intensified in a 3-bar UV-pre-ionized electric-discharge pre-amplifier. Then, a semiconductor optical switch, which is controlled by a 14-ps YAG laser, slices off a ~200 ps part of the pulse. Finally, the 200-ps pulse is sent through a CS2 Kerr cell controlled by a co-propagating, 5-ps, frequency-doubled YAG laserpulse. A polarization filter placed after the Kerr cell selects a 0.1-μJ, 5-ps seed pulse that then is raised to 10 mJ in multiple round-trip passes through a regenerative amplifier filled with a gas mixture featuring *isotopically enriched* carbon dioxide to prevent pulse splitting upon amplification. The amplifier is energized with UV-pre-ionized transverse electric discharge. Further amplification to 5 J is attained in 6 passes through a large-aperture (8x10 cm2), x-ray pre-ionized final amplifier. Field broadening prevents pulse splitting in the final amplifier despite using regular CO2 gas therein. The output is a single 5-ps pulse implying ~1 TW peak power. Table 2 summarizes the technical details of these two
(UCLA) (Haberberger et al., 2010). These systems are described briefly below.
**6.1 1-TW system at BNL's Accelerator Test Facility (BNL-ATF)**
ultimately boost the laser pulses' energy to the ~5 J level.
pulse-splitting period.
amplification stages.
**6. Existing terawatt CO2 lasers**
Fig. 13. Layout and pulse dynamics in the BNL-ATF CO2 laser system; PS: Polarizing splitter.
Table 2. Parameters of BNL-ATF laser amplifiers.
#### **6.2 15-TW system at UCLA's Neptune Laboratory**
Fig. 14 is a scheme of the CO2 laser system of the UCLA's Neptune Laboratory operating at 10.6 µm wavelength (10P branch). The 3-ps injection pulse is produced by slicing a portion of the output of a hybrid TEA CO2 laser (comprising a low-pressure smoothing tube to suppress energy modulation caused by self-mode-locking). The slicing is realized as a single-step process, using a CS2-filled Kerr cell controlled by a 3-ps pulse of a solid-state laser. The nanojoule injection pulse first is amplified in an 8-bar regenerative amplifier,
Ultrashort Pulses 159
which are the high-energy physics experiments and the proton acceleration for cancer
Achievements in solid-state laser technology can help the further development of ultrashort-pulse, high-peak-power CO2 laser systems. Modern solid-state lasers can be directly used in mid-IR systems, e.g., for controlling optical switches, pumping CO2 laser transition, or generating the ultrashort 10-µm seed pulses via nonlinear frequency conversion and parametric amplification. Apart from that, the advanced techniques initially developed for solid-state lasers (e.g. chirped pulse amplification) can be adopted
This work is supported by the US DOE contract DE-AC02-98CH10886 and by the BNL Laboratory Directed R&D (LDRD) grant #07-004. The authors are thankful to Sergei Tochitsky from UCLA's Neptune Laboratory for providing information on the Neptune's
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**9. References**
**8. Acknowledgements**
reaching milijoules energy and splitting into a train of ~7 sub-pulses separated by 18-ps intervals. The final 2.5-bar amplifier boosts the pulse energy up to 100 J, and simultaneously mostly suppresses splitting via the field-broadening effect. The output pulse consists of 2-3 sub-pulses with ~45% energy in the first of them, implying ~15 TW peak power.
Fig. 14. Layout and pulse dynamics of the UCLA's Neptune Laboratory laser system (Haberberger et al., 2010). Reproduced with permission.
Table 3 summarizes the parameters of this system's amplifiers.
Table 3. Parameters of UCLA's Neptune Laboratory laser amplifiers.
#### **7. Conclusion**
We overviewed the underlying physics and technical approaches to generating and amplifying ultrashort 10-µm pulses. Modern CO2 laser systems can generate pulses as brief as few picoseconds and as powerful as several terawatt. Potential applications, among
reaching milijoules energy and splitting into a train of ~7 sub-pulses separated by 18-ps intervals. The final 2.5-bar amplifier boosts the pulse energy up to 100 J, and simultaneously mostly suppresses splitting via the field-broadening effect. The output pulse consists of 2-3
sub-pulses with ~45% energy in the first of them, implying ~15 TW peak power.
Fig. 14. Layout and pulse dynamics of the UCLA's Neptune Laboratory laser system
Working pressure 8 bar 2.5 bar Gas mixture: [CO2]:[N2]:[He] 1:1:14 4:1:0
Active volume 1×1×60 cm3 20×35×250 cm3 Small-signal gain - 2.6 %/cm Number of passes - 3 passes Net amplification 107 105
We overviewed the underlying physics and technical approaches to generating and amplifying ultrashort 10-µm pulses. Modern CO2 laser systems can generate pulses as brief as few picoseconds and as powerful as several terawatt. Potential applications, among
**Regenerative amplifier Final amplifier**
(Haberberger et al., 2010). Reproduced with permission.
Table 3 summarizes the parameters of this system's amplifiers.
Table 3. Parameters of UCLA's Neptune Laboratory laser amplifiers.
**7. Conclusion**
which are the high-energy physics experiments and the proton acceleration for cancer therapy, call for even higher peak power.
Achievements in solid-state laser technology can help the further development of ultrashort-pulse, high-peak-power CO2 laser systems. Modern solid-state lasers can be directly used in mid-IR systems, e.g., for controlling optical switches, pumping CO2 laser transition, or generating the ultrashort 10-µm seed pulses via nonlinear frequency conversion and parametric amplification. Apart from that, the advanced techniques initially developed for solid-state lasers (e.g. chirped pulse amplification) can be adopted in CO2 laser systems.
#### **8. Acknowledgements**
This work is supported by the US DOE contract DE-AC02-98CH10886 and by the BNL Laboratory Directed R&D (LDRD) grant #07-004. The authors are thankful to Sergei Tochitsky from UCLA's Neptune Laboratory for providing information on the Neptune's CO2 laser.
#### **9. References**
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**5**
Akira Endo
*Japan*
**High Average Power Pulsed CO2 Laser**
*Research Institute for Science and Engineering, Waseda University, Tokyo*
Increase of average power of pulsed CO2 laser was required by strong demand of the semiconductor industry, in pursue of the next generation of lithography light source at 13.5nm (Endo, et.al, 2006). The target average EUV power was increased from 10W level in the beginning to several 100W levels in the recent maturing period. No existing solid state laser technology satisfies the demand of the average power as the laser driver, by counting the laser-EUV conversion efficiency around 1%. Intensive research of one decade also showed that opacity, namely self absorption of the generated EUV light is less significant in high Z plasma, driven by a longer wavelength laser. CO2 laser produced Tin plasma showed more than 4% conversion efficiency in practical target geometry. Details are recently reviewed by A.Endo (Endo, 2010) and V.Y.Banine (Banine et.al, 2011). Interested readers are advised to refer to these articles. The established architecture is shown in Fig.1 as the laser produced Tin plasma which is generated from mist target of 300μm diameter, irradiated by 15ns CO2 laser pulse. The mist target is produced from a 10μm Tin droplet after irradiation
The conversion efficiency (CE), from the input laser pulse energy to the generated EUV pulse energy at 13.5nm (2% bandwidth, 2*π* sr), is the major parameter for improvement in high average power EUV light source for better economy. Low repetition rate pulsed CO2 laser was composed of transverse discharge modules, and often employed in laser plasma
**1. Introduction**
**1.1 Background of the emerging technology**
by a solid state laser with smaller pulse energy.
Fig. 1. Schematic of Tin plasma by double pulse method
**for Short Wavelength Light Sources**
### **High Average Power Pulsed CO2 Laser for Short Wavelength Light Sources**
Akira Endo
*Research Institute for Science and Engineering, Waseda University, Tokyo Japan*
#### **1. Introduction**
162 CO2 Laser – Optimisation and Application
Yablonovich, E. (1973). Spectral broadening in the light transmitted through a rapidly
Yablonovich, E. (1974a). Short CO2 laser pulse generation by optical free induction decay. *Appl. Phys. Lett.,* Vol.25, No.10, (November 1974), pp. 580-582, ISSN 0003-6951 Yablonovich, E. (1974b). Self-phase modulation and short-pulse generation from laser-
Yakimenlo, V. & Pogorelsky, I. (2006). Polarized γ source based on Compton backscattering
0031-9007
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breakdown plasmas. *Phys. Rev. A,* Vol.10, No.5, (November 1974), pp. 1888-1895,
in a laser cavity. *Phys. Rev. ST Accel. Beams,* Vol.9, No.9, (September 2006), p.
#### **1.1 Background of the emerging technology**
Increase of average power of pulsed CO2 laser was required by strong demand of the semiconductor industry, in pursue of the next generation of lithography light source at 13.5nm (Endo, et.al, 2006). The target average EUV power was increased from 10W level in the beginning to several 100W levels in the recent maturing period. No existing solid state laser technology satisfies the demand of the average power as the laser driver, by counting the laser-EUV conversion efficiency around 1%. Intensive research of one decade also showed that opacity, namely self absorption of the generated EUV light is less significant in high Z plasma, driven by a longer wavelength laser. CO2 laser produced Tin plasma showed more than 4% conversion efficiency in practical target geometry. Details are recently reviewed by A.Endo (Endo, 2010) and V.Y.Banine (Banine et.al, 2011). Interested readers are advised to refer to these articles. The established architecture is shown in Fig.1 as the laser produced Tin plasma which is generated from mist target of 300μm diameter, irradiated by 15ns CO2 laser pulse. The mist target is produced from a 10μm Tin droplet after irradiation by a solid state laser with smaller pulse energy.
Fig. 1. Schematic of Tin plasma by double pulse method
The conversion efficiency (CE), from the input laser pulse energy to the generated EUV pulse energy at 13.5nm (2% bandwidth, 2*π* sr), is the major parameter for improvement in high average power EUV light source for better economy. Low repetition rate pulsed CO2 laser was composed of transverse discharge modules, and often employed in laser plasma
High Average Power Pulsed CO2 Laser for Short Wavelength Light Sources 165
power from the main amplifier was 7kW, but the experimentally obtained power of 5kW
The laser beam quality was measured with a ZnSe lens of 508mm focal length and a slit-scan type beam profiler (Photon Inc., NanoScan). The laser beam size at the lens focus was measured for the oscillator and amplifier, resulting in a beam quality factor M2 as 1.1. Especially, the laser beam size was identical before and after amplification, i.e. the amplification did not cause any phase distortion. Fig.3 shows a typical spatial beam profile. Fig.4 shows the temporal laser pulse profile of the amplified laser output. The pulse duration was 20 ns (FWHM) and the pedestal was below 10% of the total pulse energy. A pedestal and/or tail of the seed laser pulse could be amplified and reduce the laser gain. Back scattering light from Tin mist target is experimentally less than 10% of the input laser energy, and backward amplification must be carefully avoided by full depletion of residual laser gain.
indicated many factors for further improvement.
Fig. 3. CO2 laser beam of M2=1.1
Fig. 4. CO2 laser pulse with low pedestal
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Intensity
Intensity
Tim e 50 ns/div.
Time 50 ns/div.
Fig. 2. Schematic diagram of multi stage MOPA, short pulse CO2 laser
experiments until 90's, but gradually disappeared from laboratories after improvements of solid state pulsed lasers. It was once employed as a driver of a plasma X-ray laser from a carbon target (Suckewer, et.al., 1983).
Medium average power pulsed CO2 laser systems are very successful tools for various applications ranging from material processing of metals, glass, ceramics and epoxy, paint removal and medical or spectroscopic applications, to the generation of laser produced plasmas as UV, EUV and soft X-ray sources. One drawback is the limited repetition rate of TEA CO2 laser based source, another drawback is limited controllability of the pulse width in low pressure microwave excited lasers. Attempts were reported in early 90's to operate microwave excited CO2 laser modules in a Q-switched oscillator mode of CW 2kW device (Sakai et.al., 1994) and an oscillator-amplifier mode of CW 7kW system (Bielesch et.al., 1992). Typical performances were at the repetition rate of 4 kHz with output average power of 680 W with pulse energy of 170 mJ and pulse width in full width half maximum (FWHM) of 250 ns, and at the repetition rate of 10 kHz with average power of 800W, with pulse energy of 70 mJ, and 35 ns pulse width, respectively. Laser extraction efficiencies, however, were not very high in both cases in the short pulse mode. Commercially available short pulse CO2 laser oscillator was known typically as EOM-10 from De Maria Electro Optics Systems, Inc (now Coherent Inc). The specification was average power of 10W at 100 kHz repetition rate with 15ns pulse width. The design guideline of a multi kW short pulse CO2 laser system is characterized by high repetition rate, high pulse energy, high amplification efficiency and high beam quality. The system is based on commercial high average power CW CO2 laser modules as amplifiers.
A short pulse oscillator was installed in our laboratory as the seeder for the amplifiers. The laser was an EO Q-switched, pulse length of 15~30 ns, single P(20) line, RF pumped waveguide CO2 laser with 60 W output at a repetition rate of 100 kHz. The repetition rate was tunable as 10~140 kHz. Commercial 5 kW and 15 kW CW CO2 lasers were employed as amplifiers. Every unit was 13.56 MHz RF-excited, fast axial flow lasers from Trumpf Inc. Lasers were modified as amplifiers by replacing both cavity mirror with ZnSe windows. The 5 kW laser used a standard gas composition of CO2:N2:He=5:29:66 at 120 Torr gas pressure. The axial gas flow speed was sufficiently high to keep the laser gas temperature low inside the operational condition. The length of a single gain region was 15 cm, and 16 cylindrical gain regions were connected in series in one laser unit; the tube inner diameter was 17mm. The total length of the optical pass inside the laser was 590 cm. The laser operated at 5 kW CW output power with a M2 =1.8 beam quality. The electrical input power was 36 kW. The 15 kW laser as the main amplifier, used a standard gas composition of CO2:N2:He=2:10:48 at 150 Torr gas pressure. The length of a single gain region was 28 cm, and 16 active cylindrical gain regions were connected in series; the tube inner diameter was 30 mm. The total length of the optical pass inside the laser was 890 cm. The maximum electrical input power was 88 kW. The key parameters of the amplifier are the extraction efficiency and beam quality. A series of experiments were performed to clarify these parameters to estimate the final possible values (Hoshino et.al., 2008).
The experimental setup is shown as Fig.2 with expected output power of 10kW. The maximum average output power of 8 kW was experimentally obtained at a repetition rate of 100 kHz with 3kW input power to the main amplifier. Parasitic oscillations and/or optical coupling between amplifier modules were not significant in burst mode. It was successful to extract 5kW power in pulsed mode from CW 15kW laser. The extraction efficiency (output power-input power/ CW output power) was over 30%. Initial estimation of extractable
experiments until 90's, but gradually disappeared from laboratories after improvements of solid state pulsed lasers. It was once employed as a driver of a plasma X-ray laser from a
Medium average power pulsed CO2 laser systems are very successful tools for various applications ranging from material processing of metals, glass, ceramics and epoxy, paint removal and medical or spectroscopic applications, to the generation of laser produced plasmas as UV, EUV and soft X-ray sources. One drawback is the limited repetition rate of TEA CO2 laser based source, another drawback is limited controllability of the pulse width in low pressure microwave excited lasers. Attempts were reported in early 90's to operate microwave excited CO2 laser modules in a Q-switched oscillator mode of CW 2kW device (Sakai et.al., 1994) and an oscillator-amplifier mode of CW 7kW system (Bielesch et.al., 1992). Typical performances were at the repetition rate of 4 kHz with output average power of 680 W with pulse energy of 170 mJ and pulse width in full width half maximum (FWHM) of 250 ns, and at the repetition rate of 10 kHz with average power of 800W, with pulse energy of 70 mJ, and 35 ns pulse width, respectively. Laser extraction efficiencies, however, were not very high in both cases in the short pulse mode. Commercially available short pulse CO2 laser oscillator was known typically as EOM-10 from De Maria Electro Optics Systems, Inc (now Coherent Inc). The specification was average power of 10W at 100 kHz repetition rate with 15ns pulse width. The design guideline of a multi kW short pulse CO2 laser system is characterized by high repetition rate, high pulse energy, high amplification efficiency and high beam quality. The system is based on commercial high average power CW CO2 laser modules as amplifiers. A short pulse oscillator was installed in our laboratory as the seeder for the amplifiers. The laser was an EO Q-switched, pulse length of 15~30 ns, single P(20) line, RF pumped waveguide CO2 laser with 60 W output at a repetition rate of 100 kHz. The repetition rate was tunable as 10~140 kHz. Commercial 5 kW and 15 kW CW CO2 lasers were employed as amplifiers. Every unit was 13.56 MHz RF-excited, fast axial flow lasers from Trumpf Inc. Lasers were modified as amplifiers by replacing both cavity mirror with ZnSe windows. The 5 kW laser used a standard gas composition of CO2:N2:He=5:29:66 at 120 Torr gas pressure. The axial gas flow speed was sufficiently high to keep the laser gas temperature low inside the operational condition. The length of a single gain region was 15 cm, and 16 cylindrical gain regions were connected in series in one laser unit; the tube inner diameter was 17mm. The total length of the optical pass inside the laser was 590 cm. The laser operated at 5 kW CW output power with a M2 =1.8 beam quality. The electrical input power was 36 kW. The 15 kW laser as the main amplifier, used a standard gas composition of CO2:N2:He=2:10:48 at 150 Torr gas pressure. The length of a single gain region was 28 cm, and 16 active cylindrical gain regions were connected in series; the tube inner diameter was 30 mm. The total length of the optical pass inside the laser was 890 cm. The maximum electrical input power was 88 kW. The key parameters of the amplifier are the extraction efficiency and beam quality. A series of experiments were performed to clarify these parameters to estimate the final
The experimental setup is shown as Fig.2 with expected output power of 10kW. The maximum average output power of 8 kW was experimentally obtained at a repetition rate of 100 kHz with 3kW input power to the main amplifier. Parasitic oscillations and/or optical coupling between amplifier modules were not significant in burst mode. It was successful to extract 5kW power in pulsed mode from CW 15kW laser. The extraction efficiency (output power-input power/ CW output power) was over 30%. Initial estimation of extractable
carbon target (Suckewer, et.al., 1983).
possible values (Hoshino et.al., 2008).
power from the main amplifier was 7kW, but the experimentally obtained power of 5kW indicated many factors for further improvement.
Fig. 2. Schematic diagram of multi stage MOPA, short pulse CO2 laser
The laser beam quality was measured with a ZnSe lens of 508mm focal length and a slit-scan type beam profiler (Photon Inc., NanoScan). The laser beam size at the lens focus was measured for the oscillator and amplifier, resulting in a beam quality factor M2 as 1.1. Especially, the laser beam size was identical before and after amplification, i.e. the amplification did not cause any phase distortion. Fig.3 shows a typical spatial beam profile. Fig.4 shows the temporal laser pulse profile of the amplified laser output. The pulse duration was 20 ns (FWHM) and the pedestal was below 10% of the total pulse energy. A pedestal and/or tail of the seed laser pulse could be amplified and reduce the laser gain. Back scattering light from Tin mist target is experimentally less than 10% of the input laser energy, and backward amplification must be carefully avoided by full depletion of residual laser gain.
Fig. 3. CO2 laser beam of M2=1.1
Fig. 4. CO2 laser pulse with low pedestal
High Average Power Pulsed CO2 Laser for Short Wavelength Light Sources 167
1975). The rotational relaxation time is calculated as τr = 2.3ns for 100 Torr and a 4500C medium, while the typical laser pulse width in the example is τp = 15ns. The characteristic number τp /τr is 6.5 for 100 Torr medium, and the work of F.Rheaut deals with the case of
As a parameter study, small signal gain coefficient g0 is tentatively assumed as 1%/cm, and the saturation fluence is as 10mJ/cm2 for a 100 Torr RF pumped CO2 laser amplifier. The maximum fluence Em is then given as 10mJ/cm2 for L=1m gain length. Figure 6 shows one early experimental result of 15ns pulse amplification at 100 kHz of an axial flow CW 15kW laser. The average beam diameter was ~ 15mm, and amplification parameters were calculated as g0=0.43%/cm, and Es=8.0mJ/cm2, by numerical fitting of the experimental data to the equation (1). The fitting gave reasonable results in spite of the τp ≳τr condition (Ariga, 2007). It was concluded that the Frantz-Nodvik equation was practically usable in this semiintermediate region for characterization of the amplifier parameters. It seems reasonable by accounting the measured Es, together with an effort to increase g0 to 1%/cm,to obtain 100mJ pulse energy from the amplifier once the gain cross section A is 10cm2, or the gain length L is 10m with A=1cm2. The cross section A is depending on the amplifier design, and the gain
Fig. 6. Short pulse amplification data of 15kW CW output power module with 15ns pulse width at 100 kHz. The results were best fitted by the Frantz-Nodvik equation to give the
A short pulse amplifier is characterized by its medium gain diameter, D and gain length, L. The most common limitation of the available gain is set by the amplified spontaneous emission (ASE) on the optical axis, which deteriorates the pulse signal to noise ratio (SN), and depletes the available gain. A general mathematical formula on this phenomenon has been reported by Lowenthal et.al. (Lowental et.al,1986). On axis ASE flux is given by equation (11) from this work, for the purpose of numerical calculation. Figure 16 of the paper shows the achieved g0L with parameters L/D. It is generally advised to keep g0L less than 3 for a single pass amplifier, especially for storage lasers which have no coherent flux to extract gain simultaneously with pumping (short pulse amplification in cw pumped medium). A short pulse CO2 laser amplifier has a typical gain diameter of 1cm, an optical gain length of 1m and an aspect ratio L/D large enough to consider the configuration as one dimensional. The gain depletion effect is less significant compared to the cubic gain medium previously reported (Lowenthal, et.al. 1986), but the on axis ASE effect is similar. Parasitic
almost the same ratio for 1 atmosphere.
length L is limited by self oscillation of the amplifier.
effective value of g0 and Es.
#### **2. 10ns, 100 kHz operation of CO2 laser at 10kW average power**
CO2 molecular dynamics is the fundamental subject to understand the operational parameters of pulsed CO2 laser. Figure 5 shows a typical energy diagram of CO2 laser active medium.
Fig. 5. Energy level diagram of active CO2 laser medium
Short pulse, high repetition operational CO2 laser is the most suitable laser technology to meet the requirements of HVM EUV source. Free electron lasers (FEL) are also capable of realizing high average power, short pulse coherent beam, based on the emerging superconducting energy recovery linac (sERL) technology (Krafft, 2006). However, the pulse energy is in sub mJ range with a few ps pulse width at 75MHz repetition rate, which does not match to the plasma production specifications for EUV sources.
Short pulse CO2 laser technology was well studied by TEA discharge oscillator-amplifier configuration up until the 1990's (Decker, et.al, 1991). There were two major limitations on this scheme, namely, a low repetition rate up to 10Hz and backscatter amplification in nonsaturated amplifier medium. The gain medium in the RF pumped CO2 laser is a CO2/N2/He mixture of typically 100 Torr pressure. The CO2 molecule stores energy in the rotational-vibration mode from electric collision excitation in the 0001 band, and the typical relaxation time for the vibration is 0.5μsec, which is shorter than the pulse interval time of 10μsec for 100kHz repetition rate. The amplification that is described in this case is short pulse amplification and expressed by the Frantz-Nodvik equation (Rheaut, et.al, 1973), where Ein is the input fluence in mJ/cm2, Eout is the output fluence, Es is the saturation fluence, g0 is the small signal gain coefficient in cm-1, L is the gain medium length in cm. Maximum available fluence after single pass amplification Em is given by g0·L·Es in mJ/cm2. g0 and Es are functions of medium parameters proportional to the upper state molecule numbers as, σN\*, and inversely proportional to the radiative cross section as hν/2σ , each other. The Frantz-Nodvik equation describes the short pulse amplification as,
$$E\_{out} = E\_s \cdot \ln[1 + \exp(\mathbf{g}\_0 \cdot L)[\exp(\frac{E\_{in}}{E\_s}) - 1]] \tag{1}$$
and the equation is valid provided that the pulse duration τp is long compared to the rotational relaxation time τr , or short, namely in the case of τp τr or τp τr . CO2 laser amplification in the intermediate region, namely in the case τp ≈τr , is treated by rotational reservoir model calculations, which requires non practical numerical solutions (Harrach,
CO2 molecular dynamics is the fundamental subject to understand the operational parameters of pulsed CO2 laser. Figure 5 shows a typical energy diagram of CO2 laser active medium.
Short pulse, high repetition operational CO2 laser is the most suitable laser technology to meet the requirements of HVM EUV source. Free electron lasers (FEL) are also capable of realizing high average power, short pulse coherent beam, based on the emerging superconducting energy recovery linac (sERL) technology (Krafft, 2006). However, the pulse energy is in sub mJ range with a few ps pulse width at 75MHz repetition rate, which does
Short pulse CO2 laser technology was well studied by TEA discharge oscillator-amplifier configuration up until the 1990's (Decker, et.al, 1991). There were two major limitations on this scheme, namely, a low repetition rate up to 10Hz and backscatter amplification in nonsaturated amplifier medium. The gain medium in the RF pumped CO2 laser is a CO2/N2/He mixture of typically 100 Torr pressure. The CO2 molecule stores energy in the rotational-vibration mode from electric collision excitation in the 0001 band, and the typical relaxation time for the vibration is 0.5μsec, which is shorter than the pulse interval time of 10μsec for 100kHz repetition rate. The amplification that is described in this case is short pulse amplification and expressed by the Frantz-Nodvik equation (Rheaut, et.al, 1973), where Ein is the input fluence in mJ/cm2, Eout is the output fluence, Es is the saturation fluence, g0 is the small signal gain coefficient in cm-1, L is the gain medium length in cm. Maximum available fluence after single pass amplification Em is given by g0·L·Es in mJ/cm2. g0 and Es are functions of medium parameters proportional to the upper state molecule numbers as, σN\*, and inversely proportional to the radiative cross section as hν/2σ , each
**2. 10ns, 100 kHz operation of CO2 laser at 10kW average power**
Fig. 5. Energy level diagram of active CO2 laser medium
not match to the plasma production specifications for EUV sources.
other. The Frantz-Nodvik equation describes the short pulse amplification as,
*<sup>E</sup> E E gL <sup>E</sup>*
and the equation is valid provided that the pulse duration τp is long compared to the rotational relaxation time τr , or short, namely in the case of τp τr or τp τr . CO2 laser amplification in the intermediate region, namely in the case τp ≈τr , is treated by rotational reservoir model calculations, which requires non practical numerical solutions (Harrach,
*out s*
<sup>0</sup> ln[1 exp( )[exp( ) 1]] *in*
*s*
=⋅ + ⋅ − (1)
1975). The rotational relaxation time is calculated as τr = 2.3ns for 100 Torr and a 4500C medium, while the typical laser pulse width in the example is τp = 15ns. The characteristic number τp /τr is 6.5 for 100 Torr medium, and the work of F.Rheaut deals with the case of almost the same ratio for 1 atmosphere.
As a parameter study, small signal gain coefficient g0 is tentatively assumed as 1%/cm, and the saturation fluence is as 10mJ/cm2 for a 100 Torr RF pumped CO2 laser amplifier. The maximum fluence Em is then given as 10mJ/cm2 for L=1m gain length. Figure 6 shows one early experimental result of 15ns pulse amplification at 100 kHz of an axial flow CW 15kW laser. The average beam diameter was ~ 15mm, and amplification parameters were calculated as g0=0.43%/cm, and Es=8.0mJ/cm2, by numerical fitting of the experimental data to the equation (1). The fitting gave reasonable results in spite of the τp ≳τr condition (Ariga, 2007). It was concluded that the Frantz-Nodvik equation was practically usable in this semiintermediate region for characterization of the amplifier parameters. It seems reasonable by accounting the measured Es, together with an effort to increase g0 to 1%/cm,to obtain 100mJ pulse energy from the amplifier once the gain cross section A is 10cm2, or the gain length L is 10m with A=1cm2. The cross section A is depending on the amplifier design, and the gain length L is limited by self oscillation of the amplifier.
Fig. 6. Short pulse amplification data of 15kW CW output power module with 15ns pulse width at 100 kHz. The results were best fitted by the Frantz-Nodvik equation to give the effective value of g0 and Es.
A short pulse amplifier is characterized by its medium gain diameter, D and gain length, L. The most common limitation of the available gain is set by the amplified spontaneous emission (ASE) on the optical axis, which deteriorates the pulse signal to noise ratio (SN), and depletes the available gain. A general mathematical formula on this phenomenon has been reported by Lowenthal et.al. (Lowental et.al,1986). On axis ASE flux is given by equation (11) from this work, for the purpose of numerical calculation. Figure 16 of the paper shows the achieved g0L with parameters L/D. It is generally advised to keep g0L less than 3 for a single pass amplifier, especially for storage lasers which have no coherent flux to extract gain simultaneously with pumping (short pulse amplification in cw pumped medium). A short pulse CO2 laser amplifier has a typical gain diameter of 1cm, an optical gain length of 1m and an aspect ratio L/D large enough to consider the configuration as one dimensional. The gain depletion effect is less significant compared to the cubic gain medium previously reported (Lowenthal, et.al. 1986), but the on axis ASE effect is similar. Parasitic
High Average Power Pulsed CO2 Laser for Short Wavelength Light Sources 169
It is indicated that the thermal lens effect is not deterministic at low duty operation (2%), but significant at 100% duty operation. Filling factor Φ, is the parameter used to measure the
This is reduced at higher duty operation due to a shorter thermal lens focus length, and amplification saturation is lower at higher duty operation with reduced Φ. Specifically designed active feedback control is necessary to stabilize the beam propagation at high duty operation. Figure 8 shows an experimental example of the beam diameter at the amplifier exit, with and without active beam control (Nowak, et.al, 2008). Lower spatial quality oscillator beam causes micro lens effects in the transparent optical components, and leads to chaotic beam amplification with a higher M2 number. Design of a controlled spectrum oscillator with high quality spatial profile is important for better amplified beam quality,
Fig. 8. Measured beam diameter after output window with and without active optical
Precise control of the amplification depends on the quality of the oscillator pulse. Recent development of two different IR source technologies, namely OPA(Optical Parametric Amplifier) and QCL(Quantum Cascade Laser) are reviewed in this paragraph. Compact slab CO2 laser is employed as the gain medium of regenerative amplifier to boost the weak initial IR beam. OPA and QCL are potentially controllable in its pulse width even in the pico
Laser beam is characterized by its spectrum, and the low pressure CO2 laser medium is composed of many vibration-rotational lines. The P(20) line is the single laser line in normal oscillation conditions at 10.6μm wavelength. The rotational relaxation time is calculated as τr = 2.3ns for a 100 Torr and 4500C medium, which is not negligibly small compared to the typical laser pulse width τp = 15ns. Electrically excited energy distributes in many other rotational modes of CO2 molecules, and collisional relaxation to the P(20) line is limited
Φ= beam volume/gain volume (3)
usability of the gain region by the propagating beam,
**3. New generation oscillator technologies**
and efficient amplification.
feedback.
second range.
oscillation is experienced in actual experiments and this is often the practical limit for full amplification. The phenomena are strictly dependant on the device design, namely partial reflection from laser wall, optics holders or leakage through isolators. It is an issue to be treated for each laser system, but the physical fundamental is the same as the double pass case by Lowenthal et.al.
It is preferable to employ gaseous saturable absorbers like SF6 in the multi stage amplifiers, to avoid damage to solid-state saturable absorbers like p-doped Germanium (Haglund, et.al. 1981). The typical switching threshold in the gaseous saturable absorbers is 10mJ/cm2. Also, the optical beam delivery system requires optimization to efficiently depress pulse pedestal. An electro optical switch, which uses solid-state material like CdTe, is employed in the lower average power stage to realize better noise depression (Slattery, et.al. 1975). The damage threshold is dependant on the laser parameters such as pulse width, fluence, average power and beam uniformity. Laser beam containing hot spots can cause damage to the material surface even at much lower fluence. Solid-state materials suffer stronger thermal lens effects as shown by the following formula (Koechner, 1999).
$$f = \frac{KA}{P\_a} \left(\frac{1}{2}\frac{dn}{dT} + \alpha C\_{r,\theta} n\_0^3 + \frac{\alpha r\_0 \left(n\_0 - 1\right)}{L}\right)^{-1} \tag{2}$$
The first term is the temperature dependant refractive index change, the second term is stress induced refractive index change, and the final term is temperature dependant surface modification. Thermal lensing is the most significant phenomenon in the power amplifier stage with optical components, especially for ZnSe or Diamond windows. The effective focus depends on the beam power, and active feedback is necessary to realize stable beam propagation throughout the whole laser optical path. Figure 7 shows a model calculation of the thermal loading effect to the beam propagation through 2 stage amplifiers of axial flow type of 5kW CW output, with an input of pulses in 20ns at 100 kHz repetition rate input with 60W average power (Nowak, et.al, 2008).
Fig. 7. Calculated results of the amplified beam diameter behavior depending on the operational duty. Input power is 60W at a distance of 0m, and the output optics of the first 5kW amplifier is at a distance of 10m. The input optics of the second module is at a distance of 11m. These windows suffer from strong thermal lensing effects which lead to beam diameter fluctuation.
oscillation is experienced in actual experiments and this is often the practical limit for full amplification. The phenomena are strictly dependant on the device design, namely partial reflection from laser wall, optics holders or leakage through isolators. It is an issue to be treated for each laser system, but the physical fundamental is the same as the double pass
It is preferable to employ gaseous saturable absorbers like SF6 in the multi stage amplifiers, to avoid damage to solid-state saturable absorbers like p-doped Germanium (Haglund, et.al. 1981). The typical switching threshold in the gaseous saturable absorbers is 10mJ/cm2. Also, the optical beam delivery system requires optimization to efficiently depress pulse pedestal. An electro optical switch, which uses solid-state material like CdTe, is employed in the lower average power stage to realize better noise depression (Slattery, et.al. 1975). The damage threshold is dependant on the laser parameters such as pulse width, fluence, average power and beam uniformity. Laser beam containing hot spots can cause damage to the material surface even at much lower fluence. Solid-state materials suffer stronger
3 0 0
−
(2)
, 0 1 1
φ α − = +α +
The first term is the temperature dependant refractive index change, the second term is stress induced refractive index change, and the final term is temperature dependant surface modification. Thermal lensing is the most significant phenomenon in the power amplifier stage with optical components, especially for ZnSe or Diamond windows. The effective focus depends on the beam power, and active feedback is necessary to realize stable beam propagation throughout the whole laser optical path. Figure 7 shows a model calculation of the thermal loading effect to the beam propagation through 2 stage amplifiers of axial flow type of 5kW CW output, with an input of pulses in 20ns at 100 kHz repetition rate input
thermal lens effects as shown by the following formula (Koechner, 1999).
2 *<sup>r</sup>*
Fig. 7. Calculated results of the amplified beam diameter behavior depending on the operational duty. Input power is 60W at a distance of 0m, and the output optics of the first 5kW amplifier is at a distance of 10m. The input optics of the second module is at a distance of 11m. These windows suffer from strong thermal lensing effects which lead to beam
*f C n*
*KA dn r n*
*P dT L*
( ) <sup>1</sup>
*a*
with 60W average power (Nowak, et.al, 2008).
diameter fluctuation.
case by Lowenthal et.al.
It is indicated that the thermal lens effect is not deterministic at low duty operation (2%), but significant at 100% duty operation. Filling factor Φ, is the parameter used to measure the usability of the gain region by the propagating beam,
$$\spadesuit = \textbf{beam volume} / \text{gain volume} \tag{3}$$
This is reduced at higher duty operation due to a shorter thermal lens focus length, and amplification saturation is lower at higher duty operation with reduced Φ. Specifically designed active feedback control is necessary to stabilize the beam propagation at high duty operation. Figure 8 shows an experimental example of the beam diameter at the amplifier exit, with and without active beam control (Nowak, et.al, 2008). Lower spatial quality oscillator beam causes micro lens effects in the transparent optical components, and leads to chaotic beam amplification with a higher M2 number. Design of a controlled spectrum oscillator with high quality spatial profile is important for better amplified beam quality, and efficient amplification.
Fig. 8. Measured beam diameter after output window with and without active optical feedback.
#### **3. New generation oscillator technologies**
Precise control of the amplification depends on the quality of the oscillator pulse. Recent development of two different IR source technologies, namely OPA(Optical Parametric Amplifier) and QCL(Quantum Cascade Laser) are reviewed in this paragraph. Compact slab CO2 laser is employed as the gain medium of regenerative amplifier to boost the weak initial IR beam. OPA and QCL are potentially controllable in its pulse width even in the pico second range.
Laser beam is characterized by its spectrum, and the low pressure CO2 laser medium is composed of many vibration-rotational lines. The P(20) line is the single laser line in normal oscillation conditions at 10.6μm wavelength. The rotational relaxation time is calculated as τr = 2.3ns for a 100 Torr and 4500C medium, which is not negligibly small compared to the typical laser pulse width τp = 15ns. Electrically excited energy distributes in many other rotational modes of CO2 molecules, and collisional relaxation to the P(20) line is limited
High Average Power Pulsed CO2 Laser for Short Wavelength Light Sources 171
Fig. 10. Absolute (●,▼) and relative (X) energy extraction from a main amplifier tube pumped by a CW RF discharge with discharge power 88 kW. The input beam shape is Gaussian with diameter D=18mm. The energy extraction is plotted to the input pulse energy. The symbol -X- is the relative gain ratio of single P20 line to 4 lines (P16-P22)
It is understood that the high conversion efficiency of EUV light comes from the UTA (unresolved transition arrays) of highly ionized high Z plasma. The peak wavelength of the UTA is depending on the Z number, and further shorter wavelength plasma has a similar physical behavior (Li, et.al. 2011). The first generation EUV source at 13.5nm wavelength works well up to 11nm node of semiconductor mass production within the next one decade. A possibility to switch to a shorter wavelength (BEUV: beyond EUV) is required to be studied in advance (Banine, et.al, 2010). Availability of high reflectivity mirror at 6.x nm region initiated basic research to find candidate heavy elements with high UTA emission in this wavelength region (Churilov, et.al, 2009). Intensive research has revealed that the UTA emission has peak intensity from Gadolinium at 6.7nm wavelength comparably efficient to that of Tin at 13.5nm. Lower density plasma is favorable for less opacity effect, and short pulse CO2 laser is again the best driver (Higashiguchi et.al, 2011). The optimum plasma temperature increases from 40eV for Sn at 13.5nm, to 150eV for Gd at 6.x nm. The laser
T ∝ ( I0 λ2 ) 2/3 (4) where I0 is the laser peak intensity, and λ is the laser wavelength (Ramis et.al, 1983). It is understood that a short pulse CO2 laser is better fitted to higher plasma temperature due to its longer wavelength. Optimum laser pulse width for highest CE for Sn at 13.5nm is typically 15nsec, but higher temperature Gd plasma dissipates faster, and shorter pulse length may be required for highest CE. Minimum sustainable pulse width in low pressure CO2 laser amplifier is estimated from rotational gain bandwidth Δν(Abrams, 1974) as,
Δν = 7.58 ( φCO2 + 0.73φN2 + 0.64φHe ) x P(300/T)1/2 (5)
amplification
**4. Further developments**
plasma temperature is expressed as
during the amplified pulse period. Figure 9 shows the spectrum structure of the laser lines, with a broad continuous spectrum from a solid state seeder overlapped, to fully extract the stored energy. Recent advanced nonlinear laser technology is at the stage of operating a broad band optical parametric oscillator (OPA) at the center wavelength of 10.6μm, with more than 10mW at 150kHz (Light Conversion, "ORPHEUS", 2011). This specification is enough to seed a single transverse mode CO2 laser oscillator.
Fig. 9. CO2 laser spectrum structure overlapped with a broad OPA seeder spectrum
Multiline amplification was calculated to evaluate the effectiveness of the low pressure CO2 laser at 15kW CW output power. The beam diameter was assumed to be 18mm. Numerical result shows the amplification enhancement, with 4 lines composed of P(16,18,20,22) as 1.3 times higher than the single P(20) amplification case. Figure 10 shows the calculation result of the gain Γ. Emerging quantum cascade lasers (QCL) are available, which can generate specific lines of the P band of the CO2 laser (Cascade Technologies, LS03D, 2009) and possibly seed a CO2 laser oscillator with a discrete spectrum. QCL lasers are thus ideal as compact and robust seed sources. They can be accurately tuned to particular gain lines of the CO2 medium with sufficient accuracy. QCL lasers are capable of tens of mW output power at typical pulse durations of 10ns, providing good bandwidth matching to a lasing line in a typical CO2 medium. A QCL can provide at least 3 orders of magnitude higher power per self oscillation lines of a small oscillator, thus relaxing the requirement of the roundtrip gain and the number of roundtrips in the seeded oscillator, thereby improving power output and stability. The theoretical prediction of Fig.10 was recently confirmed by QCL multiline seeded pulses in a large slab amplifier (Nowak, 2011).
Multiline amplification effectively enhances small signal gain g0 compared to CW gain, and saturation fluence Es, by improving the spectrum factor, and this leads to the enhancement of maximum available flux Em. The final optical limit, typically 1J/cm2, is given by the optical damage of the output window, which is more than one magnitude higher than the available Em. The small signal gain g0 and saturation fluence Es are the two basic parameters to characterize for any amplifiers (DeaAutels, et.al. 2003). It is reasonable to expect double enhancement of Em to 20mJ/cm2 after optimization of amplifier parameters, and the available beam energy as 200mJ with the effective gain volume as LA = 1000 cm3 (1 litter). Typical repetition rate of 100 kHz gives the average output power as 20kW, after meeting all requirements described in this article.
Fig. 10. Absolute (●,▼) and relative (X) energy extraction from a main amplifier tube pumped by a CW RF discharge with discharge power 88 kW. The input beam shape is Gaussian with diameter D=18mm. The energy extraction is plotted to the input pulse energy. The symbol -X- is the relative gain ratio of single P20 line to 4 lines (P16-P22) amplification
#### **4. Further developments**
170 CO2 Laser – Optimisation and Application
during the amplified pulse period. Figure 9 shows the spectrum structure of the laser lines, with a broad continuous spectrum from a solid state seeder overlapped, to fully extract the stored energy. Recent advanced nonlinear laser technology is at the stage of operating a broad band optical parametric oscillator (OPA) at the center wavelength of 10.6μm, with more than 10mW at 150kHz (Light Conversion, "ORPHEUS", 2011). This specification is
Fig. 9. CO2 laser spectrum structure overlapped with a broad OPA seeder spectrum
QCL multiline seeded pulses in a large slab amplifier (Nowak, 2011).
requirements described in this article.
Multiline amplification was calculated to evaluate the effectiveness of the low pressure CO2 laser at 15kW CW output power. The beam diameter was assumed to be 18mm. Numerical result shows the amplification enhancement, with 4 lines composed of P(16,18,20,22) as 1.3 times higher than the single P(20) amplification case. Figure 10 shows the calculation result of the gain Γ. Emerging quantum cascade lasers (QCL) are available, which can generate specific lines of the P band of the CO2 laser (Cascade Technologies, LS03D, 2009) and possibly seed a CO2 laser oscillator with a discrete spectrum. QCL lasers are thus ideal as compact and robust seed sources. They can be accurately tuned to particular gain lines of the CO2 medium with sufficient accuracy. QCL lasers are capable of tens of mW output power at typical pulse durations of 10ns, providing good bandwidth matching to a lasing line in a typical CO2 medium. A QCL can provide at least 3 orders of magnitude higher power per self oscillation lines of a small oscillator, thus relaxing the requirement of the roundtrip gain and the number of roundtrips in the seeded oscillator, thereby improving power output and stability. The theoretical prediction of Fig.10 was recently confirmed by
Multiline amplification effectively enhances small signal gain g0 compared to CW gain, and saturation fluence Es, by improving the spectrum factor, and this leads to the enhancement of maximum available flux Em. The final optical limit, typically 1J/cm2, is given by the optical damage of the output window, which is more than one magnitude higher than the available Em. The small signal gain g0 and saturation fluence Es are the two basic parameters to characterize for any amplifiers (DeaAutels, et.al. 2003). It is reasonable to expect double enhancement of Em to 20mJ/cm2 after optimization of amplifier parameters, and the available beam energy as 200mJ with the effective gain volume as LA = 1000 cm3 (1 litter). Typical repetition rate of 100 kHz gives the average output power as 20kW, after meeting all
enough to seed a single transverse mode CO2 laser oscillator.
It is understood that the high conversion efficiency of EUV light comes from the UTA (unresolved transition arrays) of highly ionized high Z plasma. The peak wavelength of the UTA is depending on the Z number, and further shorter wavelength plasma has a similar physical behavior (Li, et.al. 2011). The first generation EUV source at 13.5nm wavelength works well up to 11nm node of semiconductor mass production within the next one decade. A possibility to switch to a shorter wavelength (BEUV: beyond EUV) is required to be studied in advance (Banine, et.al, 2010). Availability of high reflectivity mirror at 6.x nm region initiated basic research to find candidate heavy elements with high UTA emission in this wavelength region (Churilov, et.al, 2009). Intensive research has revealed that the UTA emission has peak intensity from Gadolinium at 6.7nm wavelength comparably efficient to that of Tin at 13.5nm. Lower density plasma is favorable for less opacity effect, and short pulse CO2 laser is again the best driver (Higashiguchi et.al, 2011). The optimum plasma temperature increases from 40eV for Sn at 13.5nm, to 150eV for Gd at 6.x nm. The laser plasma temperature is expressed as
$$\mathbf{T} \ll \left(\mathbf{I}\_0 \lambda^2\right) 2^{\mathbf{\upbeta}} \tag{4}$$
where I0 is the laser peak intensity, and λ is the laser wavelength (Ramis et.al, 1983). It is understood that a short pulse CO2 laser is better fitted to higher plasma temperature due to its longer wavelength. Optimum laser pulse width for highest CE for Sn at 13.5nm is typically 15nsec, but higher temperature Gd plasma dissipates faster, and shorter pulse length may be required for highest CE. Minimum sustainable pulse width in low pressure CO2 laser amplifier is estimated from rotational gain bandwidth Δν(Abrams, 1974) as,
$$
\Delta \mathbf{v} = 7.58 \left( \mathbf{q}\_{\rm CO2} + 0.73 \mathbf{q}\_{\rm N2} + 0.64 \mathbf{q}\_{\rm He} \right) \times \mathbf{P} (\mathbf{300}/\mathbf{T})^{1/2} \tag{5}
$$
High Average Power Pulsed CO2 Laser for Short Wavelength Light Sources 173
Laser-Compton scattering photon spectrum has a peak in the forward direction at a
2
γ
λ
*L*
where γ and β are Lorentz factors, λL the laser undulation period (laser wavelength), K the K parameter of the undulator which is equivalent to the laser intensity parameter, and Φ the colliding angle. The spectrum depends on the angular distribution; the wavelength λ is
<sup>1</sup> <sup>θ</sup> *<sup>p</sup>*
It is seen that higher γ electron beam produces less divergent light. Figure 12 shows the relationship between electron beam energy and maximum (forward) photon energy for both laser wavelengths by Nd:YAG laser (1.06µm) and CO2 laser (10.6µm). As a result of Fig. 12, the required electron beam energy is 3.2MeV (1.06µm) and 10.2MeV (10.6µm) in order to produce 6.7nm SXR. It makes large difference to treat such a low energy electron beam. The Lorentz factor γ is 6.3 and 20.0, respectively, which means 3 times better directivity with
The general formula of obtainable photon flux N0 is calculated in the normal collision by the
*c e N Np <sup>N</sup> r* <sup>σ</sup> <sup>∝</sup> <sup>π</sup>
where σc is the Compton cross section (6.7 x 10-25 cm2), Ne the total electron number, Np the
Fig. 12. Laser-Compton photon energy vs electron beam energy. Comparison 1064nm
(Nd:YAG) with 10600nm (CO2) wavelength as the photon target.
2
*cos*
<sup>=</sup> + (9)
λ−λ <sup>=</sup> γ λ (10)
(11)
*K*
*p*
(1 ) <sup>2</sup> 2 (1 )
+
βφ
*p*
λ
wavelength;
emitted at
10.6µm laser than that of 1.06µm.
<sup>0</sup> <sup>2</sup> <sup>4</sup>
total photon number, and r the interaction area radius.
following expression.
where φ is the partial ratio of each component gas, P is the total pressure in Torr, and T is the gas temperature in K. Δν is given as
$$
\Delta \mathbf{v} = 424 \text{ MHz} \tag{6}
$$
for typical gas parameters as
$$\begin{array}{l} \text{CO}\_{2}: \text{N}\_{2}: \text{He} = 1:1:8 \\\\ \text{P} = 100 \text{ Torr} \\\\ \text{T} = \text{ 450 K} \end{array} \tag{7}$$
The minimum pulse width is estimated from the Fourier transform limit of a Gaussian pulse as
$$
\Delta \mathbf{v} \cdot \Delta \mathbf{t} = 0.44 \tag{8}
$$
and the resulting Δt is around 1 nsec. The present oscillator is a QCL seeded Q-switched, cavity dumped laser based on a RF pumped low pressure CO2 laser. Typical out put pulse width is 15nsec at 100 kHz repetition rate with 5W average power. Shorter pulse width is available by various methods like electro-optical or laser pulse slicing, depending on the requirement from future plasma experiments. Careful optical design of amplifiers can sustain the amplified pulse width for the requirement by dispersion compensation.
Another important field where 10μm wavelength is effective for short wavelength light generation, is the laser Compton X-ray generation. It is already well studied on the optimization of the laser-Compton hard X-ray source by single shot base (John, 1998, Endo, 2001). Experimental results agreed well with theoretical predictions. Highest peak brightness is obtained in the case of counter propagating laser pulse and electron beam bunch, in the minimum focusing before nonlinear threshold. The new short wavelength light source is now well matured to demonstrate single-shot phase contrast bio imaging in hard X-ray region (Oliva, et.al, 2010).
The major challenge of the laser Compton source in the EUV/SXR region is the lower electron beam voltage, which in turn results in a larger interaction cross section. Figure 11 describes the schematic of the laser-Compton interaction between electron beam and laser.
Fig. 11. Schematic of laser-Compton scattering process
172 CO2 Laser – Optimisation and Application
where φ is the partial ratio of each component gas, P is the total pressure in Torr, and T is
CO2 : N2 : He = 1:1:8
T = 450 K The minimum pulse width is estimated from the Fourier transform limit of a Gaussian pulse
and the resulting Δt is around 1 nsec. The present oscillator is a QCL seeded Q-switched, cavity dumped laser based on a RF pumped low pressure CO2 laser. Typical out put pulse width is 15nsec at 100 kHz repetition rate with 5W average power. Shorter pulse width is available by various methods like electro-optical or laser pulse slicing, depending on the requirement from future plasma experiments. Careful optical design of amplifiers can
Another important field where 10μm wavelength is effective for short wavelength light generation, is the laser Compton X-ray generation. It is already well studied on the optimization of the laser-Compton hard X-ray source by single shot base (John, 1998, Endo, 2001). Experimental results agreed well with theoretical predictions. Highest peak brightness is obtained in the case of counter propagating laser pulse and electron beam bunch, in the minimum focusing before nonlinear threshold. The new short wavelength light source is now well matured to demonstrate single-shot phase contrast bio imaging in
The major challenge of the laser Compton source in the EUV/SXR region is the lower electron beam voltage, which in turn results in a larger interaction cross section. Figure 11 describes the schematic of the laser-Compton interaction between electron beam and laser.
sustain the amplified pulse width for the requirement by dispersion compensation.
Δν = 424 MHz (6)
P = 100 Torr (7)
Δν·Δt = 0.44 (8)
the gas temperature in K. Δν is given as
hard X-ray region (Oliva, et.al, 2010).
Fig. 11. Schematic of laser-Compton scattering process
for typical gas parameters as
as
Laser-Compton scattering photon spectrum has a peak in the forward direction at a wavelength;
$$\lambda\_p = \frac{\lambda\_\perp (1 + \frac{K^2}{2})}{2\gamma^2 (1 + \beta \cos \phi)} \tag{9}$$
where γ and β are Lorentz factors, λL the laser undulation period (laser wavelength), K the K parameter of the undulator which is equivalent to the laser intensity parameter, and Φ the colliding angle. The spectrum depends on the angular distribution; the wavelength λ is emitted at
$$\Theta = \frac{1}{\gamma} \sqrt{\frac{\lambda - \lambda\_p}{\lambda\_p}} \tag{10}$$
It is seen that higher γ electron beam produces less divergent light. Figure 12 shows the relationship between electron beam energy and maximum (forward) photon energy for both laser wavelengths by Nd:YAG laser (1.06µm) and CO2 laser (10.6µm). As a result of Fig. 12, the required electron beam energy is 3.2MeV (1.06µm) and 10.2MeV (10.6µm) in order to produce 6.7nm SXR. It makes large difference to treat such a low energy electron beam. The Lorentz factor γ is 6.3 and 20.0, respectively, which means 3 times better directivity with 10.6µm laser than that of 1.06µm.
The general formula of obtainable photon flux N0 is calculated in the normal collision by the following expression.
$$N\_0 \approx \frac{\sigma\_\epsilon N\_\epsilon N\_p}{4\pi r^2} \tag{11}$$
where σc is the Compton cross section (6.7 x 10-25 cm2), Ne the total electron number, Np the total photon number, and r the interaction area radius.
Fig. 12. Laser-Compton photon energy vs electron beam energy. Comparison 1064nm (Nd:YAG) with 10600nm (CO2) wavelength as the photon target.
High Average Power Pulsed CO2 Laser for Short Wavelength Light Sources 175
1
where Reff is �R�R�. As described above, higher reflectivity provides a higher enhancement super-cavity. Particularly, the loss, which includes both absorption and scattering, on the reflection coating is critical issue for storing a high power laser beam as described above. Such a high quality optical mirror was difficult for far infrared wavelength, however, there are now some products usable for super-cavity mirrors (Ophir Optics, 2009). Super-cavity for 10.6μm laser pulse can be achieved with the enhancement of about 600 by using best mirrors available. Figure 14 shows the calculated transmission, reflection and stored power of the super-cavity as a function of the phase advance in one cavity circulation i.e. supercavity length. The dotted line shows the reflection/transmission from the cavity and the solid line is stored power inside the super-cavity, assuming as input power is 1. There is no transmission light because Mirror 2 transmission is 0%. The enhancement of 600 is achieved by this super-cavity. The precision of cavity length adjustment is one issue for stable operation. The requirement is one-order relaxed due to the wavelength in case of CO2 laser, thus the stable operation with enhancement of 600 and more can be easily achieved from our experiences in 1μm laser storage. Critical issue for higher enhancement is to obtain the extremely low loss and high reflectivity mirrors, which is the key R&D of multi-layer
Fig. 14. Calculated results of CO2 laser super-cavity as a function of phase advance in one revolution of cavity. 2 π phase advance corresponds to λ/2 cavity length mismatch with the
Concerning a small waist achievement, our source requires 40μm waist (2σ). The waist of
*L L cav cav <sup>w</sup>*
where λ is wavelength of laser, Lcav cavity length, ρ curvature of cavity mirror. While high enhancement is easier, small waist cavity is difficult for 10.6μm laser as described in Eq. (15). However the two-mirror, Fabry-Perot cavity would be difficult to achieve small waist due to the cavity structure is confocal, we are developing a concentric, four-mirror super-cavity (Y.Honda, et.al. 2009). This technique can reduce the mirror alignment requirements as two order magnitude. We estimate that 40μm waist can be achieved using concentric super-cavity.
(2 ) 2
<sup>λ</sup> ρ − <sup>=</sup> π (15)
2 0 π
*eff eff*
<sup>=</sup> − (14)
*R R*
F
coatings with high resistance substrates.
laser frequency.
super-cavity is described as;
It is useful to calculate standard SXR photon numbers obtainable in ideal parameters in both cases. As described in Eq. (11), the SXR number is proportional to the laser photon number. The approach to increase the photon average flux is to increase Ne, Np and decrease r, but there are instrumental limitations to realize these simultaneously. The practical limitation of laser average power is determined by a damage on optical components, which is determined by average and peak intensity (W/cm2). It is suggested that the usage of 10.6µm CO2 laser has an advantage to produce one order larger number of SXR photons by the same intensity compared with 1µm solid state lasers. Another limitation is the onset of the nonlinear threshold of the higher harmonics generation, which is evident over 1017W/cm2 laser irradiation intensity (Kumita, 2008).
Usual approach is to increase the repetition rate of the event, and the obtainable photon average flux is expressed as;
$$\mathbf{N} = \mathbf{f} \times \mathbf{N}\_0 \tag{12}$$
where f is the repetition frequency. Characterization of the laser-Compton X-ray source has been undertaken with f as 1-10 Hz typically. High flux mode requires f in the range from kHz to MHz region.
It is under development of pulsed solid state laser storage in an optical super-cavity for laser-Compton X-ray sources (Sakaue, 2010, 2011). The enhancement inside the optical cavity was 600, in which the finess was more than 2000, and the waist of 60μm (2σ) was stably achieved using a 1μm wavelength Nd:Vanadium mode-locked laser with repetition rate 357MHz, pulse width 7ps, and average power 7W. Schematic of super-cavity is shown in Figure 13.
Fig. 13. Schematic of laser storage super-cavity.
Design and optimization is described here on a super-cavity for storing 10.6μm CO2 laser pulses. Super-cavity requires high reflectivity and high transmittance mirror i.e. ultra-low loss mirror as an input and high reflectivity mirror as an output for high enhancement. The enhancement is presented by using cavity finesse (F) as (Hodgson, et.al, 2005);
$$\mathbf{S}\_{\text{cav}} = \frac{\mathbf{F}}{\pi} \tag{13}$$
It is noted that the assumed cavity length is perfectly matched with input laser light. Finesse (F) is given by;
It is useful to calculate standard SXR photon numbers obtainable in ideal parameters in both cases. As described in Eq. (11), the SXR number is proportional to the laser photon number. The approach to increase the photon average flux is to increase Ne, Np and decrease r, but there are instrumental limitations to realize these simultaneously. The practical limitation of laser average power is determined by a damage on optical components, which is determined by average and peak intensity (W/cm2). It is suggested that the usage of 10.6µm CO2 laser has an advantage to produce one order larger number of SXR photons by the same intensity compared with 1µm solid state lasers. Another limitation is the onset of the nonlinear threshold of the higher harmonics generation, which is evident over 1017W/cm2
Usual approach is to increase the repetition rate of the event, and the obtainable photon
where f is the repetition frequency. Characterization of the laser-Compton X-ray source has been undertaken with f as 1-10 Hz typically. High flux mode requires f in the range from
It is under development of pulsed solid state laser storage in an optical super-cavity for laser-Compton X-ray sources (Sakaue, 2010, 2011). The enhancement inside the optical cavity was 600, in which the finess was more than 2000, and the waist of 60μm (2σ) was stably achieved using a 1μm wavelength Nd:Vanadium mode-locked laser with repetition rate 357MHz, pulse width 7ps, and average power 7W. Schematic of super-cavity is shown
Design and optimization is described here on a super-cavity for storing 10.6μm CO2 laser pulses. Super-cavity requires high reflectivity and high transmittance mirror i.e. ultra-low loss mirror as an input and high reflectivity mirror as an output for high enhancement. The
> cav <sup>F</sup> <sup>S</sup> <sup>=</sup> <sup>π</sup>
It is noted that the assumed cavity length is perfectly matched with input laser light. Finesse
enhancement is presented by using cavity finesse (F) as (Hodgson, et.al, 2005);
N f = × *N*0 (12)
(13)
laser irradiation intensity (Kumita, 2008).
Fig. 13. Schematic of laser storage super-cavity.
average flux is expressed as;
kHz to MHz region.
in Figure 13.
(F) is given by;
$$\mathbf{F} = \frac{\pi \sqrt{R\_{\text{eff}}}}{1 - R\_{\text{eff}}} \tag{14}$$
where Reff is �R�R�. As described above, higher reflectivity provides a higher enhancement super-cavity. Particularly, the loss, which includes both absorption and scattering, on the reflection coating is critical issue for storing a high power laser beam as described above. Such a high quality optical mirror was difficult for far infrared wavelength, however, there are now some products usable for super-cavity mirrors (Ophir Optics, 2009). Super-cavity for 10.6μm laser pulse can be achieved with the enhancement of about 600 by using best mirrors available. Figure 14 shows the calculated transmission, reflection and stored power of the super-cavity as a function of the phase advance in one cavity circulation i.e. supercavity length. The dotted line shows the reflection/transmission from the cavity and the solid line is stored power inside the super-cavity, assuming as input power is 1. There is no transmission light because Mirror 2 transmission is 0%. The enhancement of 600 is achieved by this super-cavity. The precision of cavity length adjustment is one issue for stable operation. The requirement is one-order relaxed due to the wavelength in case of CO2 laser, thus the stable operation with enhancement of 600 and more can be easily achieved from our experiences in 1μm laser storage. Critical issue for higher enhancement is to obtain the extremely low loss and high reflectivity mirrors, which is the key R&D of multi-layer coatings with high resistance substrates.
Fig. 14. Calculated results of CO2 laser super-cavity as a function of phase advance in one revolution of cavity. 2 π phase advance corresponds to λ/2 cavity length mismatch with the laser frequency.
Concerning a small waist achievement, our source requires 40μm waist (2σ). The waist of super-cavity is described as;
$$\left|w\_0^2 = \frac{\lambda}{\pi} \frac{\sqrt{L\_{\rm cav}(2\mathfrak{p} - L\_{\rm cav})}}{2}\right.\tag{15}$$
where λ is wavelength of laser, Lcav cavity length, ρ curvature of cavity mirror. While high enhancement is easier, small waist cavity is difficult for 10.6μm laser as described in Eq. (15). However the two-mirror, Fabry-Perot cavity would be difficult to achieve small waist due to the cavity structure is confocal, we are developing a concentric, four-mirror super-cavity (Y.Honda, et.al. 2009). This technique can reduce the mirror alignment requirements as two order magnitude. We estimate that 40μm waist can be achieved using concentric super-cavity.
High Average Power Pulsed CO2 Laser for Short Wavelength Light Sources 177
Ariga,T 2007 Development of a short pulse and high average power CO2 laser for EUV
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Bielesch U Budde M Fischbach M Freisinger B Schaefer J H Uhlenbusch J and Viol W 1992
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Decker,J.E. Lagace,S. Berube,J. Beaudoin,Y. Lin,S.L. (1991); Stable operation of a powerful 3-
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Endo,A.; Hoshino, H.; Ariga, T. & Miura, T. (2006). High power pulsed CO2 laser for EUV
Haglund,R.F. Nowak,A.V. Czuchlewski,S.J. (1981); Gaseous saturable absorbers for the helios CO2 laser system, IEEE Quantum Electron. QE17, pp1799-1808 Harrach, R.J. (1975); Effect of rotational and intramode vibrational coupling on short pulse
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Higashiguchi,T Yugami,N Dunne,P O'Sullivan,G (2011) Scaling of laser produced
The first preliminary experimental results were obtained by using a single transverse mode, CW 10W CO2 laser in a two mirror super cavity (Sakaue, et.al. 2011). The reflectivity of the input mirror was 99.5% with 15m curvature, and the CW CO2 laser was operated with 10W maximum power of single longitudinal mode. The obtained transmitted light is shown on the oscilloscope with a sweeping voltage signal to the Piezo driver. The highest peak signal corresponds to the fundamental transverse mode, followed by higher spatial modes. The measured Finesse was around 300, which is almost half of the calculated value 624. Measured beam waist was 2.1mm, compared to the calculated value 1.8mm. The experiment showed a relatively stable result of the optical storage cavity in the CO2 laser wavelength. Next step is planned as a demonstration of the optical storage with picosecond pulses.
Fig. 15. Experimental setup and first result of CO2 laser storage. Signal train corresponds to the transmission light with cavity length interval of 5 μm.
#### **5. Conclusion**
High average power, short pulse width CO2 laser is originated in the EUV light source research in the beginning, but expanding its application to universal short wavelength plasma and non plasma sources. Reliable gas laser amplifiers with various geometrical structures are now employed with advanced solid state, semiconductor seeders to control its wavelength more precisely, and with advanced optics to enhance its pulsed average power to unprecedented level.
The author deeply expresses his thanks to his colleagues in the trials of research and development in EUV program. Early study on the CO2 laser technology was successfully driven by Dr. T. Miura. He contributed also in the application of OPA technology as the broadband seeder for CO2 laser oscillators. QCL was successfully studied as a seeder for precision control of the lasing lines by Dr. K. M. Nowak. Dr. H. Mizoguchi of Gigaphoton Inc. kindly gave me a chance to write this overview. The author deeply appreciates coworkers in Waseda University, especially Dr. K. Sakaue and Professor M. Washio in the CO2 laser super-cavity program.
#### **6. References**
Abrams, R.L 1974 Broadening coefficients for the P(20) laser transition; Appl.Phys.Lett. 25, pp609-611
The first preliminary experimental results were obtained by using a single transverse mode, CW 10W CO2 laser in a two mirror super cavity (Sakaue, et.al. 2011). The reflectivity of the input mirror was 99.5% with 15m curvature, and the CW CO2 laser was operated with 10W maximum power of single longitudinal mode. The obtained transmitted light is shown on the oscilloscope with a sweeping voltage signal to the Piezo driver. The highest peak signal corresponds to the fundamental transverse mode, followed by higher spatial modes. The measured Finesse was around 300, which is almost half of the calculated value 624. Measured beam waist was 2.1mm, compared to the calculated value 1.8mm. The experiment showed a relatively stable result of the optical storage cavity in the CO2 laser wavelength. Next step is planned as a demonstration of the optical storage with picosecond pulses.
Fig. 15. Experimental setup and first result of CO2 laser storage. Signal train corresponds to
High average power, short pulse width CO2 laser is originated in the EUV light source research in the beginning, but expanding its application to universal short wavelength plasma and non plasma sources. Reliable gas laser amplifiers with various geometrical structures are now employed with advanced solid state, semiconductor seeders to control its wavelength more precisely, and with advanced optics to enhance its pulsed average power
The author deeply expresses his thanks to his colleagues in the trials of research and development in EUV program. Early study on the CO2 laser technology was successfully driven by Dr. T. Miura. He contributed also in the application of OPA technology as the broadband seeder for CO2 laser oscillators. QCL was successfully studied as a seeder for precision control of the lasing lines by Dr. K. M. Nowak. Dr. H. Mizoguchi of Gigaphoton Inc. kindly gave me a chance to write this overview. The author deeply appreciates coworkers in Waseda University, especially Dr. K. Sakaue and Professor M. Washio in the CO2
Abrams, R.L 1974 Broadening coefficients for the P(20) laser transition; Appl.Phys.Lett. 25,
the transmission light with cavity length interval of 5 μm.
**5. Conclusion**
to unprecedented level.
laser super-cavity program.
pp609-611
**6. References**
**6**
*India*
Rakesh Kumar Soni
**Diffusion Cooled V-Fold CO2 Laser**
*Raja Ramanna Centre for Advanced Technology, Indore (M.P.)*
A laser is light amplifier. The acronym LASER stands for Light Amplification by Stimulated Emission of Radiation. It is an electromagnetic radiation with wavelength ranging from ultraviolet to infrared. The fundamental concept of laser operation was first introduced by Einstein in 1917 in one of his three papers on the quantum theory of radiation (Einstein 1917). Almost half a century later, in 1960, T.H. Maiman was the first person to demonstrate the laser by using a ruby crystal. It is a coherent, convergent and monochromatic beam of light. Lasers have various applications in various fields and to appreciate the competency of a laser radiation it is essential to comprehend the basic operation mechanism and properties of laser radiation. The fundamental concept of laser operation is stimulated emission. The three processes required to produce the high energy laser beam are: (a) population inversion, (b) stimulated emission and (c) amplification. Population inversion is a necessary condition for stimulated emission and corresponds to a non-equilibrium distribution of electrons such that the higher energy states have a larger number of electrons than the lower energy states. The process of achieving the population inversion by exciting the electrons to the higher energy states is referred to as pumping (Svelto and Hanna 1989). In general, population inversion is achieved by optical pumping and electrical pumping. In optical pumping, gas-filled flash lamps are most popular. Flash lamps are essentially glass or quartz tubes filled with gases such as xenon and krypton. Some wavelength of the flash (emission spectrum of flash lamp) matches with the absorption characteristics of the active laser medium facilitating population inversion. This is used in solid-state lasers like ruby and Nd:YAG (yttrium–aluminum–garnet). The basic differences between lasers and other light sources are the characteristics often used to describe a laser: (i) the output beam is narrow (ii) the light is monochromatic and (iii) the emission is coherent. The laser light is categorized by different properties and many applications of lasers use these properties. These properties are: (a) mono-chromaticity (b) collimation (c) coherence (d) brightness or radiance (e) focal spot size (f) low divergence (g) transverse modes and (g) temporal modes.
After the demonstration of the first ruby laser, the laser action has been demonstrated in many materials. Lasers are generally classified depending on the physical nature of the active medium used: (I) solid-state lasers (II) gas lasers (III) semiconductor lasers and (IV) dye lasers. It is beyond the purview of this chapter to describe the principles of operation of all these lasers. Here only gas laser systems and typically V-fold CO2 laser is explained.
**1. Introduction**
**2. Gas lasers**
plasma UTA emission down to 3 nm for next generation lithography and short wavelength imaging, *Proceeding of SPIE Optics+Photonics* vol.8139, San Diego,CA,August 2011
### **6**
### **Diffusion Cooled V-Fold CO2 Laser**
#### Rakesh Kumar Soni
*Raja Ramanna Centre for Advanced Technology, Indore (M.P.) India*
#### **1. Introduction**
178 CO2 Laser – Optimisation and Application
Lowenthal, D.D. Egglestone,J.M. (1986); ASE effects in small aspect ratio laser oscillators and
Nowak, K.M.; Suganuma, T.; Endo, A.; Sumitani, A.; Goryachkin, D.A.; Romanov, N.A.;
Oliva,P. Carpinelli,M. Golosio,B. Delogu,P. Endrizzi,M. Park,J. Pogorelsky,I. Yakimenko,V.
Ramis,R (1983) Electron temperature versus laser intensity times wavelength squared: a
Rheault,F. Lachambre,J.L. Gilbert,J. Fortin,R. Blanchard,M (1973); Saturation properties of TEA-CO2 amplifiers in the nanosecond pulse regime, Opt.Commun. 8, pp132-135 Sakai,T. & Hamada,N. (1994). Q-switched CO2 laser using intense pulsed RF discharge and
Sakaue, K Araki, S Fukuda, M Higashi, Y Honda, Y Sasao, N Shimizu, H Taniguchi, T
Sakaue, K Endo,A and Washio,M (2011) Development of a 10μm optical storage cavity, *2011*
Sakaue, K Washio, M Araki, S K Fukuda, M Higashi, Y Honda, Y Omori, T Taniguchi, T
Slattery, J.E. Thompson,J.S. Schroeder,J.B. (1975); Thermal pulse damage thresholds in
Suckewer, S.; Skinner, C.; Voorhees, D.; Milchberg, D.; Keane, C. & Semet, A. (1983).
Yorozu, M Yang, J Okada, Y Yanagida, T Sakai, F Ito, S and Endo, A (2003) Spatial beam
magnetically confined plasma column, *IEEE-QE* 19 pp1855-1860
comparison of theory and experiments, Nucl.Fusion 23739
Germany, September 1994, SPIE
scattering, Nucl.Instrum.Meth. A637 S107-S111
Scattering of Electron and Laser Beams, Laser Phys. 16 pp267-271
cadmium telluride, Appl.Opt. 14, pp2234-2237
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Diego,CA,August 2011
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plasma UTA emission down to 3 nm for next generation lithography and short wavelength imaging, *Proceeding of SPIE Optics+Photonics* vol.8139, San
amplifiers with nonsaturable absorption, IEEE J.Quantum Electron. QE-22, pp1165-
Sherstobitov, V.E.; Kovalchuk, L.V.; Rodionov, A.Y. (2008). Efficient and compact short pulse MOPA system for laser-produced-plasma extreme-UV sources employing RF-discharge slab-waveguide CO2 amplifiers, *Proceedings of SPIE High-Power Laser Ablation,* vol.7005, ISBN: 9780819472069, Taos, NM, April 2008, SPIE Nowak,K.M. (2011). Towards 20kW CO2 laser system for Sn-LPP EUV source, *2011*
*International Workshop on EUV and Soft X-Ray Sources*, Dublin, Ireland November 7-
Williams,O. Rosenzweig,J (2010), Quantitative evaluation of single-shot inline phase contrast imaging using an inverse Compton x-ray source, Appl.Phys.Lett. 97,
high speed rotating chopper, *Proceedings of SPIE Gas Flow and Chemical Lasers: Tenth International Symposium*, vol.2502, pp. 25-30, ISBN: 9780819418609, Friedrichshafen,
Urakawa, J and Washio, M (2011) Development of a laser pulse storage technique in an optical super-cavity for a compact X-ray source based on laser-Compton
*International Workshop on EUV and Soft X-Ray Sources*, Dublin, Ireland November 7-
Terunuma, N Urakawa, J and Sasao, N (2009) Observation of pulsed x-ray trains produced by laser-electron Compton scatterings, Rev.Sci.Instrum. 80 123304 1-7
Population inversion and gain measurements for soft X-ray laser development in a
profile of the femtosecond X-ray pulses generated by a laser-Compton scheme,
A laser is light amplifier. The acronym LASER stands for Light Amplification by Stimulated Emission of Radiation. It is an electromagnetic radiation with wavelength ranging from ultraviolet to infrared. The fundamental concept of laser operation was first introduced by Einstein in 1917 in one of his three papers on the quantum theory of radiation (Einstein 1917). Almost half a century later, in 1960, T.H. Maiman was the first person to demonstrate the laser by using a ruby crystal. It is a coherent, convergent and monochromatic beam of light. Lasers have various applications in various fields and to appreciate the competency of a laser radiation it is essential to comprehend the basic operation mechanism and properties of laser radiation. The fundamental concept of laser operation is stimulated emission. The three processes required to produce the high energy laser beam are: (a) population inversion, (b) stimulated emission and (c) amplification. Population inversion is a necessary condition for stimulated emission and corresponds to a non-equilibrium distribution of electrons such that the higher energy states have a larger number of electrons than the lower energy states. The process of achieving the population inversion by exciting the electrons to the higher energy states is referred to as pumping (Svelto and Hanna 1989). In general, population inversion is achieved by optical pumping and electrical pumping. In optical pumping, gas-filled flash lamps are most popular. Flash lamps are essentially glass or quartz tubes filled with gases such as xenon and krypton. Some wavelength of the flash (emission spectrum of flash lamp) matches with the absorption characteristics of the active laser medium facilitating population inversion. This is used in solid-state lasers like ruby and Nd:YAG (yttrium–aluminum–garnet). The basic differences between lasers and other light sources are the characteristics often used to describe a laser: (i) the output beam is narrow (ii) the light is monochromatic and (iii) the emission is coherent. The laser light is categorized by different properties and many applications of lasers use these properties. These properties are: (a) mono-chromaticity (b) collimation (c) coherence (d) brightness or radiance (e) focal spot size (f) low divergence (g) transverse modes and (g) temporal modes.
#### **2. Gas lasers**
After the demonstration of the first ruby laser, the laser action has been demonstrated in many materials. Lasers are generally classified depending on the physical nature of the active medium used: (I) solid-state lasers (II) gas lasers (III) semiconductor lasers and (IV) dye lasers. It is beyond the purview of this chapter to describe the principles of operation of all these lasers. Here only gas laser systems and typically V-fold CO2 laser is explained.
Diffusion Cooled V-Fold CO2 Laser 181
Laser Type Linear Power Density (W/m) Max. Power (W) Efficiency (%) HeNe 0.1 1 0.1 Argon 1-10 50 0.1 CO2 60-80 1200 15-20
The CO2 laser is a gas discharge device which operates by electric excitation. The active medium in a CO2 laser is a mixture of carbon dioxide, nitrogen, and helium. Each gas plays a distinct role. Carbon dioxide is the light emitter. The CO2 molecules are excited so they vibrate in three different types such as symmetric stretching, bending, and asymmetric stretching (Fig. 1). The molecules then lose part of the excitation energy by dropping to one of two other, lower energy vibrational states as shown in Fig.2. Once the molecules have emitted their laser photons, they continue to drop down the energy-level ladder until they reach the ground state. The nitrogen molecules help to excite CO2 to the upper laser level. Nitrogen molecules are excited first. This is most often done with high voltage direct current, but may also be accomplished by radio frequency excitation. Energy level of the nitrogen molecule is nearly resembles to the (001) vibrational levels of CO2 molecule. Laser transition takes place between initial level (001) and final levels (100) and (020), resulting in 10.6 and 9.6 μm laser radiations, respectively. The nitrogen molecules mechanically transfer energy to CO2 molecules via collisions. In practice, the presence of N2 significantly enhances laser operation, and that gas is almost always present in CO2 lasers. Helium plays a dual role. It serves as a buffer gas to aid in heat transfer and helps the CO2 molecules drop from the lower laser levels to the ground state, thus maintaining the population inversion needed for laser operation. However, the laser radiation at 10.6 μm is the strongest and forms the most usual mode of operation. This process is efficient only if the carbon dioxide is cold, so that its energy levels match that of the nitrogen. High-power systems use elaborate heat exchangers to keep the gas cool. The type of CO2 lasers as slow flow, transverse or cross flow and fast axial flow determines the properties of a CO2 laser. CO2 lasers are capable of both continuous wave (CW) and pulsed operation (Wilson and Hawkes 1987) and in most
Table 2. Comparison of Gas Lasers
**2.2 Excitation mechanism of CO2 lasers**
systems; the electric excitation is controlled to do this.
Fig. 1. Vibrational Modes of CO2 Molecule
The first gas laser, a helium-neon type, conceived and developed by Ali Javan. It was demonstrated for the first time on December 12, 1960, at Bell Telephone Laboratories in Murray Hill, New Jersey. Gas lasers have certain advantages such as homogeneous medium, easy transportation for replenishment, cooling and relatively inexpensive. However, due to physical nature of the gases (low densities), a large volume of gas is required to achieve the significant population inversion for laser action. Hence, gas lasers are usually relatively larger than the solid-state lasers. Gas lasers can be classified into atomic, ionic, and molecular lasers depending on whether the laser transitions are taking place between the energy levels of atoms, ions, and molecules respectively. There are several laser systems in each class. Only some of the typical gas lasers and their wavelengths are shown below in Table-1.
Table 1. Gas lasers and Their Wavelengths
#### **2.1 Carbon dioxide lasers**
C.K.N. Patel in 1964 working at Bell laboratories made the most efficient gas laser, known as carbon dioxide (CO2) laser. The carbon dioxide laser is one of the most versatile type laser on the market today and most widely used materials processing laser. Also, they are efficient and inexpensive in terms of cost per unit power. It emits infrared radiation between 9 and 11 micro-meters (μm), either at a single line selected by the user or on the strongest lines in un-tuned cavities. It can produce continuous output powers ranging from well under 1 watt (W) for scientific applications to many kilowatts (kW) for material processing. It can generate pulses from the nanosecond to millisecond regimes. Custom-made CO2 lasers have produced continuous beams of hundreds of kilowatts for military laser weapon research (Hecht, 1984) or nanosecond-long pulses of 40 kilojoules (kJ) for research in laserinduced nuclear fusion (Los Alamos National Laboratory, 1982). This versatility comes from the fact that there are several distinct types of carbon dioxide lasers. Thus users see several distinct types, such as waveguide, low-power sealed-tube, high-power flowing-gas, and pulsed transversely excited CO2 lasers. The great interest in carbon dioxide lasers stems from their continuous power capability, high efficiency and ease of construction. Table-2 illustrates their advantages over other gas lasers.
The first gas laser, a helium-neon type, conceived and developed by Ali Javan. It was demonstrated for the first time on December 12, 1960, at Bell Telephone Laboratories in Murray Hill, New Jersey. Gas lasers have certain advantages such as homogeneous medium, easy transportation for replenishment, cooling and relatively inexpensive. However, due to physical nature of the gases (low densities), a large volume of gas is required to achieve the significant population inversion for laser action. Hence, gas lasers are usually relatively larger than the solid-state lasers. Gas lasers can be classified into atomic, ionic, and molecular lasers depending on whether the laser transitions are taking place between the energy levels of atoms, ions, and molecules respectively. There are several laser systems in each class. Only some of the typical gas lasers and their wavelengths are
> Laser Type Wavelength (nm) ArF 191 KrF 249 XeCl 308 HeCd 325, 441.5 XeF 351 Argon 488, 514.5 Copper vapor 510.6, 578.2 Krypton 520–676 Gold vapor 628 HeNe 632.8 CO2 10,600
C.K.N. Patel in 1964 working at Bell laboratories made the most efficient gas laser, known as carbon dioxide (CO2) laser. The carbon dioxide laser is one of the most versatile type laser on the market today and most widely used materials processing laser. Also, they are efficient and inexpensive in terms of cost per unit power. It emits infrared radiation between 9 and 11 micro-meters (μm), either at a single line selected by the user or on the strongest lines in un-tuned cavities. It can produce continuous output powers ranging from well under 1 watt (W) for scientific applications to many kilowatts (kW) for material processing. It can generate pulses from the nanosecond to millisecond regimes. Custom-made CO2 lasers have produced continuous beams of hundreds of kilowatts for military laser weapon research (Hecht, 1984) or nanosecond-long pulses of 40 kilojoules (kJ) for research in laserinduced nuclear fusion (Los Alamos National Laboratory, 1982). This versatility comes from the fact that there are several distinct types of carbon dioxide lasers. Thus users see several distinct types, such as waveguide, low-power sealed-tube, high-power flowing-gas, and pulsed transversely excited CO2 lasers. The great interest in carbon dioxide lasers stems from their continuous power capability, high efficiency and ease of construction. Table-2
shown below in Table-1.
Table 1. Gas lasers and Their Wavelengths
illustrates their advantages over other gas lasers.
**2.1 Carbon dioxide lasers**
Table 2. Comparison of Gas Lasers
#### **2.2 Excitation mechanism of CO2 lasers**
The CO2 laser is a gas discharge device which operates by electric excitation. The active medium in a CO2 laser is a mixture of carbon dioxide, nitrogen, and helium. Each gas plays a distinct role. Carbon dioxide is the light emitter. The CO2 molecules are excited so they vibrate in three different types such as symmetric stretching, bending, and asymmetric stretching (Fig. 1). The molecules then lose part of the excitation energy by dropping to one of two other, lower energy vibrational states as shown in Fig.2. Once the molecules have emitted their laser photons, they continue to drop down the energy-level ladder until they reach the ground state. The nitrogen molecules help to excite CO2 to the upper laser level. Nitrogen molecules are excited first. This is most often done with high voltage direct current, but may also be accomplished by radio frequency excitation. Energy level of the nitrogen molecule is nearly resembles to the (001) vibrational levels of CO2 molecule. Laser transition takes place between initial level (001) and final levels (100) and (020), resulting in 10.6 and 9.6 μm laser radiations, respectively. The nitrogen molecules mechanically transfer energy to CO2 molecules via collisions. In practice, the presence of N2 significantly enhances laser operation, and that gas is almost always present in CO2 lasers. Helium plays a dual role. It serves as a buffer gas to aid in heat transfer and helps the CO2 molecules drop from the lower laser levels to the ground state, thus maintaining the population inversion needed for laser operation. However, the laser radiation at 10.6 μm is the strongest and forms the most usual mode of operation. This process is efficient only if the carbon dioxide is cold, so that its energy levels match that of the nitrogen. High-power systems use elaborate heat exchangers to keep the gas cool. The type of CO2 lasers as slow flow, transverse or cross flow and fast axial flow determines the properties of a CO2 laser. CO2 lasers are capable of both continuous wave (CW) and pulsed operation (Wilson and Hawkes 1987) and in most systems; the electric excitation is controlled to do this.
Fig. 1. Vibrational Modes of CO2 Molecule
Diffusion Cooled V-Fold CO2 Laser 183
recombination reaction. Such measures can be used to produce sealed CO2 lasers which can operate for up to several thousand hours before their output seriously degrades. Sometimes hydrogen or water to the gas mixture is added so that it can regenerate CO2 by the carbon monoxide produced by the discharge. In traditional sealed CO2 lasers, the maximum output power possible with this longitudinal discharge is about 50 W per meter of cavity length, and maximum continuous-wave output is about 100 W. A new methodology is radiofrequency (RF) discharge transverse to the tube axis. This design does not require high-voltage electrodes and offers some other advantages, including the ability to electronically control output at rates to 10 kilohertz (kHz), lower operating voltage and potentially lower tube cost. On the other hand, RF power supplies are more complex and less efficient than DC supplies. RF excitation has been growing in popularity for sealed-tube CO2 lasers. It can generate more power because it can excite a broader area than a DC discharge, but it also works well at low powers. All sealed-tube CO2 lasers are limited in output by the difficulty in removing heat.
This type of laser structure is efficient way to produce a compact CW CO2 laser. It consists of two transverse radio-frequency (RF) electrodes separated by insulating sections. An RF power supply is connected to the electrodes to provide a high-frequency alternating field across the electrodes within the bore region. The waveguide modes access the entire gain volume since the modes reflect off the discharge walls in a zigzag fashion. The waveguide itself traverses the laser length in a zigzag. Waveguide lasers are a type of sealed CO2 laser in which the inner diameter of a sealed CO2 laser is shrunk to a few millimeters and the tube
> Minimize Voltage Variation Permitting uniform pumping
The waveguide design limits diffraction losses that would otherwise impair operation of a narrow-tube laser. The tube normally is sealed with a gas reservoir separate from the waveguide itself. Waveguide lasers may be excited by DC discharges or intense RF fields. Waveguides may be made of metal, dielectric or combinations of the two. The waveguide laser is very attractive for low powers, particularly under about 50 W. It provides a good beam quality. It can operate continuously or pulsed and can be readily tuned to many discrete lines in the CO2 spectrum. Its size is comparable to the size of a helium-neon laser
is constructed in the form of a waveguide, as shown in Fig. 4.
**2.3.2 Waveguide lasers**
Fig. 4. Waveguide Laser
but able to generate power in watts.
The energy level diagram for the operation of CO2 laser is shown in Fig.2.
Fig. 2. Energy Level Diagram of CO2 Laser
#### **2.3 Types of CO2 lasers**
#### **2.3.1 Sealed-tube lasers**
The sealed-tube CO2 laser is a glass tube filled with CO2, He, and N2, with mirrors forming a resonant cavity, as shown in Fig.3.
Fig. 3. Sealed Tube Laser
Electrodes are placed near the two ends of the tube. Proper gas mixtures are filled in the tube and seal it. A high voltage is applied to the electrodes to pass a discharge through the gas. A sealed CO2 laser with an ordinary gas mixture would stop operating within a few minutes. The electric discharge in the tube breaks down the CO2 in CO and O2. Catalyst is added in the path to regenerate CO2. Nickel cathode (at 300°C) can catalyze the
The sealed-tube CO2 laser is a glass tube filled with CO2, He, and N2, with mirrors forming a
Electrodes are placed near the two ends of the tube. Proper gas mixtures are filled in the tube and seal it. A high voltage is applied to the electrodes to pass a discharge through the gas. A sealed CO2 laser with an ordinary gas mixture would stop operating within a few minutes. The electric discharge in the tube breaks down the CO2 in CO and O2. Catalyst is added in the path to regenerate CO2. Nickel cathode (at 300°C) can catalyze the
The energy level diagram for the operation of CO2 laser is shown in Fig.2.
Fig. 2. Energy Level Diagram of CO2 Laser
**2.3 Types of CO2 lasers 2.3.1 Sealed-tube lasers**
Fig. 3. Sealed Tube Laser
resonant cavity, as shown in Fig.3.
recombination reaction. Such measures can be used to produce sealed CO2 lasers which can operate for up to several thousand hours before their output seriously degrades. Sometimes hydrogen or water to the gas mixture is added so that it can regenerate CO2 by the carbon monoxide produced by the discharge. In traditional sealed CO2 lasers, the maximum output power possible with this longitudinal discharge is about 50 W per meter of cavity length, and maximum continuous-wave output is about 100 W. A new methodology is radiofrequency (RF) discharge transverse to the tube axis. This design does not require high-voltage electrodes and offers some other advantages, including the ability to electronically control output at rates to 10 kilohertz (kHz), lower operating voltage and potentially lower tube cost. On the other hand, RF power supplies are more complex and less efficient than DC supplies. RF excitation has been growing in popularity for sealed-tube CO2 lasers. It can generate more power because it can excite a broader area than a DC discharge, but it also works well at low powers. All sealed-tube CO2 lasers are limited in output by the difficulty in removing heat.
#### **2.3.2 Waveguide lasers**
This type of laser structure is efficient way to produce a compact CW CO2 laser. It consists of two transverse radio-frequency (RF) electrodes separated by insulating sections. An RF power supply is connected to the electrodes to provide a high-frequency alternating field across the electrodes within the bore region. The waveguide modes access the entire gain volume since the modes reflect off the discharge walls in a zigzag fashion. The waveguide itself traverses the laser length in a zigzag. Waveguide lasers are a type of sealed CO2 laser in which the inner diameter of a sealed CO2 laser is shrunk to a few millimeters and the tube is constructed in the form of a waveguide, as shown in Fig. 4.
Fig. 4. Waveguide Laser
The waveguide design limits diffraction losses that would otherwise impair operation of a narrow-tube laser. The tube normally is sealed with a gas reservoir separate from the waveguide itself. Waveguide lasers may be excited by DC discharges or intense RF fields. Waveguides may be made of metal, dielectric or combinations of the two. The waveguide laser is very attractive for low powers, particularly under about 50 W. It provides a good beam quality. It can operate continuously or pulsed and can be readily tuned to many discrete lines in the CO2 spectrum. Its size is comparable to the size of a helium-neon laser but able to generate power in watts.
Diffusion Cooled V-Fold CO2 Laser 185
flow lasers because the gas moves very quickly through the discharge zone. After leaving the discharge zone, the gas is cooled by heat exchanger. The fast axial-flow laser has become the most common industrial CO2 laser in the power range of 500 W to 5 kW, because of short resonator and small floor space required. Besides the advantages, these lasers have
In transverse flow lasers, gas flow direction, electric discharge and direction of laser cavity axis are in three mutually perpendicular directions as shown in Fig.7. It can produce very
The gas flows across a much wider region and recycled by passing it through a system which regenerates CO2 and adds some fresh gas to the mixture. In this laser, beam mode structure and beam symmetry are considerably poorer than in fast or slow axial-flow lasers.
At the end of the 1960s, the gas-dynamic laser was an important breakthrough that made it possible for the first time to reach power levels of 100 kW or more. Basic structure of gas dynamic laser is shown in Fig.8. In gas dynamic lasers the gas is flowed in the transverse direction to the laser axis. Laser gas which is initially at a pressure of several atmospheres is heated electrically or thermally to excite the molecules and population inversion takes place. The high speed pumps are used to rapidly flow the gas. It is then allowed to expand supersonically through an expansion nozzle into a low-pressure region. This expansion causes the gas to supercool and thereby provide rapid relaxation of the lower laser level from the highest rotational states to the lowest rotational states, leaving a population inversion of those empty higher lying rotational states with respect to the upper laser level. A laser beam is extracted from the gas by placing a pair of mirrors on opposite sides of the expansion chamber. Lasers of this design have produced CW output powers greater than 100 kW. This type of excitation was developed primarily for military applications, but
lower-power versions have found applications in materials processing.
some limitations of complex system design and poor mode quality.
**2.3.5 Transverse flow laser**
Fig. 7. Transverse Flow Laser
**2.3.6 Gas dynamic laser**
high power of the order of 10 kW per meter.
#### **2.3.3 Longitudinal (axial) slow flow laser**
These lasers are operated as conventional gas discharge lasers in the form of long, narrow, cylindrically shaped glass enclosures with electrodes at opposite ends from which the discharge excitation current is introduced as shown in Fig.5. These lasers can be either pulsed or continuous wave and can have lengths of up to several meters. In some versions the discharge enclosure is sealed off and in other versions the gas flows through the tube longitudinally and can be re-circulated to conserve the gases. A water coolant jacket usually surrounds the discharge region. Electric discharge is applied along the tube's axis.
Fig. 5. Longitudinal (Axial) Slow Flow Laser
Low gas pressure and low consumption of gas by recycling methods are some of the salient features of this laser. Slow axial-flow CO2 lasers produce continuous-wave output proportional to the tube length. Average or continuous power of about 500 W can be produced by folding the laser beam with mirrors through multiple tube segments. This also makes the system compact and the design is simple enough. Heat is removed by conduction mode of heat transfer. Laser gases transfer its heat to the walls of the tube and ultimately that heat can be removed by water circulation or other coolant around the tube.
#### **2.3.4 Fast axial flow laser**
The efficiency of axial flow lasers can be increased dramatically by using a pump or turbine to move the gas rapidly through the discharge area as shown in Fig.6.
Fig. 6. Fast Axial Flow Laser
This design allows short resonators to produce relatively high powers; 800 W/m is a typical value of power per unit length. Excitation usually is with a longitudinal discharge, as in slow axial-flow lasers, but some fast axial-flow lasers are powered by radio-frequency discharges. The main advantage of the fast flow is that it cools the laser gas better than slow-
These lasers are operated as conventional gas discharge lasers in the form of long, narrow, cylindrically shaped glass enclosures with electrodes at opposite ends from which the discharge excitation current is introduced as shown in Fig.5. These lasers can be either pulsed or continuous wave and can have lengths of up to several meters. In some versions the discharge enclosure is sealed off and in other versions the gas flows through the tube longitudinally and can be re-circulated to conserve the gases. A water coolant jacket usually
Low gas pressure and low consumption of gas by recycling methods are some of the salient features of this laser. Slow axial-flow CO2 lasers produce continuous-wave output proportional to the tube length. Average or continuous power of about 500 W can be produced by folding the laser beam with mirrors through multiple tube segments. This also makes the system compact and the design is simple enough. Heat is removed by conduction mode of heat transfer. Laser gases transfer its heat to the walls of the tube and ultimately
The efficiency of axial flow lasers can be increased dramatically by using a pump or turbine
This design allows short resonators to produce relatively high powers; 800 W/m is a typical value of power per unit length. Excitation usually is with a longitudinal discharge, as in slow axial-flow lasers, but some fast axial-flow lasers are powered by radio-frequency discharges. The main advantage of the fast flow is that it cools the laser gas better than slow-
that heat can be removed by water circulation or other coolant around the tube.
to move the gas rapidly through the discharge area as shown in Fig.6.
surrounds the discharge region. Electric discharge is applied along the tube's axis.
**2.3.3 Longitudinal (axial) slow flow laser**
Fig. 5. Longitudinal (Axial) Slow Flow Laser
**2.3.4 Fast axial flow laser**
Fig. 6. Fast Axial Flow Laser
flow lasers because the gas moves very quickly through the discharge zone. After leaving the discharge zone, the gas is cooled by heat exchanger. The fast axial-flow laser has become the most common industrial CO2 laser in the power range of 500 W to 5 kW, because of short resonator and small floor space required. Besides the advantages, these lasers have some limitations of complex system design and poor mode quality.
#### **2.3.5 Transverse flow laser**
In transverse flow lasers, gas flow direction, electric discharge and direction of laser cavity axis are in three mutually perpendicular directions as shown in Fig.7. It can produce very high power of the order of 10 kW per meter.
Fig. 7. Transverse Flow Laser
The gas flows across a much wider region and recycled by passing it through a system which regenerates CO2 and adds some fresh gas to the mixture. In this laser, beam mode structure and beam symmetry are considerably poorer than in fast or slow axial-flow lasers.
#### **2.3.6 Gas dynamic laser**
At the end of the 1960s, the gas-dynamic laser was an important breakthrough that made it possible for the first time to reach power levels of 100 kW or more. Basic structure of gas dynamic laser is shown in Fig.8. In gas dynamic lasers the gas is flowed in the transverse direction to the laser axis. Laser gas which is initially at a pressure of several atmospheres is heated electrically or thermally to excite the molecules and population inversion takes place. The high speed pumps are used to rapidly flow the gas. It is then allowed to expand supersonically through an expansion nozzle into a low-pressure region. This expansion causes the gas to supercool and thereby provide rapid relaxation of the lower laser level from the highest rotational states to the lowest rotational states, leaving a population inversion of those empty higher lying rotational states with respect to the upper laser level. A laser beam is extracted from the gas by placing a pair of mirrors on opposite sides of the expansion chamber. Lasers of this design have produced CW output powers greater than 100 kW. This type of excitation was developed primarily for military applications, but lower-power versions have found applications in materials processing.
Diffusion Cooled V-Fold CO2 Laser 187
transverse gas flow, depending on power levels. The prime attractions of TEA lasers are their generation of short, intense pulses and the extraction of high power per unit volume of laser gas. High-pressure operation also broadens emission lines, permitting the use of mode locking techniques to generate pulses lasting about 1 nanosecond. It allows tuning over
Table-3 illustrate a comparison among details of attainable laser power per cubic cm of
CO2 Laser System Power Scaling (W/m)
In the previous paragraphs, we studied about a brief history of lasers and some details about the CO2 lasers. Here we are going to study about the topic of this chapter i.e. "V-fold diffusion cooled laser" in detail. Fig. 10 is a real photograph of 500 W diffusion cooled CO2 laser indigenously developed at Department of Atomic Energy, Raja Ramanna Centre for
V-fold laser is also a type of CO2 laser with some salient features. The name V-fold is given to this laser because of its resonator geometry which is V-folded resonator. Basically this laser is slow flow diffusion cooled CO2 laser. Convection accompanied by conduction is the mode of heat transfer of this laser. Compare to convective cooled lasers, diffusion cooled laser devoid of bulky heat exchangers and blowers. It makes laser head more attractive, compact & simple in the power range of 300-500 W. In the diffusion cooled laser the laser power can be scaled up by increasing the discharge length at the rate of 50 W/m. We
Sealed-off systems 70 Slow flow systems 100 Fast flow systems 800 Pulsed system (TEA Laser) 1.2 TW pulse
Table 3. Comparison of Power Scaling of Different Types of CO2 Lasers
most of the CO2 wavelength range.
active volume in the different types of CO2 lasers.
**3. V-fold diffusion cooled CO2 laser**
Advanced Technology, Indore, MP, India.
Fig. 10. Photograph of Diffusion Cooled V-fold CO2 Laser
Fig. 8. Gas Dynamic Laser
#### **2.3.7 Transversely Excited Atmospheric (TEA) flow laser**
These lasers operate at high total gas pressures of 1 atmosphere or more in order to benefit from obtaining a much higher energy output per unit volume of gas. A schematic of TEA CO2 laser is shown in Fig.9.
Fig. 9. Transversely Excited Atmospheric (TEA) Flow Laser
Extremely high voltages are required initially to ionize the gas and thereby initiate the discharge process to operate the laser at high pressure. Due to the high gas pressure, arcing tends to form within the discharge. In a transverse discharge, the two electrodes are placed parallel to each other over the length of the discharge and a high voltage is applied across the electrodes. Pre-ionization is used to ionize the space between the electrodes uniformly before applying the high voltage. With this pre-ionization, the discharge can then proceed in a uniform fashion over the entire electrode assembly rather than forming a narrow highcurrent arc at just one location. The pre-ionization is produced by flashes of ultraviolet light from a row of pre-ionizing UV spark discharges. Such lasers can produce many joules of energy for unit discharge volume. Tens of nanoseconds to microseconds pulse can be produced by passing electric pulses through the gas in a direction transverse to the laser cavity axis. TEA lasers are available in versions with sealed tubes, slow or fast axial flow, or
These lasers operate at high total gas pressures of 1 atmosphere or more in order to benefit from obtaining a much higher energy output per unit volume of gas. A schematic of TEA
Extremely high voltages are required initially to ionize the gas and thereby initiate the discharge process to operate the laser at high pressure. Due to the high gas pressure, arcing tends to form within the discharge. In a transverse discharge, the two electrodes are placed parallel to each other over the length of the discharge and a high voltage is applied across the electrodes. Pre-ionization is used to ionize the space between the electrodes uniformly before applying the high voltage. With this pre-ionization, the discharge can then proceed in a uniform fashion over the entire electrode assembly rather than forming a narrow highcurrent arc at just one location. The pre-ionization is produced by flashes of ultraviolet light from a row of pre-ionizing UV spark discharges. Such lasers can produce many joules of energy for unit discharge volume. Tens of nanoseconds to microseconds pulse can be produced by passing electric pulses through the gas in a direction transverse to the laser cavity axis. TEA lasers are available in versions with sealed tubes, slow or fast axial flow, or
Fig. 8. Gas Dynamic Laser
CO2 laser is shown in Fig.9.
**2.3.7 Transversely Excited Atmospheric (TEA) flow laser**
Fig. 9. Transversely Excited Atmospheric (TEA) Flow Laser
transverse gas flow, depending on power levels. The prime attractions of TEA lasers are their generation of short, intense pulses and the extraction of high power per unit volume of laser gas. High-pressure operation also broadens emission lines, permitting the use of mode locking techniques to generate pulses lasting about 1 nanosecond. It allows tuning over most of the CO2 wavelength range.
Table-3 illustrate a comparison among details of attainable laser power per cubic cm of active volume in the different types of CO2 lasers.
Table 3. Comparison of Power Scaling of Different Types of CO2 Lasers
### **3. V-fold diffusion cooled CO2 laser**
In the previous paragraphs, we studied about a brief history of lasers and some details about the CO2 lasers. Here we are going to study about the topic of this chapter i.e. "V-fold diffusion cooled laser" in detail. Fig. 10 is a real photograph of 500 W diffusion cooled CO2 laser indigenously developed at Department of Atomic Energy, Raja Ramanna Centre for Advanced Technology, Indore, MP, India.
Fig. 10. Photograph of Diffusion Cooled V-fold CO2 Laser
V-fold laser is also a type of CO2 laser with some salient features. The name V-fold is given to this laser because of its resonator geometry which is V-folded resonator. Basically this laser is slow flow diffusion cooled CO2 laser. Convection accompanied by conduction is the mode of heat transfer of this laser. Compare to convective cooled lasers, diffusion cooled laser devoid of bulky heat exchangers and blowers. It makes laser head more attractive, compact & simple in the power range of 300-500 W. In the diffusion cooled laser the laser power can be scaled up by increasing the discharge length at the rate of 50 W/m. We
Diffusion Cooled V-Fold CO2 Laser 189
The above relation is valid only when the rise in laser gas temperature ΔT ~250°C, without bottlenecking at the lower laser level and maintaining a stable and uniform discharge. The temperature above 250°C populates the lower laser level and destroys population inversion. From Eq.(5), a larger mass flow rate is required for higher laser power. Mass flow rate depends upon area of discharge zone, gas flow velocity or gas mixture density. Since the density for a gas mixture is constant at a particular pressure. So increasing either area of discharge zone or gas flow velocity can only increase power. Discharge Area (A) is the function of electrode separation or discharge height (d) and discharge length (L). So the laser power would increase with the increase of d or L. But it is observed that the maximum discharge current, discharge voltage and the laser power remained almost constant for different electrode separations (d). This is because of the electric field would remain constant to maintain the same discharge current. Laser power may also increase with the discharge length (L) but we found that on increasing the length after a certain optimum value, power decreases due to saturation and due to predominance of cavity losses. Also there are limitations of space and alignment on increasing the discharge length. Therefore length cannot be increased after a certain optimum value to increase the power. Thus, after certain value, increasing either discharge length (L) or electrode separation (d) cannot increase laser power i.e. the discharge area cannot be increased too much. Thus to increase the power gas flow velocity may be increased. So to achieve more gas flow velocity, higher capacity pumps/blowers with high discharge and high pressure are required. An effective heat exchanger is needed to dissipate the heat and to keep the gas temperature below 250°C
CO2 Laser Systems Power (kW/m3)
Transverse Flow 1500 Fast Axial Flow 3000 Slab Laser 3300
Diffusion Cooled (length scaling) 500
<sup>η</sup> − 1� . �. C�. ΔT. V�. �. � � 1�� �� (5)
In a convective cooled laser, the laser power can be scaled up with the following equation:
P� = � <sup>η</sup>
Where
� = electro-optic efficiency, ρ = laser gas density,
in discharge zone.
Table 4. Power per Unit Volume of Laser Gas
C� = specific heat of laser gas, ΔT = rise in laser gas temperature, �� = flow velocity of laser gas, L = discharge length and
d = discharge height or electrode separation, �� = mass flow rate of gas through discharge zone
adopted symmetric concave resonator geometry to reduce diffraction loss. V-folding over a cylindrical surface minimizes the astigmatism effect. We obtained more than 380 W laser power in a 7.5 meter discharge length.
#### **3.1 Design considerations**
In order to design a V-fold CO2 laser, the physical dimensions of the active volume, gas flow velocity, output coupling of optical resonator are to be decided. The desired output power *Po* can be calculated for the required volume of the active medium, if we know the typical input power density *Pin* that can be dissipated in the homogeneous and stable discharge. *Pin* depends on several factors such as electrode design, gas mixture, its pressure, excitation method, gas flow velocity and its uniformity. Following considerations are taken into account in determining the design parameters such as the discharge length, discharge aperture, optimum reflectivity and gas pressure.
$$T\_{opt} = 1 - \exp\left[-2L\left\{\left(\mathbf{g}\_0 \times a\right)^{1/2} - a\right\}\right] \tag{1}$$
iii. The small signal gain is usually experimentally measured and it is in the range of 0.5 to 1% per cm. In optimum laser design it can be seen that the transmissivity (*T*) is almost constant, independent of small signal gain and the laser power. We can write for the intra-cavity intensity *Ic* incident on the output coupler as:
$$\mathbf{l\_c = l\_s \times g\_0 \times L} \tag{2}$$
$$\frac{\mathbf{I\_c}}{\mathbf{I\_s}} = \mathbf{g\_0} \times \mathbf{L} \tag{3}$$
iv. The damage threshold of the output coupler limits the maximum value of *Ic* and thus the maximum value of g� × L is also limited. In the optimum laser design the intracavity losses a×L is kept minimum and this is also independent of laser power. Usually the total intra-cavity loss should not be more than 5% of total gain. Thus, g� × L and a × L being constant, the optimum transmittivity T��� is also constant. For the typical values of I�and I� are about 300-500 W/cm2 and 1 kW/cm2 respectively. g� × L is in the range of 2-3 in high power lasers. For these conditions:
$$\text{T}\_{\text{opt}} \approx 50 - 60\% \tag{4}$$
v. Minimum diffraction loss in the resonator criterion should also be considered in designing the V-fold resonator.
In a convective cooled laser, the laser power can be scaled up with the following equation:
$$\text{P}\_{\text{L}} = \left[\frac{\eta}{\eta - 1}\right], \rho. \text{C}\_{\text{p}}.\Delta \text{T. V}. \text{L}. \text{d} \approx \ 120 \ \dot{M} \tag{5}$$
Where
188 CO2 Laser – Optimisation and Application
adopted symmetric concave resonator geometry to reduce diffraction loss. V-folding over a cylindrical surface minimizes the astigmatism effect. We obtained more than 380 W laser
In order to design a V-fold CO2 laser, the physical dimensions of the active volume, gas flow velocity, output coupling of optical resonator are to be decided. The desired output power *Po* can be calculated for the required volume of the active medium, if we know the typical input power density *Pin* that can be dissipated in the homogeneous and stable discharge. *Pin* depends on several factors such as electrode design, gas mixture, its pressure, excitation method, gas flow velocity and its uniformity. Following considerations are taken into account in determining the design parameters such as the discharge length, discharge
i. The maximum laser power density should be less than the damage threshold of optical elements, however, it should be more than the saturation intensity Is which is proportional to ��, where p is gas pressure in mbar and n = 2 in slow flow laser. The damage threshold intensity of the ZnSe mirror, usually used as output coupler in CO2 lasers is about 2 kW/cm2. Considering this the incident intensity *Ic* on the output
ii. The optimum output coupling or transmissivity (*T*) of the resonator can be estimated with the knowledge of the discharge length '*L*' small signal gain '*go*' and the intra-cavity
iii. The small signal gain is usually experimentally measured and it is in the range of 0.5 to 1% per cm. In optimum laser design it can be seen that the transmissivity (*T*) is almost constant, independent of small signal gain and the laser power. We can write for the
> I� I�
iv. The damage threshold of the output coupler limits the maximum value of *Ic* and thus the maximum value of g� × L is also limited. In the optimum laser design the intracavity losses a×L is kept minimum and this is also independent of laser power. Usually the total intra-cavity loss should not be more than 5% of total gain. Thus, g� × L and a × L being constant, the optimum transmittivity T��� is also constant. For the typical values of I�and I� are about 300-500 W/cm2 and 1 kW/cm2 respectively. g� × L
v. Minimum diffraction loss in the resonator criterion should also be considered in
���� = � − ��� �−�� ��g� × ��� �� − ��� (1)
I� = I� × g� × L (2)
T��� ≈ 50 − 60% (4)
= g� × L (3)
power in a 7.5 meter discharge length.
aperture, optimum reflectivity and gas pressure.
losses (*a*) by the following relation:
designing the V-fold resonator.
coupler should be maintained at around 1.0 kW/cm2.
intra-cavity intensity *Ic* incident on the output coupler as:
is in the range of 2-3 in high power lasers. For these conditions:
**3.1 Design considerations**
� = electro-optic efficiency, ρ = laser gas density, C� = specific heat of laser gas, ΔT = rise in laser gas temperature, �� = flow velocity of laser gas, L = discharge length and d = discharge height or electrode separation, �� = mass flow rate of gas through discharge zone
The above relation is valid only when the rise in laser gas temperature ΔT ~250°C, without bottlenecking at the lower laser level and maintaining a stable and uniform discharge. The temperature above 250°C populates the lower laser level and destroys population inversion. From Eq.(5), a larger mass flow rate is required for higher laser power. Mass flow rate depends upon area of discharge zone, gas flow velocity or gas mixture density. Since the density for a gas mixture is constant at a particular pressure. So increasing either area of discharge zone or gas flow velocity can only increase power. Discharge Area (A) is the function of electrode separation or discharge height (d) and discharge length (L). So the laser power would increase with the increase of d or L. But it is observed that the maximum discharge current, discharge voltage and the laser power remained almost constant for different electrode separations (d). This is because of the electric field would remain constant to maintain the same discharge current. Laser power may also increase with the discharge length (L) but we found that on increasing the length after a certain optimum value, power decreases due to saturation and due to predominance of cavity losses. Also there are limitations of space and alignment on increasing the discharge length. Therefore length cannot be increased after a certain optimum value to increase the power. Thus, after certain value, increasing either discharge length (L) or electrode separation (d) cannot increase laser power i.e. the discharge area cannot be increased too much. Thus to increase the power gas flow velocity may be increased. So to achieve more gas flow velocity, higher capacity pumps/blowers with high discharge and high pressure are required. An effective heat exchanger is needed to dissipate the heat and to keep the gas temperature below 250°C in discharge zone.
Table 4. Power per Unit Volume of Laser Gas
Diffusion Cooled V-Fold CO2 Laser 191
jacketed construction. Inner tubes have outer diameter 12 mm, inner diameter 9 mm and length 750 mm. Outer (jacket) tubes are having 22 mm OD (Fig.13). Outer tubes also have ports for water inlet and outlet. Water flows through the annular space between inner and outer tube. A chiller unit supplies water at a total flow rate 12 lit/min and 15°C in water
ANODE BLOCK
WATER OUTLET
jackets for cooling of gases.
Fig. 11. Components of V-fold CO2 Laser
Fig. 12. Photograph of Anode Block
WATER INLET
Fig. 13. Schematic of Water Jacket
Table 5. Discharge Volume to Total Volume Ratio of Different Types of CO2 Lasers
As we go from diffusion-cooled lasers to convective cooled lasers, the power-scaling move from length to volume. From calculation, slab lasers give more power per cubic meter of laser gas compared to various types of CO2 lasers. Following tables shows the laser power output for unit volume of laser gas (Table-4) and typical volume of discharge region to total volume in percentage (Table-5) for various types of CO2 laser.
From above two tables, it is concluded that the maximum power could be achieved in slab laser and power is moderate in transverse flow laser. In all other laser, except transverse flow lasers, the power scaling up to multi-kilowatt is not easy. The laser power depends on length of active medium (diffusion cooled) or area of discharge electrode (in slab laser) but in transverse flow lasers, power is scaled-up by volume so it is relatively easy. From the above data, it is clear that the power per unit laser gas and discharge volume to total volume ratio is maximum for slab laser. So, if we somehow move from transverse configuration to Slab (area) or diffusion cooled configuration then we can definitely enhance the power of our laser.
#### **3.2 Construction of V-fold laser**
The complete laser assembly is mounted on a 3 meter long aluminum pipe (Fig.11). Outer diameter of aluminum pipe is 200 mm. Since the whole laser assembly is mounted on this pipe only therefore best possible straightness of pipe was required. It is very difficult to get single pipe of 3 meter length and straightness 1 mm therefore the whole pipe is casted in 3 segments, each of 1 meter. All the three segments are welded with straightness in 1 mm. To maintain the straightness and rigidity, both the ends are joined with a flange and tie rod. The aluminum pipe is supported at the ends by a support system made of stainless steel plate of 10 mm thick. Bottom of support system is bolted with the support table. There is no middle support for the pipe due to assembly constraints of glass tube. Since the straightness of tube is very important we calculated the deflection at the mid-point of pipe and it is found that the deflection is insignificant. Five rings of stainless steel 304 (SS304) are inserted in the pipe.
Anode support ring is supporting the anode part of this laser at the center. Additional rings of nylon are also placed near to this central ring to give extra support to the joint of glass tube and anode block. Anode block is made of metalon-6 which acts as insulator (Fig.12). Anode pins made of SS304 are placed at the center separated/isolated by metalon-6 tube. The anode block contains two anodes at each end. Anodes are made of stainless steel. Viton® O-rings are used in between glass tube and anode for sealing. Gas inlet ports are also provided on the anode block. Gas flows from the anode block to cathode through the glass tube. Low thermal expansion borosilicate glass tubes are used. These tubes have
Fast Axial Flow 10 Transverse Flow 14 Diffusion Cooled 20 Slab Laser 27
Table 5. Discharge Volume to Total Volume Ratio of Different Types of CO2 Lasers
volume in percentage (Table-5) for various types of CO2 laser.
our laser.
in the pipe.
**3.2 Construction of V-fold laser**
As we go from diffusion-cooled lasers to convective cooled lasers, the power-scaling move from length to volume. From calculation, slab lasers give more power per cubic meter of laser gas compared to various types of CO2 lasers. Following tables shows the laser power output for unit volume of laser gas (Table-4) and typical volume of discharge region to total
From above two tables, it is concluded that the maximum power could be achieved in slab laser and power is moderate in transverse flow laser. In all other laser, except transverse flow lasers, the power scaling up to multi-kilowatt is not easy. The laser power depends on length of active medium (diffusion cooled) or area of discharge electrode (in slab laser) but in transverse flow lasers, power is scaled-up by volume so it is relatively easy. From the above data, it is clear that the power per unit laser gas and discharge volume to total volume ratio is maximum for slab laser. So, if we somehow move from transverse configuration to Slab (area) or diffusion cooled configuration then we can definitely enhance the power of
The complete laser assembly is mounted on a 3 meter long aluminum pipe (Fig.11). Outer diameter of aluminum pipe is 200 mm. Since the whole laser assembly is mounted on this pipe only therefore best possible straightness of pipe was required. It is very difficult to get single pipe of 3 meter length and straightness 1 mm therefore the whole pipe is casted in 3 segments, each of 1 meter. All the three segments are welded with straightness in 1 mm. To maintain the straightness and rigidity, both the ends are joined with a flange and tie rod. The aluminum pipe is supported at the ends by a support system made of stainless steel plate of 10 mm thick. Bottom of support system is bolted with the support table. There is no middle support for the pipe due to assembly constraints of glass tube. Since the straightness of tube is very important we calculated the deflection at the mid-point of pipe and it is found that the deflection is insignificant. Five rings of stainless steel 304 (SS304) are inserted
Anode support ring is supporting the anode part of this laser at the center. Additional rings of nylon are also placed near to this central ring to give extra support to the joint of glass tube and anode block. Anode block is made of metalon-6 which acts as insulator (Fig.12). Anode pins made of SS304 are placed at the center separated/isolated by metalon-6 tube. The anode block contains two anodes at each end. Anodes are made of stainless steel. Viton® O-rings are used in between glass tube and anode for sealing. Gas inlet ports are also provided on the anode block. Gas flows from the anode block to cathode through the glass tube. Low thermal expansion borosilicate glass tubes are used. These tubes have
CO2 Laser Systems Typical volume of discharge region
compared to total volume (%)
jacketed construction. Inner tubes have outer diameter 12 mm, inner diameter 9 mm and length 750 mm. Outer (jacket) tubes are having 22 mm OD (Fig.13). Outer tubes also have ports for water inlet and outlet. Water flows through the annular space between inner and outer tube. A chiller unit supplies water at a total flow rate 12 lit/min and 15°C in water jackets for cooling of gases.
Fig. 11. Components of V-fold CO2 Laser
Fig. 12. Photograph of Anode Block
Fig. 13. Schematic of Water Jacket
Diffusion Cooled V-Fold CO2 Laser 193
cathode blocks, which are connected to a rotary vane vacuum pump of pumping speed of 500 lit/min. Pressure, temperature and gas mixture have been optimized for the maximum output power. Optimum gas pressure is 30 mbar. In the diffusion cooled laser the laser power can be scaled up by increasing the discharge length at the rate of 50 W/m. With the increase of discharge length and therefore optical resonator length, the Fresnel number NF = r2 / λ.l where r, l and λ are the radius of mirror clear aperture, resonator length and laser wavelength respectively. NF reduces and with this the diffraction loss increases. Due to this the input power in a laser with plano-concave resonator did not scale up with discharge length beyond 3-4 meters. We adopted the symmetric concave resonator geometry to reduce diffraction loss and V-folding over a cylindrical surface instead of a flat surface for laying the discharge tubes to minimize the astigmatism effect. Each section of V-fold laser has about 1.5 meter discharge length, distance between two mirrors is 2.5 meter. All resonator mirrors i.e. rear reflector, ZnSe output coupler and all folding mirrors are having concave surface of 5 meter ROC. Since, the laser mode formed in any section are sustained in all the other sections therefore the length of one section determines the Fresnel number. Corresponding to the resultant Fresnel number the diffraction loss is low. Introduction of curved folding mirrors through a small folding angle of 5° could introduce considerable aberration due to astigmatism after large number of folding. In order to minimize the overall effect of astigmatism, the tubes were mounted on a cylindrical surface instead of a flat surface to have ~2π folding. The central supporting aluminum pipe due to high moment of inertia have minimum deflection thus minimizes the misalignment. With a fully reflecting mirror on the left and a partially transmitting mirror on the right, the device becomes a Vfold laser which radiates in the far infrared at 10.6 microns. Till date, 420 W power in 10.5
All gas discharges operated in the glow discharge region have electrical characteristics similar to those indicated in Fig.16. The voltage and current values and the exact shape of the curve depend on the type of gases, gas pressure and the length & diameter of the
meter discharge length is obtained from this laser system.
Fig. 16. Voltage-Current Curve of a Gaseous Discharge
**VOLTAGE**
**CURRENT**
**3.4 Electrical characteristics of V-fold laser**
discharge tube.
The jacketed glass tube is supported by cathode block which is ultimately supported by a plate and ring over the aluminum pipe (Fig.14.). Two glass tubes in V-shape are supported by a cathode block on one side. Cathode block is made of SS304 have the advantage of low scaling problem caused by electrical discharge. A mirror holder is connected on the other side of the cathode block through a glass tube of 45 mm OD (Fig.14 & 15). Each mirror holder consists of one mirror and they are placed at the extreme ends on both sides. Mirror holder assembly is also supported on pipe through a ring and plate. Rear mirrors and folding mirrors are made of OFHC Copper substrate of 25 mm diameter and radius of curvature (ROC) 5 meter. Mirrors are gold coated with ∼99% reflectivity. Two micrometer screws are fitted on the back side of the each mirror holder to align the laser beam. Alignment is the most critical part of this laser. The alignment accuracy of 0.5 mrad mirror tilt was targeted and achieved by the micrometer screw. Output power is obtained through a ZnSe output coupler having concave geometry of ROC 5 meter and 17% reflectivity.
Fig. 14. Cathode Block
Fig. 15. Mirror Holder
#### **3.3 Working of V-fold laser**
The working principle of the laser is similar to other CO2 lasers. The gas mixture of CO2, N2, and He enters in each discharge tube at its center and flows symmetrically towards the
The jacketed glass tube is supported by cathode block which is ultimately supported by a plate and ring over the aluminum pipe (Fig.14.). Two glass tubes in V-shape are supported by a cathode block on one side. Cathode block is made of SS304 have the advantage of low scaling problem caused by electrical discharge. A mirror holder is connected on the other side of the cathode block through a glass tube of 45 mm OD (Fig.14 & 15). Each mirror holder consists of one mirror and they are placed at the extreme ends on both sides. Mirror holder assembly is also supported on pipe through a ring and plate. Rear mirrors and folding mirrors are made of OFHC Copper substrate of 25 mm diameter and radius of curvature (ROC) 5 meter. Mirrors are gold coated with ∼99% reflectivity. Two micrometer screws are fitted on the back side of the each mirror holder to align the laser beam. Alignment is the most critical part of this laser. The alignment accuracy of 0.5 mrad mirror tilt was targeted and achieved by the micrometer screw. Output power is obtained through a ZnSe output coupler having concave geometry of ROC 5 meter and 17% reflectivity.
CATHODE BLOCK
The working principle of the laser is similar to other CO2 lasers. The gas mixture of CO2, N2, and He enters in each discharge tube at its center and flows symmetrically towards the
Fig. 14. Cathode Block
MICROMETER
MIRROR HOLDER
SCREW
Fig. 15. Mirror Holder
**3.3 Working of V-fold laser**
cathode blocks, which are connected to a rotary vane vacuum pump of pumping speed of 500 lit/min. Pressure, temperature and gas mixture have been optimized for the maximum output power. Optimum gas pressure is 30 mbar. In the diffusion cooled laser the laser power can be scaled up by increasing the discharge length at the rate of 50 W/m. With the increase of discharge length and therefore optical resonator length, the Fresnel number NF = r2 / λ.l where r, l and λ are the radius of mirror clear aperture, resonator length and laser wavelength respectively. NF reduces and with this the diffraction loss increases. Due to this the input power in a laser with plano-concave resonator did not scale up with discharge length beyond 3-4 meters. We adopted the symmetric concave resonator geometry to reduce diffraction loss and V-folding over a cylindrical surface instead of a flat surface for laying the discharge tubes to minimize the astigmatism effect. Each section of V-fold laser has about 1.5 meter discharge length, distance between two mirrors is 2.5 meter. All resonator mirrors i.e. rear reflector, ZnSe output coupler and all folding mirrors are having concave surface of 5 meter ROC. Since, the laser mode formed in any section are sustained in all the other sections therefore the length of one section determines the Fresnel number. Corresponding to the resultant Fresnel number the diffraction loss is low. Introduction of curved folding mirrors through a small folding angle of 5° could introduce considerable aberration due to astigmatism after large number of folding. In order to minimize the overall effect of astigmatism, the tubes were mounted on a cylindrical surface instead of a flat surface to have ~2π folding. The central supporting aluminum pipe due to high moment of inertia have minimum deflection thus minimizes the misalignment. With a fully reflecting mirror on the left and a partially transmitting mirror on the right, the device becomes a Vfold laser which radiates in the far infrared at 10.6 microns. Till date, 420 W power in 10.5 meter discharge length is obtained from this laser system.
#### **3.4 Electrical characteristics of V-fold laser**
All gas discharges operated in the glow discharge region have electrical characteristics similar to those indicated in Fig.16. The voltage and current values and the exact shape of the curve depend on the type of gases, gas pressure and the length & diameter of the discharge tube.
Fig. 16. Voltage-Current Curve of a Gaseous Discharge
Diffusion Cooled V-Fold CO2 Laser 195
resistance. Pre-ionization initiates discharge in all the tubes simultaneously and maintain it stable at even low currents. Ballast resistor is required to control the current flowing in the circuit, as discharge has a negative dynamic resistance; hence ballast resistance is an important parameter in getting a uniform stable discharge. If ballast resistance is not proper, it may result in large flow of current, which may result in formation of arcs and no laser action take place. Moreover we require discharge intensity equal in all zones, if instability creeps into one zone, it will affect the other zone and we will not get uniformity in the discharge. If the ballast resistance is of high value, there will be much of power losses in the ballast resistors. We experimented with four different values of ballast resistors. They are 140, 249, 300 and 191 kΩ. With 140 kΩ we could not get the required current density for maximum output optical power. The other three gave us stable discharge and the optimum current in each discharge zone is found to be 26 mA. We finally used the 191 kΩ resistor in
Design of a suitable optical resonator is needed to extract the laser power from the annular discharge region and also to provide the feedback to the laser. Resonators are classified
The simplest optical resonator (The Fabry-Perot resonator or confocal) consists of a pair of plane or spherical mirrors located opposite one another. They are cantered to a common optical axis and are aligned perpendicular to this axis. For lasers in the low to medium power range (1 mW - 200 W), the hemispherical resonator is mainly used and for high
depending on beam stability inside the resonator and named as follows:
our circuit considering the maximum overall efficiency of 10.6%.
Fig. 17. Schematic of Power Supply of V-fold Laser
**3.6 Laser resonator of V-fold laser**
I. Stable II. Unstable
Before ionization, the current through the gas is essentially zero. Increasing the voltage on the gas results in a small pre-breakdown current due to a very small amount of easily ionized matter, which is always present in a gas near room temperature (point A). Increasing the applied voltage further will increase this current slightly until the breakdown voltage is reached (point B). At this voltage level, a significant number of atoms become ionized because of the high electric field present in the gas. The free negative electrons are attracted toward the anode and the heavier positive ions toward the cathode. This increases conductivity of the gas and lowers the electrical resistance of the discharge. The electrons are sufficiently accelerated by the electric field to free other electrons through collisions with gas atoms or molecules. Thus, as current increases (from point C to point D), ionization increases and voltage across the discharge tube decreases. This means that an increase in current results in a decrease in resistance. This property of gas discharges is called negative dynamic resistance. This does not mean that the resistance of the tube is a negative value, but that the slope of the voltage-current curve has a negative value. Current through the gas will increase until it is limited by some other electrical component in the circuit or until the power supply can no longer sustain the current. In the case of low-current CW devices such as He-Ne laser tubes, the current is limited at a lower level (point C). In the flashlamps of pulsed solid-state lasers, current is allowed to increase to a value of many kilo-amps (point E) before energy stored in the capacitors is exhausted.
#### **3.5 Power supply of V-fold laser**
The Pulser/Sustainer technique is utilized for the production of uniform electrical discharge in the glow discharge regime. The Pulser/Sustainer concept produces pressure and volume scalable plasmas by essentially applying two successive discharges to the gas. The first fast high-voltage pulse creates the electron density uniformly between its electrodes using only a small amount of energy. However a second discharge applies the proper voltage to this plasma to tune the electrons to a temperature sufficiently high for efficient laser pumping but not high enough to generate any appreciable further increase in electron density. Thus, the dominant amount of energy is put into the gas (by the sustainer) exactly where it is desired (vibration excitation of N2 and CO2) without triggering. Such plasma instabilities as arcs and sparking are usually associated with substantial ionization rates. The plasma is then with two "knobs"- one controlling electron density, the other electron temperature. The result is a stable uniform tuned high-power-density plasma that is not wall controlled and, hence a high power efficient N2/CO2 laser. To realize this concept we have used a 25 kV DC Power supply, 500 mA of current and a pulser with 9 kV of peak voltage, 2 µsec pulse and 5 kHz frequency. The schematic circuit diagram of laser power supply is shown in the Fig.17. An experiment was also performed to know the minimum pulse energy required per pulse to create the uniform discharge. This was studied by the use of another pulser which was available to us with peak voltage of 6 kV, 5 kHz frequency and with variable pulse width. By changing the pulse width we got the situation where we got the uniform smooth discharge. To initiate the discharge in all tubes simultaneously, pre-ionization technique has been adopted. For pre-ionization, a high frequency pulser of peak voltage 6 kV and repetition rate 2-5 kHz has been developed. Pulse width can be varied from 2 to 8 μsec. Pulser is connected to the anode pins by a DC power supply of 30 kV / 750 mA rating through a capacitor of 1.7 nF to block the high voltage DC excitation current. Thick film noninductive resistors of 191 kΩ are used between DC Supply and anode pins as ballast
Before ionization, the current through the gas is essentially zero. Increasing the voltage on the gas results in a small pre-breakdown current due to a very small amount of easily ionized matter, which is always present in a gas near room temperature (point A). Increasing the applied voltage further will increase this current slightly until the breakdown voltage is reached (point B). At this voltage level, a significant number of atoms become ionized because of the high electric field present in the gas. The free negative electrons are attracted toward the anode and the heavier positive ions toward the cathode. This increases conductivity of the gas and lowers the electrical resistance of the discharge. The electrons are sufficiently accelerated by the electric field to free other electrons through collisions with gas atoms or molecules. Thus, as current increases (from point C to point D), ionization increases and voltage across the discharge tube decreases. This means that an increase in current results in a decrease in resistance. This property of gas discharges is called negative dynamic resistance. This does not mean that the resistance of the tube is a negative value, but that the slope of the voltage-current curve has a negative value. Current through the gas will increase until it is limited by some other electrical component in the circuit or until the power supply can no longer sustain the current. In the case of low-current CW devices such as He-Ne laser tubes, the current is limited at a lower level (point C). In the flashlamps of pulsed solid-state lasers, current is allowed to increase to a value of many kilo-amps (point
The Pulser/Sustainer technique is utilized for the production of uniform electrical discharge in the glow discharge regime. The Pulser/Sustainer concept produces pressure and volume scalable plasmas by essentially applying two successive discharges to the gas. The first fast high-voltage pulse creates the electron density uniformly between its electrodes using only a small amount of energy. However a second discharge applies the proper voltage to this plasma to tune the electrons to a temperature sufficiently high for efficient laser pumping but not high enough to generate any appreciable further increase in electron density. Thus, the dominant amount of energy is put into the gas (by the sustainer) exactly where it is desired (vibration excitation of N2 and CO2) without triggering. Such plasma instabilities as arcs and sparking are usually associated with substantial ionization rates. The plasma is then with two "knobs"- one controlling electron density, the other electron temperature. The result is a stable uniform tuned high-power-density plasma that is not wall controlled and, hence a high power efficient N2/CO2 laser. To realize this concept we have used a 25 kV DC Power supply, 500 mA of current and a pulser with 9 kV of peak voltage, 2 µsec pulse and 5 kHz frequency. The schematic circuit diagram of laser power supply is shown in the Fig.17. An experiment was also performed to know the minimum pulse energy required per pulse to create the uniform discharge. This was studied by the use of another pulser which was available to us with peak voltage of 6 kV, 5 kHz frequency and with variable pulse width. By changing the pulse width we got the situation where we got the uniform smooth discharge. To initiate the discharge in all tubes simultaneously, pre-ionization technique has been adopted. For pre-ionization, a high frequency pulser of peak voltage 6 kV and repetition rate 2-5 kHz has been developed. Pulse width can be varied from 2 to 8 μsec. Pulser is connected to the anode pins by a DC power supply of 30 kV / 750 mA rating through a capacitor of 1.7 nF to block the high voltage DC excitation current. Thick film noninductive resistors of 191 kΩ are used between DC Supply and anode pins as ballast
E) before energy stored in the capacitors is exhausted.
**3.5 Power supply of V-fold laser**
resistance. Pre-ionization initiates discharge in all the tubes simultaneously and maintain it stable at even low currents. Ballast resistor is required to control the current flowing in the circuit, as discharge has a negative dynamic resistance; hence ballast resistance is an important parameter in getting a uniform stable discharge. If ballast resistance is not proper, it may result in large flow of current, which may result in formation of arcs and no laser action take place. Moreover we require discharge intensity equal in all zones, if instability creeps into one zone, it will affect the other zone and we will not get uniformity in the discharge. If the ballast resistance is of high value, there will be much of power losses in the ballast resistors. We experimented with four different values of ballast resistors. They are 140, 249, 300 and 191 kΩ. With 140 kΩ we could not get the required current density for maximum output optical power. The other three gave us stable discharge and the optimum current in each discharge zone is found to be 26 mA. We finally used the 191 kΩ resistor in our circuit considering the maximum overall efficiency of 10.6%.
Fig. 17. Schematic of Power Supply of V-fold Laser
#### **3.6 Laser resonator of V-fold laser**
Design of a suitable optical resonator is needed to extract the laser power from the annular discharge region and also to provide the feedback to the laser. Resonators are classified depending on beam stability inside the resonator and named as follows:
The simplest optical resonator (The Fabry-Perot resonator or confocal) consists of a pair of plane or spherical mirrors located opposite one another. They are cantered to a common optical axis and are aligned perpendicular to this axis. For lasers in the low to medium power range (1 mW - 200 W), the hemispherical resonator is mainly used and for high
Diffusion Cooled V-Fold CO2 Laser 197
0
0 <sup>1</sup> <sup>2</sup> <sup>2</sup> <sup>2</sup> πω R z1
λ z = +
<sup>1</sup> <sup>2</sup> <sup>2</sup>
(8)
(9)
0
Equation 1 and 2 gives the value of ω0 and ω is 2.72 mm and 3.153 mm respectively.
The parameters that affect such optimization for flowing gas systems are:
should be prepared to perform experimental exploration of his own system.
few tricks in aligning this particular laser. Step by step, they are as follows:
• Optical mode control, wavelength control, and output coupling
Performance of CO2 lasers may be optimized in several ways: maximize multimode power; maximize single- mode power; maximize efficiency; and/or minimize size and complexity.
Optimization is by no means simple, because the various parameters are strongly interrelated. All results, therefore, should be viewed only as indicative of performance trends. The engineer
Aligning this laser was very challenging job for us. Since the inner diameter of the discharge tube is 9 mm, we require alignment accuracy in microns. Since small amount of misalignment can lead to appreciable loss in output power, a great deal of work was done in making the system rigid. Height or position of the glass CO2 laser tube should never change because any small movement throws it out of alignment and this could take days to realign. Instead, change the laser system by varying the mirror orientations, grating orientation and He-Ne laser orientation. The idea is to make the two mirrors at the ends of the laser cavity reflect a beam back-and forth many times without striking the walls of the tube. There are a
i. Make sure that there is no high voltage at the electrodes of the laser tube by checking
0
where
= = = = =
2
• Tube length, diameter and wall temperature • Gas mixture, pressure, and flow speed
• Electrical discharge control and current density
**3.7.1 Alignment procedure of V-fold laser**
that the power supply is turned off.
R radius of curvature of the mirror
**3.7 Optimization of V-fold laser**
ω beam radius at the mirrors ω minimum spot size z distance from the waist λ wavelength of CO laser
<sup>λ</sup> <sup>z</sup> ω ω <sup>1</sup> <sup>2</sup> πω = +
power laser both stable and unstable types of resonator are used. There are many combinations depending on their stability criteria given below:
$$0 \le \mathbf{g}\_1 \mathbf{g}\_2 \le 1 \quad \text{stability condition} \tag{6}$$
$$\mathbf{g}\_l = \mathbf{1} - \frac{L}{R\_l} \qquad \text{g-parameter} \tag{7}$$
Where
L = Length of resonator,
R*i* = Radius of curvature of resonator
We use the resonator mostly which satisfy this condition. The stability curve shown below represents that which resonator is preferred in stability criteria.
Fig. 18. Resonator Stability Curve
In our present laser we are using a concave–concave type resonator (where 2L=R) in a Vfold manner. Resonator mirrors for visible laser are generally made of glass but in CO2 laser the radiation is of 10.6 µm which comes in infrared region and this wavelength is absorbed by glass. So a special type of output coupler made up of ZnSe is generally used. The V-Fold laser resonator is a stable resonator comprising of concave mirrors of radius of curvature of 5 meter. The distance between the mirrors is 2.5 meter. Concave mirrors keep the beam bound inside the cavity and tends to reduce the diffraction losses. For a Gaussian beam to exist in a resonator, its wave fronts must fit exactly into the curvature of the mirrors. Thus beam radius at the waist and at the mirrors can be found out using the following equation:
$$\mathbf{u} = \mathbf{u}\_0 \left[ 1 + \left( \frac{\lambda \mathbf{z}}{\mathbf{n} \, \mathbf{a}\_0^2} \right)^2 \right]^{\frac{1}{2}} \tag{8}$$
$$\mathbf{R} = \mathbf{z} \left[ \mathbf{1} + \left( \frac{\mathbf{n} \, \alpha\_0^2}{\lambda \, \mathbf{z}} \right)^2 \right]^{\frac{1}{2}} \tag{9}$$
where
196 CO2 Laser – Optimisation and Application
power laser both stable and unstable types of resonator are used. There are many
� ��
We use the resonator mostly which satisfy this condition. The stability curve shown below
�� <sup>g</sup>�g� = 1
In our present laser we are using a concave–concave type resonator (where 2L=R) in a Vfold manner. Resonator mirrors for visible laser are generally made of glass but in CO2 laser the radiation is of 10.6 µm which comes in infrared region and this wavelength is absorbed by glass. So a special type of output coupler made up of ZnSe is generally used. The V-Fold laser resonator is a stable resonator comprising of concave mirrors of radius of curvature of 5 meter. The distance between the mirrors is 2.5 meter. Concave mirrors keep the beam bound inside the cavity and tends to reduce the diffraction losses. For a Gaussian beam to exist in a resonator, its wave fronts must fit exactly into the curvature of the mirrors. Thus beam radius at the waist and at the mirrors can be found out using the following
<sup>g</sup>� =1−
0 ≤ <sup>g</sup>�g� ≤ 1 stability condition (6)
g-parameter (7)
<sup>g</sup>� =1−
� ��
combinations depending on their stability criteria given below:
represents that which resonator is preferred in stability criteria.
<sup>g</sup>� =1−
�
<sup>g</sup>�g� = 1
Where
L = Length of resonator,
R*i* = Radius of curvature of resonator
Fig. 18. Resonator Stability Curve
equation:
ω beam radius at the mirrors =
0 ω minimum spot size =
z distance from the waist =
2 λ wavelength of CO laser =
R radius of curvature of the mirror =
Equation 1 and 2 gives the value of ω0 and ω is 2.72 mm and 3.153 mm respectively.
#### **3.7 Optimization of V-fold laser**
Performance of CO2 lasers may be optimized in several ways: maximize multimode power; maximize single- mode power; maximize efficiency; and/or minimize size and complexity. The parameters that affect such optimization for flowing gas systems are:
Optimization is by no means simple, because the various parameters are strongly interrelated. All results, therefore, should be viewed only as indicative of performance trends. The engineer should be prepared to perform experimental exploration of his own system.
#### **3.7.1 Alignment procedure of V-fold laser**
Aligning this laser was very challenging job for us. Since the inner diameter of the discharge tube is 9 mm, we require alignment accuracy in microns. Since small amount of misalignment can lead to appreciable loss in output power, a great deal of work was done in making the system rigid. Height or position of the glass CO2 laser tube should never change because any small movement throws it out of alignment and this could take days to realign. Instead, change the laser system by varying the mirror orientations, grating orientation and He-Ne laser orientation. The idea is to make the two mirrors at the ends of the laser cavity reflect a beam back-and forth many times without striking the walls of the tube. There are a few tricks in aligning this particular laser. Step by step, they are as follows:
i. Make sure that there is no high voltage at the electrodes of the laser tube by checking that the power supply is turned off.
Diffusion Cooled V-Fold CO2 Laser 199
c. Reflection losses - Whenever light is incident on a transparent surface, some portion of it always is reflected. Brewster windows and antireflection coatings greatly reduce this
d. Diffraction loss - Part of the laser light may pass over the edges of the mirror or strike the edges of the aperture and be removed from the beam. This is the largest loss factor in many lasers. When a light beam passes through a limiting aperture, the waves at the edge of the beam bend outward slightly, causing the beam to diverge. This phenomenon is termed "diffraction". When laser light moves, diffraction occurs at the aperture and the beam diverges. When the beam returns to the aperture after reflection from the mirror, its diameter is larger than the diameter of the aperture and the edges of the beam are blocked. The portion of the beam that does pass through the aperture is
e. Absorption Loss – This loss occurs due to the mirrors either fully or partially reflecting. No mirror is considered to be the 100% reflecting mirror and some part of incident laser get absorbed in the mirror. So as the number of mirrors will increase, the loss will also
In order to ensure the high-power and stable CO2 laser operation, misalignment sensitivity has to be known. The power and stability of the laser greatly depends on the misalignment of the optical resonator. In such type of resonator in which a V-fold resonator is used, misalignment is the main cause of reduction in power. So the effect of mirror misalignment of folded resonators is investigated experimentally and compared to first-order perturbation theory. An expression *D* is derived, which characterizes the misalignment sensitivity of any folded resonator. It is proved experimentally that this misalignment sensitivity depends on
The misalignment sensitivity of a resonator is defined as the sensitivity with which the diffraction losses or the output power are changed due to mirror tilt. By adapting the diameter of the TEM00 mode to the diameter of the active medium, the efficiency of a laser oscillator can be increased considerably. This requires either a large mirror distance *L* or an optical resonator operating near the limit of stability. In either case the resonator becomes
diffracted again and experiences additional loss on the next pass.
loss of light but cannot eliminate it entirely.
**3.9 Misalignment sensitivity of V-fold laser**
Fig. 19. Misaligned Spherical Resonator
the effective resonator length *L\** and the *gi* parameters only.
increase.
#### **3.7.2 Power scaling of V-fold laser**
The output power of the laser scales up with the input power and input electrical power is limited by two factors. First is the rise in laser gas temperature and second is discharge instability. The most common being the ionization thermal instability. For efficient and reliable laser operation the input power density should be smaller and determined by the cooling and the discharge stabilization processes. In V-fold laser, the maximum input power density is limited by the heating effect and not by the discharge instability. Also, laser power in a V-fold diffusion cooled laser is directly proportional to the discharge length and is independent of the tube diameter and gas pressure. Thus, the laser power in V-fold diffusion cooled CO2 laser can be scaled up by increasing the active length only and it has been incorporated by introducing several discharge tubes arranged optically in series.
#### **3.8 Losses in optical cavities of V-fold laser**
The following factors contribute to losses within the optical cavities of the lasers:
ii. Set up a He-Ne laser alongside the cavity with a pin hole exiting the He-Ne Laser. Use two mirrors and direct the beam down the center bore of the CO2 laser tube. The He-Ne laser beam should be positioned on the center of the mirrors for adjustment purposes. In the beginning, blank off the back mirror with a piece of paper so that reflections don't
iii. Direct the He-Ne beam through the middle of the output mirror (the first mirror it
iv. Adjust the mirrors until the He-Ne laser beam goes through the middle of the bore without reflecting off the walls of the tube. It may not look as if it goes through the middle of the Brewster windows, and it may not go exactly through the middle of the
v. Remove the paper blocking the back mirror and adjust the mirror so that the reflection is centered on the output port of the He-Ne laser (it is easier to align if you place a card
vii. Blank off the output port of the He-Ne laser with a fire brick to protect it from the CO2 beam. Place the power detector in front of the CO2 output port and place a fire brick behind the detector. Whenever you change scales on the power meter, you should reset
The output power of the laser scales up with the input power and input electrical power is limited by two factors. First is the rise in laser gas temperature and second is discharge instability. The most common being the ionization thermal instability. For efficient and reliable laser operation the input power density should be smaller and determined by the cooling and the discharge stabilization processes. In V-fold laser, the maximum input power density is limited by the heating effect and not by the discharge instability. Also, laser power in a V-fold diffusion cooled laser is directly proportional to the discharge length and is independent of the tube diameter and gas pressure. Thus, the laser power in V-fold diffusion cooled CO2 laser can be scaled up by increasing the active length only and it has been incorporated by introducing several discharge tubes arranged optically in
The following factors contribute to losses within the optical cavities of the lasers:
a. Misalignment of the mirrors - If the mirrors of the cavity are not aligned properly with the optical axis, the beam will not be contained within the cavity, but will move farther
b. Dirty optics - Dust, dirt, fingerprints and scratches on optical surfaces scatter the laser
with a small hole punched in it at the output port of the Helium- Neon laser). vi. Now adjust the output mirror so that the inner surface reflection of that mirror (the bigger, dimmer one of the two) is centered on the back mirror reflection spot at the Helium- Neon laser. Fringes can usually be seen on the reflections when the two are aligned (Fabry-Perot interferometer). Alignment is pretty much complete. It may take
passes through). You will see more than one dot reflecting back.
output mirror. Going down the center of the bore is the most important.
confuse matters the set-up.
you a day or two to get to this point.
**3.8 Losses in optical cavities of V-fold laser**
toward one edge of the cavity after each reflection.
light and cause permanent damage to the optical surfaces.
**3.7.2 Power scaling of V-fold laser**
it to zero.
series.
#### **3.9 Misalignment sensitivity of V-fold laser**
In order to ensure the high-power and stable CO2 laser operation, misalignment sensitivity has to be known. The power and stability of the laser greatly depends on the misalignment of the optical resonator. In such type of resonator in which a V-fold resonator is used, misalignment is the main cause of reduction in power. So the effect of mirror misalignment of folded resonators is investigated experimentally and compared to first-order perturbation theory. An expression *D* is derived, which characterizes the misalignment sensitivity of any folded resonator. It is proved experimentally that this misalignment sensitivity depends on the effective resonator length *L\** and the *gi* parameters only.
Fig. 19. Misaligned Spherical Resonator
The misalignment sensitivity of a resonator is defined as the sensitivity with which the diffraction losses or the output power are changed due to mirror tilt. By adapting the diameter of the TEM00 mode to the diameter of the active medium, the efficiency of a laser oscillator can be increased considerably. This requires either a large mirror distance *L* or an optical resonator operating near the limit of stability. In either case the resonator becomes
Diffusion Cooled V-Fold CO2 Laser 201
*V* ( ) *w aw aw* ( ) ( ) 2 2 <sup>2</sup>
Generally a resonator has limiting apertures on both mirrors. Then the loss factor by tilting
*V VV i j i ii ji* ( )1 2
1 12 .exp 2
For small losses (1-*Vji*, 1-*Vii* << 1), Eq. (16) combined with Eq. (17) can be approximated by
*a a a a V V*
<sup>1</sup> 1 exp 2 exp 2 <sup>2</sup> =− − + −
For minimizing diffraction losses on the one hand and preventing multimode oscillation on the other hand, it is reasonable to use pinhole radii a bit larger than the beam radii.
> *io i i <sup>S</sup> V V <sup>D</sup>*
α
*j*
1 2 \* 2 1 2
*i <sup>L</sup> <sup>g</sup> g g <sup>D</sup>*
Equation (20) represents the diffraction loss factor *Vi* per resonator bounce, if mirror *Si* is
additional losses of 10% are caused. This gives a clear idea of the meaning of the misalignment sensitivity *Di*. However, the low-gain lasers are affected much more by an
*i*
π
λ
resonator length *L*\* and the *gi* parameters. If a mirror is tilted by an angle
<sup>2</sup> 1
= − <sup>−</sup>
*S* 2 2 2
( )
*i.* Misalignment sensitivity *Di*, which according to Eq. (22) depends on the
3 2 1 2
*g g g*
1 1
exp2 1
*ji j j*
*a a*
2 2 2 2 exp 2 exp 2 <sup>Δ</sup> <sup>Δ</sup> ≅ − − + <sup>−</sup>
2 2 2
*j j j*
*ww w*
*ji j j ii i i*
*j j j ii i*
*j i*
2 2
*i j a a*
*w w*
*ww w ww w*
Δ =− + <sup>−</sup>
*w* = beam diameter of the TEM00 field pattern at the pinhole, and
*ji*
*V*
*Vo* is the loss factor of the aligned system with
*o*
*V*
Combining the above equations, we finally get
Where
*a* = pinhole radius,
mirror *Si* is given by:
*i o*
tilted by
α
*V* = loss factor per resonator bounce.
=− + Δ 1 12 exp 2<sup>−</sup> (15)
= ≠ . (16)
(17)
(19)
(20)
α
*= 1*/*Di*,
*V S <sup>o</sup>* ( ) <sup>2</sup> =− − 1 exp 2 (21)
<sup>+</sup> <sup>=</sup> <sup>−</sup> (22)
(18)
very sensitive to a misalignment of the mirrors. From symmetry we may deduce that the increase of diffraction loss due to misalignment is proportional to the square of the mirrortilting angle αoi. Therefore, a suitable expression for the loss factor *Vi* per resonator bounce is:
$$V\_l = V\_o[1 - (a\_l/a\_{ol})^2] \tag{10}$$
Where *i* indicates mirror Si, which is tilted by an angle αi with respect to the resonator axis (see Fig.19). The misalignment sensitivity of the resonator is characterized by αoi. In the following sections the relation between αoi and the resonator parameters is investigated experimentally and theoretically.
#### **3.9.1 Background**
There are few papers dealing with the influence of misalignment on diffraction losses. Numerical calculations were carried out for special systems such as symmetric or confocal resonators and plane-plane resonators using first-order perturbation theory. But they assume that the aperture of the system does not disturb the field distribution of the infinite mirror. The laser oscillator consists of two spherical mirrors, radii of curvature *R1* and *R2* in a distance (*L)* and refractive index. It is assumed to be homogeneous. The mode properties of the resonator are characterized by the effective length *L\** and the *gi* parameters. For infinite mirrors, the spot size of the TEM00 mode is given by:
$$\mathcal{W}\_{i}^{2} = \frac{\mathcal{\lambda}L^{\*}}{\pi} \left(\frac{\mathcal{g}\_{i}}{\mathcal{g}\_{i} \left(1 - \mathcal{g}\_{1}\mathcal{g}\_{2}\right)}\right)^{\mathcal{W}^{2}} \tag{11}$$
The resonator axis is defined by the two centers of mirror curvature *M1* and *M2*. If mirror *Si* is tilted by an angle α*i*, the resonator axis is rotated by an angle θ*<sup>i</sup>*, and the centers of the field intensity patterns are shifted. A simple geometric consideration delivers the relations:
$$\theta\_i = \alpha\_i \frac{1 - g\_i}{1 - g\_1 g\_2} \tag{12}$$
$$
\Delta\_{il} = \alpha\_i \mathbf{g}\_i \mathbf{L}^" \;/\left(1 - \mathbf{g}\_1 \mathbf{g}\_2\right) \tag{13}
$$
$$
\Delta\_{\psi} = \alpha\_{\rangle} \mathbf{L}^\* \;/\left(\mathbf{1} - \mathbf{g}\_1 \mathbf{g}\_2\right) \qquad i \neq j \tag{14}
$$
Δ*ij* means the displacement of the intensity pattern at mirror *Si*, if mirror *Sj* is tilted by α*j*. Near the limit of stability ( ) *g g*1 2 → 1 , the beam steering angle *<sup>i</sup>* θ and the displacement Δ*ij* may become considerably large. Nevertheless, as long as infinite mirrors are considered, the resonator remains aligned, and there are no diffraction losses. But if a limiting aperture is inserted into the resonator, e.g., the active medium or a mode selecting pinhole, diffraction losses occur and increase rapidly with increasing mirror tilt angle. Tilting a mirror is equivalent to a displacement of the pinhole. For a system with only one pinhole, Berger et al calculated the dependence of diffraction loss factor *V* on the pinhole displacement (Δ). A first-order perturbation theory for the TEM00 mode delivers:
$$V = 1 - \left[1 + 2\left(\Delta / w\right)^2 \left(a / w\right)^2\right] \exp\left[-2\left(a / w\right)^2\right] \tag{15}$$
Where
200 CO2 Laser – Optimisation and Application
very sensitive to a misalignment of the mirrors. From symmetry we may deduce that the increase of diffraction loss due to misalignment is proportional to the square of the mirrortilting angle αoi. Therefore, a suitable expression for the loss factor *Vi* per resonator bounce is:
�� � ���1 − ���� � ���
Where *i* indicates mirror Si, which is tilted by an angle αi with respect to the resonator axis (see Fig.19). The misalignment sensitivity of the resonator is characterized by αoi. In the following sections the relation between αoi and the resonator parameters is investigated
There are few papers dealing with the influence of misalignment on diffraction losses. Numerical calculations were carried out for special systems such as symmetric or confocal resonators and plane-plane resonators using first-order perturbation theory. But they assume that the aperture of the system does not disturb the field distribution of the infinite mirror. The laser oscillator consists of two spherical mirrors, radii of curvature *R1* and *R2* in a distance (*L)* and refractive index. It is assumed to be homogeneous. The mode properties of the resonator are characterized by the effective length *L\** and the *gi* parameters. For infinite
> ( ) *j*
> > *i*
*g g g*1 2
1 1
( ) *ii i i g L g g* \* Δ= −
( ) *ij jL g g i j* \* Δ= − ≠
Δ*ij* means the displacement of the intensity pattern at mirror *Si*, if mirror *Sj* is tilted by α*j*.
may become considerably large. Nevertheless, as long as infinite mirrors are considered, the resonator remains aligned, and there are no diffraction losses. But if a limiting aperture is inserted into the resonator, e.g., the active medium or a mode selecting pinhole, diffraction losses occur and increase rapidly with increasing mirror tilt angle. Tilting a mirror is equivalent to a displacement of the pinhole. For a system with only one pinhole, Berger et al calculated the dependence of diffraction loss factor *V* on the pinhole displacement (Δ). A
1 2 1
*g gg* 1 2 \*
*i*
The resonator axis is defined by the two centers of mirror curvature *M1* and *M2*. If mirror *Si*
<sup>=</sup> <sup>−</sup>
*<sup>L</sup> <sup>g</sup> <sup>W</sup>*
intensity patterns are shifted. A simple geometric consideration delivers the relations:
*i i*
θ α
α
α
Near the limit of stability ( ) *g g*1 2 → 1 , the beam steering angle *<sup>i</sup>*
first-order perturbation theory for the TEM00 mode delivers:
λ
π
experimentally and theoretically.
mirrors, the spot size of the TEM00 mode is given by:
*i*
is tilted by an angle α*i*, the resonator axis is rotated by an angle
2
**3.9.1 Background**
�� (10)
(11)
*<sup>i</sup>*, and the centers of the field
and the displacement Δ*ij*
θ
<sup>−</sup> <sup>=</sup> − (12)
/ 1 1 2 (13)
/ 1 1 2 (14)
θ
*a* = pinhole radius,
*w* = beam diameter of the TEM00 field pattern at the pinhole, and
*V* = loss factor per resonator bounce.
Generally a resonator has limiting apertures on both mirrors. Then the loss factor by tilting mirror *Si* is given by:
$$V\_i = \left(V\_{ii} \cdot V\_{ji}\right)^{\sharp 2} \qquad \qquad i \neq j \tag{16}$$
$$V\_{ji} = 1 - \left[ 1 + 2\left(\frac{\Delta\_{ji}}{w\_j}\right)^2 \left(\frac{a\_j}{w\_j}\right)^2 \right] \cdot \exp\left[-2\left(\frac{a\_j}{w\_j}\right)^2\right] \tag{17}$$
For small losses (1-*Vji*, 1-*Vii* << 1), Eq. (16) combined with Eq. (17) can be approximated by
$$V\_i \equiv V\_o - \left\{ \left[ \left( \frac{\Delta\_{\vec{\mu}}}{w\_j} \right) \left( \frac{a\_j}{w\_j} \right) \right]^2 \exp \left[ -2 \left( \frac{a\_j}{w\_j} \right)^2 \right] + \left[ \left( \frac{\Delta\_{\vec{\mu}}}{w\_i} \right) \left( \frac{a\_i}{w\_i} \right) \right]^2 \exp \left[ -2 \left( \frac{a\_i}{w\_i} \right)^2 \right] \right\} \tag{18}$$
*Vo* is the loss factor of the aligned system with
$$V\_o = 1 - \frac{1}{2} \left\{ \exp \left[ -2 \left( \frac{a\_i}{w\_i} \right)^2 \right] + \exp \left[ -2 \left( \frac{a\_j}{w\_j} \right)^2 \right] \right\} \tag{19}$$
For minimizing diffraction losses on the one hand and preventing multimode oscillation on the other hand, it is reasonable to use pinhole radii a bit larger than the beam radii. Combining the above equations, we finally get
$$V\_i = V\_o \left(1 - \alpha\_i^2 \frac{S^2}{\exp 2S^2 - 1} D\_i^2\right) \tag{20}$$
$$V\_o = 1 - \exp\left(-\mathfrak{D}S^2\right) \tag{21}$$
$$D\_i^2 = \frac{\pi L^\*}{\lambda} \left(\frac{\mathcal{g}\_j}{\mathcal{g}\_i}\right)^{\mathcal{Y}^2} \frac{1 + \mathcal{g}\_1 \mathcal{g}\_2}{\left(1 - \mathcal{g}\_1 \mathcal{g}\_2\right)^{\mathcal{Y}^2}}\tag{22}$$
Equation (20) represents the diffraction loss factor *Vi* per resonator bounce, if mirror *Si* is tilted by α*i.* Misalignment sensitivity *Di*, which according to Eq. (22) depends on the resonator length *L*\* and the *gi* parameters. If a mirror is tilted by an angle α *= 1*/*Di*, additional losses of 10% are caused. This gives a clear idea of the meaning of the misalignment sensitivity *Di*. However, the low-gain lasers are affected much more by an
Diffusion Cooled V-Fold CO2 Laser 203
coupling loss of resonator & long gain length. The radiation, which begins from the output coupler-end, sees the round trip gain while the radiation which begins from the rear mirror; sees only single trip, and the starting intensity of radiation in the first case is relatively smaller than that in the second case. Therefore the misalignment in first case (output coupler) has relatively less effect on the laser power build up compared to the misalignment of the second case (rear reflector). Furthermore, the experimental results indicate that sensitivity
SL – Single limb, DL – Double limb, CC – Concave-Concave resonator, PC – Plano-Concave
parameter '*D'* is a suitable parameter to describe the alignment stability of a resonator.
S.No. Type of Resonator Active Medium Length (cm) D (mrad) 1 Concave-Concave 150 1.687 2 Concave-Concave 300 2.378 3 Plano-Concave 150 2.286 4 Plano-Concave 300 3.223
Table 6. Theoretical Value of Misalignment Sensitivity Parameter '*D'*
Note: Micro-meters are numbered 1 & 2 in anticlockwise direction.
resonator, M1 – Micrometer1, M2 – Micrometer2
Fig. 20. Misalignment in Single Limb for Reflector
Fig. 21. Misalignment in Single Limb for Output Coupler
additional loss of 10% than the high-gain lasers. Thus, misalignment sensitivities of different resonator configurations may be compared if their gains are the same. If both mirrors are misaligned, the losses proportional to *Di* 2 are summed up. Therefore, the misalignment of the complete system is defined as *DDD* ( )1 2 2 2 = +1 2 and is given by:
$$D = \left[ \left( \frac{\pi L \, ^\star}{\lambda} \right) \frac{1 + g\_1 g\_2}{\left( 1 - g\_1 g\_2 \right)^{3/2}} \frac{\left| g\_1 + g\_2 \right|}{\left( g\_1 g\_2 \right)^{3/2}} \right]^{3/2} \tag{23}$$
Where, 'D' is a number characterizing any spherical resonator with respect to its sensitivity against mirror tilting. High value of *D* means high misalignment sensitivity. The most insensitive resonator is the symmetric con-focal one with *g1=g2=0.*
$$D\_0 = \left(\frac{2\pi L}{\mathcal{A}}\right)^{\text{y2}} \tag{24}$$
But, from the stability diagram, we learn that *g1=g2=0* represents a discontinuity. Small deviations from symmetry may cause high losses and high misalignment sensitivity.
#### **3.9.2 Experimental investigation**
The power and stability of a laser system is mainly governed by the misalignment sensitivity of optical resonator. To ensure stable and high power from laser system misalignment sensitivity has to be known. The effect of reflector and output coupler misalignment for concave -concave & Plano-concave resonators in single and double limbs of V-fold laser are investigated experimentally and compared to first-order perturbation theory. Eq.23 is used to quantify the misalignment sensitivity of the V-fold laser resonator. It is proved experimentally that this misalignment sensitivity depends on the effective resonator length *L\** and the *gi* parameters only. High value of *D* means high misalignment sensitivity. The influence of mirror misalignment on laser output and field distribution was investigated by various authors. Experiment was carried out for four different arrangements.
Laser was operated with all these arrangements and then misaligned with the help of micrometer screw fitted on the backside of the optics. These four arrangements gave the misalignment characteristics for the single and double limb as well as Plano-concave and concave-concave resonator. Power was measured in the best-aligned condition then graphs were plotted for laser power v/s misalignment (Fig.20). The experimental results are verified by theoretical calculation of the misalignment sensitivity parameter '*D'* (Table-6).
Misalignment sensitivity increases with L\* i.e. no. of limbs. It is also observed that the planoconcave resonator is more sensitive to misalignment then the concave-concave resonator (Fig.20 & 22). It is also interesting to observe that the output coupler is less sensitive to misalignment compare to the rear concave reflector (Fig.21 & 23). This is due to very high
additional loss of 10% than the high-gain lasers. Thus, misalignment sensitivities of different resonator configurations may be compared if their gains are the same. If both mirrors are
*<sup>L</sup> g g g g <sup>D</sup>*
*<sup>L</sup> <sup>D</sup>*
0
deviations from symmetry may cause high losses and high misalignment sensitivity.
<sup>+</sup> <sup>+</sup> <sup>=</sup> −
Where, 'D' is a number characterizing any spherical resonator with respect to its sensitivity against mirror tilting. High value of *D* means high misalignment sensitivity. The most
> 2 \* π
<sup>=</sup>
But, from the stability diagram, we learn that *g1=g2=0* represents a discontinuity. Small
The power and stability of a laser system is mainly governed by the misalignment sensitivity of optical resonator. To ensure stable and high power from laser system misalignment sensitivity has to be known. The effect of reflector and output coupler misalignment for concave -concave & Plano-concave resonators in single and double limbs of V-fold laser are investigated experimentally and compared to first-order perturbation theory. Eq.23 is used to quantify the misalignment sensitivity of the V-fold laser resonator. It is proved experimentally that this misalignment sensitivity depends on the effective resonator length *L\** and the *gi* parameters only. High value of *D* means high misalignment sensitivity. The influence of mirror misalignment on laser output and field distribution was investigated by various authors. Experiment was carried out for four different arrangements.
Laser was operated with all these arrangements and then misaligned with the help of micrometer screw fitted on the backside of the optics. These four arrangements gave the misalignment characteristics for the single and double limb as well as Plano-concave and concave-concave resonator. Power was measured in the best-aligned condition then graphs were plotted for laser power v/s misalignment (Fig.20). The experimental results are verified by theoretical calculation of the misalignment sensitivity parameter '*D'* (Table-6). Misalignment sensitivity increases with L\* i.e. no. of limbs. It is also observed that the planoconcave resonator is more sensitive to misalignment then the concave-concave resonator (Fig.20 & 22). It is also interesting to observe that the output coupler is less sensitive to misalignment compare to the rear concave reflector (Fig.21 & 23). This is due to very high
λ
\* 1 1
( ) ( )
*gg gg*
1 2 1 2 32 12 12 12
1 2
2 are summed up. Therefore, the misalignment of
(24)
(23)
1 2
misaligned, the losses proportional to *Di*
**3.9.2 Experimental investigation**
a. Single limb with concave-concave resonator b. Single limb with Plano-concave resonator
c. Double limbs with concave-concave resonator and d. Double limbs with Plano-concave resonator.
the complete system is defined as *DDD* ( )1 2 2 2 = +1 2 and is given by:
insensitive resonator is the symmetric con-focal one with *g1=g2=0.*
π
λ
Table 6. Theoretical Value of Misalignment Sensitivity Parameter '*D'*
coupling loss of resonator & long gain length. The radiation, which begins from the output coupler-end, sees the round trip gain while the radiation which begins from the rear mirror; sees only single trip, and the starting intensity of radiation in the first case is relatively smaller than that in the second case. Therefore the misalignment in first case (output coupler) has relatively less effect on the laser power build up compared to the misalignment of the second case (rear reflector). Furthermore, the experimental results indicate that sensitivity parameter '*D'* is a suitable parameter to describe the alignment stability of a resonator.
SL – Single limb, DL – Double limb, CC – Concave-Concave resonator, PC – Plano-Concave resonator, M1 – Micrometer1, M2 – Micrometer2
Note: Micro-meters are numbered 1 & 2 in anticlockwise direction.
Fig. 20. Misalignment in Single Limb for Reflector
Fig. 21. Misalignment in Single Limb for Output Coupler
Diffusion Cooled V-Fold CO2 Laser 205
Decreasing reflectivity to extract more power increases the overall loss of the system, requiring greater pumping power to reach threshold. Increasing the output coupler reflectivity increases the cavity photon life time, thereby increasing the photon loss and
There must be an optimum reflectivity of an output coupler at which the radiant output power will be a maximum. This part reports the variation of output power as a function of output coupler reflectivity and active medium length for a V-fold diffusion cooled CO2 gas laser. A relationship (Eq.26) is used for optimum transmission coefficient of the output
( ) *opt*
0
*<sup>a</sup> T g La g L*
1 2
1 = −
In the development of a high-power CW CO2 laser; it is a design challenge to reach high output power simultaneously with good beam quality. The problem becomes stringent in multi-fold diffusion cooled CO2 lasers that uses a stable resonator configuration, where many meters of resonator length are required to generate a few kilowatts of energy, owing to the low aspect ratio between the discharge diameter and the discharge length necessary to obtain a mono mode beam. A laser will operate satisfactorily with many possible combinations of output coupler reflectivity, provided that the gain in a single pass through the amplifier is sufficiently large to equal or exceed the mirror transmission losses (or other
Experiment is carried out to test the performance of the laser for different reflectivity of output couplers and different active medium length. We used a concave-concave resonator; consist of gold coated copper mirror and a concave ZnSe output coupler of 5 meter radius of curvature each. In our experimental set-up, we have taken five different output couplers of reflectivity 5, 10, 17, 50 and 60% and corresponding output power was measured for 1.5, 3.0, 4.5 and 6.0 meter active medium length. These results are plotted for active medium length v/s output power for different output couplers (Fig.25) & reflectivity v/s output power for above stated active medium lengths (Fig.26). Output power of diffusion cooled laser is proportional to active medium length but we can see (Fig.25) that as the length increases
1 2
(26)
0
resulting in decrease of laser output power (Fig.24).
Fig. 24. Theoretical Curve for Output Coupling Reflectance
couplers to verify experimental measurements.
losses).
Fig. 22. Misalignment in Double Limb for Reflector
Fig. 23. Misalignment in Double Limb for Output Coupler
#### **3.10 Optimum reflectivity of output coupler of V-fold laser**
In V-fold type of resonator since there are more number of limbs and each limb has different output coupling reflectivity. So the output power of a laser that can be extracted depends on the reflectivity/transmission of the output coupler (Eq.25).
$$P\_{out} = A\_b I\_s \frac{1 - R}{1 - R + \sqrt{R} \left(1/a - a\right)} \left[g\_0 l - \ln \sqrt{R \times a^2}\right] \tag{25}$$
Where
Ab = Cross section area of medium, R = Reflectivity, a = cavity losses, Is = Saturation Intensity and *g l*<sup>0</sup> = Small signal gain.
Fig. 22. Misalignment in Double Limb for Reflector
Fig. 23. Misalignment in Double Limb for Output Coupler
the reflectivity/transmission of the output coupler (Eq.25).
Where
R = Reflectivity, a = cavity losses,
Ab = Cross section area of medium,
Is = Saturation Intensity and *g l*<sup>0</sup> = Small signal gain.
( ) *out b s*
1 1
**3.10 Optimum reflectivity of output coupler of V-fold laser**
In V-fold type of resonator since there are more number of limbs and each limb has different output coupling reflectivity. So the output power of a laser that can be extracted depends on
*<sup>R</sup> P AI gl R a R R aa*
0
<sup>1</sup> ln
<sup>−</sup> <sup>=</sup> − × −+ −
2
(25)
Decreasing reflectivity to extract more power increases the overall loss of the system, requiring greater pumping power to reach threshold. Increasing the output coupler reflectivity increases the cavity photon life time, thereby increasing the photon loss and resulting in decrease of laser output power (Fig.24).
Fig. 24. Theoretical Curve for Output Coupling Reflectance
There must be an optimum reflectivity of an output coupler at which the radiant output power will be a maximum. This part reports the variation of output power as a function of output coupler reflectivity and active medium length for a V-fold diffusion cooled CO2 gas laser. A relationship (Eq.26) is used for optimum transmission coefficient of the output couplers to verify experimental measurements.
$$T\_{opt} = \left(g\_o L a\right)^{\mathcal{V}2} \left[1 - \left(\frac{a}{g\_o L}\right)^{\mathcal{V}2}\right] \tag{26}$$
In the development of a high-power CW CO2 laser; it is a design challenge to reach high output power simultaneously with good beam quality. The problem becomes stringent in multi-fold diffusion cooled CO2 lasers that uses a stable resonator configuration, where many meters of resonator length are required to generate a few kilowatts of energy, owing to the low aspect ratio between the discharge diameter and the discharge length necessary to obtain a mono mode beam. A laser will operate satisfactorily with many possible combinations of output coupler reflectivity, provided that the gain in a single pass through the amplifier is sufficiently large to equal or exceed the mirror transmission losses (or other losses).
Experiment is carried out to test the performance of the laser for different reflectivity of output couplers and different active medium length. We used a concave-concave resonator; consist of gold coated copper mirror and a concave ZnSe output coupler of 5 meter radius of curvature each. In our experimental set-up, we have taken five different output couplers of reflectivity 5, 10, 17, 50 and 60% and corresponding output power was measured for 1.5, 3.0, 4.5 and 6.0 meter active medium length. These results are plotted for active medium length v/s output power for different output couplers (Fig.25) & reflectivity v/s output power for above stated active medium lengths (Fig.26). Output power of diffusion cooled laser is proportional to active medium length but we can see (Fig.25) that as the length increases
Diffusion Cooled V-Fold CO2 Laser 207
• Make sure that everyone in the vicinity of the laser or anywhere the beam (or its reflection) may be is fully aware of the safety issues and has proper eyewear. • Provide visible and unambiguous indications that the laser is powered and the beam is on. • A kill switch is essential and should be located far enough from the laser tube so that it
• For flowing gas lasers, provide adequate ventilation. While the lasing gasses (helium, nitrogen, and carbon dioxide) are not toxic, and not very much is involved for laser operation, a leak in the gas delivery system could go undetected. CO2 in particular is heavier than air so it will displace air in an enclosed space which may result in various
• Where maintenance or repair is involved, be aware of the properties of the specific materials used for the optics and elsewhere. For example, the biohazards of zinc
In the present laser, power of 380 Watts from 7.5 m discharge length and maximum 420 W from seven limbs (10.5 meter discharge length) has been achieved. The maximum average power of 50 W/m is obtained from this laser, which is comparable to other diffusion-cooled laser developed till now. Studies have shown that dissociation of CO2 molecules increases with the increase of no of tube or discharge length. Care has been taken to have a low gas residence time to reduce the deleterious effect of CO2 dissociation. The electro-optic
The power and stability of a laser system is mainly governed by the misalignment sensitivity of the optical resonator. To ensure stable and high power from a laser system misalignment sensitivity has to be known. The experimental results indicate that sensitivity parameter D is a suitable parameter to describe the alignment stability of a resonator.
The output power of a laser that can be extracted depends on the reflectivity/transmission of the output coupler. There must be an optimum reflectivity of an output coupler at which
According to Rigrod's formula if length increases power reduces, as there are many other parameters, which are not optimized. So power goes on decreases when length increases.
Author is thankful to Sh. Mukesh Jewariya, Sh. Firoz Koser, Sh. D.D. Saha, Sh. M.B. Pote, Sh. S.V. Deshmukh, Sh. A.K. Nath, Sh. Dinesh Nagpure, Sh. Abrat Verma and all other colleagues of Laser and Material Processing Division, RRCAT, who directly or indirectly involve in design and development of this laser. Author is also thankful to Sh. Abhay
Dahotre, Narendra B. & Harimkar, Sandip P. (2008). *Laser Fabrication and Machining of Materials,* Springer Science + Business Media, ISBN 978-0-387-72343-3, USA
Kumar (IMA Section) and Sh. Arup Ratan Jana (Accelerator and Beam Physics Lab.).
is accessible in an emergency even if a total meltdown is in progress.
symptoms from nausea to asphyxiation.
selenide and beryllia.
efficiency of the laser is about 13%.
the radiant output power will be a maximum.
Beam size also affects the output power.
**6. Acknowledgement**
**7. References**
**5. Conclusion**
power increases but the rate of increase of output power decreases. This is because of diffraction losses increases with increase of length. For theoretical calculation, in order to estimate *g*<sup>0</sup> and *a* in our laser, we have used the Eq.25 of laser power in a V-fold CO2 laser. Substituting the value of laser power for three different reflectivity of the output coupler, the three unknowns i.e. *<sup>s</sup> g a I* <sup>0</sup> , & are calculated theoretically. Thus using these values in expression (Eq.26), the *Topt* is estimated to be 66% for 6 meter active medium length theoretically. Experimentally also we have observed that laser output power is 209 watts for 83% transmissivity and 150 watts for 50% transmissivity. From the above data we can predict that the optimum value of transmissivity lies somewhere between 50 & 83%.
Fig. 25. Experimental Curve: Output Power v/s Active Medium Length
Fig. 26. Experimental Curve: Output Power v/s Reflectivity
#### **4. Safety precautions**
Some general considerations when working with V-fold CO2 lasers are as follows:
power increases but the rate of increase of output power decreases. This is because of diffraction losses increases with increase of length. For theoretical calculation, in order to estimate *g*<sup>0</sup> and *a* in our laser, we have used the Eq.25 of laser power in a V-fold CO2 laser. Substituting the value of laser power for three different reflectivity of the output coupler, the three unknowns i.e. *<sup>s</sup> g a I* <sup>0</sup> , & are calculated theoretically. Thus using these values in expression (Eq.26), the *Topt* is estimated to be 66% for 6 meter active medium length theoretically. Experimentally also we have observed that laser output power is 209 watts for 83% transmissivity and 150 watts for 50% transmissivity. From the above data we can
predict that the optimum value of transmissivity lies somewhere between 50 & 83%.
Fig. 25. Experimental Curve: Output Power v/s Active Medium Length
Fig. 26. Experimental Curve: Output Power v/s Reflectivity
Some general considerations when working with V-fold CO2 lasers are as follows:
• Clearly mark and if possible, block off access to the path of the beam.
• Provide a beam stop capable of safely absorbing this power on a continuous basis.
• Reflected beams may have nearly as much power as the original and are just as dangerous. Although many common materials will block 10.6 µm, specular surfaces
**4. Safety precautions**
will reflect it quite well.
### **5. Conclusion**
In the present laser, power of 380 Watts from 7.5 m discharge length and maximum 420 W from seven limbs (10.5 meter discharge length) has been achieved. The maximum average power of 50 W/m is obtained from this laser, which is comparable to other diffusion-cooled laser developed till now. Studies have shown that dissociation of CO2 molecules increases with the increase of no of tube or discharge length. Care has been taken to have a low gas residence time to reduce the deleterious effect of CO2 dissociation. The electro-optic efficiency of the laser is about 13%.
The power and stability of a laser system is mainly governed by the misalignment sensitivity of the optical resonator. To ensure stable and high power from a laser system misalignment sensitivity has to be known. The experimental results indicate that sensitivity parameter D is a suitable parameter to describe the alignment stability of a resonator.
The output power of a laser that can be extracted depends on the reflectivity/transmission of the output coupler. There must be an optimum reflectivity of an output coupler at which the radiant output power will be a maximum.
According to Rigrod's formula if length increases power reduces, as there are many other parameters, which are not optimized. So power goes on decreases when length increases. Beam size also affects the output power.
#### **6. Acknowledgement**
Author is thankful to Sh. Mukesh Jewariya, Sh. Firoz Koser, Sh. D.D. Saha, Sh. M.B. Pote, Sh. S.V. Deshmukh, Sh. A.K. Nath, Sh. Dinesh Nagpure, Sh. Abrat Verma and all other colleagues of Laser and Material Processing Division, RRCAT, who directly or indirectly involve in design and development of this laser. Author is also thankful to Sh. Abhay Kumar (IMA Section) and Sh. Arup Ratan Jana (Accelerator and Beam Physics Lab.).
#### **7. References**
Dahotre, Narendra B. & Harimkar, Sandip P. (2008). *Laser Fabrication and Machining of Materials,* Springer Science + Business Media, ISBN 978-0-387-72343-3, USA
**7**
*Kyoto University*
*Japan*
**Heterodyne Interferometer for Measurement**
Keiichiro Urabe and Kunihide Tachibana
**of Electron Density in High-Pressure Plasmas**
Conventional material processes using plasmas generated in low pressure gaseous media are recently being transposed to high-pressure plasma processes, because of the potential of high-pressure plasmas to reduce costs for vacuum systems in industrial applications. In many kinds of high-pressure plasma sources, small-scale atmospheric-pressure plasmas (APPs) having a property of thermal non-equilibrium, have been especially attracting much interest of researchers over the last 20 years (Becker, 2005). In such a high pressure gaseous medium, generating the small-scale plasmas in mm or μm order is effective to keep its ignition voltage low and discharge behavior stable, following a famous rule on discharge ignition called a "Paschen's law" (Paschen, 1889; von Engel, 1994). Using these small-scale high-pressure plasmas, localized maskless processes, for example etching (Ichiki et al., 2004) and deposition (Babayan et al., 1998), have been reported. Also, nanomaterial synthesis was realized by the APP utilizing their property of short residence time of source particles inside the plasma (Nozaki et al., 2007). In addition to the inorganic solid material processes, process objects of the APPs are spreading toward liquids (Bruggeman & Leys, 2009) and
In characterization and comparison of plasma properties, electron density is one of the most important parameters. This is because that electrons play a major role for carrying external energy to heavy particles inside the plasma, and all other excited species can be calculated theoretically from the plasma parameters of electron density, electron energy distribution, and gas composition. Diagnostics of electron density have in the APPs have been reported by many researchers using Langmuir probe methods (Chang, 1973; Chang & Laframboise, 1976), Stark broadening measurement (Laux et al., 2003), and laser Thomson scattering measurement (Kono & Iwamoto, 2004). However, these methods have limitations for the APP measurements due to their finite sensitivities, expected perturbations or interferences, spatiotemporal resolutions, etc. For instance, the Langmuir probe is difficult be applied to the small-scale APPs since its theory in a collision dominant condition is not well developed and there are discharge perturbations by the metallic probe. The Stark broadening spectroscopic method enables us to derive electron density only for over 5×1013 cm-3 due to the large pressure broadening superposed over the Lorentzian shape (Laux et al., 2003). The laser Thomson scattering method is not applicable to molecular gases although its spatial resolution is high enough, because large Raman scattering components overlap with the
biocells (Kong et al., 2009) which cannot present in low-pressure conditions.
**1. Introduction**
Thomson scattering signal.
Endo, Masamori & Walter, Robert F. (2007). *Gas Lasers*, CRC Press, ISBN 0-8493-3553-1, USA
### **Heterodyne Interferometer for Measurement of Electron Density in High-Pressure Plasmas**
Keiichiro Urabe and Kunihide Tachibana *Kyoto University Japan*
#### **1. Introduction**
208 CO2 Laser – Optimisation and Application
Endo, Masamori & Walter, Robert F. (2007). *Gas Lasers*, CRC Press, ISBN 0-8493-3553-1, USA Freiberg, R.J. and Halsted, A. S. (1969). Properties of Low Order Transverse Modes in Argon Ion Lasers. *Applied Optics*, Vol. 8, No. 2, (1969), pp. (355-362), ISSN 1559-128X Hitz, B.; Ewing, J.J. and Hetch, J. (2005). *Introduction to laser technology* (Third Edition), John
Hoag, Ethan; Pease, Henry; Staal, John and Zar, Jacob. (1974). Performance Characteristics of
Hodgson, N. (1997). Optical resonators: fundamentals, advanced concepts and applications,
Koechner, W. (1988). Solid State Laser Engineering, Springer-Verlag, ISBN 3-540-18747-2,
Migliore, Leonard. (1996). *Laser Materials Processing*, Marcel Dekker Inc., ISBN 0-8247-9714-0,
Nath, A.K. and Golubev, V.S. (1998). Design considerations and scaling laws for high power
Scott, Marion W. and Myers, Gary D. (1984). Steady-state CO2 laser model. *Applied Optics*,
Seigman, A.E. (1986). *Lasers*, University Science Books, Mill Valley, California, USA, ISBN 0-
Soni, R.K. et al. (2005). Diffusion Cooled V-Fold CO2 Laser, *Proceedings of Fourth DAE-BRNS*
Soni, R.K. et al. (2005). Misalignment Sensitivity of a V-Fold Optical Resonator, *Proceedings of*
Soni, R.K. et al. (2005). Optimum Reflectivity for a Diffusion Cooled CO2 Laser, *Proceedings*
Svelto, Orazio. (2010). *Principles of Lasers* (Fifth Edition), Springer Science+Business Media,
Thyagarajan, K. & Ghatak, Ajoy. (2010). *Lasers: Fundamentals and Applications* (Second
Weber, H., Herziger, G. & Poprawe, R. (2006). *Laser Physics and Applications Sub volume A: Laser Fundamentals Part 2*, Springer, ISBN 978-3-540-28824-4, Germany Xinju, Lan. (2010). *Laser Technology* (Second Edition), CRC Press, ISBN 978-1-4200-9171-7,
Editiion), Springer Science+Business Media, New York, USA
Vol. 23, No. 17, (September 1984), pp. (2874-2878), ISSN 1559-128X
convective cooled CW CO2 lasers. *Pramana*, Vol. 51, No. 3-4, (September-October
*National Laser Symposium, 185 January 10-13, 2005*, ISBN 8177647342, 9788177647341,
*Fifth DAE-BRNS National Laser Symposium (NLS-5), December 7-10, 2005, page 96*,
*of Fifth DAE-BRNS National Laser Symposium (NLS-5), December 7-10, 2005, page 98*,
a 10-kW Industrial CO2 Laser System. *Applied Optics*, Vol. 13, No. 8, (August 1974),
Wiley & Sons Inc., USA
Berlin
New York, USA
935702-11-5, USA
www.springer.com
USA
pp. (1959-1964), ISSN 1559-128X
1998), pp. (463-479), 0304-4289
Mumbai (India), January 2005
ISBN , Tamil Nadu (India), December 7-10, 2005
ISBN , Tamil Nadu (India), December 2005
Springer, ISBN 3-540-76137-3, Berlin
Conventional material processes using plasmas generated in low pressure gaseous media are recently being transposed to high-pressure plasma processes, because of the potential of high-pressure plasmas to reduce costs for vacuum systems in industrial applications. In many kinds of high-pressure plasma sources, small-scale atmospheric-pressure plasmas (APPs) having a property of thermal non-equilibrium, have been especially attracting much interest of researchers over the last 20 years (Becker, 2005). In such a high pressure gaseous medium, generating the small-scale plasmas in mm or μm order is effective to keep its ignition voltage low and discharge behavior stable, following a famous rule on discharge ignition called a "Paschen's law" (Paschen, 1889; von Engel, 1994). Using these small-scale high-pressure plasmas, localized maskless processes, for example etching (Ichiki et al., 2004) and deposition (Babayan et al., 1998), have been reported. Also, nanomaterial synthesis was realized by the APP utilizing their property of short residence time of source particles inside the plasma (Nozaki et al., 2007). In addition to the inorganic solid material processes, process objects of the APPs are spreading toward liquids (Bruggeman & Leys, 2009) and biocells (Kong et al., 2009) which cannot present in low-pressure conditions.
In characterization and comparison of plasma properties, electron density is one of the most important parameters. This is because that electrons play a major role for carrying external energy to heavy particles inside the plasma, and all other excited species can be calculated theoretically from the plasma parameters of electron density, electron energy distribution, and gas composition. Diagnostics of electron density have in the APPs have been reported by many researchers using Langmuir probe methods (Chang, 1973; Chang & Laframboise, 1976), Stark broadening measurement (Laux et al., 2003), and laser Thomson scattering measurement (Kono & Iwamoto, 2004). However, these methods have limitations for the APP measurements due to their finite sensitivities, expected perturbations or interferences, spatiotemporal resolutions, etc. For instance, the Langmuir probe is difficult be applied to the small-scale APPs since its theory in a collision dominant condition is not well developed and there are discharge perturbations by the metallic probe. The Stark broadening spectroscopic method enables us to derive electron density only for over 5×1013 cm-3 due to the large pressure broadening superposed over the Lorentzian shape (Laux et al., 2003). The laser Thomson scattering method is not applicable to molecular gases although its spatial resolution is high enough, because large Raman scattering components overlap with the Thomson scattering signal.
Heterodyne Interferometer for Measurement of Electron Density in High-Pressure Plasmas 211
separated by a beam splitter, and the beam's phase change is caused by variations of the refractive index of the tested material placed only in one path. Difference of the refractive indexes between the two paths is derived from the merged beam by a second beam splitter. A heterodyne technique is a reducing method of observing frequency from light frequency to external oscillator frequency in a instrumentally manageable range. When this method is applied to the Mach-Zehnder interferometer, the light frequency in one of two paths is modulated, and in the other path the frequency is not modulated but its phase is shifted by the tested material. After merging the two laser beams, a beat signal at the modulation frequency in the merged beam can be isolated from the original output signal of a solid-state
> 2 2 2 1 2 1 2 12 <sup>0</sup> *It U t U t E E EE* () () () cos( ) *Δ t ΔΦ*
where *U*1(*t*) = *E*1cos(*ωt*+*Δωt*) is the electric field of the laser beam passing through the frequency modulating device with a modulation frequency at *Δω*, *U*2(*t*) = *E*2cos(*ωt*+*ΔΦ*) is the electric field of the beam passing through the tested material, and *ΔΦ* is the phase shift of the beam by the tested material. Components in a light frequency range are automatically eliminated from the signal because of the response-speed limitation of detectors. From the output signal, *I*(*t*), we can derive the phase shift by the tested material *ΔΦ* using a phase detecting device and a reference signal which is separated from the frequency-modulation
ω
**Detector**
**Lock-in Amp.**
**Oscilloscope**
<sup>=</sup> = + =++ − , (1)
**AOM**
ω
Fig. 1. Experimental setup of CO2-laser heterodyne interferometer in Mach-Zhender principle for measurement of electron density inside high-pressure plasma source. AOM is acousto optical modulator shifting CO2-laser frequency at frequency of RF driver's signal. DC is directional coupler taking small amplitude of RF signal driving AOM to input
**Plasma source**
**ZnSe lens**
**RF driver DC**
**Mirror**
A practical experimental setup of the CO2-laser heterodyne interferometer used in our studies is shown in Fig. 1. The original CO2 laser beam was split by a ZnSe half mirror. The beam frequency in one path was shifted an acousto-optical modulator (AOM) whose modulation frequency *Δω* = 40 MHz. In the other path, the beam was focused on a tested plasma source by a pair of ZnSe lens, and the beam phase was shifted by the tested plasma. These two beams were superposed again at another ZnSe half mirror, and their beat signal
detecting device, *I*(*t*), which is expressed in a following equation.
signal.
reference signal to lock-in amplifier.
**Half mirror**
For the purpose of diagnostics in the small-scale high-pressure plasmas, refractive-index measurement using electromagnetic (EM) waves is appropriate since they can provide plasma property's information with negligible perturbation to the plasma. Also, in the refractive-index measurement, we can ignore excitation and scattering processes that are largely dependent on gas compositions and densities, which are indispensable for the above listed other measurement methods. For typical APPs whose electron densities range from 1012 to 1015 cm-3, microwaves and millimeter-waves are suitable to detect absorption of the probing EM waves and derive the electron density from the absorption ratio (Tachibana et al., 2005(a), 2005(b); Sakai et al., 2005; Ito et al., 2010). However, diagnostics using those EM waves cannot have good spatial resolutions for the small-scale plasmas because of their diffraction limits. Meanwhile, a heterodyne interferometer of CO2 laser beam, which is a theme in this chapter, can be in a category of the refractive-index measurement methods and have a good spatial resolution at the same time. This interferometer detects the laser beam's phase shift caused by the presence of tested plasma and provides line-integrated information of electron density with a spatial resolution in sub mm order (Leipold et al., 2000; Choi et al., 2009).
This chapter is focused on descriptions of the CO2-laser heterodyne interferometer for the measurement in small-scale high-pressure plasma sources, because detailed descriptions of the interferometer for the low-pressure plasmas have been written in other chapters and textbooks (*for example* Hutchinson, 2002). In high-pressure plasmas, large contribution of gas-particle density (atoms and/or molecules in ground states) to the change of the refractive index is expected due to Joule heating in the discharge region, and this must be accurately separated from the signal in order to derive the absolute value of electron density. Therefore, we firstly explain how to divide the two components in the CO2-laser beam's phase shift, which are the phase shifts due to electron generation and gas heating. Then, the fundamental properties of our CO2-laser heterodyne interferometer, for example spatial resolution and lower limit of electron-density detection, are verified reviewing experimental measurements of the small-scale APPs driven by DC applied voltages (Choi et al., 2009). Finally, a combination measurement method composed of the CO2-laser heterodyne interferometer and a millimeter-wave transmission method is introduced as a solution of spatiotemporally resolved electron-density measurement in small-scale APPs with high-speed temporal evolution of electron density (Urabe et al., 2011).
#### **2. Fundamentals of heterodyne interferometer**
This section includes brief introduction of a Mach-Zehnder principle and a heterodyne technique used in our interferometer and theoretical descriptions of the phase shift in the CO2 laser beam induced by electrons in a tested plasma source, and explanations how to derive electron density in small-scale high-pressure plasmas eliminating influence of gas heating from total phase shift signals.
#### **2.1 Mach-Zehnder heterodyne interferometer**
Measurements of refractive index in tested materials are often done by some forms of interferometer. Most of interferometers are in Michelson, Fabry-Perot, and Mach-Zehnder configurations. Mach-Zehnder interferometer is a two-beam interferometer having two paths in which the laser beams travel in only one direction. The original laser beam is 210 CO2 Laser – Optimisation and Application
For the purpose of diagnostics in the small-scale high-pressure plasmas, refractive-index measurement using electromagnetic (EM) waves is appropriate since they can provide plasma property's information with negligible perturbation to the plasma. Also, in the refractive-index measurement, we can ignore excitation and scattering processes that are largely dependent on gas compositions and densities, which are indispensable for the above listed other measurement methods. For typical APPs whose electron densities range from 1012 to 1015 cm-3, microwaves and millimeter-waves are suitable to detect absorption of the probing EM waves and derive the electron density from the absorption ratio (Tachibana et al., 2005(a), 2005(b); Sakai et al., 2005; Ito et al., 2010). However, diagnostics using those EM waves cannot have good spatial resolutions for the small-scale plasmas because of their diffraction limits. Meanwhile, a heterodyne interferometer of CO2 laser beam, which is a theme in this chapter, can be in a category of the refractive-index measurement methods and have a good spatial resolution at the same time. This interferometer detects the laser beam's phase shift caused by the presence of tested plasma and provides line-integrated information of electron density with a spatial resolution in sub mm order (Leipold et al.,
This chapter is focused on descriptions of the CO2-laser heterodyne interferometer for the measurement in small-scale high-pressure plasma sources, because detailed descriptions of the interferometer for the low-pressure plasmas have been written in other chapters and textbooks (*for example* Hutchinson, 2002). In high-pressure plasmas, large contribution of gas-particle density (atoms and/or molecules in ground states) to the change of the refractive index is expected due to Joule heating in the discharge region, and this must be accurately separated from the signal in order to derive the absolute value of electron density. Therefore, we firstly explain how to divide the two components in the CO2-laser beam's phase shift, which are the phase shifts due to electron generation and gas heating. Then, the fundamental properties of our CO2-laser heterodyne interferometer, for example spatial resolution and lower limit of electron-density detection, are verified reviewing experimental measurements of the small-scale APPs driven by DC applied voltages (Choi et al., 2009). Finally, a combination measurement method composed of the CO2-laser heterodyne interferometer and a millimeter-wave transmission method is introduced as a solution of spatiotemporally resolved electron-density measurement in small-scale APPs
This section includes brief introduction of a Mach-Zehnder principle and a heterodyne technique used in our interferometer and theoretical descriptions of the phase shift in the CO2 laser beam induced by electrons in a tested plasma source, and explanations how to derive electron density in small-scale high-pressure plasmas eliminating influence of gas
Measurements of refractive index in tested materials are often done by some forms of interferometer. Most of interferometers are in Michelson, Fabry-Perot, and Mach-Zehnder configurations. Mach-Zehnder interferometer is a two-beam interferometer having two paths in which the laser beams travel in only one direction. The original laser beam is
with high-speed temporal evolution of electron density (Urabe et al., 2011).
**2. Fundamentals of heterodyne interferometer**
heating from total phase shift signals.
**2.1 Mach-Zehnder heterodyne interferometer**
2000; Choi et al., 2009).
separated by a beam splitter, and the beam's phase change is caused by variations of the refractive index of the tested material placed only in one path. Difference of the refractive indexes between the two paths is derived from the merged beam by a second beam splitter.
A heterodyne technique is a reducing method of observing frequency from light frequency to external oscillator frequency in a instrumentally manageable range. When this method is applied to the Mach-Zehnder interferometer, the light frequency in one of two paths is modulated, and in the other path the frequency is not modulated but its phase is shifted by the tested material. After merging the two laser beams, a beat signal at the modulation frequency in the merged beam can be isolated from the original output signal of a solid-state detecting device, *I*(*t*), which is expressed in a following equation.
$$I(t) = \left| \left. U\_1(t) + \left. U\_2(t) \right| \right|\_{\alpha=0}^2 = E\_1^2 + E\_2^2 + E\_1 E\_2 \cos(\Delta a t - \Delta \Phi) \right. \tag{1}$$
where *U*1(*t*) = *E*1cos(*ωt*+*Δωt*) is the electric field of the laser beam passing through the frequency modulating device with a modulation frequency at *Δω*, *U*2(*t*) = *E*2cos(*ωt*+*ΔΦ*) is the electric field of the beam passing through the tested material, and *ΔΦ* is the phase shift of the beam by the tested material. Components in a light frequency range are automatically eliminated from the signal because of the response-speed limitation of detectors. From the output signal, *I*(*t*), we can derive the phase shift by the tested material *ΔΦ* using a phase detecting device and a reference signal which is separated from the frequency-modulation signal.
Fig. 1. Experimental setup of CO2-laser heterodyne interferometer in Mach-Zhender principle for measurement of electron density inside high-pressure plasma source. AOM is acousto optical modulator shifting CO2-laser frequency at frequency of RF driver's signal. DC is directional coupler taking small amplitude of RF signal driving AOM to input reference signal to lock-in amplifier.
A practical experimental setup of the CO2-laser heterodyne interferometer used in our studies is shown in Fig. 1. The original CO2 laser beam was split by a ZnSe half mirror. The beam frequency in one path was shifted an acousto-optical modulator (AOM) whose modulation frequency *Δω* = 40 MHz. In the other path, the beam was focused on a tested plasma source by a pair of ZnSe lens, and the beam phase was shifted by the tested plasma. These two beams were superposed again at another ZnSe half mirror, and their beat signal
Heterodyne Interferometer for Measurement of Electron Density in High-Pressure Plasmas 213
where *A* and *B* are the specific values for gas species, *n*g0 is the gas particle density under the standard temperature and pressure (STP) conditions, and *n*g is the gas-particle density inside the tested plasma source (Allen, 1973). Considering the influence of gas-particle density on the refractive index, the phase shift of the CO2 laser beam in the high-pressure plasmas is described by two components of the electron density, *ΔΦ*e, and the gas-particle
2
ε π
*e AB ΔΦ ΔΦ ΔΦ n d n d m c n*
Table 1 lists change directions of the phase shift in the CO2-laser heterodyne interferometer. Whereas we used the gas-particle density *n*g for the calculation in Eq. (6), *ΔΦ*g can be also expressed using gas temperature in the plasma. Each component of the phase shift in the high-pressure plasma, *ΔΦ*e and *ΔΦ*g, changes to negative direction due to the electron production and the decrease of gas-particle density by the Joule heating. The separation of these two components becomes a crucial problem for accurate derivation of the electron density in practical measurements of the high-pressure plasmas, because these two components are only measured together in the heterodyne interferometer and start
Increase Decrease
Electron density Negative Positive
Gas temperature Negative Positive
Table 1. Change directions of CO2-laser beam's phase shift by changes of electron density,
For the separation of two components in the phase shift of the CO2 laser beam, difference of time constants of the two phenomena is utilized, because the change of electron density is much faster than that of gas temperature in the high-pressure discharge (Leipold et al., 2000; Choi et al., 2009). Applicability of this technique can be confirmed by experimental evidence as explained below. Also, in general, the two terms of electron density, *n*e, and gas-particle density, *n*g, can be derived directly by solving a system of two equations using a twowavelength heterodyne interferometer, because the phase-shift components due to the electron and gas-particle densities depend differently on the laser wavelength. Some groups have tried to measure both electron and gas-particle densities inside the plasma by an additional heterodyne interferometer using a He-Ne laser beam at 633 nm (Adler & Kindel,
Figure 2 shows an example waveform of the output phase-shift signal of the lock-in amplifier obtained in a small-scale high-pressure plasma source operated at 70 Torr driven by a pulsed DC voltage at 300 V with a 300-μs pulse duration. The phase-shift signal recorded in the oscilloscope is proportional to the phase shift in a specification ratio of the
gas-particle density, and gas temperature, when they increase or decrease.
density Positive Negative
λ
4
e g 2 2 e g e 0 g0
<sup>2</sup> <sup>1</sup>
= + =− + + . (6)
λ
π
λ
density, *ΔΦ*g, as a following equation.
decreasing at the same timing.
2003; Acedo et al., 2004).
Gas-particle
was detected by a HgCdTe IR detector operated at room temperature. The output signal of the IR detector, *I*(*t*), was put into a lock-in amplifier with the reference signal, cos(*Δωt*), which was divided from a RF signal driving the AOM by a directional coupler (DC). The lock-in amplifier outputs a signal proportional to the phase shift, *ΔΦ*, and its temporal evolution was recorded in an oscilloscope.
#### **2.2 Phase shift of CO2 laser beam by electrons inside plasmas**
In refractive-index measurement inside a tested plasma source having no gas temperature fluctuation, for example plasmas generated in gaseous media at very lower pressure than atmosphere, the phase shift by tested plasma, *ΔΦ*, measured using the Mach-Zehnder heterodyne interferometer becomes
$$
\Delta \Phi = \int (k\_{\text{plasma}} - k\_0) \, dl = \int (N\_{\text{plasma}} - 1) \frac{2\pi}{\mathcal{N}} \, dl \, \, \, \tag{2}
$$
where *k*plasma and *k*0 are the wavenumbers of the plasma and vacuum, *N*plasma is the refractive index of the plasma, and *λ* is the wavelength of the CO2 laser beam. Absolute electron density inside the plasma, *n*e (m), can be derived using a following relationship.
$$N\_{\rm plasma} = \sqrt{1 - \frac{\alpha\_{\rm plasma}^2}{\alpha^2}} \sqrt{\frac{\alpha}{\alpha^2} - 1} - \frac{1}{2} \left(\frac{\alpha\_{\rm plasma}^2}{\alpha^2}\right) = 1 - \frac{1}{2} \frac{n\_\circ e^2}{m\_\circ e\_0} \frac{\lambda^2}{4\pi^2 c^2} \,\,\,\tag{3}$$
where *ω*plasma and *ω* are the electron plasma frequency and the angular frequency of laser beam, *m*e is the mass of electron, *ε*0 is the permittivity of vacuum, *c* is the velocity of light (Hutchinson, 2002). We used an approximate expansion in this calculation because electron density is sufficiently smaller than gas-particle density in weakly ionized plasmas generated in laboratories. When we assume that electron density inside the tested plasma is spacially homogeneous and its length along the laser path is *d* (m), the relationship between the phase shift and the electron density becomes
$$
\Delta\Phi = (N\_{\text{plasma}} - 1)\frac{2\pi}{\lambda}d = -\frac{e^2\lambda}{4m\_e\varepsilon\_0\pi c^2}n\_ed = -\left(3.0 \times 10^{-20}\right)n\_ed \text{ (rad)},\tag{4}
$$
in the CO2-laser heterodyne interferometer at *λ* = 10.6 μm.
#### **2.3 Electron-density measurement in high-pressure plasmas**
In this subsection, influence of dense gas particles in high-pressure plasmas on the calculation of electron density from the phase shift of the CO2 laser beam, which can be ignored in the measurement of low-pressure plasmas, is introduced. It should be noted that increase of gas temperature corresponding to decrease of gas-particle density is promoted in the high-pressure plasmas because of high collision frequencies between electrons and gas particles. This change of gas-particle density results in the change of the refractive index in gaseous medium. The refractive index of the gaseous medium, *N*gas, is
$$N\_{g\text{as}} = 1 + A \left(1 + \frac{B}{\lambda^2} \right) \frac{n\_{\text{g}}}{n\_{\text{g0}}} \, , \tag{5}$$
was detected by a HgCdTe IR detector operated at room temperature. The output signal of the IR detector, *I*(*t*), was put into a lock-in amplifier with the reference signal, cos(*Δωt*), which was divided from a RF signal driving the AOM by a directional coupler (DC). The lock-in amplifier outputs a signal proportional to the phase shift, *ΔΦ*, and its temporal
In refractive-index measurement inside a tested plasma source having no gas temperature fluctuation, for example plasmas generated in gaseous media at very lower pressure than atmosphere, the phase shift by tested plasma, *ΔΦ*, measured using the Mach-Zehnder
> plasma 0 plasma <sup>2</sup> *ΔΦ* ( ) ( 1) *k k dl N dl*
where *k*plasma and *k*0 are the wavenumbers of the plasma and vacuum, *N*plasma is the refractive index of the plasma, and *λ* is the wavelength of the CO2 laser beam. Absolute electron
1 1 11 1
ω
where *ω*plasma and *ω* are the electron plasma frequency and the angular frequency of laser beam, *m*e is the mass of electron, *ε*0 is the permittivity of vacuum, *c* is the velocity of light (Hutchinson, 2002). We used an approximate expansion in this calculation because electron density is sufficiently smaller than gas-particle density in weakly ionized plasmas generated in laboratories. When we assume that electron density inside the tested plasma is spacially homogeneous and its length along the laser path is *d* (m), the relationship between the phase
> plasma 2 e e e 0 <sup>2</sup> ( 1) 3.0 10 4 *<sup>e</sup> ΔΦ N d n d n d m c*
In this subsection, influence of dense gas particles in high-pressure plasmas on the calculation of electron density from the phase shift of the CO2 laser beam, which can be ignored in the measurement of low-pressure plasmas, is introduced. It should be noted that increase of gas temperature corresponding to decrease of gas-particle density is promoted in the high-pressure plasmas because of high collision frequencies between electrons and gas particles. This change of gas-particle density results in the change of the refractive index in
gas 2
*N A*
1 1 *<sup>B</sup> <sup>n</sup>*
=+ +
λ*n*
λ
ε π
*n e <sup>N</sup>*
= − ≈− = −
2 2 2 2 plasma plasma e plasma 2 2 2 2
ω
2 2 4
( ) <sup>2</sup>
<sup>−</sup> = − =− ≈− × (rad), (4)
g
g0
density inside the plasma, *n*e (m), can be derived using a following relationship.
ω
π
λ
**2.3 Electron-density measurement in high-pressure plasmas**
gaseous medium. The refractive index of the gaseous medium, *N*gas, is
in the CO2-laser heterodyne interferometer at *λ* = 10.6 μm.
π
λ= −= − , (2)
e 0
20
, (5)
ε π
*m c*
λ
, (3)
evolution was recorded in an oscilloscope.
heterodyne interferometer becomes
shift and the electron density becomes
**2.2 Phase shift of CO2 laser beam by electrons inside plasmas**
ω
where *A* and *B* are the specific values for gas species, *n*g0 is the gas particle density under the standard temperature and pressure (STP) conditions, and *n*g is the gas-particle density inside the tested plasma source (Allen, 1973). Considering the influence of gas-particle density on the refractive index, the phase shift of the CO2 laser beam in the high-pressure plasmas is described by two components of the electron density, *ΔΦ*e, and the gas-particle density, *ΔΦ*g, as a following equation.
$$
\Delta\Phi = \Delta\Phi\_{\text{e}} + \Delta\Phi\_{\text{g}} = -\frac{e^2 \mathcal{X}}{4m\_{\text{e}}\varepsilon\_0 \pi c^2} n\_{\text{e}} d + \frac{2\pi A}{\lambda n\_{\text{g}0}} \left(1 + \frac{B}{\lambda^2}\right) n\_{\text{g}} d \tag{6}
$$
Table 1 lists change directions of the phase shift in the CO2-laser heterodyne interferometer. Whereas we used the gas-particle density *n*g for the calculation in Eq. (6), *ΔΦ*g can be also expressed using gas temperature in the plasma. Each component of the phase shift in the high-pressure plasma, *ΔΦ*e and *ΔΦ*g, changes to negative direction due to the electron production and the decrease of gas-particle density by the Joule heating. The separation of these two components becomes a crucial problem for accurate derivation of the electron density in practical measurements of the high-pressure plasmas, because these two components are only measured together in the heterodyne interferometer and start decreasing at the same timing.
Table 1. Change directions of CO2-laser beam's phase shift by changes of electron density, gas-particle density, and gas temperature, when they increase or decrease.
For the separation of two components in the phase shift of the CO2 laser beam, difference of time constants of the two phenomena is utilized, because the change of electron density is much faster than that of gas temperature in the high-pressure discharge (Leipold et al., 2000; Choi et al., 2009). Applicability of this technique can be confirmed by experimental evidence as explained below. Also, in general, the two terms of electron density, *n*e, and gas-particle density, *n*g, can be derived directly by solving a system of two equations using a twowavelength heterodyne interferometer, because the phase-shift components due to the electron and gas-particle densities depend differently on the laser wavelength. Some groups have tried to measure both electron and gas-particle densities inside the plasma by an additional heterodyne interferometer using a He-Ne laser beam at 633 nm (Adler & Kindel, 2003; Acedo et al., 2004).
Figure 2 shows an example waveform of the output phase-shift signal of the lock-in amplifier obtained in a small-scale high-pressure plasma source operated at 70 Torr driven by a pulsed DC voltage at 300 V with a 300-μs pulse duration. The phase-shift signal recorded in the oscilloscope is proportional to the phase shift in a specification ratio of the
Heterodyne Interferometer for Measurement of Electron Density in High-Pressure Plasmas 215
As explained in the previous paragraph, because the temporal changes of electron and gasparticle density can be observed only in the discharge ignition or termination timings, there is a need to pulse modulation of continuous discharge for the separation of phase-shift components measured in the CO2-laser heterodyne interferometer. In order to understand appropriate range of duration times for the pulse modulation, especially minimum duration time, it is important to measure the phase-shift signal decreasing the modulating pulse duration. Figure 3 shows waveforms of the phase-shift signal measured in our heterodyne interferometer using five pulse duration times ranging from 60 to 700 µs. From the measured waveforms, it could be confirmed that the pulse modulation with the duration time shorter than 200 µs does not give us accurate information of electron density in our setup, and we used the modulation pulse with duration times larger than 500 µs in our studies. This temporal behavior of the output signal of the heterodyne interferometer is of
Fig. 4. Calculated beam spot radii of CO2 laser beam at 10.6 µm as a function of distance from lens (Yariv, 1997). Initial spot radius at 1.0 mm and focal lengths of lens at 5 and 20 cm
*f* = 5 cm
should be determined by reference to these kinds of beam-profile evaluations.
**3. Spatial distribution of electron density in high-pressure plasmas**
Some of the specific measurement results of our CO2-laser heterodyne interferometer are introduced in this section. Influence of gas heating on detected phase-shift signals, spatial
For measuring the spatial distributions of electron density, evaluation of the spot radius of the CO2 laser beam inside the tested plasma source is required. In order to measure the spatial distribution of electron density inside the high-pressure plasmas whose scale are usually below cm order, the CO2-laser beam has to be focused in μm order using a pair of ZnSe lens as shown in Fig. 1. Assuming that the laser beam is a Gaussian beam, we can calculate a profile of the beam spot radius along the laser path from the beam parameter and focal length of the ZnSe lens using an ABCD matrix analysis (Yariv, 1997). Figrue 4 shows the beam-radius profiles after the two kinds of ZnSe lens used in our experimental study. The location and length of the tested plasma source and a scanning pitch of the laser beam
0 5 10 15 20 25 30
Distance from lens (cm)
*f* = 20 cm
course different in each IR detector and phase measuring system.
correspond to practical conditions used in our experiments.
0.0
0.2
0.4
0.6
Beam radius of CO2 laser (mm)
0.8
1.0
1.2
lock-in amplifier. Around 50 μs after the discharge ignition, the phase shift decreased rapidly in 150 μs, and then a slower change followed until the discharge termination. The 50-μs delay of the phase-shift signal is due to calculating delay time of the phase shift in the lock-in amplifier. The initial faster falling part is attributed to the increase of electron density, and the second slower part is to the decrease of gas-particle density by the Joule heating. Faster and slower slopes of the phase shift similar to the ignition timing were also observed in the termination timing. Subtracting the phase-shift component of gas-particle density, *ΔΦ*g, shown in a blue dashed-dotted curve in Fig. 2, the component of electron density, *ΔΦ*e, was obtained as shown in a red dashed curve. The absolute value of electron density can be derived from the amplitude of electron-density component.
Fig. 2. Temporal evolutions of phase-shift signal from lock-in amplifier and discharge current. Tested plasma source is small-scale discharge driven by 300-V pulsed DC high voltage at 70 Torr of He gas (Choi et al., 2009).
Fig. 3. Temporal evolutions of phase-shift signal measured changing voltage pulse duration from 60 to 700 μs (Choi et al., 2009).
lock-in amplifier. Around 50 μs after the discharge ignition, the phase shift decreased rapidly in 150 μs, and then a slower change followed until the discharge termination. The 50-μs delay of the phase-shift signal is due to calculating delay time of the phase shift in the lock-in amplifier. The initial faster falling part is attributed to the increase of electron density, and the second slower part is to the decrease of gas-particle density by the Joule heating. Faster and slower slopes of the phase shift similar to the ignition timing were also observed in the termination timing. Subtracting the phase-shift component of gas-particle density, *ΔΦ*g, shown in a blue dashed-dotted curve in Fig. 2, the component of electron density, *ΔΦ*e, was obtained as shown in a red dashed curve. The absolute value of electron
density can be derived from the amplitude of electron-density component.
ΔΦ*g*
> ΔΦ*e*
Fig. 2. Temporal evolutions of phase-shift signal from lock-in amplifier and discharge current. Tested plasma source is small-scale discharge driven by 300-V pulsed DC high
0.0 0.2 0.4 0.6 0.8 1.0
Total signal ΔΦ *e* + ΔΦ*g*
Time (ms)
Pulse duration 0.06 ms 0.10 ms 0.20 ms 0.40 ms 0.70 ms
Discharge current (mA)
Fig. 3. Temporal evolutions of phase-shift signal measured changing voltage pulse duration
0.0 0.2 0.4 0.6 0.8 1.0 1.2
Time (ms)
voltage at 70 Torr of He gas (Choi et al., 2009).
Phase-shift signal (mV)
Phase-shift signal (mV)
0
from 60 to 700 μs (Choi et al., 2009).
As explained in the previous paragraph, because the temporal changes of electron and gasparticle density can be observed only in the discharge ignition or termination timings, there is a need to pulse modulation of continuous discharge for the separation of phase-shift components measured in the CO2-laser heterodyne interferometer. In order to understand appropriate range of duration times for the pulse modulation, especially minimum duration time, it is important to measure the phase-shift signal decreasing the modulating pulse duration. Figure 3 shows waveforms of the phase-shift signal measured in our heterodyne interferometer using five pulse duration times ranging from 60 to 700 µs. From the measured waveforms, it could be confirmed that the pulse modulation with the duration time shorter than 200 µs does not give us accurate information of electron density in our setup, and we used the modulation pulse with duration times larger than 500 µs in our studies. This temporal behavior of the output signal of the heterodyne interferometer is of course different in each IR detector and phase measuring system.
Fig. 4. Calculated beam spot radii of CO2 laser beam at 10.6 µm as a function of distance from lens (Yariv, 1997). Initial spot radius at 1.0 mm and focal lengths of lens at 5 and 20 cm correspond to practical conditions used in our experiments.
For measuring the spatial distributions of electron density, evaluation of the spot radius of the CO2 laser beam inside the tested plasma source is required. In order to measure the spatial distribution of electron density inside the high-pressure plasmas whose scale are usually below cm order, the CO2-laser beam has to be focused in μm order using a pair of ZnSe lens as shown in Fig. 1. Assuming that the laser beam is a Gaussian beam, we can calculate a profile of the beam spot radius along the laser path from the beam parameter and focal length of the ZnSe lens using an ABCD matrix analysis (Yariv, 1997). Figrue 4 shows the beam-radius profiles after the two kinds of ZnSe lens used in our experimental study. The location and length of the tested plasma source and a scanning pitch of the laser beam should be determined by reference to these kinds of beam-profile evaluations.
#### **3. Spatial distribution of electron density in high-pressure plasmas**
Some of the specific measurement results of our CO2-laser heterodyne interferometer are introduced in this section. Influence of gas heating on detected phase-shift signals, spatial
Heterodyne Interferometer for Measurement of Electron Density in High-Pressure Plasmas 217
Figure 5(b) shows dependences of electron density in the short HC discharge on the discharge current measured at several He gas pressures. In the calculation of electron density from the phase shift signal, we used two assumptions that the plasma was uniform along the CO2 laser path and its effective length was 9.5 mm. The electron density in the discharge increased monotonically with the increase of the discharge current and the gas pressure. This result indicates that the CO2-laser heterodyne interferometer is able to measure appropriate dependence of electron density on the gas pressure, because the electron density inside the high-pressure plasma must be increased at higher pressure by the slower drift velocity of electrons due to frequent collisions with gas particles when the
Here, the minimum sensitivity of electron density in our CO2-laser heterodyne interferometer is introduced. The minimum electron density which we recorded in the short HC discharge was corresponds to a line integrated electron density *n*e*d* of 7×1012 cm−2 with a detected phase shift around 0.1 degree (Choi et al., 2009). However, the signal to noise ratio, for instance shown in Fig. 3, showed that the minimum sensitivity should be at least six times better than that. Therefore, it could be estimated that the minimum detectable phase shift in our system was about 0.02 degree, corresponding to *n*e*d* of 1×1012 cm−2, which is much smaller than the minimum sensitivity of Stark broadening measurement (Laux et al,
Figure 6(a) shows a schematic diagram of DC discharge in open space operated in a He gas flow ejected from a tubular cathode to a pin anode. The cylindrical tubular cathode, whose outer and inner diameters were 3.1 and 2.1 mm, and the pin anode with a 0.5-mm diameter were faced to the cathode with a 3-mm gap. The measurement point between the cathode and the anode was scanned by putting the whole discharge device on a three-dimensional mechanical movement stage. The influence of temporal evolutions of the gas-particle density on the measured phase shift can be seen in Fig. 6(b) showing the phase-shift signals observed in the atmospheric-pressure DC discharge using two different flow rates of He gas (1 and 2 L/min). This measurement result clearly suggests that the contribution of the Joule heating on temporal evolution of the phase shift becomes much greater in the discharge operated at atmospheric pressure and the gas-particle component in the phase shift, *ΔΦ*g, has large dependence on the gas flow velocity. The fast time constant and large amplitude of *ΔΦ*g in both rising and falling periods of applied voltage in lower gas flow rate was due to
In order to obtain radial distributions of electron density in the high-pressure plasmas, which cannot be evaluated by photographic observations, spatial distributions of lineintegrated electron density and inverse Abel transformation of the distributions are required. This diagnostic method of radial distribution is also applied for excited species measurements in the high-pressure plasmas by a laser absorption spectroscopy (LAS) and a laser induced fluorescence (LIF) methods (Urabe et al., 2010). Figure 7(a) shows two spatial distributions of the line-integrated electron density, e *n dl x*( ) , measured near the tubular cathode and the pin anode in the DC discharge scanning the laser beam perpendicularly to the gas-flow axis. Using the CO2-laser heterodyne interferometer with appropriate control of
discharge current is the same as that at lower pressure.
**3.2 Atmospheric-pressure DC discharge in open space**
less cooling effect by neutral gas particles.
2003).
resolutions of the heterodyne interferometer, and the combination measurement method for AC-voltage driven APPs, will be discussed in addition to the measured electron-density distributions in the tested plasma sources.
#### **3.1 Small-scale DC discharge at high pressure**
A short hollow cathode (HC) discharge tube shown in Fig. 5(a) was used for basic experiments of our CO2-laser heterodyne interferometer system with variable pressure in pure He gas. The results can be good references to measurement of other plasmas operated at atmospheric pressure introduced in following subsections. Two electrodes for anode and cathode had a 2-mm bore and a 4-mm hole length, and these electrodes were separated by a 1.5-mm thick ceramic disk also having the 2-mm bore. Two of ZnSe windows were equipped to seal the discharge region and transmit the CO2 laser beam. Gas pressure inside the small chamber was controlled in a range from several tens to hundreds Torr.
Fig. 5. (a) Cross-sectional diagram and photograph of short hollow cathode (HC) discharge. (b) Dependence of electron density on discharge current in the short HC discharge in He gas at three values of gas pressure.
resolutions of the heterodyne interferometer, and the combination measurement method for AC-voltage driven APPs, will be discussed in addition to the measured electron-density
A short hollow cathode (HC) discharge tube shown in Fig. 5(a) was used for basic experiments of our CO2-laser heterodyne interferometer system with variable pressure in pure He gas. The results can be good references to measurement of other plasmas operated at atmospheric pressure introduced in following subsections. Two electrodes for anode and cathode had a 2-mm bore and a 4-mm hole length, and these electrodes were separated by a 1.5-mm thick ceramic disk also having the 2-mm bore. Two of ZnSe windows were equipped to seal the discharge region and transmit the CO2 laser beam. Gas pressure inside
(a)
Laser path
(b) Fig. 5. (a) Cross-sectional diagram and photograph of short hollow cathode (HC) discharge. (b) Dependence of electron density on discharge current in the short HC discharge in He gas
0 50 100 150 200 250
Current (mA)
the small chamber was controlled in a range from several tens to hundreds Torr.
Gas inlet Gas outlet
He gas pressure 35 Torr 70 Torr 140 Torr
Anode Ceramic
distributions in the tested plasma sources.
at three values of gas pressure.
Electron density (1013 cm-3
)
**3.1 Small-scale DC discharge at high pressure**
Cathode
ZnSe window Figure 5(b) shows dependences of electron density in the short HC discharge on the discharge current measured at several He gas pressures. In the calculation of electron density from the phase shift signal, we used two assumptions that the plasma was uniform along the CO2 laser path and its effective length was 9.5 mm. The electron density in the discharge increased monotonically with the increase of the discharge current and the gas pressure. This result indicates that the CO2-laser heterodyne interferometer is able to measure appropriate dependence of electron density on the gas pressure, because the electron density inside the high-pressure plasma must be increased at higher pressure by the slower drift velocity of electrons due to frequent collisions with gas particles when the discharge current is the same as that at lower pressure.
Here, the minimum sensitivity of electron density in our CO2-laser heterodyne interferometer is introduced. The minimum electron density which we recorded in the short HC discharge was corresponds to a line integrated electron density *n*e*d* of 7×1012 cm−2 with a detected phase shift around 0.1 degree (Choi et al., 2009). However, the signal to noise ratio, for instance shown in Fig. 3, showed that the minimum sensitivity should be at least six times better than that. Therefore, it could be estimated that the minimum detectable phase shift in our system was about 0.02 degree, corresponding to *n*e*d* of 1×1012 cm−2, which is much smaller than the minimum sensitivity of Stark broadening measurement (Laux et al, 2003).
#### **3.2 Atmospheric-pressure DC discharge in open space**
Figure 6(a) shows a schematic diagram of DC discharge in open space operated in a He gas flow ejected from a tubular cathode to a pin anode. The cylindrical tubular cathode, whose outer and inner diameters were 3.1 and 2.1 mm, and the pin anode with a 0.5-mm diameter were faced to the cathode with a 3-mm gap. The measurement point between the cathode and the anode was scanned by putting the whole discharge device on a three-dimensional mechanical movement stage. The influence of temporal evolutions of the gas-particle density on the measured phase shift can be seen in Fig. 6(b) showing the phase-shift signals observed in the atmospheric-pressure DC discharge using two different flow rates of He gas (1 and 2 L/min). This measurement result clearly suggests that the contribution of the Joule heating on temporal evolution of the phase shift becomes much greater in the discharge operated at atmospheric pressure and the gas-particle component in the phase shift, *ΔΦ*g, has large dependence on the gas flow velocity. The fast time constant and large amplitude of *ΔΦ*g in both rising and falling periods of applied voltage in lower gas flow rate was due to less cooling effect by neutral gas particles.
In order to obtain radial distributions of electron density in the high-pressure plasmas, which cannot be evaluated by photographic observations, spatial distributions of lineintegrated electron density and inverse Abel transformation of the distributions are required. This diagnostic method of radial distribution is also applied for excited species measurements in the high-pressure plasmas by a laser absorption spectroscopy (LAS) and a laser induced fluorescence (LIF) methods (Urabe et al., 2010). Figure 7(a) shows two spatial distributions of the line-integrated electron density, e *n dl x*( ) , measured near the tubular cathode and the pin anode in the DC discharge scanning the laser beam perpendicularly to the gas-flow axis. Using the CO2-laser heterodyne interferometer with appropriate control of
Heterodyne Interferometer for Measurement of Electron Density in High-Pressure Plasmas 219
e e ( ) 2 2 <sup>1</sup> ( ) ( ) *r*
(a)
Horizontal position *x* (mm)
(b) Fig. 7. (a) Spatial distributions of line-integrated electron density measured in DC plasma jet near tubular cathode and pin anode. (b) Radial distributions of electron density derived from line-integrated electron density using inverse Abel transformation (Choi et al., 2009).
Radial position *r* (mm)
The calculated results in Fig. 7(b) indicate that total amounts of the electrons near the cathode and the anode are significantly different in this DC discharge and there must be another kind of negatively charged particles delivering the discharge current and keeping
*n r n dl x* π
∞
= −
0.0
0.5
1.0
1.5
Electron density (1014 cm-3
)
2.0
2.5
Line-integrated electron
density (1013 cm-2
)
*d dx*
Measurement position
Measurement position
Near tubular cathode Near pin anode
Near tubular cathode Near pin anode
<sup>−</sup> . (7)
*dx x r*
the beam radius around the tested plasma source, the spatial distribution of electron density with enough quality for the inverse Abel transformation can be measured.
(b)
Fig. 6. (a) Schematic diagram and photograph of atmospheric-pressure DC discharge in open space. (b) Temporal evolutions of phase-shift signal in the DC discharge measured changing He gas flow rate.
Calculation results of the inverse Abel transformation corresponding to the radial distribution of electron density, *n*e(*r*), at the measurement points in the DC discharge are shown in Fig. 7(b). The calculated radial distributions showed two different structures which were a hollow shape near the cathode and a center-peaked shape near the anode, having a good agreement with the electrode structures. The inverse Abel transformation was performed using a following equation (Lochte-Holtgreven, 1968),
∞
218 CO2 Laser – Optimisation and Application
the beam radius around the tested plasma source, the spatial distribution of electron density
Atmospheric
(a)
Gas inlet
(b) Fig. 6. (a) Schematic diagram and photograph of atmospheric-pressure DC discharge in open space. (b) Temporal evolutions of phase-shift signal in the DC discharge measured
Time (ms)
Current (mA)
He flow rate 1 L/min 2 L/min
Calculation results of the inverse Abel transformation corresponding to the radial distribution of electron density, *n*e(*r*), at the measurement points in the DC discharge are shown in Fig. 7(b). The calculated radial distributions showed two different structures which were a hollow shape near the cathode and a center-peaked shape near the anode, having a good agreement with the electrode structures. The inverse Abel transformation
was performed using a following equation (Lochte-Holtgreven, 1968),
changing He gas flow rate.
Phase-shift signal (mV)
0
with enough quality for the inverse Abel transformation can be measured.
air Laser path
Pin anode
Tubular cathode
Fig. 7. (a) Spatial distributions of line-integrated electron density measured in DC plasma jet near tubular cathode and pin anode. (b) Radial distributions of electron density derived from line-integrated electron density using inverse Abel transformation (Choi et al., 2009).
(b)
The calculated results in Fig. 7(b) indicate that total amounts of the electrons near the cathode and the anode are significantly different in this DC discharge and there must be another kind of negatively charged particles delivering the discharge current and keeping
Heterodyne Interferometer for Measurement of Electron Density in High-Pressure Plasmas 221
in the APGD, and the influence of this smoothing effect on the phase-shift signal could not be resolved. Therefore, the measured phase-shift waveform should be divided into two components only at the cut-off timing of applied voltage (around 0 ms in Fig. 8(b)). Calculation procedure of the temporally-averaged electron density after dividing the two
> Power supply
**60 mm** ZnSe
Dielectric barrier Laser path
window
Modulation signal (V)
0
1
2
3
4
5
(a)
Exponential fit of ΔΦ**g**
Electrode
(b) Fig. 8. (a) Cross-sectional diagram of parallel-plate dielectric barrier discharge (DBD). (b) Example temporal evolution of phase shift by the parallel-plate DBD together with amplitude modulation signal of 30-kHz AC applied voltage (Urabe et al., 2011).
Time (ms)
ΔΦ**e**
The spatial distribution of the electron-density component in the phase shift and the calculated temporally-averaged electron density from the four signal waveforms measured under the same condition are shown in Fig. 9. The points and both ends of error bars indicate the averaged value and the maximum and minimum values in each measurement. The electron-density distribution inside the APGD was localized near the dielectric barriers, and this result had a good agreement with reported results of computational simulations in similar geometries (Massines et al., 1998, 2003; Martens et al., 2009). The electron density near the powered electrode was approximately two times larger than that near the
components is the same as the calculation in DC discharges.
0.0
Phase shift (degree)
0.2
Vacuum chamber
its continuity. Considering that electron density near the central axis of the gas flow was not different at both measurement points, there were some losing mechanisms of the electrons outside of the He gas flow where ambient air contaminates into the flow. The electron loss was mainly due to an electron attachment process with O2 molecules presenting in the ambient air in the DC discharge, because of μm-order short diffusion lengths of electrons in the atmospheric-pressure gas conditions which could be derived from calculations of electron diffusion coefficient (Hargelaar & Pitchford, 2005) and measurement results of electron lifetime (Moselhy et al., 2003). Whereas this negative-ion density, for example O2 ions, cannot be detected by the CO2-laser heterodyne interferometer, this selectivity of negatively charged particles in the interferometer enables us to distinguish kinds and fractions of the negatively charged particle inside the high-pressure plasmas with the estimation of total amount of the charged particles from the discharge current amplitude.
#### **3.3 Pulsed AC discharge at atmospheric pressure**
For the measurement of electron density inside the high-pressure plasmas driven by kHzorder AC applied voltage, the temporal resolution of CO2-laser heterodyne interferometer using the lock-in amplifier is often insufficient for direct measurement of temporal evolutions of electron density. In order to depict spatiotemporal structures of electron density inside such plasma sources, for example dielectric barrier discharges (DBDs) (Kogelschatz, 2003; Becker et al., 2005) and ns-order short pulsed discharges (Namihira et al., 2003; Walsh & Kong, 2007), it has been confirmed that an amplitude modulation of the kHz-order applied voltage at a frequency of a few hundreds Hz and additional measurement of a millimeter-wave (mm-wave) transmission method are effective.
Figure 8(a) shows a schematic diagram of atmospheric-pressure glow discharge (APGD) tested in our group. The APGD is a major kind of DBDs generating homogeneous plasmas with atmospheric-pressure He gas and kHz-order AC applied voltage (Kanazawa et al., 1988). Powered (upper side) and grounded (lower side) stainless-steel electrodes were round, and their diameters were 60 mm. Dielectric barriers of 1-mm thick alumina were placed on the electrodes' surface and a gap distance between the two barriers was set at 6.0 mm. Whole electrode setup was installed in a vacuum chamber with a pair of ZnSe windows to control the gas compositions and pressures. In the measurement of APGD, we used a pair of ZnSe lens with longer focal length (20 cm), in order to get constant beam shape in whole discharge region with 60-mm length along the CO2 laser path. Therefore, the spatial resolution of the interferometer was worse than the measurement of small-scale DC plasmas explained in above subsections.
To divide the phase-shift signal into two components of the electron and gas-particle densities in a similar way to that used for the small-scale DC discharges, we used a squarepulse amplitude modulation at 125 Hz for the 30-kHz AC applied voltage, whose modulation-signal waveform is shown in Fig. 8(b). Using this amplitude modulation, the temporal evolution of phase shift has the fall and rise slopes in the both ends of modulation signal similar to that in the measurement of pulsed DC discharges. However, in the measurement of AC discharge, the phase-shift signal at the start-up timing of applied voltage (around −3.5 ms in the abscissa axis of Fig. 8(b)) is unsuitable for the derivation of the electron-density component in the phase shift. At this timing, the signal associated with the electron density was smoothed by the step-wise increase due to intermittent discharges
its continuity. Considering that electron density near the central axis of the gas flow was not different at both measurement points, there were some losing mechanisms of the electrons outside of the He gas flow where ambient air contaminates into the flow. The electron loss was mainly due to an electron attachment process with O2 molecules presenting in the ambient air in the DC discharge, because of μm-order short diffusion lengths of electrons in the atmospheric-pressure gas conditions which could be derived from calculations of electron diffusion coefficient (Hargelaar & Pitchford, 2005) and measurement results of electron lifetime (Moselhy et al., 2003). Whereas this negative-ion density, for example O2 ions, cannot be detected by the CO2-laser heterodyne interferometer, this selectivity of negatively charged particles in the interferometer enables us to distinguish kinds and fractions of the negatively charged particle inside the high-pressure plasmas with the estimation of total amount of the charged particles from the discharge current amplitude.
For the measurement of electron density inside the high-pressure plasmas driven by kHzorder AC applied voltage, the temporal resolution of CO2-laser heterodyne interferometer using the lock-in amplifier is often insufficient for direct measurement of temporal evolutions of electron density. In order to depict spatiotemporal structures of electron density inside such plasma sources, for example dielectric barrier discharges (DBDs) (Kogelschatz, 2003; Becker et al., 2005) and ns-order short pulsed discharges (Namihira et al., 2003; Walsh & Kong, 2007), it has been confirmed that an amplitude modulation of the kHz-order applied voltage at a frequency of a few hundreds Hz and additional
Figure 8(a) shows a schematic diagram of atmospheric-pressure glow discharge (APGD) tested in our group. The APGD is a major kind of DBDs generating homogeneous plasmas with atmospheric-pressure He gas and kHz-order AC applied voltage (Kanazawa et al., 1988). Powered (upper side) and grounded (lower side) stainless-steel electrodes were round, and their diameters were 60 mm. Dielectric barriers of 1-mm thick alumina were placed on the electrodes' surface and a gap distance between the two barriers was set at 6.0 mm. Whole electrode setup was installed in a vacuum chamber with a pair of ZnSe windows to control the gas compositions and pressures. In the measurement of APGD, we used a pair of ZnSe lens with longer focal length (20 cm), in order to get constant beam shape in whole discharge region with 60-mm length along the CO2 laser path. Therefore, the spatial resolution of the interferometer was worse than the measurement of small-scale DC
To divide the phase-shift signal into two components of the electron and gas-particle densities in a similar way to that used for the small-scale DC discharges, we used a squarepulse amplitude modulation at 125 Hz for the 30-kHz AC applied voltage, whose modulation-signal waveform is shown in Fig. 8(b). Using this amplitude modulation, the temporal evolution of phase shift has the fall and rise slopes in the both ends of modulation signal similar to that in the measurement of pulsed DC discharges. However, in the measurement of AC discharge, the phase-shift signal at the start-up timing of applied voltage (around −3.5 ms in the abscissa axis of Fig. 8(b)) is unsuitable for the derivation of the electron-density component in the phase shift. At this timing, the signal associated with the electron density was smoothed by the step-wise increase due to intermittent discharges
measurement of a millimeter-wave (mm-wave) transmission method are effective.
**3.3 Pulsed AC discharge at atmospheric pressure**
plasmas explained in above subsections.
in the APGD, and the influence of this smoothing effect on the phase-shift signal could not be resolved. Therefore, the measured phase-shift waveform should be divided into two components only at the cut-off timing of applied voltage (around 0 ms in Fig. 8(b)). Calculation procedure of the temporally-averaged electron density after dividing the two components is the same as the calculation in DC discharges.
Fig. 8. (a) Cross-sectional diagram of parallel-plate dielectric barrier discharge (DBD). (b) Example temporal evolution of phase shift by the parallel-plate DBD together with amplitude modulation signal of 30-kHz AC applied voltage (Urabe et al., 2011).
The spatial distribution of the electron-density component in the phase shift and the calculated temporally-averaged electron density from the four signal waveforms measured under the same condition are shown in Fig. 9. The points and both ends of error bars indicate the averaged value and the maximum and minimum values in each measurement. The electron-density distribution inside the APGD was localized near the dielectric barriers, and this result had a good agreement with reported results of computational simulations in similar geometries (Massines et al., 1998, 2003; Martens et al., 2009). The electron density near the powered electrode was approximately two times larger than that near the
Heterodyne Interferometer for Measurement of Electron Density in High-Pressure Plasmas 223
together with the waveform of applied voltage. The electron density increased after the positive- and negative-main pulses around 2.6 and 19.0 µs in the abscissa time axis, and there were small increases in electron density after the main pulses caused by weak
From the measurement results of the CO2-laser heterodyne interferometer and the mmwave transmission method applied to the same plasma source, spatial distribution of temporal-peak electron density can be calculated dividing the temporally-averaged electron densities by a duty ratio of plasma. The duty ratio of plasma is the ratio of temporallyaveraged electron density to temporal-peak density in the result of the mm-wave transmission measurement, and it indicates the temporally averaging effects of the CO2 laser heterodyne interferometer. In an example case of the APGD measurement shown in Figs. 9 and 10, the calculated duty ratio of plasma was 0.33. Then, the temporal-peak electron densities near the dielectric barriers were derived approximately 5×1012 cm−3 on the
In this chapter, we reviewed experimental studies for electron-density measurement in small-scale plasmas operated at high and atmospheric pressures using the CO2-laser heterodyne interferometer, from brief theoretical introduction of the interferometer to specific measurement results in the high-pressure plasma sources. It should be noted that separation of the CO2 laser beam's phase shift into two components, which are due to changes of electron and gas-particle densities, is the most important procedure for the measurement in high-pressure plasmas, and pulse modulation of applied voltage is
From the experimental results of the interferometer in high-pressure plasmas driven by pulsed DC voltage, fundamental properties of our interferometer including the minimum sensitivity of line-integrated electron density and the spatial resolution could be evaluated. Because these properties are changed due to specifications of the laser source and the phase detecting system, they must be confirmed in each setup of the interferometer before practical measurements. In addition to the interferometer, a mmwave transmission method with a good temporal resolution was used to AC-voltage driven plasmas having ns-order fast temporal behavior. This novel combination method has potentials to be applied to the refractive-index measurements requiring both spatial
Our studies on the CO2-laser heterodyne interferometer were partially supported by Grantin-Aid for Scientific Research from the MEXT of Japan and Global Center of Excellence program on Photonics and Electronics Science and Engineering at Kyoto University. The authors would like to thank Prof. Osamu Sakai, Dr. Nobuhiko Takano, Dr. Joon-Young Choi, and Dr. Yosuke Ito at Kyoto University for their substantial supports. The author U.K. would like to acknowledge support of Research Fellowship from Japan Society for the
indispensable for the separation at the rise and fall timings of the modulation signal.
side of the powered electrode and 2×1012 cm−3 on the grounded electrode.
discharges in the ringing part of applied voltage.
**4. Concluding remarks**
and temporal high resolutions.
**5. Acknowledgments**
Promotion of Science.
grounded electrode in each gas composition. This asymmetric distribution was probably caused by diffusion of the discharge current flow into the chamber wall from the powered electrode not flowing into the grounded electrode.
To get temporally resolved information of the electron density inside the APGD, we inserted a mm-wave at 55 GHz through the vacuum chamber and measured temporal evolutions of the transmitted mm-wave intensity using a pair of horn antennas. In this mm-wave transmission method, spatial distributions of electron density cannot be measured because of the diffraction limit of the mm-wave. Details of the experimental setup and the calculation procedure of spatially-averaged electron density are introduced in our previous paper (Urabe et al., 2011). The temporal evolution of spatially-averaged electron density in the APGD derived from absorption ratio of the mm-wave in the plasma is shown in Fig. 10,
Fig. 9. Spatial distribution of electron-density component in phase shift and calculated temporally-averaged electron density in APGD (Urabe et al., 2011).
Fig. 10. Temporal evolution of spatially-averaged electron density in APGD measured by mm-wave transmission method under same conditions as interferometer measurement (Fig. 9), together with applied-voltage waveform (Urabe et al., 2011).
together with the waveform of applied voltage. The electron density increased after the positive- and negative-main pulses around 2.6 and 19.0 µs in the abscissa time axis, and there were small increases in electron density after the main pulses caused by weak discharges in the ringing part of applied voltage.
From the measurement results of the CO2-laser heterodyne interferometer and the mmwave transmission method applied to the same plasma source, spatial distribution of temporal-peak electron density can be calculated dividing the temporally-averaged electron densities by a duty ratio of plasma. The duty ratio of plasma is the ratio of temporallyaveraged electron density to temporal-peak density in the result of the mm-wave transmission measurement, and it indicates the temporally averaging effects of the CO2 laser heterodyne interferometer. In an example case of the APGD measurement shown in Figs. 9 and 10, the calculated duty ratio of plasma was 0.33. Then, the temporal-peak electron densities near the dielectric barriers were derived approximately 5×1012 cm−3 on the side of the powered electrode and 2×1012 cm−3 on the grounded electrode.
#### **4. Concluding remarks**
222 CO2 Laser – Optimisation and Application
grounded electrode in each gas composition. This asymmetric distribution was probably caused by diffusion of the discharge current flow into the chamber wall from the powered
To get temporally resolved information of the electron density inside the APGD, we inserted a mm-wave at 55 GHz through the vacuum chamber and measured temporal evolutions of the transmitted mm-wave intensity using a pair of horn antennas. In this mm-wave transmission method, spatial distributions of electron density cannot be measured because of the diffraction limit of the mm-wave. Details of the experimental setup and the calculation procedure of spatially-averaged electron density are introduced in our previous paper (Urabe et al., 2011). The temporal evolution of spatially-averaged electron density in the APGD derived from absorption ratio of the mm-wave in the plasma is shown in Fig. 10,
Fig. 9. Spatial distribution of electron-density component in phase shift and calculated
0123456
0.0
0
Applied voltage (kV)
2
4
6
8
10
0.5
1.0
Temporally-averaged
electron density (1012 cm-3
)
1.5
2.0
Position (mm)
Fig. 10. Temporal evolution of spatially-averaged electron density in APGD measured by mm-wave transmission method under same conditions as interferometer measurement (Fig.
0 10 20 30 40 50
Time (μs)
temporally-averaged electron density in APGD (Urabe et al., 2011).
0.00
0.05
0.10
Electron-density component in
Spatially-averaged
electron density (1012 cm-3
)
phase shift, ΔΦe (degree)
0.15
0.20
0.25
9), together with applied-voltage waveform (Urabe et al., 2011).
electrode not flowing into the grounded electrode.
In this chapter, we reviewed experimental studies for electron-density measurement in small-scale plasmas operated at high and atmospheric pressures using the CO2-laser heterodyne interferometer, from brief theoretical introduction of the interferometer to specific measurement results in the high-pressure plasma sources. It should be noted that separation of the CO2 laser beam's phase shift into two components, which are due to changes of electron and gas-particle densities, is the most important procedure for the measurement in high-pressure plasmas, and pulse modulation of applied voltage is indispensable for the separation at the rise and fall timings of the modulation signal.
From the experimental results of the interferometer in high-pressure plasmas driven by pulsed DC voltage, fundamental properties of our interferometer including the minimum sensitivity of line-integrated electron density and the spatial resolution could be evaluated. Because these properties are changed due to specifications of the laser source and the phase detecting system, they must be confirmed in each setup of the interferometer before practical measurements. In addition to the interferometer, a mmwave transmission method with a good temporal resolution was used to AC-voltage driven plasmas having ns-order fast temporal behavior. This novel combination method has potentials to be applied to the refractive-index measurements requiring both spatial and temporal high resolutions.
#### **5. Acknowledgments**
Our studies on the CO2-laser heterodyne interferometer were partially supported by Grantin-Aid for Scientific Research from the MEXT of Japan and Global Center of Excellence program on Photonics and Electronics Science and Engineering at Kyoto University. The authors would like to thank Prof. Osamu Sakai, Dr. Nobuhiko Takano, Dr. Joon-Young Choi, and Dr. Yosuke Ito at Kyoto University for their substantial supports. The author U.K. would like to acknowledge support of Research Fellowship from Japan Society for the Promotion of Science.
Heterodyne Interferometer for Measurement of Electron Density in High-Pressure Plasmas 225
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**8**
*Russian Federation*
**Transmission of CO2 Laser Radiation Through**
A. D. Pryamikov, A. F. Kosolapov, V. G. Plotnichenko and E. M. Dianov
In this chapter we would like to highlight and analyze the main problems of transmission and propagation of CO2 laser radiation in the hollow core microstructured fibers (HC MFs). It is well known that there is a strong need for the fiber delivery systems for 10.6 μm CO2 lasers due to a wide range of CO2 laser applications in medicine, spectrometry, industry, military applications and in other fields of science and technology. Research into the possibility of the mid IR laser radiation transmission (especially, CO and CO2 lasers) with the help of optical fibers as well as with crystalline or glass cores made of different materials has been going hand with hand with the technological development. However, until recently these fibers haven't been used for industrial applications due to a relatively high level of optical losses at the lasers wavelengths and certain physicochemical properties of the fiber materials. These problems mainly arise from a low laser damage threshold, low
melting temperatures of most IR transmitting materials and their high nonlinearity.
Hollow waveguides are exempt from many problems that are common to all types of the solid waveguides in this spectral region and thus can serve as much more reliable delivery systems. Glass hollow waveguides, crystalline hollow waveguides, dielectric – coated cylindrical hollow waveguides, polycrystalline fibers are well known examples of such systems. Here we consider only the glass HC MFs with their characteristics and physical phenomena laying the basis for their waveguide mechanisms. In particular, we propose a new type of HC MF for CO2 laser radiation delivery with the cladding consisting of one row of the glass capillaries. We show that due to complicated boundary conditions and optical properties of an individual capillary it is possible to obtain low loss waveguide regimes for CO2 laser radiation. Moreover, we show that the HC MFs with a determined symmetry type of a capillary arrangement in the cladding exhibit low bend losses when such low loss
The chapter is organized as follows. In Section 2 we consider all types of proposed hollow waveguides for CO2 laser radiation transmission and give a short historical overview to highlight the problem of CO2 laser beam delivery in the hollow core waveguides. In Section 3 we consider physical mechanisms of the light guiding in the glass HC MFs with cladding consisting of capillaries due to which, in our opinion, it becomes possible to guide the light in the mid IR including CO2 laser radiation. In Section 4 we offer some numerical analyses
**1. Introduction**
waveguide regimes occur.
**Glass Hollow Core Microstructured Fibers**
*Fiber Optics Research Center of Russian Academy of Sciences*
### **Transmission of CO2 Laser Radiation Through Glass Hollow Core Microstructured Fibers**
A. D. Pryamikov, A. F. Kosolapov, V. G. Plotnichenko and E. M. Dianov *Fiber Optics Research Center of Russian Academy of Sciences Russian Federation*
#### **1. Introduction**
226 CO2 Laser – Optimisation and Application
Urabe, K.; Sakai, O. & Tachibana, K. (2011). Combined spectroscopic methods for electron-
Walsh, J.L. & Kong, M.G. (2007). 10 ns pulsed atmospheric air plasma for uniform treatment of polymeric surfaces. *Applied Physics Letters,* Vol. 91, pp. 241504-1-3 Yariv, A. (1997). *Optical Electronics in Modern Communications,* Oxford University Press, New
mixture. *Journal of Physics D: Applied Physics,* Vol. 44, pp. 115203-1-11 von Engel, A. (1994). *Ionized Gases,* American Institute of Physics, New York, USA
York, USA
density diagnostics inside atmospheric-pressure glow discharge using He/N2 gas
In this chapter we would like to highlight and analyze the main problems of transmission and propagation of CO2 laser radiation in the hollow core microstructured fibers (HC MFs). It is well known that there is a strong need for the fiber delivery systems for 10.6 μm CO2 lasers due to a wide range of CO2 laser applications in medicine, spectrometry, industry, military applications and in other fields of science and technology. Research into the possibility of the mid IR laser radiation transmission (especially, CO and CO2 lasers) with the help of optical fibers as well as with crystalline or glass cores made of different materials has been going hand with hand with the technological development. However, until recently these fibers haven't been used for industrial applications due to a relatively high level of optical losses at the lasers wavelengths and certain physicochemical properties of the fiber materials. These problems mainly arise from a low laser damage threshold, low melting temperatures of most IR transmitting materials and their high nonlinearity.
Hollow waveguides are exempt from many problems that are common to all types of the solid waveguides in this spectral region and thus can serve as much more reliable delivery systems. Glass hollow waveguides, crystalline hollow waveguides, dielectric – coated cylindrical hollow waveguides, polycrystalline fibers are well known examples of such systems. Here we consider only the glass HC MFs with their characteristics and physical phenomena laying the basis for their waveguide mechanisms. In particular, we propose a new type of HC MF for CO2 laser radiation delivery with the cladding consisting of one row of the glass capillaries. We show that due to complicated boundary conditions and optical properties of an individual capillary it is possible to obtain low loss waveguide regimes for CO2 laser radiation. Moreover, we show that the HC MFs with a determined symmetry type of a capillary arrangement in the cladding exhibit low bend losses when such low loss waveguide regimes occur.
The chapter is organized as follows. In Section 2 we consider all types of proposed hollow waveguides for CO2 laser radiation transmission and give a short historical overview to highlight the problem of CO2 laser beam delivery in the hollow core waveguides. In Section 3 we consider physical mechanisms of the light guiding in the glass HC MFs with cladding consisting of capillaries due to which, in our opinion, it becomes possible to guide the light in the mid IR including CO2 laser radiation. In Section 4 we offer some numerical analyses
Transmission of CO2 Laser Radiation Through Hollow Core Microstructured Fibers 229
core materials with a refractive index less than 1. Due to this fact, the angle of incident radiation in the core is greater than the critical angle and the light experiences the total internal reflection. An example of such fiber operating at the CO2 laser wavelength is the
For our part, in the following subsections we will consider the second group of the circular cross section hollow waveguides, previously classified as leaky waveguides, in particular, HC MFs. This group of hollow waveguides has inner wall surface with refractive indices greater than 1. Leaky hollow waveguides have metallic or dielectric layers deposited on the inside metallic, plastic or glass tubing to enhance their surface reflectivity. Creation of such dielectric coated cylindrical hollow waveguides presented a complicated technological problem since high quality reflective coatings are not compatible with circular cross section geometry. Traditional vapor deposition techniques don't produce good quality coatings on the inside of a capillary. The theoretical calculated attenuation of a dielectric coated hollow waveguide for the IR region was obtained in [Miyagi & Kawakami, 1984]. The authors have shown that the attenuation is very sensitive to the material and geometry of a dielectric film. Also, the attenuation is very sensitive to the properties of the metal under the dielectric film. A metal should have a low refractive index and a very high extinction coefficient. For example, these can be silver, nickel, copper. A dielectric should be selected with a maximum refractive index, for example, KCl, ZnSe, ZnS etc. The first demonstration of a dielectric coated cylindrical hollow waveguide was perfomed by Prof. Miyagi's group [Miyagi et. al., 1983] in 1983. The 1.2 m – long mandrel of polished aluminum tubing was coated with approximately 0.45 μm of germanium. Then, a layer of nickel up to 200 μm was deposited on the top of germanium before the aluminum mandrel was removed by leaching. The final structure was a nickel tube with optically thick dielectric layers on the inner wall. The fabricated waveguide had a core diameter of 1.5 mm and length of about 1 m. The measured attenuation was ~ 0.4 dB/m in the straight waveguide but the bend loss was very high.
Authors in [Croitoru et. al., U.S. Patent, 1990] have used polyethylene and Teflon tubing as a substrate in which thin and flexible metallic layers of Al followed by AgI were deposited. They reported attenuation in a straight waveguide of about 0.6 dB/m at the bore size value of 4 mm [Croitoru et. al., 1990]. Authors of [Morrow & Gu, 1994] reported a cylindrical hollow waveguide in which Ag and Ag halide coatings were deposited inside Ag tubes. The
At the end of this subsection, we will turn to the currently most popular hollow circular cross section waveguides for CO2 laser radiation transmission. These are hollow glass waveguides developed initially by Prof. Harrington's group [Abel et. al., 1994]. There are two main advantages of the glass tubing substrate. First, it is easier to make a long, uniform tubing from glass having considerably smoother wall surfaces than metal or plastic tubings. As a result, the scattered losses are less. The second advantage is that the technology of making glass capillary tubings is common and inexpensive. The authors fabricated a hollow glass waveguide using wet chemistry methods. First, an Ag layer was deposited on the inside of a silica glass tubing. Then, an AgI layer was formed over the metallic film. The thickness of the layer was optimized to obtain a high reflectivity at the required wavelength. A straight waveguide with a bore of 530 μm demonstrated a loss of 0.3 dB/m at CO2 laser wavelength. This fiber could maintain a loss level under 2 dB/m at a bend radius as small as
λ
= 10.6 μm which
waveguide with 1 – mm – bore size had attenuation below 0.1 dB/m at
was still below 0.8 dB/m at a bend radius up to 25 cm.
sapphire fiber with a refractive index of *n* = 0.67 [Harrington & Gregory, 1990].
and show a possibility to achieve low loss waveguide regimes in such HC MFs by careful selection of geometry parameters characterizing the fibers and a glass refractive index. Section 5 contains conclusions.
#### **2. CO2 laser radiation transmission through hollow core waveguides**
A variety of waveguides has been studied for the delivery of CO2 laser energy. In this section we consider briefly all types and properties of hollow waveguides and HC MFs for the CO2 laser radiation transmission known up to now.
#### **2.1 Historical overview of CO2 laser radiation transmission through hollow waveguides**
Rectangular core hollow waveguide structures were the first suggested for the delivery of CO2 laser radiation [Nishihara et. al., 1974]. The first publication on the IR spectral transmission measurements of a rectangular hollow waveguide dates back to [Garmire, 1976]. The hollow waveguide described was made of aluminium strips with heights greater than 0.5 mm demonstrating a delivery over 200 W of continuous CO2 laser radiation with no damage to the structure [Garmire et. al., 1979]. However, such metallic rectangular waveguides are not suitable for many practical applications due to their relatively big outer dimensions ~ 1 mm\*10mm in cross section while smaller dimensions made the loss was too high for any practical use. Thus the search for materials and waveguides more suitable for practical applications continued and, a few years later, in [Laakmann, 1987] there was proposed a way to decrease the outside dimensions of HC waveguides to under 2 mm in diameter and to increase the reflectivity of the bore inside surface. Silver was used as a substrate metallic material of the rectangular core on which several dielectric coatings were deposited. By doing so, the author succeeded in maintaining a practical transmission level for the hollow rectangular waveguides. However, as imperfections of the inside geometry and surfaces affected the transmission, the design of the rectangular hollow waveguide proposed in [Laakmann, 1987] had to be improved [Mashida et. al., 1991]. The authors proposed a hollow waveguide having the same cross section design as in [Laakmann, 1987] but with multiple dielectric coatings on the inside surfaces to increase its reflectivity. As a result, 1 – mm – core straight rectangular hollow waveguide of such construction had a loss ~ 0.1 dB/m for circularly polarized CO2 laser radiation. Moreover, the waveguide demonstrated the low loss for a bend. The next attempt to decrease loss for the rectangular core hollow waveguides was described in [Karasawa et. al., 1990]. The authors proposed and fabricated a germanium coated rectangular hollow waveguide with a cross section of 2 mm2, a length of 80 cm and a loss of less than 0.1 dB/m was fabricated. The resulting waveguide had a relatively low loss even for a bend.
However, the main disadvantage of the rectangular hollow waveguides is their relatively large outer dimensions and low flexibility which has led to a greater popularity of circular hollow waveguides. These waveguides made of glass, metal or plastic are those most commonly used today. Along general lines, circular cross section hollow waveguides for CO2 laser radiation transmission can be divided into two groups: attenuating total reflecting and leaky waveguides. The metallic or dielectric films are deposited on the inside of metallic, plastic or glass tubing. Attenuating total reflecting hollow waveguides have inner
and show a possibility to achieve low loss waveguide regimes in such HC MFs by careful selection of geometry parameters characterizing the fibers and a glass refractive index.
A variety of waveguides has been studied for the delivery of CO2 laser energy. In this section we consider briefly all types and properties of hollow waveguides and HC MFs for
Rectangular core hollow waveguide structures were the first suggested for the delivery of CO2 laser radiation [Nishihara et. al., 1974]. The first publication on the IR spectral transmission measurements of a rectangular hollow waveguide dates back to [Garmire, 1976]. The hollow waveguide described was made of aluminium strips with heights greater than 0.5 mm demonstrating a delivery over 200 W of continuous CO2 laser radiation with no damage to the structure [Garmire et. al., 1979]. However, such metallic rectangular waveguides are not suitable for many practical applications due to their relatively big outer dimensions ~ 1 mm\*10mm in cross section while smaller dimensions made the loss was too high for any practical use. Thus the search for materials and waveguides more suitable for practical applications continued and, a few years later, in [Laakmann, 1987] there was proposed a way to decrease the outside dimensions of HC waveguides to under 2 mm in diameter and to increase the reflectivity of the bore inside surface. Silver was used as a substrate metallic material of the rectangular core on which several dielectric coatings were deposited. By doing so, the author succeeded in maintaining a practical transmission level for the hollow rectangular waveguides. However, as imperfections of the inside geometry and surfaces affected the transmission, the design of the rectangular hollow waveguide proposed in [Laakmann, 1987] had to be improved [Mashida et. al., 1991]. The authors proposed a hollow waveguide having the same cross section design as in [Laakmann, 1987] but with multiple dielectric coatings on the inside surfaces to increase its reflectivity. As a result, 1 – mm – core straight rectangular hollow waveguide of such construction had a loss ~ 0.1 dB/m for circularly polarized CO2 laser radiation. Moreover, the waveguide demonstrated the low loss for a bend. The next attempt to decrease loss for the rectangular core hollow waveguides was described in [Karasawa et. al., 1990]. The authors proposed and fabricated a germanium coated rectangular hollow waveguide with a cross section of 2 mm2, a length of 80 cm and a loss of less than 0.1 dB/m was fabricated. The resulting
However, the main disadvantage of the rectangular hollow waveguides is their relatively large outer dimensions and low flexibility which has led to a greater popularity of circular hollow waveguides. These waveguides made of glass, metal or plastic are those most commonly used today. Along general lines, circular cross section hollow waveguides for CO2 laser radiation transmission can be divided into two groups: attenuating total reflecting and leaky waveguides. The metallic or dielectric films are deposited on the inside of metallic, plastic or glass tubing. Attenuating total reflecting hollow waveguides have inner
**2. CO2 laser radiation transmission through hollow core waveguides**
**2.1 Historical overview of CO2 laser radiation transmission through hollow**
the CO2 laser radiation transmission known up to now.
waveguide had a relatively low loss even for a bend.
Section 5 contains conclusions.
**waveguides**
core materials with a refractive index less than 1. Due to this fact, the angle of incident radiation in the core is greater than the critical angle and the light experiences the total internal reflection. An example of such fiber operating at the CO2 laser wavelength is the sapphire fiber with a refractive index of *n* = 0.67 [Harrington & Gregory, 1990].
For our part, in the following subsections we will consider the second group of the circular cross section hollow waveguides, previously classified as leaky waveguides, in particular, HC MFs. This group of hollow waveguides has inner wall surface with refractive indices greater than 1. Leaky hollow waveguides have metallic or dielectric layers deposited on the inside metallic, plastic or glass tubing to enhance their surface reflectivity. Creation of such dielectric coated cylindrical hollow waveguides presented a complicated technological problem since high quality reflective coatings are not compatible with circular cross section geometry. Traditional vapor deposition techniques don't produce good quality coatings on the inside of a capillary. The theoretical calculated attenuation of a dielectric coated hollow waveguide for the IR region was obtained in [Miyagi & Kawakami, 1984]. The authors have shown that the attenuation is very sensitive to the material and geometry of a dielectric film. Also, the attenuation is very sensitive to the properties of the metal under the dielectric film. A metal should have a low refractive index and a very high extinction coefficient. For example, these can be silver, nickel, copper. A dielectric should be selected with a maximum refractive index, for example, KCl, ZnSe, ZnS etc. The first demonstration of a dielectric coated cylindrical hollow waveguide was perfomed by Prof. Miyagi's group [Miyagi et. al., 1983] in 1983. The 1.2 m – long mandrel of polished aluminum tubing was coated with approximately 0.45 μm of germanium. Then, a layer of nickel up to 200 μm was deposited on the top of germanium before the aluminum mandrel was removed by leaching. The final structure was a nickel tube with optically thick dielectric layers on the inner wall. The fabricated waveguide had a core diameter of 1.5 mm and length of about 1 m. The measured attenuation was ~ 0.4 dB/m in the straight waveguide but the bend loss was very high.
Authors in [Croitoru et. al., U.S. Patent, 1990] have used polyethylene and Teflon tubing as a substrate in which thin and flexible metallic layers of Al followed by AgI were deposited. They reported attenuation in a straight waveguide of about 0.6 dB/m at the bore size value of 4 mm [Croitoru et. al., 1990]. Authors of [Morrow & Gu, 1994] reported a cylindrical hollow waveguide in which Ag and Ag halide coatings were deposited inside Ag tubes. The waveguide with 1 – mm – bore size had attenuation below 0.1 dB/m at λ = 10.6 μm which was still below 0.8 dB/m at a bend radius up to 25 cm.
At the end of this subsection, we will turn to the currently most popular hollow circular cross section waveguides for CO2 laser radiation transmission. These are hollow glass waveguides developed initially by Prof. Harrington's group [Abel et. al., 1994]. There are two main advantages of the glass tubing substrate. First, it is easier to make a long, uniform tubing from glass having considerably smoother wall surfaces than metal or plastic tubings. As a result, the scattered losses are less. The second advantage is that the technology of making glass capillary tubings is common and inexpensive. The authors fabricated a hollow glass waveguide using wet chemistry methods. First, an Ag layer was deposited on the inside of a silica glass tubing. Then, an AgI layer was formed over the metallic film. The thickness of the layer was optimized to obtain a high reflectivity at the required wavelength. A straight waveguide with a bore of 530 μm demonstrated a loss of 0.3 dB/m at CO2 laser wavelength. This fiber could maintain a loss level under 2 dB/m at a bend radius as small as
Transmission of CO2 Laser Radiation Through Hollow Core Microstructured Fibers 231
chalcogenide glasses. These glasses are composed of the chalcogen elements Se and Te with an addition of such elements as Ge, As, Sb. The transparency windows of these glasses
Modeling of BG HC PCFs made of nonsilica glasses was performed by a number of authors [Shaw et. al., 2003; Pottage et. al., 2003; Pearce et. al., 2005]. In this paper [Shaw et. al., 2003] BG HC PCFs made of As – S (refractive index ~ 2.4) and As – Se (refractive index ~ 2.8) were analyzed. It was shown that there exist several spectral regions with bandgaps for an air filling fraction > 40% in both As – S and As – Se BG HC PCFs. These BG HC PCFs have large bandgap widths at the air filling fraction of 45% to 60%. BG HC PCFs with high air filling fractions > 80% also exhibited large bandgap widths. In the author' opinion, all these results show a possibility of a light transmission in the mid IR using halcogenide BG HC PCFs. The authors of [Pottage et. al., 2003] have carried out a numerical analyses of BG HC PCFs for a wide range of refractive indices from *n* = 1.5 to *n* =3.6 and for different values of air filling fractions from 33% to 87%. They discovered a new type of the bandgap which was called type 2 bandgap at an air filling fraction ~ 60% for any glass index beyond 2. The results
Another important aspect of the problem of the mid IR radiation transmission was discussed in [Pearce et. al., 2005]. Apart from a limitation to attaining a low loss guidance in BG HC PCFs connected with the intrinsic roughness of the air glass interfaces, there is another problem connected with an existence of surface guided modes that are trapped in the core surroundings. Experimental and theoretical studies [Smith et. al., 2003; Humbert et. al., 2004; West et. al., 2004; Saitoh et. al., 2004] carried out for silica BG HC PCFs have shown that the anticrossing between dispersion curves of the surface modes and the air core modes is the main factor leading to a transmission loss in BG HC PCFs. Several methods were proposed to suppress the surface modes. The first method is used to reduce the distortion of the core by including 'fingers' of glass [West et. al., 2004]. The second method is to use thin core walls [Saitoh et. al., 2004] and the third one is to use 'antiresonance' walls [Roberts, Williams et. al., 2005]. The authors of [Pearce et. al., 2005] modeled a realistic design of distorted cores for BG HC PCFs which can guide the light in the type 2 bandgap. They have demonstrated that BG HC PCFs made of high index glass can guide a fundamental air core
In their paper [Hu & Menyuk, 2007] the authors analyzed BG HC PCFs for refractive indices between 1.4 and 2.8. They found two maxima of the relative bandgap as a function of the air filling fraction and refractive index. The authors also found that the relative bandgap and the level of loss are interrelated. When the relative bandgap increases the loss decreases and
Despite the promising results of modeling obtained in the above listed works a practical realization of BG HC PCFs made completely of chalcogenide glass for the mid IR spectral region has not been reported. The only successful realization of a photonic band gap hollow core fiber for the CO2 laser radiation transmission was 'Omniguide' fiber where the cladding is a Bragg reflector (hollow core Bragg fiber) made of soft glass and polymer [Temelkuran et. al., 2002]. The authors of [Bowden & Harrington, 2009] have studied low and high index chalcogenide glasses for their potential use in the fabrication of all glass hollow core Bragg
showed a possibility of obtaining a satisfactory guidance in such BG HC PCFs.
correspond approximately to the mid IR region 2 – 25 μm.
mode with a fraction of power in the air of up to 98%.
vice versa.
fiber.
5 cm. Hollow glass waveguides have been used successfully for a modest CO2 laser power delivery below ~ 80 W. For the higher power delivery it is necessary to place a water – cooled jacket around the guides. The highest CO2 laser power delivered through the water – cooled hollow glass waveguide with 700 μm bore was 1040 W [Nubling & Harrington, 1996]. This is comparable to CO2 laser power delivered through the water – cooled hollow metallic waveguide with 1800 μm bore which was 2700 W [Hongo et. al., 1992].
#### **2.2 CO2 laser radiation transmission through hollow core microstructured fibers**
In this subsection we will consider a new approach to solving the problem of the mid IR transmission (in particular, CO2 laser radiation) through the glass hollow core microstructured fibers (HC MFs). The possibility of the light confinement in the air core of HC MFs with the cladding consisting of two dimensional periodic array of air holes was predicted by Russell at the beginning of 1990s and theoretically demonstrated by Birks et. al. [Birks et. al., 1995]. The most advanced HC MFs are hollow core photonic crystal fibers (HC PCFs). HC PCFs in turn can be divided into two main groups. The HC PCFs from the first group guide the light by virtue of photonic band gap (BG HC PCFs). The HC PCFs from the second group have no band gaps and guide the light due to an inhibited coupling between the core guided modes and modes associated with a cladding [Benabid et. al., 2002]. They are called inhibited coupling HC PCFs (IC HC PCFs). Both types of HC PCFs have the claddings with very little solid material, usually, with a filling fraction less than 10%.
The guidance mechanism for BG HC PCFs is based on the concept of 'out of plane' band gap. The microstructure of BG HC PCF cladding consists of air holes packed in a triangular arrangement. It gives rise to a full two dimensional photonic band gap [Birks et. al., 1995]. As a result, forbidden frequencies occur for optical waves whose wave vector (axial) component is not equal to zero. Such frequency ranges constitute bands. The first experimental demonstration of light transmission in the BG HC PCF was made in 1999 [Gregan et. al., 1999]. Up to now, considerable efforts have been put forth in experimental and theoretical studies of BG HC PCFs made of silica glass [Humbert et. al., 2004; Benabid et. al., 2004]. This special interest can be partly explained by a need to find a way of yielding a loss level less than 0.2 dB/km for telecommunication spectral region. So far, the BG HC PCFs loss was reduced only to 1.2 dB/km due to intrinsic roughness of the air – glass interfaces in the structure [Roberts, Couny et. al., 2005].
As it was mentioned above, BG HC PCFs made of silica glass have claddings with very little solid material. The bandgap located between 4th and 5th bands is used for guiding in HC PCFs with such high air – filling fraction ( ≥ 80%) [Humbert et. al., 2004]. The number of each band is counted from the band with the largest value of the propagation constant of the air core mode. However, there is an important need for BG HC PCFs which can be used in the mid and far IR. BG HC PCF made of silica glass with a core diameter of 40 μm demonstrated single mode waveguide regime in a narrow transmission window near the wavelength of λ = 3.14 μm with an attenuation of ~ 2.6 dB/m [Shephard et. al., 2005]. But silica glass BG HC PCFs cannot be used for CO2 laser radiation transmission due to a very high material loss of silica. Transmission of light in the mid IR region becomes possible with BG HC PCFs made of glasses which are transparent in this spectral region such as
5 cm. Hollow glass waveguides have been used successfully for a modest CO2 laser power delivery below ~ 80 W. For the higher power delivery it is necessary to place a water – cooled jacket around the guides. The highest CO2 laser power delivered through the water – cooled hollow glass waveguide with 700 μm bore was 1040 W [Nubling & Harrington, 1996]. This is comparable to CO2 laser power delivered through the water – cooled hollow
metallic waveguide with 1800 μm bore which was 2700 W [Hongo et. al., 1992].
10%.
**2.2 CO2 laser radiation transmission through hollow core microstructured fibers**
In this subsection we will consider a new approach to solving the problem of the mid IR transmission (in particular, CO2 laser radiation) through the glass hollow core microstructured fibers (HC MFs). The possibility of the light confinement in the air core of HC MFs with the cladding consisting of two dimensional periodic array of air holes was predicted by Russell at the beginning of 1990s and theoretically demonstrated by Birks et. al. [Birks et. al., 1995]. The most advanced HC MFs are hollow core photonic crystal fibers (HC PCFs). HC PCFs in turn can be divided into two main groups. The HC PCFs from the first group guide the light by virtue of photonic band gap (BG HC PCFs). The HC PCFs from the second group have no band gaps and guide the light due to an inhibited coupling between the core guided modes and modes associated with a cladding [Benabid et. al., 2002]. They are called inhibited coupling HC PCFs (IC HC PCFs). Both types of HC PCFs have the claddings with very little solid material, usually, with a filling fraction less than
The guidance mechanism for BG HC PCFs is based on the concept of 'out of plane' band gap. The microstructure of BG HC PCF cladding consists of air holes packed in a triangular arrangement. It gives rise to a full two dimensional photonic band gap [Birks et. al., 1995]. As a result, forbidden frequencies occur for optical waves whose wave vector (axial) component is not equal to zero. Such frequency ranges constitute bands. The first experimental demonstration of light transmission in the BG HC PCF was made in 1999 [Gregan et. al., 1999]. Up to now, considerable efforts have been put forth in experimental and theoretical studies of BG HC PCFs made of silica glass [Humbert et. al., 2004; Benabid et. al., 2004]. This special interest can be partly explained by a need to find a way of yielding a loss level less than 0.2 dB/km for telecommunication spectral region. So far, the BG HC PCFs loss was reduced only to 1.2 dB/km due to intrinsic roughness of the air – glass
As it was mentioned above, BG HC PCFs made of silica glass have claddings with very little solid material. The bandgap located between 4th and 5th bands is used for guiding in HC PCFs with such high air – filling fraction ( ≥ 80%) [Humbert et. al., 2004]. The number of each band is counted from the band with the largest value of the propagation constant of the air core mode. However, there is an important need for BG HC PCFs which can be used in the mid and far IR. BG HC PCF made of silica glass with a core diameter of 40 μm demonstrated single mode waveguide regime in a narrow transmission window near the wavelength of λ = 3.14 μm with an attenuation of ~ 2.6 dB/m [Shephard et. al., 2005]. But silica glass BG HC PCFs cannot be used for CO2 laser radiation transmission due to a very high material loss of silica. Transmission of light in the mid IR region becomes possible with BG HC PCFs made of glasses which are transparent in this spectral region such as
interfaces in the structure [Roberts, Couny et. al., 2005].
chalcogenide glasses. These glasses are composed of the chalcogen elements Se and Te with an addition of such elements as Ge, As, Sb. The transparency windows of these glasses correspond approximately to the mid IR region 2 – 25 μm.
Modeling of BG HC PCFs made of nonsilica glasses was performed by a number of authors [Shaw et. al., 2003; Pottage et. al., 2003; Pearce et. al., 2005]. In this paper [Shaw et. al., 2003] BG HC PCFs made of As – S (refractive index ~ 2.4) and As – Se (refractive index ~ 2.8) were analyzed. It was shown that there exist several spectral regions with bandgaps for an air filling fraction > 40% in both As – S and As – Se BG HC PCFs. These BG HC PCFs have large bandgap widths at the air filling fraction of 45% to 60%. BG HC PCFs with high air filling fractions > 80% also exhibited large bandgap widths. In the author' opinion, all these results show a possibility of a light transmission in the mid IR using halcogenide BG HC PCFs. The authors of [Pottage et. al., 2003] have carried out a numerical analyses of BG HC PCFs for a wide range of refractive indices from *n* = 1.5 to *n* =3.6 and for different values of air filling fractions from 33% to 87%. They discovered a new type of the bandgap which was called type 2 bandgap at an air filling fraction ~ 60% for any glass index beyond 2. The results showed a possibility of obtaining a satisfactory guidance in such BG HC PCFs.
Another important aspect of the problem of the mid IR radiation transmission was discussed in [Pearce et. al., 2005]. Apart from a limitation to attaining a low loss guidance in BG HC PCFs connected with the intrinsic roughness of the air glass interfaces, there is another problem connected with an existence of surface guided modes that are trapped in the core surroundings. Experimental and theoretical studies [Smith et. al., 2003; Humbert et. al., 2004; West et. al., 2004; Saitoh et. al., 2004] carried out for silica BG HC PCFs have shown that the anticrossing between dispersion curves of the surface modes and the air core modes is the main factor leading to a transmission loss in BG HC PCFs. Several methods were proposed to suppress the surface modes. The first method is used to reduce the distortion of the core by including 'fingers' of glass [West et. al., 2004]. The second method is to use thin core walls [Saitoh et. al., 2004] and the third one is to use 'antiresonance' walls [Roberts, Williams et. al., 2005]. The authors of [Pearce et. al., 2005] modeled a realistic design of distorted cores for BG HC PCFs which can guide the light in the type 2 bandgap. They have demonstrated that BG HC PCFs made of high index glass can guide a fundamental air core mode with a fraction of power in the air of up to 98%.
In their paper [Hu & Menyuk, 2007] the authors analyzed BG HC PCFs for refractive indices between 1.4 and 2.8. They found two maxima of the relative bandgap as a function of the air filling fraction and refractive index. The authors also found that the relative bandgap and the level of loss are interrelated. When the relative bandgap increases the loss decreases and vice versa.
Despite the promising results of modeling obtained in the above listed works a practical realization of BG HC PCFs made completely of chalcogenide glass for the mid IR spectral region has not been reported. The only successful realization of a photonic band gap hollow core fiber for the CO2 laser radiation transmission was 'Omniguide' fiber where the cladding is a Bragg reflector (hollow core Bragg fiber) made of soft glass and polymer [Temelkuran et. al., 2002]. The authors of [Bowden & Harrington, 2009] have studied low and high index chalcogenide glasses for their potential use in the fabrication of all glass hollow core Bragg fiber.
Transmission of CO2 Laser Radiation Through Hollow Core Microstructured Fibers 233
Fig. 1. (left) HC MF with the cladding consisting of eight capillaries (right) HC MF with the
In [Pryamikov et. al., 2011] the authors have made a calculation mistake when they were trying to justify their assumptions on the role of the negative curvature of the core boundary. The mistake was made when calculating the waveguide loss for a dielectric tube using an analytical formula from [Marcatili & Schmeltzer, 1964]. Indeed, the loss levels for the dielectric tube and silica HC MF with the cladding consisting of solid rods with equal air core diameters are approximately the same. Despite this fact, the main conclusion of the paper remains accurate, i.e. that to obtain a long wavelength waveguide regime in the mid IR with silica HC MF it is necessary to combine two factors, namely, the negative curvature of the core boundary and the low density of eigenstates of the individual elements of the
To clarify a role of the negative curvature of the core boundary we should consider a plane wave scattering on the curved surface, for example, of a solid rod. This problem was solved by many authors [Wait, 1955; Lind & Greenberg, 1966]. Depending on the polarization state of the incident plane wave *z* – component of the electric (TM polarization) or magnetic field (TE polarization) is parallel to the incident plane. Suppose the refractive index of the solid rod is *n*1 and the outer space is *n*<sup>2</sup> . In the following, only TE polarized plane wave will be considered. Its z – component of the magnetic field can be expanded according to addition
*i n in i z*
=−∞
λ
<sup>=</sup>
<sup>∞</sup> − −
ϕ β
<sup>=</sup> is a wavevector in the outer space *k*2 2
= 0 *<sup>i</sup> Ez* , (1)
λ
λ2 2 = −
and *r* are an azimuthal and radial cylindrical
is its axial component. In such a way, the incident
ωone can
*2r)* are the Bessel
β
is its
theorem for Bessel functions and taking into account the temporal dependence as *i t e*
*z n n H H i J re e* 0 2 sin [ ( ) ]
ϕ
θ
coordinates, *H0* is an amplitude of the incident plane wave and *Jn(*
2π
β= cos
λ
θ
cladding consisting of eight solid rods.
cladding.
obtain:
where
θ
is an angle of incidence,
functions of first kind . If *k n* 2 2
transverse component and *k*<sup>2</sup>
The first work devoted to the fabrication and experimental investigation of BG HC PCF for CO2 laser radiation transmission has appeared only in 2010 [Deseveday et. al., 2010]. The authors designed BG HC PCF made of chalcogenide glass to guide the light in the air core at λ = 9.3 μm. They also fabricated two BG HC PCFs which could potentially guide CO2 laser radiation but no guidance was observed. The authors explained this fact by technological difficulties in the fabrication process. They hope to improve the process by avoiding air tightness anomalies and by decreasing the core's wall thickness.
In the next section we will represent our approach to solving the problem of CO2 laser radiation transmission through the glass HC MFs.
#### **3. Mechanisms of CO2 laser radiation transmission through the glass hollow core microstructured fiber with the cladding consisting of capillaries**
In this section, we will consider physical mechanisms and principles which enable, in our opinion, to obtain a loss level much lower than the material loss of the glass of HC MFs with a negative curvature of the core boundary [Pryamikov et. al., 2011]. The negative curvature of the core boundary is obtained by the cladding consisting of one or several rows of glass capillaries. Such microstructure design leads to a significant complication of the boundary conditions for the air - core modes. To justify our assumptions it will be necessary to consider a plane wave scattering on a cylindrical surface to show an analogy between this phenomenon and the light scattering on the plane optical diffraction grating. An analogy between discrete rotational symmetry of the capillary arrangement in the cladding and the plane diffraction grating will also be outlined. We will also consider the second main factor leading to a loss reduction of the air core modes of the HC MFs and to an increase in the width of transmission regions. It is connected with the geometry parameters of an individual capillary and the glass refractive index. In the end, we will try to justify the statement how these factors can result in the loss reduction of the air core mode in HC MF with the cladding consisting of capillaries with respect to the BG HC PCFs and kagome lattice IC HC PCFs.
#### **3.1 The cylindrical surface as a diffraction grating**
In this subsection, we will offer a reason which, in our opinion, lies behind the low loss waveguide regimes for the glass HC MFs with negative curvature of the core boundary. For the first time, an effect of the loss level decrease resulting from the negative curvature of the core boundary was observed for a large pitch kagome – lattice IC HC PCF with a hypocycloid – shaped core structure (the second group of PCFs) [Wang et. al., 2011]. We have used a simple cladding structure of the HC MF consisting of eight silica capillaries (Fig. 1(left)). Such HC MF guided light in the mid IR up to 4 μm despite of very high material losses of silica in this spectral region. In this case, the negative curvature of the core boundary was created by the capillary surfaces. Of course, such long a wavelength guiding is determined by not only the negative curvature of the core boundary but (may be to a greater extent) also by the optical properties of an individual capillary of the cladding. For example, the simple cladding structure consisting of one row of the capillaries has a lower density of eigenstates with respect to the cladding consisting of the solid rods (Fig. 1(right)).
The first work devoted to the fabrication and experimental investigation of BG HC PCF for CO2 laser radiation transmission has appeared only in 2010 [Deseveday et. al., 2010]. The authors designed BG HC PCF made of chalcogenide glass to guide the light in the air core at λ = 9.3 μm. They also fabricated two BG HC PCFs which could potentially guide CO2 laser radiation but no guidance was observed. The authors explained this fact by technological difficulties in the fabrication process. They hope to improve the process by avoiding air
In the next section we will represent our approach to solving the problem of CO2 laser
**3. Mechanisms of CO2 laser radiation transmission through the glass hollow**
In this section, we will consider physical mechanisms and principles which enable, in our opinion, to obtain a loss level much lower than the material loss of the glass of HC MFs with a negative curvature of the core boundary [Pryamikov et. al., 2011]. The negative curvature of the core boundary is obtained by the cladding consisting of one or several rows of glass capillaries. Such microstructure design leads to a significant complication of the boundary conditions for the air - core modes. To justify our assumptions it will be necessary to consider a plane wave scattering on a cylindrical surface to show an analogy between this phenomenon and the light scattering on the plane optical diffraction grating. An analogy between discrete rotational symmetry of the capillary arrangement in the cladding and the plane diffraction grating will also be outlined. We will also consider the second main factor leading to a loss reduction of the air core modes of the HC MFs and to an increase in the width of transmission regions. It is connected with the geometry parameters of an individual capillary and the glass refractive index. In the end, we will try to justify the statement how these factors can result in the loss reduction of the air core mode in HC MF with the cladding consisting of capillaries with respect to the BG HC PCFs and kagome
In this subsection, we will offer a reason which, in our opinion, lies behind the low loss waveguide regimes for the glass HC MFs with negative curvature of the core boundary. For the first time, an effect of the loss level decrease resulting from the negative curvature of the core boundary was observed for a large pitch kagome – lattice IC HC PCF with a hypocycloid – shaped core structure (the second group of PCFs) [Wang et. al., 2011]. We have used a simple cladding structure of the HC MF consisting of eight silica capillaries (Fig. 1(left)). Such HC MF guided light in the mid IR up to 4 μm despite of very high material losses of silica in this spectral region. In this case, the negative curvature of the core boundary was created by the capillary surfaces. Of course, such long a wavelength guiding is determined by not only the negative curvature of the core boundary but (may be to a greater extent) also by the optical properties of an individual capillary of the cladding. For example, the simple cladding structure consisting of one row of the capillaries has a lower density of eigenstates with respect to the cladding consisting of the solid rods (Fig.
**core microstructured fiber with the cladding consisting of capillaries**
tightness anomalies and by decreasing the core's wall thickness.
radiation transmission through the glass HC MFs.
**3.1 The cylindrical surface as a diffraction grating**
lattice IC HC PCFs.
1(right)).
Fig. 1. (left) HC MF with the cladding consisting of eight capillaries (right) HC MF with the cladding consisting of eight solid rods.
In [Pryamikov et. al., 2011] the authors have made a calculation mistake when they were trying to justify their assumptions on the role of the negative curvature of the core boundary. The mistake was made when calculating the waveguide loss for a dielectric tube using an analytical formula from [Marcatili & Schmeltzer, 1964]. Indeed, the loss levels for the dielectric tube and silica HC MF with the cladding consisting of solid rods with equal air core diameters are approximately the same. Despite this fact, the main conclusion of the paper remains accurate, i.e. that to obtain a long wavelength waveguide regime in the mid IR with silica HC MF it is necessary to combine two factors, namely, the negative curvature of the core boundary and the low density of eigenstates of the individual elements of the cladding.
To clarify a role of the negative curvature of the core boundary we should consider a plane wave scattering on the curved surface, for example, of a solid rod. This problem was solved by many authors [Wait, 1955; Lind & Greenberg, 1966]. Depending on the polarization state of the incident plane wave *z* – component of the electric (TM polarization) or magnetic field (TE polarization) is parallel to the incident plane. Suppose the refractive index of the solid rod is *n*1 and the outer space is *n*<sup>2</sup> . In the following, only TE polarized plane wave will be considered. Its z – component of the magnetic field can be expanded according to addition theorem for Bessel functions and taking into account the temporal dependence as *i t e* ω one can obtain:
$$H\_z^i = H\_0 \sin \theta [\sum\_{n=-\omega}^{\omega} i^n] I\_n(\lambda\_2 r) e^{-i\nu \rho} \left[ e^{-i\beta z} \right]$$
$$E\_z^i = 0 \quad \tag{1}$$
where θ is an angle of incidence, ϕ and *r* are an azimuthal and radial cylindrical coordinates, *H0* is an amplitude of the incident plane wave and *Jn(*λ*2r)* are the Bessel functions of first kind . If *k n* 2 2 2π λ <sup>=</sup> is a wavevector in the outer space *k*2 2 λ2 2 = − β is its transverse component and *k*<sup>2</sup> β = cosθis its axial component. In such a way, the incident
Transmission of CO2 Laser Radiation Through Hollow Core Microstructured Fibers 235
Suppose the diffraction grating extends infinitely in the *y* and *z* directions, with the period *d* in the *z* direction only. The refractive index of the outer medium is again <sup>2</sup> *n* . The magnetic
*<sup>x</sup> <sup>i</sup> ik x i z H He e <sup>y</sup>* <sup>0</sup>
Then, due to periodicity the scattered magnetic field can be represented as [Hessel & Oliner,
[ () ]
transverse components of the wavevector of the scattered field. Applying boundary conditions one obtains an infinite inhomogeneous set of simultaneous linear equations
coupled between each other as in the case of the plane wave scattering on the rod and a set of resonances Lorentzian and Fano types known as Wood anomalies [Wood, 1902; Lord Rayleigh, 1907; Hessel & Oliner, 1965] also occur. In the case of optical grating the
*<sup>n</sup> <sup>k</sup>*
In the case of the plane wave scattering on a dielectric rod there is also a conservation of the angular momentum of the light. The fields of the source plane wave are expanded into a set of the 'space' channels with an angular momentum determined by the number of *n* in *in e*<sup>−</sup>
(1) and each continuous (radiation) mode of the dielectric rod has the same angular momentum (1, 2). Note, that this is (4) of the same form as an expression (2a) for the scattered field *<sup>s</sup> Hz* in the case of the dielectric rod. Comparing expressions (2a) and (4) one can state that the process of the plane wave scattering on the rod can be represented as scattering on the diffraction quasi – grating. The surface of the dielectric rod can be
conservation of the angular momentum of the light. But instead of one incident plane wave as in the case of the plane diffraction grating (3) there is an infinite set of incident cylindrical harmonics with their own angular momentums of the light. It can also be shown that the
analogous to Wood anomalies as in the case of the plane diffraction grating. This problem and its application to the waveguide mechanism in all solid band gap fibers will be considered by us thoroughly in our future publication. In the same way, it is possible to consider a cylindrical surface of the capillary as a diffraction quasi - grating in the
The main conclusion to draw is that the plane wave incidence on a curved cylindrical surface leads to the appearance of an infinite set of cylindrical harmonics with different
*n n b a*, in expansions (2a) demonstrate the resonance behaviour
= + β.
*d* 2π
conservation of light momentum for the scattered light in the *z* - direction looks like:
*s n*
considered as an azimuthal diffraction grating with periodicity in
direction instead of *z* – direction in the case of the plane diffraction grating.
*H A ee e*
β
*y n n*
[Hessel & Oliner, 1965] for the scattered field amplitudes *An*( )
=−∞
are the amplitudes of the various spectral orders and *<sup>n</sup>*
*n x <sup>n</sup> i z <sup>s</sup> ik x <sup>d</sup> i z*
<sup>∞</sup> <sup>−</sup> <sup>−</sup>
2
π
β
<sup>=</sup> , (4)
− β
= (3)
*x*
β
*<sup>n</sup> k k*
. These amplitudes are
= −+ β
2
ϕ
*d* 2 2
are
φ
ϕ
<sup>2</sup> ( ) π
field of this incident wave of *S* polarization is represented as:
1965]:
where *An*( )
β
spectral dependencies of *s s*
plane wave is expanded into an infinite number of cylindrical harmonics due to the curvature of the cylinder surface. The z – components of the scattered field and field inside the cylinder can be expressed in the same manner:
$$H\_z^s = \[\sum\_{n=-\infty}^{\infty} b\_n^s H\_n^{(2)}(\lambda\_2 r) e^{-i\alpha \rho} \} e^{-i\beta z}$$
$$E\_z^s = \[\sum\_{n=-\infty}^{\infty} a\_n^s H\_n^{(2)}(\lambda\_2 r) e^{-i\alpha \rho} \} e^{-i\beta z} \tag{2a}$$
$$H\_z^{\text{ins}} = \[\sum\_{n=-\infty}^{\infty} b\_n f\_n(\lambda\_1 r) e^{-i\alpha \rho} \} e^{-i\beta z}$$
$$E\_z^{\text{ins}} = \[\sum\_{n=-\infty}^{\infty} a\_n f\_n(\lambda\_1 r) e^{-i\alpha \rho} \} e^{-i\beta z} \tag{2b}$$
where *H r <sup>n</sup>* (2) <sup>2</sup> ( ) λ is Hankel function, *k*2 2 λ1 1 = − β is a transverse components of the wavevector *k n* 1 1 2π λ <sup>=</sup> for the field inside the rod. On the basis of these expressions for *z* – components of the incident, scattered and inside fields it is possible to calculate ϕ and *r* components of the fields [Adler, 1952]. In other words, the field of the incident wave is represented by an infinite number of space 'channels' (harmonics) through which the energy of the incident wave is transferred to the scattered fields and the fields inside the dielectric rod. It is seen, that such sets of cylindrical harmonics (1) - (2) have a mode structure and can be considered as radiation or continuous modes of ITE (incident transverse electric) type of the individual dielectric rod [Snyder, 1971].
To calculate the coefficients *s s nnnn baba* ,,, it is necessary to apply boundary conditions for *z* and ϕ - components of the incident, scattered fields and the fields inside the cylinder. Because of the mode structure of the total field it is not necessary to solve an infinite set of simultaneous linear equations. To obtain the *n*th order coefficients one needs to solve 4\*4 inhomogeneous system of linear equations. For a solid cylinder rod it is possible to obtain analytical expression for the coefficients which includes such terms as *n n J aJ a* ' 1 1 ( )/ ( ) λ λ [ Wait, 1955]. These terms have resonances (poles) corresponding to zeros of *nJ a*<sup>1</sup> ( ) λ . It is necessary to point out that the resonances of the *n*th order coefficients of the scattered field are determined not only by *n*th order functions *nJ a*<sup>1</sup> ( ) λ but also by *nJ a* 1 1 <sup>+</sup> ( ) λ and *nJ a* 1 1 <sup>−</sup> ( ) λ due to recurrent relations for the derivatives of the Bessel functions. In other words, the different diffraction orders are coupled between each other. Due to this fact, it is possible to observe not only Lorentzian – like resonances for the spectral dependencies of absolute values of the amplitudes *s s n n b a*, but also Fano type resonances [Fano, 1961]. For example, Fano type resonances were analysed in the case of all solid band gap fibers [Steinvurzel et. al., 2006] with the cladding consisting of solid dielectric cylinders with a refractive index higher than the background.
A similar phenomenon occurs when the plane wave is scattered on optical diffraction grating. A short analysis can be carried out based on the work [Hessel & Oliner, 1965].
plane wave is expanded into an infinite number of cylindrical harmonics due to the curvature of the cylinder surface. The z – components of the scattered field and field inside
*ss in i z*
λ
<sup>∞</sup> − −
ϕ β
ϕ β
> ϕ β
ϕ β
<sup>=</sup> for the field inside the rod. On the basis of these expressions for *z* –
<sup>=</sup> (2a)
<sup>=</sup> , (2b)
*nnnn baba* ,,, it is necessary to apply boundary conditions for *z*
λ
is a transverse components of the
'
λ
λ
and *nJ a* 1 1 <sup>−</sup> ( )
λ
1 1 ( )/ ( )
λ
. It is necessary to
due to recurrent
[ Wait,
ϕ
and *r* -
*H bH re e* (2) <sup>2</sup> [ () ]
*ss in i z*
λ
<sup>∞</sup> − −
*ins in i z*
λ
<sup>=</sup>
<sup>∞</sup> − −
*H bJ re e* <sup>1</sup> [ () ]
*ins in i z*
components of the fields [Adler, 1952]. In other words, the field of the incident wave is represented by an infinite number of space 'channels' (harmonics) through which the energy of the incident wave is transferred to the scattered fields and the fields inside the dielectric rod. It is seen, that such sets of cylindrical harmonics (1) - (2) have a mode structure and can be considered as radiation or continuous modes of ITE (incident transverse electric) type of
λ
<sup>∞</sup> − −
β
- components of the incident, scattered fields and the fields inside the cylinder. Because of the mode structure of the total field it is not necessary to solve an infinite set of simultaneous linear equations. To obtain the *n*th order coefficients one needs to solve 4\*4 inhomogeneous system of linear equations. For a solid cylinder rod it is possible to obtain
*E a J re e* <sup>1</sup> [ () ]
*E aH re e* (2) <sup>2</sup> [ () ]
*z nn n*
=−∞ <sup>=</sup>
*z nn n*
=−∞
*z n n n*
*z n n n*
> λ1 1 = −
is Hankel function, *k*2 2
=−∞
components of the incident, scattered and inside fields it is possible to calculate
analytical expression for the coefficients which includes such terms as *n n J aJ a*
point out that the resonances of the *n*th order coefficients of the scattered field are determined
relations for the derivatives of the Bessel functions. In other words, the different diffraction orders are coupled between each other. Due to this fact, it is possible to observe not only Lorentzian – like resonances for the spectral dependencies of absolute values of the amplitudes
*n n b a*, but also Fano type resonances [Fano, 1961]. For example, Fano type resonances were analysed in the case of all solid band gap fibers [Steinvurzel et. al., 2006] with the cladding consisting of solid dielectric cylinders with a refractive index higher than the background.
A similar phenomenon occurs when the plane wave is scattered on optical diffraction grating. A short analysis can be carried out based on the work [Hessel & Oliner, 1965].
but also by *nJ a* 1 1 <sup>+</sup> ( )
1955]. These terms have resonances (poles) corresponding to zeros of *nJ a*<sup>1</sup> ( )
λ
=−∞
the cylinder can be expressed in the same manner:
where *H r <sup>n</sup>* (2) <sup>2</sup> ( ) λ
and ϕ
*s s*
wavevector *k n* 1 1
2π
λ
the individual dielectric rod [Snyder, 1971].
not only by *n*th order functions *nJ a*<sup>1</sup> ( )
To calculate the coefficients *s s*
Suppose the diffraction grating extends infinitely in the *y* and *z* directions, with the period *d* in the *z* direction only. The refractive index of the outer medium is again <sup>2</sup> *n* . The magnetic field of this incident wave of *S* polarization is represented as:
$$H\_y^i = H\_0 e^{k\_x x} e^{-i\beta z} \tag{3}$$
Then, due to periodicity the scattered magnetic field can be represented as [Hessel & Oliner, 1965]:
$$H\_y^s = \left[\sum\_{n=-\omega}^{\omega} A\_n(\mathcal{J}) e^{i\lambda\_n^n z} e^{-i\frac{2\pi n}{d}z}\right] e^{-i\beta z} \,, \tag{4}$$
where *An*( ) β are the amplitudes of the various spectral orders and *<sup>n</sup> x <sup>n</sup> k k d* 2 2 2 <sup>2</sup> ( ) π = −+ βare
transverse components of the wavevector of the scattered field. Applying boundary conditions one obtains an infinite inhomogeneous set of simultaneous linear equations [Hessel & Oliner, 1965] for the scattered field amplitudes *An*( ) β . These amplitudes are coupled between each other as in the case of the plane wave scattering on the rod and a set of resonances Lorentzian and Fano types known as Wood anomalies [Wood, 1902; Lord Rayleigh, 1907; Hessel & Oliner, 1965] also occur. In the case of optical grating the conservation of light momentum for the scattered light in the *z* - direction looks like:
$$k\_u^s = \beta + \frac{2\pi n}{d} \dots$$
In the case of the plane wave scattering on a dielectric rod there is also a conservation of the angular momentum of the light. The fields of the source plane wave are expanded into a set of the 'space' channels with an angular momentum determined by the number of *n* in *in e*<sup>−</sup> φ (1) and each continuous (radiation) mode of the dielectric rod has the same angular momentum (1, 2). Note, that this is (4) of the same form as an expression (2a) for the scattered field *<sup>s</sup> Hz* in the case of the dielectric rod. Comparing expressions (2a) and (4) one can state that the process of the plane wave scattering on the rod can be represented as scattering on the diffraction quasi – grating. The surface of the dielectric rod can be considered as an azimuthal diffraction grating with periodicity in ϕ - direction and conservation of the angular momentum of the light. But instead of one incident plane wave as in the case of the plane diffraction grating (3) there is an infinite set of incident cylindrical harmonics with their own angular momentums of the light. It can also be shown that the spectral dependencies of *s s n n b a*, in expansions (2a) demonstrate the resonance behaviour analogous to Wood anomalies as in the case of the plane diffraction grating. This problem and its application to the waveguide mechanism in all solid band gap fibers will be considered by us thoroughly in our future publication. In the same way, it is possible to consider a cylindrical surface of the capillary as a diffraction quasi - grating in the ϕ direction instead of *z* – direction in the case of the plane diffraction grating.
The main conclusion to draw is that the plane wave incidence on a curved cylindrical surface leads to the appearance of an infinite set of cylindrical harmonics with different
Transmission of CO2 Laser Radiation Through Hollow Core Microstructured Fibers 237
cylindrical surface of an individual capillary but also by the discrete rotational symmetry of
Summarizing the conclusions of the two previous subsections one can state that the air core modes of HC MFs with the cladding consisting of capillaries are formed by a superposition (interference) of several space cylindrical harmonics originated from the azimuthal diffraction quasi - gratings occurred in the cladding. The main factors affecting the air core mode formation are geometry parameters of an individual capillary and the type of a periodic arrangement of the capillaries in the cladding. All these space harmonics interact with the capillary walls in stronger or weaker ways depending on their radial and azimuthal distribution. In this case, the material loss of the capillary walls doesn't have the same affect on the attenuation of each harmonic. In such a way, the energy of the air core mode at the core boundary for HC MF with the cladding consisting of capillaries is distributed in much more complicated way than in the case of BG HC PCFs or kagome
BG HC PCFs and kagome lattice IC HC PCFs do not have such complicated mechanism of the air core modes formation because of a quasi continuous rotational symmetry of the core boundary and the boundaries of succeeding layers in the cladding. Kagome lattices IC HC PCFs sometimes have the discrete symmetry of the core boundary but just with a polygonal shape of the core without a negative curvature of the core boundary. The authors in [Pearce et al., 2007] have shown that the loss behavior of a hollow core fiber with the cladding consisting of concentric glass rings or hexagons explain the qualitative features in the loss curves associated with kagome lattice IC HC PCF. The hollow core photonic band gap fiber with the cladding consisting of concentric glass rings has a continuous rotational symmetry of the core modes and is analogous to solid Bragg fibers [Fevrier et. al., 2006]. In this case, the air core is formed by the boundaries of the concentric glass rings which cannot play the role of the azimuthal diffraction gratings. Applying boundary conditions to each concentric ring of the cladding one obtains a homogeneous system of linear equations determinant of which is a dispersion relation for the air core modes. Each air core mode is formed and described by only one space cylindrical harmonic with the propagation constant
determined from the corresponding dispersion relation. These space harmonics don't interact with each other's as in the case of all solid band gap fibers and Fano type resonances cannot be observed (an exception can be kagome lattice IC HC PCF with a polygonal shape of the air core). As a consequence, all energy of each air core mode is concentrated in one 'space' channel (cylindrical harmonic) which interacts with glass rings of the cladding in the
above. All resonances in high index layers for this mode are radial and can be described by
The other very important moment is that the geometry parameters of the capillaries *ins out d d*, and the glass refractive index at the determined value of *Dcore* (Fig .1(left)) should be chosen
ϕ
is determined from the dispersion relation mentioned
. This air core mode is
same way according to its azimuthal angular dependence of *in e*<sup>−</sup>
β
leaky and its imaginary part of
the ARROW model [Litchinitser et. al., 2002].
β
**3.3 Complicated boundary conditions for the air core mode of HC MF with the**
**cladding consisting of capillaries and low loss guidance**
the core boundary.
lattice IC HC PCFs.
angular momentums of the light. As the source of the incident wave is outside of a solid cylinder or a capillary one obtains an inhomogeneous set of linear equations for determining amplitudes of the scattered field harmonics. These scattered field harmonics have the same values of the angular momentums of the light as the harmonics of the incident field. Different orders of these harmonics (diffraction orders) are coupled between each other due to the properties of the Bessel functions. It leads to appearance of Wood anomalies in the spectral dependencies of their amplitudes. As a consequence, the curved cylindrical surface can be considered as an azimuthal diffraction grating with modulation in ϕ - direction [Pryamikov, to be prepared]. This fact leads to a loss of simplicity of the boundary conditions for the air core modes of the HC MF with the cladding consisting of capillaries compared with BG HC PCF, for example. Similar situation occurs if one considers the boundary conditions for the core modes of all solid photonic band gap fibers [White et. al., 2002] and Bragg fibers [Yeh et. al., 1978].
#### **3.2 Discrete rotational symmetry of the core boundary**
Another aspect of the problem of complicated boundary conditions for HC MF with the cladding consisting of *N* capillaries is that the capillary location in the cladding is also periodic in the azimuthal coordinate ϕ (Fig.1). As a result, the boundary conditions for the air core modes are also periodic. It is known that if the system transforms into itself for a set of discrete rotations δϕ = 2 / π *k N* , *k N* ∈ − (0, 1) around axial vector *z* the eigenfunctions of the system (in other words, the air core modes) can be represented as [Skorobogatiy & Yang, 2009]:
*in i z n n* (, ,) () *r z e U re* ϕ β ψ ϕ− − = , (5)
and their eigenvalues are:
$$\mathbf{X} = e^{i n \frac{2\pi k}{N}} \mathbf{.}$$
These eigenvalues are the same for any *n n Nm* = + , where *m* is an integer and the eigenfunctions characterized by an integer *n* are degenerate ones [Skorobogatiy & Yang, 2009]. As a consequence, such eigenstates of the system with a discrete rotational symmetry can be expressed as a superposition of all the degenerate states:
$$\Psi\_n(r,\varphi,z) = \left[\sum\_{m=-\omega}^{\omega} A(m)\mathcal{U}\_{n+\text{Min}}(r)e^{-i(n+\text{Min})\varphi}\right]e^{-i\beta z},\tag{6}$$
where an expression under the sum sign is a periodic function in ϕ with a period *<sup>N</sup>* 2π and *n N* ∈ − [0, 1] . Note that this is (6) of the same form as an expression (2a) and (4) for the scattered field *<sup>s</sup> Hz* in the case of the dielectric rod and the plane diffraction grating. The discrete symmetry of the core boundary gives one more type of azimuthal diffraction grating in the considered HC MFs.
In such a way, the boundary conditions for the air core modes of the HC MF with the cladding consisting of capillaries are complicated not only by the curvature of the
angular momentums of the light. As the source of the incident wave is outside of a solid cylinder or a capillary one obtains an inhomogeneous set of linear equations for determining amplitudes of the scattered field harmonics. These scattered field harmonics have the same values of the angular momentums of the light as the harmonics of the incident field. Different orders of these harmonics (diffraction orders) are coupled between each other due to the properties of the Bessel functions. It leads to appearance of Wood anomalies in the spectral dependencies of their amplitudes. As a consequence, the curved cylindrical surface
[Pryamikov, to be prepared]. This fact leads to a loss of simplicity of the boundary conditions for the air core modes of the HC MF with the cladding consisting of capillaries compared with BG HC PCF, for example. Similar situation occurs if one considers the boundary conditions for the core modes of all solid photonic band gap fibers [White et. al.,
Another aspect of the problem of complicated boundary conditions for HC MF with the cladding consisting of *N* capillaries is that the capillary location in the cladding is also
air core modes are also periodic. It is known that if the system transforms into itself for a set
the system (in other words, the air core modes) can be represented as [Skorobogatiy & Yang,
*n n* (, ,) () *r z e U re* ϕ
*k N* , *k N* ∈ − (0, 1) around axial vector *z*
*<sup>k</sup> in <sup>N</sup> <sup>e</sup>* 2π
Χ = .
These eigenvalues are the same for any *n n Nm* = + , where *m* is an integer and the eigenfunctions characterized by an integer *n* are degenerate ones [Skorobogatiy & Yang, 2009]. As a consequence, such eigenstates of the system with a discrete rotational symmetry
*r z AmU r e e* ( ) (, ,) [ ( ) () ]
*n N* ∈ − [0, 1] . Note that this is (6) of the same form as an expression (2a) and (4) for the scattered field *<sup>s</sup> Hz* in the case of the dielectric rod and the plane diffraction grating. The discrete symmetry of the core boundary gives one more type of azimuthal diffraction
In such a way, the boundary conditions for the air core modes of the HC MF with the cladding consisting of capillaries are complicated not only by the curvature of the
<sup>∞</sup> − + <sup>−</sup> +
*in i z*
β ϕ
the eigenfunctions of
(Fig.1). As a result, the boundary conditions for the
− − = , (5)
*i n Nm i z*
ϕ β
<sup>=</sup> , (6)
ϕ
with a period *<sup>N</sup>*
2πand
can be considered as an azimuthal diffraction grating with modulation in
ϕ
ψ ϕ
can be expressed as a superposition of all the degenerate states:
where an expression under the sum sign is a periodic function in
ψ ϕ
grating in the considered HC MFs.
*n n Nm m*
=−∞
2002] and Bragg fibers [Yeh et. al., 1978].
periodic in the azimuthal coordinate
δϕ = 2 / π
of discrete rotations
and their eigenvalues are:
2009]:
**3.2 Discrete rotational symmetry of the core boundary**
cylindrical surface of an individual capillary but also by the discrete rotational symmetry of the core boundary.
#### **3.3 Complicated boundary conditions for the air core mode of HC MF with the cladding consisting of capillaries and low loss guidance**
Summarizing the conclusions of the two previous subsections one can state that the air core modes of HC MFs with the cladding consisting of capillaries are formed by a superposition (interference) of several space cylindrical harmonics originated from the azimuthal diffraction quasi - gratings occurred in the cladding. The main factors affecting the air core mode formation are geometry parameters of an individual capillary and the type of a periodic arrangement of the capillaries in the cladding. All these space harmonics interact with the capillary walls in stronger or weaker ways depending on their radial and azimuthal distribution. In this case, the material loss of the capillary walls doesn't have the same affect on the attenuation of each harmonic. In such a way, the energy of the air core mode at the core boundary for HC MF with the cladding consisting of capillaries is distributed in much more complicated way than in the case of BG HC PCFs or kagome lattice IC HC PCFs.
BG HC PCFs and kagome lattice IC HC PCFs do not have such complicated mechanism of the air core modes formation because of a quasi continuous rotational symmetry of the core boundary and the boundaries of succeeding layers in the cladding. Kagome lattices IC HC PCFs sometimes have the discrete symmetry of the core boundary but just with a polygonal shape of the core without a negative curvature of the core boundary. The authors in [Pearce et al., 2007] have shown that the loss behavior of a hollow core fiber with the cladding consisting of concentric glass rings or hexagons explain the qualitative features in the loss curves associated with kagome lattice IC HC PCF. The hollow core photonic band gap fiber with the cladding consisting of concentric glass rings has a continuous rotational symmetry of the core modes and is analogous to solid Bragg fibers [Fevrier et. al., 2006]. In this case, the air core is formed by the boundaries of the concentric glass rings which cannot play the role of the azimuthal diffraction gratings. Applying boundary conditions to each concentric ring of the cladding one obtains a homogeneous system of linear equations determinant of which is a dispersion relation for the air core modes. Each air core mode is formed and described by only one space cylindrical harmonic with the propagation constant β determined from the corresponding dispersion relation. These space harmonics don't interact with each other's as in the case of all solid band gap fibers and Fano type resonances cannot be observed (an exception can be kagome lattice IC HC PCF with a polygonal shape of the air core). As a consequence, all energy of each air core mode is concentrated in one 'space' channel (cylindrical harmonic) which interacts with glass rings of the cladding in the same way according to its azimuthal angular dependence of *in e*<sup>−</sup> ϕ . This air core mode is leaky and its imaginary part of β is determined from the dispersion relation mentioned above. All resonances in high index layers for this mode are radial and can be described by the ARROW model [Litchinitser et. al., 2002].
The other very important moment is that the geometry parameters of the capillaries *ins out d d*, and the glass refractive index at the determined value of *Dcore* (Fig .1(left)) should be chosen
Transmission of CO2 Laser Radiation Through Hollow Core Microstructured Fibers 239
These capillary modes have a lower azimuthal index and a different radial dependence (Fig. 3(right)). It is worth pointing out that these two types of the individual capillary leaky modes are the main reason for occurring high loss regions for the considered HC MFs made
Fig. 2. (a) loss dependence for HC MF with *Dcore* = 220 μm and 6 capillaries in the cladding, *ins out d d* / = 0.9 (*n* = 2.4, circles), *ins out d d* / = 0.9 (*n* = 2.8, triangles), *ins out d d* / = 0.85 (*n* = 2.4, rhombuses), *ins out d d* / = 0.85 (*n* = 2.8, squares); (b) loss dependence for HC MF with *Dcore* =
Fig. 3. (left) a typical Pointing vector distribution for a capillary leaky mode in the cladding with a high azimuthal index; (right) the Pointing vector distribution for the second type of
Further, we will consider HC MF with eight capillaries in the cladding. The air core diameters and the glass refractive indices will be the same as in the case of HC MFs with six
As one can see from Fig. 4(a) HC MF with *ins out d d* / = 0.85 and *n* = 2.8 has the highest loss due to the excitation of the leaky modes with a lower azimuthal index in this spectral region (Fig. 3(b)). The HC MF with *ins out d d* / = 0.85 and *n* = 2.4 has a high loss for the same reason. Losses for HC MFs with other parameters in Fig. 4(a) are relatively low, especially, in the case of HC MF with *ins out d d* / = 0.8 and *n* = 2.4. The losses for HC MFs with *Dcore* = 320 μm are very high due to the strong coupling of the air core modes with the capillary modes having a lower azimuthal index (Fig.3(right)) in this spectral range with the exception of the
capillaries in the cladding. In Fig. 4 the loss dependencies for this HC MF are shown.
of high index glasses.
320 μm the other notations are the same as in (a).
the capillary leaky modes with a lower azimuthal index.
HC MF with *ins out d d* / = 0.85 and *n* = 2.8.
in such a way so as to excite leaky modes with high quality factor. These modes usually have high radial and azimuthal indices. To this end, the capillary wall thickness must be thin enough and comparable with the wavelength. In this case, it is possible to obtain large widths of the transmission regions for the air core modes. Moreover, it seems possible to choose all parameters of the considered HC MF including glass refractive index in such a way so as to obtain a very weak interaction (coupling) of each cylindrical harmonic constituting the air core mode with capillary walls inside the transmission regions. All these factors can give rise to a low loss waveguide regime for CO2 laser radiation. Several examples of such waveguide regimes will be given in the next section.
#### **4. Numerical modelling of HC MFs with different types of discrete rotational symmetry of the core boundary and glass composition of the capillaries**
In the following subsections four types of HC MFs with the claddings consisting of 6, 8, 10 and 12 capillaries will be considered. The calculations will be carried out for two values of the glass refractive indices *n* = 2.4, 2.8 and with three values of *ins out d d* / = 0.8, 0.85, 0.9. All calculations will be made in the narrow spectral region near λ = 10.6 μm. This fact is connected with high density of the individual capillary eigenstates (leaky modes with high azimuthal indices [Vincetti & Setti, 2010]) which occur at such high values of the glass refractive indices and individual capillary dimensions with respect to the wavelength. We will show that the way of obtaining a low loss waveguide regime in the glass HC MFs with the cladding consisting of capillaries presents a complicated multiparameter task. Unusual behaviour of the bend loss depending on the bend radii for high index glass HC MFs with the cladding consisting of capillaries will be demonstrated.
#### **4.1 Loss dependencies for HC MF with different number of capillaries in the cladding**
First, we will consider the loss dependencies for HC MF with six capillaries in the cladding. As was mentioned above, all loss dependencies are calculated in the narrow spectral range from 10.59 μm to 10.61 μm with a wavelength step equals to 1 nm. The calculations will be made for two values of the ratio of *ins out d d* / 0.85 = and 0.9 because the losses are very high at lower values of the one. In Fig. 2 these dependencies are shown for two values of the air core diameter *Dcore* = 220 μm and 320 μm.
As one can see from Fig. 2(a) HC MF with *n* = 2.4 and *ins out d d* / = 0.9 has the minimal loss level in the considered spectral range. In our opinion, it can be explained by the minimal value of density of individual capillary states. In this case, the capillary has a minimal capillary wall thickness with respect to the wavelength and the lowest refractive index. The loss dependence for HC MF with *n* = 2.8 and *ins out d d* / = 0.9 is relatively inhomogeneous and has a strong peak at λ = 10.601 μm caused by the excitation of a capillary leaky mode with high azimuthal and radial indices. This mode is shown in Fig. 3(left).
Other curves in Fig.2 (a) for *ins out d d* / = 0.85 have a higher loss level due to thicker capillary walls compared to the previous case. In Fig. 2(b) HC MF with *n* = 2.8 and *ins out d d* / = 0.85 has the maximal loss due to the excitation of the second type of the capillary leaky modes.
in such a way so as to excite leaky modes with high quality factor. These modes usually have high radial and azimuthal indices. To this end, the capillary wall thickness must be thin enough and comparable with the wavelength. In this case, it is possible to obtain large widths of the transmission regions for the air core modes. Moreover, it seems possible to choose all parameters of the considered HC MF including glass refractive index in such a way so as to obtain a very weak interaction (coupling) of each cylindrical harmonic constituting the air core mode with capillary walls inside the transmission regions. All these factors can give rise to a low loss waveguide regime for CO2 laser radiation. Several
**4. Numerical modelling of HC MFs with different types of discrete rotational symmetry of the core boundary and glass composition of the capillaries**
In the following subsections four types of HC MFs with the claddings consisting of 6, 8, 10 and 12 capillaries will be considered. The calculations will be carried out for two values of the glass refractive indices *n* = 2.4, 2.8 and with three values of *ins out d d* / = 0.8, 0.85, 0.9. All
connected with high density of the individual capillary eigenstates (leaky modes with high azimuthal indices [Vincetti & Setti, 2010]) which occur at such high values of the glass refractive indices and individual capillary dimensions with respect to the wavelength. We will show that the way of obtaining a low loss waveguide regime in the glass HC MFs with the cladding consisting of capillaries presents a complicated multiparameter task. Unusual behaviour of the bend loss depending on the bend radii for high index glass HC MFs with
**4.1 Loss dependencies for HC MF with different number of capillaries in the cladding** First, we will consider the loss dependencies for HC MF with six capillaries in the cladding. As was mentioned above, all loss dependencies are calculated in the narrow spectral range from 10.59 μm to 10.61 μm with a wavelength step equals to 1 nm. The calculations will be made for two values of the ratio of *ins out d d* / 0.85 = and 0.9 because the losses are very high at lower values of the one. In Fig. 2 these dependencies are shown for two values of the air
As one can see from Fig. 2(a) HC MF with *n* = 2.4 and *ins out d d* / = 0.9 has the minimal loss level in the considered spectral range. In our opinion, it can be explained by the minimal value of density of individual capillary states. In this case, the capillary has a minimal capillary wall thickness with respect to the wavelength and the lowest refractive index. The loss dependence for HC MF with *n* = 2.8 and *ins out d d* / = 0.9 is relatively inhomogeneous and has a strong peak at λ = 10.601 μm caused by the excitation of a capillary leaky mode
Other curves in Fig.2 (a) for *ins out d d* / = 0.85 have a higher loss level due to thicker capillary walls compared to the previous case. In Fig. 2(b) HC MF with *n* = 2.8 and *ins out d d* / = 0.85 has the maximal loss due to the excitation of the second type of the capillary leaky modes.
with high azimuthal and radial indices. This mode is shown in Fig. 3(left).
λ
= 10.6 μm. This fact is
examples of such waveguide regimes will be given in the next section.
calculations will be made in the narrow spectral region near
the cladding consisting of capillaries will be demonstrated.
core diameter *Dcore* = 220 μm and 320 μm.
These capillary modes have a lower azimuthal index and a different radial dependence (Fig. 3(right)). It is worth pointing out that these two types of the individual capillary leaky modes are the main reason for occurring high loss regions for the considered HC MFs made of high index glasses.
Fig. 2. (a) loss dependence for HC MF with *Dcore* = 220 μm and 6 capillaries in the cladding, *ins out d d* / = 0.9 (*n* = 2.4, circles), *ins out d d* / = 0.9 (*n* = 2.8, triangles), *ins out d d* / = 0.85 (*n* = 2.4, rhombuses), *ins out d d* / = 0.85 (*n* = 2.8, squares); (b) loss dependence for HC MF with *Dcore* = 320 μm the other notations are the same as in (a).
Fig. 3. (left) a typical Pointing vector distribution for a capillary leaky mode in the cladding with a high azimuthal index; (right) the Pointing vector distribution for the second type of the capillary leaky modes with a lower azimuthal index.
Further, we will consider HC MF with eight capillaries in the cladding. The air core diameters and the glass refractive indices will be the same as in the case of HC MFs with six capillaries in the cladding. In Fig. 4 the loss dependencies for this HC MF are shown.
As one can see from Fig. 4(a) HC MF with *ins out d d* / = 0.85 and *n* = 2.8 has the highest loss due to the excitation of the leaky modes with a lower azimuthal index in this spectral region (Fig. 3(b)). The HC MF with *ins out d d* / = 0.85 and *n* = 2.4 has a high loss for the same reason. Losses for HC MFs with other parameters in Fig. 4(a) are relatively low, especially, in the case of HC MF with *ins out d d* / = 0.8 and *n* = 2.4. The losses for HC MFs with *Dcore* = 320 μm are very high due to the strong coupling of the air core modes with the capillary modes having a lower azimuthal index (Fig.3(right)) in this spectral range with the exception of the HC MF with *ins out d d* / = 0.85 and *n* = 2.8.
Transmission of CO2 Laser Radiation Through Hollow Core Microstructured Fibers 241
The loss dependencies in Fig. 5 show that a low loss regime for the high index glass HC MFs with the cladding consisting of capillaries can be obtained by the right selection of many
At the end of this subsection, the loss dependencies for HC MFs with the cladding
Fig. 6. (a) loss dependence for HC MF with *Dcore* = 220 μm and 12 capillaries in the cladding, *ins out d d* / = 0.9 (*n* = 2.4, circles), *ins out d d* / = 0.9 (*n* = 2.8, triangles), *ins out d d* / = 0.85 (*n* = 2.4, rhombuses), *ins out d d* / = 0.85 (*n* = 2.8, squares), *ins out d d* / = 0.8 (*n* = 2.4, white rhombuses), *ins out d d* / = 0.8 (*n* = 2.8, white squares); (b) loss dependence for HC MF with *Dcore* = 320 μm
It has thus been shown that the achievement of a low loss waveguide regime for HC MFs with the cladding consisting of capillaries is complicated multi parameter task. All the parameters characterizing the HC MFs such as *Dcore, dins, dout, n, N* (number of the capillaries in the cladding) have an effect on the waveguide regime in the considered spectral range. In this way, two main factors affect the loss level of the HC MFs. The first is the density of eigenstates of the individual capillary and the second is the discrete rotational symmetry of the core boundary. The density of eigenstates of the individual capillary is determined by geometry parameters of a capillary and the value of a glass refractive index. The second factor is connected to the symmetry of the capillary arrangement in the cladding. By comparing the figures in this subsection one can make a conclusion that by decreasing the number of capillaries in the cladding one obtains a stronger dependence on the *Dcore*. It seems possible to find a balance between the number of capillaries and the air core diameter. With the increase in the capillary number the role of the discrete rotational symmetry
**4.2 Bend loss dependencies on a bend radius for HC MF with a different number of**
In this subsection we will consider characteristics of the bend loss behaviour for HC MFs with a different number of capillaries in the cladding. To reveal the special features of the bend loss one analyses the bend loss behaviour for HC MFs with optimal waveguide
parameters characterizing HC MF including *Dcore*.
consisting of 12 capillaries will be shown (Fig. 6).
the other notations are the same as in (a).
weakens.
**capillaries in the cladding**
Fig. 4. (a) loss dependence for HC MF with *Dcore* = 220 μm and 8 capillaries in the cladding, *ins out d d* / = 0.9 (*n* = 2.4, circles), *ins out d d* / = 0.9 (*n* = 2.8, triangles), *ins out d d* / = 0.85 (*n* = 2.4, rhombuses), *ins out d d* / = 0.85 (*n* = 2.8, squares), *ins out d d* / = 0.8 (*n* = 2.4, white rhombuses), *ins out d d* / = 0.8 (*n* = 2.8, white squares); (b) loss dependence for HC MF with *Dcore* = 320 μm the other notations are the same as in (a).
The loss dependencies for HC MFs with ten capillaries in the cladding are shown in Fig. 5. The values of *dins*, *dout* and consequently the capillary wall thicknesses are lower compared to the cases considered above. The loss level for all HC MFs (Fig. 5(a)) is very high due to a strong coupling to the individual capillary leaky modes of both types. Low loss curves correspond only to HC MFs with *ins out d d* / = 0.8, 0.85 and *n* = 2.8. The other picture is observed in the case of HC MFs with *Dcore* = 320 μm. All loss dependencies have low losses with the exception of HC MF with *ins out d d* / = 0.8 and *n* = 2.8 which has strong coupling with the cladding and, consequently, a non propagating regime in this spectral range.
Fig. 5. (a) loss dependence for HC MF with *Dcore* = 220 μm and 10 capillaries in the cladding, *ins out d d* / = 0.9 (*n* = 2.4, circles), *ins out d d* / = 0.9 (*n* = 2.8, triangles), *ins out d d* / = 0.85 (*n* = 2.4, rhombuses), *ins out d d* / = 0.85 (*n* = 2.8, squares), *ins out d d* / = 0.8 (*n* = 2.4, white rhombuses), *ins out d d* / = 0.8 (*n* = 2.8, white squares); (b) loss dependence for HC MF with *Dcore* = 320 μm the other notations are the same as in (a).
Fig. 4. (a) loss dependence for HC MF with *Dcore* = 220 μm and 8 capillaries in the cladding, *ins out d d* / = 0.9 (*n* = 2.4, circles), *ins out d d* / = 0.9 (*n* = 2.8, triangles), *ins out d d* / = 0.85 (*n* = 2.4, rhombuses), *ins out d d* / = 0.85 (*n* = 2.8, squares), *ins out d d* / = 0.8 (*n* = 2.4, white rhombuses), *ins out d d* / = 0.8 (*n* = 2.8, white squares); (b) loss dependence for HC MF with *Dcore* = 320 μm
The loss dependencies for HC MFs with ten capillaries in the cladding are shown in Fig. 5. The values of *dins*, *dout* and consequently the capillary wall thicknesses are lower compared to the cases considered above. The loss level for all HC MFs (Fig. 5(a)) is very high due to a strong coupling to the individual capillary leaky modes of both types. Low loss curves correspond only to HC MFs with *ins out d d* / = 0.8, 0.85 and *n* = 2.8. The other picture is observed in the case of HC MFs with *Dcore* = 320 μm. All loss dependencies have low losses with the exception of HC MF with *ins out d d* / = 0.8 and *n* = 2.8 which has strong coupling with
Fig. 5. (a) loss dependence for HC MF with *Dcore* = 220 μm and 10 capillaries in the cladding, *ins out d d* / = 0.9 (*n* = 2.4, circles), *ins out d d* / = 0.9 (*n* = 2.8, triangles), *ins out d d* / = 0.85 (*n* = 2.4, rhombuses), *ins out d d* / = 0.85 (*n* = 2.8, squares), *ins out d d* / = 0.8 (*n* = 2.4, white rhombuses), *ins out d d* / = 0.8 (*n* = 2.8, white squares); (b) loss dependence for HC MF with *Dcore* = 320 μm
the cladding and, consequently, a non propagating regime in this spectral range.
the other notations are the same as in (a).
the other notations are the same as in (a).
The loss dependencies in Fig. 5 show that a low loss regime for the high index glass HC MFs with the cladding consisting of capillaries can be obtained by the right selection of many parameters characterizing HC MF including *Dcore*.
At the end of this subsection, the loss dependencies for HC MFs with the cladding consisting of 12 capillaries will be shown (Fig. 6).
Fig. 6. (a) loss dependence for HC MF with *Dcore* = 220 μm and 12 capillaries in the cladding, *ins out d d* / = 0.9 (*n* = 2.4, circles), *ins out d d* / = 0.9 (*n* = 2.8, triangles), *ins out d d* / = 0.85 (*n* = 2.4, rhombuses), *ins out d d* / = 0.85 (*n* = 2.8, squares), *ins out d d* / = 0.8 (*n* = 2.4, white rhombuses), *ins out d d* / = 0.8 (*n* = 2.8, white squares); (b) loss dependence for HC MF with *Dcore* = 320 μm the other notations are the same as in (a).
It has thus been shown that the achievement of a low loss waveguide regime for HC MFs with the cladding consisting of capillaries is complicated multi parameter task. All the parameters characterizing the HC MFs such as *Dcore, dins, dout, n, N* (number of the capillaries in the cladding) have an effect on the waveguide regime in the considered spectral range. In this way, two main factors affect the loss level of the HC MFs. The first is the density of eigenstates of the individual capillary and the second is the discrete rotational symmetry of the core boundary. The density of eigenstates of the individual capillary is determined by geometry parameters of a capillary and the value of a glass refractive index. The second factor is connected to the symmetry of the capillary arrangement in the cladding. By comparing the figures in this subsection one can make a conclusion that by decreasing the number of capillaries in the cladding one obtains a stronger dependence on the *Dcore*. It seems possible to find a balance between the number of capillaries and the air core diameter. With the increase in the capillary number the role of the discrete rotational symmetry weakens.
#### **4.2 Bend loss dependencies on a bend radius for HC MF with a different number of capillaries in the cladding**
In this subsection we will consider characteristics of the bend loss behaviour for HC MFs with a different number of capillaries in the cladding. To reveal the special features of the bend loss one analyses the bend loss behaviour for HC MFs with optimal waveguide
Transmission of CO2 Laser Radiation Through Hollow Core Microstructured Fibers 243
Such resonance behaviour of the bend loss occurs due to a very high density of dielectric modes (eigenstates) of an individual capillary at such values of *n*, *dins, dout*. The energy of the air core mode of the HC MFs is tunnelled by these dielectric modes into the capillary 'airy' modes. The higher the values of *n*, *dins, dout* with respect to the wavelength the more effective tunnelling is observed and the excited 'airy' modes of the individual capillary have higher quality factor. For example, the bend loss dependence for HC MF with the cladding consisting of 8 capillaries and *Dcore* = 220 μm has no resonance peaks due to suppressing the
To confirm the above conclusions, bend losses for HC MFs with the cladding consisting of 10 and 12 capillaries were calculated (Fig. 9). As in the case of Fig. 7, HC MFs with the
Fig. 9. (a) bend loss dependence on the bend radius for HC MFs with 10 capillaries in the cladding: *Dcore* = 220 μm, *ins out d d* / = 0.85 (*n* = 2.8, squares) and *Dcore* = 320 μm, *ins out d d* / = 0.85 (*n* = 2.4, circles); (b) bend loss dependence on the bend radius for HC MFs with 12 capillaries in the cladding: *Dcore* = 220 μm, *ins out d d* / = 0.8 (*n* = 2.8, squares) and *Dcore* = 320
All curves in Fig. 9 have no resonance peaks except for the HC MF with *Dcore* = 320 μm, *ins out d d* / = 0.85 and *n* = 2.4. In this case, the dielectric capillary mode with a high azimuthal index (Fig. 3(left)) was excited at the bend radius *R* = 18 cm and a weak tunnelling process into the 'airy' mode occurred. The level of the bend losses for all considered bent HC MFs
In conclusion, one can state that the optimal waveguide regime in the spectral region near λ = 10.6 μm for HC MFs made of high index glass (*n* > 2) is possible at *N* > 8, where *N* is a number of capillaries in the cladding. In this case, the process of tunnelling of the air core modes of HC MF into the 'airy' modes of an individual capillary is suppressed due to low quality factor of the 'airy' modes and thus a low loss waveguide regime for a bend becomes
The guidance of CO2 laser radiation in HC MFs with the cladding consisting of capillaries was analysed. Two main factors affecting the waveguide mechanism in these waveguide
tunnelling through the capillary walls due to a decrease in the values of *dins, dout* or *n*.
lowest waveguide losses were taken for the bend loss calculations.
μm, *ins out d d* / = 0.8 (*n* = 2.4, circles).
possible.
**5. Conclusion**
(Fig. 9) is very close to that of the straight HC MFs.
regimes found in the previous subsection and, correspondingly, with minimal waveguide losses.
In Fig.7 the bend loss dependencies for two low loss waveguide regimes in the case of HC MF with the cladding consisting of 6 capillaries and 8 capillaries are shown.
Fig. 7. (a) bend loss dependence on the bend radius for HC MFs with 6 capillaries in the cladding: *Dcore* = 220 μm, *ins out d d* / = 0.9 (*n* = 2.4, squares) and *Dcore* = 320 μm, *ins out d d* / = 0.9 (*n* = 2.4, circles); (b) bend loss dependence on the bend radius for HC MFs with 8 capillaries in the cladding: *Dcore* = 220 μm, *ins out d d* / = 0.8 (*n* = 2.4, squares) and *Dcore* = 320 μm, *ins out d d* / = 0.85 (*n* = 2.8, circles).
As one can see from Fig. 7(a), the bend loss dependencies have resonance peaks in the case of both values of *Dcore*. Just as in the case of HC MF with 8 capillaries in the cladding this resonance behaviour exists only for HC MF with *Dcore* = 320 μm (Fig. 7(b)). These resonances are connected with the excitation of capillary eigenstates modes called 'airy' modes [Vincetti&Setti, 2010]. An example of such a resonance tunnelling with the excitation of the 'airy' mode of the individual capillary under bending is shown in Fig. 8 for HC MF with 8 capillaries in the cladding.
Fig. 8. (left) the air core mode of HC MF with 8 capillaries in the cladding begin to couple with an 'airy' mode of an individual capillary of the cladding at a certain value of the bend radius; (right) the resonance excitation of the 'airy' mode of an individual capillary occurs at a lower value of the bend radius.
Such resonance behaviour of the bend loss occurs due to a very high density of dielectric modes (eigenstates) of an individual capillary at such values of *n*, *dins, dout*. The energy of the air core mode of the HC MFs is tunnelled by these dielectric modes into the capillary 'airy' modes. The higher the values of *n*, *dins, dout* with respect to the wavelength the more effective tunnelling is observed and the excited 'airy' modes of the individual capillary have higher quality factor. For example, the bend loss dependence for HC MF with the cladding consisting of 8 capillaries and *Dcore* = 220 μm has no resonance peaks due to suppressing the tunnelling through the capillary walls due to a decrease in the values of *dins, dout* or *n*.
To confirm the above conclusions, bend losses for HC MFs with the cladding consisting of 10 and 12 capillaries were calculated (Fig. 9). As in the case of Fig. 7, HC MFs with the lowest waveguide losses were taken for the bend loss calculations.
Fig. 9. (a) bend loss dependence on the bend radius for HC MFs with 10 capillaries in the cladding: *Dcore* = 220 μm, *ins out d d* / = 0.85 (*n* = 2.8, squares) and *Dcore* = 320 μm, *ins out d d* / = 0.85 (*n* = 2.4, circles); (b) bend loss dependence on the bend radius for HC MFs with 12 capillaries in the cladding: *Dcore* = 220 μm, *ins out d d* / = 0.8 (*n* = 2.8, squares) and *Dcore* = 320 μm, *ins out d d* / = 0.8 (*n* = 2.4, circles).
All curves in Fig. 9 have no resonance peaks except for the HC MF with *Dcore* = 320 μm, *ins out d d* / = 0.85 and *n* = 2.4. In this case, the dielectric capillary mode with a high azimuthal index (Fig. 3(left)) was excited at the bend radius *R* = 18 cm and a weak tunnelling process into the 'airy' mode occurred. The level of the bend losses for all considered bent HC MFs (Fig. 9) is very close to that of the straight HC MFs.
In conclusion, one can state that the optimal waveguide regime in the spectral region near λ = 10.6 μm for HC MFs made of high index glass (*n* > 2) is possible at *N* > 8, where *N* is a number of capillaries in the cladding. In this case, the process of tunnelling of the air core modes of HC MF into the 'airy' modes of an individual capillary is suppressed due to low quality factor of the 'airy' modes and thus a low loss waveguide regime for a bend becomes possible.
#### **5. Conclusion**
242 CO2 Laser – Optimisation and Application
regimes found in the previous subsection and, correspondingly, with minimal waveguide
In Fig.7 the bend loss dependencies for two low loss waveguide regimes in the case of HC
Fig. 7. (a) bend loss dependence on the bend radius for HC MFs with 6 capillaries in the cladding: *Dcore* = 220 μm, *ins out d d* / = 0.9 (*n* = 2.4, squares) and *Dcore* = 320 μm, *ins out d d* / = 0.9 (*n* = 2.4, circles); (b) bend loss dependence on the bend radius for HC MFs with 8 capillaries in the cladding: *Dcore* = 220 μm, *ins out d d* / = 0.8 (*n* = 2.4, squares) and *Dcore* = 320 μm, *ins out d d* /
As one can see from Fig. 7(a), the bend loss dependencies have resonance peaks in the case of both values of *Dcore*. Just as in the case of HC MF with 8 capillaries in the cladding this resonance behaviour exists only for HC MF with *Dcore* = 320 μm (Fig. 7(b)). These resonances are connected with the excitation of capillary eigenstates modes called 'airy' modes [Vincetti&Setti, 2010]. An example of such a resonance tunnelling with the excitation of the 'airy' mode of the individual capillary under bending is shown in Fig. 8 for HC MF with 8
Fig. 8. (left) the air core mode of HC MF with 8 capillaries in the cladding begin to couple with an 'airy' mode of an individual capillary of the cladding at a certain value of the bend radius; (right) the resonance excitation of the 'airy' mode of an individual capillary occurs at
MF with the cladding consisting of 6 capillaries and 8 capillaries are shown.
losses.
= 0.85 (*n* = 2.8, circles).
capillaries in the cladding.
a lower value of the bend radius.
The guidance of CO2 laser radiation in HC MFs with the cladding consisting of capillaries was analysed. Two main factors affecting the waveguide mechanism in these waveguide
Transmission of CO2 Laser Radiation Through Hollow Core Microstructured Fibers 245
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structures were proposed. The first factor is connected with the representation of the curved air core boundary of the HC MFs as azimuthal diffraction quasi - gratings. These azimuthal diffraction gratings occur due to the cylindrical surface of an individual capillary of the cladding as well as to a discrete rotational symmetry of their arrangement in the cladding. In this way, the air core mode of the HC MF is formed by the interference of space cylindrical harmonics originated from the light scattering on these quasi - diffraction gratings. The process of the air core mode formation is much more complicated compared to the one for the HC MFs with continuous rotational symmetry of the air core boundary. The interaction of cylindrical harmonics forming the air core mode of the HC MF with the capillary walls is different from each other. It leads to a weakened material loss effect on the waveguide regime in comparison with the HC MF with a continuous rotational symmetry of the air core boundary, for example, IC HC PCFs. The second factor is connected with the optical properties of an individual capillary, in particular, with the density of its eigenstates (leaky modes). This factor determines the widths of the transmission regions and the level of waveguide losses. Numerical analyses have shown that a low loss waveguide regime in the spectral region near λ = 10.6 μm for the HC MFs with the cladding consisting of high index glass capillaries becomes possible. The optimisation of the HC MF structure to achieve these regimes is a complicated multiparameter task depending on all geometry parameters characterizing the HC MFs and the value of a glass refractive index. The bend loss of the HC MFs with the cladding consisting of capillaries made of high index glasses has a resonance character depending on the bend radius. To suppress such resonances it is necessary to increase the number of capillaries in the cladding. It leads to a decrease in the capillary sizes and to a decrease in the quality factor of the 'airy' modes of the capillaries.
In the end, we would like to outline the prospects of future investigations in this field. In our opinion, to improve the waveguide properties of the HC MFs it is necessary to study the process of the air core mode formation more carefully. An effect of different types of symmetries of the capillaries arrangements in the cladding and the value of *Dcore* on the level of waveguide loss should be investigated. The optical properties of an individual capillary made of high index glass, in particular, its optical eigenstates and the density of these eigenstates depending on the geometry parameters of the capillary and glass refractive indices should be studied. To achieve a low loss guidance it is necessary to perform an optimisation of the HC MFs structure. This complicated optimisation task can be performed by powerful numerical algorithms which have already been applied, for example, to optimisation of the Bragg fibers structures [Biriukov et. al., 2008]. Also, it is necessary to improve the technology of making high index glasses with a low material loss and a technology of the HC MFs fabrication. It is worth mentioning that the early experiments demonstrated the possibility of obtaining waveguide regimes for such HC MFs made of high index chalcogenide glasses at CO2 laser wavelengths [Kosolapov et. al., 2011]. As the light was well localized in the core, such fibers hold much promise for the delivery of CO2 laser radiation.
#### **6. Acknowledgment**
The authors thank Alexandra Nikolskaya for her assistance in translating this chapter into English.
#### **7. References**
244 CO2 Laser – Optimisation and Application
structures were proposed. The first factor is connected with the representation of the curved air core boundary of the HC MFs as azimuthal diffraction quasi - gratings. These azimuthal diffraction gratings occur due to the cylindrical surface of an individual capillary of the cladding as well as to a discrete rotational symmetry of their arrangement in the cladding. In this way, the air core mode of the HC MF is formed by the interference of space cylindrical harmonics originated from the light scattering on these quasi - diffraction gratings. The process of the air core mode formation is much more complicated compared to the one for the HC MFs with continuous rotational symmetry of the air core boundary. The interaction of cylindrical harmonics forming the air core mode of the HC MF with the capillary walls is different from each other. It leads to a weakened material loss effect on the waveguide regime in comparison with the HC MF with a continuous rotational symmetry of the air core boundary, for example, IC HC PCFs. The second factor is connected with the optical properties of an individual capillary, in particular, with the density of its eigenstates (leaky modes). This factor determines the widths of the transmission regions and the level of waveguide losses. Numerical analyses have shown that a low loss waveguide regime in the
glass capillaries becomes possible. The optimisation of the HC MF structure to achieve these regimes is a complicated multiparameter task depending on all geometry parameters characterizing the HC MFs and the value of a glass refractive index. The bend loss of the HC MFs with the cladding consisting of capillaries made of high index glasses has a resonance character depending on the bend radius. To suppress such resonances it is necessary to increase the number of capillaries in the cladding. It leads to a decrease in the capillary sizes
In the end, we would like to outline the prospects of future investigations in this field. In our opinion, to improve the waveguide properties of the HC MFs it is necessary to study the process of the air core mode formation more carefully. An effect of different types of symmetries of the capillaries arrangements in the cladding and the value of *Dcore* on the level of waveguide loss should be investigated. The optical properties of an individual capillary made of high index glass, in particular, its optical eigenstates and the density of these eigenstates depending on the geometry parameters of the capillary and glass refractive indices should be studied. To achieve a low loss guidance it is necessary to perform an optimisation of the HC MFs structure. This complicated optimisation task can be performed by powerful numerical algorithms which have already been applied, for example, to optimisation of the Bragg fibers structures [Biriukov et. al., 2008]. Also, it is necessary to improve the technology of making high index glasses with a low material loss and a technology of the HC MFs fabrication. It is worth mentioning that the early experiments demonstrated the possibility of obtaining waveguide regimes for such HC MFs made of high index chalcogenide glasses at CO2 laser wavelengths [Kosolapov et. al., 2011]. As the light was well localized in the core, such fibers hold much promise for the delivery of CO2
The authors thank Alexandra Nikolskaya for her assistance in translating this chapter into
and to a decrease in the quality factor of the 'airy' modes of the capillaries.
= 10.6 μm for the HC MFs with the cladding consisting of high index
spectral region near
laser radiation.
English.
**6. Acknowledgment**
λ
Transmission of CO2 Laser Radiation Through Hollow Core Microstructured Fibers 247
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**Part 3**
**Material Processing**
| doab | 2025-04-07T04:13:04.462756 | 20-4-2021 17:27 | {
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"book_id": "ffee18f3-6e2b-4d7c-b1a7-110a12a2adbc",
"url": "https://mts.intechopen.com/storage/books/2086/authors_book/authors_book.pdf",
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"title": "CO2 Laser",
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"section_idx": 4
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ffee18f3-6e2b-4d7c-b1a7-110a12a2adbc.5 | **Part 3**
**Material Processing**
248 CO2 Laser – Optimisation and Application
Vincetti L.; Setti V. (2010). Waveguide mechanism in tube lattice fibers. *Optics Express*,
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Vol.18, pp. 23133 - 23146.
*America*, Vol.68, pp. 1196 – 1201.
**9**
*Poland*
Joanna Radziejewska
**Application of Laser-Burnishing Treatment**
**for Improvement of Surface Layer Properties**
*Institute of Fundamental Technological Research, Polish Academy of Sciences, Warsaw*
Laser Beam Machining (LBM) has been successfully applied for improvement of surface layer properties of machine elements. Some of laser treatments are based on melting of surface with a laser beam. Among them is laser hardening, as well as cladding and alloying. The results of broad scope research have shown that surface roughness of elements which underwent the laser melting is too high to apply the process without an additional abrasive machining, even at the optimum parameters of the laser treatment. In most cases after the surface melting with laser beam the tension stresses are observed. That is demonstrated by the cracks in the surface layer and deterioration of its properties (Dietrich Lepski et al., 2009).
A new hybrid treatment was elaborated for laser treated materials. The treatment, combining the laser melting with the burnishing process was performed simultaneously at the laser stage. The aim of the hybrid treatment was to reduce surface roughness formed in the laser process and induce compressive stresses. A surface smoothing effect was the result of plastic deformation of the surface layer in high temperature, while a reduction of the tensile
For many years the surface burnishing has been used as smoothing and strain hardening finishing. The strain hardening, favourable compressive stress and smooth surface is obtained as a result of plastic deformation of the surface layer of elements made of homogenous material, as well as material with surface layer formed in order to obtain operating properties of superior requirements (Shiou & Hsu, 2008; Milad, 2008). The main limitation of the use of burnishing is high hardness and low plasticity of material after alloying and high roughness of surface. Przybylski, 1987, Shiou & Chen, 2003 showed that as a result of burnishing such materials undergo slight deformation due to the process. Strain hardening degree and thickness of plastic zone are small; cracks often form, whereas expected roughness could not be obtained. For this reason, the process of burnishing is not applied in industry as finishing of layers produced by laser beam. The study (Abbas & West, 1991; Arutunian et al., 1989; Meijer, 2004) demonstrated that modification of surface layers of metals by laser beam - such as hardening, alloying or cladding – provide very hard,
After laser remelting the stresses in the surface layer are generated in accordance with a hot stress model and they are mostly large tensile stresses, leading to the formation of micro-
stresses within the surface layer was due to cold work (Radziejewska&Skrzypek 2009).
resistant to abrasive wear, erosion and corrosion layers.
**1. Introduction**
### **Application of Laser-Burnishing Treatment for Improvement of Surface Layer Properties**
Joanna Radziejewska
*Institute of Fundamental Technological Research, Polish Academy of Sciences, Warsaw Poland*
#### **1. Introduction**
Laser Beam Machining (LBM) has been successfully applied for improvement of surface layer properties of machine elements. Some of laser treatments are based on melting of surface with a laser beam. Among them is laser hardening, as well as cladding and alloying. The results of broad scope research have shown that surface roughness of elements which underwent the laser melting is too high to apply the process without an additional abrasive machining, even at the optimum parameters of the laser treatment. In most cases after the surface melting with laser beam the tension stresses are observed. That is demonstrated by the cracks in the surface layer and deterioration of its properties (Dietrich Lepski et al., 2009).
A new hybrid treatment was elaborated for laser treated materials. The treatment, combining the laser melting with the burnishing process was performed simultaneously at the laser stage. The aim of the hybrid treatment was to reduce surface roughness formed in the laser process and induce compressive stresses. A surface smoothing effect was the result of plastic deformation of the surface layer in high temperature, while a reduction of the tensile stresses within the surface layer was due to cold work (Radziejewska&Skrzypek 2009).
For many years the surface burnishing has been used as smoothing and strain hardening finishing. The strain hardening, favourable compressive stress and smooth surface is obtained as a result of plastic deformation of the surface layer of elements made of homogenous material, as well as material with surface layer formed in order to obtain operating properties of superior requirements (Shiou & Hsu, 2008; Milad, 2008). The main limitation of the use of burnishing is high hardness and low plasticity of material after alloying and high roughness of surface. Przybylski, 1987, Shiou & Chen, 2003 showed that as a result of burnishing such materials undergo slight deformation due to the process. Strain hardening degree and thickness of plastic zone are small; cracks often form, whereas expected roughness could not be obtained. For this reason, the process of burnishing is not applied in industry as finishing of layers produced by laser beam. The study (Abbas & West, 1991; Arutunian et al., 1989; Meijer, 2004) demonstrated that modification of surface layers of metals by laser beam - such as hardening, alloying or cladding – provide very hard, resistant to abrasive wear, erosion and corrosion layers.
After laser remelting the stresses in the surface layer are generated in accordance with a hot stress model and they are mostly large tensile stresses, leading to the formation of micro-
Application of Laser-Burnishing Treatment for Improvement of Surface Layer Properties 253
proposed hybrid treatment for other materials and coatings. Obtaining improved features of surface layer, such as material microstructure, hardening and the compressive stresses, while reducing the amount of surface roughness, will eliminate additional finishing after the laser improving. New features of surface layer affected the basic functional properties of machine parts. Hybrid treatment improved such properties as surface roughness, contact stiffness and erosive wear mainly required in operating condition. It will extend application of the hybrid treatment on cases in which the surface layers have to meet high durability
The study was performed using the CO2 laser with the maximum power 2.5 kW. The axially-symmetric beam, of the mode close to TEM10, was focused with use of the ZnSe lens. The focal length of it was 5". The set of treatment parameters, such as laser power, feed rate, and diameter of the beam focused on the metal surface, type of shielding gas and speed of the air flow to ensure a sufficient power density to obtain the remelting and optimal results, was selected. Based on previous experience (Radziejewska&Skrzypek, 2009) the ranges of parameters of laser processing were identified in the first stage of the study. The optimization criteria, such as penetration depth, surface roughness and hardness of the
The alloying process was carried out on steel 304. Prior to the alloying a layer of Stellite 6, with a thickness of about 200 µm, was formed on the surface with plasma spraying method. Preliminary studies have shown that in this case, the alloying process takes place preferably at the following parameters: laser power 2 kW, laser beam diameter of 3 mm, feed rate of
On the laser station the burnishing process was carried out simultaneously with the alloying process. The dynamic burnishing process with use of micro-hammers was applied. The technology of micro-hammering was based on a dynamic centrifugal burnishing. For microhammering a high rotational head was developed, providing the possibility of working directly on the laser processing. Processing concept and principle of operation of the head is described in work (Radziejewska et al., 2005). In this study the modified version of the head was applied. Two rows of 8 micro-hammers allow providing greater intensity of the process and the simultaneous treatment at two different temperatures. In order to obtain more uniform deformation of surface material, the oscillation motion of the sample in a direction perpendicular to the direction of feed was introduced. The motion was generated using an oscillating table. The oscillations eliminated the problem of the formation of unfavourable geometric surface structure - the grooves occurring in earlier solution. A constant velocity, of 15 oscillations per second and the amplitude of 2 mm, was used. The small radii of microhammers allow to obtained high surface plastic deformation at low forces. The scheme of the head is shown in Figure 1a while the laser-mechanical treatment presented in Fig. 1b. Figure 1c shows the temperature distribution on the surface along x axis with the selected
requirements.
**2.1 Laser alloying**
**2. The experiment description**
resulting surface layer, were taken into consideration.
sample against the laser beam from 150-900 mm/min.
range of temperatures in which the burnishing process was conducted.
**2.2 Laser-mechanical treatment**
cracks in extreme cases (Grum & Sturm, 2004; Robinson, 1996) Anthony and Cline 1977 proved theoretically that the surface topography is characterized by relatively high asperities and study (Radziejewska, 2006) where waviness and roughness after laser alloying was examined confirmed this. Such a state of surface implies a need for an additional machining in order to improve surface smoothness.
The classical burnishing process applied after laser treatment was proposed in the works (De Hossonand & Noordhuis 1989, Ignatiev et al. 1993). The reduction of surface roughness and tensile stresses was obtained in the case of thin layer produced by laser alloying of titanium. Ignatiev et al. proposed another solution - the application of classical shot-peening process after laser hardening. As a result of shot-peening the change of stresses, from tensile stresses to compressive, in surface layer 70 µm thick was obtained.
The laser heating process is successfully applied to support the mechanical and plastic working of materials which are difficult for machining. Such a hybrid method was applied for cutting and turning of hard ceramic (Tsai & Ou, 2004). The research on local heating with laser beam during turning, milling and grinding of titanium alloys, cast iron and special steel was conducted. The hybrid treatment - laser-assisted burnishing (LAB) - was elaborated by Tian and Shin 2007. The laser heating process was applied for the burnishing of steel. It provided the reduction of the burnishing force, as well as the tool wear. It was shown that LAB can form better surface roughness and higher hardness than conventional burnishing.
In the work (Radziejewska 2007) the new method to modify surface layer, combing the laser melting with the slide burnishing, was proposed. The smoothing of surface was carried out by plastic deformation of surface layer at high temperature, whereas transformation process of stresses, from compressive to tensile stresses, was performed by plastic deformation at low temperatures. All machining operations - LBM, high and low temperature burnishing are performed simultaneously on the laser station, in one pass. Temperature changes while the cooling of material that undergoes the laser beam treatment, are used. It does not extend duration of treatment. It was stated that multiple alloying combined with slide burnishing generated compressive stresses of about – 600 MPa at the surface. Because of the adopted type of burnishing – the slide burnishing and high hardness of material, the relatively small thickness of textured zone, about 30 µm, was obtained. In the case of thick layers it can be insufficient. According to (Przybylski, 1987) high degree of strain hardening of surface is possible to provide using dynamic burnishing.
The current work presents the analysis of the plastic deformation of surface layer when the new type of the laser-burnishing process was applied. The surface layer was generated by laser alloying and dynamic burnishing. Design of new head allows for simultaneous surface burnishing in two different temperatures on the laser station providing high-intensity of the burnishing process. That allowed treatment hard and low plasticity materials. Thus such a technical solution should result in high degree of surface deformation as well as its large depth. The proposed solution is designed for thick layers, above 1mm, mostly produced by LBM.
The aim of this study was to evaluate the influence of the hybrid processing parameters on plastic deformations of surface layer and the analysis of the correlations between treatment parameters and surface layer state. Establishing relationships between process parameters and the state of the surface layer of tested material will determine the application area of the proposed hybrid treatment for other materials and coatings. Obtaining improved features of surface layer, such as material microstructure, hardening and the compressive stresses, while reducing the amount of surface roughness, will eliminate additional finishing after the laser improving. New features of surface layer affected the basic functional properties of machine parts. Hybrid treatment improved such properties as surface roughness, contact stiffness and erosive wear mainly required in operating condition. It will extend application of the hybrid treatment on cases in which the surface layers have to meet high durability requirements.
#### **2. The experiment description**
#### **2.1 Laser alloying**
252 CO2 Laser – Optimisation and Application
cracks in extreme cases (Grum & Sturm, 2004; Robinson, 1996) Anthony and Cline 1977 proved theoretically that the surface topography is characterized by relatively high asperities and study (Radziejewska, 2006) where waviness and roughness after laser alloying was examined confirmed this. Such a state of surface implies a need for an
The classical burnishing process applied after laser treatment was proposed in the works (De Hossonand & Noordhuis 1989, Ignatiev et al. 1993). The reduction of surface roughness and tensile stresses was obtained in the case of thin layer produced by laser alloying of titanium. Ignatiev et al. proposed another solution - the application of classical shot-peening process after laser hardening. As a result of shot-peening the change of stresses, from tensile
The laser heating process is successfully applied to support the mechanical and plastic working of materials which are difficult for machining. Such a hybrid method was applied for cutting and turning of hard ceramic (Tsai & Ou, 2004). The research on local heating with laser beam during turning, milling and grinding of titanium alloys, cast iron and special steel was conducted. The hybrid treatment - laser-assisted burnishing (LAB) - was elaborated by Tian and Shin 2007. The laser heating process was applied for the burnishing of steel. It provided the reduction of the burnishing force, as well as the tool wear. It was shown that LAB
In the work (Radziejewska 2007) the new method to modify surface layer, combing the laser melting with the slide burnishing, was proposed. The smoothing of surface was carried out by plastic deformation of surface layer at high temperature, whereas transformation process of stresses, from compressive to tensile stresses, was performed by plastic deformation at low temperatures. All machining operations - LBM, high and low temperature burnishing are performed simultaneously on the laser station, in one pass. Temperature changes while the cooling of material that undergoes the laser beam treatment, are used. It does not extend duration of treatment. It was stated that multiple alloying combined with slide burnishing generated compressive stresses of about – 600 MPa at the surface. Because of the adopted type of burnishing – the slide burnishing and high hardness of material, the relatively small thickness of textured zone, about 30 µm, was obtained. In the case of thick layers it can be insufficient. According to (Przybylski, 1987) high degree of strain hardening of surface is
The current work presents the analysis of the plastic deformation of surface layer when the new type of the laser-burnishing process was applied. The surface layer was generated by laser alloying and dynamic burnishing. Design of new head allows for simultaneous surface burnishing in two different temperatures on the laser station providing high-intensity of the burnishing process. That allowed treatment hard and low plasticity materials. Thus such a technical solution should result in high degree of surface deformation as well as its large depth. The proposed solution is designed for thick layers, above 1mm, mostly produced by
The aim of this study was to evaluate the influence of the hybrid processing parameters on plastic deformations of surface layer and the analysis of the correlations between treatment parameters and surface layer state. Establishing relationships between process parameters and the state of the surface layer of tested material will determine the application area of the
can form better surface roughness and higher hardness than conventional burnishing.
additional machining in order to improve surface smoothness.
stresses to compressive, in surface layer 70 µm thick was obtained.
possible to provide using dynamic burnishing.
LBM.
The study was performed using the CO2 laser with the maximum power 2.5 kW. The axially-symmetric beam, of the mode close to TEM10, was focused with use of the ZnSe lens. The focal length of it was 5". The set of treatment parameters, such as laser power, feed rate, and diameter of the beam focused on the metal surface, type of shielding gas and speed of the air flow to ensure a sufficient power density to obtain the remelting and optimal results, was selected. Based on previous experience (Radziejewska&Skrzypek, 2009) the ranges of parameters of laser processing were identified in the first stage of the study. The optimization criteria, such as penetration depth, surface roughness and hardness of the resulting surface layer, were taken into consideration.
The alloying process was carried out on steel 304. Prior to the alloying a layer of Stellite 6, with a thickness of about 200 µm, was formed on the surface with plasma spraying method. Preliminary studies have shown that in this case, the alloying process takes place preferably at the following parameters: laser power 2 kW, laser beam diameter of 3 mm, feed rate of sample against the laser beam from 150-900 mm/min.
#### **2.2 Laser-mechanical treatment**
On the laser station the burnishing process was carried out simultaneously with the alloying process. The dynamic burnishing process with use of micro-hammers was applied. The technology of micro-hammering was based on a dynamic centrifugal burnishing. For microhammering a high rotational head was developed, providing the possibility of working directly on the laser processing. Processing concept and principle of operation of the head is described in work (Radziejewska et al., 2005). In this study the modified version of the head was applied. Two rows of 8 micro-hammers allow providing greater intensity of the process and the simultaneous treatment at two different temperatures. In order to obtain more uniform deformation of surface material, the oscillation motion of the sample in a direction perpendicular to the direction of feed was introduced. The motion was generated using an oscillating table. The oscillations eliminated the problem of the formation of unfavourable geometric surface structure - the grooves occurring in earlier solution. A constant velocity, of 15 oscillations per second and the amplitude of 2 mm, was used. The small radii of microhammers allow to obtained high surface plastic deformation at low forces. The scheme of the head is shown in Figure 1a while the laser-mechanical treatment presented in Fig. 1b. Figure 1c shows the temperature distribution on the surface along x axis with the selected range of temperatures in which the burnishing process was conducted.
Application of Laser-Burnishing Treatment for Improvement of Surface Layer Properties 255
element and the laser beam axis was determined initially on the basis of calculations of temperature distribution on material surface, when a moving source of heat, which has a Gaussian distribution of energy density, was considered. The distribution in a half-space has an approximate character because of numerous simplified assumptions (Anthony&Cline, 1977). The distribution in a half-space has an approximate character because of following
The calculations were done for parameters of laser treatment, the same as those were applied in preliminary investigations. The results of calculations have been verified by microstructure changes of material. The tools interacted with material within temperature
Study of surface layer state is time-consuming. Therefore determination the effect of treatment conditions on the state of surface layer was based on the theory of planned experiment, which minimizes the number of studies (Filipowski 1996; Montgomery, 1997). An experiment based on a static, determined, multi-factorial, rotatable planned program with repetition PS/DS- λ was carried out. The aim was to find functional relations between
The selection of input variables and ranges of their variation was based on preliminary results of the laser- mechanical process taking their suitability to control the hybrid treatment as an additional criterion. As the measurable, controllable input variables the
As the output factors characterizing the state of surface layer and the effects of hybrid treatment were: change in microhardness compared to the microhardness after laser alloying, thickness of the plastic deformation zones and the ratio of thickness of the plastic deformation zones to thickness of alloyed zone. On the basis of preliminary studies the areas of treatment parameters variability and the intervals of variation of input data were determined. Indications and values of variation ranges of input data are contained in Table 1. The statistical analysis of experimental results included a selection of the regression function, a statistical verification of the approximating function adequacy and verification of significance of the approximating function coefficients. The attempts of approximation using the power function and the first-degree polynomial have been made. The correlation
reflection are constant and temperature independent,
the parameters of the hybrid treatment and the state of the surface layer.
process and temperature in the region of treatment.
simplified assumptions:
range about 200–8500C.
**2.3 Testing methods**
following quantities were considered:
of the burnishing process,
hammers on the surface,
Fig. 1. A – dynamic burnishing head, b – scheme of the station for laser-mechanic treatment: 1-laser beam, 2-sample, 3-laser path, 4-oscillation table, 5-dynamic burnishing head; c – temperature distribution on the surface, along x axis, with burnishing area.
The treatment with the head is based on cyclical impacts of the burnishing elements onto the machined surface. The micro-hammers are made of bearing steel, and their working part has radius of 1.5 mm. They are placed evenly between body shields of the head and rotary mounted on the axes, providing the swinging motion of the hammers in relation to the head as well as the rotary motion with head. The compact head enables the processing of flat surfaces and curved of small sizes. The head is designed as the smoothing and strengthening treatment of laser modified parts. The head was mounted in a grip of portable grinding tool, which is mounted on the laser treatment station together with the system of the head adjustment. The station enables controlling wide range parameters of the process:
Before basic studies of the preliminary tests were carried out in order determine the optimal position of the tool in relation to the machined surface. The distance between the burnishing element and the laser beam axis was determined initially on the basis of calculations of temperature distribution on material surface, when a moving source of heat, which has a Gaussian distribution of energy density, was considered. The distribution in a half-space has an approximate character because of numerous simplified assumptions (Anthony&Cline, 1977). The distribution in a half-space has an approximate character because of following simplified assumptions:
254 CO2 Laser – Optimisation and Application
4
Fig. 1. A – dynamic burnishing head, b – scheme of the station for laser-mechanic treatment:
**x [cm]**
The treatment with the head is based on cyclical impacts of the burnishing elements onto the machined surface. The micro-hammers are made of bearing steel, and their working part has radius of 1.5 mm. They are placed evenly between body shields of the head and rotary mounted on the axes, providing the swinging motion of the hammers in relation to the head as well as the rotary motion with head. The compact head enables the processing of flat surfaces and curved of small sizes. The head is designed as the smoothing and strengthening treatment of laser modified parts. The head was mounted in a grip of portable grinding tool, which is mounted on the laser treatment station together with the system of the head adjustment. The station enables controlling wide range parameters of the process: • impact forces on the tool surface by controlling the rational speed of the head – *Vrev* and
• temperature of the process zone due to changes of the distance between the impact of
• intensity of the surface hardening by adjusting the feed rate of the sample - *Vf,* and the
Before basic studies of the preliminary tests were carried out in order determine the optimal position of the tool in relation to the machined surface. The distance between the burnishing
1-laser beam, 2-sample, 3-laser path, 4-oscillation table, 5-dynamic burnishing head; c – temperature distribution on the surface, along x axis, with burnishing area.
its distance from the surface undergoing treatment,
micro-hammers and the axis of the laser beam - *X*,
rotational speed of micro-hammers – *Vrev*.
Vf
Vosc
1
5
c
2
3
b
a
**T [C]**
The calculations were done for parameters of laser treatment, the same as those were applied in preliminary investigations. The results of calculations have been verified by microstructure changes of material. The tools interacted with material within temperature range about 200–8500C.
#### **2.3 Testing methods**
Study of surface layer state is time-consuming. Therefore determination the effect of treatment conditions on the state of surface layer was based on the theory of planned experiment, which minimizes the number of studies (Filipowski 1996; Montgomery, 1997). An experiment based on a static, determined, multi-factorial, rotatable planned program with repetition PS/DS- λ was carried out. The aim was to find functional relations between the parameters of the hybrid treatment and the state of the surface layer.
The selection of input variables and ranges of their variation was based on preliminary results of the laser- mechanical process taking their suitability to control the hybrid treatment as an additional criterion. As the measurable, controllable input variables the following quantities were considered:
As the output factors characterizing the state of surface layer and the effects of hybrid treatment were: change in microhardness compared to the microhardness after laser alloying, thickness of the plastic deformation zones and the ratio of thickness of the plastic deformation zones to thickness of alloyed zone. On the basis of preliminary studies the areas of treatment parameters variability and the intervals of variation of input data were determined. Indications and values of variation ranges of input data are contained in Table 1.
The statistical analysis of experimental results included a selection of the regression function, a statistical verification of the approximating function adequacy and verification of significance of the approximating function coefficients. The attempts of approximation using the power function and the first-degree polynomial have been made. The correlation
Application of Laser-Burnishing Treatment for Improvement of Surface Layer Properties 257
The process of contact deformation was carried on the stand using a measuring method of the surface approach proposed by Demkin 1956. The device enables measurement the approach *a* with accuracy 1 µm as a function of applied nominal pressure *q*. Contact is realized between flat and rough surface of the sample, and the smooth and rigid surface of the counter-sample, made of sintered carbide. The counter-samples have three punches, 5mm diameter and nominal area of 58.875 mm2 each. The sample undergoes deformation under the punches. Both samples are placed in a specially constructed device, which is mounted inside the laboratory precision hydraulic press that allows applying the normal pressure up to 1000 MPa. The applied pressure are measured using the compression proving ring, while the approach of samples is measured using the inductive sensor. The results of measurements were recorded in the form of the approach value *a* [µm] for the
The study was carried out to nominal pressure 270 MPa, then the unloading to 0 N. The testing enabled to determine the values of total deformation, denoted as *a1* and *a2,* and plastic deformation, *apl*, as well as the elastic one, *ae*. It also allowed determining the curves of approach - nominal pressure relation. The values of deformation are the averaged values
Due to the large differences in the surface geometric structure, which has a significant influence on the contact stiffness, the surfaces of the samples were subjected to grinding in order to form similar surface topography. Application of the grinding allowed to eliminate the influence of differences in surface roughness on contact stiffness due to the various processes applied previously. It allowed for the analysis of properties of the surface layer, in this case the burnished layer, on the process of contact deformation. After grinding the
The slurry erosive wear test was conducted on test stand equipped with the chamber, the sprinkler head with nozzle of 10 mm diameter, the mixer and a sample holder. The pressure in the power system filled with compressed air was 5 Barr. The erosive tests were performed using 15% aqueous suspension of SiC particles. The size of SiC particles was 42.5 – 46.5 µm. The samples were placed 100 mm from the nozzle. The suspension angle between the sample and surface was 900. Before the test the samples were polished to reach the same roughness of surface. A special protection was applied to provide the erosive testing only within chosen limited area. The preliminary test allowed selecting erosive test duration. It also gave information that depths of eroded material were less than thickness of tested layers. The duration of each test was 90 s. All erosive tests were performed in the same
The erosion was determined using scanning profilometer. The eroded areas, and noneroded area that was treated as a reference surface, were 3D measured. The depth and volume of eroded region was calculated based on TalyMap Platinium 5.0 program. For each examined surface layers the testing was performed on three areas. Final erosive losses were
surface roughness measurement was repeated (Radziejewska, 2011).
determined as an average value for medium depths and volumes.
**2.3.3 Contact stiffness**
given loading *F* [N].
**2.3.4 Wear test**
conditions.
from three areas of measurement.
and significance evaluation has been determined according to criteria based on I. P. Guilford theory. The confidence level α = 0.1 was adopted. Credibility of the equations was assessed based on the following criteria:
Table 1. The input values of the hybrid treatment experiment for steel 304 alloyed with Stellite 6.
#### **2.3.1 Microstructure and macro-stress analysis**
Microstructure analysis and measurements of a size of the melted zone was performed on an optical microscope at magnifications from 50 to 1000 X, and for selected samples on a scanning microscope. For this purpose, after the laser alloying and the laser-mechanical treatment all samples were cut perpendicular to the treated surface and metallographic micro-sections were made in a direction perpendicular to the direction of feed. For selected samples the micro-sections in a direction parallel to the feed samples were made additionally. Surface analysis of chemical composition was also carried out for selected samples after the alloying and the hybrid treatment.
A study of internal macro-stresses and phase composition was conducted on Bruker's D-8 Advance diffractometer, with the Mo anode lamp. The measurements of internal macrostresses were carried out on surface at a distance of 0.3 and 0.5 mm from the surface. Calculations were performed for elasticity indexes *E*=210 GPa and ν=0.28. An evaluation of the degree of plastic deformation, caused by surface burnishing process, was carried out on the basis of changes in microhardness of the material. Measurements were made at load of 0.2 N, in the central zone of the melting for both samples - alloyed with the oscillations as well as laser-mechanically treated with oscillations. The microhardness result is an average of 5 measurements.
#### **2.3.2 Surface roughness measurements**
The surface topography was examined. The measurements were conducted on a scanning profilometer Form Talysurf after laser alloying and hybrid process performed at different parameters. Surface roughness measurements were performed for each track of the laser alloying, and the laser alloying with oscillations combined with micro-hammering. The 3D roughness measurements were conducted in central area of the melting path. The values of surface topography parameters were determined for scanned area 1.4×4 mm. The measurements were conducted at steps dx = 0.5 µm, dy = 5 µm, with the stylus radius of 2 µm. Profile measurements were carried out in the middle of the zone of the melting on measuring section equal to 4 mm, parallel to direction of the feed rate. Roughness parameters are the average values of 16 measured profiles.
#### **2.3.3 Contact stiffness**
256 CO2 Laser – Optimisation and Application
and significance evaluation has been determined according to criteria based on I. P. Guilford theory. The confidence level α = 0.1 was adopted. Credibility of the equations was assessed
> Parameter Input values Unit *Vrev* 3500 4200 5000 5950 7100 rev/min *Vf* 150 230 360 570 900 mm/min *X* 5 6 7 8,5 10 mm
Table 1. The input values of the hybrid treatment experiment for steel 304 alloyed with
Microstructure analysis and measurements of a size of the melted zone was performed on an optical microscope at magnifications from 50 to 1000 X, and for selected samples on a scanning microscope. For this purpose, after the laser alloying and the laser-mechanical treatment all samples were cut perpendicular to the treated surface and metallographic micro-sections were made in a direction perpendicular to the direction of feed. For selected samples the micro-sections in a direction parallel to the feed samples were made additionally. Surface analysis of chemical composition was also carried out for selected
A study of internal macro-stresses and phase composition was conducted on Bruker's D-8 Advance diffractometer, with the Mo anode lamp. The measurements of internal macrostresses were carried out on surface at a distance of 0.3 and 0.5 mm from the surface.
An evaluation of the degree of plastic deformation, caused by surface burnishing process, was carried out on the basis of changes in microhardness of the material. Measurements were made at load of 0.2 N, in the central zone of the melting for both samples - alloyed with the oscillations as well as laser-mechanically treated with oscillations. The
The surface topography was examined. The measurements were conducted on a scanning profilometer Form Talysurf after laser alloying and hybrid process performed at different parameters. Surface roughness measurements were performed for each track of the laser alloying, and the laser alloying with oscillations combined with micro-hammering. The 3D roughness measurements were conducted in central area of the melting path. The values of surface topography parameters were determined for scanned area 1.4×4 mm. The measurements were conducted at steps dx = 0.5 µm, dy = 5 µm, with the stylus radius of 2 µm. Profile measurements were carried out in the middle of the zone of the melting on measuring section equal to 4 mm, parallel to direction of the feed rate. Roughness
ν=0.28.
Calculations were performed for elasticity indexes *E*=210 GPa and
based on the following criteria:
Stellite 6.
**2.3.1 Microstructure and macro-stress analysis**
samples after the alloying and the hybrid treatment.
microhardness result is an average of 5 measurements.
parameters are the average values of 16 measured profiles.
**2.3.2 Surface roughness measurements**
The process of contact deformation was carried on the stand using a measuring method of the surface approach proposed by Demkin 1956. The device enables measurement the approach *a* with accuracy 1 µm as a function of applied nominal pressure *q*. Contact is realized between flat and rough surface of the sample, and the smooth and rigid surface of the counter-sample, made of sintered carbide. The counter-samples have three punches, 5mm diameter and nominal area of 58.875 mm2 each. The sample undergoes deformation under the punches. Both samples are placed in a specially constructed device, which is mounted inside the laboratory precision hydraulic press that allows applying the normal pressure up to 1000 MPa. The applied pressure are measured using the compression proving ring, while the approach of samples is measured using the inductive sensor. The results of measurements were recorded in the form of the approach value *a* [µm] for the given loading *F* [N].
The study was carried out to nominal pressure 270 MPa, then the unloading to 0 N. The testing enabled to determine the values of total deformation, denoted as *a1* and *a2,* and plastic deformation, *apl*, as well as the elastic one, *ae*. It also allowed determining the curves of approach - nominal pressure relation. The values of deformation are the averaged values from three areas of measurement.
Due to the large differences in the surface geometric structure, which has a significant influence on the contact stiffness, the surfaces of the samples were subjected to grinding in order to form similar surface topography. Application of the grinding allowed to eliminate the influence of differences in surface roughness on contact stiffness due to the various processes applied previously. It allowed for the analysis of properties of the surface layer, in this case the burnished layer, on the process of contact deformation. After grinding the surface roughness measurement was repeated (Radziejewska, 2011).
#### **2.3.4 Wear test**
The slurry erosive wear test was conducted on test stand equipped with the chamber, the sprinkler head with nozzle of 10 mm diameter, the mixer and a sample holder. The pressure in the power system filled with compressed air was 5 Barr. The erosive tests were performed using 15% aqueous suspension of SiC particles. The size of SiC particles was 42.5 – 46.5 µm. The samples were placed 100 mm from the nozzle. The suspension angle between the sample and surface was 900. Before the test the samples were polished to reach the same roughness of surface. A special protection was applied to provide the erosive testing only within chosen limited area. The preliminary test allowed selecting erosive test duration. It also gave information that depths of eroded material were less than thickness of tested layers. The duration of each test was 90 s. All erosive tests were performed in the same conditions.
The erosion was determined using scanning profilometer. The eroded areas, and noneroded area that was treated as a reference surface, were 3D measured. The depth and volume of eroded region was calculated based on TalyMap Platinium 5.0 program. For each examined surface layers the testing was performed on three areas. Final erosive losses were determined as an average value for medium depths and volumes.
Application of Laser-Burnishing Treatment for Improvement of Surface Layer Properties 259
Surface plastic deformation caused a complete reconstruction of material microstructure near the surface. The microstructure, after laser alloying, is shown in Figure 4. There are equiaxed grains of a size of several micrometers. The intensive flattening of grains in the direction perpendicular to the surface is presented, Fig.5. There are no cracks or chipping. The coating particles, which were not melted by laser, can be observed on the surface in some places. They were probably stuck in the surface of already melted and solidified material by the micro-hammers. The largest deformation of grains is in the zone about 100- 150 µm from the surface. As it was expected the varied plastic deformation thickness of the zone was determined depending on the impact of the micro-hammers on the surface and the
Fig. 4. The surface layer microstructure of steel 0H18N9 after laser alloying with Stellite 6 using oscillations; a- close to surface, b- central part of alloyed zone. The treatment
Fig. 5. The surface layer microstructure after hybrid treatment at the parameters: *P* = 2kW, *d* = 3 mm, *Vf* = 230 mm/min, *V rev*= 5950 rev/min, *X* = 6 mm, using oscillations; a- close to
The X –ray diffraction XRD showed the presence of cobalt austenite, tungsten carbides and chromium as well as chromium oxides, cobalt and cobalt ferrite. In almost all samples the dominant phase was cobalt austenite. The analysis of chemical composition in the melting
a b
a b
temperature at which the process was carried out.
parameters: *P* = 2kW, *d* = 3 mm, *Vf* = 230 mm/min; (SEM).
surface, b- central part of alloyed zone; (SEM).
#### **3. Plastic deformations due to laser-burnishing**
#### **3.1 Microstructure and size of plastic deformation zone**
Studies of the microstructure showed that both after the alloying and the hybrid treatment the surface layer is homogeneous, free of pores and micro-cracks. A very fine dendrites structure, oriented in the direction of heat dissipation, is formed. The oscillations caused an increase in the width of the melted zone in relation to the alloyed samples without oscillations by the value of oscillation amplitude. Additionally the application of oscillations resulted in more even thickness of surface layer. Figure 2a shows the shape of the melted zone, while Figure 2b presents the melted zone at same processing parameters using oscillations.
Fig. 2. Shape of melted zone after: a - laser alloying of steel 0H18N9 with Stellite 6 at treatment parameters: *P* = 2 kW, *d* = 3 mm, *Vf* = 900 mm/min; b - alloying using the oscillations in perpendicular direction.
The study proved constant thickness of the melting zone. When the treatment is conducted at the lowest speed, 150 mm/min, minor changes in the thickness of the melting, related to waves of the bottom of the zone melting, can be observed (Fig. 3).
Fig. 3. The surface layer after laser alloying of steel 0H18 N9 with Stellite 6 at treatment parameters: *P* = 2kW, *d* = 3 mm, *Vf* = 150 mm/min using the oscillations. The cross-section parallel to the direction of the sample motion.
Studies of the microstructure showed that both after the alloying and the hybrid treatment the surface layer is homogeneous, free of pores and micro-cracks. A very fine dendrites structure, oriented in the direction of heat dissipation, is formed. The oscillations caused an increase in the width of the melted zone in relation to the alloyed samples without oscillations by the value of oscillation amplitude. Additionally the application of oscillations resulted in more even thickness of surface layer. Figure 2a shows the shape of the melted zone, while Figure 2b presents the melted zone at same processing parameters using
Fig. 2. Shape of melted zone after: a - laser alloying of steel 0H18N9 with Stellite 6 at treatment parameters: *P* = 2 kW, *d* = 3 mm, *Vf* = 900 mm/min; b - alloying using the
waves of the bottom of the zone melting, can be observed (Fig. 3).
The study proved constant thickness of the melting zone. When the treatment is conducted at the lowest speed, 150 mm/min, minor changes in the thickness of the melting, related to
Fig. 3. The surface layer after laser alloying of steel 0H18 N9 with Stellite 6 at treatment parameters: *P* = 2kW, *d* = 3 mm, *Vf* = 150 mm/min using the oscillations. The cross-section
a
b
**3. Plastic deformations due to laser-burnishing 3.1 Microstructure and size of plastic deformation zone**
oscillations.
oscillations in perpendicular direction.
parallel to the direction of the sample motion.
Surface plastic deformation caused a complete reconstruction of material microstructure near the surface. The microstructure, after laser alloying, is shown in Figure 4. There are equiaxed grains of a size of several micrometers. The intensive flattening of grains in the direction perpendicular to the surface is presented, Fig.5. There are no cracks or chipping. The coating particles, which were not melted by laser, can be observed on the surface in some places. They were probably stuck in the surface of already melted and solidified material by the micro-hammers. The largest deformation of grains is in the zone about 100- 150 µm from the surface. As it was expected the varied plastic deformation thickness of the zone was determined depending on the impact of the micro-hammers on the surface and the temperature at which the process was carried out.
Fig. 4. The surface layer microstructure of steel 0H18N9 after laser alloying with Stellite 6 using oscillations; a- close to surface, b- central part of alloyed zone. The treatment parameters: *P* = 2kW, *d* = 3 mm, *Vf* = 230 mm/min; (SEM).
Fig. 5. The surface layer microstructure after hybrid treatment at the parameters: *P* = 2kW, *d* = 3 mm, *Vf* = 230 mm/min, *V rev*= 5950 rev/min, *X* = 6 mm, using oscillations; a- close to surface, b- central part of alloyed zone; (SEM).
The X –ray diffraction XRD showed the presence of cobalt austenite, tungsten carbides and chromium as well as chromium oxides, cobalt and cobalt ferrite. In almost all samples the dominant phase was cobalt austenite. The analysis of chemical composition in the melting
Application of Laser-Burnishing Treatment for Improvement of Surface Layer Properties 261
0.02
Table 2. The microhardness as function of depth for samples after hybrid treatment for
Figure 7 shows the exemplary microhardness distributions after alloying and the hybrid treatment for the samples 1 and 2. The treatment process was carried out at the same feed rate, *Vf* = 230 mm/s, and at the same temperature. Both samples exhibit a substantial increase in microhardness of the hybrid treated material in relation to only alloyed. Additionally, the diagram Fig.7 shows the differences in microhardness of alloyed layers
On the basis of the results the relative percentage increase in microhardness, caused by surface plastic deformation, was determined for each sample, and the thickness of the zones of plastic deformation was estimated under the assumption that minimal increase in the
1 4200 230 6 527 480 430 410 350 2 5950 230 6 530 520 460 430 400 3 4200 570 6 540 430 380 360 290 4 5950 570 6 610 550 480 450 370 5 4200 230 8,5 640 520 440 420 290 6 5950 230 8,5 700 570 490 440 360 7 4200 570 8,5 580 550 510 440 350 8 5950 570 8,5 630 600 540 440 400 9 3500 360 7 630 500 560 520 410 10 7100 360 7 680 570 460 410 310 11 5000 150 7 660 480 450 410 400 12 5000 900 7 570 430 410 370 270 13 5000 360 5 540 460 420 380 290 14 5000 360 10 500 450 420 410 280 15 5000 360 7 610 480 430 410 280 16 5000 360 7 670 570 380 410 280 17 5000 360 7 630 490 490 420 380 18 5000 360 7 640 480 430 410 290 19 5000 360 7 620 530 450 400 350 20 5000 360 7 610 510 440 420 290
X HV
Treatment parameter Microhardness after hybrid treatment at selected
HV 0.2
0.6 [mm]
levels from surface
HV 0.3
HV 0.4
HV
Sample
Vrev [rev/min]
variable process parameters.
generated in the same conditions.
Vf [mm/min]
zone showed the increase in concentration of Co, W, Ni, and C in relation to the core. A homogeneous distribution of elements was found in the melted zone for both groups of samples - laser alloyed and hybrid treated. In Fig.6 the exemplary surface distributions of concentration of iron and cobalt for the hybrid treated sample, at the edge of the hybrid laser path, are shown.
Fig. 6. Surface distributions of elements in the melted zone after laser-mechanical treatment: a - Fe, b – Co; (EDS-SEM).
Measurements of dimensions of the alloyed zone showed that thickness of the melted zone ranges from 0.43 mm to 0.87 mm, while its width is from 3.68 mm to 4.6 mm. The size of the zone depends on the sample feed rates. The largest size of melted zone was found for the lowest feed rate of 150 mm/min. The oscillation motion caused an increase in the width of the melting zone by the value of the oscillation amplitude, i.e. 2 mm, compared to the laser alloying without oscillations. Shape changes of the melting also resulted from the oscillations, shown in Fig. 3.
#### **3.2 Microhardness of strain hardening zone**
Table 2 presents the results of microhardness study. Microhardness values refer to five distances from the surface: 0.02, 0.2, 0.3, 0.4 and 0.6 mm. After laser alloying the surface layer material had microhardness 300 - 420 HV. In most cases the increase in microhardness was recognized at the surface compared to the bottom of the melting. This is due to the presence of fine grains in the subsurface zone. Differences in microhardness observed for the same depth of the melted zone are due to different thickness and porosity of Stellite layer deposited before the process of laser-mechanical stated. These differences determine the chemical and phase composition of the alloyed layer. In order to eliminate these distortions all calculations, related to the assessment of surface plastic deformation and thickness of the hardening zone, were carried out for one laser "path" where the zone only laser-alloyed and alloyed with micro-hammering existed.
Microhardness of layer generated by laser alloying combined with micro-hammering is 530- 670 HV at the material surface and about 400 HV at the melting of the bottom. For all tested samples the increase in microhardness at the surface can be observed. It is related to the process of burnishing. The thickness of the strain hardening zone varies depending on the burnishing force as well as the intensity and temperature of the process. In all cases it is thicker than a textured zone observed on the metallographic cross-sections.
zone showed the increase in concentration of Co, W, Ni, and C in relation to the core. A homogeneous distribution of elements was found in the melted zone for both groups of samples - laser alloyed and hybrid treated. In Fig.6 the exemplary surface distributions of concentration of iron and cobalt for the hybrid treated sample, at the edge of the hybrid
Fig. 6. Surface distributions of elements in the melted zone after laser-mechanical treatment:
Measurements of dimensions of the alloyed zone showed that thickness of the melted zone ranges from 0.43 mm to 0.87 mm, while its width is from 3.68 mm to 4.6 mm. The size of the zone depends on the sample feed rates. The largest size of melted zone was found for the lowest feed rate of 150 mm/min. The oscillation motion caused an increase in the width of the melting zone by the value of the oscillation amplitude, i.e. 2 mm, compared to the laser alloying without oscillations. Shape changes of the melting also resulted from the
Table 2 presents the results of microhardness study. Microhardness values refer to five distances from the surface: 0.02, 0.2, 0.3, 0.4 and 0.6 mm. After laser alloying the surface layer material had microhardness 300 - 420 HV. In most cases the increase in microhardness was recognized at the surface compared to the bottom of the melting. This is due to the presence of fine grains in the subsurface zone. Differences in microhardness observed for the same depth of the melted zone are due to different thickness and porosity of Stellite layer deposited before the process of laser-mechanical stated. These differences determine the chemical and phase composition of the alloyed layer. In order to eliminate these distortions all calculations, related to the assessment of surface plastic deformation and thickness of the hardening zone, were carried out for one laser "path" where the zone only
Microhardness of layer generated by laser alloying combined with micro-hammering is 530- 670 HV at the material surface and about 400 HV at the melting of the bottom. For all tested samples the increase in microhardness at the surface can be observed. It is related to the process of burnishing. The thickness of the strain hardening zone varies depending on the burnishing force as well as the intensity and temperature of the process. In all cases it is
thicker than a textured zone observed on the metallographic cross-sections.
a b
laser path, are shown.
a - Fe, b – Co; (EDS-SEM).
oscillations, shown in Fig. 3.
**3.2 Microhardness of strain hardening zone**
laser-alloyed and alloyed with micro-hammering existed.
Table 2. The microhardness as function of depth for samples after hybrid treatment for variable process parameters.
Figure 7 shows the exemplary microhardness distributions after alloying and the hybrid treatment for the samples 1 and 2. The treatment process was carried out at the same feed rate, *Vf* = 230 mm/s, and at the same temperature. Both samples exhibit a substantial increase in microhardness of the hybrid treated material in relation to only alloyed. Additionally, the diagram Fig.7 shows the differences in microhardness of alloyed layers generated in the same conditions.
On the basis of the results the relative percentage increase in microhardness, caused by surface plastic deformation, was determined for each sample, and the thickness of the zones of plastic deformation was estimated under the assumption that minimal increase in the
Application of Laser-Burnishing Treatment for Improvement of Surface Layer Properties 263
HV 0.2
1 4200 230 6 46 45 34 28 9 0.55 0.75 2 5950 230 6 71 63 44 43 38 1.2 1.62 3 4200 570 6 59 13 0 0 0 0.25 0.54 4 5950 570 6 74 67 60 50 42 1.2 2.67 5 4200 230 8.5 78 30 13 14 - 0.5 0.77 6 5950 230 8.5 89 36 32 16 38 0.7 1.08 7 4200 570 8.5 32 45 34 10 - 0.4 0.75 8 5950 570 8.5 50 43 32 13 14 0.65 1.30 9 3500 360 7 62 39 56 33 8 0.5 0.94 10 7100 360 7 100 68 53 37 3 0.55 1.00 11 5000 150 7 89 55 50 37 33 1.1 1.26 12 5000 900 7 68 39 24 12 4 0.45 1.05 13 5000 360 5 54 44 35 27 - 0.5 0.77 14 5000 360 10 32 18 2 8 4 0.25 0.43 15 5000 360 7 74 41 23 28 - 0.5 0.88 16 5000 360 7 91 78 9 17 - 0.4 0.67 17 5000 360 7 70 29 32 17 12 0.55 0.80 18 5000 360 7 83 41 23 28 - 0.5 0.82 19 5000 360 7 77 66 29 14 3 0.5 0.85 20 5000 360 7 65 34 19 17 - 0.5 0.74 Table 3. The percentage change in the microhardness value at different depths, the thickness of the plastic deformation zone, *Gpl*, and the ratio of the plastic deformation thickness to the
The influence of hybrid treatment parameters on features characterizing plastic deformation zone, e.g. thickness, change in microhardness compared to the microhardness after laser alloying and ratio of thickness of the plastic deformation zone to thickness of alloyed zone,
ΔHV = 0.53 Vrev0.66 Vf
0.6 [mm]
microhardness **[%]**
HV 0.3
HV 0.4
HV
Thickness of plastic deformati on zone [mm]
Gpl/Gla
Treatment parameter Relative change in
X HV 0.02
Sample
Vrev [rev/min]
thickness of the alloyed zone, *Gpl/Gla*.
**3.3 Statistical analysis of results**
was presented in form of the regression function.
Gpl = 0.0026 Vrev1.01 Vf
Vf [mm/min]
layer is 10% (Tab.3). The micro-hammering caused the relative increase in microhardness of SL of about 32-100% at the surface compared to microhardness of SL due to laser alloying. This effect is due to surface strain hardening. The smallest increase in microhardness, 32%, at the surface was found for the sample that was burnished at the lowest temperature when distance between the hammers and the beam axis was 10 mm. In this case, the smallest depth of the plastic deformation zone was also recognized. The increase in microhardness, over 60%, was found for most of the samples (2, 4, 5, 6, 10, 11, 15-20) burnished with large impact forces of micro-hammers on surface, rotational speeds of the head above 5000 rev/min and temperature of treatment from the middle range. For the samples (1, 13) burnished at high temperature the degree of strain hardening is the order of 40-50%, which is probably related to the partial recovery of the material at high temperature.
Fig. 7. Microhardness after: ♦ ■ laser alloying of steel 0H18 N9 with Stellite 6 treatment parameters: *P* = 2 kW, *d* = 3 mm, *Vf* = 230 mm/min and alloying combined with microhammering ▲ *X* = 6 mm, *Vrev*= 4200 rev/min, **×** *Vrev*= 5950 rev/min,
The thickness of the plastic deformation zone is from 0.25 to 1.2 mm, depending on the parameters of laser-mechanical treatment. It grows with increase of the impact forces of burnishing elements on the surface and the rise of temperature in which the process of burnishing was undergoing. This effect is due to the increase in material plasticity with temperature growth.
Plastic deformation of the surface layer caused changes in residual stresses. In order to assess the state of stresses in the surface layer the preliminary studies of internal macrostresses were performed. The selected for study samples were laser alloyed at feed rate of 230 mm/min and dynamically burnished at temperature from the upper range, *Vrev* = 5950 rev/min, *X* = 6 mm. The measurements were carried out on the surface and at the depth of 0.3 and 0.5 mm beneath the surface. The results confirmed the stresses change, from tensile +398 MPa after the alloying process to compressive stresses -800 MPa caused by the burnishing process. After the dynamic burnishing the compressive stresses across the whole depth of the melted zone were found.
layer is 10% (Tab.3). The micro-hammering caused the relative increase in microhardness of SL of about 32-100% at the surface compared to microhardness of SL due to laser alloying. This effect is due to surface strain hardening. The smallest increase in microhardness, 32%, at the surface was found for the sample that was burnished at the lowest temperature when distance between the hammers and the beam axis was 10 mm. In this case, the smallest depth of the plastic deformation zone was also recognized. The increase in microhardness, over 60%, was found for most of the samples (2, 4, 5, 6, 10, 11, 15-20) burnished with large impact forces of micro-hammers on surface, rotational speeds of the head above 5000 rev/min and temperature of treatment from the middle range. For the samples (1, 13) burnished at high temperature the degree of strain hardening is the order of 40-50%, which
0 0,2 0,4 0,6 0,8 1 1,2 1,4
distance from surface [mm]
Fig. 7. Microhardness after: ♦ ■ laser alloying of steel 0H18 N9 with Stellite 6 treatment parameters: *P* = 2 kW, *d* = 3 mm, *Vf* = 230 mm/min and alloying combined with micro-
The thickness of the plastic deformation zone is from 0.25 to 1.2 mm, depending on the parameters of laser-mechanical treatment. It grows with increase of the impact forces of burnishing elements on the surface and the rise of temperature in which the process of burnishing was undergoing. This effect is due to the increase in material plasticity with
Plastic deformation of the surface layer caused changes in residual stresses. In order to assess the state of stresses in the surface layer the preliminary studies of internal macrostresses were performed. The selected for study samples were laser alloyed at feed rate of 230 mm/min and dynamically burnished at temperature from the upper range, *Vrev* = 5950 rev/min, *X* = 6 mm. The measurements were carried out on the surface and at the depth of 0.3 and 0.5 mm beneath the surface. The results confirmed the stresses change, from tensile +398 MPa after the alloying process to compressive stresses -800 MPa caused by the burnishing process. After the dynamic burnishing the compressive stresses across the whole
hammering ▲ *X* = 6 mm, *Vrev*= 4200 rev/min, **×** *Vrev*= 5950 rev/min,
is probably related to the partial recovery of the material at high temperature.
0
100
200
300
HV0.02
temperature growth.
depth of the melted zone were found.
400
500
600
Table 3. The percentage change in the microhardness value at different depths, the thickness of the plastic deformation zone, *Gpl*, and the ratio of the plastic deformation thickness to the thickness of the alloyed zone, *Gpl/Gla*.
#### **3.3 Statistical analysis of results**
The influence of hybrid treatment parameters on features characterizing plastic deformation zone, e.g. thickness, change in microhardness compared to the microhardness after laser alloying and ratio of thickness of the plastic deformation zone to thickness of alloyed zone, was presented in form of the regression function.
$$\mathbf{G}\_{\rm pl} = 0.0026 \text{ V}\_{\rm rev} \, ^{1.01} \mathbf{V}\_{\rm f} ^{0.19} \mathbf{X} \cdot ^{1.16} \tag{1}$$
$$
\Delta \text{HV} = 0.53 \text{ V}\_{\text{rev}} \alpha \text{.66 V}\_{\text{f}} \text{-0.082 } \text{X} \text{ -0.61} \tag{2}
$$
Application of Laser-Burnishing Treatment for Improvement of Surface Layer Properties 265
Figure 9 shows the relation between the increase in material microhardness at the depth of 0.2 mm and the rotational speed of the head, and the distance from the tool axis to the laser beam at constant feed rate 360 mm/min. The microhardness growth, characterizing the degree of strain hardening at the surface, depends mainly on the rotational speed of the head and the temperature of the process zone, which is a function of the distance between
Fig. 9. Dependence of microhardness increase at a depth of 0.2 mm on rotational speed of the head and distance between the tool axis *X* and the laser beam, for fixed feed rate 360
Figure 10 shows the influence of the rotational speed of the head and the distance of the tool from surface, at a fixed feed rate *Vf* = 360 mm/min, on the *Gpl/Gla* parameter. It is evident that at the highest speed and the distances of less than 9 mm the ratio is bigger than 1. At speeds below 4000 rev/min and the distance of less than 8 mm the thickness of the strainhardening zone in relation to the thickness of the alloyed layer is less than 0.5, although it
Fig. 10. Influence of rotational speed and the distance between the beam axis and the tools on the ratio of the thickness of strain hardening zone to the thickness of alloyed zone *Gpl/Gla*
does not ensure the presence of compressive stresses in the entire alloyed zone.
the burnishing tool and the laser beam.
at a fixed feed rate *Vf* = 360 mm/min.
mm/min.
$$\mathbf{G}\_{\rm pl}/\mathbf{G}\_{\rm la} = 0.00035 \text{ V}\_{\rm rev} 1^{02} \text{ V}\_{\rm l}^{0.2} \text{ X}^{-1.09} \tag{3}$$
Table 4 contains the value of multiple correlation coefficients *R*, the value of the Fisher's number *F* and *T*-Student coefficient describing the significance of subsequent independent variables *T1, T2, T3*, and *T4*.
Table 4. The regression function for the thickness of the plastic deformation zone, Gpl, change in microhardness ,ΔHV, and ratio of thickness of the plastic deformation zones to thickness of alloyed zone Gpl/Gla of hybrid treated steel 304.
Multiple correlation coefficients of the equations are high and the relation between the studied properties is significant. For all equations, the condition *F > Fkr* is fulfilled. For the first and third equations all the factors are significant *t > tkr* at confidence level α = 0.1. Only in the case of the function 2 which shows the relation of the microhardness increase the factor *T2* describing influence of feed rate is insignificant for the assumed level of confidence.
Figure 8 shows the graphical interpretation of the thickness of strain hardening depending on the rotational speed of the burnishing head and the distance between the tool from the axis of the laser beam for fixed feed rate, *Vf* = 360 mm/s, according to the relation 1, Table 4. The strain hardening thickness almost linearly increases with speed growing and decreases with increasing distance from the axis. With increasing of the head rotational speed the intensity of the burnishing process and the impact forces of micro-hammers on machined surface increases. It induced an enlargement of the plastic deformation depth of the material in the entire range of temperatures applied in the burnishing. The effect of temperature on the depth of the plastic deformation zone is stronger for raised values of the impact forces and higher intensity of the burnishing process.
Fig. 8. Effect of rotational speed and the distance between laser beam and the head on the thickness plastic deformation zone at fixed feed rate, *Vf* = 360 mm/min.
Table 4 contains the value of multiple correlation coefficients *R*, the value of the Fisher's number *F* and *T*-Student coefficient describing the significance of subsequent independent
> Relation R F T1 T2 T3 1 0.78 8.4 3.15 1.5 3.59 2 0.68 4.6 2.65 0.85 2.45 3 0.74 6.3 2.81 1.46 2.97
Table 4. The regression function for the thickness of the plastic deformation zone, Gpl, change in microhardness ,ΔHV, and ratio of thickness of the plastic deformation zones to
describing influence of feed rate is insignificant for the assumed level of confidence.
Multiple correlation coefficients of the equations are high and the relation between the studied properties is significant. For all equations, the condition *F > Fkr* is fulfilled. For the first and third equations all the factors are significant *t > tkr* at confidence level α = 0.1. Only in the case of the function 2 which shows the relation of the microhardness increase the factor *T2*
Figure 8 shows the graphical interpretation of the thickness of strain hardening depending on the rotational speed of the burnishing head and the distance between the tool from the axis of the laser beam for fixed feed rate, *Vf* = 360 mm/s, according to the relation 1, Table 4. The strain hardening thickness almost linearly increases with speed growing and decreases with increasing distance from the axis. With increasing of the head rotational speed the intensity of the burnishing process and the impact forces of micro-hammers on machined surface increases. It induced an enlargement of the plastic deformation depth of the material in the entire range of temperatures applied in the burnishing. The effect of temperature on the depth of the plastic deformation zone is stronger for raised values of the impact forces
Fig. 8. Effect of rotational speed and the distance between laser beam and the head on the
thickness plastic deformation zone at fixed feed rate, *Vf* = 360 mm/min.
0.2 X -1.09 (3)
Gpl/Gla = 0.00035 Vrev1.02 Vf
thickness of alloyed zone Gpl/Gla of hybrid treated steel 304.
and higher intensity of the burnishing process.
variables *T1, T2, T3*, and *T4*.
Figure 9 shows the relation between the increase in material microhardness at the depth of 0.2 mm and the rotational speed of the head, and the distance from the tool axis to the laser beam at constant feed rate 360 mm/min. The microhardness growth, characterizing the degree of strain hardening at the surface, depends mainly on the rotational speed of the head and the temperature of the process zone, which is a function of the distance between the burnishing tool and the laser beam.
Fig. 9. Dependence of microhardness increase at a depth of 0.2 mm on rotational speed of the head and distance between the tool axis *X* and the laser beam, for fixed feed rate 360 mm/min.
Figure 10 shows the influence of the rotational speed of the head and the distance of the tool from surface, at a fixed feed rate *Vf* = 360 mm/min, on the *Gpl/Gla* parameter. It is evident that at the highest speed and the distances of less than 9 mm the ratio is bigger than 1. At speeds below 4000 rev/min and the distance of less than 8 mm the thickness of the strainhardening zone in relation to the thickness of the alloyed layer is less than 0.5, although it does not ensure the presence of compressive stresses in the entire alloyed zone.
Fig. 10. Influence of rotational speed and the distance between the beam axis and the tools on the ratio of the thickness of strain hardening zone to the thickness of alloyed zone *Gpl/Gla* at a fixed feed rate *Vf* = 360 mm/min.
Application of Laser-Burnishing Treatment for Improvement of Surface Layer Properties 267
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 mm
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4 mm
Profile # 1 / 11 Pt = 33.6 µm Scale = 100 µm
Profile # 1 / 16 Pt = 38 µm Scale = 100 µm
Fig. 11. The surface topography views of the laser path and section of the central part of path, and perpendicular and parallel profiles to the laser path and: a – laser alloyed surface,
b – after hybrid treatment.
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 mm
Length = 4.66 mm Pt = 255 µm Scale = 400 µm µm
µm
µm
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2 mm
Length = 3.23 mm Pt = 168 µm Scale = 300 µm µm
a
a
b
#### **4. Surface roughness after hybrid treatment**
#### **4.1 Surface topography**
The study showed that of the laser alloying combined with the burnishing can provide reduction of surface roughness in relation to the heights obtained in the laser melting process. The shape of the melted zone has improved as well. In the case of profile parameters cut-off 0.8 was used for all surfaces. In Figure 11a the examples of the surface topography views of the laser path and section of the central part of the path are shown. Below the profiles perpendicular and parallel to the direction of movement are visible. Figure 11b presents geometrical features of hybrid treated surface. Table 5 shows the 3D parameters *Sa, Sz* , their changes compared to the parameters after laser alloying *Sa(la)/Sa(h), Sz(la)/Sz(h)* and the 2D roughness parameters *Ra, RSm* of the surface which underwent hybrid treatment.
A change in shape of the melted zone due to the burnishing is noticeable. The heights of asperities at the alloying zone boundary decreased significantly, while the shape of asperities in the central area of the melting underwent "flattening" in comparison with only alloyed material.
Table 5. Surface topography and roughness parameters after the hybrid treatment.
The study showed that of the laser alloying combined with the burnishing can provide reduction of surface roughness in relation to the heights obtained in the laser melting process. The shape of the melted zone has improved as well. In the case of profile parameters cut-off 0.8 was used for all surfaces. In Figure 11a the examples of the surface topography views of the laser path and section of the central part of the path are shown. Below the profiles perpendicular and parallel to the direction of movement are visible. Figure 11b presents geometrical features of hybrid treated surface. Table 5 shows the 3D parameters *Sa, Sz* , their changes compared to the parameters after laser alloying *Sa(la)/Sa(h), Sz(la)/Sz(h)* and the 2D
A change in shape of the melted zone due to the burnishing is noticeable. The heights of asperities at the alloying zone boundary decreased significantly, while the shape of asperities in the central area of the melting underwent "flattening" in comparison with only
Process parameters 3D topography parameters 2D roughness
[µm] Sa(la)/Sa(h) S z(la) /S z(h)
Sz
1 4200 230 6 8.16 60.6 2,70 1,85 2.68 0.195 2 5950 230 6 11.1 87.6 2,00 1,28 4.83 0.201 3 4200 570 6 8.64 81.2 3,00 1,60 4.36 0.203 4 5950 570 6 10.1 77.6 2,56 1,68 3.95 0.19 5 4200 230 8.5 8.67 61.6 2,54 1,82 1.62 0.251 6 5950 230 8.5 11.4 73 1,95 1,53 1.79 0.242 7 4200 570 8.5 7.61 57.6 3,40 2,26 1.67 0.226 8 5950 570 8.5 8.4 52.8 3,08 2,46 1.87 0.282 9 3500 360 7 6.16 44.6 3,90 2,80 1.71 0.231 10 7100 360 7 9.01 66.4 2,66 1,88 2.9 0.25 11 5000 150 7 13.3 82.4 2,49 1,61 3.58 0.238 12 5000 900 7 9.93 80.2 3,55 1,90 4.44 0.207 13 5000 360 5 10.6 86.4 2,26 1,45 4.3 0.213 14 5000 360 10 7.96 51.2 3,02 2,44 0.87 0.28 15 5000 360 7 9.44 67.8 2,54 1,84 2.22 0.221 16 5000 360 7 12.1 68.6 1,98 1,82 2.44 0.219 17 5000 360 7 14.8 95 1,62 1,32 2.93 0.235 18 5000 360 7 5.97 45.6 4,02 2,74 2.03 0.234 19 5000 360 7 5.81 49.6 4,13 2,52 1.69 0.212 20 5000 360 7 7.84 57.6 3,06 2,17 2.24 0.245
parameters
RSm [mm]
Ra [µm]
roughness parameters *Ra, RSm* of the surface which underwent hybrid treatment.
Sa [µm]
Table 5. Surface topography and roughness parameters after the hybrid treatment.
**4. Surface roughness after hybrid treatment**
**4.1 Surface topography**
alloyed material.
Vrev [rev/min]
Vf [mm/min]
X [mm]
No
Fig. 11. The surface topography views of the laser path and section of the central part of path, and perpendicular and parallel profiles to the laser path and: a – laser alloyed surface, b – after hybrid treatment.
Application of Laser-Burnishing Treatment for Improvement of Surface Layer Properties 269
When analyzing the influence of process parameters on roughness sampling, *RSm,* a good correlation was also stated; *R=0.82, F=11.3.*Polynomial function, for which all the coefficients
RSm = 0.085-0.000004 Vrev+ 0.00002 Vf + 0.017 X (5) With an increase in the rotational speed of the head the roughness spacing decreases, while the increasing distance between the head and the laser beam and in the feed rate, causes lowering of *RSm*. Analysis of the equation shows that the rise of temperature of the burnishing process affects the spatial property of surface geometrical structure increasing
The analysis of the parameter *Sz(la)/Sz(h)* allows the assessment of the degree of plastic
Sz(la)/Sz(h) = 3.26+0.00039Vrev-0.00026 Vf-0.29 X (6) The biggest change in the asperity magnitude is obtained using the highest rotational speeds of the burnishing head, small feed rates and small *X* distances. At these parameters the large
The influence of the rotational speed of the head, as well as of the distance between the tool axle and the laser beam axis, on the parameters *Sz(la)/Sz(h),* for constant value of feed speed
Fig. 13. The influence of rotational speed of the head, *Vrev*, and the *X* distance on changes of
The erosive tests were performed for sample 15 after laser–mechanical treatment carried at parameters presented in Table 4. It was stated that the increase of hardness compared to only laser alloyed Stellite layer was 70% while depth of plastic deformation was about 0.5
the surface topography parameter *Sz(la)/Sz(h)* for fixed feed rate *Vf* =360 mm/min.
forces of micro-hammer impact on surface and high temperature of treatment occur.
of the equation are significant at the confidence level of 0.1, shows good fit.
deformation of surface asperities due to the micro-hammering.
the distance between micro-asperities.
*Vf* =360 mm/min, is shown in Figure 13.
**5. Surface layer properties**
**5.1 Wear resistance**
#### **4.2 Correlations between roughness and treatment parameters**
The analysis of the influence of process parameters (*X, Vf, Vrev*) on roughness parameters *Ra, RSm* and changes on topography parameters *Sz/Szh* was done, a good correlation was also stated.
The following relation between *Ra* and the treatment parameters was found:
$$\text{Ra} = 6 + 0.00037 \text{V}\_{\text{rev}} + 0.0017 \text{V}\_{\text{I}} \text{-} 0.76 \text{X} \tag{4}$$
Multiple correlation coefficients, describing the relation between *Ra* and the parameters of hybrid treatment are high *R=0.83*. The dependence between studied properties is significant: *F=12.1, F > Fkr*. All coefficients are significant at the accepted level of confidence. Figure 3 shows a graphical interpretation of polynomial function that describes the relation between *Ra* and the rotational speed of the burnishing head, *Vrev*, and the distance between the tool axle and the axis of the laser beam, *X*, for fixed feed rate *Vf* = 360 mm/s. The parameter *Ra* grows with increasing, *Vrev*, *Vf,* and it lowers with the *X* distance increasing. With rise of the rotational speed of head, *Vrev,* the intensity of the burnishing process, and the forces of impact of micro-hammers on machined surface, grow. The increase in feed rate reduces temperature and intensity of the process of burnishing that means the number of microhammer strokes per unit area, leading to roughness asperity enlargement. The increasing of the *X* distance causes temperature reduces in the zone of mechanical treatment. This is also associated with the lowering of plastic properties of material, smaller plastic deformations of surface asperities and increase of the height parameter, *Ra*. high temperature of treatment occur.
The influence of the rotational speed of the head, as well as of the distance between the tool axle and the laser beam axis, on the selected surface topography parameters, *Ra* for constant value of feed speed *Vf* =360 mm/min, are shown in Figure 12.
Fig. 12. The influence of the rotational speed of the head, *Vrev*, and the distance between tool axle and laser beam axis, *X*, on roughness parameter *Ra* for fixed feed rate *Vf* = 360 mm/min.
When analyzing the influence of process parameters on roughness sampling, *RSm,* a good correlation was also stated; *R=0.82, F=11.3.*Polynomial function, for which all the coefficients of the equation are significant at the confidence level of 0.1, shows good fit.
$$\text{RSm} = 0.085 \text{-0.000004 V}\_{\text{rev}} + 0.00002 \text{ V}\_{\text{f}} + 0.017 \text{ X} \tag{5}$$
With an increase in the rotational speed of the head the roughness spacing decreases, while the increasing distance between the head and the laser beam and in the feed rate, causes lowering of *RSm*. Analysis of the equation shows that the rise of temperature of the burnishing process affects the spatial property of surface geometrical structure increasing the distance between micro-asperities.
The analysis of the parameter *Sz(la)/Sz(h)* allows the assessment of the degree of plastic deformation of surface asperities due to the micro-hammering.
$$\text{Sz}\_{\text{(lb)}/\text{Sz}\_{\text{(lb)}}} = 3.26 \pm 0.00039 \text{V}\_{\text{rev}} \text{-0.00026 V} \text{-0.29 X} \tag{6}$$
The biggest change in the asperity magnitude is obtained using the highest rotational speeds of the burnishing head, small feed rates and small *X* distances. At these parameters the large forces of micro-hammer impact on surface and high temperature of treatment occur.
The influence of the rotational speed of the head, as well as of the distance between the tool axle and the laser beam axis, on the parameters *Sz(la)/Sz(h),* for constant value of feed speed *Vf* =360 mm/min, is shown in Figure 13.
Fig. 13. The influence of rotational speed of the head, *Vrev*, and the *X* distance on changes of the surface topography parameter *Sz(la)/Sz(h)* for fixed feed rate *Vf* =360 mm/min.
#### **5. Surface layer properties**
#### **5.1 Wear resistance**
268 CO2 Laser – Optimisation and Application
The analysis of the influence of process parameters (*X, Vf, Vrev*) on roughness parameters *Ra, RSm* and changes on topography parameters *Sz/Szh* was done, a good correlation was also
Ra = 6+0.00037Vrev + 0.0017Vf - 0.76X (4) Multiple correlation coefficients, describing the relation between *Ra* and the parameters of hybrid treatment are high *R=0.83*. The dependence between studied properties is significant: *F=12.1, F > Fkr*. All coefficients are significant at the accepted level of confidence. Figure 3 shows a graphical interpretation of polynomial function that describes the relation between *Ra* and the rotational speed of the burnishing head, *Vrev*, and the distance between the tool axle and the axis of the laser beam, *X*, for fixed feed rate *Vf* = 360 mm/s. The parameter *Ra* grows with increasing, *Vrev*, *Vf,* and it lowers with the *X* distance increasing. With rise of the rotational speed of head, *Vrev,* the intensity of the burnishing process, and the forces of impact of micro-hammers on machined surface, grow. The increase in feed rate reduces temperature and intensity of the process of burnishing that means the number of microhammer strokes per unit area, leading to roughness asperity enlargement. The increasing of the *X* distance causes temperature reduces in the zone of mechanical treatment. This is also associated with the lowering of plastic properties of material, smaller plastic deformations of surface asperities and increase of the height parameter, *Ra*. high temperature of treatment
The influence of the rotational speed of the head, as well as of the distance between the tool axle and the laser beam axis, on the selected surface topography parameters, *Ra* for constant
Fig. 12. The influence of the rotational speed of the head, *Vrev*, and the distance between tool
axle and laser beam axis, *X*, on roughness parameter *Ra* for fixed feed rate
value of feed speed *Vf* =360 mm/min, are shown in Figure 12.
**4.2 Correlations between roughness and treatment parameters**
The following relation between *Ra* and the treatment parameters was found:
stated.
occur.
*Vf* = 360 mm/min.
The erosive tests were performed for sample 15 after laser–mechanical treatment carried at parameters presented in Table 4. It was stated that the increase of hardness compared to only laser alloyed Stellite layer was 70% while depth of plastic deformation was about 0.5
Application of Laser-Burnishing Treatment for Improvement of Surface Layer Properties 271
The results of contact strain and contact stiffness are shown in Table. The analysis of the total value of elastic and plastic strain shows that the surfaces after the burnishing undergo significantly lower elastic and plastic deformation in contact with counter sample. For samples without grinding this effect is associated primarily with surface topography. In the process of burnishing the rebuilding of surface topography occurs: surface irregularities have smaller heights, while their shapes become preferable. Therefore, the real contact area is increased and the unit pressure on individual asperities is reduced. At nominal pressure of 270 MPa the plastic deformation is 88 μm for samples that were only alloyed, whereas the burnished surface is deformed by 52 μm in height. It is the effect of more profitable
Grinding process has provided similar geometric structure: the surface heights and the shape of asperities are similar for all samples. In this case, examining the process of deformation of the surface under the influence of contact pressure, the differences in elastic
> a2 [µm]
Laser alloying & grinding 40 28 12 6.75 Hybrid & grinding 24 14 10 11.25 Plasma sprayed 57 36 27 4.74 Plasma sprayed & grinding 40 28 12 7.11 Table 7. The contact strain and the contact stiffness for nominal pressures 270 MPa. The hybrid treatment parameters: *P*=2 kW, *d*=3mm *Vf* =360 mm/min, *Vrev*=5000 rev/min,
The proposed new type of hybrid treatment, using the head for dynamic burnishing and applying the oscillatory motion, has lead to large plastic deformation of surface layer of steel 304 alloyed with Stellite 6. Application of the new head with two rows of hammers highly intensified the process and enabled the burnishing at various temperatures in one operation. The forces of the micro-hammer impact were changing in a wide range. The introduction of the oscillations increased the width of the melted zone by the value of the oscillation amplitude, provided more uniform surface plastic deformation, and also allowed for obtained favourable surface topography. The design of the head allowed the treatment in high as well as low temperature in the single pass. The temperature of the burnishing was around 850-200 K, depending on treatment parameters. The studies of residual stresses have shown the temperature in the zone of plastic deformation is sufficient for transforming the tensile stresses into compressive once at the material surface even at the highest temperature
Laser alloying 138 90 88 3.38 Hybrid 80 28 52 1.96
Contact strain at nominal pressure 270 MPa
> a2e [µm]
a2pl
Contact stiffness
[µm] q=270 [MPa]
geometric structure and the strengthening of surface layer due to the burnishing.
and plastic properties of the surface layer material can be established.
Treatment
*X*=7 mm.
**6. Discussions**
of the burnishing.
**5.2 Contact stiffness**
mm. The tests were done also for the plasma sprayed Stellite 6 layer and the layer formed by laser alloying at feed rate 360 mm/min.
Fig. 14. The profiles after erosive wear test: PS-plasma sprayed Stellite 6 layer, LA- laser alloyed layer (*P*=2 kW, *d*=3 mm *Vf* =360 mm/min), Hybrid - sample 13 after laser-mechanical treatment (*P*=2 kW, *d*=2.5 mm *Vf* =360 mm/min, *Vrev* = 5000 rev/min, *X* = 5 mm).
Figure 5 shows exemplary surface profiles after erosive wear after laser-mechanical treatment, laser alloying and plasma sprayed Stellite 6 layer. The eroded surface is visible in the middle. The non-eroded side material parts were used as the reference surface. The profiles of examined surfaces show significant differences in erosive depth. The maximal depth was smaller than the thickness of tested layers. The measurement of surface topography was performed on three selected areas and the medium depth and volume per 1 mm2 was chosen for quantitative analysis. This allowed compare erosive wear resistance of different surface layers of Stellite 6. Table 6 contains the average value of depth and volume of eroded surfaces whereas Figure 5 presents average eroded materials volumes loss.
Among Stellite 6 layers produced by plasma spraying, laser alloying and laser alloying combined with burnishing; the surfaces after hybrid treatment had the highest resistance to slurry erosive wear. The examined surfaces after laser-mechanical treatment showed the average depth of erosive loss 16.2µm. Surfaces after laser alloying showed little less of erosive resistance, 13-30 %, than hybrid treated surfaces. Their erosive depth was 22 µm.
The largest erosive loss was stated for plasma sprayed Stellite 6 layer, for which the average depth was 82 µm. Its erosive resistance is five times worse than the one of hybrid treated layer.
Table 6. The average depth and volume of erosive losses of Stellite 6 layer after slurry erosive test
#### **5.2 Contact stiffness**
270 CO2 Laser – Optimisation and Application
mm. The tests were done also for the plasma sprayed Stellite 6 layer and the layer formed by
Fig. 14. The profiles after erosive wear test: PS-plasma sprayed Stellite 6 layer, LA- laser alloyed layer (*P*=2 kW, *d*=3 mm *Vf* =360 mm/min), Hybrid - sample 13 after laser-mechanical
of eroded surfaces whereas Figure 5 presents average eroded materials volumes loss.
Among Stellite 6 layers produced by plasma spraying, laser alloying and laser alloying combined with burnishing; the surfaces after hybrid treatment had the highest resistance to slurry erosive wear. The examined surfaces after laser-mechanical treatment showed the average depth of erosive loss 16.2µm. Surfaces after laser alloying showed little less of erosive resistance, 13-30 %, than hybrid treated surfaces. Their erosive depth was 22 µm.
The largest erosive loss was stated for plasma sprayed Stellite 6 layer, for which the average depth was 82 µm. Its erosive resistance is five times worse than the one of hybrid treated
[µm]
Laser alloying 22.2 0.0992 Hybrid treatment 16.2 0.0699 Plasma spraying 81.9 0.3621
Table 6. The average depth and volume of erosive losses of Stellite 6 layer after slurry
The volume of loss [mm3]
Treatment The depth of erosive loss
Figure 5 shows exemplary surface profiles after erosive wear after laser-mechanical treatment, laser alloying and plasma sprayed Stellite 6 layer. The eroded surface is visible in the middle. The non-eroded side material parts were used as the reference surface. The profiles of examined surfaces show significant differences in erosive depth. The maximal depth was smaller than the thickness of tested layers. The measurement of surface topography was performed on three selected areas and the medium depth and volume per 1 mm2 was chosen for quantitative analysis. This allowed compare erosive wear resistance of different surface layers of Stellite 6. Table 6 contains the average value of depth and volume
treatment (*P*=2 kW, *d*=2.5 mm *Vf* =360 mm/min, *Vrev* = 5000 rev/min, *X* = 5 mm).
laser alloying at feed rate 360 mm/min.
layer.
erosive test
The results of contact strain and contact stiffness are shown in Table. The analysis of the total value of elastic and plastic strain shows that the surfaces after the burnishing undergo significantly lower elastic and plastic deformation in contact with counter sample. For samples without grinding this effect is associated primarily with surface topography. In the process of burnishing the rebuilding of surface topography occurs: surface irregularities have smaller heights, while their shapes become preferable. Therefore, the real contact area is increased and the unit pressure on individual asperities is reduced. At nominal pressure of 270 MPa the plastic deformation is 88 μm for samples that were only alloyed, whereas the burnished surface is deformed by 52 μm in height. It is the effect of more profitable geometric structure and the strengthening of surface layer due to the burnishing.
Grinding process has provided similar geometric structure: the surface heights and the shape of asperities are similar for all samples. In this case, examining the process of deformation of the surface under the influence of contact pressure, the differences in elastic and plastic properties of the surface layer material can be established.
Table 7. The contact strain and the contact stiffness for nominal pressures 270 MPa. The hybrid treatment parameters: *P*=2 kW, *d*=3mm *Vf* =360 mm/min, *Vrev*=5000 rev/min, *X*=7 mm.
#### **6. Discussions**
The proposed new type of hybrid treatment, using the head for dynamic burnishing and applying the oscillatory motion, has lead to large plastic deformation of surface layer of steel 304 alloyed with Stellite 6. Application of the new head with two rows of hammers highly intensified the process and enabled the burnishing at various temperatures in one operation. The forces of the micro-hammer impact were changing in a wide range. The introduction of the oscillations increased the width of the melted zone by the value of the oscillation amplitude, provided more uniform surface plastic deformation, and also allowed for obtained favourable surface topography. The design of the head allowed the treatment in high as well as low temperature in the single pass. The temperature of the burnishing was around 850-200 K, depending on treatment parameters. The studies of residual stresses have shown the temperature in the zone of plastic deformation is sufficient for transforming the tensile stresses into compressive once at the material surface even at the highest temperature of the burnishing.
Application of Laser-Burnishing Treatment for Improvement of Surface Layer Properties 273
3. The decisive influence on the thickness of the plastic deformation zone has temperature of the material in the region of burnishing. Thickness from 0.25 to 1.2 mm of the strain hardened zone was determined depending on the applied parameters of hybrid treatment. It enables the use of the hybrid treatment for the majority of layers produced
4. The residual stress measurements showed the change in stresses within the melting zone from tensile stresses after the laser alloying to compressive ones after the hybrid
5. The hybrid treatment causes an increase in surface smoothness compared to the laser alloying. More than a threefold decrease in the average height of roughness, *Sa*, due to
6. The increase of the contact stiffness in relation to the laser alloying and the Stellite 6
7. The better slurry erosive wear resistance than for the plasma sprayed layer and the
8. The study of the correlation of treatment parameters with the state of surface layer showed that the dependences between the investigated properties are significant. Therefore, the controlling the hybrid treatment is possible and its industrial
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**8. References**
1648
ISSN 1089-7550
1220, ISSN 0267-0836
0043-1648
147, pp. 351-358, ISSN 0924-0136
the burnishing process, was observed.
applications are recommended.
The results indicate that due to the burnishing at high temperature large plastic deformation of surface layer is possible to be obtain, without cracks and other defects of loosen structure that are characteristic for the classical burnishing of hard and brittle materials.
Good correlation between the process parameters and the features of plastic deformation zones and surface roughness was found. It enables controlling of the hybrid treatment. The variable degree of plastic deformation, strain hardening and thickness of plastic deformation zone can be governed by the controlling the impact force of micro-hammers on the surface and the temperature of metal in the treated zone. The application of high temperatures lowers the hardness while the plasticity of the material undergoes increasing, which in turn provides greater degree of plastic deformation and strain hardening of material. The hardness, depth of strain hardening zone increased, and residual stresses changed. Further increase of plastic deformation is possible to be obtained by the use of higher forces of micro-hammer impact on the surface. For the tested material the strain hardening, 32-100%, at the surface was obtained. Despite high degree of deformation there were no cracks no spallings. Fine grain material, homogeneous chemical and phase composition, was found.
The thickness of the strain hardened zone varying from 0.25 to 1.2 mm was obtained. This range is similar to the thickness of the typical mostly produced by LBM alloyed and cladding layers. It was found that with proper selection of process parameters it is possible to obtain the depth of strain hardening zone greater than the depth of the melting. This ensures the presence of compressive stress across the alloyed layer and provides greater durability, especially of parts subjected to fatigue during operation life.
The results of measurement of surface topography, contact stiffness and slurry erosive wear showed that laser-mechanical treatment allows the attainment better properties than the plasma sprayed and laser alloyed Stellite 6 layers have, and it can be used in specific industrial applications.
On the basis of regression equations and well-known effect of temperature on plastic properties of the material, it is possible to select parameters of the hybrid treatment in order to obtain the expected degree and thickness of the plastic deformation zone for other materials and layers. The hybrid treatment enables to combine in a single operation the advantages of laser treatment in the form of preferred microstructure, good adhesion, beneficial chemical composition of the surface layer and burnishing treatment. It ensures the increase of material hardness, improved surface topography and favourable compressive stresses of formed layer.
#### **7. Conclusion**
#### **8. References**
272 CO2 Laser – Optimisation and Application
The results indicate that due to the burnishing at high temperature large plastic deformation of surface layer is possible to be obtain, without cracks and other defects of loosen structure
Good correlation between the process parameters and the features of plastic deformation zones and surface roughness was found. It enables controlling of the hybrid treatment. The variable degree of plastic deformation, strain hardening and thickness of plastic deformation zone can be governed by the controlling the impact force of micro-hammers on the surface and the temperature of metal in the treated zone. The application of high temperatures lowers the hardness while the plasticity of the material undergoes increasing, which in turn provides greater degree of plastic deformation and strain hardening of material. The hardness, depth of strain hardening zone increased, and residual stresses changed. Further increase of plastic deformation is possible to be obtained by the use of higher forces of micro-hammer impact on the surface. For the tested material the strain hardening, 32-100%, at the surface was obtained. Despite high degree of deformation there were no cracks no spallings. Fine grain material, homogeneous chemical and phase
The thickness of the strain hardened zone varying from 0.25 to 1.2 mm was obtained. This range is similar to the thickness of the typical mostly produced by LBM alloyed and cladding layers. It was found that with proper selection of process parameters it is possible to obtain the depth of strain hardening zone greater than the depth of the melting. This ensures the presence of compressive stress across the alloyed layer and provides greater
The results of measurement of surface topography, contact stiffness and slurry erosive wear showed that laser-mechanical treatment allows the attainment better properties than the plasma sprayed and laser alloyed Stellite 6 layers have, and it can be used in specific
On the basis of regression equations and well-known effect of temperature on plastic properties of the material, it is possible to select parameters of the hybrid treatment in order to obtain the expected degree and thickness of the plastic deformation zone for other materials and layers. The hybrid treatment enables to combine in a single operation the advantages of laser treatment in the form of preferred microstructure, good adhesion, beneficial chemical composition of the surface layer and burnishing treatment. It ensures the increase of material hardness, improved surface topography and favourable compressive
1. The hybrid treatment with the new dynamic burnishing head provides the extended range of plastic deformation of surface layer of steel 304, alloyed with Stellite 6. The increase in microhardness, caused by surface strain hardening, was 32-100% depending on the impact forces of micro-hammers on the surface and the temperature of the metal
2. Due to the treatment at high temperature, despite the high degree of plastic deformation, no cracking and spallings or other phenomena proving loosen
durability, especially of parts subjected to fatigue during operation life.
that are characteristic for the classical burnishing of hard and brittle materials.
composition, was found.
industrial applications.
stresses of formed layer.
in the treated zone.
microstructure of the material were recognized.
**7. Conclusion**
**10**
*Brazil*
**Covering with Carbon Black**
G. Vasconcelos1, D. C. Chagas1 and A. N. Dias2
*1Institute of Advanced Studies - IEAv, EFO-L, S. J. dos Campos, SP, 2University of Vale do Paraíba - UNIVAP, S. J. dos Campos, SP*
**of AISI 4340 Steel**
**and Thermal Treatment by CO2 Laser Surfaces**
The application of photo-absorbing coatings is a common practice, especially when lasers of low density power are used. These materials normally MoS2, graphite and carbon black, futher the coupling of incident radiation, reducing the losses by reflection, common to the
In a previous work, using graphite coatings, it was observed that part of the coating, after irradiation, remained on the metal surface. In pin on disc tests, it was observed a reduction in the coefficient of friction surface with this coating. REIS, J. L., (2009) improvement on the surface hardness, even using laser low of energy density power. This hardenning process was attributed to better coupling in the region of beam interaction with the metal surface. The laser hardening consists in heating and rapid cooling the steel surface. If the power density is enough, a layer on the steel surface will reach the austenitizing temperature (during heating) and then with rapid cooling, place the formation of martensites (Ganeev, R. A., 2002). The depth of the surface treated is determined by the law of thermal conductivity, where the propagation of heat occurs in a region of higher temperature to a region of lower temperature (Benedeck, J.; Shachrai, A.; Levin, L., 1980). The laser hardening allows the hardening of specific areas with controlled depth and with minimal surface deformation when compared to other methods. It also promotes, improves the mechanical properties and fatigue resistance, attraction, wear (reducing the friction factor) and increased resistance to corrosion (Dohotre, N. B., 1998; Machado, I. F., 2006). This work will evaluate the use of carbon black to replace the graphite used in the work of REIS, J. L. 2009, to eliminate the stage of solution preparation, grinding mills at high
The steel used in this work is AISI 4340. Its chemical composition was assessed by the optical spectrometer Thermo Scientific, Model ARL 3460 OES Metals Analyzer, presented in
**1. Introduction**
energy.
Table 1.
**2. Methodology**
process, when CO2 lasers are used as radiation source.
### **Covering with Carbon Black and Thermal Treatment by CO2 Laser Surfaces of AISI 4340 Steel**
G. Vasconcelos1, D. C. Chagas1 and A. N. Dias2 *1Institute of Advanced Studies - IEAv, EFO-L, S. J. dos Campos, SP, 2University of Vale do Paraíba - UNIVAP, S. J. dos Campos, SP Brazil*
#### **1. Introduction**
274 CO2 Laser – Optimisation and Application
Milad, M., Zreiba, N., Elhalolouani, F., Baradai, C. (2008). Effect of cold work on structure
Montgomery D.C. (1991). *Design and analyses of experiments,* ISBN 0471520004, 3th ed. N.Y.,
Nowicki, B. (2007). *The dynamic micro-hammering head for surface metals treatment*, Patent No P
Radziejewska, J. (2006). Surface layer morphology due to laser alloying process. *J of*
Radziejewska J., Skrzypek, S. (2009). *J of Materials Processing Technology*, Vol. 209, pp. 2047–
Radziejewska J., Skrzypek, S. (2009). Microstructure and residual stresses in surface layer of
Radziejewska J. (2011). Influence of laser-mechanical treatment on surface topography,
Robinson, J.R., Van Brussel, A.B., De Hosson, J. Th.M., Reed, R.C. (1996). X ray measurement
Sai W. Bouzid, J.L. Lebbrun, J. (2002). *J of Materials Engineering and Performance,* Vol. 12, pp.
Shiou F-J., Chen Ch-H., (2003). Freeform surface finish of plastic injection mold by using
Shiou F-J., Hsu Ch-C., (2008). Surface finishing of hardened and tempered stainless tool steel
Tsai, C.-H. , Ou, C.-H. (2004). Machining a smooth surface ceramic material by laser fracture
Tian, Y., Shin, Y.C. (2007). Laser-assisted burnishing of metals. *Int. J of Machine Tools and*
*Material Processing Technology*, Vol. 205, pp. 249-58, ISSN 0924-0136
*Manufacture,* Vol. 47(1), pp. 14-22, ISSN 0890-6955
*Technology*, Vol. 209, pp. 2047–2056, ISSN 0924-0136
Vol. A208, pp. 143-147, ISSN 0921-5093
Przybylski, W. (1986). *Burnishing technology*, ISBN 83-204-0742-7, WNT, Warsaw, Poland Radziejewska J., Kalita W., Bartoszewicz A. Modification of surface layer properties by laser
pp. 80-85, ISSN 0924-0136
Wien,
May 2005
2056
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37-40
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ISSN 0924-0136
377600, Poland
and properties of AISI stainless steel. *J of Material Processing Technology*, Vol. 303,
alloying combined with burnishing. *Proceedings of Laser Technologies in Welding and Materials Processing*, pp. 162-164, ISBN 966-8872-01-0 Katsiveli Crimea, Ukraine,
*Engineering Manufacture Part B, Proc. IMechE.,* Vol. 220, pp. 447-454, ISSN 0954-4054
simultaneously laser alloyed and burnished steel. *J of Materials Processing*
erosive wear and contact stiffness. *Materials and Design,* Vol.32 pp. 5073-5081, ISSN
of residual stresses in laser melted Ti-6Al-V alloy, *Material Science and Engineering,*
ball-burnishing process. *J of Materials Processing Technology*, Vol. 140, pp. 248–254,
using ball grinding, ball burnishing and polishing process on machine centre. *J of*
machining technique*. Materials Processing Technology,* Vol. 155–156, pp. 1797–1804,
The application of photo-absorbing coatings is a common practice, especially when lasers of low density power are used. These materials normally MoS2, graphite and carbon black, futher the coupling of incident radiation, reducing the losses by reflection, common to the process, when CO2 lasers are used as radiation source.
In a previous work, using graphite coatings, it was observed that part of the coating, after irradiation, remained on the metal surface. In pin on disc tests, it was observed a reduction in the coefficient of friction surface with this coating. REIS, J. L., (2009) improvement on the surface hardness, even using laser low of energy density power. This hardenning process was attributed to better coupling in the region of beam interaction with the metal surface. The laser hardening consists in heating and rapid cooling the steel surface. If the power density is enough, a layer on the steel surface will reach the austenitizing temperature (during heating) and then with rapid cooling, place the formation of martensites (Ganeev, R. A., 2002). The depth of the surface treated is determined by the law of thermal conductivity, where the propagation of heat occurs in a region of higher temperature to a region of lower temperature (Benedeck, J.; Shachrai, A.; Levin, L., 1980). The laser hardening allows the hardening of specific areas with controlled depth and with minimal surface deformation when compared to other methods. It also promotes, improves the mechanical properties and fatigue resistance, attraction, wear (reducing the friction factor) and increased resistance to corrosion (Dohotre, N. B., 1998; Machado, I. F., 2006). This work will evaluate the use of carbon black to replace the graphite used in the work of REIS, J. L. 2009, to eliminate the stage of solution preparation, grinding mills at high energy.
#### **2. Methodology**
The steel used in this work is AISI 4340. Its chemical composition was assessed by the optical spectrometer Thermo Scientific, Model ARL 3460 OES Metals Analyzer, presented in Table 1.
Covering with Carbon Black and Thermal Treatment by CO2 Laser Surfaces of AISI 4340 Steel 277
oxidation. Figure 3 shows the laser used and Figure 4, the diagram of the experimental set
Fig. 2. Histogram of particle size distribution of carbon black.
Fig. 3. Experimental set-up. CO2 laser. Highlighted in red rectangular box, located
up of the treatment process.
galvanometric mirrors.
Table 1. Chemical composition of steel AISI 4340 -% mass
Carbon black is formed by fine particles obtained by the process of pyrolysis or partial combustion of hydrocarbon gases or liquids. These nano-particulate structure, favors the coating with thin layers (Sector Report N 09, 1998). The shape of the particles was observed by scanning electron microscopy (SEM), Zeiss / EVO MA10, as shown in Figure 1.
Fig. 1. SEM of particles of carbon black (1320X)
The particle size of the lubricant can influence the thickness of the coating deposited and after irradiation with the laser beam. In order to determine the size distribution of particles, carbon black was subjected to particle size analysis through testing by laser diffraction (CILAS 1064L, range from 0.04 to 500μm). The results of this analysis are presented in Figure 2.
Samples of AISI 4340 steel with a thickness of 3mm and 20mm diameter, previously sanded (SiC paper 600), were coated with a solution prepared with 10g of carbon black and 0.1g of carboxilmetilcelulose in 100ml of ethanol.
This solution was mechanically mixed for 20 minutes in a plastic container with metal balls to the homogenization of the solution. Subsequently, the solution was sprayed with a pneumatic pistol on the surface of steel samples previously heated to 60°C. Then the samples are irradiated with a beam of CO2 laser (50W) and beam diameter of 300μm. In the region of action beam on the sample surfaces, we used a flow of nitrogen to prevent
Carbon black is formed by fine particles obtained by the process of pyrolysis or partial combustion of hydrocarbon gases or liquids. These nano-particulate structure, favors the coating with thin layers (Sector Report N 09, 1998). The shape of the particles was observed
The particle size of the lubricant can influence the thickness of the coating deposited and after irradiation with the laser beam. In order to determine the size distribution of particles, carbon black was subjected to particle size analysis through testing by laser diffraction (CILAS 1064L, range from 0.04 to 500μm). The results of this analysis are presented in
Samples of AISI 4340 steel with a thickness of 3mm and 20mm diameter, previously sanded (SiC paper 600), were coated with a solution prepared with 10g of carbon black and 0.1g of
This solution was mechanically mixed for 20 minutes in a plastic container with metal balls to the homogenization of the solution. Subsequently, the solution was sprayed with a pneumatic pistol on the surface of steel samples previously heated to 60°C. Then the samples are irradiated with a beam of CO2 laser (50W) and beam diameter of 300μm. In the region of action beam on the sample surfaces, we used a flow of nitrogen to prevent
by scanning electron microscopy (SEM), Zeiss / EVO MA10, as shown in Figure 1.
Table 1. Chemical composition of steel AISI 4340 -% mass
Fig. 1. SEM of particles of carbon black (1320X)
carboxilmetilcelulose in 100ml of ethanol.
*Steel 4340*
Figure 2.
*Fe C Mn Si Cr Ni Mo P S*
95.79 0.361 0.638 0.261 0.794 1.702 0.221 0.024 0.008
oxidation. Figure 3 shows the laser used and Figure 4, the diagram of the experimental set up of the treatment process.
Fig. 2. Histogram of particle size distribution of carbon black.
Fig. 3. Experimental set-up. CO2 laser. Highlighted in red rectangular box, located galvanometric mirrors.
Covering with Carbon Black and Thermal Treatment by CO2 Laser Surfaces of AISI 4340 Steel 279
The irradiated samples according to Table 2 were selected one that showed lower surface ablation and greater extension of the layer treated. These parameters were evaluated by measurements of roughness and optical microscopy (OM), respectively. Figure 6 shows a
Fig. 6. Cross section of the irradiated surface of the sample P2. Optical microscopy a) OM-
The average microhardness of AISI 4340 steel without heat treatment is 286 HV0,05. After the thermal treatment with CO2 laser, we can observe that the hardness of the material increased significantly, reaching an average of about 760 HV0,05. Figure 7 shows the microhardness profile of the cross section of the treated region. These results were obtained
**A B**
Fig. 7. Microhardness profile of cross section of the treated region after treatment via laser-
According to Figure 7, there is increased hardness, occurred due to surface hardening
**3. Results and discussions**
200X, b) OM-500X.
coated carbon black.
cross section of the treated region of the sample P2.
by means of microhardness Future-Tech / FM-700.
process, resulting in heating and cooling of the sample.
Fig. 4. Fitting of the experimental process. The laser beam is guided by a set of mirrors galvanometric controlled by software
The speed of scanning laser beam (mm/s), the resolution in pulses per inch (ppp) and number of heating cycles (NC) to be used in this experiment were selected from tests previously conducted (Chagas, D. C.; et al 2010). The Table 2 shows the parameters of the laser beam used in the treatment of the samples.
Fig. 5. Illustrates the layout of the treatment process, with laser beam, the samples previously coated with carbon black.
#### **3. Results and discussions**
278 CO2 Laser – Optimisation and Application
Fig. 4. Fitting of the experimental process. The laser beam is guided by a set of mirrors
**Samples Speed (mm/s) Resolution**
Fig. 5. Illustrates the layout of the treatment process, with laser beam, the samples
**Hardened layer**
Table 2. Parameters of laser used for surface hardening of AISI 4340.
**Coating of carbon black**
The speed of scanning laser beam (mm/s), the resolution in pulses per inch (ppp) and number of heating cycles (NC) to be used in this experiment were selected from tests previously conducted (Chagas, D. C.; et al 2010). The Table 2 shows the parameters of the
> P1 40 300 5 P2 60 300 5 P3 80 300 5
**(ppp)**
**Steel AlSi 4340**
**Laser beam**
**Steel AlSi 4340**
**Number of cicles**
galvanometric controlled by software
previously coated with carbon black.
laser beam used in the treatment of the samples.
The irradiated samples according to Table 2 were selected one that showed lower surface ablation and greater extension of the layer treated. These parameters were evaluated by measurements of roughness and optical microscopy (OM), respectively. Figure 6 shows a cross section of the treated region of the sample P2.
Fig. 6. Cross section of the irradiated surface of the sample P2. Optical microscopy a) OM-200X, b) OM-500X.
The average microhardness of AISI 4340 steel without heat treatment is 286 HV0,05. After the thermal treatment with CO2 laser, we can observe that the hardness of the material increased significantly, reaching an average of about 760 HV0,05. Figure 7 shows the microhardness profile of the cross section of the treated region. These results were obtained by means of microhardness Future-Tech / FM-700.
Fig. 7. Microhardness profile of cross section of the treated region after treatment via lasercoated carbon black.
According to Figure 7, there is increased hardness, occurred due to surface hardening process, resulting in heating and cooling of the sample.
Covering with Carbon Black and Thermal Treatment by CO2 Laser Surfaces of AISI 4340 Steel 281
The tests of atomic force microscopy (AFM) were performed to evaluate the morphology and surface topography. The results obtained by AFM indicate a possible crystallization of carbon black, coalescence of grains and the appearance of new phases from the process of
> 559.76 [nm]
0.00
2.00 um 5.00 x 5.00 um IEAv600\_5
Fig. 10. AFM image obtained by the carbon black surface after irradiation with lasers. The bright spots in the figure correspond to higher regions and the darker the lower regions,
In roughness tests obtained by AFM, there is heterogeneous across regions with different surface roughness, where in the region obtained a RA= 87.999nm and region B was obtained
The experiments conducted indicate that the use of carbon black is feasible, presenting results of microhardness, friction coefficient, tempera and extent of the treated layer, similar to results reported by REIS (2009), and also eliminates the grinding step, which is required
The use of nanoparticles of carbon black aids in the absorption of radiation incident on steel, influences the alteration of microstructure and promotes surface temperature of the surface
With the different laser parameters presented, it appears that high rates of speed in the beam of laser irradiation, the steel AISI 4340, showed no significant change in hardness, where the increase in hardness was 268 HV0.05 to 405 HV0.05. Unlike the lowest parameters that showed a homogeneous microstructure and with greater depths of layers treated, increasing by up to three times the surface hardness of steel, where the increase in hardness was 268 HV0.05 to 760 HV0.05. The extent of the treated layer, the homogeneity of the steel
In tribological test, it was observed that the uncoated sample has a higher coefficient of friction of 0.70 and the samples coated with carbon black and further treated with lasers,
and its microstructure can be controlled by varying the parameters of the laser.
according to the scale shown on the right side of the figure.
and still attached to the steel surface acting as a lubricant.
RA= 124.115nm.
**4. Conclusion**
when using graphite.
heating by laser, as shown in Figure 10.
The sample was subjected to tribological tests to evaluate the friction coefficient. The parameters used in the test were: linear velocity of 10cm/s, the track radius of 5mm, 52100 steel balls with 6mm diameter, number of rounds equal to 2000 and load of 5N. In this test, the sample is supported on a support rotation and pressed with a steel ball with known load, as shows the scheme of Figure 8.
Fig. 8. Schematic drawing of the tribological tests
Then, the sample is rotated to evaluate the friction and the results obtained from the tribological tests, are presented in Figure 9.
Fig. 9. Results of the friction coefficients of steel ball (52100) in 4340 and uncoated.
The tests of atomic force microscopy (AFM) were performed to evaluate the morphology and surface topography. The results obtained by AFM indicate a possible crystallization of carbon black, coalescence of grains and the appearance of new phases from the process of heating by laser, as shown in Figure 10.
Fig. 10. AFM image obtained by the carbon black surface after irradiation with lasers. The bright spots in the figure correspond to higher regions and the darker the lower regions, according to the scale shown on the right side of the figure.
In roughness tests obtained by AFM, there is heterogeneous across regions with different surface roughness, where in the region obtained a RA= 87.999nm and region B was obtained RA= 124.115nm.
#### **4. Conclusion**
280 CO2 Laser – Optimisation and Application
The sample was subjected to tribological tests to evaluate the friction coefficient. The parameters used in the test were: linear velocity of 10cm/s, the track radius of 5mm, 52100 steel balls with 6mm diameter, number of rounds equal to 2000 and load of 5N. In this test, the sample is supported on a support rotation and pressed with a steel ball with known
Then, the sample is rotated to evaluate the friction and the results obtained from the
Fig. 9. Results of the friction coefficients of steel ball (52100) in 4340 and uncoated.
load, as shows the scheme of Figure 8.
Fig. 8. Schematic drawing of the tribological tests
tribological tests, are presented in Figure 9.
The experiments conducted indicate that the use of carbon black is feasible, presenting results of microhardness, friction coefficient, tempera and extent of the treated layer, similar to results reported by REIS (2009), and also eliminates the grinding step, which is required when using graphite.
The use of nanoparticles of carbon black aids in the absorption of radiation incident on steel, influences the alteration of microstructure and promotes surface temperature of the surface and still attached to the steel surface acting as a lubricant.
With the different laser parameters presented, it appears that high rates of speed in the beam of laser irradiation, the steel AISI 4340, showed no significant change in hardness, where the increase in hardness was 268 HV0.05 to 405 HV0.05. Unlike the lowest parameters that showed a homogeneous microstructure and with greater depths of layers treated, increasing by up to three times the surface hardness of steel, where the increase in hardness was 268 HV0.05 to 760 HV0.05. The extent of the treated layer, the homogeneity of the steel and its microstructure can be controlled by varying the parameters of the laser.
In tribological test, it was observed that the uncoated sample has a higher coefficient of friction of 0.70 and the samples coated with carbon black and further treated with lasers,
**11**
*France*
Afia Kouadri-David
**Welding of Thin Light Alloys Sheets**
**by CO2 Laser Beam: Magnesium Alloys**
*PSM Team, European University of Brittany, France, INSA of Rennes, LGCGM*
Laser welding is an important joining technique for magnesium alloys with their increasing applications in aerospace, aircraft, automotive, electronics and other industries. In this document the research and progress in laser welding of magnesium alloys are critically reviewed from different perspectives. Some important laser processing parameters and their effects on weld quality are discussed. The microstructure, metallurgical defects and mechanical properties encountered in laser welding of magnesium alloys, such as porosity, grains size, crystallographic texture and loss of alloying elements are described. Mechanical properties of welds such as hardness, residual stresses and other important structural properties are discussed. The aim of the chapter is to review the recent progress in laser
Laser Beam Welding (LBW) consists in the laser beam focalisation on the workpiece surface. The high power density then created, induces metal ionisation and then plasma is formed. The vaporisation of the surface progressively forms a depression in the workpiece and then a keyhole, which allows the laser energy in-depth absorption. The melted metal will progressively fill the keyhole during the laser displacement, to form the weld. The two laser sources available are CO2 and Nd: YAG. Laser CO2 consists in a mixture of CO2, N2 and noble gases. The nitrogen discharges in CO2 molecules activate the laser emission. The Nd: YAG (neodymium-doped yttrium aluminium garnet) consists in Nd3+ ions inserted in YAG crystal, the excitation is supplied by laser diodes. Nd: YAG laser light (λ = 1.06 μm) has a much higher absorption degree than CO2 laser light (λ=10.6 μm). Both CO2 and Nd:YAG lasers operate in the infrared region of the electromagnetic radiation spectrum, invisible to the human eye. The Nd:YAG provides its primary light output in the near-infrared, at a wavelength of 1.06 microns. This wavelength is absorbed quite well by conductive materials, with a typical reflectance of about 20 to 30 percent for most metals. The nearinfrared radiation permits the use of standard optics to achieve focused spot sizes as small as .001" in diameter. On the other hand, the far infrared (10.6 micron) output wavelength of the CO2 laser has an initial reflectance of about 80 percent to 90 percent for most metals and requires special optics to focus the beam to a minimum spot size of .003" to .004" diam. However, whereas Nd:YAG lasers might produce power outputs up to 500 watts, CO2
welding of magnesium alloys and to provide a basis for follow-on research.
**2. General principle of laser beam welding**
**1***.* **Introduction**
have coefficient of friction of the order of 0.20, favoring better properties mechanical equipment and increase the service life.
In the trial by AFM showed that the coating presents heterogeneity throughout the area, with variations in surface roughness in different regions, possible crystallization of carbon black, coalescence of grains and the appearance of new phases resulting from via laser heating process.
#### **5. Acknowledgement**
Thanks to CNPq by financial support, the Group DEDALO-IEAv, Dr. J. R. Martinelli of the IPEN-USP and to Mr. A. Zanatta of the CCM-ITA.
#### **6. References**
### **Welding of Thin Light Alloys Sheets by CO2 Laser Beam: Magnesium Alloys**
Afia Kouadri-David *PSM Team, European University of Brittany, France, INSA of Rennes, LGCGM France*
#### **1***.* **Introduction**
282 CO2 Laser – Optimisation and Application
have coefficient of friction of the order of 0.20, favoring better properties mechanical
In the trial by AFM showed that the coating presents heterogeneity throughout the area, with variations in surface roughness in different regions, possible crystallization of carbon black, coalescence of grains and the appearance of new phases resulting from via laser
Thanks to CNPq by financial support, the Group DEDALO-IEAv, Dr. J. R. Martinelli of the
Benedeck, J.; Shachrai, A.; Levin, L., 1980, Case hardening of steel by a CO2 laser beam,
Chagas, D. C.; Dias, A. N.; Vasconcelos, G.; Antunes, E. F., 2010, Surface treatment and
Dohotre, N. B., 1998, Lasers in Surface Engineering: Surface Engineering Series, Volume 1, ASM International – The Materials Information Society, Chapter 1 and 3. Ganeev, R. A., 2002, Low-power laser hardening of steels, Journal of Materials Processing
Machado, I. F., 2006, Technological advances in steels heat treatment, Journal of Materials
Reis, J. L., 2009, Thermal treatment of AISI M2 Steel by CO2 Laser, 104f. Master Dissertation
in Physics and Chemistry, Aeronautics Institute of Technology, São José dos
covering by carbon black AISI 4340 by CO2 laser action, ISSN 1983-1544. Ativ.P&D
equipment and increase the service life.
IPEN-USP and to Mr. A. Zanatta of the CCM-ITA.
Optics and Laser Technology. October.
Processing Technology, 172, 160-173.
Sector Report N 09, 1998, Chemical Complex, Carbon Black, BNDES.
heating process.
**6. References**
**5. Acknowledgement**
IEAv, v.3, p.84.
Tech., 121, 414-419.
Campos – SP.
Laser welding is an important joining technique for magnesium alloys with their increasing applications in aerospace, aircraft, automotive, electronics and other industries. In this document the research and progress in laser welding of magnesium alloys are critically reviewed from different perspectives. Some important laser processing parameters and their effects on weld quality are discussed. The microstructure, metallurgical defects and mechanical properties encountered in laser welding of magnesium alloys, such as porosity, grains size, crystallographic texture and loss of alloying elements are described. Mechanical properties of welds such as hardness, residual stresses and other important structural properties are discussed. The aim of the chapter is to review the recent progress in laser welding of magnesium alloys and to provide a basis for follow-on research.
#### **2. General principle of laser beam welding**
Laser Beam Welding (LBW) consists in the laser beam focalisation on the workpiece surface. The high power density then created, induces metal ionisation and then plasma is formed. The vaporisation of the surface progressively forms a depression in the workpiece and then a keyhole, which allows the laser energy in-depth absorption. The melted metal will progressively fill the keyhole during the laser displacement, to form the weld. The two laser sources available are CO2 and Nd: YAG. Laser CO2 consists in a mixture of CO2, N2 and noble gases. The nitrogen discharges in CO2 molecules activate the laser emission. The Nd: YAG (neodymium-doped yttrium aluminium garnet) consists in Nd3+ ions inserted in YAG crystal, the excitation is supplied by laser diodes. Nd: YAG laser light (λ = 1.06 μm) has a much higher absorption degree than CO2 laser light (λ=10.6 μm). Both CO2 and Nd:YAG lasers operate in the infrared region of the electromagnetic radiation spectrum, invisible to the human eye. The Nd:YAG provides its primary light output in the near-infrared, at a wavelength of 1.06 microns. This wavelength is absorbed quite well by conductive materials, with a typical reflectance of about 20 to 30 percent for most metals. The nearinfrared radiation permits the use of standard optics to achieve focused spot sizes as small as .001" in diameter. On the other hand, the far infrared (10.6 micron) output wavelength of the CO2 laser has an initial reflectance of about 80 percent to 90 percent for most metals and requires special optics to focus the beam to a minimum spot size of .003" to .004" diam. However, whereas Nd:YAG lasers might produce power outputs up to 500 watts, CO2
Welding of Thin Light Alloys Sheets by CO2 Laser Beam: Magnesium Alloys 285
Then, when laser power was too low, lack of penetration was observed whereas high laser power produced laser cutting. These observations are consistent with the LBW literature. From macrostructure point of view, many authors observed the same evolutions where at low beam powers, some chevronlike pattern which is also called "ripples" (Marya et al., 2001). The mechanism of ripples formation is related to the effects of surface tension on the weld pool during solidification (D'Annessa, 1970 as cited in Cao et al., 2006). They also observed that the thickness in the weld area was slightly higher, a phenomenon which is called "crowning" or "humping". High beam powers led to deep and wide beads, and reduce both ripples and crowning (Marya et al., 2001). However, others authors showed at high beam powers, spatters and the evaporative losses would be produced. Authors (Weisheit et al., 1997) investigated the laser parameters for several magnesium alloys. They reported that for thin AZ31 plates (1, 8 mm), a 1.5 kW beam power was sufficient for achieving full penetration. In our experiences, for achieving full penetration, 3 mm AZ91
The penetration depth and weld width both increase linearly with decreasing welding speed and decrease with increasing welding speed. The obtained results in the literature and reported on the figure 2A and 2B confirm the effects of welding speed on penetration depth and weld width at different levels of power for CO2 lasers: The penetration depth and
Fig. 2. Influence of welding speed on the penetration depth (A) and on the weld width (B)
**(A) Welding speed (mm/s) (B) Welding speed (mm/s)**
Moreover, it was reported that the speed lead too either to improve or to decrease the weld quality, in particular by the formation of the defects such as cracking or the pores formation. Indeed, at low speeds, the interaction time between molten metal and surrounding air is large enough to allow pores to nucleate in large quantity, grow and escape from the molten pool as a result of buoyancy and convection flow. Moreover, when the welding speed is too slow, the bead produced by the superfluous heat exchange will extend to the side, and the
plates welds were produced at 4 kW (Kouadri & Barrallier, 2006).
weld width both decrease linearly with increasing welding speed.
for WE43 magnesium alloy (Dharhi et al., 2001a, 2001b)
**Penetration depth (mm) Bead width (mm)**
**3.2 LBW welding speed, V (mm/min)**
systems can easily supply 10,000 watts and greater. The two laser processes are differentiated by the fabrication and the shape of the beam. It is generally accepted that the heat input parameter, defined as the ratio of beam power to beam travel speed, is well suited for describing LBW process. However, our results and those of the literature show that this parameter was not convenient, and that, the effect of the laser power and the welding speed parameters have to be differentiated, in particular for the light alloys such as magnesium or aluminium alloys. As a result of these broad differences, the two laser types are usually employed for different applications. The powerful CO2 lasers overcome the high reflectance by keyholing, wherein the absorption approaches blackbody. The reflectivity of the metal is only important until the keyhole weld begins. Once the material's surface at the point of focus approaches its melting point, the reflectivity drops within microseconds.
#### **3. Optimisation and influence of CO2 laser beam parameters on thin sheets**
The present investigation is concerned with laser power, welding speed, defocusing distance and type of shielding gas and their effects on the fusion zone shape and final solidification structure of magnesium alloys.
#### **3.1 Laser power, P (kW)**
The laser power is a critical parameter to obtain a full penetration depth and to control the weld bead profile. High power density at the workpiece is crucial to achieve keyhole welding and to control the formation of welds. Studies realized in this domain showed this effect of laser power on the penetration depth and weld width. The increasing beam power led to deeper and wider beads. Figure 1 shows the effect of laser power on the penetration depth (Fig. 1A) and weld width (Fig. 1B) for WE43 alloy welded at a speed of 33 mm/s and a focused diameter of 0.25mm (Dharhi et al., 2001a, 2001b as cited in Cao et al., 2006).
Fig. 1. Influence of laser power on the penetration depth (A) and on the weld width (B) for WE43 alloy (Dharhi et al., 2001a, 2001b)
The penetration depth and weld width increased with increasing laser power due to higher power density. Our experiences showed too the same evolution: the weld width becomes larger with increasing laser power. For example the threshold power to achieve full penetration is 2,5 kW (i.e. a power density of 2 MW/cm2) for 3 mm AZ91 plates welded at a speed of 4 m/min and a focused diameter of 0.25 mm.
Then, when laser power was too low, lack of penetration was observed whereas high laser power produced laser cutting. These observations are consistent with the LBW literature. From macrostructure point of view, many authors observed the same evolutions where at low beam powers, some chevronlike pattern which is also called "ripples" (Marya et al., 2001). The mechanism of ripples formation is related to the effects of surface tension on the weld pool during solidification (D'Annessa, 1970 as cited in Cao et al., 2006). They also observed that the thickness in the weld area was slightly higher, a phenomenon which is called "crowning" or "humping". High beam powers led to deep and wide beads, and reduce both ripples and crowning (Marya et al., 2001). However, others authors showed at high beam powers, spatters and the evaporative losses would be produced. Authors (Weisheit et al., 1997) investigated the laser parameters for several magnesium alloys. They reported that for thin AZ31 plates (1, 8 mm), a 1.5 kW beam power was sufficient for achieving full penetration. In our experiences, for achieving full penetration, 3 mm AZ91 plates welds were produced at 4 kW (Kouadri & Barrallier, 2006).
#### **3.2 LBW welding speed, V (mm/min)**
284 CO2 Laser – Optimisation and Application
systems can easily supply 10,000 watts and greater. The two laser processes are differentiated by the fabrication and the shape of the beam. It is generally accepted that the heat input parameter, defined as the ratio of beam power to beam travel speed, is well suited for describing LBW process. However, our results and those of the literature show that this parameter was not convenient, and that, the effect of the laser power and the welding speed parameters have to be differentiated, in particular for the light alloys such as magnesium or aluminium alloys. As a result of these broad differences, the two laser types are usually employed for different applications. The powerful CO2 lasers overcome the high reflectance by keyholing, wherein the absorption approaches blackbody. The reflectivity of the metal is only important until the keyhole weld begins. Once the material's surface at the point of focus approaches its melting point, the reflectivity drops within microseconds.
**3. Optimisation and influence of CO2 laser beam parameters on thin sheets** The present investigation is concerned with laser power, welding speed, defocusing distance and type of shielding gas and their effects on the fusion zone shape and final
The laser power is a critical parameter to obtain a full penetration depth and to control the weld bead profile. High power density at the workpiece is crucial to achieve keyhole welding and to control the formation of welds. Studies realized in this domain showed this effect of laser power on the penetration depth and weld width. The increasing beam power led to deeper and wider beads. Figure 1 shows the effect of laser power on the penetration depth (Fig. 1A) and weld width (Fig. 1B) for WE43 alloy welded at a speed of 33 mm/s and
a focused diameter of 0.25mm (Dharhi et al., 2001a, 2001b as cited in Cao et al., 2006).
Fig. 1. Influence of laser power on the penetration depth (A) and on the weld width (B) for
**Laser power (kW) Laser power (kW)**
**Bead width (mm)**
The penetration depth and weld width increased with increasing laser power due to higher power density. Our experiences showed too the same evolution: the weld width becomes larger with increasing laser power. For example the threshold power to achieve full penetration is 2,5 kW (i.e. a power density of 2 MW/cm2) for 3 mm AZ91 plates welded at a
solidification structure of magnesium alloys.
WE43 alloy (Dharhi et al., 2001a, 2001b)
speed of 4 m/min and a focused diameter of 0.25 mm.
**(A) (B)**
**3.1 Laser power, P (kW)**
**Penetration depth (mm)**
The penetration depth and weld width both increase linearly with decreasing welding speed and decrease with increasing welding speed. The obtained results in the literature and reported on the figure 2A and 2B confirm the effects of welding speed on penetration depth and weld width at different levels of power for CO2 lasers: The penetration depth and weld width both decrease linearly with increasing welding speed.
Fig. 2. Influence of welding speed on the penetration depth (A) and on the weld width (B) for WE43 magnesium alloy (Dharhi et al., 2001a, 2001b)
Moreover, it was reported that the speed lead too either to improve or to decrease the weld quality, in particular by the formation of the defects such as cracking or the pores formation.
Indeed, at low speeds, the interaction time between molten metal and surrounding air is large enough to allow pores to nucleate in large quantity, grow and escape from the molten pool as a result of buoyancy and convection flow. Moreover, when the welding speed is too slow, the bead produced by the superfluous heat exchange will extend to the side, and the
Welding of Thin Light Alloys Sheets by CO2 Laser Beam: Magnesium Alloys 287
and lack of penetration when the power density was too low. Concerning LBW, increasing the laser power (P) and decreasing the welding speed (V) result in an increase of the power
This is also a very important technical parameter, because in a certain output power, it will decide the density of beam power which is the key factor for laser weld. But for the laser beam with high power, it is difficult to measure, what is produced by the nature of the beam diameter. For laser weld, the condition of high effective deep penetration weld is that the power density on the laser focus must exceed 106 W/cm2. We can adopt two methods to enhance the power density, one is to enhance the laser power, and the other one is to reduce the diameter of the beam. The power density has linear relation with the laser power, and has inverse-square ratio relation with beam diameter, so the effect of reducing beam diameter is better. In ours experiments, to realize full deep penetration weld of 2mm thick
For the sake of simplicity, the focal distance is defined as the distance between the focal point and the top surface of the sample. The position of focal points has an important influence on welding process and quality. The focal plane should be set where the maximum penetration depths or best process tolerances are produced. The laser welding usually needs some focus distance, because too high power density of the beam center at the laser focus is easy to vaporize and become bores. When the focus distance reduces to a certain value, the melting depth will suddenly change, which will establish necessary conditions for producing penetration pores. These most results in this domain showed that the focus distance influences not only the laser beam on the weld piece surface, but also the incidence direction of beam, so it has important influences to the melting depth and seam
In our experiences, the most acceptable weld profile was obtained at defocusing distance of – 0.2 mm for 3 mm thickness where weld bead depth / width ratio is maximum and fusion zone size is minimum. In order to obtain the optimum value, complete penetration butt welds were made using previously obtained optimum laser power (4 kW) and optimum welding speed (3 m/min) (Kouadri & Barrallier, 2006). Sound welds were achieved with a focal point on the surface, which is consistent with what we found for thin plates. The weld width increases with moving the focal point away from the surface (i.e. increasing focal distance) which was also observed by other authors. These results indicated that the most effective range of defocusing distance to get maximum penetration with acceptable weld
However, this distance has to be adjusted to obtain the best quality of welding. For example, the optimum defocusing distance to attain acceptable weld profile for 5 mm thickness was 0.4 mm under the surface of the workpiece (Cao et al., 2006). These results were consistent with the literature study (Dharhi et al., 2000, 2001, 2002 as cited in Cao et al., 2006). They studied 1–5kW CO2 laser welding of 2mm AZ91 and 4mm WE43-T6 alloys. Their results showed that an adequate weld could be obtained for a focal position on or 1mm under the
density. This tendency is consistent with all the previous studies on laser welding.
plates, we choose beam diameter of 1 mm with 4 kW CO2 at 3 m/min.
**3.4 Beam diameter**
**3.5 LBW focal distance, f (mm)**
profile lies between zero and – 1 mm.
shape.
heat influenced area will become too heat and extended, the seam metallographic structure crystal becomes thick, sometimes the cracking will appear, which will seriously influence the welding quality. When the welding speed achieves the lower limitation, the superfluous power absorption also will induce local evaporation loss and hollows. Moreover, using lower welding speeds induced no real change in the penetration depth but wider the weld width and especially the heat affected zone (HAZ) (Marya & Edwards, 2000).
From pores formation point of view, at high speeds, the pores do not have enough time to nucleate. The influence of the welding speed on pore formation was studied (Marya & Edwards, 2001). They found that the pore fraction goes to a maximum with increasing welding speed. The obtained results showed that using higher welding speeds reduced ripples but greatly increased crowning phenomena (Marya & Edwards, 2001), and the fusion zone appeared to be far more brittle (Watkins, 2003 as cited in Cao et al., 2006). Moreover, they showed a dependency between crowning and pores content, so that crowning is actually a relevant parameter to assess the weld quality. When welding speed was too high, lack of penetration was observed, whereas low welding speed produced laser cutting. This one is explained by the fact the power density increases with decreasing welding speed (Dharhi et al., 2000 as cited in Cao et al., 2006).
However these observations must be readjusted because the results depend on the nature of used magnesium alloys. Indeed, though similar welding parameters are used, various magnesium alloys exhibit different welding performance due to their different metallurgical and thermophysical properties. For example, die cast AZ91D has a lower thermal conductivity of 51 W/m K as compared with 139 W/m K for wrought AZ21A alloy. Thus, for similar welding parameters, the AZ91D alloy has a higher weld depth and weld volume compared with AZ21A alloy. It was also reported that greater penetration depth could be reached in AM50 alloy compared with AZ91 alloy welded under similar conditions using a 6kW CO2 laser (Marya & Edwards, 2001). These observations explain why it is needed to systematically investigate the laser-welding characteristics of different magnesium alloys because of the difference in their thermal properties. In the same way, it is needed to take account of the geometry and thickness of the plates for readjusting the speed of welding. Many authors reported that a welding speed of 2.5-3 m/min was suitable for thin plates, when using 1.5 kW laser beam. Therefore, welding speed above 3 m/min should be achievable during CO2 laser welding of 2 mm thick plates (Weisheit et al., 1997).
#### **3.3 LBW density (W/cm2 )**
It's the one more important parameter: the power density is one of most pivotal parameters in laser weld. When the laser power density is lower than 106 W/cm2, the laser weld belongs to category of heat exchange weld. When the laser power density achieves only 106 W/cm2, the deep penetration weld can be formed and "keyhole effects" appears.
The "keyhole effects" is closely correlative to the laser power density which is more low, the "keyhole effects" is more unstable even can not be formed, and the melting pool is also small. The melting depth of laser weld is directly correlative to the laser output power density and which is the function of incidence beam power and beam diameter. Therefore, to enhance the power density, we can enhance laser power or decrease the laser speed. A good balance had to be found to avoid laser cutting when the power density was too high and lack of penetration when the power density was too low. Concerning LBW, increasing the laser power (P) and decreasing the welding speed (V) result in an increase of the power density. This tendency is consistent with all the previous studies on laser welding.
#### **3.4 Beam diameter**
286 CO2 Laser – Optimisation and Application
heat influenced area will become too heat and extended, the seam metallographic structure crystal becomes thick, sometimes the cracking will appear, which will seriously influence the welding quality. When the welding speed achieves the lower limitation, the superfluous power absorption also will induce local evaporation loss and hollows. Moreover, using lower welding speeds induced no real change in the penetration depth but wider the weld
From pores formation point of view, at high speeds, the pores do not have enough time to nucleate. The influence of the welding speed on pore formation was studied (Marya & Edwards, 2001). They found that the pore fraction goes to a maximum with increasing welding speed. The obtained results showed that using higher welding speeds reduced ripples but greatly increased crowning phenomena (Marya & Edwards, 2001), and the fusion zone appeared to be far more brittle (Watkins, 2003 as cited in Cao et al., 2006). Moreover, they showed a dependency between crowning and pores content, so that crowning is actually a relevant parameter to assess the weld quality. When welding speed was too high, lack of penetration was observed, whereas low welding speed produced laser cutting. This one is explained by the fact the power density increases with decreasing
However these observations must be readjusted because the results depend on the nature of used magnesium alloys. Indeed, though similar welding parameters are used, various magnesium alloys exhibit different welding performance due to their different metallurgical and thermophysical properties. For example, die cast AZ91D has a lower thermal conductivity of 51 W/m K as compared with 139 W/m K for wrought AZ21A alloy. Thus, for similar welding parameters, the AZ91D alloy has a higher weld depth and weld volume compared with AZ21A alloy. It was also reported that greater penetration depth could be reached in AM50 alloy compared with AZ91 alloy welded under similar conditions using a 6kW CO2 laser (Marya & Edwards, 2001). These observations explain why it is needed to systematically investigate the laser-welding characteristics of different magnesium alloys because of the difference in their thermal properties. In the same way, it is needed to take account of the geometry and thickness of the plates for readjusting the speed of welding. Many authors reported that a welding speed of 2.5-3 m/min was suitable for thin plates, when using 1.5 kW laser beam. Therefore, welding speed above 3 m/min should be
achievable during CO2 laser welding of 2 mm thick plates (Weisheit et al., 1997).
the deep penetration weld can be formed and "keyhole effects" appears.
It's the one more important parameter: the power density is one of most pivotal parameters in laser weld. When the laser power density is lower than 106 W/cm2, the laser weld belongs to category of heat exchange weld. When the laser power density achieves only 106 W/cm2,
The "keyhole effects" is closely correlative to the laser power density which is more low, the "keyhole effects" is more unstable even can not be formed, and the melting pool is also small. The melting depth of laser weld is directly correlative to the laser output power density and which is the function of incidence beam power and beam diameter. Therefore, to enhance the power density, we can enhance laser power or decrease the laser speed. A good balance had to be found to avoid laser cutting when the power density was too high
width and especially the heat affected zone (HAZ) (Marya & Edwards, 2000).
welding speed (Dharhi et al., 2000 as cited in Cao et al., 2006).
**3.3 LBW density (W/cm2**
**)**
This is also a very important technical parameter, because in a certain output power, it will decide the density of beam power which is the key factor for laser weld. But for the laser beam with high power, it is difficult to measure, what is produced by the nature of the beam diameter. For laser weld, the condition of high effective deep penetration weld is that the power density on the laser focus must exceed 106 W/cm2. We can adopt two methods to enhance the power density, one is to enhance the laser power, and the other one is to reduce the diameter of the beam. The power density has linear relation with the laser power, and has inverse-square ratio relation with beam diameter, so the effect of reducing beam diameter is better. In ours experiments, to realize full deep penetration weld of 2mm thick plates, we choose beam diameter of 1 mm with 4 kW CO2 at 3 m/min.
#### **3.5 LBW focal distance, f (mm)**
For the sake of simplicity, the focal distance is defined as the distance between the focal point and the top surface of the sample. The position of focal points has an important influence on welding process and quality. The focal plane should be set where the maximum penetration depths or best process tolerances are produced. The laser welding usually needs some focus distance, because too high power density of the beam center at the laser focus is easy to vaporize and become bores. When the focus distance reduces to a certain value, the melting depth will suddenly change, which will establish necessary conditions for producing penetration pores. These most results in this domain showed that the focus distance influences not only the laser beam on the weld piece surface, but also the incidence direction of beam, so it has important influences to the melting depth and seam shape.
In our experiences, the most acceptable weld profile was obtained at defocusing distance of – 0.2 mm for 3 mm thickness where weld bead depth / width ratio is maximum and fusion zone size is minimum. In order to obtain the optimum value, complete penetration butt welds were made using previously obtained optimum laser power (4 kW) and optimum welding speed (3 m/min) (Kouadri & Barrallier, 2006). Sound welds were achieved with a focal point on the surface, which is consistent with what we found for thin plates. The weld width increases with moving the focal point away from the surface (i.e. increasing focal distance) which was also observed by other authors. These results indicated that the most effective range of defocusing distance to get maximum penetration with acceptable weld profile lies between zero and – 1 mm.
However, this distance has to be adjusted to obtain the best quality of welding. For example, the optimum defocusing distance to attain acceptable weld profile for 5 mm thickness was 0.4 mm under the surface of the workpiece (Cao et al., 2006). These results were consistent with the literature study (Dharhi et al., 2000, 2001, 2002 as cited in Cao et al., 2006). They studied 1–5kW CO2 laser welding of 2mm AZ91 and 4mm WE43-T6 alloys. Their results showed that an adequate weld could be obtained for a focal position on or 1mm under the
Welding of Thin Light Alloys Sheets by CO2 Laser Beam: Magnesium Alloys 289
from oxidation, combined with helium back shielding and nitrogen shielding to protect the optics. Wang et al., 2006 studied the influence of gas flow rate on weld width and reported that increasing gas flow up to 20 l/min is needed to affect the susceptibility to oxidation.
As magnesium is highly susceptible to oxidation, a protective atmosphere is required during welding. Surface cracking leading to laser welding was observed without gas protection. This is due to the oxide formation during welding in the magnesium alloys. To increase the magnesium alloy weldability, argon or helium are the most common choices. Argon is heavier than air so it provides a better shield than helium, but it ionizes easily and has much lower thermal conductivity than helium. This causes a problem with high power CO2 welding: The metal vapour emerging from the keyhole is partially ionized, with charged atoms and free electrons. The free electrons absorb some of the laser light, reducing the power available for welding. As the vapor absorbs energy, it heats up, increasing the number of free elections and further increasing absorption. Helium shield gas is more effective than argon in suppressing this effect because it cools the vapor plume and does not contribute many electrons itself. This welding gas often plays an active role in the welding process, such as increasing the welding speed and improving the mechanical properties of the joint. Weisheit et al., 1997 investigated the effectiveness of these three shielding gases
In addition, often to increase the weld quality, helium/argon mixtures combining the benefits of both gases, i.e. the higher density of argon and the higher ionization potential of helium, may be used to obtain better protection of the weld zone in CO2 laser welding. Hiraga et al., 2001 studied 1.7 mm thick AZ31B-H24 butt joints and get some improvements using argon back shielding in addition to the helium centre shielding. With these two gases, weld profile is remarkably improved where fusion zone interfaces are almost parallel to each other. The melting depth increases with the increase of gas flux, but too much gas flux will induce the surface hollow even penetration of the melting pool. Indeed, higher porosity content was observed for He gas flow higher than 50 l/min. Using Ar back shielding gas allowed us to produce sound welds at lower welding speed, reducing sag of the weld pool. Our study led to the same conclusions and sound welds were produced (Kouadri & Barrallier, 2006). Then, the optimum shielding system consists in a top helium flow superior to 20 l/min and Ar back shielding. By adding single-sided access, laser welding is even
**4. Application of laser beam CO2 on thin sheets of magnesium alloy**
The presented material is a cast magnesium alloy (AZ91D) welded by laser CO2 processing. The alloy used for the study of the laser welding is a ternary magnesium - aluminium - zinc of designation AZ91, according to standard ASTM. Laser welding of magnesium alloys appears to be a challenge itself. Indeed, the ability to produce laser welds depends on the properties of the material to be welded. Then, magnesium being characterised by quite unfavourable properties (i.e. low absorptivity of laser beams, strong tendency to oxidize, high thermal conductivity, high coefficient of thermal expansion, low melting temperature, wide solidification temperature range, high solidification shrinkage, a tendency to form low melting-point constituents, low viscosity, low surface tensions, high solubility for hydrogen
and confirmed that a helium gas flow was the best choice.
in the liquid state), processing is expected to be an issue.
more strategically advantageous.
surface of the workpiece. Focal position on the workpiece surface had the smallest weld width while the weld width became larger when the focal position deviated above or below the surface.
Then, the optimal focal point position to weld thin plates lies on the top surface of the workpiece. Indeed, Weisheit et al., (1997, 1998) investigated 2.5kW CO2 laser welding of some magnesium alloys. For thin plates (2.5 and 3 mm), the best welds according to penetration depth, aspect ratio and sag were achieved when the focal point was adjusted on the surface of workpiece, whereas for thick plates (5 and 8 mm) a position of 2mm below the surface of workpiece proved to be the best. Thus, the focal position should be moved deeper into the material for thicker work pieces and the following used process. Lehner et al., 1999 further researched the tolerance of focal position. For 3mm AZ91 and AM50 die castings welded using a 3 kW Nd:YAG laser, the best focal position is approximately 0.8mm below the workpiece surface, with a tolerance of ±0.5 mm. For 5mm material, the focal position has to shift to about 1.2±0.2mm below the surface.
#### **3.6 LBW shielding gas flow, V (l/min)**
Shielding gas selection produces a best weld quality. With the welding laser, the welding gas is flushed onto the workpiece through a nozzle system in order to protect molten and heated metal from the atmosphere. Gases have different chemical reactions and physical properties, which affect their suitability as assist gases for different welding tasks. At least three important points must be considered: tendency to form plasma, influence on mechanical properties and shielding effect.
Three main types of shielding gases are used: helium, argon and nitrogen. Helium is a gas characterized by minimum molecular weight, maximum thermal conductivity, and maximum ionization energy, thereby making it the most suitable gas for suppressing plasma formation. Argon, on the other hand, becomes ionized relatively easily and is therefore more prone to forming excessive amounts of plasma, in particular at CO2 laser power over 3 kW. Carbon dioxide and nitrogen, on the other hand, are reactive gases, which may react with the weld metal to form oxides, carbides, or nitrides and get trapped in pores. This can result in welds with deficient mechanical properties. As a result, pure carbon dioxide or nitrogen are unsuitable as welding gases in certain applications in particular for the aluminium or magnesium alloys due to the oxidation.
To reduce the plasma effect, in these cases, it is advantageous to use inert gases such as helium or argon as welding gases, because there is no reaction on the weld metal and do not affect weld metallurgy. Indeed, in general, when the laser beam interacts with the workpiece, a hole is drilled through the thickness of the material. This hole or cavity is filled with plasma and surrounded by molten metal, thus, the high energy density of the focused beam could be lost easily. Weisheit et al., 1997 investigated the effectiveness of these three shielding gases and reported that a helium gas flow was the best choice. This plasma effect was reduced as a result of the higher ionization potential of helium, and then the weld profile was improved.
These welding gases have other functions, too. It protects the focusing optics against fumes and spatters and, in the case of CO2 lasers, also controls plasma cloud formation. Leong et al., 1998 when welding 1.8 mm thick AZ31B-H4 used helium top shielding gas to protect
surface of the workpiece. Focal position on the workpiece surface had the smallest weld width while the weld width became larger when the focal position deviated above or below
Then, the optimal focal point position to weld thin plates lies on the top surface of the workpiece. Indeed, Weisheit et al., (1997, 1998) investigated 2.5kW CO2 laser welding of some magnesium alloys. For thin plates (2.5 and 3 mm), the best welds according to penetration depth, aspect ratio and sag were achieved when the focal point was adjusted on the surface of workpiece, whereas for thick plates (5 and 8 mm) a position of 2mm below the surface of workpiece proved to be the best. Thus, the focal position should be moved deeper into the material for thicker work pieces and the following used process. Lehner et al., 1999 further researched the tolerance of focal position. For 3mm AZ91 and AM50 die castings welded using a 3 kW Nd:YAG laser, the best focal position is approximately 0.8mm below the workpiece surface, with a tolerance of ±0.5 mm. For 5mm material, the focal position has
Shielding gas selection produces a best weld quality. With the welding laser, the welding gas is flushed onto the workpiece through a nozzle system in order to protect molten and heated metal from the atmosphere. Gases have different chemical reactions and physical properties, which affect their suitability as assist gases for different welding tasks. At least three important points must be considered: tendency to form plasma, influence on
Three main types of shielding gases are used: helium, argon and nitrogen. Helium is a gas characterized by minimum molecular weight, maximum thermal conductivity, and maximum ionization energy, thereby making it the most suitable gas for suppressing plasma formation. Argon, on the other hand, becomes ionized relatively easily and is therefore more prone to forming excessive amounts of plasma, in particular at CO2 laser power over 3 kW. Carbon dioxide and nitrogen, on the other hand, are reactive gases, which may react with the weld metal to form oxides, carbides, or nitrides and get trapped in pores. This can result in welds with deficient mechanical properties. As a result, pure carbon dioxide or nitrogen are unsuitable as welding gases in certain applications in particular for
To reduce the plasma effect, in these cases, it is advantageous to use inert gases such as helium or argon as welding gases, because there is no reaction on the weld metal and do not affect weld metallurgy. Indeed, in general, when the laser beam interacts with the workpiece, a hole is drilled through the thickness of the material. This hole or cavity is filled with plasma and surrounded by molten metal, thus, the high energy density of the focused beam could be lost easily. Weisheit et al., 1997 investigated the effectiveness of these three shielding gases and reported that a helium gas flow was the best choice. This plasma effect was reduced as a result of the higher ionization potential of helium, and then the weld
These welding gases have other functions, too. It protects the focusing optics against fumes and spatters and, in the case of CO2 lasers, also controls plasma cloud formation. Leong et al., 1998 when welding 1.8 mm thick AZ31B-H4 used helium top shielding gas to protect
the surface.
to shift to about 1.2±0.2mm below the surface.
**3.6 LBW shielding gas flow, V (l/min)**
mechanical properties and shielding effect.
profile was improved.
the aluminium or magnesium alloys due to the oxidation.
from oxidation, combined with helium back shielding and nitrogen shielding to protect the optics. Wang et al., 2006 studied the influence of gas flow rate on weld width and reported that increasing gas flow up to 20 l/min is needed to affect the susceptibility to oxidation.
As magnesium is highly susceptible to oxidation, a protective atmosphere is required during welding. Surface cracking leading to laser welding was observed without gas protection. This is due to the oxide formation during welding in the magnesium alloys. To increase the magnesium alloy weldability, argon or helium are the most common choices. Argon is heavier than air so it provides a better shield than helium, but it ionizes easily and has much lower thermal conductivity than helium. This causes a problem with high power CO2 welding: The metal vapour emerging from the keyhole is partially ionized, with charged atoms and free electrons. The free electrons absorb some of the laser light, reducing the power available for welding. As the vapor absorbs energy, it heats up, increasing the number of free elections and further increasing absorption. Helium shield gas is more effective than argon in suppressing this effect because it cools the vapor plume and does not contribute many electrons itself. This welding gas often plays an active role in the welding process, such as increasing the welding speed and improving the mechanical properties of the joint. Weisheit et al., 1997 investigated the effectiveness of these three shielding gases and confirmed that a helium gas flow was the best choice.
In addition, often to increase the weld quality, helium/argon mixtures combining the benefits of both gases, i.e. the higher density of argon and the higher ionization potential of helium, may be used to obtain better protection of the weld zone in CO2 laser welding. Hiraga et al., 2001 studied 1.7 mm thick AZ31B-H24 butt joints and get some improvements using argon back shielding in addition to the helium centre shielding. With these two gases, weld profile is remarkably improved where fusion zone interfaces are almost parallel to each other. The melting depth increases with the increase of gas flux, but too much gas flux will induce the surface hollow even penetration of the melting pool. Indeed, higher porosity content was observed for He gas flow higher than 50 l/min. Using Ar back shielding gas allowed us to produce sound welds at lower welding speed, reducing sag of the weld pool. Our study led to the same conclusions and sound welds were produced (Kouadri & Barrallier, 2006). Then, the optimum shielding system consists in a top helium flow superior to 20 l/min and Ar back shielding. By adding single-sided access, laser welding is even more strategically advantageous.
#### **4. Application of laser beam CO2 on thin sheets of magnesium alloy**
The presented material is a cast magnesium alloy (AZ91D) welded by laser CO2 processing. The alloy used for the study of the laser welding is a ternary magnesium - aluminium - zinc of designation AZ91, according to standard ASTM. Laser welding of magnesium alloys appears to be a challenge itself. Indeed, the ability to produce laser welds depends on the properties of the material to be welded. Then, magnesium being characterised by quite unfavourable properties (i.e. low absorptivity of laser beams, strong tendency to oxidize, high thermal conductivity, high coefficient of thermal expansion, low melting temperature, wide solidification temperature range, high solidification shrinkage, a tendency to form low melting-point constituents, low viscosity, low surface tensions, high solubility for hydrogen in the liquid state), processing is expected to be an issue.
Welding of Thin Light Alloys Sheets by CO2 Laser Beam: Magnesium Alloys 291
Al8Mn3. The typical overall microstructure of the HAZ is shown in figure 3b. The microstructure of HAZ has coarse grain polygonal Mg as the base metal. Nevertheless, eutectic grains disappeared whereas a continuous β-Al12Mg17 phase was created at grain boundary. At the fusion boundary, where a relatively large thermal gradient and small growth rate are established, the microstructure is predominantly cellular (Marya & Edwards, 2000). Grains usually grow epitaxially from the Fusion Zone (FZ) –Heat Affected
**Al8Mn3**
β**-Mg17Al12**
**(**α**-Mg +** β**-Mg17Al12) eutectic phases**
Fig. 3. a) Microstructure of the base metal, solid solution α-Mg and (α-Mg + β-Mg17Al12) eutectic phase and precipitates (OM), b) Microstructure of the Heat Affected Zone (HAZ) by
a b
**100 μm**
Fig. 4. a) Microstructure in the welding zone and in the heat affected zone by (OM), b)
a b
**80 μm**
Microstructure of the fusion zone (SEM) (Kouadri & Barrallier, 2006, 2011)
**Heat Affected zone 50-200 μm**
In the fusion zone (FZ), the microstructure is very different from the base metal (Figure 4a, 4b).
Zone (HAZ) interface.
α**-Mg phase**
SEM (Kouadri & Barrallier, 2006, 2011)
**Welded zone 10-25 μm**
The used magnesium alloy in this study were obtained by high pressure die casting under neutral gas and did not undergo heat treatment to match the conditions generally encountered in automobile applications. The provided plates were sheared to recover 3 mm-thick samples. Their edges were machined by milling. The plates were welded together side by side using a laser beam CO2, which penetrated through the thickness of the plates. The welding was performed using a 4000 W CO2 laser in an inert helium atmosphere. The speed of welding was optimised in the range of 1.0 – 4.25 m/min and the power in the range of 1 – 4 kW.
The objective of this part is to show the evolutions of the metallurgical and mechanical properties generated by the laser CO2 in thin AZ91 magnesium alloy sheets. The presented results were obtained with optimized parameters of CO2 laser beam welding. This part shows the microstructure modifications (characterization of the grain size, chemical properties and the crystallographic texture) occurring during laser welding in every zone of the welded sheet. From mechanical properties point of view, we present the evolution of the hardness and the residual stresses. These one have been performed by taking into account the crystallographic texture. The strain measurements and the characterization of the crystallographic texture have been performed using X-ray diffraction techniques. The set of results demonstrated that laser welding induces the presence of several distinct zones which have distinct microstructural and mechanical properties.
#### **4.1 Study of metallurgical properties**
#### **4.1.1 Macrostructure analysis**
From macrostructure point of view, a narrow weld joint is an important characteristic of high power density welding. The 4 kW CO2 laser welding in an inert helium atmosphere (2 bars) with a speed of welding of 2 m/min of 3mm AZ91D plates showed that the fusion zones have widths of approximately 0.8 1.6 mm (Kouadri & Barrallier, 2006). The region with a width of about 200 – 500 μm between the base metal (BM) and the fusion zone (FZ) can be recognized as the heat affected zone (HAZ). However, the width of the HAZ is defined according to the variations of laser beam parameters. For example, in the literature, the 6 kW CO2 laser welding with a speed of welding of 3.5 m/min of wrought AZ31B alloy indicated that the width of the HAZ was 50–60 µm, but can be doubled at substantially slower speeds (Leong et al., 1998; Sanders et al., 1999). These results showed that the width of the HAZ is tightly connected to laser process parameters.
#### **4.1.2 Microstructure analysis**
From microstructure point of view, the microstructure of the laser welds is characteristic of a high-speed process in which heat is rapidly extracted from the molten fusion zone by surrounding base material. In our study, the mean grain sizes of the base metal (BM) range from 50 to 200 µm (figure 3a).
The BM is heterogeneous and characterised by a mixture of a large primary α-Mg phase and of a (α-Mg + β-Mg17Al12) eutectic phases. This later constituent is a so-called abnormal eutectic (Kouadri & Barrallier, 2006, 2011; Dubé et al., 2001; Luo, 1996) because of its lamellar shape. The base metal exhibits small precipitates dispersed in the matrix but mainly located at the grain boundaries. These precipitates are β-Mg17Al12 and to a lesser degree
The used magnesium alloy in this study were obtained by high pressure die casting under neutral gas and did not undergo heat treatment to match the conditions generally encountered in automobile applications. The provided plates were sheared to recover 3 mm-thick samples. Their edges were machined by milling. The plates were welded together side by side using a laser beam CO2, which penetrated through the thickness of the plates. The welding was performed using a 4000 W CO2 laser in an inert helium atmosphere. The speed of welding was optimised in the range of 1.0 – 4.25 m/min and the power in the range
The objective of this part is to show the evolutions of the metallurgical and mechanical properties generated by the laser CO2 in thin AZ91 magnesium alloy sheets. The presented results were obtained with optimized parameters of CO2 laser beam welding. This part shows the microstructure modifications (characterization of the grain size, chemical properties and the crystallographic texture) occurring during laser welding in every zone of the welded sheet. From mechanical properties point of view, we present the evolution of the hardness and the residual stresses. These one have been performed by taking into account the crystallographic texture. The strain measurements and the characterization of the crystallographic texture have been performed using X-ray diffraction techniques. The set of results demonstrated that laser welding induces the presence of several distinct zones which
From macrostructure point of view, a narrow weld joint is an important characteristic of high power density welding. The 4 kW CO2 laser welding in an inert helium atmosphere (2 bars) with a speed of welding of 2 m/min of 3mm AZ91D plates showed that the fusion zones have widths of approximately 0.8 1.6 mm (Kouadri & Barrallier, 2006). The region with a width of about 200 – 500 μm between the base metal (BM) and the fusion zone (FZ) can be recognized as the heat affected zone (HAZ). However, the width of the HAZ is defined according to the variations of laser beam parameters. For example, in the literature, the 6 kW CO2 laser welding with a speed of welding of 3.5 m/min of wrought AZ31B alloy indicated that the width of the HAZ was 50–60 µm, but can be doubled at substantially slower speeds (Leong et al., 1998; Sanders et al., 1999). These results showed that the width
From microstructure point of view, the microstructure of the laser welds is characteristic of a high-speed process in which heat is rapidly extracted from the molten fusion zone by surrounding base material. In our study, the mean grain sizes of the base metal (BM) range
The BM is heterogeneous and characterised by a mixture of a large primary α-Mg phase and of a (α-Mg + β-Mg17Al12) eutectic phases. This later constituent is a so-called abnormal eutectic (Kouadri & Barrallier, 2006, 2011; Dubé et al., 2001; Luo, 1996) because of its lamellar shape. The base metal exhibits small precipitates dispersed in the matrix but mainly located at the grain boundaries. These precipitates are β-Mg17Al12 and to a lesser degree
have distinct microstructural and mechanical properties.
of the HAZ is tightly connected to laser process parameters.
**4.1 Study of metallurgical properties**
**4.1.1 Macrostructure analysis**
**4.1.2 Microstructure analysis**
from 50 to 200 µm (figure 3a).
of 1 – 4 kW.
Al8Mn3. The typical overall microstructure of the HAZ is shown in figure 3b. The microstructure of HAZ has coarse grain polygonal Mg as the base metal. Nevertheless, eutectic grains disappeared whereas a continuous β-Al12Mg17 phase was created at grain boundary. At the fusion boundary, where a relatively large thermal gradient and small growth rate are established, the microstructure is predominantly cellular (Marya & Edwards, 2000). Grains usually grow epitaxially from the Fusion Zone (FZ) –Heat Affected Zone (HAZ) interface.
Fig. 3. a) Microstructure of the base metal, solid solution α-Mg and (α-Mg + β-Mg17Al12) eutectic phase and precipitates (OM), b) Microstructure of the Heat Affected Zone (HAZ) by SEM (Kouadri & Barrallier, 2006, 2011)
In the fusion zone (FZ), the microstructure is very different from the base metal (Figure 4a, 4b).
Fig. 4. a) Microstructure in the welding zone and in the heat affected zone by (OM), b) Microstructure of the fusion zone (SEM) (Kouadri & Barrallier, 2006, 2011)
Welding of Thin Light Alloys Sheets by CO2 Laser Beam: Magnesium Alloys 293
Fig. 6. a) Statistical distribution of the grain sizes of AZ91 alloys in welded zone, b) Volume fraction of the α-Mg grains and of the β-Mg17Al12 precipitates in the welded zone (Kouadri
**M1**
**Volume fraction (%)**
Generally, the laser welding leads to the redistribution of chemical composition or evaporative losses in the fusion zone. In the case of magnesium alloys, the temperatures reached within keyholes are far greater than the boiling temperatures of magnesium, aluminium or zinc (Liu et al., 2000). Thus, during welding, the preferential evaporative losses consist primarily of zinc and magnesium and then aluminum. This evaporation causes a variation of chemical composition in the fusion zone, especially at high laser power
In our studies, the welding of AZ91 D plates, the chemical composition (% weight.) of each individual phase in the different phases was studied using EDS analysis. The base metal constitutes a reference state for the comparison with the welded area. In the table 1 energy dispersive spectroscopy (EDS) analysis indicated that the α-Mg grain contains up to 8.1 and 1.15 wt. % of Al and Zn, respectively. The α-phase contains ≈ 32 wt. % Al (Kouadri & Barrallier, 2006). All these results were in complete agreement with the literature on AZ91
Chemical Composition (weight %) Al Mg Zn Mn Fe Si
α*, (*α *+* β
Table 1. Chemical composition of AZ91 alloy in base metal, BM, (EDS analysis) (Kouadri &
The tables 2 and 3 show the evolution of the chemical composition in the HAZ and in the
In the HAZ, higher Al and Zn concentrations in the α-phase were measured as compared to the base metal and to the welded zone (Kouadri & Barrallier, 2006). So, this confirms that β-Mg17Al12 and Al8Mn3 precipitates were diluted in a matrix made of over-saturated α-phase.
*)* 8,1 90,6 1,15 0,15 - -
**Volume fraction in the welded zone**
**M2+ M3**
**Precipitates Principal phase: α-Mg Eutectic phase (α-Mg + β-**
**Phases**
**Al12Mg17)**
**M4**
Precipitates β 32,5 67,5 - - - - Precipitates Al8Mn3 32,8 14,5 - 52,7 - -
& Barrallier, 2010).
M1
**Volume fraction (%)**
Barrallier, 2006).
welded zone.
**4.1.4 Chemical analysis**
density which leads to a new chemical redistribution.
**Distribution of grain sizes in the welded zone**
M2
**0 10 20 30 40 50 Grain sizes (μm)**
M3
M4
alloy microstructure, which has been widely studied.
Base metal Matrix
The α-Mg microstructure is much finer ranging between 10µm and 25µm. There is β-Mg17Al12 precipitates too, clearly present and located in grain boundaries. So, the microstructure appears to be more homogenous at scale lengths of a few micrometers. This fine equiaxed grains in the fusion zones formed by cellular growth were also observed by the others authors (Cao et al., 2006) in Zr-containing ZE41A alloy. Weisheit et al., 1997, 1998 have also observed a cellular morphology in all joints except for the WE54 alloy which showed a more globular grain shape. It was further observed the equiaxed morphology in AM60B alloy occurring at low welding speeds. At higher welding speeds, however, the morphology changes from equiaxed to dendritic forms (Pastor et al., 2000). In the same way, our observations showed that the rapid cooling experienced during laser welding leads to a significant grain refinement with cellular growth in the fusion zone. In brief, the laser welding leads to a grain refinement some is the initial structure (Haferkamp et al., 1996, 1998). Only the grain morphology changes following the used laser parameters. Indeed, it was reported that the original microstructure has little influence on the fusion zone structure though magnesium alloys can be welded in different conditions (Weisheit et al., 1998).
#### **4.1.3 Distribution of grain size and volumetric fractions of phases by image analysis**
An example of the statistical distribution of the different grain sizes obtained by grain count is presented on the figure 5a.
Fig. 5. (a) Statistical distribution of the grain sizes of AZ91 alloys in the base metal. (b) Volume fraction of the α-Mg grains, of the eutectic (α-Mg grains β-Mg17Al12) phase and of the precipitates in the base metal (Kouadri & Barrallier, 2011).
This distribution shows the presence of four modes; M1, M2, M3 and M4: The precipitates (M1), whose main mode is about 10 µm and the principal α-Mg (M2 + M3) phase, whose main modes are 50 and 160 µm. Finally the principal mode of the eutectic phase (α-Mg + β-Al12Mg17) is about 220µm. The volumetric fractions calculation (figure 5b) demonstrates that the volumetric fraction of the base metal for the principal α-Mg phase is estimated at 85.4%, that of the eutectic phases at 13.8% and those of the precipitates at 0.8%. These results are in line with those in the literature and the diagram of the alloy phase AZ91 (StJohn et al., 2003). The large reduction of grains in the fusion zone is confirmed by statistical distribution of the grains (Figure 6a) where the principal mode is 16µm. The volumetric fraction of the principal phase represented by modes 2 and 3 constitutes 96% of the matrix (Figure 6b). The eutectic phase has almost disappeared in the welded zone.
Fig. 6. a) Statistical distribution of the grain sizes of AZ91 alloys in welded zone, b) Volume fraction of the α-Mg grains and of the β-Mg17Al12 precipitates in the welded zone (Kouadri & Barrallier, 2010).
#### **4.1.4 Chemical analysis**
292 CO2 Laser – Optimisation and Application
The α-Mg microstructure is much finer ranging between 10µm and 25µm. There is β-Mg17Al12 precipitates too, clearly present and located in grain boundaries. So, the microstructure appears to be more homogenous at scale lengths of a few micrometers. This fine equiaxed grains in the fusion zones formed by cellular growth were also observed by the others authors (Cao et al., 2006) in Zr-containing ZE41A alloy. Weisheit et al., 1997, 1998 have also observed a cellular morphology in all joints except for the WE54 alloy which showed a more globular grain shape. It was further observed the equiaxed morphology in AM60B alloy occurring at low welding speeds. At higher welding speeds, however, the morphology changes from equiaxed to dendritic forms (Pastor et al., 2000). In the same way, our observations showed that the rapid cooling experienced during laser welding leads to a significant grain refinement with cellular growth in the fusion zone. In brief, the laser welding leads to a grain refinement some is the initial structure (Haferkamp et al., 1996, 1998). Only the grain morphology changes following the used laser parameters. Indeed, it was reported that the original microstructure has little influence on the fusion zone structure
though magnesium alloys can be welded in different conditions (Weisheit et al., 1998).
Fig. 5. (a) Statistical distribution of the grain sizes of AZ91 alloys in the base metal. (b) Volume fraction of the α-Mg grains, of the eutectic (α-Mg grains β-Mg17Al12) phase and of
This distribution shows the presence of four modes; M1, M2, M3 and M4: The precipitates (M1), whose main mode is about 10 µm and the principal α-Mg (M2 + M3) phase, whose main modes are 50 and 160 µm. Finally the principal mode of the eutectic phase (α-Mg + β-Al12Mg17) is about 220µm. The volumetric fractions calculation (figure 5b) demonstrates that the volumetric fraction of the base metal for the principal α-Mg phase is estimated at 85.4%, that of the eutectic phases at 13.8% and those of the precipitates at 0.8%. These results are in line with those in the literature and the diagram of the alloy phase AZ91 (StJohn et al., 2003). The large reduction of grains in the fusion zone is confirmed by statistical distribution of the grains (Figure 6a) where the principal mode is 16µm. The volumetric fraction of the principal phase represented by modes 2 and 3 constitutes 96% of the matrix (Figure 6b). The
**M1**
**Volume fraction (%)**
the precipitates in the base metal (Kouadri & Barrallier, 2011).
**M**
eutectic phase has almost disappeared in the welded zone.
is presented on the figure 5a.
**M**
**M**
**Volume fraction (%)**
**Distribution of the grain sizes in the base metal**
**M**
**0 50 100 150 200 250 300 Grain sizes (μm)**
**4.1.3 Distribution of grain size and volumetric fractions of phases by image analysis** An example of the statistical distribution of the different grain sizes obtained by grain count
**Volume fraction in the base metal**
**Precipitates Principal phase α-Mg Eutectic phase (α-Mg + β-**
**M2+ M3**
**Al12Mg17) Phases**
**M4**
Generally, the laser welding leads to the redistribution of chemical composition or evaporative losses in the fusion zone. In the case of magnesium alloys, the temperatures reached within keyholes are far greater than the boiling temperatures of magnesium, aluminium or zinc (Liu et al., 2000). Thus, during welding, the preferential evaporative losses consist primarily of zinc and magnesium and then aluminum. This evaporation causes a variation of chemical composition in the fusion zone, especially at high laser power density which leads to a new chemical redistribution.
In our studies, the welding of AZ91 D plates, the chemical composition (% weight.) of each individual phase in the different phases was studied using EDS analysis. The base metal constitutes a reference state for the comparison with the welded area. In the table 1 energy dispersive spectroscopy (EDS) analysis indicated that the α-Mg grain contains up to 8.1 and 1.15 wt. % of Al and Zn, respectively. The α-phase contains ≈ 32 wt. % Al (Kouadri & Barrallier, 2006). All these results were in complete agreement with the literature on AZ91 alloy microstructure, which has been widely studied.
Table 1. Chemical composition of AZ91 alloy in base metal, BM, (EDS analysis) (Kouadri & Barrallier, 2006).
The tables 2 and 3 show the evolution of the chemical composition in the HAZ and in the welded zone.
In the HAZ, higher Al and Zn concentrations in the α-phase were measured as compared to the base metal and to the welded zone (Kouadri & Barrallier, 2006). So, this confirms that β-Mg17Al12 and Al8Mn3 precipitates were diluted in a matrix made of over-saturated α-phase.
Welding of Thin Light Alloys Sheets by CO2 Laser Beam: Magnesium Alloys 295
However, close to the surface, the AZ91 alloy exhibits two preferential orientations
Fig. 7. Evolution of the crystallographic texture (pole figure 101) from the surface to a depth of 200 µm: a) Texture to the surface, b) Texture to 50 µm, c) Texture to 150 µm, d)
**c) to 150 μm d) to 200 μm**
**a) On the surface b) to 50 μm**
The {1011} pole figure showed that a large fraction of grains are oriented with pyramidal {1011} planes parallel to the surface of the sheet. The intensity of the center of this pole figure is a tenfold improvement in intensity compared with other peaks. A close look at the position reveals that the normal axis is tilted at an angle of ± 4° from the normal sheet direction to the transverse direction and around the welding direction. The ODF calculation allows us to say that 71% of the grains have an orientation {1011} 3413 corresponding to the 3 Euler angles (ϕ1 = 65,6°, φ = 60,8°, ϕ2 = 54,4°). Likewise, the {1010} pole figure shows the presence of a smaller proportion of grains with {1010} planes parallel to the surface. Such grains show two other poles {1010} Pi and {1010} Pj , on both sides of the centre, due to the multiplicity of the hexagonal symmetry, which is equal to three. These poles are tilted at 60° around the centre. The ODF calculation allows us to say that 6% about of the grains have an orientation {1010} 0334 corresponding to the 3 Euler angles (ϕ1 = 65°, φ = 90°, ϕ2 = 60°). The most likely explanation for the variation of texture between the surface and the depth of the fusion zone is that the differences of the thermophysical and thermomechanical properties of the investigated location affect the process of solidification and plastic deformation, leading to different final out-comes. In our study, the nature of this texture has been explained by the thermodynamic conditions of minimisation of surface energies
Texture to 200 µm (Kouadri & Barrallier, 2006, 2011).
concerning 77% of the grains (Figure 7a).
In the welded zone, the chemical composition of phases was much the same as in base metal (Kouadri & Barrallier, 2006) except in a thin superficial layer close to the surface. The Alcontent in the β-phase decreases from 30 % (weight) in the BM down to 17% (weight) in the fusion zone. Likewise, we could see a strong decrease of the Al-content in every crystalline phase. We did'nt observe evaporative losses due to a good optimisation of the laser parameters. Indeed, it is known that higher energy densities lead to greater evaporative losses, increased spatter, and uneven weld beads. Thus, minimizing the irradiance incident upon the workpiece would reduce the loss of high vapor pressure elements. For example, larger reductions of both Mg and Zn were also reported at slower travel speeds (Leong et al., 1998; Sanders et al., 1999). We can conclude in the case of the magnesium alloys that there aren't evaporative losses of Mg and Zn if the laser parameters are optimized. There is only a chemical redistribution of the overall Al quantity for example in our case, due to the solidification conditions (Kouadri & Barrallier, 2006, 2011; Dubé et al., 2001; Luo, 1996). This redistribution should then be carefully controlled and optimized by manipulation of welding parameters.
Table 2. Chemical composition of AZ91 alloy in the HAZ, (EDS analysis) (Kouadri & Barrallier, 2006).
Table 3. Chemical composition of AZ91 alloy in welded metal, (EDS analysis) (Kouadri & Barrallier, 2006).
#### **4.1.5 Texture characterisation**
In general, texture develops in a metal as a result of processes such as crystallisation, plastic deformation…. The practical importance of preferred crystallographic orientation results from the dependence of many mechanical and physical properties on crystal direction. Thus, a textured material will have, in general, anisotropic values for a number of parameters including the yield strength, Young's modulus and Poisson's ratio. In order to understand how preferred crystallographic orientations might occur in laser welding, it is necessary to consider the formation and structure of the fusion zone in detail.
Initially, in our study, the base metal is characterized by a random orientation. Likewise, there is no more texture in the HAZ with 90% of the grains being randomly oriented. This is consistent with Coelho et al., 2008, study. In the fusion zone, the microstructure consists of fine and randomly oriented equiaxed dendrites nucleated. The texture analysis showed clearly that there is no texture. Indeed, ODF calculation indicates that more than 99% of the grains are randomly oriented (Kouadri & Barrallier, 2006, 2011).
In the welded zone, the chemical composition of phases was much the same as in base metal (Kouadri & Barrallier, 2006) except in a thin superficial layer close to the surface. The Alcontent in the β-phase decreases from 30 % (weight) in the BM down to 17% (weight) in the fusion zone. Likewise, we could see a strong decrease of the Al-content in every crystalline phase. We did'nt observe evaporative losses due to a good optimisation of the laser parameters. Indeed, it is known that higher energy densities lead to greater evaporative losses, increased spatter, and uneven weld beads. Thus, minimizing the irradiance incident upon the workpiece would reduce the loss of high vapor pressure elements. For example, larger reductions of both Mg and Zn were also reported at slower travel speeds (Leong et al., 1998; Sanders et al., 1999). We can conclude in the case of the magnesium alloys that there aren't evaporative losses of Mg and Zn if the laser parameters are optimized. There is only a chemical redistribution of the overall Al quantity for example in our case, due to the solidification conditions (Kouadri & Barrallier, 2006, 2011; Dubé et al., 2001; Luo, 1996). This redistribution should then be carefully controlled and optimized by manipulation of
Chemical Composition (weight %) Al Mg Zn Mn Fe Si HAZ Matrix α 8,51 90,31 1,15 0,04 -
Table 2. Chemical composition of AZ91 alloy in the HAZ, (EDS analysis) (Kouadri &
Chemical Composition (weight %) Al Mg Zn Mn Fe Si welded zone Matrix α 8,3 90,3 1,1 0,2 - 0,1
Table 3. Chemical composition of AZ91 alloy in welded metal, (EDS analysis) (Kouadri &
In general, texture develops in a metal as a result of processes such as crystallisation, plastic deformation…. The practical importance of preferred crystallographic orientation results from the dependence of many mechanical and physical properties on crystal direction. Thus, a textured material will have, in general, anisotropic values for a number of parameters including the yield strength, Young's modulus and Poisson's ratio. In order to understand how preferred crystallographic orientations might occur in laser welding, it is
Initially, in our study, the base metal is characterized by a random orientation. Likewise, there is no more texture in the HAZ with 90% of the grains being randomly oriented. This is consistent with Coelho et al., 2008, study. In the fusion zone, the microstructure consists of fine and randomly oriented equiaxed dendrites nucleated. The texture analysis showed clearly that there is no texture. Indeed, ODF calculation indicates that more than 99% of the
necessary to consider the formation and structure of the fusion zone in detail.
grains are randomly oriented (Kouadri & Barrallier, 2006, 2011).
Precipitates β 26,83 69,63 3,50 0,04 -
Precipitates β 29,9 69,1 - - - -
welding parameters.
Barrallier, 2006).
Barrallier, 2006).
**4.1.5 Texture characterisation**
However, close to the surface, the AZ91 alloy exhibits two preferential orientations concerning 77% of the grains (Figure 7a).
Fig. 7. Evolution of the crystallographic texture (pole figure 101) from the surface to a depth of 200 µm: a) Texture to the surface, b) Texture to 50 µm, c) Texture to 150 µm, d) Texture to 200 µm (Kouadri & Barrallier, 2006, 2011).
The {1011} pole figure showed that a large fraction of grains are oriented with pyramidal {1011} planes parallel to the surface of the sheet. The intensity of the center of this pole figure is a tenfold improvement in intensity compared with other peaks. A close look at the position reveals that the normal axis is tilted at an angle of ± 4° from the normal sheet direction to the transverse direction and around the welding direction. The ODF calculation allows us to say that 71% of the grains have an orientation {1011} 3413 corresponding to the 3 Euler angles (ϕ1 = 65,6°, φ = 60,8°, ϕ2 = 54,4°). Likewise, the {1010} pole figure shows the presence of a smaller proportion of grains with {1010} planes parallel to the surface. Such grains show two other poles {1010} Pi and {1010} Pj , on both sides of the centre, due to the multiplicity of the hexagonal symmetry, which is equal to three. These poles are tilted at 60° around the centre. The ODF calculation allows us to say that 6% about of the grains have an orientation {1010} 0334 corresponding to the 3 Euler angles (ϕ1 = 65°, φ = 90°, ϕ2 = 60°). The most likely explanation for the variation of texture between the surface and the depth of the fusion zone is that the differences of the thermophysical and thermomechanical properties of the investigated location affect the process of solidification and plastic deformation, leading to different final out-comes. In our study, the nature of this texture has been explained by the thermodynamic conditions of minimisation of surface energies
Welding of Thin Light Alloys Sheets by CO2 Laser Beam: Magnesium Alloys 297
due to its finer microstructure and higher volume fraction of intermetallics such as Mg17Al12
Of the same form, through the thickness at a depth around 200 μm, the results show a different evolution than the surface concerning the BM and the HAZ, the hardness is lower. In the depth, the evolution of the microhardness shows no statistical variation of the microhardness between the base metal, the heat affected zone and the welded zone contrary to the surface. We can see that the hardness between the base metal, the heat affected zone and the core of the welded zone at a depth around 200 μm stays stable with a value of about 85 HV. Though significant grain coarsening occurred in the HAZ of AZ91 alloy, the hardness in the HAZ was still almost the same as that in base metal. These results join those of literature. Weisheit et al., 1997, studied 2.5kW CO2 laser welding of cast magnesium alloys such as AZ91 AM60, ZC63, ZE41, QE22 and WE54 and wrought alloys (AZ31, AZ61, ZW3 and ZC71). For as-cast alloys, there is an increase in hardness of the FZ but little variation in hardness occurs in the HAZ. These observations demonstrate that laser welding induces particular profiles in the zones studied. These differences in hardness distribution over laser weld joints indicate the inhomogeneity of the joints following used parameters. Hardness is influenced by the laser welding parameters but too the initial chemical composition and also depends on the manufacturing process of the magnesium alloy. The choice of the laser process CO2 or Nd:YAG influences too the hardness. This last point has been demonstrated by Hiraga et al., 2001, studied 2 kW CO2 and Nd:YAG laser welding of wrought AZ31B-H24 butt joints of 1.7mm thickness. Their results showed that the Nd:YAG
laser welded fusion zone is slightly harder than the FZ produced by CO2 laser.
**Microhardness from the base metal passing on the welded zone**
**close to the surface**
Fig. 8. Hardness from the base metal passing on the heat affected zone and the welded zone close to the surface and in the thickness at a depth about 200 μm (Kouadri & Barrallier,
**-3000 -2000 -1000 0 1000 2000 3000**
**Base Metal (MB) Welded zone Base Metal (MB) (ZWS) HAZ HAZ**
(Watkins, 2003).
2011).
**60**
**70**
**80**
**90**
**100**
**110**
**120**
(Kalinyuk et al., 2003; Matysina, 1999) which results in the presence of columnar grain growth at the surface of the welded zone (Kurtz et al., 2001). Between the surface and the 200 µm depth (Figure 7b, 7c, 7d) the texture decreases to disappear completely from 200 µm. This change underlines the presence of a transition from columnar growth to equiaxial grain growth. These results showed that the laser welding led to a complex microstructure and induced high temperature and deformation gradients which may cause changes in crystalline orientations. The study of the texture evolution is then required to understand the anisotropic characteristic of the welds and its influence on mechanical properties. Compared with the literature, little study takes into account the texture due to laser welding. So our results showed that the laser welding can form a crystallographic texture and that it is necessary to study it thoroughly to apprehend the mechanical properties as well as possible.
#### **4.2 Experimental results of the mechanical properties**
#### **4.2.1 Hardness characterization**
From laser parameters point of view, hardness in the fusion zone was found to increase almost linearly with welding speed because higher welding speeds lead to a more significant refinement of the microstructure and more alloying elements into the matrix, even though hard intermetallics are reduced and more finely distributed at high cooling rates. The average hardness of CO2 laser welded joints decreases with slower welding speeds. Indeed, at low welding speeds the weld structure and hardness were nearly the same or sometimes lower as those in the base die-cast material. The decrease in the hardness of the HAZ was due to grain growth. However, these results depend too on the laser power and the nature of used alloy. In the literature, it was also reported that there was a gradual decrease in hardness of 6 kW CO2 laser welded joints from the BM to the HAZ to the FZ of AZ31BH24 alloy, with a minimum value in the FZ (Leong et al., 1998; Sanders et al., 1999). Dhahri et al., 2001 investigated WE43-T6 alloy using 5 kW CO2 laser. The hardness at the top and bottom of the welds was similar but the hardness in the middle of the bead was lower. In our studies, the 4 kW CO2 laser welding of die cast AZ91D alloy showed that there is an increase in hardness of the fusion zone but little variation in hardness occurs in the HAZ according to the localization of the measurements. Figure 8 shows an example of the hardness results, measured close to the surface on both sides of the linear weld in a profile including the base metal passing through the heat affected zone and the welded zone. The same measurement has been realized along the same profile and at a depth around 200 μm.
Close to the surface, the hardness varies from around 90 HV in the base metal and around 95 HV in the heat affected zone to 110 HV in the welded zone. The hardness in the HAZ is higher than in the BM, even though the size of the grains is identical. This augmentation of microhardness has in part been explained by the contribution of added elements and particularly the increase in the level of aluminium in this zone (10%). Other studies have also demonstrated the presence of precipitates which are formed in this zone considered to be a zone of diffusion which contributes towards augmenting the hardness (Shaw et al., 1997). In the fusion zone, compared to the base metal, the increase in hardness was probably
(Kalinyuk et al., 2003; Matysina, 1999) which results in the presence of columnar grain growth at the surface of the welded zone (Kurtz et al., 2001). Between the surface and the 200 µm depth (Figure 7b, 7c, 7d) the texture decreases to disappear completely from 200 µm. This change underlines the presence of a transition from columnar growth to equiaxial grain growth. These results showed that the laser welding led to a complex microstructure and induced high temperature and deformation gradients which may cause changes in crystalline orientations. The study of the texture evolution is then required to understand the anisotropic characteristic of the welds and its influence on mechanical properties. Compared with the literature, little study takes into account the texture due to laser welding. So our results showed that the laser welding can form a crystallographic texture and that it is necessary to study it thoroughly to apprehend the mechanical properties as
From laser parameters point of view, hardness in the fusion zone was found to increase almost linearly with welding speed because higher welding speeds lead to a more significant refinement of the microstructure and more alloying elements into the matrix, even though hard intermetallics are reduced and more finely distributed at high cooling rates. The average hardness of CO2 laser welded joints decreases with slower welding speeds. Indeed, at low welding speeds the weld structure and hardness were nearly the same or sometimes lower as those in the base die-cast material. The decrease in the hardness of the HAZ was due to grain growth. However, these results depend too on the laser power and the nature of used alloy. In the literature, it was also reported that there was a gradual decrease in hardness of 6 kW CO2 laser welded joints from the BM to the HAZ to the FZ of AZ31BH24 alloy, with a minimum value in the FZ (Leong et al., 1998; Sanders et al., 1999). Dhahri et al., 2001 investigated WE43-T6 alloy using 5 kW CO2 laser. The hardness at the top and bottom of the welds was similar but the hardness in the middle of the bead was lower. In our studies, the 4 kW CO2 laser welding of die cast AZ91D alloy showed that there is an increase in hardness of the fusion zone but little variation in hardness occurs in the HAZ according to the localization of the measurements. Figure 8 shows an example of the hardness results, measured close to the surface on both sides of the linear weld in a profile including the base metal passing through the heat affected zone and the welded zone. The same measurement has been realized along the same profile and at a depth around 200
Close to the surface, the hardness varies from around 90 HV in the base metal and around 95 HV in the heat affected zone to 110 HV in the welded zone. The hardness in the HAZ is higher than in the BM, even though the size of the grains is identical. This augmentation of microhardness has in part been explained by the contribution of added elements and particularly the increase in the level of aluminium in this zone (10%). Other studies have also demonstrated the presence of precipitates which are formed in this zone considered to be a zone of diffusion which contributes towards augmenting the hardness (Shaw et al., 1997). In the fusion zone, compared to the base metal, the increase in hardness was probably
well as possible.
μm.
**4.2.1 Hardness characterization**
**4.2 Experimental results of the mechanical properties**
due to its finer microstructure and higher volume fraction of intermetallics such as Mg17Al12 (Watkins, 2003).
Of the same form, through the thickness at a depth around 200 μm, the results show a different evolution than the surface concerning the BM and the HAZ, the hardness is lower. In the depth, the evolution of the microhardness shows no statistical variation of the microhardness between the base metal, the heat affected zone and the welded zone contrary to the surface. We can see that the hardness between the base metal, the heat affected zone and the core of the welded zone at a depth around 200 μm stays stable with a value of about 85 HV. Though significant grain coarsening occurred in the HAZ of AZ91 alloy, the hardness in the HAZ was still almost the same as that in base metal. These results join those of literature. Weisheit et al., 1997, studied 2.5kW CO2 laser welding of cast magnesium alloys such as AZ91 AM60, ZC63, ZE41, QE22 and WE54 and wrought alloys (AZ31, AZ61, ZW3 and ZC71). For as-cast alloys, there is an increase in hardness of the FZ but little variation in hardness occurs in the HAZ. These observations demonstrate that laser welding induces particular profiles in the zones studied. These differences in hardness distribution over laser weld joints indicate the inhomogeneity of the joints following used parameters. Hardness is influenced by the laser welding parameters but too the initial chemical composition and also depends on the manufacturing process of the magnesium alloy. The choice of the laser process CO2 or Nd:YAG influences too the hardness. This last point has been demonstrated by Hiraga et al., 2001, studied 2 kW CO2 and Nd:YAG laser welding of wrought AZ31B-H24 butt joints of 1.7mm thickness. Their results showed that the Nd:YAG laser welded fusion zone is slightly harder than the FZ produced by CO2 laser.
Fig. 8. Hardness from the base metal passing on the heat affected zone and the welded zone close to the surface and in the thickness at a depth about 200 μm (Kouadri & Barrallier, 2011).
Welding of Thin Light Alloys Sheets by CO2 Laser Beam: Magnesium Alloys 299
stresses. This evolution is in line with mechanical equilibrium. The welded zone exhibits residual traction stresses which are counter balanced by compression stresses in the base
Furthermore, study of the state of surface stresses demonstrates some anisotropy: the residual stresses are not equibiaxial. We observe that the longitudinal component decreases from the center of the weld zone towards the base metal, whereas the transverse component remains high before a sudden reduction. These changes occur in the thermally affected zone, and are associated with numerous factors and by many authors with a zone of relaxation. The evolution of the longitudinal stress can be connected with the heat flow resulting from the mobile heat source that follows the welding direction. We can see two explanations of these evolutions. On the one hand, the effect of temperature and cooling speed gradients arise from the anisotropic heat flow (Teng et al., 2002), on the other, the anisotropy can be the result of a shrinking structure. The negligible thermal dilation of both plates prevents the free shrinkage of the weld line along the direction of the weld line. The same applies to the transverse component. The restricted shrinkage in the transverse plane arises from the clamping of the plates during welding. Even if the influence of the clamping is hard to evaluate experimentally, digital studies have shown that the field of residual stresses is strongly influenced by the geometry of the assembly (Jensen et al., 2002; Dai & Shaw, 2003).
**4.2.2.2 Distribution of the residual stresses though the thickness of the welded zone**
Fig. 10. Longitudinal and transverse residual stresses from the welded zone close to the
**0 200 400 600 800 1000 1200 1400 1600 1800 2000**
**Decreasing in the core of the welded zone**
surface to the core of the welded zone (Kouadri & Barrallier, 2011)
The profiles of the average stresses through the thickness are plotted in the figure 10 for the
**Residual stresses in the welded zone from the surface to the core**
**longitudunal residual stresses transverse residual stresses**
metal (Pryds & Huang, 2000).
longitudinal and transversal residual stresses.
**Minimum close to the surface**
**Maximum in sub layer**
**0**
**20**
**40**
**60**
**80**
**100**
**120**
**1 2**
#### **4.2.2 Residual stresses results**
The origin of residual stresses and their evolution within a welded joint is difficult to evaluate because they are the result of a number of competing mechanisms: shrinkage changes in phase and microstructure (Dai & Shaw, 2003). In our case, the magnesium alloy does not undergo a phase transformation, as is the case for aluminium alloys. Numerous studies have shown that when this is the case residual stresses are primarily a consequence of an inhibited shrinkage in the weld line and of the modified microstructure which is linked to strong temperature gradients, and to their distribution within the material (Wagner, 1999; Cho et al., 2003; Mao et al., 2006).
#### **4.2.2.1 Distribution of the residual stresses at the surface of the assembled sheets**
The measurements were undertaken in the welded zone, perpendicular to the weld line towards the base metal. Figure 9 shows an example of the obtained results.
Fig. 9. Longitudinal and transverse residual stresses close to the surface from the base metal until the welded zone (Kouadri & Barrallier, 2011)
At the surface, the results demonstrated that the base metal presents a state of compression, whereas the weld line is submitted to residual traction stresses. This state of compression has been attributed to the nature of the cooling, linked to the moulding process. With laser welding, cooling occurs by the diffusion of heat through the outer surfaces of the plates which are in contact with the mould walls. Furthermore the machining by milling of the surface of the base metal before welding accentuates the state of compression and explains the raised values (- 120 MPa) observed at the surface of the base metal. However, in the weld line the heat is evacuated by the plates and not the free surfaces. This leads to traction
The origin of residual stresses and their evolution within a welded joint is difficult to evaluate because they are the result of a number of competing mechanisms: shrinkage changes in phase and microstructure (Dai & Shaw, 2003). In our case, the magnesium alloy does not undergo a phase transformation, as is the case for aluminium alloys. Numerous studies have shown that when this is the case residual stresses are primarily a consequence of an inhibited shrinkage in the weld line and of the modified microstructure which is linked to strong temperature gradients, and to their distribution within the material
**4.2.2.1 Distribution of the residual stresses at the surface of the assembled sheets**
towards the base metal. Figure 9 shows an example of the obtained results.
The measurements were undertaken in the welded zone, perpendicular to the weld line
**Longitudinal and Transversal residual stresses on the surface from the welded zone to the base metal**
> **longitudinal stresses Transversal stresses**
Fig. 9. Longitudinal and transverse residual stresses close to the surface from the base metal
**-200**
**-150**
**BM SZWHAZ HAZ BM**
**-100**
**-50**
**0**
**-3000 -2000 -1000 0 1000 2000 3000**
**50**
**100**
At the surface, the results demonstrated that the base metal presents a state of compression, whereas the weld line is submitted to residual traction stresses. This state of compression has been attributed to the nature of the cooling, linked to the moulding process. With laser welding, cooling occurs by the diffusion of heat through the outer surfaces of the plates which are in contact with the mould walls. Furthermore the machining by milling of the surface of the base metal before welding accentuates the state of compression and explains the raised values (- 120 MPa) observed at the surface of the base metal. However, in the weld line the heat is evacuated by the plates and not the free surfaces. This leads to traction
**4.2.2 Residual stresses results**
(Wagner, 1999; Cho et al., 2003; Mao et al., 2006).
until the welded zone (Kouadri & Barrallier, 2011)
stresses. This evolution is in line with mechanical equilibrium. The welded zone exhibits residual traction stresses which are counter balanced by compression stresses in the base metal (Pryds & Huang, 2000).
Furthermore, study of the state of surface stresses demonstrates some anisotropy: the residual stresses are not equibiaxial. We observe that the longitudinal component decreases from the center of the weld zone towards the base metal, whereas the transverse component remains high before a sudden reduction. These changes occur in the thermally affected zone, and are associated with numerous factors and by many authors with a zone of relaxation. The evolution of the longitudinal stress can be connected with the heat flow resulting from the mobile heat source that follows the welding direction. We can see two explanations of these evolutions. On the one hand, the effect of temperature and cooling speed gradients arise from the anisotropic heat flow (Teng et al., 2002), on the other, the anisotropy can be the result of a shrinking structure. The negligible thermal dilation of both plates prevents the free shrinkage of the weld line along the direction of the weld line. The same applies to the transverse component. The restricted shrinkage in the transverse plane arises from the clamping of the plates during welding. Even if the influence of the clamping is hard to evaluate experimentally, digital studies have shown that the field of residual stresses is strongly influenced by the geometry of the assembly (Jensen et al., 2002; Dai & Shaw, 2003).
#### **4.2.2.2 Distribution of the residual stresses though the thickness of the welded zone**
The profiles of the average stresses through the thickness are plotted in the figure 10 for the longitudinal and transversal residual stresses.
Fig. 10. Longitudinal and transverse residual stresses from the welded zone close to the surface to the core of the welded zone (Kouadri & Barrallier, 2011)
Welding of Thin Light Alloys Sheets by CO2 Laser Beam: Magnesium Alloys 301
However, during laser welding of magnesium alloys, therefore, some processing problems and weld defects can be encountered such as an unstable weld pool, substantial spatter a strong tendency to drop-through for large weld pools (Leong et al., 1998; Haferkamp et al., 1998; Sanders et al., 1999), sag of the weld pool (especially for thick workpiece), undercut (Dubé et al., 2001), porous oxide inclusions, loss of alloying elements (Leong et al., 1998; Sanders et al., 1999), excessive pore formation (particularly for die castings) (Pastor et al., 2000; Zhao & DebRoy, 2001) and solidification cracking (Marya & Edwards, 2000). These defects are generally decreased by a good optimization of the laser parameters. In view of the results achieved in this study, the use of high-power intensity focused CO2 laser beam with optimized parameters and careful material preparation prior to welding can produce welds with high quality for the most magnesium alloys, in particular for AZ91D of our study (Kouadri & Barrallier, 2006, 2010). Welding speed of 2 m/min and laser power of 4kW let to a full penetration of 3mm thickness welded joint. Optimum weld profile was obtained when focal point was placed on the top surface. In comparison with the literature, all the investigated magnesium alloys showed tendencies for porosity and solidification cracking particularly, at high welding speed (*≥*4m/min). Porosity was prevented by accurate cleaning of the base metal before welding and optimizing the flow rate of argon shielding gas. In order to maintain the mechanical properties when welding magnesium alloys, the heat input and time of exposure to very high temperatures must be minimized. For LBW, the laser power (P) and weld speed (V) directly influence the heat input. This relationship is
Beyond of the optimization of laser parameters, it is believed that the efficiency of CO2 laser beam welding of magnesium alloys could be improved by cleaning the workpiece surface prior to welding. This is due to increasing surface roughness that means decreasing surface reflectivity and enhancing the laser energy coupling during welding. Recent efforts on C02 laser beam welding have resolved several of the initial problems associated with the welding of magnesium alloys. Consistent and repeatable welds can now be obtained without resorting to meticulous edge preparation. Moreover, elimination or reduction of the plasma is recommended for optimal welding of magnesium. This effect of plasma formation which affects the weld quality and the optics during welding has been clarified: trouble-free operation of the optics has been achieved with the use of inert gas shielding
However, several results showed that the weldability of thin magnesium plates was significantly better with the Nd:YAG laser. These observations were attributed to the higher absorption of the Nd:YAG beam, which in turn reduced the threshold irradiance required for welding and produced a more stable weldpool. Indeed, an advantage of Nd:YAG laser processing is its shorter wavelength; consequently, because of the dependency of the material's emissivity on the wavelength, energy is absorbed by the material more readily than for the CO2 laser and a lower energy can be used for welding, allowing greater control of the heat input. This is particularly useful when working with thin materials. Recently, tremendous efforts have been made to clarify the fundamental laser weldability of different types of magnesium alloys using both Nd:YAG and CO2 lasers. It is pointed out that improvements in the laser weldability of a range of magnesium alloys are possible by increasing the power density of the focused spot, and this can be achieved through higher average powers, improved beam focusing system, and decreasing beam reflectivity on
often used to determine the heat input.
such as helium.
The study through the thickness of the welded zone shows that in general the profiles of the stresses reproduce the asymmetry of the welding process. Their behaviour in tension and their variation have in part been explained by the influence of the thermal cycle on the origin of residual stresses and their evolution within the material. The residual stresses on the face exposed to the laser beam are elevated (up to 80 MPa) whereas the opposite face creates stresses of only 23 and 7 MPa respectively for the longitudinal and transverse stresses. This effect can also be explained by the fact that using inert gas ensures very rapid cooling of the superior face whereas the inferior face cools more slowly (Dong et al., 2004).
Finally we have noticed that over the first 200 micrometers the residual stresses present a particular evolution. The intensity of the stresses is not maximum at the surface of the welded zone (78 and 45 MPa respectively for the longitudinal and transverse constraints) as expected, but at a depth of 200µm the stress is 100 MPa. The presence of this maximum stress demonstrates that there is a stress gradient between the textured layer and the heart of the isotropic welded zone. This specific evolution has partly been explained by the plastic deformation of the superficial layer. By presenting a strong texture and an important loss of aluminum, the superficial layer is more sensitive to plastic deformation in the plane compared to the heart of the weld line which is isotropic (Hsiao et al., 2000). These results can be compared to a thin coat deposit because these thin coatings are textured and the maximum stresses are found at the interface (Pina et al., 1997; Cevat Sarioglu 2006).
In our study there is a transition zone with a continual evolution in properties, in particular an evolution of the texture between the outer surface and the depth at about 200 µm. It appears that the development of this texture affects the distribution of stresses with a relaxation of the stresses at the surface and a maximum in the under layer. We explain these modifications by the fact that the level of plastic flow, related to local stresses, is dependent on grain orientation (Su et al., 2002; Agnew & Duygulu, 2005; Wu et al., 2007).
In conclusion, these results showed that the laser welding processes influence the residual stress distribution. Whereas compressive stresses are obtained in the base metal, tensile stresses are obtained in the LBWelds due to thermal gradients and high residual stresses are observed in the LBW fusion zone. These results showed too that it is important to take into account the crystallographic texture to evaluate the residual stresses.
#### **5. Conclusions**
The influence of various welding parameters during continuous wave CO2 laser beam welding of thin plates of magnesium alloys was investigated in this chapter. It is known that the weldability of such materials is usually not excellent and lasers can be utilized to achieve good quality welds. The obtained results and the realized synthesis from the literature showed that the CO2 laser welding possesses comprehensive performances such as good technology and the technology of laser welding magnesium alloys plates is well appropriated. The keyhole welding mode is likely to be encountered in the laser welding of thin sheet magnesium. The results of a detailed investigation showed the influence of different parameters of the laser which have to be tightly combined to obtain a weld quality.
The study through the thickness of the welded zone shows that in general the profiles of the stresses reproduce the asymmetry of the welding process. Their behaviour in tension and their variation have in part been explained by the influence of the thermal cycle on the origin of residual stresses and their evolution within the material. The residual stresses on the face exposed to the laser beam are elevated (up to 80 MPa) whereas the opposite face creates stresses of only 23 and 7 MPa respectively for the longitudinal and transverse stresses. This effect can also be explained by the fact that using inert gas ensures very rapid cooling of the superior face whereas the inferior face cools more slowly (Dong et al.,
Finally we have noticed that over the first 200 micrometers the residual stresses present a particular evolution. The intensity of the stresses is not maximum at the surface of the welded zone (78 and 45 MPa respectively for the longitudinal and transverse constraints) as expected, but at a depth of 200µm the stress is 100 MPa. The presence of this maximum stress demonstrates that there is a stress gradient between the textured layer and the heart of the isotropic welded zone. This specific evolution has partly been explained by the plastic deformation of the superficial layer. By presenting a strong texture and an important loss of aluminum, the superficial layer is more sensitive to plastic deformation in the plane compared to the heart of the weld line which is isotropic (Hsiao et al., 2000). These results can be compared to a thin coat deposit because these thin coatings are textured and the
maximum stresses are found at the interface (Pina et al., 1997; Cevat Sarioglu 2006).
on grain orientation (Su et al., 2002; Agnew & Duygulu, 2005; Wu et al., 2007).
account the crystallographic texture to evaluate the residual stresses.
In our study there is a transition zone with a continual evolution in properties, in particular an evolution of the texture between the outer surface and the depth at about 200 µm. It appears that the development of this texture affects the distribution of stresses with a relaxation of the stresses at the surface and a maximum in the under layer. We explain these modifications by the fact that the level of plastic flow, related to local stresses, is dependent
In conclusion, these results showed that the laser welding processes influence the residual stress distribution. Whereas compressive stresses are obtained in the base metal, tensile stresses are obtained in the LBWelds due to thermal gradients and high residual stresses are observed in the LBW fusion zone. These results showed too that it is important to take into
The influence of various welding parameters during continuous wave CO2 laser beam welding of thin plates of magnesium alloys was investigated in this chapter. It is known that the weldability of such materials is usually not excellent and lasers can be utilized to achieve good quality welds. The obtained results and the realized synthesis from the literature showed that the CO2 laser welding possesses comprehensive performances such as good technology and the technology of laser welding magnesium alloys plates is well appropriated. The keyhole welding mode is likely to be encountered in the laser welding of thin sheet magnesium. The results of a detailed investigation showed the influence of different parameters of the laser which have to be tightly combined to obtain a weld
2004).
**5. Conclusions**
quality.
However, during laser welding of magnesium alloys, therefore, some processing problems and weld defects can be encountered such as an unstable weld pool, substantial spatter a strong tendency to drop-through for large weld pools (Leong et al., 1998; Haferkamp et al., 1998; Sanders et al., 1999), sag of the weld pool (especially for thick workpiece), undercut (Dubé et al., 2001), porous oxide inclusions, loss of alloying elements (Leong et al., 1998; Sanders et al., 1999), excessive pore formation (particularly for die castings) (Pastor et al., 2000; Zhao & DebRoy, 2001) and solidification cracking (Marya & Edwards, 2000). These defects are generally decreased by a good optimization of the laser parameters. In view of the results achieved in this study, the use of high-power intensity focused CO2 laser beam with optimized parameters and careful material preparation prior to welding can produce welds with high quality for the most magnesium alloys, in particular for AZ91D of our study (Kouadri & Barrallier, 2006, 2010). Welding speed of 2 m/min and laser power of 4kW let to a full penetration of 3mm thickness welded joint. Optimum weld profile was obtained when focal point was placed on the top surface. In comparison with the literature, all the investigated magnesium alloys showed tendencies for porosity and solidification cracking particularly, at high welding speed (*≥*4m/min). Porosity was prevented by accurate cleaning of the base metal before welding and optimizing the flow rate of argon shielding gas. In order to maintain the mechanical properties when welding magnesium alloys, the heat input and time of exposure to very high temperatures must be minimized. For LBW, the laser power (P) and weld speed (V) directly influence the heat input. This relationship is often used to determine the heat input.
Beyond of the optimization of laser parameters, it is believed that the efficiency of CO2 laser beam welding of magnesium alloys could be improved by cleaning the workpiece surface prior to welding. This is due to increasing surface roughness that means decreasing surface reflectivity and enhancing the laser energy coupling during welding. Recent efforts on C02 laser beam welding have resolved several of the initial problems associated with the welding of magnesium alloys. Consistent and repeatable welds can now be obtained without resorting to meticulous edge preparation. Moreover, elimination or reduction of the plasma is recommended for optimal welding of magnesium. This effect of plasma formation which affects the weld quality and the optics during welding has been clarified: trouble-free operation of the optics has been achieved with the use of inert gas shielding such as helium.
However, several results showed that the weldability of thin magnesium plates was significantly better with the Nd:YAG laser. These observations were attributed to the higher absorption of the Nd:YAG beam, which in turn reduced the threshold irradiance required for welding and produced a more stable weldpool. Indeed, an advantage of Nd:YAG laser processing is its shorter wavelength; consequently, because of the dependency of the material's emissivity on the wavelength, energy is absorbed by the material more readily than for the CO2 laser and a lower energy can be used for welding, allowing greater control of the heat input. This is particularly useful when working with thin materials. Recently, tremendous efforts have been made to clarify the fundamental laser weldability of different types of magnesium alloys using both Nd:YAG and CO2 lasers. It is pointed out that improvements in the laser weldability of a range of magnesium alloys are possible by increasing the power density of the focused spot, and this can be achieved through higher average powers, improved beam focusing system, and decreasing beam reflectivity on
Welding of Thin Light Alloys Sheets by CO2 Laser Beam: Magnesium Alloys 303
this technique stays still in the developing stage and many mechanisms need to be studied because of many parameters which govern this process. However, with a good optimization, laser welding for the magnesium alloys seems to be the most appropriated joining technique and can promote their wider uses in aerospace, aircraft, automotive,
Agnew S.R., Duygulu Ö. (2005). Plastic anisotropy and the role of non-basal slip in
Cao, X., Jahazi, M., Immarigeon, J.P., Wallace, W. (2006). A review of laser welding
Cevat Sarioglu C. (2006). The effect of anisotropy on residual stress values and modification
Coelho, R.S., Kostka, A., Pinto, H., Riekehr, S., Koçak, M., Pyzalla, A.R. (2008).
Dai, K., Shaw L. (2003). Finite-Element Analysis of Effects of the Laser-Processed Bimaterial
Dhahri, M., Masse, J.E, Mathieu, JF, Barreau, G, Autric, M. (2000). CO2 laser welding of
Dhahri, M., Masse, J.E, Mathieu, JF, Barreau, G, Autric, M. (2000). CO2 laser welding of
Dhahri, M., Masse, J.E, Mathieu, JF, Barreau, G, Autric, M. (2001). Laser weldability of WE43
Dhahri, M., Masse, J.E, Mathieu, JF, Barreau, G, Autric, M. (2001). Laser welding of AZ91
Dhahri, M., Masse, J.E, Mathieu, JF, Barreau, G, Autric, M. (2002). Laser welding of
Dong, W., Kokawa, H., Tsukamoto, S., Sato Y. S., Ogawa, M. (2004). Mechanism governing
welds. *Materials Science and Engineering A*, Vol. 485, Issue 1-2, pp.20–30. D'Annessa, A. T. (1970). Sources and effects of growth rate fluctuation during weld metal
and HfN. *Surface and coatings technology*, Vol. 201, Issue 3- 4, pp.707-717. Cho, J.R., Conlon, K.T., Reed, R.C. (2003). Residual Stresses in an Electron Beam Weld of Ti-
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magnesium alloy AZ31B. *International Journal of Plasticity*, Vol. 21, Issue 6, pp. 1161-
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Microstructure and mechanical properties of magnesium alloy AZ31B laser beam
Component Size on Stresses and Distortion. *Metallurgical and Materials Transactions*
magnesium alloys. *In Proceedings of the SPIE: High power lasers in manufacturing*, pp.
magnesium alloys. *In Proceedings of the SPIE: High power lasers in manufacturing*, pp.
magnesium alloys for aeronautic industry. *In third LANE 2001: Laser Assisted Net*
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nitrogen absorption by steel weld metal during laser welding. *Metallurgical and*
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**6. References**
1193.
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workpiece surface. In conclusion, the weldability problems of magnesium alloys are much more easily overcome when using Nd:YAG than CO2 laser. However, our studies showed that CO2 laser is more appropriate to weld cast magnesium alloy than wrought magnesium alloy.
From technology point of view, in comparison with the traditional welding method such as arc welding processes, the laser welding has high efficiency, small welding distortion, low labor costs and convenient construction, is easy to realize automatization, and can be the effective measure to enhance the weld quality. Laser welding processes offer great benefit over other welding processes, e.g., arc welding, resistance welding, etc., since less heat is coupled into the workpiece. The low-heat input will tend to keep a very narrow HAZ then, retaining some to the strength of the material. The benefits of low-heat input and extremely rapid cooling rate, all of which help to minimize the metallurgical problems in the fusion zone. For example, high cooling rate will tend to slow down the development of blisters because of the short time in which the diffusion of hydrogen can take place. In comparison with electron beam welding, even though this process offers the advantages of a high energy density welding process, a vacuum chamber is required, which is not always practical.
All advantages explain their integration in the advanced technology industries such as in aerospace, aircraft, automotive, electronics and other industries. Indeed, today, laser beam welding is being used in an increasingly wider range of industries, from the production of medical devices and microelectronics to shipbuilding. The automotive industry, in particular, takes advantage of this technology's benefits: low heat input, small heat-affected zone (HAZ), low distortion rate, good repeatability, reduced need for post processing and high welding speed. This last point is becoming critical for a successful application in the automotive industry because the increase in welding speed provided by laser welding has resulted in the need for an automated system. Another application of the laser process is the aircraft where weight and cost reduction in civil aircrafts by replacing rivets by advanced welding techniques has now been realized for skin-stringer joints. In this context the laser beam welding technology has proved to be very suitable for a number of reasons, for example, low distortion while processing at high speeds. These benefits have made laser welding the process of choice for many applications that previously used resistance welding. Compared for example with TIG welding, the welding speed with laser is generally three times higher.
However, although laser materials processing has gained widespread acceptability, the mechanisms and main factors controlling the process remain controversial and need further theoretical and experimental studies. Further work is needed to develop the weld process parameters necessary to achieve the materials characteristics required for the use of magnesium alloys in industrial applications. Improved gas shielding requirements are expected to be critical to obtaining welds with the required materials properties. Further studies are needed to determine the parameters controlling weld quality. Indeed, laser beam welding involves many variables: laser power, welding speed, defocusing distance and type of shielding gas, any of which may have an important effect on heat flow and fluid flow in the weld pool. This in turn will affect penetration depth, shape and final solidification structure of the fusion zone. These final states affect the mechanical behaviour. It is for that this technique stays still in the developing stage and many mechanisms need to be studied because of many parameters which govern this process. However, with a good optimization, laser welding for the magnesium alloys seems to be the most appropriated joining technique and can promote their wider uses in aerospace, aircraft, automotive, electronics and other industries.
#### **6. References**
302 CO2 Laser – Optimisation and Application
workpiece surface. In conclusion, the weldability problems of magnesium alloys are much more easily overcome when using Nd:YAG than CO2 laser. However, our studies showed that CO2 laser is more appropriate to weld cast magnesium alloy than wrought magnesium
From technology point of view, in comparison with the traditional welding method such as arc welding processes, the laser welding has high efficiency, small welding distortion, low labor costs and convenient construction, is easy to realize automatization, and can be the effective measure to enhance the weld quality. Laser welding processes offer great benefit over other welding processes, e.g., arc welding, resistance welding, etc., since less heat is coupled into the workpiece. The low-heat input will tend to keep a very narrow HAZ then, retaining some to the strength of the material. The benefits of low-heat input and extremely rapid cooling rate, all of which help to minimize the metallurgical problems in the fusion zone. For example, high cooling rate will tend to slow down the development of blisters because of the short time in which the diffusion of hydrogen can take place. In comparison with electron beam welding, even though this process offers the advantages of a high energy density welding process, a vacuum chamber is required, which is not always
All advantages explain their integration in the advanced technology industries such as in aerospace, aircraft, automotive, electronics and other industries. Indeed, today, laser beam welding is being used in an increasingly wider range of industries, from the production of medical devices and microelectronics to shipbuilding. The automotive industry, in particular, takes advantage of this technology's benefits: low heat input, small heat-affected zone (HAZ), low distortion rate, good repeatability, reduced need for post processing and high welding speed. This last point is becoming critical for a successful application in the automotive industry because the increase in welding speed provided by laser welding has resulted in the need for an automated system. Another application of the laser process is the aircraft where weight and cost reduction in civil aircrafts by replacing rivets by advanced welding techniques has now been realized for skin-stringer joints. In this context the laser beam welding technology has proved to be very suitable for a number of reasons, for example, low distortion while processing at high speeds. These benefits have made laser welding the process of choice for many applications that previously used resistance welding. Compared for example with TIG welding, the welding speed with laser is
However, although laser materials processing has gained widespread acceptability, the mechanisms and main factors controlling the process remain controversial and need further theoretical and experimental studies. Further work is needed to develop the weld process parameters necessary to achieve the materials characteristics required for the use of magnesium alloys in industrial applications. Improved gas shielding requirements are expected to be critical to obtaining welds with the required materials properties. Further studies are needed to determine the parameters controlling weld quality. Indeed, laser beam welding involves many variables: laser power, welding speed, defocusing distance and type of shielding gas, any of which may have an important effect on heat flow and fluid flow in the weld pool. This in turn will affect penetration depth, shape and final solidification structure of the fusion zone. These final states affect the mechanical behaviour. It is for that
alloy.
practical.
generally three times higher.
Welding of Thin Light Alloys Sheets by CO2 Laser Beam: Magnesium Alloys 305
Mao, W.G., Zhou, Y.C., Yang L., Yu, X.H. (2006). Modeling of residual stresses variation
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Sanders, P.G., Keske, J.S., Leong, K.H., Kornecki, G. (1999). High power Nd:YAG and CO2 laser welding of magnesium. *J. Laser Appl*., Vol. 11, Issue 2, pp. 96–103. Shaw, C., Jones, H. (1997). The contributions of different alloying additions to hardening in
StJohn, D.H., Dahle, A.K., Abbott, T., Nave M.D., Qian, M. (2003).Solidification of cast magnesium alloys. *The minerals, Metals and Materials Society*, pp. 95-100. Su, S.F., Huang, J.C., Lin, H.K., Ho N.J. (2002). Electron-Beam Welding Behavior in Mg-Al-
Teng, T.L, Fung C.H., Chang, P.H. (2002). Effect of weld geometry and residual stresses on
Wagner, L. (1999). Mechanical surface treatments on titanium, aluminum and magnesium alloys. *Materials Science and Engineering A,* Vol. 263, Issue 2, pp. 210-216. Wang H.Y., Li, Z.J. (2006). Investigation of laser beam welding process of AZ61 magnesium based alloy. *Acta metallurgica sinica (english letters)*, Vol. 19, Issue 4, pp. 287–294. Watkins, K.G. (2003). Laser welding of magnesium alloys. *In H.I. Kaplan, editor, Magnesium Technology, TMS Annual Meeting and Exhibition*, pp. 153–156, San Diego. Weisheit, A., Galun, R., Mordike, B.L. (1997). Weldability of various magnesium alloys
Weisheit, Galun, R., Mordike, B.L. (1998). CO2 laser beam welding of magnesium-based
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**12**
*Iran*
Mohammadreza Riahi
**CO2 Laser and Micro-Fluidics**
*Shahid Beheshti University/Laser and Plasma Research Institute*
Microfluidic chips have attracted significant attention over the past decade due to their wide range of potential applications in the biomedical and chemical analysis field such as drug delivery, Point of care diagnostics (Jakeway et al, 2004), flow cytometry (Fu LM et al 2004; Chen & Wang, 2009; Lin et al, 2009), polymerize chain reaction (Suna et al, 2007; Sun & Kwok, 2006; Hsieh et al, 2009), electrophoresis (Fu et al, 2007, 2009) and many other
Traditionally, silicon and glass are the predominant materials employed in the design of microfluidic systems. This was primarily due to their excellent chemical, physical, electrical and optical properties. But fabrication of a microfluidic device on these materials needs standard photolithography equipments such as Reactive Ion Etching (RIE) system which are very expensive and increases the production costs specially in single-use applications
In recent years application of polymeric materials for microfluidic device fabrication is becoming more and more important. Different methods for microfluidic fabrication on polymers such as hot embossing (Gerlach et al, 2002), injection molding (Rotting et al, 2002),
Different kind of lasers such as UV (Ball et al, 2000) and Infrared lasers is used for laser micromachining of polymers. In infrared regime, CO2 laser has a predominant application
In this chapter, we will deal with application of a CO2 laser in microfluidic device fabrication. The application of CO2 laser for fabrication of a optofluidic device and
Application of the CO2 laser for microfluidic device fabrication was first proposed in 2002
CO2 laser emits radiation with the wavelength of 10.6 micrometer. A CO2 laser mostly interacts with a polymer, photo-thermally. When a CO2 laser is irradiated on a polymer surface, it is strongly absorbed and raises the temperature of the polymer. The polymer is
application of a optofluidic device for CO2 laser characterization is also presented.
soft lithography (Xia et al, 1998) and laser micromachining can be applied.
**1. Introduction**
applications.
which are desired in order to avoid contamination.
due to it's excellent absorption in polymers.
by H. Klank et al (Klank et al, 2002).
**2. Interaction of a CO2 laser with polymers**
then melted, decomposed and leaving a void in a workpiece.
high purity titanium using a Taylor-type crystal plasticity. *Acta Materialia*, Vol.55, pp. 423-432.
Zhao, H., DebRoy, T. (2001). Pore formation during laser beam welding of die cast magnesium alloy AM60B – mechanism and remedy*. Weld. J*., Vol. 80, Issue 8, pp. 204S–210S.
### **CO2 Laser and Micro-Fluidics**
### Mohammadreza Riahi
*Shahid Beheshti University/Laser and Plasma Research Institute Iran*
#### **1. Introduction**
306 CO2 Laser – Optimisation and Application
Zhao, H., DebRoy, T. (2001). Pore formation during laser beam welding of die cast
pp. 423-432.
204S–210S.
high purity titanium using a Taylor-type crystal plasticity. *Acta Materialia*, Vol.55,
magnesium alloy AM60B – mechanism and remedy*. Weld. J*., Vol. 80, Issue 8, pp.
Microfluidic chips have attracted significant attention over the past decade due to their wide range of potential applications in the biomedical and chemical analysis field such as drug delivery, Point of care diagnostics (Jakeway et al, 2004), flow cytometry (Fu LM et al 2004; Chen & Wang, 2009; Lin et al, 2009), polymerize chain reaction (Suna et al, 2007; Sun & Kwok, 2006; Hsieh et al, 2009), electrophoresis (Fu et al, 2007, 2009) and many other applications.
Traditionally, silicon and glass are the predominant materials employed in the design of microfluidic systems. This was primarily due to their excellent chemical, physical, electrical and optical properties. But fabrication of a microfluidic device on these materials needs standard photolithography equipments such as Reactive Ion Etching (RIE) system which are very expensive and increases the production costs specially in single-use applications which are desired in order to avoid contamination.
In recent years application of polymeric materials for microfluidic device fabrication is becoming more and more important. Different methods for microfluidic fabrication on polymers such as hot embossing (Gerlach et al, 2002), injection molding (Rotting et al, 2002), soft lithography (Xia et al, 1998) and laser micromachining can be applied.
Different kind of lasers such as UV (Ball et al, 2000) and Infrared lasers is used for laser micromachining of polymers. In infrared regime, CO2 laser has a predominant application due to it's excellent absorption in polymers.
In this chapter, we will deal with application of a CO2 laser in microfluidic device fabrication. The application of CO2 laser for fabrication of a optofluidic device and application of a optofluidic device for CO2 laser characterization is also presented.
#### **2. Interaction of a CO2 laser with polymers**
Application of the CO2 laser for microfluidic device fabrication was first proposed in 2002 by H. Klank et al (Klank et al, 2002).
CO2 laser emits radiation with the wavelength of 10.6 micrometer. A CO2 laser mostly interacts with a polymer, photo-thermally. When a CO2 laser is irradiated on a polymer surface, it is strongly absorbed and raises the temperature of the polymer. The polymer is then melted, decomposed and leaving a void in a workpiece.
CO2 Laser and Micro-Fluidics 309
Cheng et. al. reported that the roughness of the machined channels can be treated by thermal annealing of the samples (Cheng et al 2004). Fig. 2. shows the surface of their work
Hong et. al. also reported that the roughness of the microfluidic structures can be drastically
Fig. 2. The SEM pictures showing the rugged interior surface of the trench after laser machining (a) and smooth surface after thermal annealing (b). The AFM topography of the annealed surface is shown in the inset with full scale of 38 nm in the *Z*-axes. The viewing angle is perpendicular to the plane of the side wall (Cheng et al 2004) - Reproduced by
PMMA micro fluidic structures can then be top covered by other polymers like PMMA or poly carbonate (PC) utilizing thermal-bonding process. Thermal bonding is a process of joining two materials by the mechanism of diffusion; and unity of the materials. This process is accomplished through the application of pressure at temperature higher that the
permission of Elsevier under the license no. 283342005275.
glass temperature of the polymers.
reduced by out of focus machining of PMMA (Hong, et al, 2010).
piece before and after annealing.
Different kind of polymers can be used for microfluidic applications by taking a choice care that the fluid in the device do not interact chemically with the device. However just some of the polymers can be machined with CO2 laser. Most of the polymers leave contamination and soot when exposed to CO2 laser irradiation. For example, polycarbonate (PC) leaves a brownish residue after exposing to the CO2 laser.
Among different kind of polymers, poly methyl methacrylate (PMMA) is the most suitable polymer for CO2 laser machining. When PMMA heats up by the CO2 laser, after passing the glass temperature, the material turns into a rubbery material and by increasing the temperature, the chains are broken by depropagation process (Arisawa & Brill, 1997; Ferriol et al, 2003), and decompose with a non-charring process to it's MMA monomer which is volatile
$$Solids \to Volattices\tag{1}$$
Fig. 1 shows the decomposition process of PMMA polymer.
Fig. 1. Decomposition process of PMMA.
Decomposition of PMMA into volatile MMA monomers makes a hole in the workpiece. The shape and size of the hole, highly depends on the thermal properties of PMMA, focusing parameters, laser beam profile, exposure time and even exposure strategy. At the beginning of the exposure, the shape of the channel is very similar to the laser beam profile but as time goes up, the shape of the hole becomes more conical.
#### **3. Fabrication of a channel on PMMA utilizing a CO2 laser**
Fabrication of a channel on the surface of PMMA can be performed by scanning a CO2 laser over the surface of the workpiece. Commercial CO2 engraving systems with laser powers about a few watts to a few tens of watts and scanning speeds from a few tens of mm/sec up to a few hundreds of mm/sec, are good choices for micro channel fabrication.
By scanning the PMMA surface with CO2 laser, different channels and cavities can be fabricated. However, the ablated structures may be very rugged such that those can not be used for microfluidic structures with optical detection. Martin et al. reported that the roughness of the machined surfaces depends on the grade of the PMMA sheets. He reported roughness of 1.54 microns and 0.42 microns for two different grades of PMMA (Martin et al, 2003). Presence of the different roughness should probably be sought in the chemical additives of the different types of PMMA.
Different kind of polymers can be used for microfluidic applications by taking a choice care that the fluid in the device do not interact chemically with the device. However just some of the polymers can be machined with CO2 laser. Most of the polymers leave contamination and soot when exposed to CO2 laser irradiation. For example, polycarbonate (PC) leaves a
Among different kind of polymers, poly methyl methacrylate (PMMA) is the most suitable polymer for CO2 laser machining. When PMMA heats up by the CO2 laser, after passing the glass temperature, the material turns into a rubbery material and by increasing the temperature, the chains are broken by depropagation process (Arisawa & Brill, 1997; Ferriol et al, 2003), and decompose with a non-charring process to it's MMA monomer which is volatile
Decomposition of PMMA into volatile MMA monomers makes a hole in the workpiece. The shape and size of the hole, highly depends on the thermal properties of PMMA, focusing parameters, laser beam profile, exposure time and even exposure strategy. At the beginning of the exposure, the shape of the channel is very similar to the laser beam profile but as time
Fabrication of a channel on the surface of PMMA can be performed by scanning a CO2 laser over the surface of the workpiece. Commercial CO2 engraving systems with laser powers about a few watts to a few tens of watts and scanning speeds from a few tens of mm/sec up
By scanning the PMMA surface with CO2 laser, different channels and cavities can be fabricated. However, the ablated structures may be very rugged such that those can not be used for microfluidic structures with optical detection. Martin et al. reported that the roughness of the machined surfaces depends on the grade of the PMMA sheets. He reported roughness of 1.54 microns and 0.42 microns for two different grades of PMMA (Martin et al, 2003). Presence of the different roughness should probably be sought in the chemical
*Solids Volatiles* → (1)
brownish residue after exposing to the CO2 laser.
Fig. 1. Decomposition process of PMMA.
additives of the different types of PMMA.
goes up, the shape of the hole becomes more conical.
**3. Fabrication of a channel on PMMA utilizing a CO2 laser**
to a few hundreds of mm/sec, are good choices for micro channel fabrication.
Fig. 1 shows the decomposition process of PMMA polymer.
Cheng et. al. reported that the roughness of the machined channels can be treated by thermal annealing of the samples (Cheng et al 2004). Fig. 2. shows the surface of their work piece before and after annealing.
Hong et. al. also reported that the roughness of the microfluidic structures can be drastically reduced by out of focus machining of PMMA (Hong, et al, 2010).
Fig. 2. The SEM pictures showing the rugged interior surface of the trench after laser machining (a) and smooth surface after thermal annealing (b). The AFM topography of the annealed surface is shown in the inset with full scale of 38 nm in the *Z*-axes. The viewing angle is perpendicular to the plane of the side wall (Cheng et al 2004) - Reproduced by permission of Elsevier under the license no. 283342005275.
PMMA micro fluidic structures can then be top covered by other polymers like PMMA or poly carbonate (PC) utilizing thermal-bonding process. Thermal bonding is a process of joining two materials by the mechanism of diffusion; and unity of the materials. This process is accomplished through the application of pressure at temperature higher that the glass temperature of the polymers.
CO2 Laser and Micro-Fluidics 311
When a shape, is engraved in a raster scan mode by a CO2 laser engraving system on PMMA, the system scans a shape, row by row which each row has a certain overlap with a previous row. During the first row scan, a symmetrical V-shape channel is ablated on the PMMA surface. When the laser scans the subsequent rows, a small portion of the laser beam reflects from the wall of the channel produced by the previous scan to the bottom of the hole in the opposite side of the scanning direction as shown in Fig. 4a. After several scans, the reflected beam can ablate a considerable amount of PMMA material at the bottom of the hole at the opposite side of the scanning direction which can causes a bending shape in the
Fig. 4. Ablation of a PMMA hole with CO2 laser. a) Reflection of the laser from the walls of
It is found that the shape of the holes can be controlled by adjusting the scanning parameters such as resolution, power and scan speed. Some of the fabricated holes have very bent shapes and some are straight. Fig. 5 shows the ablated bending holes for different
an ablated hole. b) The shape of the hole after several scans (Riahi, 2012).
Fig. 5. The ablated bending holes for different scan parameters (Riahi, 2012).
**4.1 Fabrication of the bending cones**
structure (Fig. 4b).
scan parameters.
In addition to fabrication of the holes, channels and cavities, CO2 laser machining can be used to make some complicated structures, like bending holes. These structures can also be molded with other materials such as PDMS to get the negative of the PMMA structures. In the next section fabrication technique of the other complicated structure with 3D structure is presented.
#### **4. Fabrication of a 3D Mixer with CO2 laser machining of PMMA and PDMS molding**
In this section we present application of a CO2 laser for fabrication of a 3D mixer with bending cones (Riahi, 2012). Mixers are the elements in microfluidic and micro total analysis systems which are used for mixing two or more liquids in biological and chemical analyses. Mixers can be divided into the two categories, active and passive. In active mixers, an external actuation mechanism is used to mix liquids in a microfluidic chamber. In passive mixers, there is no energy consumption and the structure of these devices is simpler than that of active devices. Different schemes such as a Tesla structure (Hong et al, 2004), a T mixer (Hoe et al, 2004), a 3D serpentine (Liu et al, 2000) and twisted shapes (Bertsch et al, 2001) are also used in passive micro mixers.
The technique which is presented here is based on the application of the CO2 laser for fabrication of some bending and straight cones on PMMA followed by PDMS molding. The designed mixer is shown in Fig. 3.
Fig. 3. Schematic of the designed 3D mixer (Riahi, 2012).
In addition to fabrication of the holes, channels and cavities, CO2 laser machining can be used to make some complicated structures, like bending holes. These structures can also be molded with other materials such as PDMS to get the negative of the PMMA structures. In the next section fabrication technique of the other complicated structure with 3D structure is
**4. Fabrication of a 3D Mixer with CO2 laser machining of PMMA and PDMS**
In this section we present application of a CO2 laser for fabrication of a 3D mixer with bending cones (Riahi, 2012). Mixers are the elements in microfluidic and micro total analysis systems which are used for mixing two or more liquids in biological and chemical analyses. Mixers can be divided into the two categories, active and passive. In active mixers, an external actuation mechanism is used to mix liquids in a microfluidic chamber. In passive mixers, there is no energy consumption and the structure of these devices is simpler than that of active devices. Different schemes such as a Tesla structure (Hong et al, 2004), a T mixer (Hoe et al, 2004), a 3D serpentine (Liu et al, 2000) and twisted shapes (Bertsch et al,
The technique which is presented here is based on the application of the CO2 laser for fabrication of some bending and straight cones on PMMA followed by PDMS molding. The
presented.
**molding**
2001) are also used in passive micro mixers.
Fig. 3. Schematic of the designed 3D mixer (Riahi, 2012).
designed mixer is shown in Fig. 3.
#### **4.1 Fabrication of the bending cones**
When a shape, is engraved in a raster scan mode by a CO2 laser engraving system on PMMA, the system scans a shape, row by row which each row has a certain overlap with a previous row. During the first row scan, a symmetrical V-shape channel is ablated on the PMMA surface. When the laser scans the subsequent rows, a small portion of the laser beam reflects from the wall of the channel produced by the previous scan to the bottom of the hole in the opposite side of the scanning direction as shown in Fig. 4a. After several scans, the reflected beam can ablate a considerable amount of PMMA material at the bottom of the hole at the opposite side of the scanning direction which can causes a bending shape in the structure (Fig. 4b).
Fig. 4. Ablation of a PMMA hole with CO2 laser. a) Reflection of the laser from the walls of an ablated hole. b) The shape of the hole after several scans (Riahi, 2012).
It is found that the shape of the holes can be controlled by adjusting the scanning parameters such as resolution, power and scan speed. Some of the fabricated holes have very bent shapes and some are straight. Fig. 5 shows the ablated bending holes for different scan parameters.
Fig. 5. The ablated bending holes for different scan parameters (Riahi, 2012).
CO2 Laser and Micro-Fluidics 313
The molded PDMS structures are then stacked to each other and three steel tubes are inserted into the input and output channels and the voids are filled with PDMS to form the
Optofluidics refers to a science that uses the optical property of fluids for adjusting, measuring the properties of a device. Some examples of such devices are, liquid mirrors (Wood, 1909), liquid-crystal displays (Haas, 1983) and liquid lenses (Kuiper & Hendriks,
Several techniques are used to fabricate a tunable lens array (Dong et al, 2006; Jeong et al,
In this section we show how a CO2 laser can be used for fabrication of an optofluidic device,
The liquid microlens array is an array of tunable liquid lenses which can be used for Medical
Fig. 8. shows the basic structure of the liquid lens array which is presented here. An array of the hexagonal holes with about 2mm width each, are first fabricated on a 1mm thick PMMA sheet. A thin layer of PDMS with the thickness of about 50 microns is fabricated and placed on the array of holes. A 1mm depth reservoir with an inlet and outlet for the fluid is also
By introducing a liquid into the reservoir and changing the pressure inside, the curvature of the PDMS sheet in place of the holes changes and produces convex lenses as shown in
stereoendoscopy, Telecommunication, Optical data storage, Photonic imaging, etc.
fabricated. The whole of the collection are placed on top of each other.
Fig. 8. The structure of a tunable liquid lens array.
final mixer structure. The fabricated mixer is shown in Fig. 7.
2004).
Fig. 9.
2004; Xu et al 2009)
liquid micro lens array (Riahi, 2011).
**5. Fabrication of the structures for optofluidics applications**
#### **4.2 Ablation of the mixer structure**
To fabricate the mixer, a few straight cones and bending cones are ablated with CO2 laser on two different PMMA sheets. One of the PMMA sheets is CO2 laser cut to form a channel with two inputs and one output. The structures are then molded with PDMS and one is placed upside down on top of the other. Fig. 6. shows the fabricated channels and holes on the PMMA sheet and the molded PDMS structure.
Fig. 6. a) Fabricated structures on PMMA sheets. b) The PDMS molds of structures shown in part a. The straight and bending cones are clear (Riahi, 2012).
Fig. 7. The fabricated mixer (Riahi, 2012).
To fabricate the mixer, a few straight cones and bending cones are ablated with CO2 laser on two different PMMA sheets. One of the PMMA sheets is CO2 laser cut to form a channel with two inputs and one output. The structures are then molded with PDMS and one is placed upside down on top of the other. Fig. 6. shows the fabricated channels and holes on
Fig. 6. a) Fabricated structures on PMMA sheets. b) The PDMS molds of structures shown in
part a. The straight and bending cones are clear (Riahi, 2012).
Fig. 7. The fabricated mixer (Riahi, 2012).
**4.2 Ablation of the mixer structure**
the PMMA sheet and the molded PDMS structure.
The molded PDMS structures are then stacked to each other and three steel tubes are inserted into the input and output channels and the voids are filled with PDMS to form the final mixer structure. The fabricated mixer is shown in Fig. 7.
### **5. Fabrication of the structures for optofluidics applications**
Optofluidics refers to a science that uses the optical property of fluids for adjusting, measuring the properties of a device. Some examples of such devices are, liquid mirrors (Wood, 1909), liquid-crystal displays (Haas, 1983) and liquid lenses (Kuiper & Hendriks, 2004).
Several techniques are used to fabricate a tunable lens array (Dong et al, 2006; Jeong et al, 2004; Xu et al 2009)
In this section we show how a CO2 laser can be used for fabrication of an optofluidic device, liquid micro lens array (Riahi, 2011).
The liquid microlens array is an array of tunable liquid lenses which can be used for Medical stereoendoscopy, Telecommunication, Optical data storage, Photonic imaging, etc.
Fig. 8. shows the basic structure of the liquid lens array which is presented here. An array of the hexagonal holes with about 2mm width each, are first fabricated on a 1mm thick PMMA sheet. A thin layer of PDMS with the thickness of about 50 microns is fabricated and placed on the array of holes. A 1mm depth reservoir with an inlet and outlet for the fluid is also fabricated. The whole of the collection are placed on top of each other.
Fig. 8. The structure of a tunable liquid lens array.
By introducing a liquid into the reservoir and changing the pressure inside, the curvature of the PDMS sheet in place of the holes changes and produces convex lenses as shown in Fig. 9.
CO2 Laser and Micro-Fluidics 315
Fig. 11. The fabricated tunable liquid lens array.
Fig. 12. Imaging from the letter "B" with the fabricated liquid lens array.
Fig. 9. Mechanism of convex micro lens creation by applying pressure in a water reservoir limited by PDMS and PMMA walls.
As shown in Fig. 10, a commercial CO2 laser engraving system is used for producing the patterns on PMMA sheets. This engraving machine is also used for fabrication of the reservoir on PMMA sheets.
Fig. 10. The commercial CO2 laser engraving system in production process of an array of hexagonal holes on a PMMA sheet.
Fig. 11 shows the fabricated tunable microlens array with this technique. Fig. 12 also shows this microlens array in imaging from a "B" letter.
Fig. 9. Mechanism of convex micro lens creation by applying pressure in a water reservoir
As shown in Fig. 10, a commercial CO2 laser engraving system is used for producing the patterns on PMMA sheets. This engraving machine is also used for fabrication of the
Fig. 10. The commercial CO2 laser engraving system in production process of an array of
Fig. 11 shows the fabricated tunable microlens array with this technique. Fig. 12 also shows
limited by PDMS and PMMA walls.
hexagonal holes on a PMMA sheet.
this microlens array in imaging from a "B" letter.
reservoir on PMMA sheets.
Fig. 11. The fabricated tunable liquid lens array.
Fig. 12. Imaging from the letter "B" with the fabricated liquid lens array.
CO2 Laser and Micro-Fluidics 317
normalized intensity
normalized intensity
normalized intensity
Fig. 13. (a) Square-well grating with n1 and n2 for the refractive indices of the land and the groove. (b)Wavefront of an incoming ray immediately after passing through the grating. (c), (d), (e) Simulation results of diffraction from the grating shown in (a) for γ = 0, γ = π=2, and γ = π, respectively. On the vertical axes, the maximum intensity has been normalized to unity. (f) Results of simulation of the intensity of the 1st order of diffraction versus phase
difference (Riahi et al, 2008).
#### **6. Fabrication of a beam profiler using the optical properties of liquids**
In the previous sections we focused on the application of the CO2 laser for fabrication of the devices used in microfluidics and optofluidics. In this section we look at the application of a fluid device which is used for CO2 laser characterization. We present a device called thermally tunable grating (TTG), which can be used as a CO2 laser beam profiler.
Thermally tunable grating is a family of the gratings which some of their specifications can be adjusted by the user. The tuning ability of a diffractive grating can be divided into two categories: first, gratings in which the diffractive angle can be tuned, and second, gratings in which the intensity of diffraction orders can be modulated which are called grating light valves (GLV). Electrostatic actuation is one of the methods used in MEMS based grating light valves system (Trisnadi et al, 2004). In this grating light valve system, tiny suspended ribbons are put together to form a specular surface. Electrostatic actuation lowers some of the ribbons, and a diffractive grating is formed. Electric field actuation has also been used to actuate an electro-optically controlled liquid crystal based GLV (Chen et al, 1995).
But in TTG device, thermal method is used for actuation of a grating which contains a liquid in it's grooves. Increasing temperature, changes the refractive index of the liquid and consequently the diffraction efficiency of the grating (Riahi et al, 2008).
#### **6.1 Principle of the method**
As shown in Fig. 13a, we suppose that the grooves of a transparent binary grating with refractive index n1 are filled with another transparent material with refractive index n2. Assume that a laser beam with wavelength λ is incident on this grating. If the period of the grating is large enough compared to the wavelength of light, the rays that pass through the n1 and n2 materials will have phases φ1 and φ2, respectively and the phase difference Δφ= φ1- φ2 as shown in Fig. 13b.
By changing Δφ, the intensity of the diffraction orders is changed as shown in Fig. 13c,d,e. It can be shown that the intensity of the first order of diffraction can be calculated as follow (Riahi et al, 2009):
$$I = I\_{\text{max}} S \dot{m}^2(\frac{\Delta \varphi}{2}) \tag{2}$$
It is clear now that if n1 or n2 are changed, the intensity of the first order of diffraction changes sinusoidally (Fig. 13f).
#### **6.2 Fabrication method**
Standard lithography technique is used for fabrication of the binary grating on a glass substrate (n=1.52). As shown in Fig 14, the grooves are then filled with nitrobenzene and a thin glass sheet with 250 microns thickness is placed on it. The high boiling point (T= 210:8 °C), low specific heat capacity (1:51 J/gK), and high dn/dT (−4:6 × 10−4 K−1 at 626.58 at T= 288 K) [36] make nitrobenzene suitable for this work. The refractive index of nitrobenzene is 1.546 at 656:28nm at 293:15 K.
In the previous sections we focused on the application of the CO2 laser for fabrication of the devices used in microfluidics and optofluidics. In this section we look at the application of a fluid device which is used for CO2 laser characterization. We present a device called
Thermally tunable grating is a family of the gratings which some of their specifications can be adjusted by the user. The tuning ability of a diffractive grating can be divided into two categories: first, gratings in which the diffractive angle can be tuned, and second, gratings in which the intensity of diffraction orders can be modulated which are called grating light valves (GLV). Electrostatic actuation is one of the methods used in MEMS based grating light valves system (Trisnadi et al, 2004). In this grating light valve system, tiny suspended ribbons are put together to form a specular surface. Electrostatic actuation lowers some of the ribbons, and a diffractive grating is formed. Electric field actuation has also been used to
But in TTG device, thermal method is used for actuation of a grating which contains a liquid in it's grooves. Increasing temperature, changes the refractive index of the liquid and
As shown in Fig. 13a, we suppose that the grooves of a transparent binary grating with refractive index n1 are filled with another transparent material with refractive index n2. Assume that a laser beam with wavelength λ is incident on this grating. If the period of the grating is large enough compared to the wavelength of light, the rays that pass through the n1 and n2 materials will have phases φ1 and φ2, respectively and the phase difference Δφ=
By changing Δφ, the intensity of the diffraction orders is changed as shown in Fig. 13c,d,e. It can be shown that the intensity of the first order of diffraction can be calculated as follow
> 2 max ( ) <sup>2</sup> *I I Sin* <sup>Δ</sup>
It is clear now that if n1 or n2 are changed, the intensity of the first order of diffraction
Standard lithography technique is used for fabrication of the binary grating on a glass substrate (n=1.52). As shown in Fig 14, the grooves are then filled with nitrobenzene and a thin glass sheet with 250 microns thickness is placed on it. The high boiling point (T= 210:8 °C), low specific heat capacity (1:51 J/gK), and high dn/dT (−4:6 × 10−4 K−1 at 626.58 at T= 288 K) [36] make nitrobenzene suitable for this work. The refractive index of nitrobenzene is
ϕ
= (2)
**6. Fabrication of a beam profiler using the optical properties of liquids**
thermally tunable grating (TTG), which can be used as a CO2 laser beam profiler.
actuate an electro-optically controlled liquid crystal based GLV (Chen et al, 1995).
consequently the diffraction efficiency of the grating (Riahi et al, 2008).
**6.1 Principle of the method**
φ1- φ2 as shown in Fig. 13b.
changes sinusoidally (Fig. 13f).
1.546 at 656:28nm at 293:15 K.
**6.2 Fabrication method**
(Riahi et al, 2009):
Fig. 13. (a) Square-well grating with n1 and n2 for the refractive indices of the land and the groove. (b)Wavefront of an incoming ray immediately after passing through the grating. (c), (d), (e) Simulation results of diffraction from the grating shown in (a) for γ = 0, γ = π=2, and γ = π, respectively. On the vertical axes, the maximum intensity has been normalized to unity. (f) Results of simulation of the intensity of the 1st order of diffraction versus phase difference (Riahi et al, 2008).
CO2 Laser and Micro-Fluidics 319
Fig. 15. Diffraction order intensities at different temperatures: (a) T = 77 °C, (b) T = 108 °C, and (c) T = 140 °C. The maximum intensity is normalized to unity. (d) Experimental result of the intensity of the 1st order of diffraction versus temperature. The maximum intensity is
Fig. 16. Setup used for measurement of the temperature profile of the CO2 laser (Riahi et al,
The Image produced on the CCD camera and measured beam profile of the CO2 laser is
normalized to unity (Riahi et al, 2008).
2008).
shown in Fig. 17.
The diffraction pattern and intensity of the first order of diffraction versus temperature has been presented in Fig. 15.
#### **6.3 Measurement of the beam profile of a CO2 laser**
By changing the temperature, the intensity of the 1st order of diffraction is changed. The temperature of the TTG changes upon radiation by a CO2 laser beam. Radiation of a CO2 laser beam on a substrate warms it up and produces a temperature profile on the surface of the substrate. The temperature profile depends on the intensity profile of the laser beam. For example, if the laser profile is circular Gaussian, the temperature profile on the surface will be circular Gaussian in ideal case. Now if another visible laser is expanded and diffracted from the surface of the grating, the laser will be diffracted in different amounts from different parts of the grating, containing information on the temperature profile on the grating.
The setup shown in Fig. 16 is used to measure the beam profile of a CO2 laser. In this setup, a CO2 laser and a 658nm diode laser are made collinear with each other using a ZnSe window, and finally both lasers are irradiated on a 4mm × 4mm TTG device. The diode laser is expanded to about 3 cm diameter to cover the 4mm × 4mm TTG device with uniform intensity. The CO2 laser is passed through a shutter so that the irradiation time can be controlled. Immediately after the CO2 laser pulse, the CCD camera takes a picture from diffracted diode laser by a 4f imaging system using the 1st order of diffraction. It takes about 1 min for the device to get cool enough to repeat the experiment. The heat gun shown in Fig. 16. is used to keep the working area between point A and B as specified in Fig. 15d.
Fig. 14. Fabrication of the TTG device: (a) the grooves of the grating are filled with nitrobenzene and (b) a supporting glass is placed on the device (Riahi et al, 2008).
The diffraction pattern and intensity of the first order of diffraction versus temperature has
By changing the temperature, the intensity of the 1st order of diffraction is changed. The temperature of the TTG changes upon radiation by a CO2 laser beam. Radiation of a CO2 laser beam on a substrate warms it up and produces a temperature profile on the surface of the substrate. The temperature profile depends on the intensity profile of the laser beam. For example, if the laser profile is circular Gaussian, the temperature profile on the surface will be circular Gaussian in ideal case. Now if another visible laser is expanded and diffracted from the surface of the grating, the laser will be diffracted in different amounts from different parts of the grating, containing information on the temperature profile on the
The setup shown in Fig. 16 is used to measure the beam profile of a CO2 laser. In this setup, a CO2 laser and a 658nm diode laser are made collinear with each other using a ZnSe window, and finally both lasers are irradiated on a 4mm × 4mm TTG device. The diode laser is expanded to about 3 cm diameter to cover the 4mm × 4mm TTG device with uniform intensity. The CO2 laser is passed through a shutter so that the irradiation time can be controlled. Immediately after the CO2 laser pulse, the CCD camera takes a picture from diffracted diode laser by a 4f imaging system using the 1st order of diffraction. It takes about 1 min for the device to get cool enough to repeat the experiment. The heat gun shown in Fig. 16. is used to keep the working area between point A and B as specified
Fig. 14. Fabrication of the TTG device: (a) the grooves of the grating are filled with nitrobenzene and (b) a supporting glass is placed on the device (Riahi et al, 2008).
been presented in Fig. 15.
grating.
in Fig. 15d.
**6.3 Measurement of the beam profile of a CO2 laser**
Fig. 15. Diffraction order intensities at different temperatures: (a) T = 77 °C, (b) T = 108 °C, and (c) T = 140 °C. The maximum intensity is normalized to unity. (d) Experimental result of the intensity of the 1st order of diffraction versus temperature. The maximum intensity is normalized to unity (Riahi et al, 2008).
Fig. 16. Setup used for measurement of the temperature profile of the CO2 laser (Riahi et al, 2008).
The Image produced on the CCD camera and measured beam profile of the CO2 laser is shown in Fig. 17.
CO2 Laser and Micro-Fluidics 321
problem for real time measurement. In this part, a thermally tunable grating with fast response time is presented, which makes the real time measurements feasible (Riahi &
The principle of this method is the same as what was mentioned in the previous section except that the device becomes a reflective instead of transitive and a thin supporting glass in the device is replaced by a double side polished silicon wafer. The silicon wafer plays the role of a reflector at 532 nm (40% of reflection) and also as an optical window for the CO2 laser. But the most important characteristic of the silicon is it's high thermal diffusivity. The
However, silicon can plays a role of a heat sink during the measurements and maked the
Fig. 18. Schematic setup for real time measurement of the CO2 laser beam profile (Riahi &
To measure the beam profile of a CO2 laser, a setup as shown in Fig. 18 was used. In this setup, a CO2 laser beam incident on the grating device from the silicon side is absorbed in the grating structure and warms it up. A 532 nm laser is expanded and irradiates the grating, from the glass side. After passing through the grating, the visible light reflects back from the silicon slab and is directed to a 4*f* imaging system. A high pass spatial filter is used
The response time of this system can be measured. For this reason the same setup as in Fig. 18 is used, except that a chopper is placed in front of the CO2 laser and a fast photo-detector is used instead of the CCD camera. By chopping the CO2 laser beam, the signal of the photodetector was monitored by an oscilloscope. As seen in Fig. 19, the response time of this
*Si* = (cm^/sec). It has to be mentioned that the thermal
α
*cu* = (cm^2/sec) which is
Latifi, 2011).
Latifi, 2011)
thermal diffusivity of silicon is 0.95
to keep the first order of diffraction for imaging.
device is about 10 milliseconds.
just a bit more than for silicon.
real time measurements feasible.
α
diffusivity of copper which is used as a very good heat sink is 1.1
Fig. 17. (a) Image produced on the CCD camera. (b) 3D intensity profile of "a" will be the same as the beam profile of the CO2 laser (Riahi et al, 2008).
The followings are some errors presented in this experiment.
Some of these errors are so small to affect the beam profile, but some of them might be important and have to be corrected.
#### **7. Real time measurement of the CO2 laser beam profile utilizing TTG**
In method we presented in the previous section, after each measurement, time had to be taken for the grating to cool down and get ready for another measurement. This can be a big
Fig. 17. (a) Image produced on the CCD camera. (b) 3D intensity profile of "a" will be the
Some of these errors are so small to affect the beam profile, but some of them might be
In method we presented in the previous section, after each measurement, time had to be taken for the grating to cool down and get ready for another measurement. This can be a big
**7. Real time measurement of the CO2 laser beam profile utilizing TTG**
same as the beam profile of the CO2 laser (Riahi et al, 2008).
The followings are some errors presented in this experiment.
environmental errors
important and have to be corrected.
problem for real time measurement. In this part, a thermally tunable grating with fast response time is presented, which makes the real time measurements feasible (Riahi & Latifi, 2011).
The principle of this method is the same as what was mentioned in the previous section except that the device becomes a reflective instead of transitive and a thin supporting glass in the device is replaced by a double side polished silicon wafer. The silicon wafer plays the role of a reflector at 532 nm (40% of reflection) and also as an optical window for the CO2 laser. But the most important characteristic of the silicon is it's high thermal diffusivity. The thermal diffusivity of silicon is 0.95 α*Si* = (cm^/sec). It has to be mentioned that the thermal diffusivity of copper which is used as a very good heat sink is 1.1 α*cu* = (cm^2/sec) which is just a bit more than for silicon.
However, silicon can plays a role of a heat sink during the measurements and maked the real time measurements feasible.
Fig. 18. Schematic setup for real time measurement of the CO2 laser beam profile (Riahi & Latifi, 2011)
To measure the beam profile of a CO2 laser, a setup as shown in Fig. 18 was used. In this setup, a CO2 laser beam incident on the grating device from the silicon side is absorbed in the grating structure and warms it up. A 532 nm laser is expanded and irradiates the grating, from the glass side. After passing through the grating, the visible light reflects back from the silicon slab and is directed to a 4*f* imaging system. A high pass spatial filter is used to keep the first order of diffraction for imaging.
The response time of this system can be measured. For this reason the same setup as in Fig. 18 is used, except that a chopper is placed in front of the CO2 laser and a fast photo-detector is used instead of the CCD camera. By chopping the CO2 laser beam, the signal of the photodetector was monitored by an oscilloscope. As seen in Fig. 19, the response time of this device is about 10 milliseconds.
CO2 Laser and Micro-Fluidics 323
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#### **8. Conclusion**
In this chapter, the relation between CO2 laser and fluid applications was presented. First, application of a CO2 laser for fabrication of microfluidic and optofluidic structures on PMMA polymer was presented. Then application of a fluidic device for measurement of a characteristic of a CO2 laser was discussed.
Application of the CO2 laser for microfluidic fabrication is a simple and low cost method which can be performed by a commercial CO2 laser engraving system. This method makes the final products very cheap which are suitable for single use applications.
Also it seems that the application of the CO2 laser in microfluidics shows a good potential for fabrication of some complicated structures even 3D structures for future works.
#### **9. References**
Fig. 19. Detected signals from photo-diode when CO2 laser is chopped off and on
the final products very cheap which are suitable for single use applications.
for fabrication of some complicated structures even 3D structures for future works.
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Application of the CO2 laser for microfluidic fabrication is a simple and low cost method which can be performed by a commercial CO2 laser engraving system. This method makes
Also it seems that the application of the CO2 laser in microfluidics shows a good potential
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**8. Conclusion**
**9. References**
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**13**
*Portugal*
Rui F. M. Lobo1,2
**Infrared Lasers in Nanoscale Science**
*Faculdade de Ciencias e Tecnologia - New University of Lisbon, Caparica, 2Institute for Science and Technology of Materials and Surfaces (ICEMS)*
*1Group for Nanoscale Science and Nanotechnology (GNCN), Physics Department,*
In the nearly half a century scientists have already realized that, just as Feynman predicted, there is plenty of research room at the bottom of the matter world in a tiny universe so small that new methods for viewing it are still being discovered. Actually, nanoscience and nanotechnology have evolved into a revolutionary area of technology-based research, opening the door to precise engineering on the atomic scale and affecting everything from healthcare to the environment. Nanoscience research and education lead to nanotechnology, the manipulation of nanometer-length atoms, molecules, and supramolecular structures in order to generate larger structures with superior features. Because all natural materials and systems exist at a nanoscale level, nanotechnology impacts a variety of scientific fundamental and applied disciplines, from physics to medicine and engineering. Nanomaterials consisting of nano-sized building blocks exhibit unique and often superior properties relatively to their bulk counterpart. Due to the fact that most of the novel properties of nanomaterials are size-dependent, synthesis methods leading to better control of size, distribution and chemical content of the nanoparticles are imperative in modern
On its turn, the laser has been one of the top applied physics inventions that played a significant role in many fields of science and technology. It has been used in tackling and solving many scientific and technological problems, including interesting applications in the field of nanotechnology, biotechnology/medicine, environment, material characterization,
There are several gaseous molecules which serve as good laser media and the majority of them are simple molecules which provide emission in the ultraviolet. Infrared molecular gas lasers fall into two general categories, namely the middle- and far-infrared lasers, which
The N2 laser is known as a pulse ultraviolet laser and in addition it covers some lines in the infrared up to 8,2 μm. Normally, the pulse width is a few nanoseconds and a high-voltage power supply of 30-40 kV is necessary to excite it. The HF is a high power chemical laser media with an emission wavelength of about 2,7 μm, a laser pulse of the order of μs in duration and the output energy ranges from 1 J to more than 1 kJ per pulse. The DF and HBr chemical lasers emit larger wavelengths than the HF laser, and their output power is lower [1,2].
occur on rotational-vibrational transitions or on pure rotational transitions.
**1. Introduction**
nanotechnologies.
and energy.
### **Infrared Lasers in Nanoscale Science**
Rui F. M. Lobo1,2
*1Group for Nanoscale Science and Nanotechnology (GNCN), Physics Department, Faculdade de Ciencias e Tecnologia - New University of Lisbon, Caparica, 2Institute for Science and Technology of Materials and Surfaces (ICEMS) Portugal*
#### **1. Introduction**
324 CO2 Laser – Optimisation and Application
Liu Robin H., Mark Stremler A., Kendra V. Sharp, Michael G. Olsen, Juan G. Santiago,
Martin F. Jensen,ab Mikkel Noerholm,bc Leif Højslet Christensena and Oliver Geschkeb
Riahi M., Latifi H., and Moghimislam G. (2008), Fabrication of a thermally actuated tunable
Riahi M., Latifi H., Madani A., Moazzenzadeh A. (2009), Design and fabrication of a spatial
Rotting O, Ropke W, Becker H and Gartner C. (2002), Polymer microfabrication
Sun Y.,Y.C.Kwok (2006), Polymeric microfluidic system for DNA analysis, *Anal. Chim. Acta*
Suna Yi, Satyanarayan M.V.D., Nguyenb Nam Trung, Kwok Yien Chian (2008), Continuous
Trisnadi J. I., Carlisle C. B., and Monteverde R. (2004), Overview and applications of Grating
Wood, R. W. (1909), The mercury paraboloid as a reflecting telescope. *Astrophys. J.,* Vol. 29,
Xia Y. and Whitesides G. M. (1998), soft lithography, *Annu. Rev. Mater. Sci*., Vol. 28, pp. 153–
Xu S., Lin Yeong-Jyh, and Wu Shin-Tson (2009), Dielectric liquid microlens with wellshaped electrode, *Opt. Express,* Vol. 17, pp. 10499-10505, ISSN 1094-4087.
Vol. 48, pp. 5647-5654, ISSN: 1559-128X (print), ISSN: 2155-3165 (online) Riahi M., Latifi M. (2011), Fabrication of a 2D thermally tunable reflective grating for
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3, pp. 302–307, ISSN (printed) 1473-0197. ISSN (electronic) 1473-0189. Riahi M. (2012), Fabrication of a passive 3D mixer using CO2 laser ablation of PMMA and PDMS moldings *Microchemical Journal,* Vol. 100, pp. 14–20, ISSN 0026-265X. Riahi M. (2011), Fabrication and characterization of a tunable liquid lens array in water-
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January, San Jose, California, USA.
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Ronald J. Adrian, Hassan Aref, David J. Beebe (2000), Passive mixing in a threedimensional serpentine microchannel, *J. Microelectromech. Syst*. Vol. 9, pp.190–
(2003), Microstructure fabrication with a CO2 laser system: characterization and fabrication of cavities produced by raster scanning of the laser beam, *Lab Chip*, Vol.
PDMS sheet interface by applying pressure , 1st EOS conference on optofluidics.
grating and its application as a CO2 laser beam profile analyzer, *Appl. Opt*. Vol. 47,
light modulator using thermally tunable grating and thin film heater, *Appl. Opt*.
measuring a CO2 laser beam profile, *Optica Applicata* , Vol. 41. pp. 735-742, ISSN:
technologies, *Microsyst. Technol.* Vol. 8, pp. 32–36, ISSN: 0946-7076 (print version)
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Light Valve based optical write engines for high-speed digital imaging, presented at Photonics West2004—Micromachining and Microfabrication Symposium, 26 In the nearly half a century scientists have already realized that, just as Feynman predicted, there is plenty of research room at the bottom of the matter world in a tiny universe so small that new methods for viewing it are still being discovered. Actually, nanoscience and nanotechnology have evolved into a revolutionary area of technology-based research, opening the door to precise engineering on the atomic scale and affecting everything from healthcare to the environment. Nanoscience research and education lead to nanotechnology, the manipulation of nanometer-length atoms, molecules, and supramolecular structures in order to generate larger structures with superior features. Because all natural materials and systems exist at a nanoscale level, nanotechnology impacts a variety of scientific fundamental and applied disciplines, from physics to medicine and engineering. Nanomaterials consisting of nano-sized building blocks exhibit unique and often superior properties relatively to their bulk counterpart. Due to the fact that most of the novel properties of nanomaterials are size-dependent, synthesis methods leading to better control of size, distribution and chemical content of the nanoparticles are imperative in modern nanotechnologies.
On its turn, the laser has been one of the top applied physics inventions that played a significant role in many fields of science and technology. It has been used in tackling and solving many scientific and technological problems, including interesting applications in the field of nanotechnology, biotechnology/medicine, environment, material characterization, and energy.
There are several gaseous molecules which serve as good laser media and the majority of them are simple molecules which provide emission in the ultraviolet. Infrared molecular gas lasers fall into two general categories, namely the middle- and far-infrared lasers, which occur on rotational-vibrational transitions or on pure rotational transitions.
The N2 laser is known as a pulse ultraviolet laser and in addition it covers some lines in the infrared up to 8,2 μm. Normally, the pulse width is a few nanoseconds and a high-voltage power supply of 30-40 kV is necessary to excite it. The HF is a high power chemical laser media with an emission wavelength of about 2,7 μm, a laser pulse of the order of μs in duration and the output energy ranges from 1 J to more than 1 kJ per pulse. The DF and HBr chemical lasers emit larger wavelengths than the HF laser, and their output power is lower [1,2].
Infrared Lasers in Nanoscale Science 327
ground state takes place by collision with cold helium atoms. The resulting hot helium atoms must be cooled in order to sustain the ability to produce a population inversion in the carbon dioxide molecules. In sealed lasers, this takes place as the helium atoms strike the walls of the container. In flow-through lasers, a continuous stream of CO2 and nitrogen is excited by the plasma discharge and the hot gas mixture is exhausted from the resonator by
The CO2 laser transitions are the 961 cm-1 transition of the 10,4 μm band and the 1064 cm-1 transition of the 9,4 μm band. Owing to the symmetry of the CO2 molecule, laser transitions occur to lower energy levels whose rotational quantum numbers are even, resulting in more
In general, there are two common different types of CO2 laser configurations. In one of them (longitudinally excited laser), the CO2 laser is excited by direct current and when the pressure raised from 103 Pa to 104 Pa, a peak power is obtainable by using a pulsed discharge. This is an arc maintained by an anode and a cathode at the ends of a long discharge tube. Another possibility is the transversely excited atmospheric pressure laser (TEA), excited by an arc discharge at roughly atmospheric pressure [4]. The current in the arc flows at right angles to the axis of the laser [5]. A TEA laser is always pulsed and many CO2
Some CO2 TEA lasers have been developed with additional techniques enabling us to achieve tuneable wavelengths, and in particular may reach oscillation threshold for several atomic or molecular transitions. The laser can then simultaneously oscillate on these transitions. In order to reach single mode operation, one has to first select a single transition. Because the laser transitions are actually on vibration-rotation bands of a linear triatomic molecule CO2, the rotational structure of the P and R bands can be selected by a tuning element in the laser cavity. Because transmissive materials in the infrared are rather lossy, the frequency tuning element is almost always a diffraction grating. By rotating the diffraction grating, a particular rotational line of the vibrational transition can be selected.
A procedure towards optimization performance of a CO2 pulsed tuneable laser was developed which allows the power and the energy to be optimized [5]. The MTL3-GT is a very compact grating tuneable TEA laser version (Figure 1), and represents a significant improvement in performance and portability [6]. Combining a pulse mode with a grating tuning facility, it enables us to scan the working wavelength between 9.2 and 10.8 μm (operating on more than 60 lines), with repetition rates ranging from single-shot to 200 Hz. The maximum energy for this version is 50 mJ/pulse on the strongest lines. The MTL3-GT CO2 infrared laser works with a a gas mixture (40% He : 30% CO2 : 30% N2) and a chiller for high repetition rates. Actually, above 20 Hz, the number of HV discharges increases and the
The finest frequency selection may also be obtained through the use of an etalon.
laser needs to be cooled down in order to lower the temperature in the optical cavity.
Following an adequate procedure, the energy values could be optimized in intensity and stability, and therefore indirectly laser power. In addition, the same procedure allows to check the wavelengths of the laser emission lines in the absence of a spectrometer, using a previously established conversion table of the grating position versus line designation. With such method, many experiments can be performed in real time with simultaneous control of
than 30 laser lines in each of the two branches P and R [1,2].
pumps.
lasers are TEA lasers.
The CO2 laser is a gas laser electrically pumped, that emits in the mid-infrared. It gives a cw output at 10 μm in the infrared with a high efficiency and it is the most practical molecular laser. There are a large number of CO2 lasers, varying in structure, method of excitation and capacity, which can provide hundreds of laser lines, the main ones being between 9 and 11 μm. The output power of even a small CO2 laser is about 1 kW and large ones give over 10 kW. The usual way of obtaining single-line oscillation is to use a diffraction grating in conjunction with a laser resonator. If only mirrors are used, simultaneous oscillation on several lines in the neighborhood of 10,6 μm is commonly obtained [1,2]. Transverse excited atmosphere (TEA) CO2 lasers have a very high (about atmospheric) gas pressure. As the voltage required for a longitudinal discharge would be too high, transverse excitation is done with a series of electrodes along the tube. TEA lasers are operated in pulsed mode only, as the gas discharge would not be stable at high pressures, and are suitable for average powers of tens of kilowatts [1,2].
Although N2O and CO laser have a lower output power than the CO2 laser, they have about one hundred laser lines each in the ranges 10-11 μm and 5-6,5 μm, respectivelly (considering the main isotopic species). The molecules NH3, OCS, CS also have quite a few laser lines in the infrared. With the SO2, HCN, H2O, many laser lines are obtained in the infrared from 30 μm up to submilimeter wavelengths [1,2].
Dye lasers are convenient tunable lasers in the visible but not so far in the infrared, mainly due to the lack of appropriate dyes, and in addition, since the dye laser medium is liquid, it is very inconvenient to handle.
In face of the real advantage of the laser as a very intense heating source that can be applied to a very small area, the most significant areas in which the CO2 laser has shown remarkable applications are in the general fields of materials processing and medical applications. This includes cutting, cauterizing, drilling, material removal, melting, welding, alloying, hardening, surgery, cancer treatment and so forth.
The carbon dioxide laser, invented by Patel [3], operates on rotational-vibrational transitions and is still one of the most useful among all the infrared molecular lasers. In general, it is one of the most powerful lasers currently available. It operates in the middle infrared wavelength region with the principal wavelength bands centering around 9.4 and 10.6 micrometers. It is also quite efficient, with a ratio of output power to pump power as large as 20%. It can operate at very high pressures because the energies of the upper laser levels are much closer to the ground state of the CO2 molecule than are energies of the upper laser levels of atomic lasers and so the electron temperature can be much lower, thereby allowing higher operating gas pressures. Higher operating gas pressures means a much greater population in the upper laser level per unit volume of the laser discharge and therefore much higher power output per unit volume of laser gain media. These lasers have produced cw powers of greater than 100 kW and pulsed energies of as much as 10 kJ. The gain occurs on a range of rotational-vibrational transitions that are dominated by either Doppler broadening or pressure broadening, depending upon the gas pressure.
Although laser radiation is obtainable with pure CO2 gas, the usual CO2 laser uses a mixture of He, N2 and CO2. The population inversion in the laser is achieved by a sequence of fundamental processes starting by vibrational excitation of nitrogen molecules in the electric discharge. The nitrogen molecules are left in a lower excited state and their transition to
The CO2 laser is a gas laser electrically pumped, that emits in the mid-infrared. It gives a cw output at 10 μm in the infrared with a high efficiency and it is the most practical molecular laser. There are a large number of CO2 lasers, varying in structure, method of excitation and capacity, which can provide hundreds of laser lines, the main ones being between 9 and 11 μm. The output power of even a small CO2 laser is about 1 kW and large ones give over 10 kW. The usual way of obtaining single-line oscillation is to use a diffraction grating in conjunction with a laser resonator. If only mirrors are used, simultaneous oscillation on several lines in the neighborhood of 10,6 μm is commonly obtained [1,2]. Transverse excited atmosphere (TEA) CO2 lasers have a very high (about atmospheric) gas pressure. As the voltage required for a longitudinal discharge would be too high, transverse excitation is done with a series of electrodes along the tube. TEA lasers are operated in pulsed mode only, as the gas discharge would not be stable at high pressures, and are suitable for average
Although N2O and CO laser have a lower output power than the CO2 laser, they have about one hundred laser lines each in the ranges 10-11 μm and 5-6,5 μm, respectivelly (considering the main isotopic species). The molecules NH3, OCS, CS also have quite a few laser lines in the infrared. With the SO2, HCN, H2O, many laser lines are obtained in the infrared from 30
Dye lasers are convenient tunable lasers in the visible but not so far in the infrared, mainly due to the lack of appropriate dyes, and in addition, since the dye laser medium is liquid, it
In face of the real advantage of the laser as a very intense heating source that can be applied to a very small area, the most significant areas in which the CO2 laser has shown remarkable applications are in the general fields of materials processing and medical applications. This includes cutting, cauterizing, drilling, material removal, melting, welding, alloying,
The carbon dioxide laser, invented by Patel [3], operates on rotational-vibrational transitions and is still one of the most useful among all the infrared molecular lasers. In general, it is one of the most powerful lasers currently available. It operates in the middle infrared wavelength region with the principal wavelength bands centering around 9.4 and 10.6 micrometers. It is also quite efficient, with a ratio of output power to pump power as large as 20%. It can operate at very high pressures because the energies of the upper laser levels are much closer to the ground state of the CO2 molecule than are energies of the upper laser levels of atomic lasers and so the electron temperature can be much lower, thereby allowing higher operating gas pressures. Higher operating gas pressures means a much greater population in the upper laser level per unit volume of the laser discharge and therefore much higher power output per unit volume of laser gain media. These lasers have produced cw powers of greater than 100 kW and pulsed energies of as much as 10 kJ. The gain occurs on a range of rotational-vibrational transitions that are dominated by either Doppler
Although laser radiation is obtainable with pure CO2 gas, the usual CO2 laser uses a mixture of He, N2 and CO2. The population inversion in the laser is achieved by a sequence of fundamental processes starting by vibrational excitation of nitrogen molecules in the electric discharge. The nitrogen molecules are left in a lower excited state and their transition to
broadening or pressure broadening, depending upon the gas pressure.
powers of tens of kilowatts [1,2].
is very inconvenient to handle.
μm up to submilimeter wavelengths [1,2].
hardening, surgery, cancer treatment and so forth.
ground state takes place by collision with cold helium atoms. The resulting hot helium atoms must be cooled in order to sustain the ability to produce a population inversion in the carbon dioxide molecules. In sealed lasers, this takes place as the helium atoms strike the walls of the container. In flow-through lasers, a continuous stream of CO2 and nitrogen is excited by the plasma discharge and the hot gas mixture is exhausted from the resonator by pumps.
The CO2 laser transitions are the 961 cm-1 transition of the 10,4 μm band and the 1064 cm-1 transition of the 9,4 μm band. Owing to the symmetry of the CO2 molecule, laser transitions occur to lower energy levels whose rotational quantum numbers are even, resulting in more than 30 laser lines in each of the two branches P and R [1,2].
In general, there are two common different types of CO2 laser configurations. In one of them (longitudinally excited laser), the CO2 laser is excited by direct current and when the pressure raised from 103 Pa to 104 Pa, a peak power is obtainable by using a pulsed discharge. This is an arc maintained by an anode and a cathode at the ends of a long discharge tube. Another possibility is the transversely excited atmospheric pressure laser (TEA), excited by an arc discharge at roughly atmospheric pressure [4]. The current in the arc flows at right angles to the axis of the laser [5]. A TEA laser is always pulsed and many CO2 lasers are TEA lasers.
Some CO2 TEA lasers have been developed with additional techniques enabling us to achieve tuneable wavelengths, and in particular may reach oscillation threshold for several atomic or molecular transitions. The laser can then simultaneously oscillate on these transitions. In order to reach single mode operation, one has to first select a single transition.
Because the laser transitions are actually on vibration-rotation bands of a linear triatomic molecule CO2, the rotational structure of the P and R bands can be selected by a tuning element in the laser cavity. Because transmissive materials in the infrared are rather lossy, the frequency tuning element is almost always a diffraction grating. By rotating the diffraction grating, a particular rotational line of the vibrational transition can be selected. The finest frequency selection may also be obtained through the use of an etalon.
A procedure towards optimization performance of a CO2 pulsed tuneable laser was developed which allows the power and the energy to be optimized [5]. The MTL3-GT is a very compact grating tuneable TEA laser version (Figure 1), and represents a significant improvement in performance and portability [6]. Combining a pulse mode with a grating tuning facility, it enables us to scan the working wavelength between 9.2 and 10.8 μm (operating on more than 60 lines), with repetition rates ranging from single-shot to 200 Hz. The maximum energy for this version is 50 mJ/pulse on the strongest lines. The MTL3-GT CO2 infrared laser works with a a gas mixture (40% He : 30% CO2 : 30% N2) and a chiller for high repetition rates. Actually, above 20 Hz, the number of HV discharges increases and the laser needs to be cooled down in order to lower the temperature in the optical cavity.
Following an adequate procedure, the energy values could be optimized in intensity and stability, and therefore indirectly laser power. In addition, the same procedure allows to check the wavelengths of the laser emission lines in the absence of a spectrometer, using a previously established conversion table of the grating position versus line designation. With such method, many experiments can be performed in real time with simultaneous control of
Infrared Lasers in Nanoscale Science 329
unreliable, tedious and very sensitive to the rotation speed of the micrometer and stability of the energy signal. Thus, two acquisitions were made for each repetition rate, one by turning the micrometer clockwise and another counter clockwise. An average of them was calculated and definitive energy values were registered. This procedure was repeated in the opposite direction, in order to obtain an average, and also to confirm the reproducibility of the result. It was actually confirmed for every emission line and several values of repetition
Acquisitions recorded without concerns about the external factors and in different days revealed instability in energy values for each repetition rate measured (singleshot, 5 Hz, 10 Hz or 20 Hz), as displayed in Figure 2 (A). For the other three emission bands available (10R, 9P and 9R), the problem is also present. A variation in the power measured was observed in all possible cases, emission bands and repetition rates available. The values could vary from 1 mJ up to 5 mJ for energy and 10 mW to 50 mW for power (repetition rate was 10 Hz in this measurement). The final acquisitions were recorded taking into account the improved procedure regarding the verification of a correspondence between the micrometer drive readings and wavelengths. It can be observed in Figure 2 (B) that the energy values for each repetition rates available were smoother and without significant
Fig. 2. Relationship between emission line, energy and repetition rates (10P emission band-repetition rates single-shot, 5 Hz, 10 Hz and 20 Hz)
This has also been verified for the power values measured for the same repetition rates. The values still vary with the new procedure but in a much lower interval, between 0.5 mJ and 1 mJ for energy, and 5 mW and 10 mW for power values. The same improvement was verified for higher repetition rates up to 100 Hz. The confirmation could be observed not only for the 10P emission band (Figure 3), but also for the other three emission bands available (10R, 9P
rates [5].
deviations [5].
and 9R) [5].
power/energy and wavelength, and taking advantage of the full laser power for each selected wavelength.
Fig. 1. Schematics of the MTL3-GT TEA laser from Edinburgh Instruments [6]
One could observe, after improving the procedure, that energy values are more stable in all four emission bands (9P, 9R, 10P and 10R). This behaviour was also observed regardless of the repetition rate, even for higher ones around 100 Hz. Besides energy, power was also measured and improved following the same procedure. This procedure can also be used on other infrared lasers with some minor adaptations regarding the software and energy detectors used.
In order to overcome several error sources which are the causes of non-reproducibility in these type of lasers, the procedure used a continuous measurement of the energy line, making use of an infrared detector and power meter acquisition software. Such a display method reflects the inherent error associated with the grating tuning motion and therefore the micrometer hysteresis. The method is suitable to obtain the energy and power values for each emission laser TEA CO2 line optimized. The experimental set-up consists of the tuneable TEA CO2 laser, a pyroelectric energy detector connected to a handheld power/energy meter and a computer for acquisition purposes [5].
Since the laser is tuneable by wavelength, some specific emission lines of the CO2 molecule can be selected, making use of a micrometer. The correspondence between such emission lines and the micrometer driving position can be previously verified with an infrared spectrometer, in order to check those mentioned in the user's manual. Changing the position of the micrometer, one varies the angular position of the diffraction grid. This allows to scan among several emission lines, and so to choose the working wavelength.
Using a graphite target block, the pulse shape can be observed while the micrometer is moving. When the correct position is achieved, the focus should be round and symmetric (≈5 mm in diameter), displaying a strong luminosity and without sudden changes for consecutive shots. However, this method proved to be somewhat inaccurate and not very user friendly. To overcome these drawbacks, one must look at the real-time graphic line display of energy on the computer and follow its behaviour during the micrometer rotation, as well. The higher value of the energy line display corresponds, for each wavelength, to the desired position of the micrometer. This can be confirmed at any time by crossing the laser beam with the graphite target [5]. However, the micrometer hysteresis makes the procedure
power/energy and wavelength, and taking advantage of the full laser power for each
Fig. 1. Schematics of the MTL3-GT TEA laser from Edinburgh Instruments [6]
power/energy meter and a computer for acquisition purposes [5].
among several emission lines, and so to choose the working wavelength.
One could observe, after improving the procedure, that energy values are more stable in all four emission bands (9P, 9R, 10P and 10R). This behaviour was also observed regardless of the repetition rate, even for higher ones around 100 Hz. Besides energy, power was also measured and improved following the same procedure. This procedure can also be used on other infrared lasers with some minor adaptations regarding the software and energy
In order to overcome several error sources which are the causes of non-reproducibility in these type of lasers, the procedure used a continuous measurement of the energy line, making use of an infrared detector and power meter acquisition software. Such a display method reflects the inherent error associated with the grating tuning motion and therefore the micrometer hysteresis. The method is suitable to obtain the energy and power values for each emission laser TEA CO2 line optimized. The experimental set-up consists of the tuneable TEA CO2 laser, a pyroelectric energy detector connected to a handheld
Since the laser is tuneable by wavelength, some specific emission lines of the CO2 molecule can be selected, making use of a micrometer. The correspondence between such emission lines and the micrometer driving position can be previously verified with an infrared spectrometer, in order to check those mentioned in the user's manual. Changing the position of the micrometer, one varies the angular position of the diffraction grid. This allows to scan
Using a graphite target block, the pulse shape can be observed while the micrometer is moving. When the correct position is achieved, the focus should be round and symmetric (≈5 mm in diameter), displaying a strong luminosity and without sudden changes for consecutive shots. However, this method proved to be somewhat inaccurate and not very user friendly. To overcome these drawbacks, one must look at the real-time graphic line display of energy on the computer and follow its behaviour during the micrometer rotation, as well. The higher value of the energy line display corresponds, for each wavelength, to the desired position of the micrometer. This can be confirmed at any time by crossing the laser beam with the graphite target [5]. However, the micrometer hysteresis makes the procedure
selected wavelength.
detectors used.
unreliable, tedious and very sensitive to the rotation speed of the micrometer and stability of the energy signal. Thus, two acquisitions were made for each repetition rate, one by turning the micrometer clockwise and another counter clockwise. An average of them was calculated and definitive energy values were registered. This procedure was repeated in the opposite direction, in order to obtain an average, and also to confirm the reproducibility of the result. It was actually confirmed for every emission line and several values of repetition rates [5].
Acquisitions recorded without concerns about the external factors and in different days revealed instability in energy values for each repetition rate measured (singleshot, 5 Hz, 10 Hz or 20 Hz), as displayed in Figure 2 (A). For the other three emission bands available (10R, 9P and 9R), the problem is also present. A variation in the power measured was observed in all possible cases, emission bands and repetition rates available. The values could vary from 1 mJ up to 5 mJ for energy and 10 mW to 50 mW for power (repetition rate was 10 Hz in this measurement). The final acquisitions were recorded taking into account the improved procedure regarding the verification of a correspondence between the micrometer drive readings and wavelengths. It can be observed in Figure 2 (B) that the energy values for each repetition rates available were smoother and without significant deviations [5].
Fig. 2. Relationship between emission line, energy and repetition rates (10P emission band-repetition rates single-shot, 5 Hz, 10 Hz and 20 Hz)
This has also been verified for the power values measured for the same repetition rates. The values still vary with the new procedure but in a much lower interval, between 0.5 mJ and 1 mJ for energy, and 5 mW and 10 mW for power values. The same improvement was verified for higher repetition rates up to 100 Hz. The confirmation could be observed not only for the 10P emission band (Figure 3), but also for the other three emission bands available (10R, 9P and 9R) [5].
Infrared Lasers in Nanoscale Science 331
allows one to follow the formation rate of clusters and complexes during the adiabatic expansion. Selective photodissociation of van der Waals clusters by infrared lasers could be
Ca\*(3PJ) + HCl → CaCl(X;v'',J'') + H The reaction with the ground state Ca(1S0) is endothermic and this is why excited Ca atoms are required. When interrogating the centre of the reaction cell with a tuneable cw laser, Laser Induced Fluorescence (LIF) emission is observed on transitions in the CaCl(A-X) band system [2]. An example of a fraction of the related LIF excitation spectrum is shown in Figure
Fig. 4. LIF spectroscopy of the beam-gas reaction, revealing part of the rotational level
Important analytical applications are represented by measurements of the internal-state distribution of reaction products with LIF and spectroscopic investigations of collisioninduced energy-transfer processes. The high output power of pulsed CO2 lasers allows excitation of high vibrational levels by multiphoton absorption, which eventually may lead to the dissociation of the excited molecule. In some favorable cases the excited molecules or the dissociation fragments can even selectively react with other added components. Such selectively initiated chemical reactions can be induced by CO2 lasers which are particularly
As an example, let us consider the synthesis of SF5NF2 by multiphoton absorption of CO2 photons in a mixture of S2F10 and N2F4, which proceeds according to the following scheme:
> ν→ 2SF5
ν→ 2NF2
SF5 + NF2 → SF5NF2 This laser-driven reaction proceeds much more quickly than the conventional high-
S2F10 + *nh*
N2F4 + *nh*
temperature synthesis without laser, even at the lower temperature of 350 K.
used for isotope separation [1].
population of the reaction product.
advantageous due to their large electrical efficiency.
4.
A typical example of a beam-gas collision is the process
Fig. 3. Power versus repetition rate for 10P emission band.
Along this book chapter, several examples of CO2 lasers applications to nanoscale science and nanotechnology, are explored and generally explained. These include examples in different topics, namely molecular photodynamics, tailored-size nanoparticles production, optical spectroscopy of nanopowders, infrared irradiation of nanostructures, desorption kinetics, photodynamic therapy, among others.
#### **2. Laser spectroscopy and photodynamics**
The combination of pulsed lasers, pulsed molecular beams and time-of-flight mass spectrometry represents a powerful technique for studying excitation, ionization and fragmentation of wanted molecules out of a large variety of different species present in a molecular beam [7]. The success of these two combined techniques is mainly due to the increase in the spectral resolution of absorption and fluorescence spectra by using collimated molecular beams with reduced transverse velocity components, and also to the fact that internal cooling of molecules during adiabatic expansion of supersonic beams compresses their population distribution into the lowest vibrational-rotational levels. This particular aspect greatly reduces the number of absorbing levels and results in a huge simplification of the absorption spectrum [7].
In addition, the low translational temperature achieved in supersonic beams allows the generation and observation of loosely bound van der Waals complexes and clusters. The collision-free conditions in molecular beams after their expansion into a vacuum chamber facilitates saturation of absorbing levels, since no collisions refill a level depleted by optical pumping. This makes Doppler-free saturation spectroscopy feasible even at low cw laser intensities [1].
The structure of molecular complexes in their electronic ground state can be obtained from direct infrared laser absorption spectroscopy in pulsed supersonic-slit jet expansions. This
Along this book chapter, several examples of CO2 lasers applications to nanoscale science and nanotechnology, are explored and generally explained. These include examples in different topics, namely molecular photodynamics, tailored-size nanoparticles production, optical spectroscopy of nanopowders, infrared irradiation of nanostructures, desorption
The combination of pulsed lasers, pulsed molecular beams and time-of-flight mass spectrometry represents a powerful technique for studying excitation, ionization and fragmentation of wanted molecules out of a large variety of different species present in a molecular beam [7]. The success of these two combined techniques is mainly due to the increase in the spectral resolution of absorption and fluorescence spectra by using collimated molecular beams with reduced transverse velocity components, and also to the fact that internal cooling of molecules during adiabatic expansion of supersonic beams compresses their population distribution into the lowest vibrational-rotational levels. This particular aspect greatly reduces the number of absorbing levels and results in a huge
In addition, the low translational temperature achieved in supersonic beams allows the generation and observation of loosely bound van der Waals complexes and clusters. The collision-free conditions in molecular beams after their expansion into a vacuum chamber facilitates saturation of absorbing levels, since no collisions refill a level depleted by optical pumping. This makes Doppler-free saturation spectroscopy feasible even at low cw laser
The structure of molecular complexes in their electronic ground state can be obtained from direct infrared laser absorption spectroscopy in pulsed supersonic-slit jet expansions. This
Fig. 3. Power versus repetition rate for 10P emission band.
kinetics, photodynamic therapy, among others.
simplification of the absorption spectrum [7].
intensities [1].
**2. Laser spectroscopy and photodynamics**
allows one to follow the formation rate of clusters and complexes during the adiabatic expansion. Selective photodissociation of van der Waals clusters by infrared lasers could be used for isotope separation [1].
A typical example of a beam-gas collision is the process
$$\text{Ca}^\*(\text{°P}\_\text{I}) \text{ + HCl} \rightarrow \text{CaCl}(\text{X}; \text{v}^{\text{\textquotedblleft}}, \text{J}^{\text{\textquotedblright}}) + \text{H}^+$$
The reaction with the ground state Ca(1S0) is endothermic and this is why excited Ca atoms are required. When interrogating the centre of the reaction cell with a tuneable cw laser, Laser Induced Fluorescence (LIF) emission is observed on transitions in the CaCl(A-X) band system [2]. An example of a fraction of the related LIF excitation spectrum is shown in Figure 4.
Fig. 4. LIF spectroscopy of the beam-gas reaction, revealing part of the rotational level population of the reaction product.
Important analytical applications are represented by measurements of the internal-state distribution of reaction products with LIF and spectroscopic investigations of collisioninduced energy-transfer processes. The high output power of pulsed CO2 lasers allows excitation of high vibrational levels by multiphoton absorption, which eventually may lead to the dissociation of the excited molecule. In some favorable cases the excited molecules or the dissociation fragments can even selectively react with other added components. Such selectively initiated chemical reactions can be induced by CO2 lasers which are particularly advantageous due to their large electrical efficiency.
As an example, let us consider the synthesis of SF5NF2 by multiphoton absorption of CO2 photons in a mixture of S2F10 and N2F4, which proceeds according to the following scheme:
$$\begin{array}{c} \mathrm{S\_2F\_{10} + } \eta h \nu \to \mathrm{2SF\_5} \\\\ \mathrm{N\_2F\_4 + } \eta h \nu \to \mathrm{2NF\_2} \\\\ \mathrm{SF\_5 + NF\_2} \to \mathrm{SF\_5NF\_2} \end{array}$$
This laser-driven reaction proceeds much more quickly than the conventional hightemperature synthesis without laser, even at the lower temperature of 350 K.
Infrared Lasers in Nanoscale Science 333
τ v-T - relaxation time for vibration-translation transfer, i.e, the time needed to reach the
The first process can occur without collisions, but the other two are necesserelly collisional and therefore are pressure dependent. Considering vexc as the vibrational excitation velocity of a molecule by multiphoton ionization (i.e, 1/vexc will be the excitation time, which depends on radiation intensity and on the vibrational transition cross-section), a comparison of vexc with the several excitation velocities, gives rise to four different
The excitation spectrum obtained by LIF of the CN fragment (produced by multiphoton absorption in the infrared with a high power CO2 laser) of the gas H2C=CHCN, shows clearly the rotational fine structure of the (0,0) band of the CN violet emission, which allows to conclude that the rotational distribution is statistical and characterized by a certain Boltzmann temperature, confirming that the excitation energy is statistically redistributed in
The ability to energize a specific molecular bond and thereby promote a certain desired reaction pathway, has been a widely pursued goal, called mode-selective control in molecular physics. Actually, tunable infrared lasers are very convenient tools to divert a reaction from its dominant thermal pathway toward an envisaged possible product. However, the surplus of vibrational energy tends to be redistributed rapidly within a molecule. An initially excited, high-frequency localized mode can quickly de-excite by transferring its energy into combinations of lower frequency modes. In large molecules, in condensed phases, and at surfaces, huge numbers of low-frequency modes can accept energy, and energy randomization is very rapid (generally on the picosecond time scale or faster). This way, energy does not remain localized in a bond for a sufficiently long time to influence a chemical reaction. Therefore, the resulting chemistry is thermal rather than selective, which leads to the breaking of the weakest bond or to the reaction of the most reactive site. However, in small molecules with sparse vibrational modes, only a few or even
τintra v-v' - relaxation time for intramolecular vibrational energy transfer τv-v' - relaxation time for vibrational energy transfer among distinct molecules
complete thermal equilibrium
electromagnetic field.
the dissociation by multiphoton absorption [9].
situations:
modes.
system up.
Another example of CO2 laser-initiated reactions is the gas-phase telomerization of methyliodide CF3I with C2F4, which represents an exothermic radical chain reaction
$$\text{(CF}\_3\text{I)}^\* + \text{nC}\_2\text{F}\_4 \rightarrow \text{CF}\_3\text{(C}\_2\text{F}\_4\text{)}\_\text{nI} \tag{\text{n} = 1, 2, 3}$$
producing CF3(C2F4)nI with low values of n. The CO2 laser is in near resonance with the ν2+ν3 band of CF3I. The quantum yield for this reaction increases with increasing pressure in the irradiated cell [2].
The infrared lasers have the advantage that the contribution of scattering losses to the total beam attenuation is much smaller than in the visible range. For measurements of very low concentrations, on the other hand, visible dye lasers may be more advantageous because of the larger absorption cross sections for electronic transitions and the higher detector sensitivity.
The applications of lasers to chemical reactions in gas phase are usually classified in two categories: laser induced chemical reactions and laser catalyzed chemical reactions. In the first ones, the laser supplies all the energy thermodinamically needed for the occurence of the reaction and they correspond typically to unimolecular processes (dissociation by multiphotonic absorption); in the second ones (typically bimolecular reactions) only a partial energy amount is supplied and then reaction proceeds by itself. The dissociation by multiphotonic absorption has seen a huge growth in the last decades [8] due to the availability of high power infrared lasers and important technological applications, like isotopic separation. As an example, since in SF6 a mixture exists of 32SF6 and 34SF6, an infrared CO2 laser with λ = 10.61 μm only gives rise to the excitation of vibrational states of 32SF6 but not those of 34SF6; thus, when the continuum of 32SF6 vibrational states is reached after the absorption of 25 photons, only dissociation into 32SF5 and F is produced. This dissociation is fast and corresponds to a statistical mechanism. On its turn, when the wavelength is tuned to 10.82 μm, the dissociation takes place in the 34SF6 molecules.
The observation of dissociation phenomena in molecular beam apparatuses proves unequivocally that it is unimolecular and non-collisional, as it was shown through energetic and angular distributions of the SF5 fragment formed in the dissociation of a SF6 molecular beam, by a CO2 laser pulse of 5 J/cm3 [2]. Actually the results are consistent with RRKM unimolecular theory predictions. In this theory, it is assumed that energy is statistically distributed among the several available modes before dissociation takes place. This means that excitation energy is not localized in just one or a few modes, because in such cases, it will be not possible to reproduce with RRKM theory the above mentioned experimental results; in addition, the mean lifetime would be not about 10-8s (as predicted by RRKM) but much smaller [2].
The dissociation of a polyatomic molecule by multiphotonic absorption is in fact a statistical process (i.e, non-selective), and allows to consider distinct types of selectivity with lasers, based on the existing relation between the different relaxation times which are involved in a vibrationally excited molecule:
τintra v-v' << τv-v' << τ v-T
where:
Another example of CO2 laser-initiated reactions is the gas-phase telomerization of methyl-
(CF3I)\* + nC2F4 → CF3(C2F4)nI (n = 1, 2, 3) producing CF3(C2F4)nI with low values of n. The CO2 laser is in near resonance with the ν2+ν3 band of CF3I. The quantum yield for this reaction increases with increasing pressure in
The infrared lasers have the advantage that the contribution of scattering losses to the total beam attenuation is much smaller than in the visible range. For measurements of very low concentrations, on the other hand, visible dye lasers may be more advantageous because of the larger absorption cross sections for electronic transitions and the higher detector
The applications of lasers to chemical reactions in gas phase are usually classified in two categories: laser induced chemical reactions and laser catalyzed chemical reactions. In the first ones, the laser supplies all the energy thermodinamically needed for the occurence of the reaction and they correspond typically to unimolecular processes (dissociation by multiphotonic absorption); in the second ones (typically bimolecular reactions) only a partial energy amount is supplied and then reaction proceeds by itself. The dissociation by multiphotonic absorption has seen a huge growth in the last decades [8] due to the availability of high power infrared lasers and important technological applications, like isotopic separation. As an example, since in SF6 a mixture exists of 32SF6 and 34SF6, an infrared CO2 laser with λ = 10.61 μm only gives rise to the excitation of vibrational states of 32SF6 but not those of 34SF6; thus, when the continuum of 32SF6 vibrational states is reached after the absorption of 25 photons, only dissociation into 32SF5 and F is produced. This dissociation is fast and corresponds to a statistical mechanism. On its turn, when the
wavelength is tuned to 10.82 μm, the dissociation takes place in the 34SF6 molecules.
The observation of dissociation phenomena in molecular beam apparatuses proves unequivocally that it is unimolecular and non-collisional, as it was shown through energetic and angular distributions of the SF5 fragment formed in the dissociation of a SF6 molecular beam, by a CO2 laser pulse of 5 J/cm3 [2]. Actually the results are consistent with RRKM unimolecular theory predictions. In this theory, it is assumed that energy is statistically distributed among the several available modes before dissociation takes place. This means that excitation energy is not localized in just one or a few modes, because in such cases, it will be not possible to reproduce with RRKM theory the above mentioned experimental results; in addition, the mean lifetime would be not about 10-8s (as predicted by RRKM) but
The dissociation of a polyatomic molecule by multiphotonic absorption is in fact a statistical process (i.e, non-selective), and allows to consider distinct types of selectivity with lasers, based on the existing relation between the different relaxation times which are involved in a
τintra v-v' << τv-v' << τ v-T
iodide CF3I with C2F4, which represents an exothermic radical chain reaction
the irradiated cell [2].
sensitivity.
much smaller [2].
where:
vibrationally excited molecule:
τintra v-v' - relaxation time for intramolecular vibrational energy transfer
τv-v' - relaxation time for vibrational energy transfer among distinct molecules
τ v-T - relaxation time for vibration-translation transfer, i.e, the time needed to reach the complete thermal equilibrium
The first process can occur without collisions, but the other two are necesserelly collisional and therefore are pressure dependent. Considering vexc as the vibrational excitation velocity of a molecule by multiphoton ionization (i.e, 1/vexc will be the excitation time, which depends on radiation intensity and on the vibrational transition cross-section), a comparison of vexc with the several excitation velocities, gives rise to four different situations:
The excitation spectrum obtained by LIF of the CN fragment (produced by multiphoton absorption in the infrared with a high power CO2 laser) of the gas H2C=CHCN, shows clearly the rotational fine structure of the (0,0) band of the CN violet emission, which allows to conclude that the rotational distribution is statistical and characterized by a certain Boltzmann temperature, confirming that the excitation energy is statistically redistributed in the dissociation by multiphoton absorption [9].
The ability to energize a specific molecular bond and thereby promote a certain desired reaction pathway, has been a widely pursued goal, called mode-selective control in molecular physics. Actually, tunable infrared lasers are very convenient tools to divert a reaction from its dominant thermal pathway toward an envisaged possible product. However, the surplus of vibrational energy tends to be redistributed rapidly within a molecule. An initially excited, high-frequency localized mode can quickly de-excite by transferring its energy into combinations of lower frequency modes. In large molecules, in condensed phases, and at surfaces, huge numbers of low-frequency modes can accept energy, and energy randomization is very rapid (generally on the picosecond time scale or faster). This way, energy does not remain localized in a bond for a sufficiently long time to influence a chemical reaction. Therefore, the resulting chemistry is thermal rather than selective, which leads to the breaking of the weakest bond or to the reaction of the most reactive site. However, in small molecules with sparse vibrational modes, only a few or even
Infrared Lasers in Nanoscale Science 335
k(T) of the individual elementary reactions which occur on the molecular level have to be known. For example, the above mentioned H2/O2 combustion reaction consists of 38 elementary reactions involving a variety of reactive intermediates like H, O atoms and OH radicals [14]. Laser pump-and-probe techniques, which combine pulsed laser photolysis for reactive species generation with time-resolved laser-induced fluorescence (LIF) detection for reaction products, have paved the way for detailed studies of the molecular dynamics of the
The experimental possibilities for studying processes in technical combustion devices have expanded a lot in recent years as a result of the development of various pulsed high-power laser sources which provide high temporal, spectral and spatial resolution. Laser spectroscopic methods are important for non-intensive measurements in systems where complex chemical kinetics are coupled with transport processes. One of the key factors for improving the performance of many technical combustion devices is an optimum control of the ignition process. Optimized reproducible ignition ensures an efficient and safe operation. Actually, experimental studies on CO2 laser-induced thermal ignition of CH3OH/O2 mixtures have been performed. In a quartz cell equipped with SrF2 windows, CH3OH/O2 mixtures are ignited using a cw CO2 laser in the pulsed mode. The coincidence of the 9P(12) CO2 laser line in the (001)-(020) band with the R(12) CO stretch fundamental band of the methanol molecule at 9.6 μm allows controlled heating and ignition of the mixture. OH radicals formed during flame propagation were excited in the (v' = 3, v\* = 0) vibrational band of the OH (A2Σ+ - X2Π) transition around 248 nm using two tunable KrF excimer lasers (laser wavelength tunable in the range 247.9 - 248.9 nm with a bandwidth of typically 0.5 cm-1). The time delay between the two excimer laser pulses is 100 ns, in order to separate the signals induced by them. The fluorescence is collected using achromatic UV lens. Reflection filters are used to spectrally isolate the (v' = 3, v'' = 2) fluorescence band of the OH radical for detection. Fluorescence is detected by gated image-intensified CCD cameras. Excitation of two different optical transitions starting from the N'' = 8 and N'' = 11 rotational levels of the OH (X2Π - v'' = 0) vibrational state, allowed the measurement of spatially corresponding LIF image pairs. Assuming a Boltzmann distribution for the population of the OH (X2Π - v'' = 0) rotational states, the ratio of the two OH fluorescence
Interest in infrared laser-induced chemical reactions centered on the quest for mode selectivity was sparked by work on laser isotope separation in the 1970s. In much of this work it is common to find a strongly increasing yield with fluence. However, it is much less common to find an increase with pressure. The most dramatic example of an increasing yield with pressure is the IR laser-induced reaction of isomerization of methyl isocyanid to acetonitrile. [15]. It was then demonstrated that absorption and dissociation can be significantly enhanced through collisions and such reaction exhibits a sharp threshold pressure above which nearly complete isomerization occurs in a single pulse. Thus, the laser-initiated isomerization of methyl isocyanide is an ideal reaction for examination of collision-induced energy transfer phenomena. For the fluence dependence experiments, the laser was operated on the P(20) 944.19 cm-1 transition of the 10.6-μm band. CaF2 flats and KCl windows were used to attenuate the beam for the lower fluence data. For the wavelength data, the laser was operated on the P(6) through P(34) lines of the 10.6-pm band (corresponding to the maximum of the R through the maximum of the P branch of the ν4(C-
elementary reactions [1,2].
images can be converted into a OH temperature field [1,2].
zero combinations of low-frequency modes can accept the energy, and so the lifetime of the initially excited mode may be sufficiently long to allow mode-selective chemistry. This was already demonstrated for the outcome of the gas-phase reaction of H atoms with singly deuterated water (HOD) that can be controlled through laser excitation of specific HOD vibrational modes [10] and it is also illustrated in Figure 5 for a molecular case where one can take profit of the C - Cl bond being stronger than C - Br one.
Fig. 5. LIF spectroscopy of the beam-gas reaction, revealing part of the rotational level population of the reaction product.
From the few attempts at IR mode-selective desorption of molecules from surfaces, that have been reported, perhaps the most successfull was the experimental findings of Liu et al [11], where the authors first created an absorbed layer of about 15% H atoms and 85% D atoms on an Si(111) surface. They then irradiated the surface with a free-electron laser tuned to the 4,8 μm Si-H stretching mode. They found that almost all desorbing atoms were in the form of H2 and less than 5% of desorbing molecules were HD or D2. This result rules out any local heating mechanism, which would produce a statistical mixture (2% H2, 26% HD and 72% D2). Largely as a result of the layer-molecular beam experiments of Y. T. Lee et al [12] the interpretation of the IR-MPD is now almost entirely clarified. The key to the understanding was the determination, under collision-free conditions, of the translational energy distribution of the photofragments as a function of laser intensity and, separately, laser fluence. It is found, indeed, that a suitable tailored statistical (RRKM) theory of unimolecular dissociation can explain most of the observations. All the evidence from these experiments suggests that mode selective molecular dissociation is only possible to achieve using a fast (ps) intense laser pulse.
Thermodynamic laws can be used to determine the equilibrium state of chemical reaction systems like the H2/O2 combustion (2H2 + O2= 2H2O). If chemical reactions are fast compared to all other physical processes (molecular diffusion, heat conduction and flow) thermodynamics alone allows the local description of even complex systems [13]. However, in most cases, chemistry occurs on time scales which are comparable with those of molecular transport. Thus, chemical kinetics information is required, e.g rate coefficients
zero combinations of low-frequency modes can accept the energy, and so the lifetime of the initially excited mode may be sufficiently long to allow mode-selective chemistry. This was already demonstrated for the outcome of the gas-phase reaction of H atoms with singly deuterated water (HOD) that can be controlled through laser excitation of specific HOD vibrational modes [10] and it is also illustrated in Figure 5 for a molecular case where one can
Fig. 5. LIF spectroscopy of the beam-gas reaction, revealing part of the rotational level
From the few attempts at IR mode-selective desorption of molecules from surfaces, that have been reported, perhaps the most successfull was the experimental findings of Liu et al [11], where the authors first created an absorbed layer of about 15% H atoms and 85% D atoms on an Si(111) surface. They then irradiated the surface with a free-electron laser tuned to the 4,8 μm Si-H stretching mode. They found that almost all desorbing atoms were in the form of H2 and less than 5% of desorbing molecules were HD or D2. This result rules out any local heating mechanism, which would produce a statistical mixture (2% H2, 26% HD and 72% D2). Largely as a result of the layer-molecular beam experiments of Y. T. Lee et al [12] the interpretation of the IR-MPD is now almost entirely clarified. The key to the understanding was the determination, under collision-free conditions, of the translational energy distribution of the photofragments as a function of laser intensity and, separately, laser fluence. It is found, indeed, that a suitable tailored statistical (RRKM) theory of unimolecular dissociation can explain most of the observations. All the evidence from these experiments suggests that mode selective molecular dissociation is only possible to achieve using a fast
Thermodynamic laws can be used to determine the equilibrium state of chemical reaction systems like the H2/O2 combustion (2H2 + O2= 2H2O). If chemical reactions are fast compared to all other physical processes (molecular diffusion, heat conduction and flow) thermodynamics alone allows the local description of even complex systems [13]. However, in most cases, chemistry occurs on time scales which are comparable with those of molecular transport. Thus, chemical kinetics information is required, e.g rate coefficients
take profit of the C - Cl bond being stronger than C - Br one.
population of the reaction product.
(ps) intense laser pulse.
k(T) of the individual elementary reactions which occur on the molecular level have to be known. For example, the above mentioned H2/O2 combustion reaction consists of 38 elementary reactions involving a variety of reactive intermediates like H, O atoms and OH radicals [14]. Laser pump-and-probe techniques, which combine pulsed laser photolysis for reactive species generation with time-resolved laser-induced fluorescence (LIF) detection for reaction products, have paved the way for detailed studies of the molecular dynamics of the elementary reactions [1,2].
The experimental possibilities for studying processes in technical combustion devices have expanded a lot in recent years as a result of the development of various pulsed high-power laser sources which provide high temporal, spectral and spatial resolution. Laser spectroscopic methods are important for non-intensive measurements in systems where complex chemical kinetics are coupled with transport processes. One of the key factors for improving the performance of many technical combustion devices is an optimum control of the ignition process. Optimized reproducible ignition ensures an efficient and safe operation. Actually, experimental studies on CO2 laser-induced thermal ignition of CH3OH/O2 mixtures have been performed. In a quartz cell equipped with SrF2 windows, CH3OH/O2 mixtures are ignited using a cw CO2 laser in the pulsed mode. The coincidence of the 9P(12) CO2 laser line in the (001)-(020) band with the R(12) CO stretch fundamental band of the methanol molecule at 9.6 μm allows controlled heating and ignition of the mixture. OH radicals formed during flame propagation were excited in the (v' = 3, v\* = 0) vibrational band of the OH (A2Σ+ - X2Π) transition around 248 nm using two tunable KrF excimer lasers (laser wavelength tunable in the range 247.9 - 248.9 nm with a bandwidth of typically 0.5 cm-1). The time delay between the two excimer laser pulses is 100 ns, in order to separate the signals induced by them. The fluorescence is collected using achromatic UV lens. Reflection filters are used to spectrally isolate the (v' = 3, v'' = 2) fluorescence band of the OH radical for detection. Fluorescence is detected by gated image-intensified CCD cameras. Excitation of two different optical transitions starting from the N'' = 8 and N'' = 11 rotational levels of the OH (X2Π - v'' = 0) vibrational state, allowed the measurement of spatially corresponding LIF image pairs. Assuming a Boltzmann distribution for the population of the OH (X2Π - v'' = 0) rotational states, the ratio of the two OH fluorescence images can be converted into a OH temperature field [1,2].
Interest in infrared laser-induced chemical reactions centered on the quest for mode selectivity was sparked by work on laser isotope separation in the 1970s. In much of this work it is common to find a strongly increasing yield with fluence. However, it is much less common to find an increase with pressure. The most dramatic example of an increasing yield with pressure is the IR laser-induced reaction of isomerization of methyl isocyanid to acetonitrile. [15]. It was then demonstrated that absorption and dissociation can be significantly enhanced through collisions and such reaction exhibits a sharp threshold pressure above which nearly complete isomerization occurs in a single pulse. Thus, the laser-initiated isomerization of methyl isocyanide is an ideal reaction for examination of collision-induced energy transfer phenomena. For the fluence dependence experiments, the laser was operated on the P(20) 944.19 cm-1 transition of the 10.6-μm band. CaF2 flats and KCl windows were used to attenuate the beam for the lower fluence data. For the wavelength data, the laser was operated on the P(6) through P(34) lines of the 10.6-pm band (corresponding to the maximum of the R through the maximum of the P branch of the ν4(C-
Infrared Lasers in Nanoscale Science 337
features one single peak centered at 1042.2 cm−1. This is consistent with a cyclic structure in which all three methanol molecules are equivalent. Higher cluster spectra are characterized
LIF detection is also used for various ultrasensitive techniques by probing reagents that are either autofluorescing or tagged with a fluorescent dye molecule. Applying different microscopic techniques with tight spatial and spectral filtering, various groups have directly visualized a variety of single fluorescent dye molecules (rhodamines and coumarins), dissolved in liquids by using coherent one- and two-photon excitation. Photophysical parameters and photobleaching play a crucial role for the accuracy of single-molecule detection by LIF and for the high sensitivity of fluorescence spectroscopy. Key properties for a fluorescent dye are its absorption coefficient, fluorescence- and photobleaching quantum yield. Photobleaching is a dynamic irreversible process in which fluorescent molecules undergo photoinduced chemical destruction upon absorption of light, thus losing their ability to fluoresce. Thus, for every absorption process there is a certain fixed probability φ<sup>b</sup> of the molecule to be bleached. The probability P to survive n absorption cycles and become
P (survival of n absorptions) = (1- φb)n φ<sup>b</sup> This corresponds to the geometric distribution, which is the discrete counterpart of the exponential distribution. Due to this exponential nature of the photodestruction process with the standard deviation (1- φb)/φb , the relative fluctuation of the number of detected photons due to a single molecule transit can be as high as 100% [18]. The mean number of
μ = (1- φb)/φb = 1/φ<sup>b</sup> Photobleaching is the ultimate limit of fluorescence-based single-molecule spectroscopy,
Unfortunatelly, the total number of absorbed molecules cannot be measured directly and precisely under single-molecule-detection, making it impossible to determine φb. However, the above definition can be expressed in kinetic terms. If a dye solution is illuminated, it is possible to measure the number of irreversibly photobleached molecules as a decrease in the dye concentration c(t) with time t. Under appropriate conditions the rate of this decrease is proportional to c(t), and so photobleaching reaction can be treated as a quasi-unimolecular
dc(t)/dt = -kb c(t) → c(t) = c(0) e-kbt The rate constant kb is dependent on cw laser irradiance, and the study of this dependence can lead to the evaluation of φb. The photostability of many organic dyes in organic solvents
Pursuing the goal of single-molecule spectroscopy where ultra-low concentrations of the fluorophore in water (< 10 pM) are used, we should focus on photoreactions occuring under these conditions. In single-molecule spectroscopy, photostability and fluorescence
by single peaks gradually blue-shifted with respect to the trimer line [17].
bleached in the (n+1)th cycle is given by:
reaction:
is higher than in water.
survived absorption cycles μ is equal to the standard deviation:
and the quantum yield of photobleaching is defined by the ratio:
φb = number of photobleached molecules/total number of absorbed molecules
N) stretch of methyl isocyanide) with an average energy per pulse of 1.05 J at the sample. Analysis for reaction was performed by monitoring the 2165 cm-1 ν2(C=N) stretch with a grating spectrophotometer. To determine the relationship between fluence and threshold pressure, the fluence was kept constant while the pressure was varied so as to bracket the threshold. Then the process was repeated with a new fluence. The results indicate a linear variation of the fluence with the inverse of the pressure. Analysis of the fluence dependence of the threshold pressure indicates that the inverse of the threshold pressure is directly proportional to the average number of photons absorbed per molecule. This balance between incident fluence (or average number of photons absorbed) and threshold pressure can be understood in terms of the usual model of increasing yield with fluence. Hence, if the yield is increased owing to an increase in fluence, the threshold pressure should exhibit a concomitant decrease, as is observed. The multiphoton absorption spectrum as well as the wavelength dependence of the threshold pressure reflects the structure of the linear absorption spectrum [1].
Interest in using infrared laser radiation for studying charge transfer processes at surfaces relies on the possibility of exciting vibrational modes of adsorbate molecules. It is wellknown that vibrational excitation is very important in promoting endoergic gas-phase chemical reactions, as well as in controlling chemical processes occurring in adsorbed layers. The use of IR lasers for initiating gas-surface reactions when the gas consists of polyatomic molecules allows us to put, at the gas-surface interface, a large amount of energy due to IR multiphoton absorption. As is known from gas-phase experiments, at rather moderate for IR C02 laser energy fluences of about 1 J/cm2, it is possible to excite to high vibrationally excited states (up to energy levels E > 1 eV) practically all irradiated molecule. Therefore, in spite of a rather small value of C02 laser quantum (0.1-0.12 eV) and longer pulse duration (>100 ns), one can induce effective charge transfer.
The use of a pulsed TEA C02 laser allowed us to apply the time-of-flight (TOF) technique for the detection of ion signal and, as a result, to distinguish between molecular negative ions and electron emission. A TEA C02 laser line tunable in the range 9-11 μm was used for the excitation of molecules at the Ba surface. The laser beam was directed to the Ba surface perpendicular to the SF6 beam direction via a ZnSe window in the HVC and a KBr window in the UHVC [16]. The negative molecular ion signal was shown to be very sensitive to the SF6 molecular absorption (to the exciting C02 laser frequency). Thus, enhancement factors of 10 or 4 were found for 10P(20) versus 10R(20) and 10P(16) versus 10P(22) lines, respectively. This support the vibrational selectivity (vibrational enhancement) of the SF6 + Ba gas-surface IR laser-photoinduced ionization process [16].
Regarding applications in cluster spectroscopy, some experiments have been performed, in particular infrared photodissociation by crossing a continuous supersonic molecular beam of small methanol clusters with the radiation of a pulsed CO2 laser [17] . Subsequent scattering by a secondary He beam disperses the cluster beam and allows the off-axis detection of selected cluster species, undisturbed by ionizer fragmentation artifacts. In the region of the ν8 C-O stretching vibration, the dependence of IR photon absorption on laser frequency and fluence is investigated as a function of cluster size [17]. The predissociation spectrum of the dimer shows two distinct peaks at 1026.5 and 1051.6 cm−1 which correspond to the excitation of the two non-equivalent monomers in the dimer. The trimer spectrum
N) stretch of methyl isocyanide) with an average energy per pulse of 1.05 J at the sample. Analysis for reaction was performed by monitoring the 2165 cm-1 ν2(C=N) stretch with a grating spectrophotometer. To determine the relationship between fluence and threshold pressure, the fluence was kept constant while the pressure was varied so as to bracket the threshold. Then the process was repeated with a new fluence. The results indicate a linear variation of the fluence with the inverse of the pressure. Analysis of the fluence dependence of the threshold pressure indicates that the inverse of the threshold pressure is directly proportional to the average number of photons absorbed per molecule. This balance between incident fluence (or average number of photons absorbed) and threshold pressure can be understood in terms of the usual model of increasing yield with fluence. Hence, if the yield is increased owing to an increase in fluence, the threshold pressure should exhibit a concomitant decrease, as is observed. The multiphoton absorption spectrum as well as the wavelength dependence of the threshold pressure reflects the structure of the linear
Interest in using infrared laser radiation for studying charge transfer processes at surfaces relies on the possibility of exciting vibrational modes of adsorbate molecules. It is wellknown that vibrational excitation is very important in promoting endoergic gas-phase chemical reactions, as well as in controlling chemical processes occurring in adsorbed layers. The use of IR lasers for initiating gas-surface reactions when the gas consists of polyatomic molecules allows us to put, at the gas-surface interface, a large amount of energy due to IR multiphoton absorption. As is known from gas-phase experiments, at rather moderate for IR C02 laser energy fluences of about 1 J/cm2, it is possible to excite to high vibrationally excited states (up to energy levels E > 1 eV) practically all irradiated molecule. Therefore, in spite of a rather small value of C02 laser quantum (0.1-0.12 eV) and longer pulse duration
The use of a pulsed TEA C02 laser allowed us to apply the time-of-flight (TOF) technique for the detection of ion signal and, as a result, to distinguish between molecular negative ions and electron emission. A TEA C02 laser line tunable in the range 9-11 μm was used for the excitation of molecules at the Ba surface. The laser beam was directed to the Ba surface perpendicular to the SF6 beam direction via a ZnSe window in the HVC and a KBr window in the UHVC [16]. The negative molecular ion signal was shown to be very sensitive to the SF6 molecular absorption (to the exciting C02 laser frequency). Thus, enhancement factors of 10 or 4 were found for 10P(20) versus 10R(20) and 10P(16) versus 10P(22) lines, respectively. This support the vibrational selectivity (vibrational enhancement) of the SF6 + Ba gas-surface
Regarding applications in cluster spectroscopy, some experiments have been performed, in particular infrared photodissociation by crossing a continuous supersonic molecular beam of small methanol clusters with the radiation of a pulsed CO2 laser [17] . Subsequent scattering by a secondary He beam disperses the cluster beam and allows the off-axis detection of selected cluster species, undisturbed by ionizer fragmentation artifacts. In the region of the ν8 C-O stretching vibration, the dependence of IR photon absorption on laser frequency and fluence is investigated as a function of cluster size [17]. The predissociation spectrum of the dimer shows two distinct peaks at 1026.5 and 1051.6 cm−1 which correspond to the excitation of the two non-equivalent monomers in the dimer. The trimer spectrum
absorption spectrum [1].
(>100 ns), one can induce effective charge transfer.
IR laser-photoinduced ionization process [16].
features one single peak centered at 1042.2 cm−1. This is consistent with a cyclic structure in which all three methanol molecules are equivalent. Higher cluster spectra are characterized by single peaks gradually blue-shifted with respect to the trimer line [17].
LIF detection is also used for various ultrasensitive techniques by probing reagents that are either autofluorescing or tagged with a fluorescent dye molecule. Applying different microscopic techniques with tight spatial and spectral filtering, various groups have directly visualized a variety of single fluorescent dye molecules (rhodamines and coumarins), dissolved in liquids by using coherent one- and two-photon excitation. Photophysical parameters and photobleaching play a crucial role for the accuracy of single-molecule detection by LIF and for the high sensitivity of fluorescence spectroscopy. Key properties for a fluorescent dye are its absorption coefficient, fluorescence- and photobleaching quantum yield. Photobleaching is a dynamic irreversible process in which fluorescent molecules undergo photoinduced chemical destruction upon absorption of light, thus losing their ability to fluoresce. Thus, for every absorption process there is a certain fixed probability φ<sup>b</sup> of the molecule to be bleached. The probability P to survive n absorption cycles and become bleached in the (n+1)th cycle is given by:
#### P (survival of n absorptions) = (1- φb)n φ<sup>b</sup>
This corresponds to the geometric distribution, which is the discrete counterpart of the exponential distribution. Due to this exponential nature of the photodestruction process with the standard deviation (1- φb)/φb , the relative fluctuation of the number of detected photons due to a single molecule transit can be as high as 100% [18]. The mean number of survived absorption cycles μ is equal to the standard deviation:
$$\mu = (1 - \phi\_{\rm b}) / \phi\_{\rm b} = 1 / \phi\_{\rm b}$$
Photobleaching is the ultimate limit of fluorescence-based single-molecule spectroscopy, and the quantum yield of photobleaching is defined by the ratio:
φb = number of photobleached molecules/total number of absorbed molecules
Unfortunatelly, the total number of absorbed molecules cannot be measured directly and precisely under single-molecule-detection, making it impossible to determine φb. However, the above definition can be expressed in kinetic terms. If a dye solution is illuminated, it is possible to measure the number of irreversibly photobleached molecules as a decrease in the dye concentration c(t) with time t. Under appropriate conditions the rate of this decrease is proportional to c(t), and so photobleaching reaction can be treated as a quasi-unimolecular reaction:
$$\mathbf{\dot{c}} \mathbf{\dot{c}}(\mathbf{t})/\mathbf{\dot{d}t} = \mathbf{\dot{-}} \mathbf{\dot{s}}\_b \mathbf{c}(\mathbf{t}) \quad \rightarrow \quad \mathbf{c}(\mathbf{t}) = \mathbf{c}(0) \text{ } \mathbf{e}^\text{-k} \mathbf{b}^\text{ } \mathbf{t}$$
The rate constant kb is dependent on cw laser irradiance, and the study of this dependence can lead to the evaluation of φb. The photostability of many organic dyes in organic solvents is higher than in water.
Pursuing the goal of single-molecule spectroscopy where ultra-low concentrations of the fluorophore in water (< 10 pM) are used, we should focus on photoreactions occuring under these conditions. In single-molecule spectroscopy, photostability and fluorescence
Infrared Lasers in Nanoscale Science 339
and ability to maintain steep temperature gradients allows for precise control of the nucleation and growth rates favoring the formation of very fine and uniform powders. When reaction occurs in the gas phase, far from polluting walls very pure nano-scale materials may be prepared, and so conditions are created permitting the homogeneous nucleation of particles by condensation from a supersaturated vapor phase. In addition, the
Pulsed laser ablation of materials in aqueous solutions of surfactants can also lead in some cases to the formation of ultrafine particles as in the case of TiO2 crystalline anatase 3 nm nanoparticles, using the third harmonic of a Nd:YAG laser (355 nm) operating at 10 Hz [19]. Laser-induced production of silicon nanoparticles has been mostly based on the global
SiH4 (g) + nhν → Si(s) + 2H2 (g) Silane strongly absorbs in the ν4 band at 10.5 μm resonant with the P(20) line of the 00º1-10º0 transition at 944.19 cm-1 of the CO2 laser [20]. Experiments have been performed in high vacuum with a pulsed TEA CO2 laser at fluences that varied between 0.5 J/cm2 and 150 J/cm2 (using different converging focal distance infrared lenses). The shape and size of the nanoparticles is then examined by electron microscopy (SEM and TEM) and Atomic Force Microscopy (AFM), and their structure by X-ray diffraction [21,22]. Figure 7 schematically describes the experimental set-up for laser pyrolysis (chemical decomposition by heat in the absence of oxygen). Silane gas is laminarilly flowing through the center of the laser pyrolysis reactor, surrounded by another laminar flow of helium. The focalized laser pulse (Δt = 100 ns, *I* = 30-40 mJ/pulse) decompose silane into silicon and hydrogen atoms which then recombine to form molecular hydrogen. Since a nozzle is placed close to the reaction zone, the silicon atoms and small silicon clusters are extracted in the majority helium supersonic expansion (like an ultra-fast cooler at a rate of 109 Ks-1), giving rise by condensation to larger clusters and nanoparticles. Their formation is processing in bursts of some nanoseconds, according to the laser pulses characteristics. The nanoparticles size selection is assured by a synchronized chopper with the laser pulses, and the size distribution is measured in-situ by time-of-flight mass spectrometry (TOFMS). It is also possible to deposit these silicon
laser processing is cleaner.
nanocrystals on a removable target mica substrate [21,22].
Fig. 7. Laser pirolysis set-up for size-selected nanoparticles production [21,22].
reaction
saturation of the fluorophores impose limitations on the achievable fluorescence flow and the resulting signal-to-background ratio.
The rate constants for excitation from a state *i* to a state *f* are proportional to the irradiance *I* [W/cm2] and to the absorption cross section Qif(λ) [cm2] at a wavelength λ:
$$\mathbf{k}\_{\rm{Tif}}(\boldsymbol{\chi}) = \mathbf{I} \, \mathbf{Q}\_{\rm{if}}(\boldsymbol{\chi}) \, \boldsymbol{\chi}$$
where = γ/(hc), being *c* the velocity of light in vacuum and *h* the Planck constant.
Fluorescence saturation follows from the fact that a molecule cannot be in an electronically excited state and in the ground state at the same time; i.e. a single molecule can emit only a limited number of fluorescence photons in a certain time interval. Thus, the saturation characteristics of the fluorescent flows are determined by ground state depletion due to the finite excited state lifetimes of the S1 and T1 states (Figure 6).
Fig. 6. Electronic Energy Diagram of a dye molecule with 5 electronic levels: S - singlet states; T - triplet states
The main reasons for using in life sciences near-infrared and infrared dyes as infrared fluorophores are threefold (although they photobleach more readily than dyes emitting in the visible):
Most of these dyes are cyanines, and they display a common problem which is their stability in biological fluids (composed mainly of water). They tend to aggregate, and so contribute to the quenching of emission. One method of preventing such aggregation is to isolate the dyes by encapsulation (in a nanobubble or a liposome) or to use chemical functionalization.
#### **3. Nanoparticles and carbon nanotechnology**
Given the importance of nanoscale particles in present technology, size distribution is a fundamental aspect as a quality control parameter. The production of nanoparticles using laser-induced gas-phase reactions techniques assures in general narrow size distributions contrarily to chemical techniques (such as precipitation or sol-gel processing) or to usual vapor-phase methods (furnace-heated vapor or arc-plasma), while the low reaction volume
saturation of the fluorophores impose limitations on the achievable fluorescence flow and
The rate constants for excitation from a state *i* to a state *f* are proportional to the irradiance *I*
kTif(λ) = I Qif(λ) γ
Fluorescence saturation follows from the fact that a molecule cannot be in an electronically excited state and in the ground state at the same time; i.e. a single molecule can emit only a limited number of fluorescence photons in a certain time interval. Thus, the saturation characteristics of the fluorescent flows are determined by ground state depletion due to the
Fig. 6. Electronic Energy Diagram of a dye molecule with 5 electronic levels: S - singlet
The main reasons for using in life sciences near-infrared and infrared dyes as infrared fluorophores are threefold (although they photobleach more readily than dyes emitting in
Most of these dyes are cyanines, and they display a common problem which is their stability in biological fluids (composed mainly of water). They tend to aggregate, and so contribute to the quenching of emission. One method of preventing such aggregation is to isolate the dyes by encapsulation (in a nanobubble or a liposome) or to use chemical functionalization.
Given the importance of nanoscale particles in present technology, size distribution is a fundamental aspect as a quality control parameter. The production of nanoparticles using laser-induced gas-phase reactions techniques assures in general narrow size distributions contrarily to chemical techniques (such as precipitation or sol-gel processing) or to usual vapor-phase methods (furnace-heated vapor or arc-plasma), while the low reaction volume
often limited by the auto-fluorescence background, is significantly improved. - the laser wavelength also produce reduced scattering in the biological tissue, and thus increase both the penetration depth and the efficiency of collection of emission. - available low-cost and compact NIR and IR diode lasers (e.g. 650 nm, 800 nm, 970 nm,
[W/cm2] and to the absorption cross section Qif(λ) [cm2] at a wavelength λ:
finite excited state lifetimes of the S1 and T1 states (Figure 6).
etc...) can be used as excitation sources for these dyes.
**3. Nanoparticles and carbon nanotechnology**
where = γ/(hc), being *c* the velocity of light in vacuum and *h* the Planck constant.
the resulting signal-to-background ratio.
states; T - triplet states
the visible):
and ability to maintain steep temperature gradients allows for precise control of the nucleation and growth rates favoring the formation of very fine and uniform powders. When reaction occurs in the gas phase, far from polluting walls very pure nano-scale materials may be prepared, and so conditions are created permitting the homogeneous nucleation of particles by condensation from a supersaturated vapor phase. In addition, the laser processing is cleaner.
Pulsed laser ablation of materials in aqueous solutions of surfactants can also lead in some cases to the formation of ultrafine particles as in the case of TiO2 crystalline anatase 3 nm nanoparticles, using the third harmonic of a Nd:YAG laser (355 nm) operating at 10 Hz [19].
Laser-induced production of silicon nanoparticles has been mostly based on the global reaction
$$\text{SiH}\_4\text{ (g)} + \text{nhv} \rightarrow \text{Si(s)} + 2\text{H}\_2\text{(g)}$$
Silane strongly absorbs in the ν4 band at 10.5 μm resonant with the P(20) line of the 00º1-10º0 transition at 944.19 cm-1 of the CO2 laser [20]. Experiments have been performed in high vacuum with a pulsed TEA CO2 laser at fluences that varied between 0.5 J/cm2 and 150 J/cm2 (using different converging focal distance infrared lenses). The shape and size of the nanoparticles is then examined by electron microscopy (SEM and TEM) and Atomic Force Microscopy (AFM), and their structure by X-ray diffraction [21,22]. Figure 7 schematically describes the experimental set-up for laser pyrolysis (chemical decomposition by heat in the absence of oxygen). Silane gas is laminarilly flowing through the center of the laser pyrolysis reactor, surrounded by another laminar flow of helium. The focalized laser pulse (Δt = 100 ns, *I* = 30-40 mJ/pulse) decompose silane into silicon and hydrogen atoms which then recombine to form molecular hydrogen. Since a nozzle is placed close to the reaction zone, the silicon atoms and small silicon clusters are extracted in the majority helium supersonic expansion (like an ultra-fast cooler at a rate of 109 Ks-1), giving rise by condensation to larger clusters and nanoparticles. Their formation is processing in bursts of some nanoseconds, according to the laser pulses characteristics. The nanoparticles size selection is assured by a synchronized chopper with the laser pulses, and the size distribution is measured in-situ by time-of-flight mass spectrometry (TOFMS). It is also possible to deposit these silicon nanocrystals on a removable target mica substrate [21,22].
Fig. 7. Laser pirolysis set-up for size-selected nanoparticles production [21,22].
Infrared Lasers in Nanoscale Science 341
IR-active, graphite-like *E*1*<sup>u</sup>*mode (also known as the *G* band) originating from the *sp*2 hybridized carbon. The absorbance peak at 1200 cm−1, is a disorder-induced one phonon absorbance band (D band-from the *sp*3-hybridized carbon), which has been also observed in neutron irradiated diamonds [24]. This lattice mode arises from the disruption of the translational symmetry of the diamond lattice. In general, the presence of CH*<sup>x</sup>* groups (evidenced in the IR active bands in the range of 3000 cm−1) and non-conjugated carboxylic carbonyl groups (peak around 1725 cm−1) can benefit a number of bio-sensing applications
Fig. 9. Typical infrared absorbance spectrum obtained on 60 nm multiwall carbon
Production of single-wall carbon nanotubes (SWNT) by the laser-ablation technique using graphite, pitch and coke as carbonaceous feedstock materials has been reported [26]. This has been done with a 250W continuous-wave CO2-laser at a wavelength of 10.6 *μ*m and varying the nature and the concentration of the metal catalyst, the type and pressure of the buffergas as well as the laser conditions. The amount of SWNT material obtained is much higher when using graphite as a precursor than in the case of coke and carbonaceous feedstock [26]. Laser-assisted chemical vapour deposition (LCVD) for the formation and growth of carbon nanotubes has also been performed using a medium-power continuous-wave CO2 laser to
acetylene and to simultaneously heat a silicon substrate on which the carbon nanotubes were grown [27]. Electron microscopy (TEM and HRTEM) as well as atomic force microscopy (AFM) were used to analyze the as-grown films and samples specially prepared on TEM grids and AFM substrates. Carbon nanotubes with different structures (straight, curved and even branched), including single- and multi-walled nanotubes were observed. Some nanotubes were found to be partially filled with a solid material (probably metallic iron)
Using a CO2 laser perpendicularly directed onto a silicon substrate, sensitized mixtures of iron pentacarbonyl vapour and acetylene were pyrolyzed in a flow reactor. The method involves the heating of both the gas phase and the substrate by IR radiation. The carbon
by offering a simple route to nanotube functionalization [25].
irradiate a sensitized mixture of Fe(CO)5 vapour and
that seems to catalyze the nanotube growth [27].
nanotubes [25]
The kinetic energy of the clusters is small (less than 0.4 eV per atom for 4 nm size particles, which corresponds to 10% of the bonding energy) and so a Low Energy Cluster Beam Deposition (LECBD) is taking place, without changing the cluster properties in gas phase. Due to the geometry of the system, a distribution of nanoparticle sizes also appears on the substrate, and it can be guaranteed that all the sizes correspond to the same air exposition history. By varying some experimental parameters (pressure, flow, laser power, delay between laser pulse and chopper slit) it is possible to control the size distribution of the silicon nanoparticles deposited onto the substrate, for ultramicroscopic analysis (Figure 8).
Fig. 8. AFM image of size-selected silicon nanoparticles together with their size distributions measured by TOFMS and AFM[21,22].
The advantage of using a TEA CO2 laser in the pyrolysis becomes clear for the fine tuning wavelength adjust, in order to optimize the production process of several other types of nanoparticles [22].
Several synthesis reactors geometries based on the vaporization of a target (graphite/metal catalyst pellet) inside a oven at a fixed temperature (above 1000 K) by continuous CO2 laser beam (λ = 10.6 μm) have been developed to produce several types of carbon nanotubes. The laser power can be varied from 100 W to 1600 W and the temperature of the target is measured with an optical pyrometer. In general a inert gas flow carries away the solid particles formed in the laser ablation process which are then collected on a filter [23].
A typical mid-IR to near-IR absorbance spectrum taken on uniformly dispersed, purified CNTs (grown by CVD-mostly MWNT) at room temperature is displayed in Figure 9 [24]. In the measured IR absorbance spectrum, a prime intensity peak is seen at 1584 cm−1. This is an
The kinetic energy of the clusters is small (less than 0.4 eV per atom for 4 nm size particles, which corresponds to 10% of the bonding energy) and so a Low Energy Cluster Beam Deposition (LECBD) is taking place, without changing the cluster properties in gas phase. Due to the geometry of the system, a distribution of nanoparticle sizes also appears on the substrate, and it can be guaranteed that all the sizes correspond to the same air exposition history. By varying some experimental parameters (pressure, flow, laser power, delay between laser pulse and chopper slit) it is possible to control the size distribution of the silicon nanoparticles deposited onto the substrate, for ultramicroscopic analysis (Figure 8).
Fig. 8. AFM image of size-selected silicon nanoparticles together with their size distributions
The advantage of using a TEA CO2 laser in the pyrolysis becomes clear for the fine tuning wavelength adjust, in order to optimize the production process of several other types of
Several synthesis reactors geometries based on the vaporization of a target (graphite/metal catalyst pellet) inside a oven at a fixed temperature (above 1000 K) by continuous CO2 laser beam (λ = 10.6 μm) have been developed to produce several types of carbon nanotubes. The laser power can be varied from 100 W to 1600 W and the temperature of the target is measured with an optical pyrometer. In general a inert gas flow carries away the solid
A typical mid-IR to near-IR absorbance spectrum taken on uniformly dispersed, purified CNTs (grown by CVD-mostly MWNT) at room temperature is displayed in Figure 9 [24]. In the measured IR absorbance spectrum, a prime intensity peak is seen at 1584 cm−1. This is an
particles formed in the laser ablation process which are then collected on a filter [23].
measured by TOFMS and AFM[21,22].
nanoparticles [22].
IR-active, graphite-like *E*1*<sup>u</sup>*mode (also known as the *G* band) originating from the *sp*2 hybridized carbon. The absorbance peak at 1200 cm−1, is a disorder-induced one phonon absorbance band (D band-from the *sp*3-hybridized carbon), which has been also observed in neutron irradiated diamonds [24]. This lattice mode arises from the disruption of the translational symmetry of the diamond lattice. In general, the presence of CH*<sup>x</sup>* groups (evidenced in the IR active bands in the range of 3000 cm−1) and non-conjugated carboxylic carbonyl groups (peak around 1725 cm−1) can benefit a number of bio-sensing applications by offering a simple route to nanotube functionalization [25].
Fig. 9. Typical infrared absorbance spectrum obtained on 60 nm multiwall carbon nanotubes [25]
Production of single-wall carbon nanotubes (SWNT) by the laser-ablation technique using graphite, pitch and coke as carbonaceous feedstock materials has been reported [26]. This has been done with a 250W continuous-wave CO2-laser at a wavelength of 10.6 *μ*m and varying the nature and the concentration of the metal catalyst, the type and pressure of the buffergas as well as the laser conditions. The amount of SWNT material obtained is much higher when using graphite as a precursor than in the case of coke and carbonaceous feedstock [26].
Laser-assisted chemical vapour deposition (LCVD) for the formation and growth of carbon nanotubes has also been performed using a medium-power continuous-wave CO2 laser to irradiate a sensitized mixture of Fe(CO)5 vapour and
acetylene and to simultaneously heat a silicon substrate on which the carbon nanotubes were grown [27]. Electron microscopy (TEM and HRTEM) as well as atomic force microscopy (AFM) were used to analyze the as-grown films and samples specially prepared on TEM grids and AFM substrates. Carbon nanotubes with different structures (straight, curved and even branched), including single- and multi-walled nanotubes were observed. Some nanotubes were found to be partially filled with a solid material (probably metallic iron) that seems to catalyze the nanotube growth [27].
Using a CO2 laser perpendicularly directed onto a silicon substrate, sensitized mixtures of iron pentacarbonyl vapour and acetylene were pyrolyzed in a flow reactor. The method involves the heating of both the gas phase and the substrate by IR radiation. The carbon
Infrared Lasers in Nanoscale Science 343
LCVD technique in general has several prominent advantages including high deposition rates that are favorable for scale-up production of CNTs. In contrast to the standard LCVD system, in which the in situ thermal decarbonylation of Fe(CO)5 is used for obtaining Fe nanoparticles C-LCVD employs the catalytic activity of pre-deposited metal-based nanoparticles. For a better control of CNT growth conditions, it proved to be advantageous to carry out separately the catalyst deposition and carbon nanotube growth. Thus, as a main advantage, C-LCVD allows *ex-situ* prepared metal-based particles with the desired properties and dispersion degree to provide the nucleation conditions for the growth of CNTs [28]. The temperature is measured with a thermocouple positioned behind the substrate. Since the temperature of the area which is irradiated by the laser is expected to be considerably higher than the average temperature of the substrate holder, the temperature inside the laser spot is measured optically with a pyrometer. Under a total gas pressure of about 80 mbar, the temperature in the laser spot, measured with the pyrometer could reach values between 800 and 900 C, while the thermocouple indicates values that are about 100 C lower. Mainly depending on the ethylene concentration, nanotube mean diameters between 10 and 60 nm were found. By increasing ethylene precursor flow rate, not only larger mean diameters of the CNTs were found but also the distribution of the CNT diameters became
Cementite (Fe3C) is of great technological importance for the mechanical properties of steels and iron alloys and for its role as catalyst to produce various hydrocarbons (including olefins, from CO2 and H2) and preferred catalysts for carbon fibers, nanotubes and nanoparticles due to their low nanometric mean sizes and narrow size distributions. Pure Fe3C nanomaterials have also been obtained by the pyrolysis of methyl methacrylate monomer, ethylene (as sensitizer), and iron pentacarbonyl (vapors) in a suitable range of laser intensities (by irradiating the same reactive mixture with a lower intensity radiation, the chemical content of the produced nanoparticles shifts towards mixtures of iron and iron oxides). Such nanopowders exhibited core (Fe3C)–shell polymer-based morphologies and their magnetic properties are likely to display high values for the saturation
Semiconductor photocatalysts have been used in different applications, and the combination of high photocatalytic activity, high stability and the benefit of environmental friendliness makes titanium dioxide the material of choice for such applications. In order to enhance the performance of this material for industrial purposes, coupling of TiO2 with other semiconductors and immobilization of TiO2 on porous materials have been studied as means for improving its photocatalytic activity. Nanocomposites of TiO2 and multiwalled carbon nanotubes were prepared and deposited by sol–gel spin coating on borosilicate substrates and sintered in air at 300 ºC. Further irradiation of the films with different CO2 laser intensities was carried out in order to crystallize TiO2 in the anatase form while preserving the MWNT's structure. The laser irradiation changed the crystal structure of the coatings and also affected the wettability and photocatalytic activity of the films. The anatase phase was only observed when a minimum laser intensity of 12*.*5 W/m2 was used [29]. The contact angle decreased with the enhancement of the laser intensity. The photocatalytic activity of the films was determined from the degradation of a stearic acid layer deposited on the films. It was observed that the addition of carbon nanotubes themselves increases the photocatalytic activity of TiO2 films. This efficiency is
broader [28].
magnetization.
nanotubes were formed via the catalyzing action of the fine iron particles produced in the same experiment by the decomposition of the organometallic precursor molecules [27]. The reactant gas, a mixture of iron pentacarbonyl vapour (Fe(CO)5), ethylene (C2H4) and acetylene (C2H2), is admitted to the reaction cell through a rectangular nozzle, creating a gas flow close and parallel to the Si substrate and being pumped from the opposite side. The flow of ethylene is directed through a bubbler containing liquid iron pentacarbonyl at room temperature (and 27 Torr vapour pressure). Thus, the ethylene serves as carrier gas for the iron pentacarbonyl. The third gas, acetylene, is supplied by an extra line. Before entering the flow reactor, the gases are mixed in a small mixing vessel. The iron nanoparticles, needed to catalyze the formation of carbon nanotubes from carbon-containing precursors, are obtained by decomposing Fe(CO)5 during the laser-induced reaction. Ethylene gas, introduced into the gaseous atmosphere, serves also as a sensitizer activating the laser reaction and speeding up the Fe(CO)5 dissociation. C2H4 has a resonant absorption at the CO2 laser emission wavelength (10.6 μm) and is characterized by a rather high dissociation energy. Under the present conditions, C2H4 is only expected to collisionally exchange its internal energy with the other precursor molecules that do not absorb the CO2 laser radiation, thus heating the entire gas mixture [27].
The pressure inside the reaction chamber was keptat a constant value of 150 Torr. A flow of argon (500 sccm) was used to avoid contamination of the ZnSe entrance window during irradiation. At the heated surface and interface, iron pentacarbonyl is the first molecule to undergo dissociation, which can proceed until bare iron is obtained. Carbon nanotubes with straight, curved or even branched structures have been identified by ultramicroscopy. While some nanotubes were hollow, many of them were found to be partially or totally filled with nanoparticles most probably being metallic iron [27].
Films of vertically aligned MWCNTs of extremely high packing density were produced by this technique under very clean hydrocarbon supply conditions. Using an open-air pyrolitic LCVD system in which the role of gas-phase reactions are minimized, the growth of highly oriented and aligned single- and multiwall carbon nanotubes have been reported [28]. The
nanotubes were formed via the catalyzing action of the fine iron particles produced in the same experiment by the decomposition of the organometallic precursor molecules [27]. The reactant gas, a mixture of iron pentacarbonyl vapour (Fe(CO)5), ethylene (C2H4) and acetylene (C2H2), is admitted to the reaction cell through a rectangular nozzle, creating a gas flow close and parallel to the Si substrate and being pumped from the opposite side. The flow of ethylene is directed through a bubbler containing liquid iron pentacarbonyl at room temperature (and 27 Torr vapour pressure). Thus, the ethylene serves as carrier gas for the iron pentacarbonyl. The third gas, acetylene, is supplied by an extra line. Before entering the flow reactor, the gases are mixed in a small mixing vessel. The iron nanoparticles, needed to catalyze the formation of carbon nanotubes from carbon-containing precursors, are obtained by decomposing Fe(CO)5 during the laser-induced reaction. Ethylene gas, introduced into the gaseous atmosphere, serves also as a sensitizer activating the laser reaction and speeding up the Fe(CO)5 dissociation. C2H4 has a resonant absorption at the CO2 laser emission wavelength (10.6 μm) and is characterized by a rather high dissociation energy. Under the present conditions, C2H4 is only expected to collisionally exchange its internal energy with the other precursor molecules that do not absorb the CO2 laser radiation, thus heating the
Fig. 10. Experimental set-up for the deposition of carbon nanotubes by LCVD
nanoparticles most probably being metallic iron [27].
The pressure inside the reaction chamber was keptat a constant value of 150 Torr. A flow of argon (500 sccm) was used to avoid contamination of the ZnSe entrance window during irradiation. At the heated surface and interface, iron pentacarbonyl is the first molecule to undergo dissociation, which can proceed until bare iron is obtained. Carbon nanotubes with straight, curved or even branched structures have been identified by ultramicroscopy. While some nanotubes were hollow, many of them were found to be partially or totally filled with
Films of vertically aligned MWCNTs of extremely high packing density were produced by this technique under very clean hydrocarbon supply conditions. Using an open-air pyrolitic LCVD system in which the role of gas-phase reactions are minimized, the growth of highly oriented and aligned single- and multiwall carbon nanotubes have been reported [28]. The
entire gas mixture [27].
Adapted from [27]
LCVD technique in general has several prominent advantages including high deposition rates that are favorable for scale-up production of CNTs. In contrast to the standard LCVD system, in which the in situ thermal decarbonylation of Fe(CO)5 is used for obtaining Fe nanoparticles C-LCVD employs the catalytic activity of pre-deposited metal-based nanoparticles. For a better control of CNT growth conditions, it proved to be advantageous to carry out separately the catalyst deposition and carbon nanotube growth. Thus, as a main advantage, C-LCVD allows *ex-situ* prepared metal-based particles with the desired properties and dispersion degree to provide the nucleation conditions for the growth of CNTs [28]. The temperature is measured with a thermocouple positioned behind the substrate. Since the temperature of the area which is irradiated by the laser is expected to be considerably higher than the average temperature of the substrate holder, the temperature inside the laser spot is measured optically with a pyrometer. Under a total gas pressure of about 80 mbar, the temperature in the laser spot, measured with the pyrometer could reach values between 800 and 900 C, while the thermocouple indicates values that are about 100 C lower. Mainly depending on the ethylene concentration, nanotube mean diameters between 10 and 60 nm were found. By increasing ethylene precursor flow rate, not only larger mean diameters of the CNTs were found but also the distribution of the CNT diameters became broader [28].
Cementite (Fe3C) is of great technological importance for the mechanical properties of steels and iron alloys and for its role as catalyst to produce various hydrocarbons (including olefins, from CO2 and H2) and preferred catalysts for carbon fibers, nanotubes and nanoparticles due to their low nanometric mean sizes and narrow size distributions. Pure Fe3C nanomaterials have also been obtained by the pyrolysis of methyl methacrylate monomer, ethylene (as sensitizer), and iron pentacarbonyl (vapors) in a suitable range of laser intensities (by irradiating the same reactive mixture with a lower intensity radiation, the chemical content of the produced nanoparticles shifts towards mixtures of iron and iron oxides). Such nanopowders exhibited core (Fe3C)–shell polymer-based morphologies and their magnetic properties are likely to display high values for the saturation magnetization.
Semiconductor photocatalysts have been used in different applications, and the combination of high photocatalytic activity, high stability and the benefit of environmental friendliness makes titanium dioxide the material of choice for such applications. In order to enhance the performance of this material for industrial purposes, coupling of TiO2 with other semiconductors and immobilization of TiO2 on porous materials have been studied as means for improving its photocatalytic activity. Nanocomposites of TiO2 and multiwalled carbon nanotubes were prepared and deposited by sol–gel spin coating on borosilicate substrates and sintered in air at 300 ºC. Further irradiation of the films with different CO2 laser intensities was carried out in order to crystallize TiO2 in the anatase form while preserving the MWNT's structure. The laser irradiation changed the crystal structure of the coatings and also affected the wettability and photocatalytic activity of the films. The anatase phase was only observed when a minimum laser intensity of 12*.*5 W/m2 was used [29]. The contact angle decreased with the enhancement of the laser intensity. The photocatalytic activity of the films was determined from the degradation of a stearic acid layer deposited on the films. It was observed that the addition of carbon nanotubes themselves increases the photocatalytic activity of TiO2 films. This efficiency is
Infrared Lasers in Nanoscale Science 345
matrix improved the photocatalytic activity of TiO2 coatings, as has been demonstrated by the degradation of a stearic acid layer deposited on the films [29]. In addition, higher CO2 laser intensities during the sintering implies enhanced photocatalytic activity of the
Laser Induced Breakdown Spectroscopy (LIBS) and Laser Induced Incandescence (LII) are two suitable techniques for analyzing small particles. In LII a pulsed laser rapidly heats the particles and by monitoring the rate of decay of the resulting incandescent radiation, one can extract particle size information, as the rate is related to the size of the particle. In order to get information on the particles chemical composition LIBS must be used instead. With it, the pulsed laser is tightly focused on the sample to induce a breakdown (microspark) of the material, and so by monitoring the emission of light from this plasma one can gather information about chemical compositions. Sensor technology is now able to capture light between 200 and 940 nm, a region where all elements emit. Since LIBS data is generated in real-time (response time of 1 second or less), one keep track of rapid changes in the composition of the particles during the actual production run. LIBS is very sensitive, having a resolution in the femtogram region and capable of detecting as few as 100
Irradiating the surface of a solid with a laser, material can be ablated in a controlled way by optimizing intensity and pulse duration of the laser (laser ablation). Depending on the laser wavelength, the ablation is dominated by thermal evaporation (using a CO2 laser) or photochemical processes (using an excimer laser). Laser-spectroscopic diagnostics can distinguish between the two processes. Excitation spectroscopy or resonant two-photon ionization of the sputtered atoms, molecules, clusters, nanoparticles or even microparticles
In addition, the velocity distribution of particles emitted from the surface can be obtained from the Doppler shifts and broadning of the absorption lines, and their internal energy distribution from the intensity ratios of different vibrational-rotational transitions. With a pulsed ablation laser, the measured time delay between ablation pulses and probe laser
nanocomposites.
particles/cm3.
allows their identification (Figure 12).
Fig. 12. Laser ablation from a surface
even improved when high CO2 laser intensities are used during the sintering of the coatings [29].
The high aspect ratio combined with high mechanical and chemical stability of carbon nanotubes could help in enhancing the photocatalytic activity of TiO2. Nanocomposites of TiO2/CNTs were prepared by the addition of –NH2 functionalized, 10 nm outer diameter MWNTs to the TiO2-based sol, in a concentration of 3 mg/ml. After a fine dispersion of the nanotubes was achieved using a high shear processor, the sol was deposited on borosilicate glass substrates by a spin coating technique (2000 rpm, 10 s) [29].
Fig. 11. Raman spectra in the ranges (a) 100-800 and (b) 300-3000 cm-1 of TiO2/MWNT coatings not irradiated and irradiated with different CO2 laser intensities [29]
Raman spectra of TiO2/MWNT coatings not irradiated and irradiated with different CO2 laser intensities are shown in Figure 11. Non-irradiated films sintered in air at 300◦C have shown no Raman bands, as is clear from Figure 11 (a). Higher sintering temperatures were applied to the coatings inorder to obtain anatase TiO2 phase structure, but oxidation of the MWNTs took place, limiting the study of the composites. Therefore, CO2 laser irradiation was applied to the coatings with different density outputs as an alternative sintering process. This process is very fast, and because it is pulsed (highly spatially limited heated zone), it has not damaged the CNT's structure. Films irradiated with lasers of intensity lower than 12*.*8 Wm−2 have shown similar results to non-irradiated coatings in the range 100–1000 cm−<sup>1</sup> [29]. However, a sharp and intense Raman band at 144 cm−1 corresponding to anatase TiO2 phase was observed when such minimum laser intensity was applied during the sintering of the coatings, suggesting that the local temperature obtained using 12*.*8 Wm−<sup>2</sup> is sufficient to crystallize the coating. Figure 11 (b) shows D and G bands corresponding to the presence of MWNTs at 1319–1328 and 1592–1601 cm−1, respectively. The D band corresponds to defects present in carbonaceous materials. The addition of CNTs to the TiO2
even improved when high CO2 laser intensities are used during the sintering of the
The high aspect ratio combined with high mechanical and chemical stability of carbon nanotubes could help in enhancing the photocatalytic activity of TiO2. Nanocomposites of TiO2/CNTs were prepared by the addition of –NH2 functionalized, 10 nm outer diameter MWNTs to the TiO2-based sol, in a concentration of 3 mg/ml. After a fine dispersion of the nanotubes was achieved using a high shear processor, the sol was deposited on borosilicate
Fig. 11. Raman spectra in the ranges (a) 100-800 and (b) 300-3000 cm-1 of TiO2/MWNT
Raman spectra of TiO2/MWNT coatings not irradiated and irradiated with different CO2 laser intensities are shown in Figure 11. Non-irradiated films sintered in air at 300◦C have shown no Raman bands, as is clear from Figure 11 (a). Higher sintering temperatures were applied to the coatings inorder to obtain anatase TiO2 phase structure, but oxidation of the MWNTs took place, limiting the study of the composites. Therefore, CO2 laser irradiation was applied to the coatings with different density outputs as an alternative sintering process. This process is very fast, and because it is pulsed (highly spatially limited heated zone), it has not damaged the CNT's structure. Films irradiated with lasers of intensity lower than 12*.*8 Wm−2 have shown similar results to non-irradiated coatings in the range 100–1000 cm−<sup>1</sup> [29]. However, a sharp and intense Raman band at 144 cm−1 corresponding to anatase TiO2 phase was observed when such minimum laser intensity was applied during the sintering of the coatings, suggesting that the local temperature obtained using 12*.*8 Wm−<sup>2</sup> is sufficient to crystallize the coating. Figure 11 (b) shows D and G bands corresponding to the presence of MWNTs at 1319–1328 and 1592–1601 cm−1, respectively. The D band corresponds to defects present in carbonaceous materials. The addition of CNTs to the TiO2
coatings not irradiated and irradiated with different CO2 laser intensities [29]
glass substrates by a spin coating technique (2000 rpm, 10 s) [29].
coatings [29].
matrix improved the photocatalytic activity of TiO2 coatings, as has been demonstrated by the degradation of a stearic acid layer deposited on the films [29]. In addition, higher CO2 laser intensities during the sintering implies enhanced photocatalytic activity of the nanocomposites.
Laser Induced Breakdown Spectroscopy (LIBS) and Laser Induced Incandescence (LII) are two suitable techniques for analyzing small particles. In LII a pulsed laser rapidly heats the particles and by monitoring the rate of decay of the resulting incandescent radiation, one can extract particle size information, as the rate is related to the size of the particle. In order to get information on the particles chemical composition LIBS must be used instead. With it, the pulsed laser is tightly focused on the sample to induce a breakdown (microspark) of the material, and so by monitoring the emission of light from this plasma one can gather information about chemical compositions. Sensor technology is now able to capture light between 200 and 940 nm, a region where all elements emit. Since LIBS data is generated in real-time (response time of 1 second or less), one keep track of rapid changes in the composition of the particles during the actual production run. LIBS is very sensitive, having a resolution in the femtogram region and capable of detecting as few as 100 particles/cm3.
Irradiating the surface of a solid with a laser, material can be ablated in a controlled way by optimizing intensity and pulse duration of the laser (laser ablation). Depending on the laser wavelength, the ablation is dominated by thermal evaporation (using a CO2 laser) or photochemical processes (using an excimer laser). Laser-spectroscopic diagnostics can distinguish between the two processes. Excitation spectroscopy or resonant two-photon ionization of the sputtered atoms, molecules, clusters, nanoparticles or even microparticles allows their identification (Figure 12).
Fig. 12. Laser ablation from a surface
In addition, the velocity distribution of particles emitted from the surface can be obtained from the Doppler shifts and broadning of the absorption lines, and their internal energy distribution from the intensity ratios of different vibrational-rotational transitions. With a pulsed ablation laser, the measured time delay between ablation pulses and probe laser
Infrared Lasers in Nanoscale Science 347
absorbed by cancer cells. During the surgery, the light beam is positioned at the tumor site,
A sensitizer in chemoluminescence is a chemical compound, capable of light emission after it has received energy from a molecule, which became excited previously in the chemical reaction. A good example is when an alkaline solution of sodium hypochlorite and a
ClO-(aq) + H2O2(aq) → O2\*(g) + H+(aq) + Cl-(aq) + OH-(aq) O2\*is excited oxygen - meaning, one or more electrons in the O2 molecule have been promoted to higher-energy molecular orbitals. Hence, oxygen produced by this chemical reaction somehow 'absorbed' the energy released by the reaction and became excited. This energy state is unstable, therefore it will return to the ground state by lowering its energy. It
New types of photosensitizers used in photodynamic therapy, which are based on photon upconverting nanoparticles, have been developed. Such photosensitizers are excitable with infrared irradiation, which has several times larger tissue penetration depth than the currently available ones. Photon upconverting materials convert lower-energy light to higher-energy light through excitation with multiple photons. For instance, such materials would adsorb infrared irradiation and emit visible light to further excite the photosensitizing molecules. Photon upconverting nanoparticles (PUNPs) play here a crucial role. They can be first coated with a porous, thin layer of silica through sol-gel reaction. During the coating process, photosensitizing molecules with high absorbance in the spectral window matching the emission of the PUNPs are doped, so that the resulting silica layer contains a certain amount of these photosensitizing molecules. Finally, an antibody, specific to antigens expressed on the target cell surface, is covalently attached to the silica-coated nanoparticles. When the thus-prepared nanoparticles are irradiated by infrared light, emission from the PUNPs will be absorbed by the photosensitizing molecules coated on their surfaces. Subsequently, excited photosensitizing molecules will interact with surrounding groundstate molecular oxygen, generating singlet oxygen, leading to oxidative damage of the neighboring cells to which the nanoparticles are attached via specific antigen-
PUNPs made from NaYF4:Yb3+,Er3+ have been recognized as one of the most efficient
When excited by an infrared (974 nm) source, strong visible bands appear around 537 nm and 635 nm. Merocyanine 540 (M-540) is used as photosensitizing molecule, and doped into the silica layer during the coating process. M-540 is a molecule that can produce singlet oxygen and other reactive oxygen species, and has been used before in photodynamic therapy as a photosensitizer with a visible light source. Both the emission spectrum of NaYF4:Yb3+,Er3+ nanoparticles and the absorption spectrum of M-540 show a good overlap
which then activates the drug that kills the cancer cells, thus photodynamic therapy.
concentrated solution of hydrogen peroxide are mixed, a reaction occurs:
can do that in more than one way:
antibody binding.
photon upconverting phosphors [30].
transferring energy to another molecule
pulses allows the determination of the velocity distribution. Resonant two-photon ionization in combination with a TOFMS gives the mass spectrum. It is common to observe a broad mass range of clusters. The question is whether these clusters were emitted from the solid or whether they were formed by collisions in the evaporated cloud just after emission. Measurements of the vibrational energy distributions can give an answer. If the mean vibrational energy is much higher than the temperature of the solid, the molecules were formed in the gas phase, where an insufficient number of collisions cannot fully transfer the internal energy of molecules formed by recombination of sputtered atoms into kinetic energy.
Whereas laser ablation of graphite yields thermalized C2 molecules with a rotationalvibrational energy distribution following a Boltzmann distribution at the temperature T of the solid, ablation of electrical insulators, such as AlO, produces AlO molecules with a large kinetic energy (≈1 eV), but a "rotational temperature" of only 500 K.
#### **4. Technological applications**
Photodynamic therapy (PDT) is used clinically to treat a wide range of medical conditions, including malignant cancers, and is recognised as a treatment strategy which is both minimally invasive and minimally toxic. Photosensitization is a process of transferring the energy of absorbed light. After absorption, the energy is transferred to the (chosen) reactants. This is part of the work of photochemistry in general. In particular this process is commonly employed where reactions require light sources of certain wavelengths that are not readily available. For example, mercury absorbs radiation at 1849 and 2537 angstroms, and the source is often high-intensity mercury lamps. It is a commonly used sensitizer. When mercury vapor is mixed with ethylene, and the compound is irradiated with a mercury lamp, this results in the photodecomposition of ethylene to acetylene. This occurs on absorption of light to yield excited state mercury atoms, which are able to transfer this energy to the ethylene molecules, and are in turn deactivated to their initial energy state.
In order to achieve the selective destruction of the target biological area using PDT while leaving normal tissues untouched, the photosensitizer can be applied locally to the target area. For instance, in the treatment of skin conditions, including acne, psoriasis, and also skin cancers, the photosensitizer can be locally excited by a light source. In the local treatment of internal tissues and cancers, after photosensitizers have been administered intravenously, light can be delivered to the target area using endoscopes and fiber optic catheters. Compared to normal tissues, most types of cancers are especially active in both the uptake and accumulation of photosensitizers agents, which makes cancers especially vulnerable to PDT. Since photosensitizers can also have a high affinity for vascular endothelial cells, PDT can be targetted to the blood carrying vasculature that supplies nutrients to tumours, increasing further the destruction of tumours.
The optimal spectral window for biological tissue penetration of irradiation is around 800 nm to 1 μm. The use of CO2 laser relies on water (the largest constituent of most biological tissues) absorbing strongly at 10.6 μm; this strong absorption leads to a shorter optical penetration depth (≈13 μm), limiting its use to extremely thin tissues. A patient would be given a photo sensitive drug (photofrin) containing cancer killing substances which are
pulses allows the determination of the velocity distribution. Resonant two-photon ionization in combination with a TOFMS gives the mass spectrum. It is common to observe a broad mass range of clusters. The question is whether these clusters were emitted from the solid or whether they were formed by collisions in the evaporated cloud just after emission. Measurements of the vibrational energy distributions can give an answer. If the mean vibrational energy is much higher than the temperature of the solid, the molecules were formed in the gas phase, where an insufficient number of collisions cannot fully transfer the internal energy of molecules formed by recombination of sputtered atoms into kinetic
Whereas laser ablation of graphite yields thermalized C2 molecules with a rotationalvibrational energy distribution following a Boltzmann distribution at the temperature T of the solid, ablation of electrical insulators, such as AlO, produces AlO molecules with a large
Photodynamic therapy (PDT) is used clinically to treat a wide range of medical conditions, including malignant cancers, and is recognised as a treatment strategy which is both minimally invasive and minimally toxic. Photosensitization is a process of transferring the energy of absorbed light. After absorption, the energy is transferred to the (chosen) reactants. This is part of the work of photochemistry in general. In particular this process is commonly employed where reactions require light sources of certain wavelengths that are not readily available. For example, mercury absorbs radiation at 1849 and 2537 angstroms, and the source is often high-intensity mercury lamps. It is a commonly used sensitizer. When mercury vapor is mixed with ethylene, and the compound is irradiated with a mercury lamp, this results in the photodecomposition of ethylene to acetylene. This occurs on absorption of light to yield excited state mercury atoms, which are able to transfer this energy to the ethylene molecules, and are in turn deactivated to their initial energy state.
In order to achieve the selective destruction of the target biological area using PDT while leaving normal tissues untouched, the photosensitizer can be applied locally to the target area. For instance, in the treatment of skin conditions, including acne, psoriasis, and also skin cancers, the photosensitizer can be locally excited by a light source. In the local treatment of internal tissues and cancers, after photosensitizers have been administered intravenously, light can be delivered to the target area using endoscopes and fiber optic catheters. Compared to normal tissues, most types of cancers are especially active in both the uptake and accumulation of photosensitizers agents, which makes cancers especially vulnerable to PDT. Since photosensitizers can also have a high affinity for vascular endothelial cells, PDT can be targetted to the blood carrying vasculature that supplies
The optimal spectral window for biological tissue penetration of irradiation is around 800 nm to 1 μm. The use of CO2 laser relies on water (the largest constituent of most biological tissues) absorbing strongly at 10.6 μm; this strong absorption leads to a shorter optical penetration depth (≈13 μm), limiting its use to extremely thin tissues. A patient would be given a photo sensitive drug (photofrin) containing cancer killing substances which are
kinetic energy (≈1 eV), but a "rotational temperature" of only 500 K.
nutrients to tumours, increasing further the destruction of tumours.
**4. Technological applications**
energy.
absorbed by cancer cells. During the surgery, the light beam is positioned at the tumor site, which then activates the drug that kills the cancer cells, thus photodynamic therapy.
A sensitizer in chemoluminescence is a chemical compound, capable of light emission after it has received energy from a molecule, which became excited previously in the chemical reaction. A good example is when an alkaline solution of sodium hypochlorite and a concentrated solution of hydrogen peroxide are mixed, a reaction occurs:
$$\text{ClO-(aq)} + \text{H}\_2\text{O}\_2(\text{aq}) \rightarrow \text{O}\_2\text{\*(g)} + \text{H}^\*(\text{aq}) + \text{Cl}\cdot(\text{aq}) + \text{OH}\cdot(\text{aq})$$
O2\*is excited oxygen - meaning, one or more electrons in the O2 molecule have been promoted to higher-energy molecular orbitals. Hence, oxygen produced by this chemical reaction somehow 'absorbed' the energy released by the reaction and became excited. This energy state is unstable, therefore it will return to the ground state by lowering its energy. It can do that in more than one way:
New types of photosensitizers used in photodynamic therapy, which are based on photon upconverting nanoparticles, have been developed. Such photosensitizers are excitable with infrared irradiation, which has several times larger tissue penetration depth than the currently available ones. Photon upconverting materials convert lower-energy light to higher-energy light through excitation with multiple photons. For instance, such materials would adsorb infrared irradiation and emit visible light to further excite the photosensitizing molecules. Photon upconverting nanoparticles (PUNPs) play here a crucial role. They can be first coated with a porous, thin layer of silica through sol-gel reaction. During the coating process, photosensitizing molecules with high absorbance in the spectral window matching the emission of the PUNPs are doped, so that the resulting silica layer contains a certain amount of these photosensitizing molecules. Finally, an antibody, specific to antigens expressed on the target cell surface, is covalently attached to the silica-coated nanoparticles. When the thus-prepared nanoparticles are irradiated by infrared light, emission from the PUNPs will be absorbed by the photosensitizing molecules coated on their surfaces. Subsequently, excited photosensitizing molecules will interact with surrounding groundstate molecular oxygen, generating singlet oxygen, leading to oxidative damage of the neighboring cells to which the nanoparticles are attached via specific antigenantibody binding.
PUNPs made from NaYF4:Yb3+,Er3+ have been recognized as one of the most efficient photon upconverting phosphors [30].
When excited by an infrared (974 nm) source, strong visible bands appear around 537 nm and 635 nm. Merocyanine 540 (M-540) is used as photosensitizing molecule, and doped into the silica layer during the coating process. M-540 is a molecule that can produce singlet oxygen and other reactive oxygen species, and has been used before in photodynamic therapy as a photosensitizer with a visible light source. Both the emission spectrum of NaYF4:Yb3+,Er3+ nanoparticles and the absorption spectrum of M-540 show a good overlap
Infrared Lasers in Nanoscale Science 349
signaling plant defence mechanisms [32]. The technique is based on the photoacoustic effect, i.e. the generation of acoustic waves as a consequence of light absorption. The absorption of photons of a suitable wavelength and energy by the gas molecules excites them to a higher ro-vibrational state. The absorbed energy is subsequently transferred by intermolecular collisions to translational energy, and thereby to heat. When a gas sample is collected in a closed cell, the heating of the gas molecules will increase the cell pressure. Hence, by modulating the light intensity pressure variations are produced that generate a sound wave, which can be detected with a sensitive microphone. A schematic view of a typical LPAS
experimental system for the detection of volatile molecules is shown in Figure 15.
Fig. 14. Photoluminescence spectra of NaYF4:Yb3+,Tm3+nanoparticles before and after being
Fig. 15. Schematic view of a LPAS set-up for the detection of volatile molecule emission
The photoacoustic signal depends on the number of absorbing molecules present in the gas, the absorption strength of the molecules at a specific light frequency, and the intensity of the light. Then, for trace gas detection, the light source should have a narrow bandwidth and be tuneable (in order to match the specific molecular absorption feature), and it should have a high intensity to ensure a good signal-to-noise ratio. Since the absorption processes of interest involve ro-vibrational transitions, it is normally necessary to work in the IR region. In this spectral range each molecule has its own *fingerprint* absorption spectrum,whose strength can vary rapidly over a short wavelength interval. Specifically, the preferred range for spectroscopic applications lies in the range 3–20 μm. Specifically, CO2 and CO lasers serve as the most frequently used light sources for photoacoustic detection of gases because
coated with Ru(bpy)3-doped silica [31]
from plants.
between the nanoparticles' emission and M-540's absorption [30]. The photon upconverting property of the nanoparticles was not affected by the silica coating, as confirmed by their photoluminescence spectrum. The presence of M-540 in the silica coating could be readily confirmed by the change in color of the nanoparticles, to slightly yellowish.
Photosensitizers drugs, should ideally be specific to the target, highly effective in producing reactive oxygen species (ROS) when exposed to appropriate illumination, and excitable by a wavelength close to the near-infrared region (800 nm to 1 μm),where tissue penetration of the illumination is at a maximum.
Regarding the last desired feature, single photons with infrared wavelengths are usually too weak energetically to generate reactive oxygen species (1O2). Thus, multiphoton excitation would be needed for infrared light to be used as illumination source. It has been reported the synthesis and characterization of a type of nanomaterial capable of generating 1O2 under continuous wave infrared excitation, based on photon upconverting nanoparticles (PUNPs). The results demonstrate that such nanoparticles have great potential for becoming a new type of versatile PDT drugs for photodynamic therapy [31]. Although photon upconverting materials do not directly produce ROS, one utilizes the fact that they adsorb infrared photons and emit visible ones to further excite the photosensitizing molecules, thus indirectly causing the photosensitizing molecules to generate 1O2 under infrared excitation. The design and synthesis of the PUNP-based photosensitizers follow a schema depicted in Figure 13.
Fig. 13. PUNP-based photosensitizer preparation.
The core is a NaYF4:Yb3+,Tm3+ nanoparticle, a photon upconverting material capable of emitting blue light (≈ 477 nm) upon excitation by an infrared light source (≈ 975 nm). The nanoparticle was then coated by a thin layer of tris(bipyridine)ruthenium(II)-doped silica which generates 1O2. The NaYF4:Yb3+,Tm3+ nanoparticles were synthesized by a microemulsion method [31].
The photoluminescence spectra of the NaYF4:Yb3+,Tm3+ nanoparticles before and after beingcoated with Ru(bpy)3-doped silica, under 975 nm excitation, are shown in Figure 14.
One of the most sensitive methods to detect volatile compounds released by the plants is laser photoacoustic spectroscopy (LPAS), which allows the identification of many molecules
between the nanoparticles' emission and M-540's absorption [30]. The photon upconverting property of the nanoparticles was not affected by the silica coating, as confirmed by their photoluminescence spectrum. The presence of M-540 in the silica coating could be readily
Photosensitizers drugs, should ideally be specific to the target, highly effective in producing reactive oxygen species (ROS) when exposed to appropriate illumination, and excitable by a wavelength close to the near-infrared region (800 nm to 1 μm),where tissue penetration of
Regarding the last desired feature, single photons with infrared wavelengths are usually too weak energetically to generate reactive oxygen species (1O2). Thus, multiphoton excitation would be needed for infrared light to be used as illumination source. It has been reported the synthesis and characterization of a type of nanomaterial capable of generating 1O2 under continuous wave infrared excitation, based on photon upconverting nanoparticles (PUNPs). The results demonstrate that such nanoparticles have great potential for becoming a new type of versatile PDT drugs for photodynamic therapy [31]. Although photon upconverting materials do not directly produce ROS, one utilizes the fact that they adsorb infrared photons and emit visible ones to further excite the photosensitizing molecules, thus indirectly causing the photosensitizing molecules to generate 1O2 under infrared excitation. The design and synthesis of the PUNP-based photosensitizers follow a schema depicted in
The core is a NaYF4:Yb3+,Tm3+ nanoparticle, a photon upconverting material capable of emitting blue light (≈ 477 nm) upon excitation by an infrared light source (≈ 975 nm). The nanoparticle was then coated by a thin layer of tris(bipyridine)ruthenium(II)-doped silica which generates 1O2. The NaYF4:Yb3+,Tm3+ nanoparticles were synthesized by a
The photoluminescence spectra of the NaYF4:Yb3+,Tm3+ nanoparticles before and after beingcoated with Ru(bpy)3-doped silica, under 975 nm excitation, are shown in Figure 14. One of the most sensitive methods to detect volatile compounds released by the plants is laser photoacoustic spectroscopy (LPAS), which allows the identification of many molecules
confirmed by the change in color of the nanoparticles, to slightly yellowish.
the illumination is at a maximum.
Fig. 13. PUNP-based photosensitizer preparation.
microemulsion method [31].
Figure 13.
signaling plant defence mechanisms [32]. The technique is based on the photoacoustic effect, i.e. the generation of acoustic waves as a consequence of light absorption. The absorption of photons of a suitable wavelength and energy by the gas molecules excites them to a higher ro-vibrational state. The absorbed energy is subsequently transferred by intermolecular collisions to translational energy, and thereby to heat. When a gas sample is collected in a closed cell, the heating of the gas molecules will increase the cell pressure. Hence, by modulating the light intensity pressure variations are produced that generate a sound wave, which can be detected with a sensitive microphone. A schematic view of a typical LPAS experimental system for the detection of volatile molecules is shown in Figure 15.
Fig. 14. Photoluminescence spectra of NaYF4:Yb3+,Tm3+nanoparticles before and after being coated with Ru(bpy)3-doped silica [31]
Fig. 15. Schematic view of a LPAS set-up for the detection of volatile molecule emission from plants.
The photoacoustic signal depends on the number of absorbing molecules present in the gas, the absorption strength of the molecules at a specific light frequency, and the intensity of the light. Then, for trace gas detection, the light source should have a narrow bandwidth and be tuneable (in order to match the specific molecular absorption feature), and it should have a high intensity to ensure a good signal-to-noise ratio. Since the absorption processes of interest involve ro-vibrational transitions, it is normally necessary to work in the IR region. In this spectral range each molecule has its own *fingerprint* absorption spectrum,whose strength can vary rapidly over a short wavelength interval. Specifically, the preferred range for spectroscopic applications lies in the range 3–20 μm. Specifically, CO2 and CO lasers serve as the most frequently used light sources for photoacoustic detection of gases because
Infrared Lasers in Nanoscale Science 351
deliver the laser's energy to heat the bonded cut and are used for controlling the temperature. They also make it possible to bond tissues inside the body. Sutures or stitches are not water tight, and blood or urine can pass through cuts, causing severe infection. Laser-bonded tissues heal faster, with less scarring. Even using today's microsurgery techniques, the treated wounds are open to infection, and the patient is inevitably left with permanent and unsightly scars. The near-infrared light is just the right wavelength to excite vibrations in chemical bonds in the water molecules (via first-overtone excitation in the OH-
Keeping the heat from the laser at exactly the right temperature for optimal wound healing, allows surgeons to seal cuts both on our skin and inside our bodies with less scarring, and less exposure to infection. When the laser begins to overheat and risks burning the tissue, the device reduces laser power, and if the temperature is too low to complete a closure, laser
There is also an enormous potential of a CO2-laser system for rapidly producing polymer microfluidic structures. The common polymer poly (methyl methacrylate) (PMMA) absorbs IR light in the 2.8–25 µm wavelength band, so CO2 lasers have been used in recent years for fabricating microfluidic devices from it, with channel widths of a few hundred micrometers. The narrowest produced channel was 85 μm wide. A solvent-assisted thermal bonding method proved to be the most time-efficient one. These systems provide a cost effective alternative to UV-laser systems and they are especially useful in microfluidic prototyping
Furthermore, surface heat treatment in glasses and ceramics, using CO2 lasers, has drawn the attention to several technological applications, such as lab-on-a-chip devices, diffraction gratings and microlenses. Microlens fabrication on a glass surface has been studied mainly due to its importance in optical devices, as fiber coupling and CCD signal enhancement. Using microlens arrays, recorded on the glass surface, can enable the bidimensional codification for product identification. This would allow the production of codes without any residues (like the fine powder generated by laser ablation) and resistance to an aggressive environment, such as sterilization processes. Microlens arrays can be fabricated using a continuous wave CO2 laser, focused on the surface of flat commercial soda-lime
Silicon micromachining is a very important technology in microfabrication and microelectromechanical system (MEMS) industry. Nd:YAG laser has a wavelength of 1.06 μm, which is adsorbed by silicon, and is easily used for direct silicon machining. But the cost is very high. Although CO2 laser is cheap, its wavelength of 10.64 μm is not absorbed by silicon. However, a silicon sample put on the top of a glass, instead of pure silicon, is used for CO2 laser micromachining. The silicon on the top of a glass may absorb the CO2 laser and become able to be etched, even through the wafer. Commercial available air-cooled CO2 laser equipment can be used with a maximum laser power of 30 W. A glass below the silicon changes the absorption of silicon to CO2 laser during machining. The silicon on the top of a glass may be etched by CO2 laser even through the wafer due to the absorption variation. The etching depth increases with the pass number at constant laser power and scanning
stretch manifold); the vibrations quickly turn into heat.
power is increased appropriately.
silicate glass substrates.
speed.
due to the very short cycle time of production.
they provide relatively high CW powers, typically up to 100 W and 20 W respectively, over this wavelength region. LPAS shows a large versatility of applications not only in plant science, but also in other fields, e.g. in environmental chemistry [33]. It has been shown to be a reliable method for the detection of ethylene in several plant physiological processes at parts per trillion concentration levels (e.g. from a cherry tomato under different conditions) [34].
One of the major analytical problems with fruit and vegetable samples is the detection and identification of non-volatile organic compounds present in low concentration levels, as happens for most of the phytoalexins produced by plants. Mass spectrometry is widely used in the analysis of such compounds, providing exact mass identification. However, the difficulty with their unequivocal identification and quantitative detection lies in their volatilization into the gas phase prior to injection into the analyser. This constitutes particular problems for thermally labile samples, as they rapidly decompose upon heating. To circumvent this difficulty a wide range of techniques have been applied for non-volatile compound analysis, including LD (Laser Desorption). Recently, LD methods have been developed in which the volatilization and ionization steps are separated, providing higher sample sensitivity. In particular, REMPI-TOFMS is considered to be one of the most powerful methods for trace component analysis in complex matrices [2]. The high selectivity of REMPI-TOFMS stems from the combination of the mass-selective detection with the resonant ionization process, i.e. the ionization is achieved by absorption of two or more laser photons through a resonant, intermediate state. This condition provides a second selectivity to the technique, namely laser wavelength-selective ionization. In addition, it shows an easy control of the molecular fragmentation by the laser intensity and the possibility of simultaneous analysis of different components present in a matrix.
As an example, it is possible to perform fast and direct analysis of non-volatile compounds in fruit and vegetables, particularly *trans*-resveratrol in grapes and vine leaves. The method is based on the combination of LD followed by REMPI and TOFMS detection, often identified by its sum of acronyms, i.e. LD-REMPI-TOFMS [35]. *Trans*-Resveratrol is an antioxidant compound naturally produced in a huge number of plants, including grapes. Analysis of *trans*-resveratrol is generally carried out by high-performance liquid chromatography. Its analysis in grapes and wines requires the use of pre-concentration prior to analysis and/or multi-solvent extraction techniques, due to the complexity of the matrices and to the low concentration of the analyte. The extraction methods generally employed are liquid extraction with organic solvents or solid-phase extraction. It is generally accepted that the sample preparation is the limiting step in *trans*-resveratrol analysis, not only because of the need for costly and time-consuming operations, but also because of the error sources introduced during this operation. These error sources can largely be overcome when applying the method of LD-REMPI-TOFMS. The experimental set-up used in this analysis method basically consists of two independent high vacuum chambers; the first chamber is used for both laser desorption and laser post-ionization of the samples, and the second chamber for TOFMS [2].
Some other relevant technological applications of infrared lasers will be generally described here in the following paragraphs.
Katzir was the first researcher to apply the carbon dioxide laser, coupled to optical fibers made from silver halide, for wound closure under a tight temperature control. The fibers
they provide relatively high CW powers, typically up to 100 W and 20 W respectively, over this wavelength region. LPAS shows a large versatility of applications not only in plant science, but also in other fields, e.g. in environmental chemistry [33]. It has been shown to be a reliable method for the detection of ethylene in several plant physiological processes at parts per trillion concentration levels (e.g. from a cherry tomato under different conditions) [34]. One of the major analytical problems with fruit and vegetable samples is the detection and identification of non-volatile organic compounds present in low concentration levels, as happens for most of the phytoalexins produced by plants. Mass spectrometry is widely used in the analysis of such compounds, providing exact mass identification. However, the difficulty with their unequivocal identification and quantitative detection lies in their volatilization into the gas phase prior to injection into the analyser. This constitutes particular problems for thermally labile samples, as they rapidly decompose upon heating. To circumvent this difficulty a wide range of techniques have been applied for non-volatile compound analysis, including LD (Laser Desorption). Recently, LD methods have been developed in which the volatilization and ionization steps are separated, providing higher sample sensitivity. In particular, REMPI-TOFMS is considered to be one of the most powerful methods for trace component analysis in complex matrices [2]. The high selectivity of REMPI-TOFMS stems from the combination of the mass-selective detection with the resonant ionization process, i.e. the ionization is achieved by absorption of two or more laser photons through a resonant, intermediate state. This condition provides a second selectivity to the technique, namely laser wavelength-selective ionization. In addition, it shows an easy control of the molecular fragmentation by the laser intensity and the possibility of
simultaneous analysis of different components present in a matrix.
chamber for TOFMS [2].
here in the following paragraphs.
As an example, it is possible to perform fast and direct analysis of non-volatile compounds in fruit and vegetables, particularly *trans*-resveratrol in grapes and vine leaves. The method is based on the combination of LD followed by REMPI and TOFMS detection, often identified by its sum of acronyms, i.e. LD-REMPI-TOFMS [35]. *Trans*-Resveratrol is an antioxidant compound naturally produced in a huge number of plants, including grapes. Analysis of *trans*-resveratrol is generally carried out by high-performance liquid chromatography. Its analysis in grapes and wines requires the use of pre-concentration prior to analysis and/or multi-solvent extraction techniques, due to the complexity of the matrices and to the low concentration of the analyte. The extraction methods generally employed are liquid extraction with organic solvents or solid-phase extraction. It is generally accepted that the sample preparation is the limiting step in *trans*-resveratrol analysis, not only because of the need for costly and time-consuming operations, but also because of the error sources introduced during this operation. These error sources can largely be overcome when applying the method of LD-REMPI-TOFMS. The experimental set-up used in this analysis method basically consists of two independent high vacuum chambers; the first chamber is used for both laser desorption and laser post-ionization of the samples, and the second
Some other relevant technological applications of infrared lasers will be generally described
Katzir was the first researcher to apply the carbon dioxide laser, coupled to optical fibers made from silver halide, for wound closure under a tight temperature control. The fibers deliver the laser's energy to heat the bonded cut and are used for controlling the temperature. They also make it possible to bond tissues inside the body. Sutures or stitches are not water tight, and blood or urine can pass through cuts, causing severe infection. Laser-bonded tissues heal faster, with less scarring. Even using today's microsurgery techniques, the treated wounds are open to infection, and the patient is inevitably left with permanent and unsightly scars. The near-infrared light is just the right wavelength to excite vibrations in chemical bonds in the water molecules (via first-overtone excitation in the OHstretch manifold); the vibrations quickly turn into heat.
Keeping the heat from the laser at exactly the right temperature for optimal wound healing, allows surgeons to seal cuts both on our skin and inside our bodies with less scarring, and less exposure to infection. When the laser begins to overheat and risks burning the tissue, the device reduces laser power, and if the temperature is too low to complete a closure, laser power is increased appropriately.
There is also an enormous potential of a CO2-laser system for rapidly producing polymer microfluidic structures. The common polymer poly (methyl methacrylate) (PMMA) absorbs IR light in the 2.8–25 µm wavelength band, so CO2 lasers have been used in recent years for fabricating microfluidic devices from it, with channel widths of a few hundred micrometers. The narrowest produced channel was 85 μm wide. A solvent-assisted thermal bonding method proved to be the most time-efficient one. These systems provide a cost effective alternative to UV-laser systems and they are especially useful in microfluidic prototyping due to the very short cycle time of production.
Furthermore, surface heat treatment in glasses and ceramics, using CO2 lasers, has drawn the attention to several technological applications, such as lab-on-a-chip devices, diffraction gratings and microlenses. Microlens fabrication on a glass surface has been studied mainly due to its importance in optical devices, as fiber coupling and CCD signal enhancement. Using microlens arrays, recorded on the glass surface, can enable the bidimensional codification for product identification. This would allow the production of codes without any residues (like the fine powder generated by laser ablation) and resistance to an aggressive environment, such as sterilization processes. Microlens arrays can be fabricated using a continuous wave CO2 laser, focused on the surface of flat commercial soda-lime silicate glass substrates.
Silicon micromachining is a very important technology in microfabrication and microelectromechanical system (MEMS) industry. Nd:YAG laser has a wavelength of 1.06 μm, which is adsorbed by silicon, and is easily used for direct silicon machining. But the cost is very high. Although CO2 laser is cheap, its wavelength of 10.64 μm is not absorbed by silicon. However, a silicon sample put on the top of a glass, instead of pure silicon, is used for CO2 laser micromachining. The silicon on the top of a glass may absorb the CO2 laser and become able to be etched, even through the wafer. Commercial available air-cooled CO2 laser equipment can be used with a maximum laser power of 30 W. A glass below the silicon changes the absorption of silicon to CO2 laser during machining. The silicon on the top of a glass may be etched by CO2 laser even through the wafer due to the absorption variation. The etching depth increases with the pass number at constant laser power and scanning speed.
Infrared Lasers in Nanoscale Science 353
nanoparticle production, surgery and biomedicine, nanoanalysis, nanomaterials processing
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Several techniques are available that allow the size distribution of an aerosol to be determined in real time, but the determination of chemical composition, which has traditionally been done by impaction methods, is slow and yields only an average composition of the ensemble of particles in a given size range. It is essential, therefore, that new techniques be developed to allow the characterization of both the physical properties and chemical composition of aerosols, and that these operate on a time-scale that allows changes in the aerosol composition to be determined in real time. Several variants of the aerosol TOFMS (ATOFMS) instrument have been described in the literature [2]. The principle of the most sophisticated instrument reported to date employs two laser systems; first, a tuneable IR laser (OPO) is used to desorb material selectively from the particle, and then a second (VUV) laser is used to ionize the molecules that are produced [2]. With this approach, greater control over the particle ablation and ionization steps is possible, and by using low IR laser energy for the first evaporation step it is possible to depth profile heterogeneously mixed aerosol particles. Molecular information can be obtained by tuning the laser energy to just above the threshold required for desorption.
#### **5. Conclusions**
Several applications have been demonstrated for CO2 lasers, despite it is impossible to use photoelectric emission to detect this radiation (photon energy of about 0.1 eV is only about five times room temperature, and cryogenically cooled photoconductors are necessary to achieve fast low-level detection) and little engineering has been done in the mid-infrared region.
All applications mentioned require a stable, single-frequency source of radiation. A kilowatt of radiation at 10 microns, focused down to its diffraction limit, is a power density of 1 gigawatt per square cm. Because most materials absorb at 10 microns, considerable interest has been shown in CO2 lasers in many applications, and any problem which requires controlled surface heating or burning might find a potential solution with the CO2 laser.
The MTL3-GT CO2 laser ability to combine the laser pulsed mode with tune-ability introduces new perspectives to perform different experiments, which require a suitable, reliable and user-friendly procedure. With the method described in this work, many experiments can be performed in real time with simultaneous control of power*/*energy and wavelength, and taking advantage of the full laser power for each selected wavelength. One could observe, after improving the procedure, that energy values are more stable in all four emission bands (9P, 9R, 10P and 10R). This behavior was also observed regardless of the repetition rate, even for higher ones around 100 Hz. Besides energy, power was also measured and improved following the same procedure. This procedure can also be used on other infrared lasers with some minor adaptations regarding the software and energy detectors used. The observation of the energy line variation in real time is very important for understanding the behavior of the laser under certain external conditions and also to make sure that internal mechanisms are also error free.
It was demonstrated that CO2 laser applications in the fields of nanoscience and nanotechnology are very promising, with particular relevance on spectroscopy, photodynamic kinetics and photodynamic therapy, ultra-pure and size-selected nanoparticle production, surgery and biomedicine, nanoanalysis, nanomaterials processing and composite nano-engineered catalysts.
#### **6. References**
352 CO2 Laser – Optimisation and Application
Several techniques are available that allow the size distribution of an aerosol to be determined in real time, but the determination of chemical composition, which has traditionally been done by impaction methods, is slow and yields only an average composition of the ensemble of particles in a given size range. It is essential, therefore, that new techniques be developed to allow the characterization of both the physical properties and chemical composition of aerosols, and that these operate on a time-scale that allows changes in the aerosol composition to be determined in real time. Several variants of the aerosol TOFMS (ATOFMS) instrument have been described in the literature [2]. The principle of the most sophisticated instrument reported to date employs two laser systems; first, a tuneable IR laser (OPO) is used to desorb material selectively from the particle, and then a second (VUV) laser is used to ionize the molecules that are produced [2]. With this approach, greater control over the particle ablation and ionization steps is possible, and by using low IR laser energy for the first evaporation step it is possible to depth profile heterogeneously mixed aerosol particles. Molecular information can be obtained by tuning the laser energy to
Several applications have been demonstrated for CO2 lasers, despite it is impossible to use photoelectric emission to detect this radiation (photon energy of about 0.1 eV is only about five times room temperature, and cryogenically cooled photoconductors are necessary to achieve fast low-level detection) and little engineering has been done in the mid-infrared
All applications mentioned require a stable, single-frequency source of radiation. A kilowatt of radiation at 10 microns, focused down to its diffraction limit, is a power density of 1 gigawatt per square cm. Because most materials absorb at 10 microns, considerable interest has been shown in CO2 lasers in many applications, and any problem which requires controlled surface heating or burning might find a potential solution with the CO2 laser.
The MTL3-GT CO2 laser ability to combine the laser pulsed mode with tune-ability introduces new perspectives to perform different experiments, which require a suitable, reliable and user-friendly procedure. With the method described in this work, many experiments can be performed in real time with simultaneous control of power*/*energy and wavelength, and taking advantage of the full laser power for each selected wavelength. One could observe, after improving the procedure, that energy values are more stable in all four emission bands (9P, 9R, 10P and 10R). This behavior was also observed regardless of the repetition rate, even for higher ones around 100 Hz. Besides energy, power was also measured and improved following the same procedure. This procedure can also be used on other infrared lasers with some minor adaptations regarding the software and energy detectors used. The observation of the energy line variation in real time is very important for understanding the behavior of the laser under certain external conditions and also to make
It was demonstrated that CO2 laser applications in the fields of nanoscience and nanotechnology are very promising, with particular relevance on spectroscopy, photodynamic kinetics and photodynamic therapy, ultra-pure and size-selected
just above the threshold required for desorption.
sure that internal mechanisms are also error free.
**5. Conclusions**
region.
**Part 4**
**Medical Applications**
## **Part 4**
**Medical Applications**
354 CO2 Laser – Optimisation and Application
[32] F. J. M. Harren and J. Reuss, *Photoacoustic Spectroscopy* in Encyclopedia of Applied
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*Phys. B*, 71 (2000) 601.
**14**
*Korea*
**Clinical Application of CO2 Laser**
The carbon dioxide (CO2) laser was first introduced in 1964 by Patel and has been extensively used in the next two decades as an incision tool in increasingly wide areas, such as neurosurgery, dermatology and plastic surgery, otorhinolaryngology, ophthalmology, gynecology, and general surgery. In 1984, its reliability resulted in its approval by the U.S. Food and Drug Administration, and thus, medical use of lasers became more prevalent. Currently, the CO2 laser is considered an indispensable piece of diagnostic and therapeutic
The CO2 laser produces a beam of infrared light with the principal wavelength bands centering at 10,600 nanometers. Collisional energy transfer between the nitrogen and the carbon dioxide molecule causes vibrational excitation of the carbon dioxide, with sufficient efficiency to lead to the desired population inversion necessary for laser operation. It is easy to actively Q-switch a CO2 laser by means of a rotating mirror or an electro-optic switch,
CO2 lasers are attracting attention as cutting tools. They are able to seal lymphatic and blood vessels less than 0.5-mm wide and can reduce intraoperative bleeding and the occurrence of postoperative swelling. CO2 lasers emit a longer wavelength than those transmitted by other types of lasers. Their penetration depth of 0.03 mm is very safe. Coagulation in small blood vessels, as well as sealing of lymphatic and small peripheral nerves, have been reported in
The CO2 laser also offers more comfort to patients by reducing intraoperative bleeding and postoperative edema, facilitating the process of wound healing after surgery. The boundaries between the tissues receiving heat damage and the surrounding intact tissue are very well defined. A CO2 laser can evaporate through the surrounding tissue without physical force, sealing the vessel and minimizing bleeding; thus, it is useful when a bloodless view is required during surgery. Moreover, wounds can be treated in a sterile
Regarding its disadvantages, the equipment is expensive, operators require time to become familiar with it, and the sophisticated operation is technically difficult. Therefore, more repetitions are required to gain the necessary experience and practice. In addition, there is a risk of fire if the laser is used improperly. It can also damage the cornea; thus, eye protection is needed for the surgeon and the patient. Because the gas discharged from the vaporization of tissue contains an excess of CO2 or virus particles, it can be harmful to the human body.
giving rise to Q-switched peak powers up to gigawatts (GW) of peak power.
experimental studies using CO2 lasers; this sealing alleviates postoperative pain.
manner because of high-temperature evaporation of tissue lesions.
**1. Introduction**
equipment.
Hyeong-Seok Oh and Jin-Sung Kim
*Wooridul Spine Hospital, Seoul*
### **Clinical Application of CO2 Laser**
Hyeong-Seok Oh and Jin-Sung Kim *Wooridul Spine Hospital, Seoul Korea*
#### **1. Introduction**
The carbon dioxide (CO2) laser was first introduced in 1964 by Patel and has been extensively used in the next two decades as an incision tool in increasingly wide areas, such as neurosurgery, dermatology and plastic surgery, otorhinolaryngology, ophthalmology, gynecology, and general surgery. In 1984, its reliability resulted in its approval by the U.S. Food and Drug Administration, and thus, medical use of lasers became more prevalent. Currently, the CO2 laser is considered an indispensable piece of diagnostic and therapeutic equipment.
The CO2 laser produces a beam of infrared light with the principal wavelength bands centering at 10,600 nanometers. Collisional energy transfer between the nitrogen and the carbon dioxide molecule causes vibrational excitation of the carbon dioxide, with sufficient efficiency to lead to the desired population inversion necessary for laser operation. It is easy to actively Q-switch a CO2 laser by means of a rotating mirror or an electro-optic switch, giving rise to Q-switched peak powers up to gigawatts (GW) of peak power.
CO2 lasers are attracting attention as cutting tools. They are able to seal lymphatic and blood vessels less than 0.5-mm wide and can reduce intraoperative bleeding and the occurrence of postoperative swelling. CO2 lasers emit a longer wavelength than those transmitted by other types of lasers. Their penetration depth of 0.03 mm is very safe. Coagulation in small blood vessels, as well as sealing of lymphatic and small peripheral nerves, have been reported in experimental studies using CO2 lasers; this sealing alleviates postoperative pain.
The CO2 laser also offers more comfort to patients by reducing intraoperative bleeding and postoperative edema, facilitating the process of wound healing after surgery. The boundaries between the tissues receiving heat damage and the surrounding intact tissue are very well defined. A CO2 laser can evaporate through the surrounding tissue without physical force, sealing the vessel and minimizing bleeding; thus, it is useful when a bloodless view is required during surgery. Moreover, wounds can be treated in a sterile manner because of high-temperature evaporation of tissue lesions.
Regarding its disadvantages, the equipment is expensive, operators require time to become familiar with it, and the sophisticated operation is technically difficult. Therefore, more repetitions are required to gain the necessary experience and practice. In addition, there is a risk of fire if the laser is used improperly. It can also damage the cornea; thus, eye protection is needed for the surgeon and the patient. Because the gas discharged from the vaporization of tissue contains an excess of CO2 or virus particles, it can be harmful to the human body.
Clinical Application of CO2 Laser 359
surgery (Nerubay, Caspi et al. 1997; Hellinger 1999; Houck 2006). Nerubay et al. reported that 50 patients with low back and radicular pains were successfully treated by percutaneous laser nucleolysis using a CO2 laser (Nerubay, Caspi et al. 1997), and successful vaporization of the disk was accomplished in animal models (Stein, Sedlacek et al. 1990).
Considering the similarity between the disk and the meniscus (Whipple, Caspari et al. 1984), we cite studies on the effect of the CO2 laser on the meniscus. According to these research results, there was a considerable proliferation of cells resembling chondrocytes after 2 weeks of the CO2 laser treatment and there was definitely an increase in the production of ground substance and immature collagen fibers after 4 weeks; the collagen had become well reorganized into a logical orientation, resembling the normal architecture of fibrocartilage,
These animal and clinical studies strongly support the claim that CO2 lasers can safely and feasibly be used for the removal of protruded disks and discal cysts. Moreover, the CO2 laser, when attached to an operating microscope, allows for quick and easy removal of the
Laser removes disk material by vaporization (Stein, Sedlacek et al. 1990) and consequently lowers intradiskal pressure (Gropper, Robertson et al. 1984). In spine surgery, the use of a laser has advantages over scalpel use in terms of precision; the ability to be used on delicate tissues; minimal tissue manipulation; and less bleeding, swelling, and trauma (Jeon, Lee et al. 2007). It is especially useful in the small spaces involved in herniated disks (Kim, Choi et al. 2009). Therefore, a laser is an effective tool for performing a minimally invasive spinal surgery with percutaneous and open spinal procedures (Ahn, Lee et al. 2005; Lee, Ahn et al. 2006; Lee, Ahn et al. 2006; Jeon, Lee et al. 2007; Lee, Ahn et al. 2008; Kim, Choi et al. 2009;
In the Wooridul Hospital, CO2 laser-equipped surgical microscopes have been used for open lumbar microdiscectomy since December 1991 (Fig. 1). These microscopes coaxially align the invisible CO2 laser beam with a visible helium-neon laser beam and can focus exactly on and evaporate the target disk material by the commonly used 20- to 30-W single-pulse mode laser. Therefore, we aimed to determine whether a CO2 laser-equipped surgical microscope
Lee et al. (Lee and Lee 2011) reported that the CO2 laser-assisted microdiscectomy could be an effective alternative to conventional microdiscectomy techniques. Because the CO2 laser enabled effective removal of extraforaminal lumbar disk herniation(EFLDH) via a narrow extraforaminal operative corridor without excessive loss of the facet joint and/or the par interarticularis, a thorough decompression of the extraforaminal and/or the foraminal zone was achieved while preserving spinal stability (Fig. 2). Thirty-one patients exhibited a marked reduction in leg pain immediately after the surgery. No patient complained of persistent severe leg pain in the perioperative period. In the present study, reherniation occurred in 1 patient (3.6%) at the 1-year follow-up. The CO2 laser is also believed to decrease reherniation after discectomy owing to laser-induced metaplasia. (Kim, Choi et al.
after 10 weeks (Benjamin, Qin et al. 1995).
**2.2.1 Disk herniation**
Kim and Lee 2009).
2009)
is a useful tool for microdiscectomy.
discal cyst and, if needed, easy vaporization of disk material.
#### **2. Clinical application in neurosurgery**
The CO2 laser is most widely used in the field of neurosurgery for removal and evaporation of tumors located in difficult surgical fields, such as the base of the skull, ventricles, brainstem, and spinal cord.
#### **2.1 Brain tumor surgery**
The CO2 laser has been used in brain microsurgery after Steller et al. (Stellar, Polanyi et al. 1970) had first successfully used it in removing a recurrent glioma in 1969. The most ideal treatment of a brain tumor is minimizing damage to the normal brain tissue and removing only the tumor area. To overcome the surgical difficulty of avoiding damage to the brain tissue, a special instrument was developed. Theoretically, lasers have several advantages. First, although the surgical field is narrow, it makes surgery possible. Other small-sized surgical approaches are facilitated to minimize injury to normal brain tissue. Second, brain retraction is minimized, thus causing less damage to normal brain tissue. Third, laser beam minimizes injury to surrounding tissues and enables removal of a tumor with less thermal injury. Fourth, lasers have a coagulating property that lessens bleeding of the surgical field. Fifth, operation time is shortened (Tew and Tobler 1983; Krishnamurthy and Powers 1994).
The CO2 laser is the main instrument used in brain surgery. It has the advantage of rapidly removing separated tumor cells and exact irradiation of target cells by a microsurgical technique where the CO2 laser is installed with a microscope. However, as energy cannot pass through an optical fiber, it is inconvenient to use the equipment. It has limited function in bleeding control, as control of bleeding is not possible in a vessel with a diameter 0.5 mm, necessitating the use of the equipment in conjunction with other equipments for severe bleeding management (Heppner 1978; Ascher and Heppner 1984; Deruty, Pelissou-Guyotat et al. 1993).
The CO2 laser is most widely used in the field of neurosurgery, and it is mainly used in the removal of tumors by evaporation where surgical approach of the tumor site is difficult. It is common opinion that the CO2 laser is most effective with skull base, ventricular, brainstem, and spinal cord tumors (Powers, Cush et al. 1991; Origitano and Reichman 1993). In particular, it is most effective in removing a meningioma that is relatively hard or has less vascular distribution to be calcified. In addition, it is suitable for removing a low-grade glioma that is relatively rigid (Deruty, Pelissou-Guyotat et al. 1993).
The Nd:YAG laser has the advantage that energy can be passed to thinner fiberoptic cables and excellent clotting function is possible at a 3-mm vessel. Therefore, it has been reported as a valid technique of removing brain tumors having greater vascular distribution and cerebral vascular malformation (Beck 1980). The combolaser has been developed in recent years by Fasano et al. (Glasscock, Jackson et al. 1981) and has been applied in surgery. It is composed of CO2 and Nd:YAG lasers, combining the advantages of both. It works by first emitting Nd:YAG energy to the tumor for clotting, followed by tumor removal by evaporation using the CO2 laser (Beck 1980; Glasscock, Jackson et al. 1981).
#### **2.2 Spine surgery**
Since the first trial of Nd:YAG in a lumbar disk surgery in 1986 (Choy, Case et al. 1987), there have been many reports about the usefulness of different kinds of lasers in disk
The CO2 laser is most widely used in the field of neurosurgery for removal and evaporation of tumors located in difficult surgical fields, such as the base of the skull, ventricles,
The CO2 laser has been used in brain microsurgery after Steller et al. (Stellar, Polanyi et al. 1970) had first successfully used it in removing a recurrent glioma in 1969. The most ideal treatment of a brain tumor is minimizing damage to the normal brain tissue and removing only the tumor area. To overcome the surgical difficulty of avoiding damage to the brain tissue, a special instrument was developed. Theoretically, lasers have several advantages. First, although the surgical field is narrow, it makes surgery possible. Other small-sized surgical approaches are facilitated to minimize injury to normal brain tissue. Second, brain retraction is minimized, thus causing less damage to normal brain tissue. Third, laser beam minimizes injury to surrounding tissues and enables removal of a tumor with less thermal injury. Fourth, lasers have a coagulating property that lessens bleeding of the surgical field. Fifth, operation time is shortened (Tew and Tobler 1983; Krishnamurthy and Powers 1994). The CO2 laser is the main instrument used in brain surgery. It has the advantage of rapidly removing separated tumor cells and exact irradiation of target cells by a microsurgical technique where the CO2 laser is installed with a microscope. However, as energy cannot pass through an optical fiber, it is inconvenient to use the equipment. It has limited function in bleeding control, as control of bleeding is not possible in a vessel with a diameter 0.5 mm, necessitating the use of the equipment in conjunction with other equipments for severe bleeding management (Heppner 1978; Ascher and Heppner 1984; Deruty, Pelissou-Guyotat
The CO2 laser is most widely used in the field of neurosurgery, and it is mainly used in the removal of tumors by evaporation where surgical approach of the tumor site is difficult. It is common opinion that the CO2 laser is most effective with skull base, ventricular, brainstem, and spinal cord tumors (Powers, Cush et al. 1991; Origitano and Reichman 1993). In particular, it is most effective in removing a meningioma that is relatively hard or has less vascular distribution to be calcified. In addition, it is suitable for removing a low-grade
The Nd:YAG laser has the advantage that energy can be passed to thinner fiberoptic cables and excellent clotting function is possible at a 3-mm vessel. Therefore, it has been reported as a valid technique of removing brain tumors having greater vascular distribution and cerebral vascular malformation (Beck 1980). The combolaser has been developed in recent years by Fasano et al. (Glasscock, Jackson et al. 1981) and has been applied in surgery. It is composed of CO2 and Nd:YAG lasers, combining the advantages of both. It works by first emitting Nd:YAG energy to the tumor for clotting, followed by tumor removal by
Since the first trial of Nd:YAG in a lumbar disk surgery in 1986 (Choy, Case et al. 1987), there have been many reports about the usefulness of different kinds of lasers in disk
glioma that is relatively rigid (Deruty, Pelissou-Guyotat et al. 1993).
evaporation using the CO2 laser (Beck 1980; Glasscock, Jackson et al. 1981).
**2. Clinical application in neurosurgery**
brainstem, and spinal cord.
**2.1 Brain tumor surgery**
et al. 1993).
**2.2 Spine surgery**
surgery (Nerubay, Caspi et al. 1997; Hellinger 1999; Houck 2006). Nerubay et al. reported that 50 patients with low back and radicular pains were successfully treated by percutaneous laser nucleolysis using a CO2 laser (Nerubay, Caspi et al. 1997), and successful vaporization of the disk was accomplished in animal models (Stein, Sedlacek et al. 1990).
Considering the similarity between the disk and the meniscus (Whipple, Caspari et al. 1984), we cite studies on the effect of the CO2 laser on the meniscus. According to these research results, there was a considerable proliferation of cells resembling chondrocytes after 2 weeks of the CO2 laser treatment and there was definitely an increase in the production of ground substance and immature collagen fibers after 4 weeks; the collagen had become well reorganized into a logical orientation, resembling the normal architecture of fibrocartilage, after 10 weeks (Benjamin, Qin et al. 1995).
These animal and clinical studies strongly support the claim that CO2 lasers can safely and feasibly be used for the removal of protruded disks and discal cysts. Moreover, the CO2 laser, when attached to an operating microscope, allows for quick and easy removal of the discal cyst and, if needed, easy vaporization of disk material.
#### **2.2.1 Disk herniation**
Laser removes disk material by vaporization (Stein, Sedlacek et al. 1990) and consequently lowers intradiskal pressure (Gropper, Robertson et al. 1984). In spine surgery, the use of a laser has advantages over scalpel use in terms of precision; the ability to be used on delicate tissues; minimal tissue manipulation; and less bleeding, swelling, and trauma (Jeon, Lee et al. 2007). It is especially useful in the small spaces involved in herniated disks (Kim, Choi et al. 2009). Therefore, a laser is an effective tool for performing a minimally invasive spinal surgery with percutaneous and open spinal procedures (Ahn, Lee et al. 2005; Lee, Ahn et al. 2006; Lee, Ahn et al. 2006; Jeon, Lee et al. 2007; Lee, Ahn et al. 2008; Kim, Choi et al. 2009; Kim and Lee 2009).
In the Wooridul Hospital, CO2 laser-equipped surgical microscopes have been used for open lumbar microdiscectomy since December 1991 (Fig. 1). These microscopes coaxially align the invisible CO2 laser beam with a visible helium-neon laser beam and can focus exactly on and evaporate the target disk material by the commonly used 20- to 30-W single-pulse mode laser. Therefore, we aimed to determine whether a CO2 laser-equipped surgical microscope is a useful tool for microdiscectomy.
Lee et al. (Lee and Lee 2011) reported that the CO2 laser-assisted microdiscectomy could be an effective alternative to conventional microdiscectomy techniques. Because the CO2 laser enabled effective removal of extraforaminal lumbar disk herniation(EFLDH) via a narrow extraforaminal operative corridor without excessive loss of the facet joint and/or the par interarticularis, a thorough decompression of the extraforaminal and/or the foraminal zone was achieved while preserving spinal stability (Fig. 2). Thirty-one patients exhibited a marked reduction in leg pain immediately after the surgery. No patient complained of persistent severe leg pain in the perioperative period. In the present study, reherniation occurred in 1 patient (3.6%) at the 1-year follow-up. The CO2 laser is also believed to decrease reherniation after discectomy owing to laser-induced metaplasia. (Kim, Choi et al. 2009)
Clinical Application of CO2 Laser 361
Owing to the steep learning curve of PELD (Lee and Lee 2008), the modified
In the study by Kims et al (Kim 2010), 21 cases of rLDH, which caused the same symptoms and signs as those of virgin lumbar disk herniations, were excised successfully with
The author used CO2 laser during modified lumbar microdiscectomy and reported that using the technique, surgeons can focus the laser beam exactly on the target adhesion scar for adhesiolysis and vaporization and then quickly and easily dissect the adhesion scar tissue. In his results, no approach-related or CO2 laser-related complications developed. In our opinion, the reason that no incidental durotomy occurred in our series is the precise and
Fig. 3. A. Operative view showing granulation tissue and recurrent lumbar disc herniation located ventromedially to the L5 nerve root (black asterisk) B. The small tip of the CO2 laser
Fig. 4. Operative view presenting easily access narrow ventral part of nerve root using CO2 laser where blunt scalpel couldn't access, with a slight gentle retraction of nerve root
microdiscectomy is still more popularity.
modified microdiscectomy using a CO2 laser.
gentle dissection using the CO2 laser (Fig. 3 A.B).
A. B.
could be seen on the protruded disc (black arrow).
Fig. 1. Photograph of a CO2 laser-equipped surgical microscope.
Fig. 2. Intraoperative photomicrographs depicting CO2 laser-assisted microdiscectomy for EFLDH at the L5/S1 level. Left. Photomicrograph taken after exposure of the L5 dorsal root ganglion (A:upper border of the sacral ala; D:herniated disc; F: the lateral L5-S1 facet joint, G:the L5 dorsal root ganglion; and T:the lower border of the L5 transverse process). Right: EFLDH being removed by CO2 laser with gentle retraction of L5 dorsal root ganglion. Note the red He-Ne beam in the surgical field.
#### **2.2.2 Recurrent disk herniation**
There are various surgical treatments for recurrent lumbar disk herniation (rLDH), including revision microdiscectomy, lumbar fusion with or without instrumentation (Choi, Lee et al. 2008), and recently, some minimally invasive methods, such as percutaneous endoscopic lumbar discectomy (PELD) (Ahn, Lee et al. 2004), have also been developed. They noted that favorable pain relief was achieved in most patients through this procedure.
Fig. 1. Photograph of a CO2 laser-equipped surgical microscope.
the red He-Ne beam in the surgical field.
**2.2.2 Recurrent disk herniation**
Fig. 2. Intraoperative photomicrographs depicting CO2 laser-assisted microdiscectomy for EFLDH at the L5/S1 level. Left. Photomicrograph taken after exposure of the L5 dorsal root ganglion (A:upper border of the sacral ala; D:herniated disc; F: the lateral L5-S1 facet joint, G:the L5 dorsal root ganglion; and T:the lower border of the L5 transverse process). Right: EFLDH being removed by CO2 laser with gentle retraction of L5 dorsal root ganglion. Note
There are various surgical treatments for recurrent lumbar disk herniation (rLDH), including revision microdiscectomy, lumbar fusion with or without instrumentation (Choi, Lee et al. 2008), and recently, some minimally invasive methods, such as percutaneous endoscopic lumbar discectomy (PELD) (Ahn, Lee et al. 2004), have also been developed. They noted that favorable pain relief was achieved in most patients through this procedure. Owing to the steep learning curve of PELD (Lee and Lee 2008), the modified microdiscectomy is still more popularity.
In the study by Kims et al (Kim 2010), 21 cases of rLDH, which caused the same symptoms and signs as those of virgin lumbar disk herniations, were excised successfully with modified microdiscectomy using a CO2 laser.
The author used CO2 laser during modified lumbar microdiscectomy and reported that using the technique, surgeons can focus the laser beam exactly on the target adhesion scar for adhesiolysis and vaporization and then quickly and easily dissect the adhesion scar tissue. In his results, no approach-related or CO2 laser-related complications developed. In our opinion, the reason that no incidental durotomy occurred in our series is the precise and gentle dissection using the CO2 laser (Fig. 3 A.B).
Fig. 3. A. Operative view showing granulation tissue and recurrent lumbar disc herniation located ventromedially to the L5 nerve root (black asterisk) B. The small tip of the CO2 laser could be seen on the protruded disc (black arrow).
Fig. 4. Operative view presenting easily access narrow ventral part of nerve root using CO2 laser where blunt scalpel couldn't access, with a slight gentle retraction of nerve root
Clinical Application of CO2 Laser 363
Direct anterior decompression by corpectomy followed by fusion should be the proper choice of surgical treatment of this multi-level OPLL than indirect decompression by posterior
The rationale of preferring the anterior approach is based on evidence that the compressive elements are located anterior to the spinal cord in 75% of cases, and therapeutic benefit can be obtained by directly approaching these lesions(Cusick 1991). The degree of cervical myelopathy caused by OPLL is also reported to be influenced not only by static compression from the ossification mass, but also by abnormal intervertebral mobility at the
Despite these theoretical advantages, anterior corpectomy has been reported to be fraught with iatrogenic deterioration of the neurological state, and complications such as spinal fluid fistula or graft problem. Naturally, it will be more technically demanding if the OPLL is involved at multiple cervical levels, and treatment success will depend heavily on a less
In Lee at al report, the authors concluded that direct anterior cervical corpectomy using the CO2 laser resulted in a better recovery of neurological deficit, and adequate decompression of the spinal canal and maintenance of cervical regional lordosis at the operated level for patients with multilevel cervical OPLL. Assuming the surgeon can employ safe anterior microsurgical tools combined CO2 laser and decompression method, proceeding with direct decompressive corpectomy rather than indirect, inadequate laminoplasty is recommended if
They expected that a focused laser beam could vaporize the OPLL and even produce a positive effect. A 5-W pulse, single-pulse mode laser was sufficient to vaporize a thinned OPLL or an osteophyte, as it is known to penetrate only the outer table of the bone(Neblett
Fig. 6. An illustration showing the tope view (A) and side view(B) of the surgical technique
microdissector (d), which is held between the OPLL. (p) and the dura to avoid laser-induced
used to remove the densely adhered OPLL using the CO2 laser (L) with the angled
laminoplasy.
traumatic manipulation.
1992).(Fig.6)
damage to the dura.
responsible level(s)(Onari, Akiyama et al. 2001).
the patient's preoperative status is appropriate.
Because an epidural or perineural scar tissue may hinder the dissection using the modified microdiscectomy, increasing the risk of incidental durotomy or iatrogenic neural injury, the CO2 laser can help surgeons make more precise and safe dissections of the scar tissue than when using a blunt scalpel. Calcifications around recurrent disk fragments are often seen, which may also hinder surgeons to dissect safely. However, with the aid of the CO2 laser, surgeons can evaporate the calcified portion of the disk without excessive retraction of the nerve root via a narrow operative corridor (Lee, Ahn et al. 2008). Moreover, with a slight gentle retraction of the nerve root, surgeons can easily access the narrow ventral part of the nerve root using the CO2 laser, where a blunt scalpel could not (Fig. 4).
#### **2.2.3 Discal cyst**
Many kinds of surgical methods have been introduced for the treatment of discal cysts. Most discal cysts reported have been treated by open surgical excision (Chiba, Toyama et al. 2001; Lee, Lee et al. 2006) or with some direct intervention, such as computed tomography-guided aspiration and steroid injection (Kang, Liu et al. 2008). Recently, it was reported that a discal cyst was treated with a minimally invasive technique using PELD (Min 2006).
Kim et al. (Kim and Lee 2009) reported that the CO2 laser, when attached to an operating microscope, allows for quick and easy removal of a discal cyst and, if needed, easy vaporization of disk material. In his study, 14 cases of discal cyst that caused the same symptoms and signs as those of lumbar disk herniations were excised successfully by open surgery using a CO2 laser.
After the intraoperative removal of the discal cyst, the authors found the communication hole between the cyst and the protruded disk. They then used the heat energy produced by CO2 lasering and removed the pulled-out disk fragment, if any existed, after pushing into the disk space with a right-angled probe (Fig. 5 A.B).
Fig. 5. (A) Distinct communication-like hole between the cyst and intervertebral disc (white arrow) and (B) small tip of laser on the protruded disc (white arrow)
#### **2.2.4 Cervical ossification of ligamentum flavum (OPLL)**
The choice of a surgical approach for multi-level cervical OPLL is still a controversial issue.
Because an epidural or perineural scar tissue may hinder the dissection using the modified microdiscectomy, increasing the risk of incidental durotomy or iatrogenic neural injury, the CO2 laser can help surgeons make more precise and safe dissections of the scar tissue than when using a blunt scalpel. Calcifications around recurrent disk fragments are often seen, which may also hinder surgeons to dissect safely. However, with the aid of the CO2 laser, surgeons can evaporate the calcified portion of the disk without excessive retraction of the nerve root via a narrow operative corridor (Lee, Ahn et al. 2008). Moreover, with a slight gentle retraction of the nerve root, surgeons can easily access the narrow ventral part of the
Many kinds of surgical methods have been introduced for the treatment of discal cysts. Most discal cysts reported have been treated by open surgical excision (Chiba, Toyama et al. 2001; Lee, Lee et al. 2006) or with some direct intervention, such as computed tomography-guided aspiration and steroid injection (Kang, Liu et al. 2008). Recently, it was reported that a discal cyst was treated with a minimally invasive technique using
Kim et al. (Kim and Lee 2009) reported that the CO2 laser, when attached to an operating microscope, allows for quick and easy removal of a discal cyst and, if needed, easy vaporization of disk material. In his study, 14 cases of discal cyst that caused the same symptoms and signs as those of lumbar disk herniations were excised successfully by open
After the intraoperative removal of the discal cyst, the authors found the communication hole between the cyst and the protruded disk. They then used the heat energy produced by CO2 lasering and removed the pulled-out disk fragment, if any existed, after pushing into
Fig. 5. (A) Distinct communication-like hole between the cyst and intervertebral disc (white
The choice of a surgical approach for multi-level cervical OPLL is still a controversial issue.
arrow) and (B) small tip of laser on the protruded disc (white arrow)
**2.2.4 Cervical ossification of ligamentum flavum (OPLL)**
nerve root using the CO2 laser, where a blunt scalpel could not (Fig. 4).
**2.2.3 Discal cyst**
PELD (Min 2006).
surgery using a CO2 laser.
the disk space with a right-angled probe (Fig. 5 A.B).
Direct anterior decompression by corpectomy followed by fusion should be the proper choice of surgical treatment of this multi-level OPLL than indirect decompression by posterior laminoplasy.
The rationale of preferring the anterior approach is based on evidence that the compressive elements are located anterior to the spinal cord in 75% of cases, and therapeutic benefit can be obtained by directly approaching these lesions(Cusick 1991). The degree of cervical myelopathy caused by OPLL is also reported to be influenced not only by static compression from the ossification mass, but also by abnormal intervertebral mobility at the responsible level(s)(Onari, Akiyama et al. 2001).
Despite these theoretical advantages, anterior corpectomy has been reported to be fraught with iatrogenic deterioration of the neurological state, and complications such as spinal fluid fistula or graft problem. Naturally, it will be more technically demanding if the OPLL is involved at multiple cervical levels, and treatment success will depend heavily on a less traumatic manipulation.
In Lee at al report, the authors concluded that direct anterior cervical corpectomy using the CO2 laser resulted in a better recovery of neurological deficit, and adequate decompression of the spinal canal and maintenance of cervical regional lordosis at the operated level for patients with multilevel cervical OPLL. Assuming the surgeon can employ safe anterior microsurgical tools combined CO2 laser and decompression method, proceeding with direct decompressive corpectomy rather than indirect, inadequate laminoplasty is recommended if the patient's preoperative status is appropriate.
They expected that a focused laser beam could vaporize the OPLL and even produce a positive effect. A 5-W pulse, single-pulse mode laser was sufficient to vaporize a thinned OPLL or an osteophyte, as it is known to penetrate only the outer table of the bone(Neblett 1992).(Fig.6)
Fig. 6. An illustration showing the tope view (A) and side view(B) of the surgical technique used to remove the densely adhered OPLL using the CO2 laser (L) with the angled microdissector (d), which is held between the OPLL. (p) and the dura to avoid laser-induced damage to the dura.
Clinical Application of CO2 Laser 365
After destructive treatment by CO2 laser at skin tumor was universal, it was known that aging skin was reformed as causing shrinkage in the recovery process after resurfacing. Laser resurfacing has been used as representative of treatment of aging of the skin in spite of
There was effort to reduce the inconvenience of resurfacing by conventional laser using for rejuvenation and minimized the downtime. In 1991 Dr Shimon Dckhouse developed intense pulsed light(IPL) emiting to single pulse from multiple optical energy and introduced advantage for various clinical effect. So it was introduced the concept of non-ablative
Since introduction of the infrared wavelength range of equipment in 2006, AFR concept of a number of devices are being launched as merging ablation and FP using CO2 and Er:YAG laser. Recently, laser equipment of IR, CO2, and Er:YAG are coexisting. The concept of FP having advantage of being safely usable of high powered energy is proliferated broadly to
CO2 laser is used to treatment of antiaging as removing by ablation of aging tissue and
Though resurfacing using existing CO2 laser has many discomfort as ablation of total skin, CO2 fractional laser is focus to treat only fine territory partially. So it is enable to treat safely epidermis and dermis though more high energy than existing treatment is transferred. Because the wound can be restored quickly and easily from surrounding normal skin
**3.1.2 The development and change of fractional photothermolysis(FP)**
accelerating to regeneration of dermis by transmission of thermal stimulus.
2. The period of CO2 laser resurfacing
long downtime from early 1990.
3. Introduction of NAR
rejuvenation(NAR).
all territory of laser.
**3.2 Treatment principle of CO2 fractional laser**
though injured at epidermis and dermis by laser (Fig. 8.9).
Fig. 8. The basic concept of Fractional Photothermolysis
#### **2.2.5 Complication**
Previously, a case of major vessel injury involving perforation of the iliac artery during CO2 laser-assisted lumbar microdiscectomy, caused by prolonged irradiation of the CO2 laser into the deep anterior disc space, has been reported.(Jeon, Lee et al. 2007) Avoiding point focusing of the CO 2 laser on the surface of the anterior annulus, as well as injecting a small amount of saline at the bottom of the intradiscal space during laser ablation, can prevent the occurrence of such a complication. (Jeon, Lee et al. 2007)
And during this procedure, keeping the surgical field moistened was important to minimize the risk of inadvertent injury, as water can absorb CO2 laser energy immediately (Choi, Lee et al. 2005). (Fig.7)
Fig. 7. Preoperative axial MRI at the L5-S1 level. White arrow indicates the direction of the carbon dioxide laser beam. A, External iliac artery. B, Internal iliac artery. C, Common iliac vein. D, Herniated disc fragment.
### **3. Clinical application in dermatology (Jung 2008)**
#### **3.1 Evolution of rejuvenation using lasers. (Alexiades-Armenakas, Dover et al. 2008; Jih and Kimyai-Asadi 2008)**
It is described about the evolution of CO2 fractional lasers utilized in the treatment of aging on a typical appliance, equipmental characteristics, clinical utilization in the future development direction,
#### **3.1.1 Historical background of fractional photothermolysis(FP)**
1. The period of introduction of concept of SPTL
Since Dr Rox Anderson represented the concept of selective photothermolysis(SPTL) In 1983, specific treatment methods of selectively targeting chromophore like melanin and hemoglobin have been developed clinically.
Previously, a case of major vessel injury involving perforation of the iliac artery during CO2 laser-assisted lumbar microdiscectomy, caused by prolonged irradiation of the CO2 laser into the deep anterior disc space, has been reported.(Jeon, Lee et al. 2007) Avoiding point focusing of the CO 2 laser on the surface of the anterior annulus, as well as injecting a small amount of saline at the bottom of the intradiscal space during laser ablation, can prevent the
And during this procedure, keeping the surgical field moistened was important to minimize the risk of inadvertent injury, as water can absorb CO2 laser energy immediately (Choi, Lee
Fig. 7. Preoperative axial MRI at the L5-S1 level. White arrow indicates the direction of the carbon dioxide laser beam. A, External iliac artery. B, Internal iliac artery. C, Common iliac
**3.1 Evolution of rejuvenation using lasers. (Alexiades-Armenakas, Dover et al. 2008;**
It is described about the evolution of CO2 fractional lasers utilized in the treatment of aging on a typical appliance, equipmental characteristics, clinical utilization in the future
Since Dr Rox Anderson represented the concept of selective photothermolysis(SPTL) In 1983, specific treatment methods of selectively targeting chromophore like melanin and
occurrence of such a complication. (Jeon, Lee et al. 2007)
**2.2.5 Complication**
et al. 2005). (Fig.7)
vein. D, Herniated disc fragment.
**Jih and Kimyai-Asadi 2008)**
development direction,
**3. Clinical application in dermatology (Jung 2008)**
**3.1.1 Historical background of fractional photothermolysis(FP)**
1. The period of introduction of concept of SPTL
hemoglobin have been developed clinically.
2. The period of CO2 laser resurfacing
After destructive treatment by CO2 laser at skin tumor was universal, it was known that aging skin was reformed as causing shrinkage in the recovery process after resurfacing. Laser resurfacing has been used as representative of treatment of aging of the skin in spite of long downtime from early 1990.
3. Introduction of NAR
There was effort to reduce the inconvenience of resurfacing by conventional laser using for rejuvenation and minimized the downtime. In 1991 Dr Shimon Dckhouse developed intense pulsed light(IPL) emiting to single pulse from multiple optical energy and introduced advantage for various clinical effect. So it was introduced the concept of non-ablative rejuvenation(NAR).
#### **3.1.2 The development and change of fractional photothermolysis(FP)**
Since introduction of the infrared wavelength range of equipment in 2006, AFR concept of a number of devices are being launched as merging ablation and FP using CO2 and Er:YAG laser. Recently, laser equipment of IR, CO2, and Er:YAG are coexisting. The concept of FP having advantage of being safely usable of high powered energy is proliferated broadly to all territory of laser.
#### **3.2 Treatment principle of CO2 fractional laser**
CO2 laser is used to treatment of antiaging as removing by ablation of aging tissue and accelerating to regeneration of dermis by transmission of thermal stimulus.
Though resurfacing using existing CO2 laser has many discomfort as ablation of total skin, CO2 fractional laser is focus to treat only fine territory partially. So it is enable to treat safely epidermis and dermis though more high energy than existing treatment is transferred. Because the wound can be restored quickly and easily from surrounding normal skin though injured at epidermis and dermis by laser (Fig. 8.9).
Fig. 8. The basic concept of Fractional Photothermolysis
Clinical Application of CO2 Laser 367
Fig. 10. The main difference between fractional infrared and fractional CO2 laser.
Fig. 11. The wound healing process of fractional CO2 laser treatment
The water absorption rate is in the order of Er: YAG, CO2, and IR (Fig. 12)
CO2 and Er:YAG lasers have higher water absorption rates compared to IR (1064~1600nm) equipment under the same condition (Fig. 12), and most of the energy disappears during the ablation and vaporization process. Therefore, they have less lateral heat diffusion to surrounding tissues when the laser is irradiated and can minimize heat accumulation inside the dermis. In other words, IR equipment accumulates relatively more heat in dermis
**3.2.1.1 Difference in tissue reaction according to wavelength**
Infrared (Er:Glass, 1,400-1,600nm)
CO2: 10,640 nm Er:YAG: 2,940 nm
tissues.
1. Water absorption
2. Lateral Heat Diffusion
Fig. 9. The basic concept of Fractional CO2 resurfacing: advantages.
You shall awaken warning to attach a laser cover at an operating room entrance as you use a CO2 laser if you enforce an operation.
An enough exhaust device shall install because a lot of extensions occur, and you disturb an operation visual field, and you pollute air when you vaporize an organization.
#### **3.2.1 Differences between the fractional infrared laser (1064~1600nm) and the CO2 fractional laser (10,600nm)**
CO2 laser ablates tissue, such as the epidermis and dermis, resulting in tissue damage. This outcome differs completely from the Fraxal tissue reaction, which occurs when a cut is treated by the existing infrared laser (Fig. 10). However, no comparative study has been conducted on the recovery process of infrared rays (IR) and CO2 laser. We know that CO2 laser damages the dermo-epidermal junction, causing severe inflammation at the outset, which in turn results in edema and erythema reactions. We also understand that the lesions damaged by CO2 laser ablation is first filled with keratonocyte within 48 hours and replaced by dermis through the remodeling process, a process that can be continued even after three months(Hantash, Bedi et al. 2007) (Fig. 11).
Fig. 9. The basic concept of Fractional CO2 resurfacing: advantages.
CO2 laser if you enforce an operation.
months(Hantash, Bedi et al. 2007) (Fig. 11).
**fractional laser (10,600nm)**
You shall awaken warning to attach a laser cover at an operating room entrance as you use a
An enough exhaust device shall install because a lot of extensions occur, and you disturb an
CO2 laser ablates tissue, such as the epidermis and dermis, resulting in tissue damage. This outcome differs completely from the Fraxal tissue reaction, which occurs when a cut is treated by the existing infrared laser (Fig. 10). However, no comparative study has been conducted on the recovery process of infrared rays (IR) and CO2 laser. We know that CO2 laser damages the dermo-epidermal junction, causing severe inflammation at the outset, which in turn results in edema and erythema reactions. We also understand that the lesions damaged by CO2 laser ablation is first filled with keratonocyte within 48 hours and replaced by dermis through the remodeling process, a process that can be continued even after three
**3.2.1 Differences between the fractional infrared laser (1064~1600nm) and the CO2**
operation visual field, and you pollute air when you vaporize an organization.
Fig. 10. The main difference between fractional infrared and fractional CO2 laser.
Fig. 11. The wound healing process of fractional CO2 laser treatment
### **3.2.1.1 Difference in tissue reaction according to wavelength**
Infrared (Er:Glass, 1,400-1,600nm) CO2: 10,640 nm Er:YAG: 2,940 nm
1. Water absorption
The water absorption rate is in the order of Er: YAG, CO2, and IR (Fig. 12)
2. Lateral Heat Diffusion
CO2 and Er:YAG lasers have higher water absorption rates compared to IR (1064~1600nm) equipment under the same condition (Fig. 12), and most of the energy disappears during the ablation and vaporization process. Therefore, they have less lateral heat diffusion to surrounding tissues when the laser is irradiated and can minimize heat accumulation inside the dermis. In other words, IR equipment accumulates relatively more heat in dermis tissues.
Clinical Application of CO2 Laser 369
The potential of erythema and pigmentation is related to the damage level of the epidermis and dermis-epidermis joint area as well as the inflammatory reaction due to heat stimulation on the dermis. The risk decreases as fractional treatment convergence is decreased, with the proper convergence of fractional treatment being 20%. Since the risk increases in proportion to the level of heat damage, the CO2 fractional laser can theoretically reduce epidermis damage and dermis heat damage because it has less lateral heat diffusion compared to the IR laser, which in turn will reduce the risk of erythema and pigmentation if
Theoretically, the CO2 fractional laser minimizes pain during procedures. Since most of the heat is lost during the tissue ablation process performed after laser irradiation, it has less lateral heat diffusion, which in turn reduces heat accumulation in tissues compared to IR equipment. Therefore, we expect less pain if we use the CO2 fractional laser. In addition, short pulse duration, smaller spot size, and shorter irradiation time on the skin can reduce pain. Therefore, it is theoretically possible to actualize the CO2 fractional laser with very little pain. However, this laser can damage the epidermis and dermis joint, which will cause
Since the CO2 laser is a continuous wave laser, output is written in watts. Most CO2 lasers can control parameters such as watt and pulse duration independently. Therefore, the same amount of energy (J) can be irradiated while the depth is controlled by watt level and the coagulation range (heat damage range) can be controlled by pulse duration (Fig. 13). These are very important strong points of the CO2 laser, differing from IR equipment, which changes power and pulse duration according to the J (energy) level. That is, IR equipments cannot independently control factors since power and pulse duration is simultaneously
5. Risk of erythema and pigmentation
6. Level of Pain
it can produce a quality laser beam and be irradiated.
**3.2.1.2 Can parameter be independently controlled?**
increased as J (energy) is increased.
J (Energy) = Watt (Power)\* Time (pulse duration)
Fig. 13. Parameter controllability
more severe initial inflammation reaction and edema and burn feeling.
Fig. 12. Water absorption curve.
3. Shrinkage (Rosenberg, Brito et al. 1999; Fitzpatrick, Rostan et al. 2000; Jun, Harris et al. 2003; Zelickson, Kist et al. 2004; Hantash, Bedi et al. 2007)
Shrinkage is a tissue reaction related to the tightening effect that occurs due to tissue shortening after laser irradiation. So far, it is known to progress in the collagen denaturation zone (reversible thermal damage zone) (Fig. 10). Even though no comparative studies have been conducted on this process, IR equipment accumulates more heat inside dermis tissues under the same condition, which is why we think shrinkage will happen more with IR equipment. However, it is hard to estimate if more shrinkage is directly related to the rejuvenation effect, such as tissue tightening or lifting.
4. Penetration Depth
IR can penetrate 1.0~1.5 mm maximum. However, it is believed that the Er:YAG or CO2 lasers penetrate less than the IR laser since they absorb more water. According to recent studies, the CO2 laser can penetrate as deeply as the IR laser if the wattage is increased, the size of the beam is reduced, and the quality beam is irradiated vertically on skin. Due to the CO2 laser's characteristics, if the laser is repeatedly stacked on the same area, tissues can be constantly vaporized so that the laser can penetrate deeply enough. However, repetitive and deep penetration performed several times can cause unwanted excessive heat stimulation, leading to a higher chance of side effects. Therefore, it is right to consider the depth of single beam irradiation as a standard.
3. Shrinkage (Rosenberg, Brito et al. 1999; Fitzpatrick, Rostan et al. 2000; Jun, Harris et al.
Shrinkage is a tissue reaction related to the tightening effect that occurs due to tissue shortening after laser irradiation. So far, it is known to progress in the collagen denaturation zone (reversible thermal damage zone) (Fig. 10). Even though no comparative studies have been conducted on this process, IR equipment accumulates more heat inside dermis tissues under the same condition, which is why we think shrinkage will happen more with IR equipment. However, it is hard to estimate if more shrinkage is directly related to the
IR can penetrate 1.0~1.5 mm maximum. However, it is believed that the Er:YAG or CO2 lasers penetrate less than the IR laser since they absorb more water. According to recent studies, the CO2 laser can penetrate as deeply as the IR laser if the wattage is increased, the size of the beam is reduced, and the quality beam is irradiated vertically on skin. Due to the CO2 laser's characteristics, if the laser is repeatedly stacked on the same area, tissues can be constantly vaporized so that the laser can penetrate deeply enough. However, repetitive and deep penetration performed several times can cause unwanted excessive heat stimulation, leading to a higher chance of side effects. Therefore, it is right to consider the depth of single
2003; Zelickson, Kist et al. 2004; Hantash, Bedi et al. 2007)
rejuvenation effect, such as tissue tightening or lifting.
Fig. 12. Water absorption curve.
4. Penetration Depth
beam irradiation as a standard.
5. Risk of erythema and pigmentation
The potential of erythema and pigmentation is related to the damage level of the epidermis and dermis-epidermis joint area as well as the inflammatory reaction due to heat stimulation on the dermis. The risk decreases as fractional treatment convergence is decreased, with the proper convergence of fractional treatment being 20%. Since the risk increases in proportion to the level of heat damage, the CO2 fractional laser can theoretically reduce epidermis damage and dermis heat damage because it has less lateral heat diffusion compared to the IR laser, which in turn will reduce the risk of erythema and pigmentation if it can produce a quality laser beam and be irradiated.
6. Level of Pain
Theoretically, the CO2 fractional laser minimizes pain during procedures. Since most of the heat is lost during the tissue ablation process performed after laser irradiation, it has less lateral heat diffusion, which in turn reduces heat accumulation in tissues compared to IR equipment. Therefore, we expect less pain if we use the CO2 fractional laser. In addition, short pulse duration, smaller spot size, and shorter irradiation time on the skin can reduce pain. Therefore, it is theoretically possible to actualize the CO2 fractional laser with very little pain. However, this laser can damage the epidermis and dermis joint, which will cause more severe initial inflammation reaction and edema and burn feeling.
#### **3.2.1.2 Can parameter be independently controlled?**
Since the CO2 laser is a continuous wave laser, output is written in watts. Most CO2 lasers can control parameters such as watt and pulse duration independently. Therefore, the same amount of energy (J) can be irradiated while the depth is controlled by watt level and the coagulation range (heat damage range) can be controlled by pulse duration (Fig. 13). These are very important strong points of the CO2 laser, differing from IR equipment, which changes power and pulse duration according to the J (energy) level. That is, IR equipments cannot independently control factors since power and pulse duration is simultaneously increased as J (energy) is increased.
J (Energy) = Watt (Power)\* Time (pulse duration) Fig. 13. Parameter controllability
Clinical Application of CO2 Laser 371
After Fraxal acquired a patent on moving method irradiation, the equipment produced later on mostly adapted the stamp method. This method can be classified into two divisions. The most common method is micro lens array (the beam goes through the lens and is simultaneously irradiated after being divided into several fine beams, like LUX1540 and Affirm) that treats the parts by stamping microbeams. The other one is also a stamp method, but uses the scanner method rather than lens array and irradiates beams in order. The scanner method can be classified in two manners. The sequential type irradiates beams sequentially adjacently. Alternative type irradiates one line and skips the close line and irradiates the next line. Random type irradiates with no order. Theoretically, the sequential type includes a high possibility of heat accumulation due to adjacent irradiation. The random type can be a safer treatment since it minimizes heat accumulation. However, it is
Density is the portion of treated area where the laser beam is irradiated. Density is considered to be low when there are many normal tissues left around the area. If density is too low, it is safe but can be ineffective. However, when density is high, there are less normal tissues, which is meaningless since fractional technology's strength is safe and fast recovery. Therefore, we need to determine the density within proper range, considering the purpose of the treatment as well as the characteristics of the equipment. If more than two passes of treatment is done right after the first pass, density increases in proportion. However, it can weaken its safety as it causes excessive heat accumulation and repetitive irradiation on the same area. Therefore, it is safer to obtain the intended density from the
We can observe various equipment characteristics, such as spot size, pitch, pitch control possibility and range, controllable range of irradiation time, watt range, scanning method scanning range, and scanning shape. However, the most important evaluation factor should be: "How superior is each beam's characteristic?" In fact, the users are not fully aware of CO2 laser equipment's characteristics. For example, if this equipment includes a scanner with various modes with an inconsistent quality beam, inconsistent irradiation direction or penetration depth, it can be a low-priced CO2 laser with a scanner. It is too much to expect this type of equipment to irradiate the quality beam uniformly on skin with detailed control over lateral heat diffusion and to penetrate into the skin with the depth one wants and
(*Longitudinally excited, Transversely excited, Gas dynamic, Waveguide laser*)
2. Stamp method: micro lens array and scanner type
first pass rather than acquiring it from repetitive passes.
not clear if it has clinical significance.
**3.2.2.4 Characteristics of the beam**
obtain satisfactory results safely.
● Type of resonator and beam
● Individual control of pulse duration
Table 1. Factors Affecting the Beam Quality of CO2 Laser
● Durability of resonator
● Beam window ● Calibration ● Auto-detection
**3.2.2.3 Density**
Due to these characteristics, the CO2 fractional laser can treat very deep layers and offers a better recovery process after treatment as it allows high power (deep penetration) and short pulse duration (minimal lateral heat diffusion to the surrounding area), creates a very narrow vertical ablation zone, and forms a limited lateral heat diffusion zone (Fig. 14).
Fig. 14. Vertical and horizontal view of Fractional CO2 laser.
#### **3.2.2 Basic considerations for the fractional laser**
#### **3.2.2.1 Spot size**
The spot size of the laser refers to the diameter of irradiated beam. As the spot gets bigger, the re-epithelization process takes longer, which in turn cases a longer downtime. In fact, if the spot size is 140um, re-epithelization of the dermo-epidermal junction takes less than 36 hours. It takes two to four days for 300*μ*m, three to five days for 500*μ*m, and five to ten days for over 1.25mm. Therefore, it is important to make the spot small to facilitate safe treatment and fewer inconveniences. Since there is a limit to the minimum spot size that can be actualized physically, it is impossible to reduce the size below a certain point. If the spot is too small, a lot more lasers should be irradiated on a certain area to get converge that is required for proper treatment. In that case, we cannot exclude excessive heat accumulation, which is why we cannot say a smaller spot size is always advantageous.
#### **3.2.2.2 Laser beam irradiation methods**
There are various methods of irradiating several spots on a certain area. They can be classified into the moving method and the stamp method according to the type of equipment being utilized.
1. Moving method
The moving method is used for Fraxal repair that has a function that can realize regular spot converge by controlling beam irradiation speed regardless of moving speed and eCO2 equipment that has fixed beam irradiation speed by moving the handpiece constantly.
Due to these characteristics, the CO2 fractional laser can treat very deep layers and offers a better recovery process after treatment as it allows high power (deep penetration) and short pulse duration (minimal lateral heat diffusion to the surrounding area), creates a very narrow vertical ablation zone, and forms a limited lateral heat diffusion zone (Fig. 14).
The spot size of the laser refers to the diameter of irradiated beam. As the spot gets bigger, the re-epithelization process takes longer, which in turn cases a longer downtime. In fact, if the spot size is 140um, re-epithelization of the dermo-epidermal junction takes less than 36 hours. It takes two to four days for 300*μ*m, three to five days for 500*μ*m, and five to ten days for over 1.25mm. Therefore, it is important to make the spot small to facilitate safe treatment and fewer inconveniences. Since there is a limit to the minimum spot size that can be actualized physically, it is impossible to reduce the size below a certain point. If the spot is too small, a lot more lasers should be irradiated on a certain area to get converge that is required for proper treatment. In that case, we cannot exclude excessive heat accumulation,
There are various methods of irradiating several spots on a certain area. They can be classified into the moving method and the stamp method according to the type of
The moving method is used for Fraxal repair that has a function that can realize regular spot converge by controlling beam irradiation speed regardless of moving speed and eCO2 equipment that has fixed beam irradiation speed by moving the handpiece constantly.
Fig. 14. Vertical and horizontal view of Fractional CO2 laser.
which is why we cannot say a smaller spot size is always advantageous.
**3.2.2 Basic considerations for the fractional laser**
**3.2.2.2 Laser beam irradiation methods**
equipment being utilized.
1. Moving method
**3.2.2.1 Spot size**
2. Stamp method: micro lens array and scanner type
After Fraxal acquired a patent on moving method irradiation, the equipment produced later on mostly adapted the stamp method. This method can be classified into two divisions. The most common method is micro lens array (the beam goes through the lens and is simultaneously irradiated after being divided into several fine beams, like LUX1540 and Affirm) that treats the parts by stamping microbeams. The other one is also a stamp method, but uses the scanner method rather than lens array and irradiates beams in order. The scanner method can be classified in two manners. The sequential type irradiates beams sequentially adjacently. Alternative type irradiates one line and skips the close line and irradiates the next line. Random type irradiates with no order. Theoretically, the sequential type includes a high possibility of heat accumulation due to adjacent irradiation. The random type can be a safer treatment since it minimizes heat accumulation. However, it is not clear if it has clinical significance.
#### **3.2.2.3 Density**
Density is the portion of treated area where the laser beam is irradiated. Density is considered to be low when there are many normal tissues left around the area. If density is too low, it is safe but can be ineffective. However, when density is high, there are less normal tissues, which is meaningless since fractional technology's strength is safe and fast recovery. Therefore, we need to determine the density within proper range, considering the purpose of the treatment as well as the characteristics of the equipment. If more than two passes of treatment is done right after the first pass, density increases in proportion. However, it can weaken its safety as it causes excessive heat accumulation and repetitive irradiation on the same area. Therefore, it is safer to obtain the intended density from the first pass rather than acquiring it from repetitive passes.
#### **3.2.2.4 Characteristics of the beam**
We can observe various equipment characteristics, such as spot size, pitch, pitch control possibility and range, controllable range of irradiation time, watt range, scanning method scanning range, and scanning shape. However, the most important evaluation factor should be: "How superior is each beam's characteristic?" In fact, the users are not fully aware of CO2 laser equipment's characteristics. For example, if this equipment includes a scanner with various modes with an inconsistent quality beam, inconsistent irradiation direction or penetration depth, it can be a low-priced CO2 laser with a scanner. It is too much to expect this type of equipment to irradiate the quality beam uniformly on skin with detailed control over lateral heat diffusion and to penetrate into the skin with the depth one wants and obtain satisfactory results safely.
Table 1. Factors Affecting the Beam Quality of CO2 Laser
<sup>●</sup> Type of resonator and beam (*Longitudinally excited, Transversely excited, Gas dynamic, Waveguide laser*)
Clinical Application of CO2 Laser 373
because CO2 laser can seal the lymphatics located at cutting plane and directly seal at peripheral vessel small sized less than 0.5mm, you can support patient to more comfort by accelerating wound healing process and by decreasing intraoperative bleeding and
This CO2 laser has definite advantage that it is very clear between the tissue which has thermal injury and no damaged tissue located at surrounding area, and it enable to cut tissue by not putting to physical stress to surrounding tissue. It has profit at the procedure by need of bloodless surgical field because of minimizing bleeding. And it enable to treat
In palatoplasty and pharyngeal flap operation, compared to conventional method, complication caused by intra- and postoperative bleeding was decreased and there was no difference between CO2 using and conventional method in aspect of wound healing. And
In patients of shortening of frenulum, CO2 laser is available to frenotomy (Yoon CH 1998) In blepharoplasty using CO2 laser, it was first reported by Baker in 1984(Baker, Muenzler et al. 1984). Using CO2 laser, blepharoplasty was processed safely and more enhanced to decrease bleeding, operation time, edema, time to heal. Mittelman et al (Mittelman and Apfelberg 1990) evaluated as safe method in spite of having risks of eyeball injury,
Extramammary Paget's disease is eczema like disease accompanied mainly by itching of anus and genitalia. It was found very much among a middle age or patients of prime of manhood with the past history which a treatment of skin clinic was failed in during long periods. About this disease, invasive method was surgical excision and topical ointment using 5-fluorouracil, radiotherapy. Recently it is the trend that an interest of the treatment
In wound healing, Low-powered CO2 laser helps to induce the synthesis of DNA by give effects to permeability of cell membrane, and to activate fibroblast and condrocyte, and to accelerate to absorp the hematom and to remove necrotic tissue, and to help to healing
In acne scar, dermabrasion using CO2 laser minimizing the common complication by conventional method and is enable to control the depth of peeling by depth of acne scar. So is enable to avoid to complication of hypertrophic scar and keloid etc.. It is available to process peeling safely and deeply. So recovery toward a daily life is fast as managements after operations is convenient because of decreasing of postoperative edema and pain by
CO2 laser is infrared having 10,600 nm wavelength. It has vibrational energy and enable to control to transfer through the mirror. The diameter of focus is about 1mm by
antiseptically to wound as vaporize the tissue by high temperature.
CO2 laser has merits to decrease hospitalization (Song IC 1998).
breakaway from its course, fire at operation room, burn injury of skin.
using CO2 laser and Nd:YAG laer is rising (Ewing 1991; Yoon ES 2000)
In atropic scar, pulsed CO2 laser having high power was utilized.
process of bone and cartilage (Tsai, Huang et al. 1997).
disconnecting at nerve terminals (Yoon ES 1998).
**5.1 Laryngeal microsurgery using CO2 laser**
**5. Clinical application in otorhinolaryngology**
postoperative edema, contusion.
Therefore, when comparing CO2 fractional laser performance, the most important factor that shows the biggest deviation is quality of beam. The following are the major factors that determine beam quality (Table 1).
#### **3.3 Clinical application of CO2 fractional laser**
Laser induced regeneration by vaporizing aging tissue at the epidermis and dermis. It can effectively lead to regrowth and remodeling in the process, so it can be applied broadly to a variety of issues (scar, pigmentation, texture so on) at epidermis and dermis and it was also effective to improve the aging skin because laser can treated from 0.2mm in depth to 1mm or more at dermis.
However, it is limited to case reflected in the previously mentioned characteristics of the CO2 when the CO2 laser beam to penetrate the organization is elaborately controlled.
Because it can not be told that there was less possibility of erythema and pigmentation if not uncontrolled beam.
By such advantage, CO2 fractional laser can be utilized to various indications of epidermis and dermis. (Table 2)
Since it was initially introduced in 2006, CO2 fractional laser has been made up a large development and evolution for a short period of time, and future developments are expected as follows. (Table 3)
Table 3. Future Development of CO2 Fractional Laser
#### **4. Clinical application in plastic surgery**
You decrease a pain after operations by a laser in case of constancy as the way that used a CO2 laser compares to the way that used a knife as you seal the peripheral nerve edge. And
Therefore, when comparing CO2 fractional laser performance, the most important factor that shows the biggest deviation is quality of beam. The following are the major factors that
Laser induced regeneration by vaporizing aging tissue at the epidermis and dermis. It can effectively lead to regrowth and remodeling in the process, so it can be applied broadly to a variety of issues (scar, pigmentation, texture so on) at epidermis and dermis and it was also effective to improve the aging skin because laser can treated from 0.2mm in depth to 1mm
However, it is limited to case reflected in the previously mentioned characteristics of the CO2 when the CO2 laser beam to penetrate the organization is elaborately controlled.
Because it can not be told that there was less possibility of erythema and pigmentation if not
By such advantage, CO2 fractional laser can be utilized to various indications of epidermis
Since it was initially introduced in 2006, CO2 fractional laser has been made up a large development and evolution for a short period of time, and future developments are
● Variable parameter ● Surface cooling for epidermal preservation
You decrease a pain after operations by a laser in case of constancy as the way that used a CO2 laser compares to the way that used a knife as you seal the peripheral nerve edge. And
determine beam quality (Table 1).
or more at dermis.
uncontrolled beam.
and dermis. (Table 2)
expected as follows. (Table 3)
2. Surface and deeper structure
1. Superficial problem
3. Dermal problem
● Wrinkles
**3.3 Clinical application of CO2 fractional laser**
● Pigmentary lesions ● Pore
● Pigmentary lesions: refractory PIH, cloasma, tattoo
● Texture & aging skin ● Laxity(hand, neck)
● Scar ● Striae distensa
Table 2. Clinical Indications of CO2 Fractional Laser Treatment
● Increasing dermal shrinkage ● Deeper penetration ● Pattern of beam ● Quality of beam
● Controllable, smaller spot ● Powerful
Table 3. Future Development of CO2 Fractional Laser
**4. Clinical application in plastic surgery**
because CO2 laser can seal the lymphatics located at cutting plane and directly seal at peripheral vessel small sized less than 0.5mm, you can support patient to more comfort by accelerating wound healing process and by decreasing intraoperative bleeding and postoperative edema, contusion.
This CO2 laser has definite advantage that it is very clear between the tissue which has thermal injury and no damaged tissue located at surrounding area, and it enable to cut tissue by not putting to physical stress to surrounding tissue. It has profit at the procedure by need of bloodless surgical field because of minimizing bleeding. And it enable to treat antiseptically to wound as vaporize the tissue by high temperature.
In palatoplasty and pharyngeal flap operation, compared to conventional method, complication caused by intra- and postoperative bleeding was decreased and there was no difference between CO2 using and conventional method in aspect of wound healing. And CO2 laser has merits to decrease hospitalization (Song IC 1998).
In patients of shortening of frenulum, CO2 laser is available to frenotomy (Yoon CH 1998)
In blepharoplasty using CO2 laser, it was first reported by Baker in 1984(Baker, Muenzler et al. 1984). Using CO2 laser, blepharoplasty was processed safely and more enhanced to decrease bleeding, operation time, edema, time to heal. Mittelman et al (Mittelman and Apfelberg 1990) evaluated as safe method in spite of having risks of eyeball injury, breakaway from its course, fire at operation room, burn injury of skin.
Extramammary Paget's disease is eczema like disease accompanied mainly by itching of anus and genitalia. It was found very much among a middle age or patients of prime of manhood with the past history which a treatment of skin clinic was failed in during long periods. About this disease, invasive method was surgical excision and topical ointment using 5-fluorouracil, radiotherapy. Recently it is the trend that an interest of the treatment using CO2 laser and Nd:YAG laer is rising (Ewing 1991; Yoon ES 2000)
In wound healing, Low-powered CO2 laser helps to induce the synthesis of DNA by give effects to permeability of cell membrane, and to activate fibroblast and condrocyte, and to accelerate to absorp the hematom and to remove necrotic tissue, and to help to healing process of bone and cartilage (Tsai, Huang et al. 1997).
In atropic scar, pulsed CO2 laser having high power was utilized.
In acne scar, dermabrasion using CO2 laser minimizing the common complication by conventional method and is enable to control the depth of peeling by depth of acne scar. So is enable to avoid to complication of hypertrophic scar and keloid etc.. It is available to process peeling safely and deeply. So recovery toward a daily life is fast as managements after operations is convenient because of decreasing of postoperative edema and pain by disconnecting at nerve terminals (Yoon ES 1998).
#### **5. Clinical application in otorhinolaryngology**
#### **5.1 Laryngeal microsurgery using CO2 laser**
CO2 laser is infrared having 10,600 nm wavelength. It has vibrational energy and enable to control to transfer through the mirror. The diameter of focus is about 1mm by
Clinical Application of CO2 Laser 375
CO2 laser has been developed until nowadays. At present, CO2 laser is considered as essential instrument in medicine. In future, we think that CO2 laser may be more developed
Thank to corresponding Dr. Jun-Sung Kim for helping to support academic basis. And thank to Dr. Sang-Ho Lee for permitting the chance to writing in Wooridul Hospital. Also thank to Dr. Chan-Woo Jung at Leaders dermatologic clinics in South Korea for supporting dermatologic description and figure. Special thank to Mrs. Park Min for supporting
Ahn, Y., S. H. Lee, et al. (2004). "Percutaneous endoscopic lumbar discectomy for recurrent
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**6. Conclusion**
**8. References**
**7. Acknowledgment**
dedication of this manuscript.
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for achieving the goal and more generalized.
handpiece and about 200-250μm by micromanipulator from focus distance of 400mm. It was used by controlling the focus distance by using purpose, and it enable to cut like common scalpel. If defocused beam is emitted from more longer distance than focus distance, tissue coagulation was available as decreasing per unit of energy (Ossoff, Coleman et al. 1994).
Laryngeal microsurgery using CO2 laser has many advantages more than conventional surgical method. CO2 laser enables the surgeon to remove the exact desired pathology through controlling of power and emission time, which is necessary to preserve normal function. In situations where it is necessary to remove the pathology of a tubed organ, direct contact of the excised area and disturbance of the surgical field can be avoided. A bloodless operation is possible by the unique ability of the CO2 laser to cauterize even arteries and veins as small as 0.5mm in diameter. Safety margins can be secured more easily when removing a tumor. A local inflammatory response is extremely small after an operation, and there is a little hyperplasia of a granulation tissue and scar formation during healing processes. In case of a huge tumor, CO2 laser can help reduce the amount of tissue removed, which in turn can assist in maintenance of laryngeal function (Hall 1971; Norris and Mullarky 1982).
Possible anesthesia methods for laser microlaryngosurgery are intubation and nonintubated anesthesia etc. Rarely it is possible by Jet ventilation or tracheostomy. In cases of intubated anesthesia, protective tubes such as laser-Shield, Bivoan, and Malincrodt tubes can be used. The cuff of the intubation tube must be protected with a cottonoid soaked in a solution of normal saline during an operation. Non intubated anesthesia may be used when the visual field during an operation is obstructed by an anesthesia tube. This is achieved by removing the tube during surgery at 100% O2 saturation, and quickly reinserting the tube after 2-3 minutes when the O2 saturation begins to fall. Tumors located posterior to the larynx may be removed by inserting a laryngoscope behind the intubation tube (Fried 1984; Shapshay, Beamis et al. 1989).
When using a laser for laryngomicrosurgery, laser power should be set between 1 to 10 watts according the type of tissue to be removed or type of surgery. The most frequently used setting is a microscope magnified 16 to 25 times, laser power of 2 watts, and focus of 250 to 400*μ*m. If the target tissue is too hard and blood vessels are too small, super pulse or ultrapulse methods should be used with a smaller focus (Ossoff, Coleman et al. 1994).
Carbonization of cancer pathology can be visualized more clearly during an operation with a continuous mode laser and can help discriminate between normal and cancer cells. A laryngeal forceps or suction tube should be used to pull the target tissue before using a laser to cut more cleanly while reducing carbonization or thermal injury.
CO2 laser can be used in a variety of laryngeal diseases. First, benign laryngeal diseases such as laryngeal nodule, Reinke's edema, laryngeal cyst, granuloma, papilloma, angioma, septum and so on. Secondly, pre-cancerous pathology like white keratosis. Thirdly, it is partly applied in a treatment of malignant laryngeal pathology.
CO2 laser can also be used to prevent asphyxia in various laryngeal diseases causing airway obstruction such as laryngeal and tracheal stenosis.
#### **6. Conclusion**
374 CO2 Laser – Optimisation and Application
handpiece and about 200-250μm by micromanipulator from focus distance of 400mm. It was used by controlling the focus distance by using purpose, and it enable to cut like common scalpel. If defocused beam is emitted from more longer distance than focus distance, tissue coagulation was available as decreasing per unit of energy (Ossoff,
Laryngeal microsurgery using CO2 laser has many advantages more than conventional surgical method. CO2 laser enables the surgeon to remove the exact desired pathology through controlling of power and emission time, which is necessary to preserve normal function. In situations where it is necessary to remove the pathology of a tubed organ, direct contact of the excised area and disturbance of the surgical field can be avoided. A bloodless operation is possible by the unique ability of the CO2 laser to cauterize even arteries and veins as small as 0.5mm in diameter. Safety margins can be secured more easily when removing a tumor. A local inflammatory response is extremely small after an operation, and there is a little hyperplasia of a granulation tissue and scar formation during healing processes. In case of a huge tumor, CO2 laser can help reduce the amount of tissue removed, which in turn can assist in maintenance of laryngeal function (Hall 1971; Norris and
Possible anesthesia methods for laser microlaryngosurgery are intubation and nonintubated anesthesia etc. Rarely it is possible by Jet ventilation or tracheostomy. In cases of intubated anesthesia, protective tubes such as laser-Shield, Bivoan, and Malincrodt tubes can be used. The cuff of the intubation tube must be protected with a cottonoid soaked in a solution of normal saline during an operation. Non intubated anesthesia may be used when the visual field during an operation is obstructed by an anesthesia tube. This is achieved by removing the tube during surgery at 100% O2 saturation, and quickly reinserting the tube after 2-3 minutes when the O2 saturation begins to fall. Tumors located posterior to the larynx may be removed by inserting a laryngoscope behind the intubation tube (Fried 1984;
When using a laser for laryngomicrosurgery, laser power should be set between 1 to 10 watts according the type of tissue to be removed or type of surgery. The most frequently used setting is a microscope magnified 16 to 25 times, laser power of 2 watts, and focus of 250 to 400*μ*m. If the target tissue is too hard and blood vessels are too small, super pulse or ultrapulse methods should be used with a smaller focus (Ossoff, Coleman et al. 1994).
Carbonization of cancer pathology can be visualized more clearly during an operation with a continuous mode laser and can help discriminate between normal and cancer cells. A laryngeal forceps or suction tube should be used to pull the target tissue before using a laser
CO2 laser can be used in a variety of laryngeal diseases. First, benign laryngeal diseases such as laryngeal nodule, Reinke's edema, laryngeal cyst, granuloma, papilloma, angioma, septum and so on. Secondly, pre-cancerous pathology like white keratosis. Thirdly, it is
CO2 laser can also be used to prevent asphyxia in various laryngeal diseases causing airway
to cut more cleanly while reducing carbonization or thermal injury.
partly applied in a treatment of malignant laryngeal pathology.
obstruction such as laryngeal and tracheal stenosis.
Coleman et al. 1994).
Mullarky 1982).
Shapshay, Beamis et al. 1989).
CO2 laser has been developed until nowadays. At present, CO2 laser is considered as essential instrument in medicine. In future, we think that CO2 laser may be more developed for achieving the goal and more generalized.
#### **7. Acknowledgment**
Thank to corresponding Dr. Jun-Sung Kim for helping to support academic basis. And thank to Dr. Sang-Ho Lee for permitting the chance to writing in Wooridul Hospital. Also thank to Dr. Chan-Woo Jung at Leaders dermatologic clinics in South Korea for supporting dermatologic description and figure. Special thank to Mrs. Park Min for supporting dedication of this manuscript.
#### **8. References**
Clinical Application of CO2 Laser 377
Lee, D. Y. and S. H. Lee (2011). "Carbon dioxide (CO2) laser-assisted microdiscectomy for
Lee, H. K., D. H. Lee, et al. (2006). "Discal cyst of the lumbar spine: MR imaging features."
Lee, S. H., Y. Ahn, et al. (2006). "Immediate pain improvement is a useful predictor of long-
Lee, S. H., Y. Ahn, et al. (2008). "Laser-assisted anterior cervical corpectomy versus posterior
Mittelman, H. and D. B. Apfelberg (1990). "Carbon dioxide laser blepharoplasty--advantages
Neblett, C. R. (1992). *Laser and the cervical spine*. Philadelphia, Lippincott Williams & Wilkins. Nerubay, J., I. Caspi, et al. (1997). "Percutaneous laser nucleolysis of the intervertebral lumbar disc. An experimental study." *Clin Orthop Relat Res*(337): 42-44. Norris, C. W. and M. B. Mullarky (1982). "Experimental skin incision made with the carbon
Onari, K., N. Akiyama, et al. (2001). "Long-term follow-up results of anterior interbody
Origitano, T. C. and O. H. Reichman (1993). "Photodynamic therapy for intracranial
Ossoff, R. H., J. A. Coleman, et al. (1994). "Clinical applications of lasers in otolaryngology--
Powers, S. K., S. S. Cush, et al. (1991). "Stereotactic intratumoral photodynamic therapy for
Rosenberg, G. J., M. A. Brito, Jr., et al. (1999). "Long-term histologic effects of the CO2 laser."
Shapshay, S. M., J. F. Beamis, Jr., et al. (1989). "Total cervical tracheal stenosis: treatment by laser, dilation, and stenting." *Ann Otol Rhinol Laryngol* 98(11): 890-895. Song IC, P. W., Kil MS (1998). "Laser assisted palatoplasty and pharyngeal flap." J Korean
Stein, E., T. Sedlacek, et al. (1990). "Acute and chronic effects of bone ablation with a pulsed
Stellar, S., T. G. Polanyi, et al. (1970). "Experimental studies with the carbon dioxide laser as
Tew, J. M., Jr. and W. D. Tobler (1983). "The laser: history, biophysics, and neurosurgical
Tsai, C. L., L. L. Huang, et al. (1997). "Effect of CO2 laser on healing of cultured meniscus."
longitudinal ligament." *Spine (Phila Pa 1976)* 26(5): 488-493.
head and neck surgery." *Lasers Surg Med* 15(3): 217-248.
*Plast Reconstr Surg* 104(7): 2239-2244; discussion 2245-2236.
holmium laser." *Lasers Surg Med* 10(4): 384-388.
applications." *Clin Neurosurg* 31: 506-549.
a neurosurgical instrument." *Med Biol Eng* 8(6): 549-558.
fusion applied for cervical myelopathy due to ossification of the posterior
neoplasms: development of an image-based computer-assisted protocol for photodynamic therapy of intracranial neoplasms." *Neurosurgery* 32(4): 587-595;
recurrent malignant brain tumors." *Neurosurgery* 29(5): 688-695; discussion 695-686.
posterior longitudinal ligament." *Photomed Laser Surg* 26(2): 119-127. Min, J., Chung, B.J., and Lee, S.H. (2006). "Endoscopically managed synovial cyst of the
disc herniation." *Photomed Laser Surg* 24(4): 508-513.
lumbar spine." *Korean Journal of Spine* 3: 242-245.
and disadvantages." *Ann Plast Surg* 24(1): 1-6.
dioxide laser." *Laryngoscope* 92(4): 416-419.
discussion 595-586.
*Soc Plast Reconstr Surg* 25: 252.
*Lasers Surg Med* 20(2): 172-178.
531-535.
*Clin Imaging* 30(5): 326-330.
extraforaminal lumbar disc herniation at the L5-S1 level." *Photomed Laser Surg* 29(8):
term favorable outcome after percutaneous laser disc decompression for cervical
laminoplasty for cervical myelopathic patients with multilevel ossification of the
Ewing, T. L. (1991). "Paget's disease of the vulva treated by combined surgery and laser."
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*korean society for laser medicine and surgery* 12(1): 90-99.
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*Gynecol Oncol* 43(2): 137-140.
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207.
222-225.
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83-87.
126-167.
*Med* 73(6): 864-870.
**15**
*USA*
**CO2 Laser:**
*1University Of California San Francisco,*
*3Lutheran Medical Center/UCSF,*
**Evidence Based Applications in Dentistry**
Pinalben Viraparia1,2, Joel M. White1 and Ram M. Vaderhobli1,3
Ever since Kumar Patel introduced lasers in 1960s', researchers have been looking into its possible applications in the field of dentistry. Researchers have investigated the effects of laser radiation on teeth, bone, pulp and oral mucosal tissues (Taylor, Shklar, & Roeber, 1965). CO2 lasers have been used extensively in medical field and the first laser to be approved by FDA for dental application was Nd:YAG (Neodymium-Yttrium-Aluminum-Garnet) in 1990s. Since then many types of lasers including CO2, Er:YAG (Erbium-Yttrium-Aluminum-Garnet), Diode, Er Cr:YSGG (Erbium-Chromium-Yttrium-Scallium-Gallium-Garnet) have been approved for dental use. FDA approved Er:YAG for dental hard tissue in 1997 and has approved other types of lasers for soft and hard tissue procedures in many area of dentistry. Many authors have reported the use of Carbon Dioxide (CO2) lasers for soft tissue applications in dentistry (Pick & Pecaro, 1987a; White et al., 1998). The Food and Drug Administration (FDA) granted clearance for marketing CO2 lasers for soft tissue procedures such as frenectomy, gingivectomy, biopsies, and removal of benign and malignant lesions because CO2 laser energy is well absorbed by water. Specific indications for use in dentistry include apthous ulcer treatment, coagulation of extraction sites, sulcular debridement and intraoral soft tissue surgeries such as ablating, incising, and excising (U.S. FDA 510(k)
In this chapter, we will discuss basic design, tissue interactions, evidence based clinical
The growth of CO2 laser applications in Dentistry has grown substantially with its wavelength bands ranging from 9.4 and 10.6 micrometers. The laser medium consists of water or air cooled gas discharge (Carbon dioxide, nitrogen, hydrogen, xenon, helium) that helps in producing a beam of infrared light by activating the footswitch. The original CO2 lasers were continuous wave or interrupted pulse durations of about 0.5 sec to 50 msec with non contact delivery and large beam diameters up to 1mm and larger. Because, the delivery
**1. Introduction**
marketing clearance) as shown in Table 1.
applications, and future of dental applications of CO2 lasers.
**2. Basic equipment design & tissue interactions of CO2 laser**
*2 Lutheran Medical Center-Advanced Education in General Dentistry,*
*Dept. Preventive and Restorative Dental Sciences, UCSF*
### **CO2 Laser: Evidence Based Applications in Dentistry**
Pinalben Viraparia1,2, Joel M. White1 and Ram M. Vaderhobli1,3
*1University Of California San Francisco, 2 Lutheran Medical Center-Advanced Education in General Dentistry, 3Lutheran Medical Center/UCSF, Dept. Preventive and Restorative Dental Sciences, UCSF USA*
#### **1. Introduction**
378 CO2 Laser – Optimisation and Application
Whipple, T. L., R. B. Caspari, et al. (1984). "Laser subtotal meniscectomy in rabbits." *Lasers*
Yoon CH, R. Y., Park HS, Kim HJ (1998). "Comparative study between using CO2-laser and classic method in frenulotomy." *J Korean Soc Plast Reconstr Surg* 25: 1475. Yoon ES, C. J., Han SK, Kim WK (2000). "Treatment of extramammary Paget's disease using
Yoon ES, K. S., Ahn DS, Park SH (1998). "CO2 laser resurfacing of acne scar." *J Korean Soc*
Zelickson, B. D., D. Kist, et al. (2004). "Histological and ultrastructural evaluation of the
effects of a radiofrequency-based nonablative dermal remodeling device: a pilot
the CO2 laser." *J Korean Soc Plast Reconstr Surg* 27: 169.
*Surg Med* 3(4): 297-304.
*Aesth Plast Reconstr Surg* 4: 381.
study." *Arch Dermatol* 140(2): 204-209.
Ever since Kumar Patel introduced lasers in 1960s', researchers have been looking into its possible applications in the field of dentistry. Researchers have investigated the effects of laser radiation on teeth, bone, pulp and oral mucosal tissues (Taylor, Shklar, & Roeber, 1965). CO2 lasers have been used extensively in medical field and the first laser to be approved by FDA for dental application was Nd:YAG (Neodymium-Yttrium-Aluminum-Garnet) in 1990s. Since then many types of lasers including CO2, Er:YAG (Erbium-Yttrium-Aluminum-Garnet), Diode, Er Cr:YSGG (Erbium-Chromium-Yttrium-Scallium-Gallium-Garnet) have been approved for dental use. FDA approved Er:YAG for dental hard tissue in 1997 and has approved other types of lasers for soft and hard tissue procedures in many area of dentistry.
Many authors have reported the use of Carbon Dioxide (CO2) lasers for soft tissue applications in dentistry (Pick & Pecaro, 1987a; White et al., 1998). The Food and Drug Administration (FDA) granted clearance for marketing CO2 lasers for soft tissue procedures such as frenectomy, gingivectomy, biopsies, and removal of benign and malignant lesions because CO2 laser energy is well absorbed by water. Specific indications for use in dentistry include apthous ulcer treatment, coagulation of extraction sites, sulcular debridement and intraoral soft tissue surgeries such as ablating, incising, and excising (U.S. FDA 510(k) marketing clearance) as shown in Table 1.
In this chapter, we will discuss basic design, tissue interactions, evidence based clinical applications, and future of dental applications of CO2 lasers.
#### **2. Basic equipment design & tissue interactions of CO2 laser**
The growth of CO2 laser applications in Dentistry has grown substantially with its wavelength bands ranging from 9.4 and 10.6 micrometers. The laser medium consists of water or air cooled gas discharge (Carbon dioxide, nitrogen, hydrogen, xenon, helium) that helps in producing a beam of infrared light by activating the footswitch. The original CO2 lasers were continuous wave or interrupted pulse durations of about 0.5 sec to 50 msec with non contact delivery and large beam diameters up to 1mm and larger. Because, the delivery
CO2 Laser: Evidence Based Applications in Dentistry 381
Fig. 1. CO2 laser hand piece with different tips marketed by GPT Dental
Numerous studies have been done pre and post FDA approval to improve the technique and provide best practice guidelines for the clinicians. A report published by American Dental Association (ADA) in 2001 describing the challenges in the future of oral health care mentioned the role of laser applications. The report specifically mentioned that more clinical research and technical developments in CO2 laser delivery systems will promise to expand its clinical applications beyond soft tissue procedures (Seldin, 2001). Although CO2 laser 10.6micron wavelength is absorbed by water and even though 9.3micron is absorbed in hydroxyapetite, it is primarily a soft tissue laser (Convissar & Goldstein, 2003). Even before CO2 laser received FDA approval for soft tissue procedures in 1990's, many studies have looked at its hard tissue applications in 1980's. Table 2. Lists CO2 laser application in
One of the earlier case series by Pick and colleagues reported soft tissue procedures using Sharplan 743 CO2 and Xanar Ambulase lasers. In a Clinical trial 250 patients were treated for conditions ranging from, gingival hyperplasia, benign and malignant lesions (along with conventional surgery), incisional & excisional biopsy, red-white lesions, and haemorrhagic
**3. Evidence based clinical applications**
various dental procedures (Sulewski JG).
**3.1 Soft tissue procedures**
mode is non-contact this results in lack of tactile sensation to the operator. Previous studies with these continuous wave CO2 lasers showed a variety of structural and ultrasonic changes of the hard tooth structure. These included cracking, flaking, crater formation, charring, melting, and recrystallizaton due to the highly efficient absorption of CO2 wavelengths by the apatite mineral of hard tissues (Boehm, Rich, Webster, & Janke, 1977; J. D. B. Featherstone & D. G. A. Nelson, 1987; McCormack, 1995; Stern & Sognnaes, 1964; Stern, Vahl, & Sognnaes, 1972). All dental tissues have different absorption coefficient for various wavelength depending on water, blood, pigment, and mineral content. For example, Nd:YAG and Diode lasers are absorbed by dark pigments making them ideal for soft tissue procedures. Tissue component that maximally absorbs CO2 wavelength is water followed by apatite (Gouw-Soares et al., 2004). Because of this CO2 lasers have been proven to be the gold standard for intra-oral soft tissue applications for decades. Thermal effects and various parameters settings of CO2 lasers have also been studied extensively (Leighty, Pogrel, Goodis, & White, 1991; Malmström, McCormack, Fried, & Featherstone, 2001). These studies indicated that application of CO2 laser created unacceptable thermal damage to adjacent tissue. Because of these reasons early CO2 laser system had been limited by their continuous wave operations and delivery system constraints. Lasers parameters such as power, repetition rate, average power and highest peak power play a role in surgical and collateral effects. Studies have concluded that high repetition rate, high peak power and lower average power yield favourable clinical results (Wilder-Smith, Dang, & Kurosaki, 1997).
Table 1. Examples of CO2 lasers available in market for dental use
Due to the lack of tactile sensation, their use in hard-tissue applications is not favorable. With new technologies, dental laser manufacturers now claim to have shorter pulse durations (as short as 150 microsecond pulse duration) with beam diameters of as small as 100 microns. This allows for cooling of tissues between pulses and results in minimal thermal damage. These lasers are now marketed for soft tissue intraoral procedures as described earlier. The laser is usually equipped with various hand pieces and tips of differing diameter for tissue ablation as shown in Figure 1. The hand piece is usually the size of a dental drill and the spot size that is emitted from these hand pieces allows for greater accuracy resulting in minimal damage to the surrounding tissues. As these lasers operate the best in pulse or super pulse infrared mode, they are able to remove precise amount of tissues with each pulse emission.
mode is non-contact this results in lack of tactile sensation to the operator. Previous studies with these continuous wave CO2 lasers showed a variety of structural and ultrasonic changes of the hard tooth structure. These included cracking, flaking, crater formation, charring, melting, and recrystallizaton due to the highly efficient absorption of CO2 wavelengths by the apatite mineral of hard tissues (Boehm, Rich, Webster, & Janke, 1977; J. D. B. Featherstone & D. G. A. Nelson, 1987; McCormack, 1995; Stern & Sognnaes, 1964; Stern, Vahl, & Sognnaes, 1972). All dental tissues have different absorption coefficient for various wavelength depending on water, blood, pigment, and mineral content. For example, Nd:YAG and Diode lasers are absorbed by dark pigments making them ideal for soft tissue procedures. Tissue component that maximally absorbs CO2 wavelength is water followed by apatite (Gouw-Soares et al., 2004). Because of this CO2 lasers have been proven to be the gold standard for intra-oral soft tissue applications for decades. Thermal effects and various parameters settings of CO2 lasers have also been studied extensively (Leighty, Pogrel, Goodis, & White, 1991; Malmström, McCormack, Fried, & Featherstone, 2001). These studies indicated that application of CO2 laser created unacceptable thermal damage to adjacent tissue. Because of these reasons early CO2 laser system had been limited by their continuous wave operations and delivery system constraints. Lasers parameters such as power, repetition rate, average power and highest peak power play a role in surgical and collateral effects. Studies have concluded that high repetition rate, high peak power and lower average power yield favourable clinical results (Wilder-Smith, Dang, & Kurosaki, 1997).
> Most Absorption
N/A 9.6 Appetite Hard tissue No N/A 9.3 Appetite Hard Tissue No Smart CO2 10.6 Water Soft Tissue Yes CO2DENTA 10.6 Water Soft Tissue Yes
Due to the lack of tactile sensation, their use in hard-tissue applications is not favorable. With new technologies, dental laser manufacturers now claim to have shorter pulse durations (as short as 150 microsecond pulse duration) with beam diameters of as small as 100 microns. This allows for cooling of tissues between pulses and results in minimal thermal damage. These lasers are now marketed for soft tissue intraoral procedures as described earlier. The laser is usually equipped with various hand pieces and tips of differing diameter for tissue ablation as shown in Figure 1. The hand piece is usually the size of a dental drill and the spot size that is emitted from these hand pieces allows for greater accuracy resulting in minimal damage to the surrounding tissues. As these lasers operate the best in pulse or super pulse infrared mode, they are able to remove precise amount of
Recommended Use
10.6 Water Soft Tissue Yes
FDA Approval
Device Trade Name
Opus 20 Dental laser system
tissues with each pulse emission.
Wavelength (micro mm)
Table 1. Examples of CO2 lasers available in market for dental use
Fig. 1. CO2 laser hand piece with different tips marketed by GPT Dental
#### **3. Evidence based clinical applications**
Numerous studies have been done pre and post FDA approval to improve the technique and provide best practice guidelines for the clinicians. A report published by American Dental Association (ADA) in 2001 describing the challenges in the future of oral health care mentioned the role of laser applications. The report specifically mentioned that more clinical research and technical developments in CO2 laser delivery systems will promise to expand its clinical applications beyond soft tissue procedures (Seldin, 2001). Although CO2 laser 10.6micron wavelength is absorbed by water and even though 9.3micron is absorbed in hydroxyapetite, it is primarily a soft tissue laser (Convissar & Goldstein, 2003). Even before CO2 laser received FDA approval for soft tissue procedures in 1990's, many studies have looked at its hard tissue applications in 1980's. Table 2. Lists CO2 laser application in various dental procedures (Sulewski JG).
#### **3.1 Soft tissue procedures**
One of the earlier case series by Pick and colleagues reported soft tissue procedures using Sharplan 743 CO2 and Xanar Ambulase lasers. In a Clinical trial 250 patients were treated for conditions ranging from, gingival hyperplasia, benign and malignant lesions (along with conventional surgery), incisional & excisional biopsy, red-white lesions, and haemorrhagic
CO2 Laser: Evidence Based Applications in Dentistry 383
Hard tissue applications of continuous wave CO2 lasers have been limited due to thermal damage, charring effect and resultant rough tooth surface. Due to its high absorption overlap with phosphate in enamel apetite crystal, all radiation is absorbed in thin enamel (<10 um). This makes heat transfer as the main way of energy transport leading to thermal damage to pulp (Wigdor et al., 1995). Conversely studies have shown that high reflectivity (9%-50%) at 9.3- and 9.6um wavelengths may pose a safety concern and it requires accurate knowledge of radiation dose while doing treatment (Fried, Glena, Featherstone, & Seka, 1997). In 1990's TEA (Transversely excited atmospheric pressure) 10.6um pulsed (0.1-2usec) CO2 laser had the best reported success in ablating dental hard tissue. However, high plasma induction with TEA CO2 laser posed problems with decreased ablation efficiency and damage due to shock wave rendering it unacceptable for clinical use (Wigdor et al., 1995). Since the 9.6micrometer CO2 laser wavelength is highly absorbed in appetite crystals, it presents a future potential for its applications in cavity preparation, apicectomies and other hard tissue procedures. In vitro study using Scanning Electron Microscopic (SEM) images reported cleaner dentinal surface with fusion and recrystallized dentine following apicectomy and root treatment with pulsed TEA 9.6micrometer CO2 laser (Gouw-Soares et al., 2004). Contrasting results were reported in a more recent SEM analysis study. The study compared the marginal permeability and dentinal surface texture following apicectomy performed with burs and CO2 laser. Authors attributed rougher surface and less favourable marginal fit following CO2 laser treatment to the use of continuous wave mode, no cooling agent and less experience of the operator with CO2 laser (Lustosa-Pereira et al., 2011).
Advantages of using CO2 laser for periodontal procedures have been accepted by American Academy of Periodontology in its position paper. Its ability to provide dry surgical field and haemostasis has been proven useful in periodontal surgical procedures. Additionally CO2 laser use has shown mixed results when used for periodontal pocket debridement in addition to mechanical debridement, pocket reduction, attachment gain, decreased microorganisms, and guided tissue regeneration cases (Convissar & Goldstein, 2003; Matthews, 2010; Wigdor et al., 1995). Porcine mandible study evaluating efficacy of newer micropulse 10.6um CO2 laser showed clinically acceptable results in coagulation, incision depth and width, time required to perform procedure, with minimal hard tissue damage on accidental exposure but surface melting with direct exposure to laser (Vaderhobli, White, Le, Ho, & Jordan, 2010). Other studies have also reported thermal side effects like dentin cracking, carbonization, and melting following CO2 laser use on root surfaces (Matthews,
Preventive uses of CO2 laser have been well researched. Literature suggests that 9.3 and 9.6um wavelength (pulse width <100usec) at a specified pulse rate has higher efficiency than 10.6um in heating dentin/enamel surface leading to desired crystallization and fusion of surface layer for sealing effect (McCormack, Fried, Featherstone, Glena, & Seka, 1995; Wigdor et al., 1995). A case report by Dederich, suggested to use 15W for 0.2sec duration to
**3.2 Endodontics (apicoectomy, root canal debridement)**
**3.3 Periodontal procedures**
**3.4 Other restorative uses**
2010).
and coagulation disorders. They concluded that CO2 laser provided bloodless field, less post-operative discomfort, tissue coagulation, and better accessibility in some areas of oral cavity compared to scalpel surgery (Pick & Pecaro, 1987b). The advantages compared to scalpel wounds also included site-specific wound sterilization; minimal swelling and scarring but slower healing; reduced necessity for suturing; decreased incidence of mechanical trauma; shorter operative time; favorable patient acceptance; decreased use of local anesthesia; and little or no postoperative pain (Pick & Powell, 1993; White et al., 2002; Wigdor et al., 1995). Literature also reported increased levels of hyaluronic acid in laser wounds compared to scalpel wounds, a chemical that plays a key role in wound repair (Pogrel, Pham, Guntenhoner, & Stern, 1993). With increased use of CO2 lasers clinically, adjacent tissue interaction and damage has been an issue. Studies have reported on chemical and thermal interaction of CO2 lasers with surrounding tissue. In vitro study using 9.3 micrometers Duolase CO2 laser (Medical Optics Inc.) investigated variations in incision depth and width, collateral damage, and bone charring using continuous mode (1-9W average power; 1-10W peak power; 0.5-500Hz; 1, 20, 200miliseconds), superpulse (1-7W average power; 20W peak power; 170-1170hz; 300microseconds) and optipulse (0.72-1.20W average power; 60-100W peak power; 10-40Hz; 300microseconds) mode with various parameter combinations. They concluded that superpulse and optipulse mode with lower average powers and higher peak powers created narrow and deep cuts. Also, almost no charring was noticed with optipulse mode. Optipulse mode reduced the collateral damage by the factor of 10 compared to continuous mode (Wilder-Smith et al., 1997).
Table 2. CO2 Laser soft tissue applications
and coagulation disorders. They concluded that CO2 laser provided bloodless field, less post-operative discomfort, tissue coagulation, and better accessibility in some areas of oral cavity compared to scalpel surgery (Pick & Pecaro, 1987b). The advantages compared to scalpel wounds also included site-specific wound sterilization; minimal swelling and scarring but slower healing; reduced necessity for suturing; decreased incidence of mechanical trauma; shorter operative time; favorable patient acceptance; decreased use of local anesthesia; and little or no postoperative pain (Pick & Powell, 1993; White et al., 2002; Wigdor et al., 1995). Literature also reported increased levels of hyaluronic acid in laser wounds compared to scalpel wounds, a chemical that plays a key role in wound repair (Pogrel, Pham, Guntenhoner, & Stern, 1993). With increased use of CO2 lasers clinically, adjacent tissue interaction and damage has been an issue. Studies have reported on chemical and thermal interaction of CO2 lasers with surrounding tissue. In vitro study using 9.3 micrometers Duolase CO2 laser (Medical Optics Inc.) investigated variations in incision depth and width, collateral damage, and bone charring using continuous mode (1-9W average power; 1-10W peak power; 0.5-500Hz; 1, 20, 200miliseconds), superpulse (1-7W average power; 20W peak power; 170-1170hz; 300microseconds) and optipulse (0.72-1.20W average power; 60-100W peak power; 10-40Hz; 300microseconds) mode with various parameter combinations. They concluded that superpulse and optipulse mode with lower average powers and higher peak powers created narrow and deep cuts. Also, almost no charring was noticed with optipulse mode. Optipulse mode reduced the collateral damage
by the factor of 10 compared to continuous mode (Wilder-Smith et al., 1997).
Oral Medicine Aphthous ulcer treatment, Biopsies (incisional/excisional),
procedures Crown lengthening (soft tissue only), Tissue retraction for impression
Abscess incision and drainage, Hemostatic assistance, Fibroma removal, Oral papillectomy, Exposure of nonerupted or partially erupted teeth, Implant recovery, Lesion (tumor) removal, Vestibuloplasty, Frenectomy, Frenotomy, Operculectomy, coagulation
Leukoplakia
Sulcular debridement, Gingival excision/incision, laser assisted new attachment procedure, Gingivectomy/gingivoplasty
Pulpotomy, Pulpotomy, as an adjunct to root canal therapy and retreatment cases, Removal of filling material such as gutta-percha or resin
resin curing, teeth whitening agent activation, caries detection, pit and fissure sealants, enamel treatment to increase caries resistance, enamel etching for resin bonding procedures, caries removal, tissue ablation
Area of Dentistry Procedure
Oral & maxillofacial surgery
Pre-prosthetic
Periodontal procedures
Endodontics
Restorative uses
Table 2. CO2 Laser soft tissue applications
#### **3.2 Endodontics (apicoectomy, root canal debridement)**
Hard tissue applications of continuous wave CO2 lasers have been limited due to thermal damage, charring effect and resultant rough tooth surface. Due to its high absorption overlap with phosphate in enamel apetite crystal, all radiation is absorbed in thin enamel (<10 um). This makes heat transfer as the main way of energy transport leading to thermal damage to pulp (Wigdor et al., 1995). Conversely studies have shown that high reflectivity (9%-50%) at 9.3- and 9.6um wavelengths may pose a safety concern and it requires accurate knowledge of radiation dose while doing treatment (Fried, Glena, Featherstone, & Seka, 1997). In 1990's TEA (Transversely excited atmospheric pressure) 10.6um pulsed (0.1-2usec) CO2 laser had the best reported success in ablating dental hard tissue. However, high plasma induction with TEA CO2 laser posed problems with decreased ablation efficiency and damage due to shock wave rendering it unacceptable for clinical use (Wigdor et al., 1995). Since the 9.6micrometer CO2 laser wavelength is highly absorbed in appetite crystals, it presents a future potential for its applications in cavity preparation, apicectomies and other hard tissue procedures. In vitro study using Scanning Electron Microscopic (SEM) images reported cleaner dentinal surface with fusion and recrystallized dentine following apicectomy and root treatment with pulsed TEA 9.6micrometer CO2 laser (Gouw-Soares et al., 2004). Contrasting results were reported in a more recent SEM analysis study. The study compared the marginal permeability and dentinal surface texture following apicectomy performed with burs and CO2 laser. Authors attributed rougher surface and less favourable marginal fit following CO2 laser treatment to the use of continuous wave mode, no cooling agent and less experience of the operator with CO2 laser (Lustosa-Pereira et al., 2011).
#### **3.3 Periodontal procedures**
Advantages of using CO2 laser for periodontal procedures have been accepted by American Academy of Periodontology in its position paper. Its ability to provide dry surgical field and haemostasis has been proven useful in periodontal surgical procedures. Additionally CO2 laser use has shown mixed results when used for periodontal pocket debridement in addition to mechanical debridement, pocket reduction, attachment gain, decreased microorganisms, and guided tissue regeneration cases (Convissar & Goldstein, 2003; Matthews, 2010; Wigdor et al., 1995). Porcine mandible study evaluating efficacy of newer micropulse 10.6um CO2 laser showed clinically acceptable results in coagulation, incision depth and width, time required to perform procedure, with minimal hard tissue damage on accidental exposure but surface melting with direct exposure to laser (Vaderhobli, White, Le, Ho, & Jordan, 2010). Other studies have also reported thermal side effects like dentin cracking, carbonization, and melting following CO2 laser use on root surfaces (Matthews, 2010).
#### **3.4 Other restorative uses**
Preventive uses of CO2 laser have been well researched. Literature suggests that 9.3 and 9.6um wavelength (pulse width <100usec) at a specified pulse rate has higher efficiency than 10.6um in heating dentin/enamel surface leading to desired crystallization and fusion of surface layer for sealing effect (McCormack, Fried, Featherstone, Glena, & Seka, 1995; Wigdor et al., 1995). A case report by Dederich, suggested to use 15W for 0.2sec duration to
CO2 Laser: Evidence Based Applications in Dentistry 385
Fried, D., Glena, R. E., Featherstone, J. D., & Seka, W. (1997). Permanent and transient
Gouw-Soares, S., Stabholz, A., Lage-Marques, J. L., Zezell, D. M., Groth, E. B., & Eduardo, C.
Leighty, S. M., Pogrel, M. A., Goodis, H. E., & White, J. M. (1991). Thermal effects of the carbon dioxide laser on teeth. Lasers in the life sciences, 4(2), 93-102. Luk, K., Tam, L., & Hubert, M. (2004). Effect of light energy on peroxide tooth bleaching. J
Lustosa-Pereira, A. C., Pozza, D. H., Cunha, A., Dedavid, B. A., Duarte-de Moraes, J. F., &
Malmström, H. S., McCormack, S. M., Fried, D., & Featherstone, J. D. B. (2001). Effect of CO2
Matthews, D. C. (2010). Seeing the Light--the truth about soft tissue lasers and nonsurgical
McCormack, S. M. (1995). Scanning electron microscope observations of CO2 laser effects on
McCormack, S. M., Fried, D., Featherstone, J. D., Glena, R. E., & Seka, W. (1995). Scanning
Melcer, J. (1986). Latest treatment in dentistry by means of the CO2 laser beam. Lasers Surg
N/A. (1998). Laser-assisted bleaching: an update. ADA Council on Scientific Affairs. J Am
Obata, A., Tsumura, T., Niwa, K., Ashizawa, Y., Deguchi, T., & Ito, M. (1999). Super pulse CO2 laser for bracket bonding and debonding. Eur J Orthod, 21(2), 193-198. Pick, R. M., & Pecaro, B. C. (1987a). Use of the CO2 laser in soft tissue dental surgery. Lasers
Pick, R. M., & Pecaro, B. C. (1987b). Use of the CO2 laser in soft tissue dental surgery. Lasers
Pick, R. M., & Powell, G. L. (1993). Laser in dentistry. Soft-tissue procedures. Dent Clin
Pogrel, M. A., Pham, H. D., Guntenhoner, M., & Stern, R. (1993). Profile of hyaluronidase
Seldin, L. W. (2001). Future of Dentistry: Today's Vision: Tomorrow's Reality: American
Stern, R. H., & Sognnaes, R. F. (1964). Laser beam effect on dental hard tissues. J Dent Res,
Dental Association, Health Policy Resources Center.
activity distinguishes carbon dioxide laser from scalpel wound healing. Ann Surg,
dental enamel. Journal of Dental Research, 74(10), 1702-1708.
Gerhardt-de Oliveira, M. (2011). Analysis of the morphology and composition of tooth apices apicectomized using three different ablation techniques. Med Oral
laser on pulpal temperature and surface morphology: an in vitro study. Journal of
electron microscope observations of CO2 laser effects on dental enamel. J Dent Res,
9.3, 9.6, 10.3, and 10.6 microns and at fluences of 1-20 J/cm2. [In Vitro
Research Support, U.S. Gov't, P.H.S.]. Lasers Surg Med, 20(1), 22-31.
Am Dent Assoc, 135(2), 194-201; quiz 228-199.
periodontal therapy. J Can Dent Assoc, 76, a30.
Patol Oral Cir Bucal, 16(2), e225-230.
Dentistry, 29(8), 521-529.
74(10), 1702-1708.
Med, 6(4), 396-398.
Dent Assoc, 129(10), 1484-1487.
in Surgery and Medicine, 7(2).
Surg Med, 7(2), 207-213.
North Am, 37(2), 281-296.
217(2), 196-200.
43(5).
10.1089/104454704774076190
changes in the reflectance of CO2 laser-irradiated dental hard tissues at lambda =
P. (2004). Comparative study of dentine permeability after apicectomy and surface treatment with 9.6 microm TEA CO2 and Er:YAG laser irradiation. [Research Support, Non-U.S. Gov't]. J Clin Laser Med Surg, 22(2), 129-139. doi:
achieve dentinal sealing effect with CO2 laser without detrimental effect to pulpal tissue (Dederich, 1999). Furthermore, researchers showed that pulsed CO2 laser produced >1000 celsius temperature increase at the surface, enough to melt and recrystalize enamel and minimal changes deeper than 40um which is critical to avoid collateral damage (J. D. Featherstone & D. G. Nelson, 1987; Wigdor et al., 1995). For dental decay, CO2 lasers have mixed results ranging from thermal damage, dentin/pulp sterilization, and mineralization under treated surface (Melcer, 1986). Surface etching with CO2 laser showed 300% increased in dentin resin bond but no change in enamel bonding (Obata et al., 1999; Wigdor et al., 1995). Superpulse CO2 laser produced fastest debonding of orthodontic brackets (Obata et al., 1999). CO2 laser has also been used in otherwise hopeless prognosis cases of vertical root fracture with radiographical success at one-year follow-up. Teeth bleaching agent activation with CO2 laser causes higher temperature changes and due to lack of controlled clinical trials, ADA does not support its clinical use (Luk, Tam, & Hubert, 2004; N/A, 1998).
#### **4. Future of dental applications of CO2 lasers**
The future looks promising for CO2 laser use in the field of dentistry. We need more clinical research specifically randomized clinical trials to evaluate effectiveness of CO2 laser compared to traditional methods. The evidence will help develop standard clinical guidelines for practicing dentists.
#### **5. Conclusion**
Currently, CO2 lasers have been used widely in dentistry for soft tissue procedures. More research will help provide practice guidelines. Clinical research including randomized trials are needed to provide specifications for parameter settings, delivery mode, and other guidelines for hard tissue procedures. More information regarding shorter wavelength CO2 lasers in recent years makes future of CO2 laser promising in dentistry.
#### **6. Acknowledgment**
We would like to thank Lutheram Medical Center- Department of Dental Medicine for providing support for this project.
#### **7. References**
achieve dentinal sealing effect with CO2 laser without detrimental effect to pulpal tissue (Dederich, 1999). Furthermore, researchers showed that pulsed CO2 laser produced >1000 celsius temperature increase at the surface, enough to melt and recrystalize enamel and minimal changes deeper than 40um which is critical to avoid collateral damage (J. D. Featherstone & D. G. Nelson, 1987; Wigdor et al., 1995). For dental decay, CO2 lasers have mixed results ranging from thermal damage, dentin/pulp sterilization, and mineralization under treated surface (Melcer, 1986). Surface etching with CO2 laser showed 300% increased in dentin resin bond but no change in enamel bonding (Obata et al., 1999; Wigdor et al., 1995). Superpulse CO2 laser produced fastest debonding of orthodontic brackets (Obata et al., 1999). CO2 laser has also been used in otherwise hopeless prognosis cases of vertical root fracture with radiographical success at one-year follow-up. Teeth bleaching agent activation with CO2 laser causes higher temperature changes and due to lack of controlled clinical
trials, ADA does not support its clinical use (Luk, Tam, & Hubert, 2004; N/A, 1998).
The future looks promising for CO2 laser use in the field of dentistry. We need more clinical research specifically randomized clinical trials to evaluate effectiveness of CO2 laser compared to traditional methods. The evidence will help develop standard clinical
Currently, CO2 lasers have been used widely in dentistry for soft tissue procedures. More research will help provide practice guidelines. Clinical research including randomized trials are needed to provide specifications for parameter settings, delivery mode, and other guidelines for hard tissue procedures. More information regarding shorter wavelength CO2
We would like to thank Lutheram Medical Center- Department of Dental Medicine for
Boehm, R., Rich, J., Webster, J., & Janke, S. (1977). Thermal stress effects and surface cracking associated with laser use on human teeth. J Biomech Eng, 99, 189-194. Convissar, R. A., & Goldstein, E. E. (2003). An overview of lasers in dentistry. [Case Reports,
Dederich, D. N. (1999). CO2 laser fusion of a vertical root fracture. J Am Dent Assoc, 130(8),
Featherstone, J. D., & Nelson, D. G. (1987). Laser effects on dental hard tissues. [Research
Featherstone, J. D. B., & Nelson, D. G. A. (1987). Laser effects on dental hard tissues.
lasers in recent years makes future of CO2 laser promising in dentistry.
Support, U.S. Gov't, P.H.S.]. Adv Dent Res, 1(1), 21-26.
**4. Future of dental applications of CO2 lasers**
guidelines for practicing dentists.
**5. Conclusion**
**6. Acknowledgment**
**7. References**
providing support for this project.
1195-1199.
Review]. Gen Dent, 51(5), 436-440.
Advances in Dental Research, 1(1), 21-26.
**16**
*Iran*
Nasrin Zand
**Non-Thermal, Non-Ablative CO2 Laser Therapy**
CO2 laser has been used as a very useful device in surgery for ablation, coagulation and cutting the tissues for the last four decades. It is interesting to know that this high power laser can also be used as a therapeutic laser for immediate pain reduction in some oral lesions without any visible side effects such as ulceration, erosion formation and even erythema.
Recently few case reports and clinical trials have been published about using CO2 laser in non-ablative manner to reduce pain in oral lesions. In these studies, the oral painful lesions were irradiated through a layer of transparent, non-anesthetic gel with high water content to reduce the beam absorption by the soft tissue. The patients reported immediate and significant pain relief after laser irradiation. The procedure was painless and anesthesia was not required. This technique was called non-thermal, Non-Ablative CO2 Laser Therapy (NACLT). The results of powermetry and thermometry demonstrated the low power nature of NACLT. However there are some differences between analgesic effects of NACLT and
To provide a comprehensive understanding of NACLT, this chapter is organized in several sections. First, due to low level therapeutic nature of NACLT, conventional low power therapeutic lasers, their biological effects and their pain relieving properties are reviewed. Then, a discussion about the interesting analgesic effects of CO2 lasers is presented. In the next section, NACLT as a new low level laser therapy procedure and its pain relieving applications in painful oral lesions is discussed. Finally, the presumed mechanisms of
Low-level laser (or light) therapy (LLLT) has been investigated and used clinically for over 40 years. However, it is only in relatively recent times that LLLT has become scientifically
and clinically accepted by even a fraction of the medical community (Hamblin 2010).
the other classical low power lasers which will be discussed in the next sections.
**1. Introduction**
analgesic effects of NACLT are covered.
**2.1 History**
**2. Low level laser therapy (laser phototherapy)**
**(NACLT): A New Approach to Relieve Pain in**
*Academic Center for Education, Culture and Research (ACECR), Tehran*
**Some Painful Oral Diseases**
*Iranian Center for Medical Lasers (ICML),*
### **Non-Thermal, Non-Ablative CO2 Laser Therapy (NACLT): A New Approach to Relieve Pain in Some Painful Oral Diseases**
Nasrin Zand *Iranian Center for Medical Lasers (ICML), Academic Center for Education, Culture and Research (ACECR), Tehran Iran*
#### **1. Introduction**
386 CO2 Laser – Optimisation and Application
Stern, R. H., Vahl, J., & Sognnaes, R. F. (1972). Lased enamel: ultrastructural observations of pulsed carbon dioxide laser effects. Journal of Dental Research, 51(2), 455-460. Sulewski JG, (2010). Making the most of 16th Annual conference and exhibition: A practical orientation for attendees. Academy of Laser Dentistry, Miami, Florida. Taylor, R., Shklar, G., & Roeber, F. (1965). THE EFFECTS OF LASER RADIATION ON
Vaderhobli, R. M., White, J. M., Le, C., Ho, S., & Jordan, R. (2010). In vitro study of the soft
White, J. M., Chaudhry, S. I., Kudler, J. J., Sekandari, N., Schoelch, M. L., & Silverman Jr, S.
White, J. M., Gekelman, D., Shin, K. B., Park, J. S., Swenson, T. O., Rouse, B. P., . . . Oto, M. G.
Wigdor, H. A., Walsh, J. T., Jr., Featherstone, J. D., Visuri, S. R., Fried, D., & Waldvogel, J. L.
Wilder-Smith, P., Dang, J., & Kurosaki, T. (1997). Investigating the range of surgical effects
Oral Surg Oral Med Oral Pathol, 19, 786-795.
laser medicine & surgery, 16(6), 299.
Review]. Lasers Surg Med, 16(2), 103-133.
of applied scientific research?
Lasers Surg Med, 42(3), 257-263. doi: 10.1002/lsm.20888
TEETH, DENTAL PULP, AND ORAL MUCOSA OF EXPERIMENTAL ANIMALS.
tissue effects of microsecond-pulsed CO(2) laser parameters during soft tissue incision and sulcular debridement. [In Vitro, Research Support, Non-U.S. Gov't].
(1998). Nd: YAG and CO2 laser therapy of oral mucosal lesions. Journal of clinical
(2002). Laser interaction with dental soft tissues: What do we know from our years
(1995). Lasers in dentistry. [Case Reports, Research Support, Non-U.S. Gov't, Research Support, U.S. Gov't, Non-P.H.S., Research Support, U.S. Gov't, P.H.S.,
on soft tissue produced by a carbon dioxide laser. J Am Dent Assoc, 128(5), 583-588.
CO2 laser has been used as a very useful device in surgery for ablation, coagulation and cutting the tissues for the last four decades. It is interesting to know that this high power laser can also be used as a therapeutic laser for immediate pain reduction in some oral lesions without any visible side effects such as ulceration, erosion formation and even erythema.
Recently few case reports and clinical trials have been published about using CO2 laser in non-ablative manner to reduce pain in oral lesions. In these studies, the oral painful lesions were irradiated through a layer of transparent, non-anesthetic gel with high water content to reduce the beam absorption by the soft tissue. The patients reported immediate and significant pain relief after laser irradiation. The procedure was painless and anesthesia was not required. This technique was called non-thermal, Non-Ablative CO2 Laser Therapy (NACLT). The results of powermetry and thermometry demonstrated the low power nature of NACLT. However there are some differences between analgesic effects of NACLT and the other classical low power lasers which will be discussed in the next sections.
To provide a comprehensive understanding of NACLT, this chapter is organized in several sections. First, due to low level therapeutic nature of NACLT, conventional low power therapeutic lasers, their biological effects and their pain relieving properties are reviewed. Then, a discussion about the interesting analgesic effects of CO2 lasers is presented. In the next section, NACLT as a new low level laser therapy procedure and its pain relieving applications in painful oral lesions is discussed. Finally, the presumed mechanisms of analgesic effects of NACLT are covered.
#### **2. Low level laser therapy (laser phototherapy)**
#### **2.1 History**
Low-level laser (or light) therapy (LLLT) has been investigated and used clinically for over 40 years. However, it is only in relatively recent times that LLLT has become scientifically and clinically accepted by even a fraction of the medical community (Hamblin 2010).
Non-Thermal, Non-Ablative CO2 Laser Therapy (NACLT):
2002; Gigo-Benato, Geuna et al. 2005).
(Hamblin, Waynant et al. 2006).
**2.3 Pain relieving effects of low level therapeutic lasers**
A New Approach to Relieve Pain in Some Painful Oral Diseases 389
level (therapeutic laser) in NACLT, too). Some of the researchers favour the term "laser
Low level laser (or light) therapy (LLLT) is the application of light (usually a low power laser or LED in the range of 1mW – 500mW) to a pathology to promote wound healing and tissue repair, reduce inflammation and relieve pain. The light is typically of narrow spectral width in the red or near infrared spectrum (600nm – 1000nm); at power densities (between 1mw-5W/cm2) (Huang, Chen et al. 2009), not associated with macroscopic thermal effects, in contrast to thermally mediated surgical applications (Chow, David et al. 2007). In using high power surgical lasers, the collimation of laser light leads to the emission of a narrow, intense beam of light and is used for precise tissue destruction (photothermal effect). However, in LLLT, light radiation intensities are so low that the resulting biological effects are ascribable to physical or chemical changes associated with the interaction of cells and tissues with the laser radiation, and not simply to a result of heating (Snyder, Byrnes et al.
The main areas of medicine where laser phototherapy has a known and major role are as follows: promoting wound healing, tissue repair and prevention of tissue death, relief of inflammation in chronic diseases and injuries with its associated pain and edema, relief of
The first law of photobiology states that for low power visible light to have any effect on a living biological system, the photons must be absorbed by electronic absorption bands belonging to some molecular photoacceptors, or chromophores (Sutherland 2002; Huang, Chen et al. 2009). Red and near infrared light is absorbed by photoreceptors contained in the protein components of the respiratory chain located in mitochondria, in particular cytochrome c oxidase and flavoproteins like NADH-dehydrogenase. This can lead to a short time activation of respiratory chain and oxidation of NADH pool leading to changes in the redox state of both mitochondria and cytoplasm, leading to increased ATP production, and biological responses at the cellular level through cascades of biochemical reactions (Karu 1989; Karu, Pyatibrat et al. 2004; Karu and Kolyakov 2005). These effects in turn lead to increased cell proliferation and migration, modulation in levels of cytokines, growth factors, inflammatory mediators, and increased tissue oxygenation. The results of these biochemical and cellular changes in animals and patients lead to valuable biological effects such as promoting wound healing and tissue repair, relief of inflammation, pain reduction, and amelioration of damage after heart attacks, stroke, nerve injury and even retinal toxicity
Low-level laser therapy (LLLT) is increasingly recognized as an appropriate option for pain relief. In fact, it is for this indication that biostimulative lasers have been approved for marketing by the U.S. Food and Drug Administration through the premarket notification/510(k) (Gigo-Benato, Geuna et al. 2005). Many studies have demonstrated the efficacy of phototherapy in various pain syndromes (Tuner and Hode 2010). Responding to the increasing levels of evidence, the World Health Organization's Committee of the Decay
neurogenic pain and some neurological problems (Hamblin, Waynant et al. 2006).
phototherapy (LPT)" which is an emerging terminology (Tuner and Hode 2010).
**2.2 A brief review on biological effects of low level therapeutic lasers**
The history of the use of laser phototherapy in medicine goes back to the late 1960s, only eight years after the invention of the first laser (Ruby laser) by Theodore Maiman. In 1967, Endre Mester in Semmelweis University, Budapest, Hungary decided to test if laser radiation might cause cancer in mice. He shaved the dorsal hair of the mice, divided them into two groups and irradiated the shaved areas with a low powered ruby laser (694-nm) in one group. They did not get cancer and to his surprise the hair on the treated group grew back more quickly than the untreated group. This was the first demonstration of "laser biostimulation" (Hamblin, Waynant et al. 2006).
In early 1960's, the first low level laser, Helium-Neon was developed by Professor Ali Javan. It emits visible, red light with a wavelength of 632.8nm. This low power laser has been used extensively in experimental and therapeutic studies. Today, the semiconductor lasers, including InGaAlP lasers (633-700nm), GaAlAs lasers (780-890nm, invisible, near infrared area), GaAs laser (904nm, invisible, near infrared area) are widely used by researchers and clinicians.
LLLT originally thought to be a peculiar property of laser light (soft or cold lasers); the subject has now broadened, using non-coherent light (light-emitting diodes, LEDs). Today, medical treatment with coherent-light sources (lasers) or noncoherent light (LEDs) has passed through its childhood and adolescence (Hamblin, Waynant et al. 2006). Currently, low-level laser (or light) therapy (LLLT) is practiced as part of physical therapy in many parts of the world. Although LLLT was used mainly for wound healing and pain relief, the medical applications of LLLT have broadened to include diseases such as stroke, myocardial infarction, and degenerative or traumatic brain disorders (Hashmi, Huang et al. 2010).
Although many experimental and clinical studies have reported the positive effects of phototherapy to promote wound healing , pain relief and anti-inflammatory effects, some negative reports also have been published, further confounding the issue (Demidova-Rice, Salomatina et al. 2007), for instance regarding the application of laser phototherapy on wound healing (Posten, Wrone et al. 2005). This controversy seems to be due to two main reasons; first of all, the basic biochemical mechanisms underlying these biological effects are not completely understood. Secondly, the complexity of rationally choosing amongst a large number of laser irradiation parameters (such as wavelength, fluence, power density, pulse structure and treatment timing), inappropriate anatomical treatment location and concurrent patient medication (such as steroidal and non-steroidal anti-inflammatories which can inhibit healing) has led to conflicting results and publication of a number of unfavourable, as well as many favourable, studies. In particular a biphasic dose response has been frequently observed where low levels of light have a much better effect than higher levels (Gigo-Benato, Geuna et al. 2005; Aimbire, Albertini et al. 2006; Hamblin, Waynant et al. 2006; Goncalves, Souza et al. 2007; Huang, Chen et al. 2009; Hamblin 2010).
It should be noticed that LLLT has a diversified terminology. It is also called "cold laser", "soft laser", "biostimulation" , "photobiomodulation", "low intensity laser therapy" , "low energy laser therapy", "laser phototherapy (LPT)", "laser therapy", and "non-ablative irradiation". Some investigators state that using frequent terms, such as "low power laser therapy" is misleading, since high power lasers, too, can be used for laser phototherapy (Tuner and Hode 2010) (as we will discuss in the next sections, CO2 laser is applied as a low
The history of the use of laser phototherapy in medicine goes back to the late 1960s, only eight years after the invention of the first laser (Ruby laser) by Theodore Maiman. In 1967, Endre Mester in Semmelweis University, Budapest, Hungary decided to test if laser radiation might cause cancer in mice. He shaved the dorsal hair of the mice, divided them into two groups and irradiated the shaved areas with a low powered ruby laser (694-nm) in one group. They did not get cancer and to his surprise the hair on the treated group grew back more quickly than the untreated group. This was the first demonstration of "laser
In early 1960's, the first low level laser, Helium-Neon was developed by Professor Ali Javan. It emits visible, red light with a wavelength of 632.8nm. This low power laser has been used extensively in experimental and therapeutic studies. Today, the semiconductor lasers, including InGaAlP lasers (633-700nm), GaAlAs lasers (780-890nm, invisible, near infrared area), GaAs laser (904nm, invisible, near infrared area) are widely used by researchers and
LLLT originally thought to be a peculiar property of laser light (soft or cold lasers); the subject has now broadened, using non-coherent light (light-emitting diodes, LEDs). Today, medical treatment with coherent-light sources (lasers) or noncoherent light (LEDs) has passed through its childhood and adolescence (Hamblin, Waynant et al. 2006). Currently, low-level laser (or light) therapy (LLLT) is practiced as part of physical therapy in many parts of the world. Although LLLT was used mainly for wound healing and pain relief, the medical applications of LLLT have broadened to include diseases such as stroke, myocardial infarction, and degenerative or traumatic brain disorders (Hashmi, Huang et
Although many experimental and clinical studies have reported the positive effects of phototherapy to promote wound healing , pain relief and anti-inflammatory effects, some negative reports also have been published, further confounding the issue (Demidova-Rice, Salomatina et al. 2007), for instance regarding the application of laser phototherapy on wound healing (Posten, Wrone et al. 2005). This controversy seems to be due to two main reasons; first of all, the basic biochemical mechanisms underlying these biological effects are not completely understood. Secondly, the complexity of rationally choosing amongst a large number of laser irradiation parameters (such as wavelength, fluence, power density, pulse structure and treatment timing), inappropriate anatomical treatment location and concurrent patient medication (such as steroidal and non-steroidal anti-inflammatories which can inhibit healing) has led to conflicting results and publication of a number of unfavourable, as well as many favourable, studies. In particular a biphasic dose response has been frequently observed where low levels of light have a much better effect than higher levels (Gigo-Benato, Geuna et al. 2005; Aimbire, Albertini et al. 2006; Hamblin, Waynant et
al. 2006; Goncalves, Souza et al. 2007; Huang, Chen et al. 2009; Hamblin 2010).
It should be noticed that LLLT has a diversified terminology. It is also called "cold laser", "soft laser", "biostimulation" , "photobiomodulation", "low intensity laser therapy" , "low energy laser therapy", "laser phototherapy (LPT)", "laser therapy", and "non-ablative irradiation". Some investigators state that using frequent terms, such as "low power laser therapy" is misleading, since high power lasers, too, can be used for laser phototherapy (Tuner and Hode 2010) (as we will discuss in the next sections, CO2 laser is applied as a low
biostimulation" (Hamblin, Waynant et al. 2006).
clinicians.
al. 2010).
level (therapeutic laser) in NACLT, too). Some of the researchers favour the term "laser phototherapy (LPT)" which is an emerging terminology (Tuner and Hode 2010).
#### **2.2 A brief review on biological effects of low level therapeutic lasers**
Low level laser (or light) therapy (LLLT) is the application of light (usually a low power laser or LED in the range of 1mW – 500mW) to a pathology to promote wound healing and tissue repair, reduce inflammation and relieve pain. The light is typically of narrow spectral width in the red or near infrared spectrum (600nm – 1000nm); at power densities (between 1mw-5W/cm2) (Huang, Chen et al. 2009), not associated with macroscopic thermal effects, in contrast to thermally mediated surgical applications (Chow, David et al. 2007). In using high power surgical lasers, the collimation of laser light leads to the emission of a narrow, intense beam of light and is used for precise tissue destruction (photothermal effect). However, in LLLT, light radiation intensities are so low that the resulting biological effects are ascribable to physical or chemical changes associated with the interaction of cells and tissues with the laser radiation, and not simply to a result of heating (Snyder, Byrnes et al. 2002; Gigo-Benato, Geuna et al. 2005).
The main areas of medicine where laser phototherapy has a known and major role are as follows: promoting wound healing, tissue repair and prevention of tissue death, relief of inflammation in chronic diseases and injuries with its associated pain and edema, relief of neurogenic pain and some neurological problems (Hamblin, Waynant et al. 2006).
The first law of photobiology states that for low power visible light to have any effect on a living biological system, the photons must be absorbed by electronic absorption bands belonging to some molecular photoacceptors, or chromophores (Sutherland 2002; Huang, Chen et al. 2009). Red and near infrared light is absorbed by photoreceptors contained in the protein components of the respiratory chain located in mitochondria, in particular cytochrome c oxidase and flavoproteins like NADH-dehydrogenase. This can lead to a short time activation of respiratory chain and oxidation of NADH pool leading to changes in the redox state of both mitochondria and cytoplasm, leading to increased ATP production, and biological responses at the cellular level through cascades of biochemical reactions (Karu 1989; Karu, Pyatibrat et al. 2004; Karu and Kolyakov 2005). These effects in turn lead to increased cell proliferation and migration, modulation in levels of cytokines, growth factors, inflammatory mediators, and increased tissue oxygenation. The results of these biochemical and cellular changes in animals and patients lead to valuable biological effects such as promoting wound healing and tissue repair, relief of inflammation, pain reduction, and amelioration of damage after heart attacks, stroke, nerve injury and even retinal toxicity (Hamblin, Waynant et al. 2006).
#### **2.3 Pain relieving effects of low level therapeutic lasers**
Low-level laser therapy (LLLT) is increasingly recognized as an appropriate option for pain relief. In fact, it is for this indication that biostimulative lasers have been approved for marketing by the U.S. Food and Drug Administration through the premarket notification/510(k) (Gigo-Benato, Geuna et al. 2005). Many studies have demonstrated the efficacy of phototherapy in various pain syndromes (Tuner and Hode 2010). Responding to the increasing levels of evidence, the World Health Organization's Committee of the Decay
Non-Thermal, Non-Ablative CO2 Laser Therapy (NACLT):
**3. Pain relieving effects of carbon dioxide lasers**
Mansouri et al. 2009; Zand, Najafi et al. 2010).
(Tuner and Hode 2010).
two main groups:
1996; Tuner and Hode 2010)*.*
(Duncavage and Ossoff 1986).
A New Approach to Relieve Pain in Some Painful Oral Diseases 391
• Systemic effect, some researchers propose that laser phototherapy has both a local effect in the area treated by laser light, and a systemic effect through the release of metabolites
Laser phototherapy is a relatively new application of carbon dioxide lasers, in spite of the fact that papers on the subject were published as early as the mid-eighties (Tuner and Hode 2010). CO2 laser biostimulative and pain relieving effects can be assessed in the following
• The lower post operative pain following CO2 laser surgery compared to conventional surgery, which is attributed to the simultaneous low level laser therapy (photobiomodulation) effects of high power CO2 laser irradiation (Tuner and Hode 2010). • Application of CO2 laser, as a phototherapeutic laser. "For using CO2 laser as a low power therapeutic laser, one needs to make the beam wide enough not to burn. Another option is to scan rapidly over the lesion with a narrow beam. Therefore the power density or average power is kept low enough to avoid burning and their incident energy and power density are set within the low intensity biostimulative laser therapy range" (Tuner and Hode 2010). NACLT is a new technique, in which CO2 laser is used in non-thermal, non-ablative manner as a biomodulative, low intensity laser for immediate and significant pain relief. (Zand, Ataie-Fashtami et al. 2009; Zand,
This section is organized as follows. At first some clinical studies which demonstrate the pain relieving effects of high power surgical CO2 lasers are reviewed, and then low power biostimulative CO2 laser studies are briefly reviewed. In the next section, NACLT as a new
Some investigators who used the classical thermal effects of CO2 laser (vaporization, cutting and coagulation) reported less post-operative pain following CO2 laser surgery, and a reduced requirement for post-operative analgesics (Duncavage and Ossoff 1986; Colvard and Kuo 1991; Demidov, Rykov et al. 1992; Chia, Darzi et al. 1995; Kaplan, Kott et al. 1996; Andre 2003).
Kaplan, one of the pioneers of CO2 laser surgery, attributed the excellent healing and lower post operative pain experienced with CO2 laser surgery compared to conventional surgery to the simultaneous laser therapy effects of CO2 laser irradiation. Kaplan stated that laser surgery and laser therapy should be regarded *as two sides of the same coin* (Kaplan, Kott et al.
Duncavage reported that the advantages of the CO2 laser surgery included homeostasis, precise visualization, and less edema and pain than the conventional techniques
Colvard and Kuo used high-power, ablative CO2 laser at a power output of 4 W under local anesthesia for painful minor aphthous ulcers of 14 patients. In all, 88.8% of the patients in the study were completely pain free following anesthetic resolution, and none of the
patients required post-operative medication for pain relief. (Colvard and Kuo 1991).
low power, biomodulative laser therapy protocol for pain reduction is introduced.
**3.1 Pain relieving effects of carbon dioxide laser as a high power surgical laser**
of the Bone and Joint has also recently incorporated LLLT into guidelines for treatment of neck pain (Haldeman, Carroll et al. 2008; Chow, Armati et al. 2011).
*"Efficacy of LLLT in painful clinical conditions has been established by several recent systematic reviews and meta-analyses [level 1 evidence, according to the Australian Government, NHMRC (1999)] (Chow, David et al. 2007). This level of evidence relates to chronic neck pain(Chow and Barnsley 2005), tendinitis (Bjordal, Couppe et al. 2001), chronic joint disorders (Bjordal, Couppe et al. 2003), and chronic pain (Enwemeka, Parker et al. 2004) . Randomized controlled trials (RCTs) provide level II evidence for the efficacy of laser therapy in chronic low back pain (Umegaki, Tanaka et al. 1989; Soriano and Rios 1998; Basford, Sheffield et al. 1999) . In other reviews of laser therapy for painful conditions such as rheumatoid arthritis (Brosseau, Robinson et al. 2005) and musculoskeletal pain (Gam, Thorsen et al. 1993; De Bie, De. Vet et al. 1998), the evidence is equivocal. Such variability in outcomes may be due to the multiplicity of parameters used, including wavelengths, energy, and power densities, with differing frequencies of application(Chow and Barnsley 2005)." (Chow, David et al. 2007)*
#### **2.4 Mechanisms of analgesic effects of low level laser therapy**
The basic biological mechanisms behind the analgesic effects of conventional LLLT are not completely understood. Some of the explanations for these pain relieving effects are as follows:
of the Bone and Joint has also recently incorporated LLLT into guidelines for treatment of
*"Efficacy of LLLT in painful clinical conditions has been established by several recent systematic reviews and meta-analyses [level 1 evidence, according to the Australian Government, NHMRC (1999)] (Chow, David et al. 2007). This level of evidence relates to chronic neck pain(Chow and Barnsley 2005), tendinitis (Bjordal, Couppe et al. 2001), chronic joint disorders (Bjordal, Couppe et al. 2003), and chronic pain (Enwemeka, Parker et al. 2004) . Randomized controlled trials (RCTs) provide level II evidence for the efficacy of laser therapy in chronic low back pain (Umegaki, Tanaka et al. 1989; Soriano and Rios 1998; Basford, Sheffield et al. 1999) . In other reviews of laser therapy for painful conditions such as rheumatoid arthritis (Brosseau, Robinson et al. 2005) and musculoskeletal pain (Gam, Thorsen et al. 1993; De Bie, De. Vet et al. 1998), the evidence is equivocal. Such variability in outcomes may be due to the multiplicity of parameters used, including wavelengths, energy, and power densities, with differing frequencies of*
The basic biological mechanisms behind the analgesic effects of conventional LLLT are not completely understood. Some of the explanations for these pain relieving effects are as follows: • Reversible blockage of action potential generation of nociceptive signals in primary afferent neurons and specific reversible inhibition and functional impairment of Aδ and C fibers, which transmit nociceptive stimuli (Wakabayashi, Hamba et al. 1993; Kasai, Kono et al. 1996; Orchardson, Peacock et al. 1997; Chow, Armati et al. 2011). • Increase in β-endorphin synthesis and release (Labajos 1988; Montesinos 1988;
• Inhibiting cyclooxygenase, interrupting conversion of arachidonic acid in to prostaglandins, especially prostaglandin E2 (PGE2) (Shimizu, Yamaguchi et al. 1995;
• Suppression of Substance P, a neuropeptide associated with nociception (Ohno 1997). • Suppression of bradykinin activity, a pro-inflammatory neuropeptide that irritates nociceptors and is a key element in clinical pain and the associated inflammation
• Increased production of serotonin, affecting negatively neurotransmission (Tuner and
• Decreased inflammation and subsequently decreased inflammatory sensitization of
• Improvement of local microcirculation, increased tissue oxygenation, shift of metabolism from anaerobic to aerobic pathways, decreased production of acidic
• Increased lymphatic flow and consequently reducing edema, which leads to decreased
• Involvement of nitric oxide in analgesic effects of therapeutic lasers (Mrowiec 1997) • Single oxygen production, which in small amounts, is very important in biochemical
• Increased synaptic activity of acetylcholine esterase (Simunovic 2000)
processes and may be important in biostimulation
metabolites which stimulate the pain receptors
neck pain (Haldeman, Carroll et al. 2008; Chow, Armati et al. 2011).
*application(Chow and Barnsley 2005)." (Chow, David et al. 2007)*
**2.4 Mechanisms of analgesic effects of low level laser therapy**
Hagiwara, Iwasaka et al. 2007).
Mizutani, Musya et al. 2004).
Hode 2010)
(Maeda 1989; Jimbo, Noda et al. 1998)
small-diameter afferent nerve endings
sensitization of pain receptors
• Systemic effect, some researchers propose that laser phototherapy has both a local effect in the area treated by laser light, and a systemic effect through the release of metabolites (Tuner and Hode 2010).
#### **3. Pain relieving effects of carbon dioxide lasers**
Laser phototherapy is a relatively new application of carbon dioxide lasers, in spite of the fact that papers on the subject were published as early as the mid-eighties (Tuner and Hode 2010). CO2 laser biostimulative and pain relieving effects can be assessed in the following two main groups:
This section is organized as follows. At first some clinical studies which demonstrate the pain relieving effects of high power surgical CO2 lasers are reviewed, and then low power biostimulative CO2 laser studies are briefly reviewed. In the next section, NACLT as a new low power, biomodulative laser therapy protocol for pain reduction is introduced.
#### **3.1 Pain relieving effects of carbon dioxide laser as a high power surgical laser**
Some investigators who used the classical thermal effects of CO2 laser (vaporization, cutting and coagulation) reported less post-operative pain following CO2 laser surgery, and a reduced requirement for post-operative analgesics (Duncavage and Ossoff 1986; Colvard and Kuo 1991; Demidov, Rykov et al. 1992; Chia, Darzi et al. 1995; Kaplan, Kott et al. 1996; Andre 2003).
Kaplan, one of the pioneers of CO2 laser surgery, attributed the excellent healing and lower post operative pain experienced with CO2 laser surgery compared to conventional surgery to the simultaneous laser therapy effects of CO2 laser irradiation. Kaplan stated that laser surgery and laser therapy should be regarded *as two sides of the same coin* (Kaplan, Kott et al. 1996; Tuner and Hode 2010)*.*
Duncavage reported that the advantages of the CO2 laser surgery included homeostasis, precise visualization, and less edema and pain than the conventional techniques (Duncavage and Ossoff 1986).
Colvard and Kuo used high-power, ablative CO2 laser at a power output of 4 W under local anesthesia for painful minor aphthous ulcers of 14 patients. In all, 88.8% of the patients in the study were completely pain free following anesthetic resolution, and none of the patients required post-operative medication for pain relief. (Colvard and Kuo 1991).
Non-Thermal, Non-Ablative CO2 Laser Therapy (NACLT):
for the treatment of chronic pharyngitis (Nicola and Nicola 1994).
of treatment protocols deserves further studies (Longo, Simunovic et al. 1997).
and Hode 2010).
and Hode 2010).
**4. NACLT (Non-Ablative CO2 Laser Therapy)**
aggravation of the oral lesions (Elad, Or et al. 2003).
A New Approach to Relieve Pain in Some Painful Oral Diseases 393
their incident energy and power density were set within the laser therapy range by spreading out the beam over such a large surface that the laser did not cause burning (Tuner
Nicola used CO2 low power laser treating chronic pharyngitis. 85 patients with non-specific chronic pharyngitis were elected to be treated: Group Ι, 40 patients with predominance of hyperaemic aspect; and group II, 45 patients, predominance of hypertrophied aspect. Both groups were treated for 8 to 10 sessions. They concluded that this method was very suitable
In another study, 846 patients with different types of fibromyositic rheumatisms were submitted to defocalized laser therapy from 1980 to 1995. They employed Diodes and CO2 lasers. Control groups were used to compare results with those of traditional methods. Results were evaluated on the basis of subjective (such as local pain) and objective criteria. On the whole, results were positive in comparison with other methods both as regards recovery time and persistence of results. Approximately 2/3 of the patients benefited from the treatment indicated that there were greater advantages in use of laser over other presently available methods. Longo and his collogues recommended that standardalization
The CO2-laser can also be used as an acupuncture tool. Simulation of acupuncture points has been carried out both with biostimulating power densities (e.g.100mW/cm2) and burning/coagulation/ evaporation power densities. Some clinics state that CO2 lasers give better results on acupuncture points than HeNe lasers. "As the CO2 laser's beam cannot penetrate more than around 0.5 mm into tissue, the effects must be due to the influence of the laser energy on the cells encountered, so that signal substances are released and then circulate in the organism. This indirectly confirms the hypothesis that conventional laser therapy has both a local effect in the area treated by laser light, and a systemic effect through the release of metabolites. It is well known that these kinds of secondary effect also occur at the traditional wavelengths of 633, 830, and 904 nm" (Tuner
Recently, there have been few reports about using CO2 laser in non-ablative manner to reduce pain in painful mucosal lesions (Elad, Or et al. 2003; Sharon-Buller and Sela 2004; Zand, Ataie-Fashtami et al. 2009; Zand, Mansouri et al. 2009; Zand, Najafi et al. 2010).
In a report, Elad et al suggested that CO2 laser treatment could be of benefit to control pain in severe oral chronic graft-versus-host-disease (Elad, Or et al. 2003). In this study, the oral lesions of four patients were irradiated by CO2 laser during 17 sessions. The CO2 laser was applied, over mucosal lesions, using 1W power for 2-3s/1mm2. The treated mucosa was kept wet during the process. The treatment was pain free, and anesthesia was not required. The average VAS scores before, during, and immediately after CO2 laser treatment were 8.09, 3.47, and 4.88 respectively. There was no visual damage to the oral mucosa and no
In another case report, aphthous ulcers of two patients were irradiated with CO2 laser at 1.0- 1.5 W power, with a defocused hand piece for 5 seconds. Before laser irradiation, a thin film
Demidov and his colleagues used high-energy CO2 laser as a laser scalpel in 120 cases of breast surgery including 70 operations for cancer. They reported reduced pain sensitivity in the region of the wounds in the postoperative period (Demidov, Rykov et al. 1992).
In another study Chia reported that high-power CO2 laser haemorrhoidectomy was associated with a reduced requirement for post-operative analgesia (Chia, Darzi et al. 1995).
Andre used classical high power CO2 laser at 10–15 W in continuous mode under local anesthesia for treating ingrowing nails of 302 patients. He reported that ingrowing nails were easily operated with the CO2 laser; bleeding was minimal, infection was rare, the wounds healed without exhudative drainage and cosmetic results were good. In addition the immediate post-operative pain was less severe than after classical nail surgery with scalpel (Andre 2003).
In another study, Tada et al. compared the clinical effects and postoperative course of the scanning CO2 laser surgery and conventional surgical method to evaluate the clinical effectiveness of the former for the treatment of ingrowing nail deformities. They demonstrated that statistically, the operating time and the duration of postoperative pain were reduced significantly by the scanning CO2 laser. Furthermore, patients treated with CO2 laser were able to return to daily life significantly sooner (Tada, Hatoko et al. 2004).
Kaviani et al. investigated whether the CO2 laser surgery was superior to conventional surgical techniques for minor breast surgery in a randomized clinical trial. They demonstrated that application of the CO2 laser in breast mass biopsy had some advantages, including a lower requirement for local anesthetic and a lower rate of intraoperative bleeding; however it did not reduce the postoperative pain grade severity (Kaviani, Fateh et al. 2008).
Demetriades used ablative CO2 laser in painful oral aphthous ulcer of a patient with Behçet's Syndrome. His experience showed transient pain relief following ablative CO2 laser irradiation (Demetriades, Hansford et al. 2009).
#### **3.2 Pain relieving effects of carbon dioxide laser as a low level (therapeutic) laser**
It is interesting to know that in addition to classical low level therapeutic lasers, surgical lasers could also be used as therapeutic instruments, for example; defocused CO2 laser 10,600 nm, defocused ruby laser 694 nm and defocused Nd:YAG laser 1064 nm can be used for photobiomodulation. "When high power laser are used for biomodulation, one only needs to make the beam wide enough not to burn. The patient will then feel only a mild heat. An alternative is to scan rapidly over the lesion with a narrow beam. Therefore the power density or average power is kept low enough to avoid burning and their incident energy and power density are set within the low intensity laser therapy range" (Tuner and Hode 2010).
The famous investigation of Mester with a low powered ruby laser (694-nm) on the shaved areas of the mice and quickly growing back of hairs can be an example of using a surgical laser as a therapeutic, biostimulative one (please see 2.1. History).
At Uppsala Academic Hospital, a CO2-laser was tested successfully for biostimulative treatment of epicondylitis. This method was called EDL (Emitted Defocused Laser-light). It should be noted, however, that CO2-lasers were not used as surgical lasers in this study;
Demidov and his colleagues used high-energy CO2 laser as a laser scalpel in 120 cases of breast surgery including 70 operations for cancer. They reported reduced pain sensitivity in
In another study Chia reported that high-power CO2 laser haemorrhoidectomy was associated with a reduced requirement for post-operative analgesia (Chia, Darzi et al. 1995). Andre used classical high power CO2 laser at 10–15 W in continuous mode under local anesthesia for treating ingrowing nails of 302 patients. He reported that ingrowing nails were easily operated with the CO2 laser; bleeding was minimal, infection was rare, the wounds healed without exhudative drainage and cosmetic results were good. In addition the immediate post-operative pain was less severe than after classical nail surgery with
In another study, Tada et al. compared the clinical effects and postoperative course of the scanning CO2 laser surgery and conventional surgical method to evaluate the clinical effectiveness of the former for the treatment of ingrowing nail deformities. They demonstrated that statistically, the operating time and the duration of postoperative pain were reduced significantly by the scanning CO2 laser. Furthermore, patients treated with CO2 laser were able to return to daily life significantly sooner (Tada, Hatoko et al. 2004).
Kaviani et al. investigated whether the CO2 laser surgery was superior to conventional surgical techniques for minor breast surgery in a randomized clinical trial. They demonstrated that application of the CO2 laser in breast mass biopsy had some advantages, including a lower requirement for local anesthetic and a lower rate of intraoperative bleeding; however
Demetriades used ablative CO2 laser in painful oral aphthous ulcer of a patient with Behçet's Syndrome. His experience showed transient pain relief following ablative CO2 laser
It is interesting to know that in addition to classical low level therapeutic lasers, surgical lasers could also be used as therapeutic instruments, for example; defocused CO2 laser 10,600 nm, defocused ruby laser 694 nm and defocused Nd:YAG laser 1064 nm can be used for photobiomodulation. "When high power laser are used for biomodulation, one only needs to make the beam wide enough not to burn. The patient will then feel only a mild heat. An alternative is to scan rapidly over the lesion with a narrow beam. Therefore the power density or average power is kept low enough to avoid burning and their incident energy and power density are set within the low intensity laser therapy range" (Tuner and
The famous investigation of Mester with a low powered ruby laser (694-nm) on the shaved areas of the mice and quickly growing back of hairs can be an example of using a surgical
At Uppsala Academic Hospital, a CO2-laser was tested successfully for biostimulative treatment of epicondylitis. This method was called EDL (Emitted Defocused Laser-light). It should be noted, however, that CO2-lasers were not used as surgical lasers in this study;
laser as a therapeutic, biostimulative one (please see 2.1. History).
**3.2 Pain relieving effects of carbon dioxide laser as a low level (therapeutic) laser**
it did not reduce the postoperative pain grade severity (Kaviani, Fateh et al. 2008).
irradiation (Demetriades, Hansford et al. 2009).
the region of the wounds in the postoperative period (Demidov, Rykov et al. 1992).
scalpel (Andre 2003).
Hode 2010).
their incident energy and power density were set within the laser therapy range by spreading out the beam over such a large surface that the laser did not cause burning (Tuner and Hode 2010).
Nicola used CO2 low power laser treating chronic pharyngitis. 85 patients with non-specific chronic pharyngitis were elected to be treated: Group Ι, 40 patients with predominance of hyperaemic aspect; and group II, 45 patients, predominance of hypertrophied aspect. Both groups were treated for 8 to 10 sessions. They concluded that this method was very suitable for the treatment of chronic pharyngitis (Nicola and Nicola 1994).
In another study, 846 patients with different types of fibromyositic rheumatisms were submitted to defocalized laser therapy from 1980 to 1995. They employed Diodes and CO2 lasers. Control groups were used to compare results with those of traditional methods. Results were evaluated on the basis of subjective (such as local pain) and objective criteria. On the whole, results were positive in comparison with other methods both as regards recovery time and persistence of results. Approximately 2/3 of the patients benefited from the treatment indicated that there were greater advantages in use of laser over other presently available methods. Longo and his collogues recommended that standardalization of treatment protocols deserves further studies (Longo, Simunovic et al. 1997).
The CO2-laser can also be used as an acupuncture tool. Simulation of acupuncture points has been carried out both with biostimulating power densities (e.g.100mW/cm2) and burning/coagulation/ evaporation power densities. Some clinics state that CO2 lasers give better results on acupuncture points than HeNe lasers. "As the CO2 laser's beam cannot penetrate more than around 0.5 mm into tissue, the effects must be due to the influence of the laser energy on the cells encountered, so that signal substances are released and then circulate in the organism. This indirectly confirms the hypothesis that conventional laser therapy has both a local effect in the area treated by laser light, and a systemic effect through the release of metabolites. It is well known that these kinds of secondary effect also occur at the traditional wavelengths of 633, 830, and 904 nm" (Tuner and Hode 2010).
### **4. NACLT (Non-Ablative CO2 Laser Therapy)**
Recently, there have been few reports about using CO2 laser in non-ablative manner to reduce pain in painful mucosal lesions (Elad, Or et al. 2003; Sharon-Buller and Sela 2004; Zand, Ataie-Fashtami et al. 2009; Zand, Mansouri et al. 2009; Zand, Najafi et al. 2010).
In a report, Elad et al suggested that CO2 laser treatment could be of benefit to control pain in severe oral chronic graft-versus-host-disease (Elad, Or et al. 2003). In this study, the oral lesions of four patients were irradiated by CO2 laser during 17 sessions. The CO2 laser was applied, over mucosal lesions, using 1W power for 2-3s/1mm2. The treated mucosa was kept wet during the process. The treatment was pain free, and anesthesia was not required. The average VAS scores before, during, and immediately after CO2 laser treatment were 8.09, 3.47, and 4.88 respectively. There was no visual damage to the oral mucosa and no aggravation of the oral lesions (Elad, Or et al. 2003).
In another case report, aphthous ulcers of two patients were irradiated with CO2 laser at 1.0- 1.5 W power, with a defocused hand piece for 5 seconds. Before laser irradiation, a thin film
Non-Thermal, Non-Ablative CO2 Laser Therapy (NACLT):
2009; Zand, Najafi et al. 2010).
**4.1 NACLT protocol**
burning.
evaluated in further studies.
A New Approach to Relieve Pain in Some Painful Oral Diseases 395
There are some differences between analgesic effects of NACLT and the other classical low power lasers. The analgesic effect in LLLT is usually gradual, cumulative, and multi-session (Pinheiro, Cavalcanti et al. 1998; Gur, Karakoc et al. 2002; Gur, Sarac et al. 2004; Nes and Posso 2005; Chow, Heller et al. 2006; Djavid, Mehrdad et al. 2007; Bjordal, Bensadoun et al. 2011; Iwatsuki, Yoshimine et al. 2011; Ribeiro, de Aguiar et al. 2011). In contrast, the pain relieving effect of NACLT is immediate, dramatic and more sustained than conventional phototherapeutic lasers, so that immediately after NACLT, the patients of the studies have been able to eat and drink easily (Zand, Ataie-Fashtami et al. 2009; Zand, Mansouri et al.
At first, all standard precautions of laser utilization should be considered. Before laser irradiation, the patient and surgical staff should be given appropriate protective eye shields and eye glasses matched to the laser wavelength (10,600 nm) to protect inadvertent laser impact.1 Before laser irradiation, a layer of a transparent, non-anesthetic gel with high water content is placed on the lesion. In our studies, we use a transparent gel (Abzar Darman Co., Iran) with 87.5% water content, with a thickness of 3–4 mm on the lesion, is used. The CO2 laser is operated at 1W power, with a de-focused hand piece, 5–6 mm distant from the mucosal surface, in continuous mode, scanning rapidly over the lesion with a circular motion. The irradiation time depends on the size of the lesion. For example, in our studies the irradiation time for minor aphthous ulcers is about 5-10 seconds. When using NACLT for larger lesions, such as; pemphigus vulgaris, every one centimeter square of the lesion has been irradiated for 5 seconds in each pass, and repeated immediately if the contact pain of the lesion persists. Between the passes, the prior gel should be wiped gently and replaced by a new layer of gel, otherwise the water content of the gel will decrease which may lead to increasing the beam absorption by the lesion and subsequent tissue injury and even
1 In NACLT studies, we used eye glasses matched to the CO2 laser wavelength (10,600 nm), but we presume that it might be safer to use eye glasses matched to both the 10,600 nm and the guiding beam to protect the eyes from the reflected beam from the surface of the gel, the presumption that should be
Fig. 1. Minor aphthous ulcers before and immediately after NACLT
of Elmex Gel (a transparent gel with high water content) was placed on the lesions to reduce the beam absorption by the soft tissue. Anesthesia was not required since the treatment was not painful. The patients reported immediate pain relief after laser irradiation (Sharon-Buller and Sela 2004).
In these two reports, water (Elad, Or et al. 2003) and transparent gel with high water content (Sharon-Buller and Sela 2004) were used to reduce the beam absorption by the soft tissue.
These interesting results encouraged us to conduct a randomized controlled clinical trial to confirm the pain-relieving effect of CO2 laser in minor aphthous ulcers as a prototype of painful oral lesions (Zand, Ataie-Fashtami et al. 2009). The results of this clinical trial demonstrated that a single session of low-intensity, non-thermal, CO2 laser irradiation could reduce pain in minor aphthous ulcers immediately and significantly, with no visible side effects (Zand, Ataie-Fashtami et al. 2009), the technique was called NACLT (Non-Ablative CO2 Laser Therapy) afterwards.
In order to use the CO2 laser as a phototherapeutic laser for NACLT, the CO2 laser beam is irradiated through a thick layer of transparent gel with high water content to reduce the beam absorption by the tissue. In addition, the CO2 laser is operated with a de-focused hand piece 5–6 mm distant from the mucosal surface, scanning rapidly over the lesion with circular motion (Zand, Ataie-Fashtami et al. 2009; Zand, Mansouri et al. 2009; Zand, Najafi et al. 2010). The results of the powermetry have shown that the final laser power output, after passing through the gel, is reduced to 2-5 mW, which is in the range of low power lasers. Thermometry has also shown no significant temperature rise on the surface of the ulcers and under the gel layer during the laser irradiation, supporting the low power nature of the applied CO2 laser in NACLT(Zand, Ataie-Fashtami et al. 2009). It appears that due to high water content of the gel, it absorbs CO2 laser irradiation considerably, resulting in significant drops in the power output, by a factor of 200-500. In fact by irradiation of CO2 laser through a transparent gel with high water content, CO2 laser can be used as a nondestructive, non-thermal laser to reduce pain in some oral lesions immediately and significantly. This technique was called non-ablative CO2 laser therapy (NACLT), in order to avoid any confusion with classical high power thermal CO2 laser effects. This technique could also be called non-thermal CO2 laser therapy (NTCLT) to avoid misinterpretation with fractional non-ablative lasers used for skin rejuvenation (Zand, Ataie-Fashtami et al. 2009).
NACLT is a pain free procedure and neither systemic nor topical anesthesia is required. The patients don't feel warmth in their lesions during the procedure, in contrast to conventional defocused CO2 laser therapy in which the patients feel mild warmth. Up to now, in the series of studies about the analgesic effects of NACLT, we have observed no visual effects of thermal damage to the oral mucosa such as tissue ablation, ulceration, erythema or aggravation of the lesions following the careful application of the technique (Zand, Ataie-Fashtami et al. 2009; Zand, Mansouri et al. 2009; Zand, Najafi et al. 2010). So that the beforeafter NACLT pictures of the lesions cannot be differentiated from each other easily (Figure 1). Since there is no tissue ablation and plume formation during NACLT, in contrast to the classical ablative CO2 laser surgery, it seems rational to conclude that this procedure has no potential for carrying viral particles to the surgeon and other operating room staff (Zand, Ataie-Fashtami et al. 2009).
There are some differences between analgesic effects of NACLT and the other classical low power lasers. The analgesic effect in LLLT is usually gradual, cumulative, and multi-session (Pinheiro, Cavalcanti et al. 1998; Gur, Karakoc et al. 2002; Gur, Sarac et al. 2004; Nes and Posso 2005; Chow, Heller et al. 2006; Djavid, Mehrdad et al. 2007; Bjordal, Bensadoun et al. 2011; Iwatsuki, Yoshimine et al. 2011; Ribeiro, de Aguiar et al. 2011). In contrast, the pain relieving effect of NACLT is immediate, dramatic and more sustained than conventional phototherapeutic lasers, so that immediately after NACLT, the patients of the studies have been able to eat and drink easily (Zand, Ataie-Fashtami et al. 2009; Zand, Mansouri et al. 2009; Zand, Najafi et al. 2010).
Fig. 1. Minor aphthous ulcers before and immediately after NACLT
#### **4.1 NACLT protocol**
394 CO2 Laser – Optimisation and Application
of Elmex Gel (a transparent gel with high water content) was placed on the lesions to reduce the beam absorption by the soft tissue. Anesthesia was not required since the treatment was not painful. The patients reported immediate pain relief after laser irradiation (Sharon-
In these two reports, water (Elad, Or et al. 2003) and transparent gel with high water content (Sharon-Buller and Sela 2004) were used to reduce the beam absorption by the soft tissue.
These interesting results encouraged us to conduct a randomized controlled clinical trial to confirm the pain-relieving effect of CO2 laser in minor aphthous ulcers as a prototype of painful oral lesions (Zand, Ataie-Fashtami et al. 2009). The results of this clinical trial demonstrated that a single session of low-intensity, non-thermal, CO2 laser irradiation could reduce pain in minor aphthous ulcers immediately and significantly, with no visible side effects (Zand, Ataie-Fashtami et al. 2009), the technique was called NACLT (Non-Ablative
In order to use the CO2 laser as a phototherapeutic laser for NACLT, the CO2 laser beam is irradiated through a thick layer of transparent gel with high water content to reduce the beam absorption by the tissue. In addition, the CO2 laser is operated with a de-focused hand piece 5–6 mm distant from the mucosal surface, scanning rapidly over the lesion with circular motion (Zand, Ataie-Fashtami et al. 2009; Zand, Mansouri et al. 2009; Zand, Najafi et al. 2010). The results of the powermetry have shown that the final laser power output, after passing through the gel, is reduced to 2-5 mW, which is in the range of low power lasers. Thermometry has also shown no significant temperature rise on the surface of the ulcers and under the gel layer during the laser irradiation, supporting the low power nature of the applied CO2 laser in NACLT(Zand, Ataie-Fashtami et al. 2009). It appears that due to high water content of the gel, it absorbs CO2 laser irradiation considerably, resulting in significant drops in the power output, by a factor of 200-500. In fact by irradiation of CO2 laser through a transparent gel with high water content, CO2 laser can be used as a nondestructive, non-thermal laser to reduce pain in some oral lesions immediately and significantly. This technique was called non-ablative CO2 laser therapy (NACLT), in order to avoid any confusion with classical high power thermal CO2 laser effects. This technique could also be called non-thermal CO2 laser therapy (NTCLT) to avoid misinterpretation with fractional non-ablative lasers used for skin rejuvenation (Zand, Ataie-Fashtami et al.
NACLT is a pain free procedure and neither systemic nor topical anesthesia is required. The patients don't feel warmth in their lesions during the procedure, in contrast to conventional defocused CO2 laser therapy in which the patients feel mild warmth. Up to now, in the series of studies about the analgesic effects of NACLT, we have observed no visual effects of thermal damage to the oral mucosa such as tissue ablation, ulceration, erythema or aggravation of the lesions following the careful application of the technique (Zand, Ataie-Fashtami et al. 2009; Zand, Mansouri et al. 2009; Zand, Najafi et al. 2010). So that the beforeafter NACLT pictures of the lesions cannot be differentiated from each other easily (Figure 1). Since there is no tissue ablation and plume formation during NACLT, in contrast to the classical ablative CO2 laser surgery, it seems rational to conclude that this procedure has no potential for carrying viral particles to the surgeon and other operating room staff (Zand,
Buller and Sela 2004).
CO2 Laser Therapy) afterwards.
2009).
Ataie-Fashtami et al. 2009).
At first, all standard precautions of laser utilization should be considered. Before laser irradiation, the patient and surgical staff should be given appropriate protective eye shields and eye glasses matched to the laser wavelength (10,600 nm) to protect inadvertent laser impact.1 Before laser irradiation, a layer of a transparent, non-anesthetic gel with high water content is placed on the lesion. In our studies, we use a transparent gel (Abzar Darman Co., Iran) with 87.5% water content, with a thickness of 3–4 mm on the lesion, is used. The CO2 laser is operated at 1W power, with a de-focused hand piece, 5–6 mm distant from the mucosal surface, in continuous mode, scanning rapidly over the lesion with a circular motion. The irradiation time depends on the size of the lesion. For example, in our studies the irradiation time for minor aphthous ulcers is about 5-10 seconds. When using NACLT for larger lesions, such as; pemphigus vulgaris, every one centimeter square of the lesion has been irradiated for 5 seconds in each pass, and repeated immediately if the contact pain of the lesion persists. Between the passes, the prior gel should be wiped gently and replaced by a new layer of gel, otherwise the water content of the gel will decrease which may lead to increasing the beam absorption by the lesion and subsequent tissue injury and even burning.
1 In NACLT studies, we used eye glasses matched to the CO2 laser wavelength (10,600 nm), but we presume that it might be safer to use eye glasses matched to both the 10,600 nm and the guiding beam to protect the eyes from the reflected beam from the surface of the gel, the presumption that should be evaluated in further studies.
Non-Thermal, Non-Ablative CO2 Laser Therapy (NACLT):
**4.2.1.2 NACLT and minor oral aphthous stomatitis**
**4.2.1.3 NACLT and major oral aphthous stomatitis**
Fig. 3. Major aphthous ulcer
Fashtami et al. 2009).
A New Approach to Relieve Pain in Some Painful Oral Diseases 397
A randomized controlled clinical trial was designed to evaluate the pain relieving effects of a single-session of NACLT in minor recurrent aphthous stomatitis as a prototype of painful oral ulcers. Fifteen patients, each with two discrete aphthous ulcers, were included. One of the ulcers was randomly allocated to be treated with NACLT and the other one served as a placebo. In each patient, the laser lesion was treated with NACLT, while the placebo lesion was irradiated with the same laser, but with an inactive probe. The patients scored and recorded the pain severity of their lesions on a 10-grade visual analogue scale (VAS) up to 4 days post operatively. In the laser group, the pain severity scores of the lesions were dramatically declined immediately after irradiation (p<0.001), whereas there were no changes in the mean scores in the placebo lesions at the same time. The reduction in pain scores was significantly greater in the laser group than in the placebo group in all of the follow up periods (p<0.001). The procedure itself was not painful, so anesthesia was not required. The patients reported no warmth in their lesions during laser treatment. There was no visual effect of thermal damage to the oral mucosa such as ablation, coagulation or erythema. The results showed that a single-session of NACLT reduced pain in minor aphthous ulcers immediately and significantly, without any visible side effects (Zand, Ataie-
A pilot randomized controlled clinical trial was designed to evaluate the analgesic effects of a single-session of NACLT in major recurrent aphthous ulcers. Five patients, each with two discrete major aphthous ulcers were included. One of the ulcers was randomly allocated to be treated with NACLT and the other one served as a placebo. The lesions in laser group were irradiated with CO2 laser (λ = 10,600 nm; Lancet-2, Russia) through a thick layer of transparent, non-anesthetic gel (Abzar Darman Co., Iran) with 87.5% water content, with a thickness of 3–4 mm. The CO2 laser was operated at 1W power, with a de-focused hand piece, 5–6 mm distant from the mucosal surface, in continuous mode, scanning rapidly over the lesion with circular motion. The patients' idiopathic (non-contact) and contact pain severity scores were recorded before and immediately after NACLT. These scores were also recorded up to 4 days post- operatively. The results of the study demonstrated that in the laser group, both the non-contact and contact pain severity scores of the lesions were dramatically declined immediately after irradiation (p<0.001), whereas there were no
#### **4.2 NACLT applications in clinical studies**
#### **4.2.1 Recurrent oral aphthous stomatitis**
#### **4.2.1.1 Definition**
Recurrent aphthous stomatitis (RAS) is a common oral disorder of uncertain etiopathogenesis (Scully, Gorsky et al. 2003), characterized by painful, round or ovoid ulcers with circumscribed margins, yellowish fibrinoid base, surrounded with erythematous haloes. The lesions typically first presenting in childhood or adolescence, recur at varying intervals throughout life (Jurge, Kuffer et al. 2006). The frequency and severity of the ulcerations usually decreases with age (Arikan, Birol et al. 2006). RAS occurs worldwide although it appears most common in the developed world (Jurge, Kuffer et al. 2006).
Recurrent aphthous stomatitis (RAS) is classified into three clinical forms, namely minor (miRAS), major (maRAS) and herpetiformis. Minor aphthous ulcers, which comprise over 80–90% of cases (Shashy and Ridley 2000), are less than 1 cm in diameter, last up to 7–14 days, and they heal without scar formation. Major aphthous ulcers are over 1 cm in diameter, and their healing may take 20 to 30 days at a time and often heal with scarring. Herpetiform ulcers (HUs) are multiple, clustered, 1–3 mm lesions that may coalesce into larger ulcers. They typically heal within 15 days (Prolo P 2006).
Although the exact underlying pathophysiology of RAS is not completely known, some evidences propose that aphthous ulcers are related to a focal immune dysfunction in which T lymphocytes have a significant role (Shashy and Ridley 2000; Jurge, Kuffer et al. 2006). Many etiologic, predisposing, and precipitating factors, such as genetic factors, immunologic problems, trauma, hypersensitivity to foods and drugs, hormonal changes, hematological deficiencies, cessation of smoking, and psychological stresses have been propsed (Shashy and Ridley 2000; Arikan, Birol et al. 2006).
Since there is no consensus regarding the cause of recurrent oral aphthous ulcers, it is difficult to have completely effective treatment or prevention (Shashy and Ridley 2000). There are currently few agents that have been found in randomized controlled clinical trials to cure aphthous ulcers (Jurge, Kuffer et al. 2006). As a result, the management of RAS is directed largely toward symptomatic relief. The main problem with aphthous ulcers is their pain which may be so severe. Many different therapeutic agents, including topical corticosteroids, mouth rinses, antibiotics, local anesthetic gels or pastilles, and treatment modalities, such as silver nitrate cautery and cryotherapy, have been tried for pain control in miRAS patients (Alidaee, Taheri et al. 2005; Arikan, Birol et al. 2006).
Fig. 2. Minor aphthous ulcer
Fig. 3. Major aphthous ulcer
396 CO2 Laser – Optimisation and Application
Recurrent aphthous stomatitis (RAS) is a common oral disorder of uncertain etiopathogenesis (Scully, Gorsky et al. 2003), characterized by painful, round or ovoid ulcers with circumscribed margins, yellowish fibrinoid base, surrounded with erythematous haloes. The lesions typically first presenting in childhood or adolescence, recur at varying intervals throughout life (Jurge, Kuffer et al. 2006). The frequency and severity of the ulcerations usually decreases with age (Arikan, Birol et al. 2006). RAS occurs worldwide
Recurrent aphthous stomatitis (RAS) is classified into three clinical forms, namely minor (miRAS), major (maRAS) and herpetiformis. Minor aphthous ulcers, which comprise over 80–90% of cases (Shashy and Ridley 2000), are less than 1 cm in diameter, last up to 7–14 days, and they heal without scar formation. Major aphthous ulcers are over 1 cm in diameter, and their healing may take 20 to 30 days at a time and often heal with scarring. Herpetiform ulcers (HUs) are multiple, clustered, 1–3 mm lesions that may coalesce into
Although the exact underlying pathophysiology of RAS is not completely known, some evidences propose that aphthous ulcers are related to a focal immune dysfunction in which T lymphocytes have a significant role (Shashy and Ridley 2000; Jurge, Kuffer et al. 2006). Many etiologic, predisposing, and precipitating factors, such as genetic factors, immunologic problems, trauma, hypersensitivity to foods and drugs, hormonal changes, hematological deficiencies, cessation of smoking, and psychological stresses have been
Since there is no consensus regarding the cause of recurrent oral aphthous ulcers, it is difficult to have completely effective treatment or prevention (Shashy and Ridley 2000). There are currently few agents that have been found in randomized controlled clinical trials to cure aphthous ulcers (Jurge, Kuffer et al. 2006). As a result, the management of RAS is directed largely toward symptomatic relief. The main problem with aphthous ulcers is their pain which may be so severe. Many different therapeutic agents, including topical corticosteroids, mouth rinses, antibiotics, local anesthetic gels or pastilles, and treatment modalities, such as silver nitrate cautery and cryotherapy, have been tried for pain control in
although it appears most common in the developed world (Jurge, Kuffer et al. 2006).
larger ulcers. They typically heal within 15 days (Prolo P 2006).
propsed (Shashy and Ridley 2000; Arikan, Birol et al. 2006).
miRAS patients (Alidaee, Taheri et al. 2005; Arikan, Birol et al. 2006).
Fig. 2. Minor aphthous ulcer
**4.2 NACLT applications in clinical studies 4.2.1 Recurrent oral aphthous stomatitis**
**4.2.1.1 Definition**
#### **4.2.1.2 NACLT and minor oral aphthous stomatitis**
A randomized controlled clinical trial was designed to evaluate the pain relieving effects of a single-session of NACLT in minor recurrent aphthous stomatitis as a prototype of painful oral ulcers. Fifteen patients, each with two discrete aphthous ulcers, were included. One of the ulcers was randomly allocated to be treated with NACLT and the other one served as a placebo. In each patient, the laser lesion was treated with NACLT, while the placebo lesion was irradiated with the same laser, but with an inactive probe. The patients scored and recorded the pain severity of their lesions on a 10-grade visual analogue scale (VAS) up to 4 days post operatively. In the laser group, the pain severity scores of the lesions were dramatically declined immediately after irradiation (p<0.001), whereas there were no changes in the mean scores in the placebo lesions at the same time. The reduction in pain scores was significantly greater in the laser group than in the placebo group in all of the follow up periods (p<0.001). The procedure itself was not painful, so anesthesia was not required. The patients reported no warmth in their lesions during laser treatment. There was no visual effect of thermal damage to the oral mucosa such as ablation, coagulation or erythema. The results showed that a single-session of NACLT reduced pain in minor aphthous ulcers immediately and significantly, without any visible side effects (Zand, Ataie-Fashtami et al. 2009).
#### **4.2.1.3 NACLT and major oral aphthous stomatitis**
A pilot randomized controlled clinical trial was designed to evaluate the analgesic effects of a single-session of NACLT in major recurrent aphthous ulcers. Five patients, each with two discrete major aphthous ulcers were included. One of the ulcers was randomly allocated to be treated with NACLT and the other one served as a placebo. The lesions in laser group were irradiated with CO2 laser (λ = 10,600 nm; Lancet-2, Russia) through a thick layer of transparent, non-anesthetic gel (Abzar Darman Co., Iran) with 87.5% water content, with a thickness of 3–4 mm. The CO2 laser was operated at 1W power, with a de-focused hand piece, 5–6 mm distant from the mucosal surface, in continuous mode, scanning rapidly over the lesion with circular motion. The patients' idiopathic (non-contact) and contact pain severity scores were recorded before and immediately after NACLT. These scores were also recorded up to 4 days post- operatively. The results of the study demonstrated that in the laser group, both the non-contact and contact pain severity scores of the lesions were dramatically declined immediately after irradiation (p<0.001), whereas there were no
Non-Thermal, Non-Ablative CO2 Laser Therapy (NACLT):
Managing aphthous ulcers: Laser Treatment Applied
CO2-laser treatment of ulcerative lesions
Relieving pain in minor aphthous stomatitis by a single session of non-thermal carbon dioxide laser irradiation
Analgesic effects of single session of Non-Ablative CO2 Laser Therapy (NACLT) in major aphthous ulcers: (a preliminary study)
**4.2.2 Behçet's disease**
**4.2.2.1 Definition**
A New Approach to Relieve Pain in Some Painful Oral Diseases 399
Title Author Study Type of irradiation
Before-after clinical trial 1991
Case report 2004
Randomized controlled clinical trial (RCT) 2008
Randomized controlled clinical trial (RCT) 2009
Behçet's disease (BD) which is classified among vasculitides is a complex, multisystem inflammatory disease characterized by oral and genital aphthae, cutaneous lesions, arthritis, ocular, gastrointestinal, and neurologic manifestations (Meador, Ehrlich et al. 2002; Suzuki
The most common clinical feature is the presence of recurrent and usually painful mucocutaneous ulcers (Lin and Liang 2006). Oral aphthosis is the most frequent and
Colvard M, Kuo P
> Sharon-Buller A, et al.
Zand N, Ataiefashtami L. et al.
Zand N, Ataiefashtami L. et al.
Table 1. Irradiation of aphthous ulcers with CO2 laser
Kurokawa and Suzuki 2004; Lin and Liang 2006).
Need to
surgery + 18
Ablative CO2 laser
CO2 laser irradiation of the lesions through a thin film of transparent gel with high water content
CO2 laser Irradiation of the lesions through a thick (3-4mm) layer of transparent gel with high water content; (NACLT)
CO2 laser Irradiation of the lesions through a thick (3-4mm) layer of transparent gel with high water content; (NACLT)
Anesthesia
Number of
\_ 2
\_ 15
patients
changes in the mean scores in the placebo lesions at the same time. The reduction in pain scores was significantly greater in the laser group than in the placebo group in all of the follow up periods (p<0.001). There were not any visible side effects following NACLT. None of the patients reported warmth feeling in their lesions during laser treatment. The results of the study suggested that a single-session of NACLT could reduce pain in major aphthous ulcers immediately and significantly, without any visible side effects (Zand, Ataie-Fashtami et al. 2009). This study is in progress.
#### **4.2.1.4 Literature review**
Colvard and Kuo evaluated the potential efficacy of the high-power, surgical CO2 laser for pain relief in 28 painful minor aphthous ulcers of 14 patients. Their anesthetic protocol included pre-operative pain medication (oral administration of ketoprofen) and local anesthesia by infiltration of 1:200,000 2% isocaine with 1:200000 neocobefrin to overcome the painful nature of the procedure. During the procedure, CO2 laser was used as a classical, ablative manner with power output 4 W and as much necrotic tissue as possible was removed. Over all 88.8% of the patients were completely pain free following anesthetic resolution, and none of the patients required post-operative medication for pain relief. The authors concluded that CO2 laser should be included as an alternative modality for the treatment of miRAS, due to its ability to reduce or eliminate pain (Colvard and Kuo 1991). In this study, CO2 laser was used in classical, high power ablative manner. However the post operative analgesic effects of the procedure demonstrated the simultaneous biomodulative effects of CO2 laser irradiation. The same concept Kaplan stated that laser surgery and low level laser therapy should be regarded as two sides of the same coin.
Fekrazad et al. evaluated the effects of Nd: YAG laser (power: 3 W, energy: 100 mj, pulse repetition rate: 30Hz, irradiation time: 60 sec) in 138 patients with aphtous ulcers. The patients were randomly assigned into three groups, as follows: (1) treatment with a focalized beam; (2) treatment with a non-focalised beam and (3) placebo treatment. In group (1) the laser beam was administered from a distance of 6 mm from the centre of the ulcer without using a clear and defined point of irradiation. In group (2) a well defined point beam of the laser was irradiated from a distance of 2 mm from the center of the ulcer, in a helical fashion. In group (3) the HeNe Laser was used as placebo with inactive probe. In group (1) and (2) a significant reduction of pain was observed compared to group (3). The duration of pain and the duration of recovery period were shortest in group (2) (Fekrazad, Jafari et al. 2006).
De Souza TO et al. assessed the effect of low-level laser therapy on pain control and the repair of recurrent aphthous stomatitis. Twenty patients with recurrent aphthous ulcers were divided into two groups. The patients in Group I (n = 5) treated with topical triamcinolone acetonide and the patients in Group II (n = 15) received laser treatment with an InGaA1P diode laser (670 nm, 50 mW, 3 J/cm2 per point) in daily sessions on consecutive days. All patients were assessed daily, and the following clinical parameters were determined during each session: pain intensity before and after treatment and clinical measurement of lesion size. The results showed that 75% of the patients reported a reduction in pain in the same session after laser treatment, and total regression of the lesion occurred after 4 days. Total regression in the corticosteroid group was from 5 to 7 days. They concluded that LLLT with these laser parameters demonstrated analgesic and healing effects with regard to recurrent aphthous stomatitis (De Souza, Martins et al. 2010).
changes in the mean scores in the placebo lesions at the same time. The reduction in pain scores was significantly greater in the laser group than in the placebo group in all of the follow up periods (p<0.001). There were not any visible side effects following NACLT. None of the patients reported warmth feeling in their lesions during laser treatment. The results of the study suggested that a single-session of NACLT could reduce pain in major aphthous ulcers immediately and significantly, without any visible side effects (Zand, Ataie-Fashtami
Colvard and Kuo evaluated the potential efficacy of the high-power, surgical CO2 laser for pain relief in 28 painful minor aphthous ulcers of 14 patients. Their anesthetic protocol included pre-operative pain medication (oral administration of ketoprofen) and local anesthesia by infiltration of 1:200,000 2% isocaine with 1:200000 neocobefrin to overcome the painful nature of the procedure. During the procedure, CO2 laser was used as a classical, ablative manner with power output 4 W and as much necrotic tissue as possible was removed. Over all 88.8% of the patients were completely pain free following anesthetic resolution, and none of the patients required post-operative medication for pain relief. The authors concluded that CO2 laser should be included as an alternative modality for the treatment of miRAS, due to its ability to reduce or eliminate pain (Colvard and Kuo 1991). In this study, CO2 laser was used in classical, high power ablative manner. However the post operative analgesic effects of the procedure demonstrated the simultaneous biomodulative effects of CO2 laser irradiation. The same concept Kaplan stated that laser surgery and low
Fekrazad et al. evaluated the effects of Nd: YAG laser (power: 3 W, energy: 100 mj, pulse repetition rate: 30Hz, irradiation time: 60 sec) in 138 patients with aphtous ulcers. The patients were randomly assigned into three groups, as follows: (1) treatment with a focalized beam; (2) treatment with a non-focalised beam and (3) placebo treatment. In group (1) the laser beam was administered from a distance of 6 mm from the centre of the ulcer without using a clear and defined point of irradiation. In group (2) a well defined point beam of the laser was irradiated from a distance of 2 mm from the center of the ulcer, in a helical fashion. In group (3) the HeNe Laser was used as placebo with inactive probe. In group (1) and (2) a significant reduction of pain was observed compared to group (3). The duration of pain and the duration of recovery period were shortest in group (2) (Fekrazad,
De Souza TO et al. assessed the effect of low-level laser therapy on pain control and the repair of recurrent aphthous stomatitis. Twenty patients with recurrent aphthous ulcers were divided into two groups. The patients in Group I (n = 5) treated with topical triamcinolone acetonide and the patients in Group II (n = 15) received laser treatment with an InGaA1P diode laser (670 nm, 50 mW, 3 J/cm2 per point) in daily sessions on consecutive days. All patients were assessed daily, and the following clinical parameters were determined during each session: pain intensity before and after treatment and clinical measurement of lesion size. The results showed that 75% of the patients reported a reduction in pain in the same session after laser treatment, and total regression of the lesion occurred after 4 days. Total regression in the corticosteroid group was from 5 to 7 days. They concluded that LLLT with these laser parameters demonstrated analgesic and healing effects with regard to recurrent aphthous stomatitis (De Souza, Martins et al. 2010).
level laser therapy should be regarded as two sides of the same coin.
et al. 2009). This study is in progress.
**4.2.1.4 Literature review**
Jafari et al. 2006).
Table 1. Irradiation of aphthous ulcers with CO2 laser
#### **4.2.2 Behçet's disease**
#### **4.2.2.1 Definition**
Behçet's disease (BD) which is classified among vasculitides is a complex, multisystem inflammatory disease characterized by oral and genital aphthae, cutaneous lesions, arthritis, ocular, gastrointestinal, and neurologic manifestations (Meador, Ehrlich et al. 2002; Suzuki Kurokawa and Suzuki 2004; Lin and Liang 2006).
The most common clinical feature is the presence of recurrent and usually painful mucocutaneous ulcers (Lin and Liang 2006). Oral aphthosis is the most frequent and
Non-Thermal, Non-Ablative CO2 Laser Therapy (NACLT):
was required. The study is in progress.
**4.2.2.2 NACLT and oral aphthous ulcers of Behçet's disease**
participate in the study according the inclusion/exclusion criteria.
**4.2.2.3 NACLT and genital aphthous ulcers of Behçet's disease**
ulcers of Behçet's disease without any complications.
significant pain relieving effect.
A New Approach to Relieve Pain in Some Painful Oral Diseases 401
A pilot before-after clinical trial was designed to evaluate the analgesic effects of a singlesession of NACLT in painful aphthous ulcers of Behçet's disease. Up until the time of this publication,, three patients with known Behcet's disease have been eligible and consented to
Four painful oral aphthous ulcers of the three patients were treated by NACLT. The pain severity of the lesions were dramatically declined immediately after irradiation (p<0.001). This analgesic effect was consistently sustained during the five days follow-up periods. Up until the time of this publication, the results of this pilot study suggest that a single session of NACLT could relieve pain in oral aphthous ulcers of Behçet's disease immediately and significantly without visible side effects of thermal damage or aggravation of the lesions. Similar to the other NACLT studies, the procedure itself was pain free and no anesthesia
In another case report that is being published, the extremely painful genital aphthous ulcers of a 23-year-old female with Behcet's disease were irradiated by NACLT. Before laser irradiation the pain of the lesions was so severe which impeded daily functions, such as sitting, walking, and even sleeping and did not respond to conventional analgesics. The non-contact and contact visual analogue scale (VAS) pain scores of the left genital ulcer were 8 and 10 and the scores of the right sided ulcer were 6 and 10 respectively. Immediately after NACLT of the ulcers and its surrounding erythematous rim, the contact pain of the lesions relieved completely (so that she could even walk downstairs without difficulties). Similar to the other prior NACLT investigations, there were no visual side effects of thermal damage to the lesions, such as tissue ablation or aggravation of the lesions following NACLT. The procedure was painless and neither systemic nor local anesthesia was required. This analgesic effect of NACLT was sustained during the healing period and she experienced no problem in daily functions and did not require topical or systemic analgesics. She just had mild burning sensation during urination for the first three days which relieved after healing of the lesions. It should be noted that concomitantly, treatment with prednisolone 30mg/day and colchicine 2mg/ day was initiated in the hospital. Additionally, the depth of the genital ulcers decreased remarkably two days after NACLT. The ulcers healed completely within 11 days which seemed to be much shorter than what was expected. Interestingly in spite of the large size of the ulcers, they left a very small (6mm) scar. The results of this case report suggest that NACLT could be potentially considered as an alternative method for pain relief in painful genital aphthous
It should be noticed that Behçet's syndrome is a serious multisystem disease, which in some cases it may lead to systemic complications such as; severe ocular problems (even blindness), intestinal, central nervous system,… involvement. Therefore the patients must be warned that NACLT should not substitute the systemic therapy of the disease in spite of its
Certainly, controlled clinical trials with larger sample sizes are necessary to further evaluate the efficacy and safety of NACLT in reducing the pain of oral and genital aphthous ulcers of
constant manifestation of Behçet's disease (Davatchi, Shahram et al. 2005) and usually the initial presenting symptom in most, if not all, patients (Lin and Liang 2006) . The distinct difference between the clinical features of aphthous ulcers of RAS and Behçet's Syndrome remains unclear. The aphthous ulcers of Behçet's disease are typically painful punched-out ulcers with a white yellowish fibrinoid base, surrounded with erythematous halo. They range in size from a few millimeters to 2 cm. These ulcers typically heal spontaneously within 1 to 3 weeks,usually without scarring (Ghate and Jorizzo 1999; Lin and Liang 2006).
Genital ulceration occurs in approximately 75% of the patients with Behçet's disease (Lin and Liang 2006). The genital aphthous ulcers are morphologically similar to the oral ulcers, except that lesions are usually larger, more painful, heal more slowly, recur less frequently and can have scarring tendency (Davatchi, Shahram et al. 2005; Lin and Liang 2006). In females they are often larger than 10 mm, and deeper than oral lesions. They are localized on the vulva, vagina, and rarely cervix. The giant aphthous lesion of the vulva is frequent, causing dysfunction and leaving sometimes indelible cicatrix. In males, genital aphthosis is often seen on the scrotum, but may be seen also on the shaft of penis or on the meatus. Sometimes they become giant lesions (Davatchi, Shahram et al. 2005) . Genital ulcerations of Behçet's disease may be very painful, exert a negative impact on the patient's quality of life, and these lesions are often refractory to multiple treatments(Kasugai, Watanabe et al. 2010).
Treatment of Behçet's disease is based on the clinical symptoms and severity of systemic involvement, including topical therapies as well as colchicine, dapsone, thalidomide, and immunosuppressants, interferon-alpha/beta, anti-tumor necrosis factor antibody, the latter specially in treatment for the cases with poor prognosis including eye, intestine, vessel and central nervous system involvement (Suzuki Kurokawa and Suzuki 2004).
The mucosal lesions, especially genital lesions can often become refractory to multiple treatments and present challenges to physicians. Topical or intralesional corticosteroids, oral pentoxifylline, sucralfate, dapsone, colchicine, and systemic low-dose corticosteroids, used either alone or in combination, are safe and having varying evidence for effect in mild to moderate mucocutaneous disease. Azathioprine or methotrexate can be used if the lesions are refractory to the previously mentioned therapies. Tumor necrosis factor (TNF) inhibitors such as infliximab or etanercept should be considered as the next step treatments . Tacrolimus, cyclosporine, and interferon-alpha-2a should be used generally only if TNF inhibitors have failed as a result of their toxicitie*s* (Lin and Liang 2006)*.*
Fig. 4. Behçet's Disease
constant manifestation of Behçet's disease (Davatchi, Shahram et al. 2005) and usually the initial presenting symptom in most, if not all, patients (Lin and Liang 2006) . The distinct difference between the clinical features of aphthous ulcers of RAS and Behçet's Syndrome remains unclear. The aphthous ulcers of Behçet's disease are typically painful punched-out ulcers with a white yellowish fibrinoid base, surrounded with erythematous halo. They range in size from a few millimeters to 2 cm. These ulcers typically heal spontaneously within 1 to 3 weeks,usually without scarring (Ghate and Jorizzo 1999; Lin and Liang 2006). Genital ulceration occurs in approximately 75% of the patients with Behçet's disease (Lin and Liang 2006). The genital aphthous ulcers are morphologically similar to the oral ulcers, except that lesions are usually larger, more painful, heal more slowly, recur less frequently and can have scarring tendency (Davatchi, Shahram et al. 2005; Lin and Liang 2006). In females they are often larger than 10 mm, and deeper than oral lesions. They are localized on the vulva, vagina, and rarely cervix. The giant aphthous lesion of the vulva is frequent, causing dysfunction and leaving sometimes indelible cicatrix. In males, genital aphthosis is often seen on the scrotum, but may be seen also on the shaft of penis or on the meatus. Sometimes they become giant lesions (Davatchi, Shahram et al. 2005) . Genital ulcerations of Behçet's disease may be very painful, exert a negative impact on the patient's quality of life, and these lesions are often refractory to multiple treatments(Kasugai, Watanabe et al. 2010). Treatment of Behçet's disease is based on the clinical symptoms and severity of systemic involvement, including topical therapies as well as colchicine, dapsone, thalidomide, and immunosuppressants, interferon-alpha/beta, anti-tumor necrosis factor antibody, the latter specially in treatment for the cases with poor prognosis including eye, intestine, vessel and
central nervous system involvement (Suzuki Kurokawa and Suzuki 2004).
inhibitors have failed as a result of their toxicitie*s* (Lin and Liang 2006)*.*
Fig. 4. Behçet's Disease
The mucosal lesions, especially genital lesions can often become refractory to multiple treatments and present challenges to physicians. Topical or intralesional corticosteroids, oral pentoxifylline, sucralfate, dapsone, colchicine, and systemic low-dose corticosteroids, used either alone or in combination, are safe and having varying evidence for effect in mild to moderate mucocutaneous disease. Azathioprine or methotrexate can be used if the lesions are refractory to the previously mentioned therapies. Tumor necrosis factor (TNF) inhibitors such as infliximab or etanercept should be considered as the next step treatments . Tacrolimus, cyclosporine, and interferon-alpha-2a should be used generally only if TNF
#### **4.2.2.2 NACLT and oral aphthous ulcers of Behçet's disease**
A pilot before-after clinical trial was designed to evaluate the analgesic effects of a singlesession of NACLT in painful aphthous ulcers of Behçet's disease. Up until the time of this publication,, three patients with known Behcet's disease have been eligible and consented to participate in the study according the inclusion/exclusion criteria.
Four painful oral aphthous ulcers of the three patients were treated by NACLT. The pain severity of the lesions were dramatically declined immediately after irradiation (p<0.001). This analgesic effect was consistently sustained during the five days follow-up periods. Up until the time of this publication, the results of this pilot study suggest that a single session of NACLT could relieve pain in oral aphthous ulcers of Behçet's disease immediately and significantly without visible side effects of thermal damage or aggravation of the lesions. Similar to the other NACLT studies, the procedure itself was pain free and no anesthesia was required. The study is in progress.
#### **4.2.2.3 NACLT and genital aphthous ulcers of Behçet's disease**
In another case report that is being published, the extremely painful genital aphthous ulcers of a 23-year-old female with Behcet's disease were irradiated by NACLT. Before laser irradiation the pain of the lesions was so severe which impeded daily functions, such as sitting, walking, and even sleeping and did not respond to conventional analgesics. The non-contact and contact visual analogue scale (VAS) pain scores of the left genital ulcer were 8 and 10 and the scores of the right sided ulcer were 6 and 10 respectively. Immediately after NACLT of the ulcers and its surrounding erythematous rim, the contact pain of the lesions relieved completely (so that she could even walk downstairs without difficulties). Similar to the other prior NACLT investigations, there were no visual side effects of thermal damage to the lesions, such as tissue ablation or aggravation of the lesions following NACLT. The procedure was painless and neither systemic nor local anesthesia was required. This analgesic effect of NACLT was sustained during the healing period and she experienced no problem in daily functions and did not require topical or systemic analgesics. She just had mild burning sensation during urination for the first three days which relieved after healing of the lesions. It should be noted that concomitantly, treatment with prednisolone 30mg/day and colchicine 2mg/ day was initiated in the hospital. Additionally, the depth of the genital ulcers decreased remarkably two days after NACLT. The ulcers healed completely within 11 days which seemed to be much shorter than what was expected. Interestingly in spite of the large size of the ulcers, they left a very small (6mm) scar. The results of this case report suggest that NACLT could be potentially considered as an alternative method for pain relief in painful genital aphthous ulcers of Behçet's disease without any complications.
It should be noticed that Behçet's syndrome is a serious multisystem disease, which in some cases it may lead to systemic complications such as; severe ocular problems (even blindness), intestinal, central nervous system,… involvement. Therefore the patients must be warned that NACLT should not substitute the systemic therapy of the disease in spite of its significant pain relieving effect.
Certainly, controlled clinical trials with larger sample sizes are necessary to further evaluate the efficacy and safety of NACLT in reducing the pain of oral and genital aphthous ulcers of
Non-Thermal, Non-Ablative CO2 Laser Therapy (NACLT):
**4.2.3 Pemphigus vulgaris**
2005; Bystryn and Rudolph 2005).
during conventional systemic therapy.
Fig. 5. Pemphigus vulgaris
**4.2.3.2 NACLT and oral lesions of pemphigus vulgaris**
**4.2.3.1 Definition**
A New Approach to Relieve Pain in Some Painful Oral Diseases 403
Pemphigus Vulgaris (PV) is a rare, potentially life-threatening, autoimmune blistering disease of the skin and mucous membranes. Although the disease can affect anyone, it is most prevalent in people of Mediterranean or Jewish ancestry(Bystryn and Rudolph 2005). The prevalence of the disease is 30/100,000 and annual incidence has been reported between 1 and 5 in 100,000 according to different studies in Iran (Chams-Davatchi, Valikhani et al. 2005; Asilian, Yoosefi et al. 2006). The lesions are characterized by intra-epidermal vesicles with acantholysis and an intact basal layer. In the majority of patients, painful mucous membrane erosions are the presenting sign of pemphigus vulgaris and may be the only sign for weeks to months before any bullous skin lesion develops. The mucous membranes most often affected are those of the oral cavity, in which intact blisters are rare, probably because they are fragile and break easily, leaving scattered and often extensive erosions. The lesions are usually multiple, superficial, and irregular in shape, and arise from mucosa of healthy appearance. Although any surface can be involved, the most common sites are the buccal
Oral lesions in pemphigus vulgaris may be so painful during the active period of the disease that may interfere with their eating, drinking and even speaking (Black, Mignogna et al.
High doses of systemic corticosteroids plus immunosuppressive agents have dramatically declined the mortality rate of the disease. Understandably, owing to the life threatening nature of PV, the main focus of the peer reviewed literature has been on suppression and remission of PV (Rashid and Candido 2008). However, remission is not instantaneous and takes time to achieve. This delay in remission allows ample opportunity for complications to develop, secondary to the pain associated with PV. This can be highlighted by cases of repeated dehydration and malnutrition seen in PV patients (Rashid and Candido 2008). Therefore it seems necessary to obtain new modalities for pain control of these oral lesions
A pilot before-after clinical trial was designed to evaluate the analgesic effects of application of a single session of NACLT in oral lesions of PV. Thirty eight painful oral lesions of ten patients with PV were irradiated with CO2 laser by NACLT protocol. The patients scored and recorded the pain severity of their lesions on a visual analogue scale (VAS) up to 7 days
and labial mucosa, the palate, and the tongue (Bystryn and Rudolph 2005).
Behcet's disease. In addition such studies can demonstrate whether NACLT could accelerate wound healing in these lesions and specially prevent scar formation in genital aphthous ulcers of Behcet's disease or not.
#### **4.2.2.4 Literature review**
Demetriades used ablative CO2 laser in four painful oral aphthous ulcers of a patient with Behçet's Syndrome. Before laser irradiation, the lesions were infiltrated with a minimal amount of lidocaine 2% with 1:100,000 epinephrine. A CO2-laser set at 2W superpulse mode with a 0.4 mm ceramic tip was used, in a defocused way to lightly char the surface of the ulcers. The patient tolerated the procedure well. On subsequent follow-up, one week after the procedure, the patient reported considerable relief of symptoms on most of the treated ulcers. The oropharyngeal ulcer displayed only moderate response, but the patient reported an overall improvement of his quality of life (Demetriades, Hansford et al. 2009).
Table 2. Irradiation of aphthous ulcers of Behcet's disease with CO2 laser
#### **4.2.3 Pemphigus vulgaris**
#### **4.2.3.1 Definition**
402 CO2 Laser – Optimisation and Application
Behcet's disease. In addition such studies can demonstrate whether NACLT could accelerate wound healing in these lesions and specially prevent scar formation in genital aphthous
Demetriades used ablative CO2 laser in four painful oral aphthous ulcers of a patient with Behçet's Syndrome. Before laser irradiation, the lesions were infiltrated with a minimal amount of lidocaine 2% with 1:100,000 epinephrine. A CO2-laser set at 2W superpulse mode with a 0.4 mm ceramic tip was used, in a defocused way to lightly char the surface of the ulcers. The patient tolerated the procedure well. On subsequent follow-up, one week after the procedure, the patient reported considerable relief of symptoms on most of the treated ulcers. The oropharyngeal ulcer displayed only moderate response, but the patient reported an overall improvement of his quality of life (Demetriades, Hansford et
Title Author Study Type of
Demetriades M et al
Zand N, Fateh M. Et al.
Zand N, Fateh M. Et al.
Table 2. Irradiation of aphthous ulcers of Behcet's disease with CO2 laser
irradiation
Ablative defocused CO2-laser irradiation
NACLT \_
NACLT -
Case report 2009
Case report, under publish
Pilot beforeafter clinical trial/ under publish
Need to
Anesthesia
+
ulcers of Behcet's disease or not.
General manifestations of Behçet's syndrome and the success of CO2-laser as treatment for oral lesions: A review of the literature and case presentation
Relieving pain in painful genital ulcers of Behcet's disease by a single session of non thermal, Non-Ablative CO2 Laser Therapy(NACLT): A Case Report
Immediate pain relief of oral aphthous ulcers of Behcet's disease by nonthermal, Non-Ablative CO2 Laser Therapy (NACLT)
**4.2.2.4 Literature review**
al. 2009).
Pemphigus Vulgaris (PV) is a rare, potentially life-threatening, autoimmune blistering disease of the skin and mucous membranes. Although the disease can affect anyone, it is most prevalent in people of Mediterranean or Jewish ancestry(Bystryn and Rudolph 2005). The prevalence of the disease is 30/100,000 and annual incidence has been reported between 1 and 5 in 100,000 according to different studies in Iran (Chams-Davatchi, Valikhani et al. 2005; Asilian, Yoosefi et al. 2006). The lesions are characterized by intra-epidermal vesicles with acantholysis and an intact basal layer. In the majority of patients, painful mucous membrane erosions are the presenting sign of pemphigus vulgaris and may be the only sign for weeks to months before any bullous skin lesion develops. The mucous membranes most often affected are those of the oral cavity, in which intact blisters are rare, probably because they are fragile and break easily, leaving scattered and often extensive erosions. The lesions are usually multiple, superficial, and irregular in shape, and arise from mucosa of healthy appearance. Although any surface can be involved, the most common sites are the buccal and labial mucosa, the palate, and the tongue (Bystryn and Rudolph 2005).
Oral lesions in pemphigus vulgaris may be so painful during the active period of the disease that may interfere with their eating, drinking and even speaking (Black, Mignogna et al. 2005; Bystryn and Rudolph 2005).
High doses of systemic corticosteroids plus immunosuppressive agents have dramatically declined the mortality rate of the disease. Understandably, owing to the life threatening nature of PV, the main focus of the peer reviewed literature has been on suppression and remission of PV (Rashid and Candido 2008). However, remission is not instantaneous and takes time to achieve. This delay in remission allows ample opportunity for complications to develop, secondary to the pain associated with PV. This can be highlighted by cases of repeated dehydration and malnutrition seen in PV patients (Rashid and Candido 2008). Therefore it seems necessary to obtain new modalities for pain control of these oral lesions during conventional systemic therapy.
#### **4.2.3.2 NACLT and oral lesions of pemphigus vulgaris**
A pilot before-after clinical trial was designed to evaluate the analgesic effects of application of a single session of NACLT in oral lesions of PV. Thirty eight painful oral lesions of ten patients with PV were irradiated with CO2 laser by NACLT protocol. The patients scored and recorded the pain severity of their lesions on a visual analogue scale (VAS) up to 7 days
Non-Thermal, Non-Ablative CO2 Laser Therapy (NACLT):
**4.2.4 Post chemotherapy oral mucositis**
producing a "pseudomembrane" (Sonis 2004).
Fig. 6. Post Docetaxole chemotherapy oral mucositis
**4.2.4.2 NACLT and mild to moderate post chemotherapy oral mucositis**
A pilot before-after clinical trial was designed to evaluate the effects of single session of nonthermal, Non-ablative CO2 Laser therapy (NACLT) to reduce pain in mild to moderate oral mucositis following breast cancer chemotherapy with Docetaxole. Six patients were included and their oral lesions were irradiated by NACLT. The patients reported the idiopathic (non-contact) and contact pain of their lesions on VAS (visual analogue scale) before and immediately after laser and up to 7 days post operatively. The results of the
**4.2.4.1 Definition**
(Arora, Pai et al. 2008).
A New Approach to Relieve Pain in Some Painful Oral Diseases 405
Oral mucositis is a common, debilitating, and potentially serious complication of chemoradiotherapy. Studies have shown that mucositis will develop in about 40% of chemotherapy patients, 80% of bone marrow transplant patients, and 100% of patients treated with radiotherapy to the head and neck (Berger & Kilroy 1997; Sonis et al. 1999).
It presents as erythema, edema, ulceration, bleeding along with pain. The pain of the lesions is aggravated by the patient's swallowing and normal oral functioning. Consequently, oral intake difficulties lead to loss of weight. The progression of oral lesions and their impact on the patient's general condition may require nasogastric tube feeding or temporary discontinuation of the treatment or modification of the therapeutic plan (Arora, Pai et al. 2008). Pathologic evaluation of mucositis reveals mucosal thinning leading to a shallow ulcer thought to be caused by inflammation and depletion of the epithelial basal layer with subsequent denudation and bacterial infection. The wound healing response to this injury is characterized by inflammatory cell infiltration, interstitial exudate, fibrin and cell debris
Various preventive measures causing alteration of mucosa (cryotherapy, allopurinol, pilocarpine, leucovorin), modification of mucosal proliferation (beta-carotene, glutamine, cytokines), and antimicrobial or anti-inflammatory action (chlorhexidine, corticosteroids) have been tried. General oral care, diet, topical mucosal coating agents (sucralfate, magnesium hydroxide), topical anesthetics, and systemic analgesics ( opiods and non opioids) (Arora, Pai et al. 2008), recombinant human keratinocyte growth factor (palifermin) and Amifostine have also been suggested (Kuhn, Porto et al. 2009). However, currently no definitive preventive or therapeutic intervention exists that is completely successful at preventing oral mucositis and treatment for this complication has thus been symptomatic
post operatively. Immediately after NACLT, the severity of idiopathic (non-contact) and contact pain were dramatically declined (p<0.001), so that the patients could eat and drink without any difficulties. This analgesic effect was sustained during follow-up periods. There was no visual effect of thermal damage to the oral mucosa or aggravation of the lesions. The results of this pilot study suggested that a single session of NACLT could reduce pain in oral lesions of pemphigus vulgaris immediately and significantly, without visible side effects (Zand, Mansouri et al. 2009). We recommend that in further studies, the pain severity of the lesions should be followed up for longer periods of time.
*It should be noted that due to the life threatening nature of PV without appropriate systemic treatment, the patients must be warned that NACLT should not alter their conventional treatment at all, in spite of its significant analgesic effect, as we instructed our patients to comply with their prescribed medical regimen.*
#### **4.2.3.3 Literature review**
In a case report, the oral lesions of two patients with recalcitrant oral pemphigus vulgaris (who were under systemic treatment) were irradiated with CO2 laser at 1-1.5 W in a defocused mode for 5-10 seconds. The patients reported no pain after treatment. Recall examination after 1 month, 3 months and 5 month revealed complete healing of the lesions with no recurrence (Bhardwaj, Joshi et al. 2010).
The pictures of the paper demonstrate the themal, ablative nature within the procedure. Its pain relieving effects can be explained by its simultaneous biomodulative effects of CO2 laser irradiation.
Table 3. Irradiation of oral lesions of pemphigus vulgaris with CO2 laser
#### **4.2.4 Post chemotherapy oral mucositis**
#### **4.2.4.1 Definition**
404 CO2 Laser – Optimisation and Application
post operatively. Immediately after NACLT, the severity of idiopathic (non-contact) and contact pain were dramatically declined (p<0.001), so that the patients could eat and drink without any difficulties. This analgesic effect was sustained during follow-up periods. There was no visual effect of thermal damage to the oral mucosa or aggravation of the lesions. The results of this pilot study suggested that a single session of NACLT could reduce pain in oral lesions of pemphigus vulgaris immediately and significantly, without visible side effects (Zand, Mansouri et al. 2009). We recommend that in further studies, the
*It should be noted that due to the life threatening nature of PV without appropriate systemic treatment, the patients must be warned that NACLT should not alter their conventional treatment at all, in spite of its significant analgesic effect, as we instructed our patients to comply with their*
In a case report, the oral lesions of two patients with recalcitrant oral pemphigus vulgaris (who were under systemic treatment) were irradiated with CO2 laser at 1-1.5 W in a defocused mode for 5-10 seconds. The patients reported no pain after treatment. Recall examination after 1 month, 3 months and 5 month revealed complete healing of the lesions
The pictures of the paper demonstrate the themal, ablative nature within the procedure. Its pain relieving effects can be explained by its simultaneous biomodulative effects of CO2
> Before-after clinical trial 2009
Case report 2010
irradiation
CO2 laser Irradiation of the lesions in a defocused mode (thermal)
NACLT \_
Need to
Anesthesia
?
Number of
patients
Ten patients/38 lesions
Two patients/? lesions
pain severity of the lesions should be followed up for longer periods of time.
Title Author Study Type of
Zand, N., Mansouri, P. et al.
Bhardwaj, A. et al.
Table 3. Irradiation of oral lesions of pemphigus vulgaris with CO2 laser
*prescribed medical regimen.* **4.2.3.3 Literature review**
laser irradiation.
Relieving pain in painful oral lesions of pemphigus vulgaris by a single session, Non-ablative 10600 nm CO2 Laser irradiation (pilot study )
Management of recalcitrant oral pemphigus vulgaris with CO2 laser-Report of two cases
with no recurrence (Bhardwaj, Joshi et al. 2010).
Oral mucositis is a common, debilitating, and potentially serious complication of chemoradiotherapy. Studies have shown that mucositis will develop in about 40% of chemotherapy patients, 80% of bone marrow transplant patients, and 100% of patients treated with radiotherapy to the head and neck (Berger & Kilroy 1997; Sonis et al. 1999).
It presents as erythema, edema, ulceration, bleeding along with pain. The pain of the lesions is aggravated by the patient's swallowing and normal oral functioning. Consequently, oral intake difficulties lead to loss of weight. The progression of oral lesions and their impact on the patient's general condition may require nasogastric tube feeding or temporary discontinuation of the treatment or modification of the therapeutic plan (Arora, Pai et al. 2008).
Pathologic evaluation of mucositis reveals mucosal thinning leading to a shallow ulcer thought to be caused by inflammation and depletion of the epithelial basal layer with subsequent denudation and bacterial infection. The wound healing response to this injury is characterized by inflammatory cell infiltration, interstitial exudate, fibrin and cell debris producing a "pseudomembrane" (Sonis 2004).
Various preventive measures causing alteration of mucosa (cryotherapy, allopurinol, pilocarpine, leucovorin), modification of mucosal proliferation (beta-carotene, glutamine, cytokines), and antimicrobial or anti-inflammatory action (chlorhexidine, corticosteroids) have been tried. General oral care, diet, topical mucosal coating agents (sucralfate, magnesium hydroxide), topical anesthetics, and systemic analgesics ( opiods and non opioids) (Arora, Pai et al. 2008), recombinant human keratinocyte growth factor (palifermin) and Amifostine have also been suggested (Kuhn, Porto et al. 2009). However, currently no definitive preventive or therapeutic intervention exists that is completely successful at preventing oral mucositis and treatment for this complication has thus been symptomatic (Arora, Pai et al. 2008).
Fig. 6. Post Docetaxole chemotherapy oral mucositis
#### **4.2.4.2 NACLT and mild to moderate post chemotherapy oral mucositis**
A pilot before-after clinical trial was designed to evaluate the effects of single session of nonthermal, Non-ablative CO2 Laser therapy (NACLT) to reduce pain in mild to moderate oral mucositis following breast cancer chemotherapy with Docetaxole. Six patients were included and their oral lesions were irradiated by NACLT. The patients reported the idiopathic (non-contact) and contact pain of their lesions on VAS (visual analogue scale) before and immediately after laser and up to 7 days post operatively. The results of the
Non-Thermal, Non-Ablative CO2 Laser Therapy (NACLT):
the promoting wound healing effects of NACLT.
2009). (Please see also 4. NACLT)
effect.
performed frequently.
A New Approach to Relieve Pain in Some Painful Oral Diseases 407
results of which are being published. In addition we have used NACLT in few ulcerated and non-ulcerated skin diseases with variable degrees of success. In some lesions, such as post herpetic neuralgia, NACLT acts like other conventional therapeutic lasers, in which the pain relieving effect is completely short standing and needs the several sessions of NACLT to be
In addition, we have evaluated the effects of NACLT in promoting wound healing in few studies with variable degrees of success. Although in few studies, NACLT has shown some valuable results in this field we are not yet ready to express our view in this regard. Certainly, controlled clinical trials with larger sample sizes are necessary to further evaluate
*It should be mentioned that since the biological effects of NACLT and their mechanisms are not fully known, it seems ethically questionable to use NACLT in diseases with malignant potential, such as*
In order to develop an understanding of the mechanisms of analgesic effect of NACLT, powermetry and thermometry were performed in prior studies, the results of which demonstrated the low power nature of the applied CO2 laser (Zand, Ataie-Fashtami et al.
Since the analgesic effect of NACLT is immediate, we assume that at least in part, physiological neural changes such as blockage of action potential generation and conduction of nociceptive signals in primary afferent neurons might take part in this analgesic effect (Zand, Ataie-Fashtami et al. 2009). Destruction of nerve endings is less probable to be induced by NACLT, because, even in the studies in which CO2 laser has been used as a surgical scalpel, there have been no statistically significant differences in the number of intact peripheral nerve structures in laser-treated sites in comparison with sites treated with electrocautery and scalpel (Rocha, Pinheiro et al. 2001). It is not known, whether the other mechanisms such as increase in β-endorphin synthesis and release, changes in bradykinin, prostaglandins, substance P, serotonin, acetylcholine, nitric oxide, singlet oxygen production, and the other biochemical events- which have been proposed to play a part in pain relieving effect of conventional low power lasers - are responsible for analgesic effect of NACLT or not. (Please see also 2.4.Mechanisms of analgesic effects of low power laser therapy) Further basic studies are necessary to elucidate the mechanisms of this analgesic
On the other hand, there are some differences between analgesic effects of NACLT and the other classical low power lasers. The analgesic effect in LLLT is usually gradual, cumulative, and multi-session (Pinheiro, Cavalcanti et al. 1998; Gur, Karakoc et al. 2002; Gur, Sarac et al. 2004; Nes and Posso 2005; Chow, Heller et al. 2006; Djavid, Mehrdad et al. 2007; Bjordal, Bensadoun et al. 2011; Iwatsuki, Yoshimine et al. 2011; Ribeiro, de Aguiar et al. 2011). In contrast, the pain relieving effect of NACLT is immediate, dramatic and more sustained than conventional low level therapeutic lasers (Zand, Ataie-Fashtami et al. 2009; Zand, Mansouri et al. 2009; Zand, Najafi et al. 2010). Therefore one could presume that the mechanisms of analgesic effect of NACLT might have some differences from that for conventional low power lasers, an assumption that should be assessed in further basic studies.
*oral lichen planus, consequently we have not assessed the effects of NACLT in such illnesses.*
**4.3 Presumed mechanisms of pain relieving effects of NACLT**
study showed that dramatically after NACLT, the severity of pain declined immediately and it was sustained during follow-up periods (P<0.001). Similar to the other NACLT clinical trials, the procedure itself was painless and anesthesia was not required. There was no visible side effect such as ulceration, erosion and even erythema following NACLT. The results of the study suggested that single session of NACLT could reduce pain in lesions of mild/moderate post Docetaxole oral mucositis immediately and dramatically without visible side effects (Zand, Najafi et al. 2010).
#### **4.2.4.3 A brief literature review**
Some studies have shown that laser phototherapy (LPT) can be useful in prevention or treatment of oral mucositis. The principle behind using laser phototherapy (LPT) is that it accelerates wound healing and has anti-inflammatory effects (Arora, Pai et al. 2008). In addition, pain relieving properties of low power lasers seem to be of value in management of painful lesions of oral mucositis (Arora, Pai et al. 2008).
Some researchers state that prophylactic laser application seems more successful than curative laser application, although the reason is not entirely clear (Arora, Pai et al. 2008). Most studies of LLLT in cancer patients have focused on oral mucositis prevention (Bensadoun, Franquin et al. 1994; Bensadoun, Franquin et al. 1999; Rubenstein, Peterson et al. 2004; Bensadoun, Le Page et al. 2006; Kuhn, Porto et al. 2009; Clarkson, Worthington et al. 2010; Bjordal, Bensadoun et al. 2011) . Since the subjects of this section refers to analgesic effects of low power lasers in established chemotherapy-induced oral mucositis (COM), we briefly review some articles in which LLLT has been used for relieving pain in patients with chemotherapy-induced oral mucositis and not the prophylactic laser protocols.
Cauwels and Martens evaluated the capacity of analgesic effect and wound healing of low level laser therapy in 16 children suffering from chemotherapy-induced oral mucositis. All children were treated using a GaAlAs diode laser (wavelength: 830 nm, potency: 150 mW). The energy released was adapted according to the severity of the oral lesions. The same protocol was repeated every 48 hours until healing of each lesion occurred. The results of the study demonstrated that immediately after irradiation of the oral mucositis, pain relief was noticed. Depending on the severity of oral mucositis, on average, 2.5 treatments per lesion in a period of 1 week were sufficient to heal a mucositis lesion. They concluded that LLLT could reduce the severity and duration of mucositis and to relieve pain significantly (Cauwels and Martens 2011).
Nes and Posso investigated the pain relieving effect of LLLT among 13 patients who have developed moderate chemotherapy-induced oral mucositis. The laser used was GaAlAs (830 nm, power: 250 mW, energy density: 35 J cm-2). The patients were treated during a 5-day period, and the pain was measured before and after each laser application. There was a significant ( *P*= 0.007) 67% decrease in the daily average experience of pain felt before and after each treatment, confirming that LLLT can relieve pain among patients who have developed chemotherapy-induced oral mucositis (Nes and Posso 2005).
#### **4.2.5 Other NACLT studies**
We have evaluated the pain relieving effects of NACLT in some other painful mucosal lesions such as painful oral lesions of Stevens Johnson Syndrome, etc. as case reports, the performed frequently.
406 CO2 Laser – Optimisation and Application
study showed that dramatically after NACLT, the severity of pain declined immediately and it was sustained during follow-up periods (P<0.001). Similar to the other NACLT clinical trials, the procedure itself was painless and anesthesia was not required. There was no visible side effect such as ulceration, erosion and even erythema following NACLT. The results of the study suggested that single session of NACLT could reduce pain in lesions of mild/moderate post Docetaxole oral mucositis immediately and dramatically without
Some studies have shown that laser phototherapy (LPT) can be useful in prevention or treatment of oral mucositis. The principle behind using laser phototherapy (LPT) is that it accelerates wound healing and has anti-inflammatory effects (Arora, Pai et al. 2008). In addition, pain relieving properties of low power lasers seem to be of value in management
Some researchers state that prophylactic laser application seems more successful than curative laser application, although the reason is not entirely clear (Arora, Pai et al. 2008). Most studies of LLLT in cancer patients have focused on oral mucositis prevention (Bensadoun, Franquin et al. 1994; Bensadoun, Franquin et al. 1999; Rubenstein, Peterson et al. 2004; Bensadoun, Le Page et al. 2006; Kuhn, Porto et al. 2009; Clarkson, Worthington et al. 2010; Bjordal, Bensadoun et al. 2011) . Since the subjects of this section refers to analgesic effects of low power lasers in established chemotherapy-induced oral mucositis (COM), we briefly review some articles in which LLLT has been used for relieving pain in patients with
Cauwels and Martens evaluated the capacity of analgesic effect and wound healing of low level laser therapy in 16 children suffering from chemotherapy-induced oral mucositis. All children were treated using a GaAlAs diode laser (wavelength: 830 nm, potency: 150 mW). The energy released was adapted according to the severity of the oral lesions. The same protocol was repeated every 48 hours until healing of each lesion occurred. The results of the study demonstrated that immediately after irradiation of the oral mucositis, pain relief was noticed. Depending on the severity of oral mucositis, on average, 2.5 treatments per lesion in a period of 1 week were sufficient to heal a mucositis lesion. They concluded that LLLT could reduce the severity and duration of mucositis and to relieve pain significantly
Nes and Posso investigated the pain relieving effect of LLLT among 13 patients who have developed moderate chemotherapy-induced oral mucositis. The laser used was GaAlAs (830 nm, power: 250 mW, energy density: 35 J cm-2). The patients were treated during a 5-day period, and the pain was measured before and after each laser application. There was a significant ( *P*= 0.007) 67% decrease in the daily average experience of pain felt before and after each treatment, confirming that LLLT can relieve pain among patients who have
We have evaluated the pain relieving effects of NACLT in some other painful mucosal lesions such as painful oral lesions of Stevens Johnson Syndrome, etc. as case reports, the
developed chemotherapy-induced oral mucositis (Nes and Posso 2005).
chemotherapy-induced oral mucositis and not the prophylactic laser protocols.
visible side effects (Zand, Najafi et al. 2010).
of painful lesions of oral mucositis (Arora, Pai et al. 2008).
**4.2.4.3 A brief literature review**
(Cauwels and Martens 2011).
**4.2.5 Other NACLT studies**
In addition, we have evaluated the effects of NACLT in promoting wound healing in few studies with variable degrees of success. Although in few studies, NACLT has shown some valuable results in this field we are not yet ready to express our view in this regard. Certainly, controlled clinical trials with larger sample sizes are necessary to further evaluate the promoting wound healing effects of NACLT.
*It should be mentioned that since the biological effects of NACLT and their mechanisms are not fully known, it seems ethically questionable to use NACLT in diseases with malignant potential, such as oral lichen planus, consequently we have not assessed the effects of NACLT in such illnesses.*
#### **4.3 Presumed mechanisms of pain relieving effects of NACLT**
In order to develop an understanding of the mechanisms of analgesic effect of NACLT, powermetry and thermometry were performed in prior studies, the results of which demonstrated the low power nature of the applied CO2 laser (Zand, Ataie-Fashtami et al. 2009). (Please see also 4. NACLT)
Since the analgesic effect of NACLT is immediate, we assume that at least in part, physiological neural changes such as blockage of action potential generation and conduction of nociceptive signals in primary afferent neurons might take part in this analgesic effect (Zand, Ataie-Fashtami et al. 2009). Destruction of nerve endings is less probable to be induced by NACLT, because, even in the studies in which CO2 laser has been used as a surgical scalpel, there have been no statistically significant differences in the number of intact peripheral nerve structures in laser-treated sites in comparison with sites treated with electrocautery and scalpel (Rocha, Pinheiro et al. 2001). It is not known, whether the other mechanisms such as increase in β-endorphin synthesis and release, changes in bradykinin, prostaglandins, substance P, serotonin, acetylcholine, nitric oxide, singlet oxygen production, and the other biochemical events- which have been proposed to play a part in pain relieving effect of conventional low power lasers - are responsible for analgesic effect of NACLT or not. (Please see also 2.4.Mechanisms of analgesic effects of low power laser therapy) Further basic studies are necessary to elucidate the mechanisms of this analgesic effect.
On the other hand, there are some differences between analgesic effects of NACLT and the other classical low power lasers. The analgesic effect in LLLT is usually gradual, cumulative, and multi-session (Pinheiro, Cavalcanti et al. 1998; Gur, Karakoc et al. 2002; Gur, Sarac et al. 2004; Nes and Posso 2005; Chow, Heller et al. 2006; Djavid, Mehrdad et al. 2007; Bjordal, Bensadoun et al. 2011; Iwatsuki, Yoshimine et al. 2011; Ribeiro, de Aguiar et al. 2011). In contrast, the pain relieving effect of NACLT is immediate, dramatic and more sustained than conventional low level therapeutic lasers (Zand, Ataie-Fashtami et al. 2009; Zand, Mansouri et al. 2009; Zand, Najafi et al. 2010). Therefore one could presume that the mechanisms of analgesic effect of NACLT might have some differences from that for conventional low power lasers, an assumption that should be assessed in further basic studies.
Non-Thermal, Non-Ablative CO2 Laser Therapy (NACLT):
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In some ulcerated oral lesions such as aphthous ulcers, the pain of the lesions derives from inflammatory sensitization of small-diameter afferent nerve endings that form a plexus at the junction of the epithelial and subepithelial layers. Branches of this plexus extend upward, into the epithelial layer; producing a superficial, focal, inflammatory lesion that is directly associated with exposed sensory nerve endings. Therefore, in such ulcers, CO2 laser irradiation can reach the exposed nerve endings easily and as we assume, for example, block the action potential generation and conduction of nociceptive signals in primary afferent neurons. On the other hand, in other under-publish studies, we have used NACLT for reducing pain in few non-ulcerated lesions, such as pre-aphthous lesions with moderate-good results. As the CO2 laser's beam has a very limited depth of penetration in to the tissue, explaining this analgesic effect of NACLT seems more complex.
Tuner and Hode state that the therapeutic effects of defocused CO2 laser must be due to the influence of the laser energy on the cells encountered, so that signal substances are released and then circulate in the organism. This indirectly confirms the hypothesis that conventional laser therapy has both a local effect in the area treated by laser light, and a systemic effect through the release of metabolites (Tuner and Hode 2010). We don't know whether such mechanisms may at least in part, take part in the analgesic effect of NACLT or not.
Further fundamental studies are necessary to elucidate the mechanisms of analgesic effect of NACLT.
#### **5. Conclusion**
CO2 laser has been used as a very useful high power, thermal laser in surgery for cutting, ablation and coagulation of the tissues for many years. In contrast, in non- thermal, Non-Ablative CO2 Laser Therapy (NACLT), the CO2 laser is used as a low level (phototherapeutic) laser to reduce pain in some oral mucosal lesions without any visual effects of thermal damage to the oral mucosa such as ablation, ulceration or aggravation of the lesions. The results of powermetry and thermometry have demonstrated the low power nature of the applied CO2 laser in NACLT.
As discussed above, in order to use the CO2 laser as a phototherapeutic laser for NACLT, the CO2 laser beam is irradiated through a thick layer of transparent, non-anesthetic gel with high water content. In addition, the CO2 laser is operated with a de-focused hand piece, scanning rapidly over the lesion with circular motion. With these considerations, CO2 laser can be used as a non-destructive, non-thermal, phototherapeutic laser (NACLT) to reduce pain in some oral mucosal lesions immediately and dramatically, so that after NACLT, the patients of the studies have been able to eat and drink easily at once. So far, in the series of NACLT studies, we have not observed any visible side effects following careful performance of the technique.
Certainly, controlled clinical trials with larger sample sizes will be able to prove the analgesic effects of NACLT more definitely. We recommend that in further studies, the pain severity of the lesions would be followed up for longer periods of time.
In addition, it should be emphasized that in serious diseases such as pemphigus vulgaris, Behcet's disease, etc., the patients must be warned that NACLT should not alter their conventional systemic treatment in spite of its significant analgesic effect.
#### **6. References**
408 CO2 Laser – Optimisation and Application
In some ulcerated oral lesions such as aphthous ulcers, the pain of the lesions derives from inflammatory sensitization of small-diameter afferent nerve endings that form a plexus at the junction of the epithelial and subepithelial layers. Branches of this plexus extend upward, into the epithelial layer; producing a superficial, focal, inflammatory lesion that is directly associated with exposed sensory nerve endings. Therefore, in such ulcers, CO2 laser irradiation can reach the exposed nerve endings easily and as we assume, for example, block the action potential generation and conduction of nociceptive signals in primary afferent neurons. On the other hand, in other under-publish studies, we have used NACLT for reducing pain in few non-ulcerated lesions, such as pre-aphthous lesions with moderate-good results. As the CO2 laser's beam has a very limited depth of penetration
Tuner and Hode state that the therapeutic effects of defocused CO2 laser must be due to the influence of the laser energy on the cells encountered, so that signal substances are released and then circulate in the organism. This indirectly confirms the hypothesis that conventional laser therapy has both a local effect in the area treated by laser light, and a systemic effect through the release of metabolites (Tuner and Hode 2010). We don't know whether such
Further fundamental studies are necessary to elucidate the mechanisms of analgesic effect of
CO2 laser has been used as a very useful high power, thermal laser in surgery for cutting, ablation and coagulation of the tissues for many years. In contrast, in non- thermal, Non-Ablative CO2 Laser Therapy (NACLT), the CO2 laser is used as a low level (phototherapeutic) laser to reduce pain in some oral mucosal lesions without any visual effects of thermal damage to the oral mucosa such as ablation, ulceration or aggravation of the lesions. The results of powermetry and thermometry have demonstrated the low power
As discussed above, in order to use the CO2 laser as a phototherapeutic laser for NACLT, the CO2 laser beam is irradiated through a thick layer of transparent, non-anesthetic gel with high water content. In addition, the CO2 laser is operated with a de-focused hand piece, scanning rapidly over the lesion with circular motion. With these considerations, CO2 laser can be used as a non-destructive, non-thermal, phototherapeutic laser (NACLT) to reduce pain in some oral mucosal lesions immediately and dramatically, so that after NACLT, the patients of the studies have been able to eat and drink easily at once. So far, in the series of NACLT studies, we have not observed any visible side effects following careful
Certainly, controlled clinical trials with larger sample sizes will be able to prove the analgesic effects of NACLT more definitely. We recommend that in further studies, the pain
In addition, it should be emphasized that in serious diseases such as pemphigus vulgaris, Behcet's disease, etc., the patients must be warned that NACLT should not alter their
severity of the lesions would be followed up for longer periods of time.
conventional systemic treatment in spite of its significant analgesic effect.
in to the tissue, explaining this analgesic effect of NACLT seems more complex.
mechanisms may at least in part, take part in the analgesic effect of NACLT or not.
NACLT.
**5. Conclusion**
nature of the applied CO2 laser in NACLT.
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**0**
**17**
*Italy*
**Protons Acceleration by CO**2 **Laser Pulses and**
Pasquale Londrillo, Graziano Servizi, Andrea Sgattoni, Stefano Sinigardi,
The acceleration of electrons with the high electric fields generated in a plasma by a very intense laser pulse was proposed over forty years ago [Tajima & Dawson (1979)], but only the advent of the chirped pulse amplification CPA [Mourou et al. (2006)] allowed to increase the laser power and intensity up to the required values. The continuous progress, since a decade, of compact Ti:Sa lasers allowed 1 GeV good quality electron beams to be generated [Leemans et al. (2006)]. The optical acceleration of protons and ions has been also actively investigated. The highest energy of protons, 60 MeV, has been reached with short high energy pulses of Nd:Yag lasers, developed for inertial fusion [Snavely (2000)]. With compact ultrashort Ti:Sa laser pulses intensities of 1021 W/cm2 are reached and proton beams with energy up to a few
The targets are typically thin metal foils and the acceleration is achieved in the TNSA regime (Target Normal Sheath Acceleration). The laser, interacting with an overcritical plasma, cannot propagate through it and heats the electrons on the surface of the target. A large number of "hot" electrons is hence produced and they are are accelerated in the forward direction and can cross the target. When reaching the rear surface they create an intense electrostatic field which accelerates the protons present on the surface [Passoni & Lontano (2004)]. The energy spectrum is exponential and the angular spread is significant so that the beam is not suitable for free propagation. Energy selection and collimation may reduce the intensity below the threshold required for any application. However other acceleration mechanisms have been considered such as the radiation pressure dominated regime RPA (Radiation Pressure Acceleration), where two distinct mechanism act depending on the thickness *h* of the target. If *h* ∼ *λ* the hole boring regime with break-up of the electron density wave is active [Macchi et al. (2005)], whereas for ultrathin targets the acceleration mechanism is the same as for the relativistic mirror [Londrillo et al. (2010); Macchi et al. (2009)] and high efficiencies can be reached. This regime was recently experimentally observed using a few nanometers carbon targets, a circularly polarized laser pulse with *λ* ∼ 1*μ*m and very high contrast [Henig (2009)]. The efficiency of the RPA should be higher than TNSA and the proton bunches should have a small energy and angular spread; however the requirements of circular polarization and high contrast of the laser pulse render this regime not easily achievable. The subcritical or
**1. Introduction**
tens of MeV are obtained [Zeil et al. (2010)].
**Perspectives for Medical Applications**
Marco Sumini and Giorgio Turchetti *Università di Bologna, INFN Sezione di Bologna*
### **Protons Acceleration by CO**2 **Laser Pulses and Perspectives for Medical Applications**
Pasquale Londrillo, Graziano Servizi, Andrea Sgattoni, Stefano Sinigardi, Marco Sumini and Giorgio Turchetti *Università di Bologna, INFN Sezione di Bologna Italy*
#### **1. Introduction**
414 CO2 Laser – Optimisation and Application
Zand, N., P. Mansouri, et al. (2009). Relieving pain in painful oral lesions of pemphigus
Zand, N., S. Najafi, et al. (2010). "NACLT ( Non-ablative CO2 laser therapy ): a new
chemotherapy ( a pilot study ) " *EJC supplements* 8(3): 166.
*medicine*. Harbor. 41: 67-68.
vulgaris by a single session, Non-ablative 10600 nm CO2 Laser irradiation ( pilot study ). *The 29th Annual conference of the American Society for Lasers in surgery and*
approach to relieve pain in mild to moderate oral mucositis following breast cancer
The acceleration of electrons with the high electric fields generated in a plasma by a very intense laser pulse was proposed over forty years ago [Tajima & Dawson (1979)], but only the advent of the chirped pulse amplification CPA [Mourou et al. (2006)] allowed to increase the laser power and intensity up to the required values. The continuous progress, since a decade, of compact Ti:Sa lasers allowed 1 GeV good quality electron beams to be generated [Leemans et al. (2006)]. The optical acceleration of protons and ions has been also actively investigated. The highest energy of protons, 60 MeV, has been reached with short high energy pulses of Nd:Yag lasers, developed for inertial fusion [Snavely (2000)]. With compact ultrashort Ti:Sa laser pulses intensities of 1021 W/cm2 are reached and proton beams with energy up to a few tens of MeV are obtained [Zeil et al. (2010)].
The targets are typically thin metal foils and the acceleration is achieved in the TNSA regime (Target Normal Sheath Acceleration). The laser, interacting with an overcritical plasma, cannot propagate through it and heats the electrons on the surface of the target. A large number of "hot" electrons is hence produced and they are are accelerated in the forward direction and can cross the target. When reaching the rear surface they create an intense electrostatic field which accelerates the protons present on the surface [Passoni & Lontano (2004)]. The energy spectrum is exponential and the angular spread is significant so that the beam is not suitable for free propagation. Energy selection and collimation may reduce the intensity below the threshold required for any application. However other acceleration mechanisms have been considered such as the radiation pressure dominated regime RPA (Radiation Pressure Acceleration), where two distinct mechanism act depending on the thickness *h* of the target. If *h* ∼ *λ* the hole boring regime with break-up of the electron density wave is active [Macchi et al. (2005)], whereas for ultrathin targets the acceleration mechanism is the same as for the relativistic mirror [Londrillo et al. (2010); Macchi et al. (2009)] and high efficiencies can be reached. This regime was recently experimentally observed using a few nanometers carbon targets, a circularly polarized laser pulse with *λ* ∼ 1*μ*m and very high contrast [Henig (2009)].
The efficiency of the RPA should be higher than TNSA and the proton bunches should have a small energy and angular spread; however the requirements of circular polarization and high contrast of the laser pulse render this regime not easily achievable. The subcritical or
number, were accelerated with a very small energy spread [Haberberger, Tochitsky, Fiuza, Gong, Fonseca, Silva, Mori & Joshi (2011); Haberberger, Tochitsky, Gong & Joshi (2011)]. This result inserts the CO2 lasers among the possible candidates for the production of proton beams for medical therapy, possibly combined with a post acceleration device. The CO2 lasers have high efficiency and produce almost circularly polarized light which is suited for acceleration mechanisms like RPA. In particular the possibility of using gas jets at under-critical or slightly overcritical density opens very interesting perspectives because of the high repetition rate allowed, the absence of debris (opposed to the case of solid thin targets) jointly with the
Protons Acceleration by CO2 Laser Pulses and Perspectives for Medical Applications 417
We can now study the very same physical problem using a CO2 laser instead of a Ti:Sa laser keeping the dynamics of the interaction unchanged. This can be done keeping unchanged the adimensional parameters which characterize the laser-plasma: the ratio of the plasma density over critical density (*n*/*nc*) and the normalized vector potential (*a*). This correspond to consider a CO2 laser pulse with the same peak power but ten times longer (same number of wave cycle) and a laser waist proportional to the wavelength. The plasma density is hundred times lower and the total volume interested by the acceleration is three order of magnitude larger than the case of a Ti:Sa laser. If we assume a definite fraction of the protons in the corresponding volume are accelerated and the density is kept at the critical value, then the number of accelerated protons is proportional to *λ*. If coupled with a high repetition rate, long wavelength pulses may offer a further advantage to reach the doses required by therapy. In the present note we shall review the basic mechanisms for laser acceleration to present the related scaling laws and compare the results one expects from small (1 *μ*) and large (10 *μ*) wavelength pulses. Systematic 2D and 3D simulations were performed with the high order PIC code ALaDyn [Benedetti et al. (2008)] developed by the university of Bologna to provide quantitative results in addition to the qualitative results of scaling laws. We shall also discuss
The paper consists of six sections: after this introduction, in section 2 we recall the basic features and parameters of the laser beam, in section 3 the TNSA regime is reviewed, in section 4 the RPA regime is presented, in section 5 the acceleration on under-critical target is discussed, in section 6 we discuss the transport of the optically accelerated proton bunch, in
A laser pulse is described by an electromagnetic wave packet, which is a solution of Maxwell's
in the vacuum where source charges and currents are absent *ρ* = 0, **j** = 0. The sources arise when the pulse propagates in material medium creating a plasma. The scalar and vector
**B** div **B** = 0
**E** div **E** = 4*πρ* (1)
advantages of circular polarization.
the transport of a protons beam through an optical system.
section 7 we analyze the perspectives for therapy.
rot **<sup>E</sup>** <sup>=</sup> <sup>−</sup><sup>1</sup>
rot **<sup>B</sup>** <sup>=</sup> <sup>4</sup>*<sup>π</sup>*
*c ∂ ∂t*
*<sup>c</sup>* **<sup>j</sup>** <sup>+</sup> 1 *c ∂ ∂t*
**2. Laser beam interaction with matter**
equations
quasi-critical targets are also a possible alternative because the laser-plasma energy coupling can be very high with a much higher energy transfer from the laser pulse to the plasma: a different regime may be achieved where the laser drills a channel and a strong electron current is created on its trail. At the exit from the plasma it creates a magnetic azimuthal field and a longitudinal electric field. As a consequence, the protons are efficiently accelerated and nicely collimated [Bulanov (2010); Nakamura, Bulanov, Esirkepov & Kando (2010); Naumova & Bulanov (2002); Yogo (2008)]. Experimental results with near critical targets confirmed the theoretical and simulation results on the enhancement of maximum energy and reduction of angular spread with respect to the TNSA acceleration mechanism [Fukuda & Bulanov (2009)].
The possibility of reaching energies close to the threshold of 60 MeV for cancer therapy with compact Ti:Sa laser system has stimulated several projects dedicated to medical applications. Indeed the protons or ions deposit most of their energy at the end of their range and are biologically more effective with respect to electrons or X rays since they allow to spare nearby healthy tissues. However the cyclotrons and synchrotrons currently used require large and expensive infrastructures. The use of laser acceleration opens a perspective for more compact and cheaper devices suitable to be installed on a regional scale. Two possible strategies are being considered:
The hybrid acceleration scheme does not require to develop new laser systems but only the improvement of targets and the design of a transport system capable of shaping the beam in such a way to render it suitable for injection. Simulations and experiments are presently being developed to explore the feasibility of transport of an optically accelerated protons bunch reaching the beam quality required for irradiation and for injection into a post-acceleration device [Melone (2011); Nishiuchi (2010); Schollmeier (2008)]. The reduction of the energy spread with a longitudinal phase space rotation provided by a synchronized RF was also proved [Noda (2008)].
Recently, new protons acceleration experiments have been performed taking advantage of short pulses of long wavelength *λ* = 10*μ*m produced by CO2 lasers. This approach provides a parallel research pathway which offers some advantages. Being the wavelength one order of magnitude larger than the optical values typical of Ti:Sa or Nd laser, the plasma critical density for *λ* = 10*μ*m is about 10<sup>19</sup> cm−<sup>3</sup> which can be reached ionizing a supersonic gas-jet. The CO2 lasers deliver a pulse with a native quasi circular polarization which is interesting for triggering the RPA regime at lower intensities.
Recent experiments showed that using a 1 TW pulse of 1 J interacting with a solid target, protons can be accelerated up to 1 MeV with exponential energy spectrum and wide angular spread typical of TNSA regime [Pogorelsky (2010b)]. On a gas jet at the critical density a quasi monochromatic beam of protons at 2 MeV was obtained suggesting that a RPA mechanism is dominating [Pogorelsky (2010a)]. With a train of short pulses and 100 J total energy a different acceleration mechanism was achieved and protons of 25 MeV, even though a low 2 Will-be-set-by-IN-TECH
quasi-critical targets are also a possible alternative because the laser-plasma energy coupling can be very high with a much higher energy transfer from the laser pulse to the plasma: a different regime may be achieved where the laser drills a channel and a strong electron current is created on its trail. At the exit from the plasma it creates a magnetic azimuthal field and a longitudinal electric field. As a consequence, the protons are efficiently accelerated and nicely collimated [Bulanov (2010); Nakamura, Bulanov, Esirkepov & Kando (2010); Naumova & Bulanov (2002); Yogo (2008)]. Experimental results with near critical targets confirmed the theoretical and simulation results on the enhancement of maximum energy and reduction of angular spread with respect to the TNSA acceleration mechanism [Fukuda & Bulanov (2009)]. The possibility of reaching energies close to the threshold of 60 MeV for cancer therapy with compact Ti:Sa laser system has stimulated several projects dedicated to medical applications. Indeed the protons or ions deposit most of their energy at the end of their range and are biologically more effective with respect to electrons or X rays since they allow to spare nearby healthy tissues. However the cyclotrons and synchrotrons currently used require large and expensive infrastructures. The use of laser acceleration opens a perspective for more compact and cheaper devices suitable to be installed on a regional scale. Two possible strategies are
A) increase the power of the laser system in order to reach energies in the 100-200 MeV energy
B) use the laser system as injector into a DTL linac to increase the energy starting from 10 MeV [Antici (2011)] or to inject a 30 MeV laser accelerated protons bunch, into a high field compact linac in order to raise the energy up to 100 or 200 MeV [Londrillo et al. (2011)]. The hybrid acceleration scheme does not require to develop new laser systems but only the improvement of targets and the design of a transport system capable of shaping the beam in such a way to render it suitable for injection. Simulations and experiments are presently being developed to explore the feasibility of transport of an optically accelerated protons bunch reaching the beam quality required for irradiation and for injection into a post-acceleration device [Melone (2011); Nishiuchi (2010); Schollmeier (2008)]. The reduction of the energy spread with a longitudinal phase space rotation provided by a synchronized RF was also
Recently, new protons acceleration experiments have been performed taking advantage of short pulses of long wavelength *λ* = 10*μ*m produced by CO2 lasers. This approach provides a parallel research pathway which offers some advantages. Being the wavelength one order of magnitude larger than the optical values typical of Ti:Sa or Nd laser, the plasma critical density for *λ* = 10*μ*m is about 10<sup>19</sup> cm−<sup>3</sup> which can be reached ionizing a supersonic gas-jet. The CO2 lasers deliver a pulse with a native quasi circular polarization which is interesting
Recent experiments showed that using a 1 TW pulse of 1 J interacting with a solid target, protons can be accelerated up to 1 MeV with exponential energy spectrum and wide angular spread typical of TNSA regime [Pogorelsky (2010b)]. On a gas jet at the critical density a quasi monochromatic beam of protons at 2 MeV was obtained suggesting that a RPA mechanism is dominating [Pogorelsky (2010a)]. With a train of short pulses and 100 J total energy a different acceleration mechanism was achieved and protons of 25 MeV, even though a low
range [Bulanov (2008); Hofmann (2011); Murakami (2008)]
being considered:
proved [Noda (2008)].
for triggering the RPA regime at lower intensities.
number, were accelerated with a very small energy spread [Haberberger, Tochitsky, Fiuza, Gong, Fonseca, Silva, Mori & Joshi (2011); Haberberger, Tochitsky, Gong & Joshi (2011)]. This result inserts the CO2 lasers among the possible candidates for the production of proton beams for medical therapy, possibly combined with a post acceleration device. The CO2 lasers have high efficiency and produce almost circularly polarized light which is suited for acceleration mechanisms like RPA. In particular the possibility of using gas jets at under-critical or slightly overcritical density opens very interesting perspectives because of the high repetition rate allowed, the absence of debris (opposed to the case of solid thin targets) jointly with the advantages of circular polarization.
We can now study the very same physical problem using a CO2 laser instead of a Ti:Sa laser keeping the dynamics of the interaction unchanged. This can be done keeping unchanged the adimensional parameters which characterize the laser-plasma: the ratio of the plasma density over critical density (*n*/*nc*) and the normalized vector potential (*a*). This correspond to consider a CO2 laser pulse with the same peak power but ten times longer (same number of wave cycle) and a laser waist proportional to the wavelength. The plasma density is hundred times lower and the total volume interested by the acceleration is three order of magnitude larger than the case of a Ti:Sa laser. If we assume a definite fraction of the protons in the corresponding volume are accelerated and the density is kept at the critical value, then the number of accelerated protons is proportional to *λ*. If coupled with a high repetition rate, long wavelength pulses may offer a further advantage to reach the doses required by therapy.
In the present note we shall review the basic mechanisms for laser acceleration to present the related scaling laws and compare the results one expects from small (1 *μ*) and large (10 *μ*) wavelength pulses. Systematic 2D and 3D simulations were performed with the high order PIC code ALaDyn [Benedetti et al. (2008)] developed by the university of Bologna to provide quantitative results in addition to the qualitative results of scaling laws. We shall also discuss the transport of a protons beam through an optical system.
The paper consists of six sections: after this introduction, in section 2 we recall the basic features and parameters of the laser beam, in section 3 the TNSA regime is reviewed, in section 4 the RPA regime is presented, in section 5 the acceleration on under-critical target is discussed, in section 6 we discuss the transport of the optically accelerated proton bunch, in section 7 we analyze the perspectives for therapy.
#### **2. Laser beam interaction with matter**
A laser pulse is described by an electromagnetic wave packet, which is a solution of Maxwell's equations
$$\begin{aligned} \text{rot}\,\mathbf{E} &= -\frac{1}{c}\frac{\partial}{\partial t}\mathbf{B} & \text{div}\,\mathbf{B} = 0\\ \text{rot}\,\mathbf{B} &= \frac{4\pi}{c}\mathbf{j} + \frac{1}{c}\frac{\partial}{\partial t}\mathbf{E} & \text{div}\,\mathbf{E} = 4\pi\rho \end{aligned} \tag{1}$$
in the vacuum where source charges and currents are absent *ρ* = 0, **j** = 0. The sources arise when the pulse propagates in material medium creating a plasma. The scalar and vector
x
x
t=1
x
*γ* =
*∂ f ∂***r** + *e***E** +
x
x
*∂ f*
**p**
x
B\_y t=1
x
*<sup>∂</sup>***<sup>p</sup>** <sup>=</sup> <sup>0</sup> (11)
*<sup>m</sup><sup>γ</sup> <sup>f</sup>*(**r**, **<sup>p</sup>**, *<sup>t</sup>*) *<sup>d</sup>***<sup>p</sup>** (12)
*mpc*<sup>2</sup> (13)
x
Energy t=1
E\_x t=1
2
z
2
z
A t=1
2
z
1/2
Fig. 1. Gaussian wave packet for the function *A* defined by eq. (7, 9), for *σ<sup>x</sup>* = 0.25, *σ<sup>z</sup>* = 1, *k*<sup>0</sup> = 4, *c* = 1, at different times (left figures). Electric field *Ex*, magnetic field *By* and energy at
> <sup>1</sup> <sup>+</sup> **<sup>p</sup>**<sup>2</sup> *m*2*c*<sup>2</sup>
Given *N* particles we introduce the phase space density *f*(**r**, **p**, *t*), which evolves according to
where the fields are solution of the Maxwell's equations, the sources being defined by
*f*(**r**, **p**, *t*) *d***p j** = *e*
The set (1), (11), (12) forms the Maxwell-Vlasov equations and provides the dynamical setting to investigate the laser plasma interaction. In actual computations the number *N* of numerical particles, used to sample the phase space density, is considerably lower than the number *N*ph of physical particles and the masses and charges are *N*ph/*N* times larger with respect to the masses and charges of the physical particles.The equations of motion are not affected since they depend only on the ratio *e*/*m*. The computation of charge densities and currents requires
The basic parameter of the electromagnetic wave is a dimensionless quantity which gives the ratio between the electromagnetic energy and the electron (or proton) rest mass energy
*mc*<sup>2</sup> *ap* <sup>=</sup> *eA*
*e mc<sup>γ</sup>* **<sup>p</sup>** <sup>×</sup> **<sup>B</sup>**
2
Protons Acceleration by CO2 Laser Pulses and Perspectives for Medical Applications 419
z
t=2
t=0.5
2
z
where *e* is the electric charge and *γ* is the relativistic factor
*∂ f <sup>∂</sup><sup>t</sup>* <sup>+</sup> **<sup>p</sup>** *mγ*
an interpolation procedure with smooth functions like splines,
*<sup>a</sup>* <sup>=</sup> *eA*
*ρ*(**r**, *t*) = *e*
2
z
t=0
2
z
time *t* = 1 (right figures).
the Liouville equation
**2.1 Basic parameters**
2
z
potential defined by
$$\mathbf{B} = \text{rot}\,\mathbf{A} \qquad \qquad \mathbf{E} = -\text{grad}\,\Phi - \frac{1}{c}\frac{\partial}{\partial t}\mathbf{A} \tag{2}$$
identically satisfy he first two equations. Choosing a gauge such that
$$
\mathbf{div}\,\mathbf{A} + \frac{1}{c}\Phi = 0\tag{3}
$$
the last two equations become the wave equations
$$\begin{aligned} \Delta \mathbf{A} - \frac{1}{c^2} \frac{\partial^2}{\partial t^2} \mathbf{A} &= -\frac{4\pi}{c} \mathbf{j} \\ \Delta \Phi - \frac{1}{c^2} \frac{\partial^2}{\partial t^2} \Phi &= -4\pi \rho \end{aligned} \tag{4}$$
In the vacuum *ρ* = 0 and we may choose Φ = 0. The gauge equation in this case reads div **A** = 0 and the electric field is given by **<sup>E</sup>** <sup>=</sup> <sup>−</sup>*c*−1*∂***A**/*∂t*. If we consider a two dimensional wave propagating in the *x*, *z* plane we may set *Ax* = *∂A*/*∂z* and *Az* = −*∂A*/*∂x* where *A* = *A*(*x*, *z*) so that div **A** = 0 is identically satisfied. In this case the electric field is given by
$$E\_X = -\frac{1}{c} \frac{\partial}{\partial t} \frac{\partial}{\partial z} A \qquad E\_y = 0 \qquad E\_z = \frac{1}{c} \frac{\partial}{\partial t} \frac{\partial}{\partial x} A \tag{5}$$
and the magnetic field
$$B\_{\mathbf{X}} = 0 \qquad B\_{\mathbf{Y}} = \left(\frac{\partial^2}{\partial \mathbf{x}^2} + \frac{\partial^2}{\partial z^2}\right) A \qquad B\_{\mathbf{Z}} = \mathbf{0} \tag{6}$$
Given an initial Gaussian wave field specified by
$$A(\mathbf{x}, z, \mathbf{0}) = A\_0(\mathbf{x}, z) \equiv \frac{e^{-\mathbf{x}^2 / 2\sigma\_\mathbf{x}^2}}{\sqrt{2\pi\sigma\_\mathbf{x}^2}} \frac{e^{-z^2 / 2\sigma\_\mathbf{z}^2}}{\sqrt{2\pi\sigma\_\mathbf{z}^2}} \cos(k\_0 z) \tag{7}$$
The evolution at time *t* is obtained by computing its Fourier transform and by taking into account that *A* satisfies the wave equation
$$
\Delta A - \frac{1}{c^2} \frac{\partial^2}{\partial t^2} A = 0 \tag{8}
$$
The result for the propagating wave packet is given by
$$A(\mathbf{x}, z, t) = \frac{1}{(2\pi)^2} \int\_{-\infty}^{+\infty} dk\_{\mathbf{x}} \int\_{-\infty}^{+\infty} dk\_{\mathbf{z}} \, e^{-\sigma\_{\mathbf{z}}^2 k\_{\mathbf{x}}^2/2} e^{-\sigma\_{\mathbf{z}}^2 (k\_{\mathbf{z}} - k\_0)^2/2} \cos(\mathbf{x} k\_{\mathbf{x}} + z k\_{\mathbf{z}} - \omega t) \tag{9}$$
When the wave interacts with a medium it ionizes it if the intensity is sufficiently high and the charged particles move according to the equations of motion
$$\frac{d\mathbf{r}}{dt} = \frac{\mathbf{p}}{m\gamma} \qquad\qquad\qquad \frac{d\mathbf{p}}{dt} = e\mathbf{E} + \frac{e}{mc\gamma} \mathbf{p} \times \mathbf{B} \tag{10}$$
Fig. 1. Gaussian wave packet for the function *A* defined by eq. (7, 9), for *σ<sup>x</sup>* = 0.25, *σ<sup>z</sup>* = 1, *k*<sup>0</sup> = 4, *c* = 1, at different times (left figures). Electric field *Ex*, magnetic field *By* and energy at time *t* = 1 (right figures).
where *e* is the electric charge and *γ* is the relativistic factor
$$\gamma = \left(1 + \frac{\mathbf{p}^2}{m^2 c^2}\right)^{1/2}$$
Given *N* particles we introduce the phase space density *f*(**r**, **p**, *t*), which evolves according to the Liouville equation
$$\left(\frac{\partial f}{\partial t} + \frac{\mathbf{p}}{m\gamma} \frac{\partial f}{\partial \mathbf{r}} + \left(e\mathbf{E} + \frac{e}{mc\gamma} \mathbf{p} \times \mathbf{B}\right) \frac{\partial f}{\partial \mathbf{p}} = 0\tag{11}$$
where the fields are solution of the Maxwell's equations, the sources being defined by
$$
\rho(\mathbf{r},t) = e \int f(\mathbf{r}, \mathbf{p}, t) \, d\mathbf{p} \qquad \qquad \mathbf{j} = e \int \frac{\mathbf{p}}{m\gamma} f(\mathbf{r}, \mathbf{p}, t) \, d\mathbf{p} \tag{12}
$$
The set (1), (11), (12) forms the Maxwell-Vlasov equations and provides the dynamical setting to investigate the laser plasma interaction. In actual computations the number *N* of numerical particles, used to sample the phase space density, is considerably lower than the number *N*ph of physical particles and the masses and charges are *N*ph/*N* times larger with respect to the masses and charges of the physical particles.The equations of motion are not affected since they depend only on the ratio *e*/*m*. The computation of charge densities and currents requires an interpolation procedure with smooth functions like splines,
#### **2.1 Basic parameters**
4 Will-be-set-by-IN-TECH
1 *c*
*<sup>∂</sup>t*<sup>2</sup> **<sup>A</sup>** <sup>=</sup> <sup>−</sup>4*<sup>π</sup>*
*<sup>∂</sup>t*<sup>2</sup> <sup>Φ</sup> <sup>=</sup> <sup>−</sup>4*πρ*
*A Ey* <sup>=</sup> <sup>0</sup> *Ez* <sup>=</sup> <sup>1</sup>
*∂*2 *∂z*<sup>2</sup>
*x*
*e*−*z*2/2*σ*<sup>2</sup> *z* <sup>2</sup>*πσ*<sup>2</sup> *z*
<sup>2</sup>*πσ*<sup>2</sup> *x*
The evolution at time *t* is obtained by computing its Fourier transform and by taking into
<sup>Δ</sup>*<sup>A</sup>* <sup>−</sup> <sup>1</sup> *c*2 *∂*2
> *dkz e* <sup>−</sup>*σ*<sup>2</sup> *<sup>x</sup> k*<sup>2</sup> *<sup>x</sup>*/2 *e* <sup>−</sup>*σ*<sup>2</sup>
When the wave interacts with a medium it ionizes it if the intensity is sufficiently high and
*d***p**
*dt* <sup>=</sup> *<sup>e</sup>***<sup>E</sup>** <sup>+</sup>
*e*
In the vacuum *ρ* = 0 and we may choose Φ = 0. The gauge equation in this case reads div **A** = 0 and the electric field is given by **<sup>E</sup>** <sup>=</sup> <sup>−</sup>*c*−1*∂***A**/*∂t*. If we consider a two dimensional wave propagating in the *x*, *z* plane we may set *Ax* = *∂A*/*∂z* and *Az* = −*∂A*/*∂x* where *A* = *A*(*x*, *z*)
*c* **j**
*c ∂ ∂t ∂ ∂x*
*c ∂ ∂t*
Φ = 0 (3)
**A** (2)
*A* (5)
*A Bz* = 0 (6)
*<sup>∂</sup>t*<sup>2</sup> *<sup>A</sup>* <sup>=</sup> <sup>0</sup> (8)
*<sup>z</sup>* (*kz*−*k*0)2/2 cos(*xkx* <sup>+</sup> *zkz* <sup>−</sup> *<sup>ω</sup>t*) (9)
*mc<sup>γ</sup>* **<sup>p</sup>** <sup>×</sup> **<sup>B</sup>** (10)
cos(*k*<sup>0</sup> *z*) (7)
(4)
**<sup>B</sup>** <sup>=</sup> rot **A E** <sup>=</sup> <sup>−</sup>grad <sup>Φ</sup> <sup>−</sup> <sup>1</sup>
div **A** +
<sup>Δ</sup>**<sup>A</sup>** <sup>−</sup> <sup>1</sup> *c*2 *∂*2
ΔΦ <sup>−</sup> <sup>1</sup> *c*2 *∂*2
so that div **A** = 0 is identically satisfied. In this case the electric field is given by
*∂*<sup>2</sup> *<sup>∂</sup>x*<sup>2</sup> <sup>+</sup>
identically satisfy he first two equations. Choosing a gauge such that
the last two equations become the wave equations
*Ex* <sup>=</sup> <sup>−</sup><sup>1</sup> *c ∂ ∂t ∂ ∂z*
Given an initial Gaussian wave field specified by
account that *A* satisfies the wave equation
(2*π*)<sup>2</sup>
*<sup>A</sup>*(*x*, *<sup>z</sup>*, *<sup>t</sup>*) = <sup>1</sup>
The result for the propagating wave packet is given by
+∞ −∞
*d***r** *dt* <sup>=</sup> **<sup>p</sup>** *mγ*
*dkx*
the charged particles move according to the equations of motion
+∞ −∞
*Bx* = 0 *By* =
*<sup>A</sup>*(*x*, *<sup>z</sup>*, 0) = *<sup>A</sup>*0(*x*, *<sup>z</sup>*) <sup>≡</sup> *<sup>e</sup>*−*x*2/2*σ*<sup>2</sup>
potential defined by
and the magnetic field
The basic parameter of the electromagnetic wave is a dimensionless quantity which gives the ratio between the electromagnetic energy and the electron (or proton) rest mass energy
$$a = \frac{eA}{mc^2} \qquad \qquad a\_p = \frac{eA}{m\_p c^2} \tag{13}$$
**3. The TNSA regime**
acceleration is observed.
given by
This regime is observed when the laser beam interacts with a metallic foil whose electron density is largely overcritical *n* � *nc*, the thickness *h* of the foil is large with respect to the skin depth *h* � *<sup>s</sup>* and the polarization is linear. When the laser pulse interacts with the overcritical plasma it is reflected by the target. A sizable fraction of its energy can be absorbed by the electrons of the plasma by linear or nonlinear mechanisms. These electrons can travel through the target and expand around both the front and the rear side. The thickness of
Protons Acceleration by CO2 Laser Pulses and Perspectives for Medical Applications 421
and Θ refers to the temperature of the electrons whose density is *n*. The protons which are present on both surfaces as part of the contaminants deposited on the target (hydrocarbons, water vapors) are accelerated by the electrostatic field built up by the expanding electron cloud preferentially along the normal to the surfaces. If the laser contrast is high enough the front side of the target may be preserved intact until the main part of the pulse interacts with the plasma and a substantially symmetric acceleration takes place in both forward direction (from the rear side) and backward direction (protons from the front side). On the other hand, if the front surface is destroyed by the prepulses or by the laser pedestal, only the forward
We can consider the motion of a single electron in a plane e.m. wave. Assuming that the propagation is along the *z* axis and the vector potential has only the *Ay* component, defining *<sup>a</sup>* <sup>=</sup> *eAy*/(*mc*2) the generalized momenta are *Px* <sup>=</sup> 0, *Py* <sup>=</sup> *py* <sup>−</sup> *mca*, *Pz* <sup>=</sup> *<sup>p</sup>*. If we have a particle in an external field then *Py* = 0 because it is so initially and it is conserved. When collective effects are present we consider a fluid approximation assuming that �*Py*� = 0 which implies �*py*� = *mca*. The longitudinal motion is a coherent one given by the ponderomotive force, whereas the transverse one is random and we may assume the temperature Θ to be
so that for *<sup>a</sup>* � 1 we have *kb*<sup>Θ</sup> � *a mc*2. As a consequence the Debye length *<sup>λ</sup><sup>D</sup>* is given by
1 <sup>4</sup>*<sup>π</sup> rc <sup>n</sup>* <sup>=</sup> *<sup>a</sup>*
where *n*<sup>0</sup> denotes the electron density. An estimate of the electrostatic field is obtained by supposing the electrons charge distribution obeys Boltzmann statistics so that *n* = *n*<sup>0</sup> *eeV*/(*kB*Θ)
1 + tan2
*<sup>h</sup>* <sup>−</sup> *<sup>z</sup> λ<sup>D</sup>* √2
*λ*2
*d*2*V*
*<sup>V</sup>* <sup>=</sup> *<sup>T</sup>*
simply the potential energy at the origin and can be expressed as
*<sup>e</sup>* log
and *V* satisfies the Poisson-Boltzmann equation
which must be solved with the conditions *V*(*h*) = *V*�
The longitudinal electric field is given by *Ez* = −*V*�
*<sup>D</sup>* <sup>=</sup> *kB*<sup>Θ</sup> *mc*<sup>2</sup>
*kB*<sup>Θ</sup> <sup>=</sup> *<sup>T</sup>* <sup>=</sup> *mc*2[(<sup>1</sup> <sup>+</sup> *<sup>a</sup>*2)1/2 <sup>−</sup> <sup>1</sup>] (21)
*dz*<sup>2</sup> <sup>=</sup> <sup>4</sup>*<sup>π</sup> <sup>n</sup>*<sup>0</sup> *<sup>e</sup>* exp (*eV*/(*kB*Θ)) (23)
(*h*) = 0. The solution is given by
(*z*); the maximum protons energy *E*max is
(24)
<sup>4</sup>*<sup>π</sup> rc <sup>n</sup>* (22)
*<sup>D</sup>* <sup>=</sup> *kB*Θ/(4*<sup>π</sup> <sup>e</sup>*2*n*)
the electron cloud is estimated equal to some Debye lengths *λ<sup>D</sup>* where *λ*<sup>2</sup>
When *a* ∼ 1 the electron becomes relativistic since the energy acquired from the wave is comparable with its rest energy. The intensity is related to the electromagnetic energy by
$$\frac{I}{c} = \frac{B^2 + E^2}{8\pi} = \pi \frac{A^2}{\lambda^2} \tag{14}$$
and consequently letting *rc* = *e*2/(*mc*2) = 3 10−<sup>13</sup> cm be the classical electron radius we have
$$a^2 = \frac{r\_c}{\pi \, mc^3} \, \lambda^2 I = \frac{I}{mc^3 n\_c} \tag{15}$$
where *nc* is the critical density defined below (19). A frequently used formula follows
$$a = 0.85 \, 10^{-9} I^{1/2} (\text{W/cm}^2) \, \lambda (\mu \text{m}) \tag{16}$$
The most relevant plasma parameter is the electron density which determines the plasma oscillations frequency *ω<sup>p</sup>*
$$
\omega\_p^2 = \frac{4\pi e^2 n}{m} = 4\pi r\_c c^2 n \tag{17}
$$
where *n* = *ρ*/*e* is the electron density, and *ρ* is the charge density. The plasma is an active optical medium and its refraction index is
$$m\_{\text{refr}} = \left(1 - \frac{\omega\_p^2}{\omega^2}\right)^{1/2} \tag{18}$$
For *ω<sup>p</sup>* < *ω* the medium is transparent. The density *nc* at which the medium becomes opaque is called critical density and is given by *ω<sup>p</sup>* = *ω* namely
$$n\_{\mathcal{L}} = \frac{\omega^2}{4\pi c^2 r\_{\mathcal{L}}} \simeq \frac{\pi}{\lambda^2 r\_{\mathcal{L}}} = \frac{10^{21}}{\lambda^2 (\mu \text{m})} \text{ cm}^{-3} \tag{19}$$
When the plasma is overcritical the wave becomes evanescent and decays exponentially. The decay length, called skin depth, is given by
$$\ell\_s = \frac{\lambda}{2\pi} \left(\frac{\omega\_p^2}{\omega^2} - 1\right)^{-1/2} = \frac{\lambda}{2\pi} \left(\frac{n}{n\_c} - 1\right)^{-1/2} \tag{20}$$
If we assume the waist to be a fixed multiple of the wavelength, *w* = *κλ* then *a*<sup>2</sup> is proportional to the pulse power and does not depend on the wavelength. On the contrary the critical density in proportional to *λ*<sup>−</sup>2, and is *nc* = 10−<sup>19</sup> cm−<sup>3</sup> for a CO2 laser pulse. This means that a gas jet, from which a plasma density in the range of 1018 <sup>÷</sup> 1020 cm−<sup>3</sup> can be obtained, provides a medium with quasi critical electron density. Moreover since the pulse length is in the range of hundreds of microns, the millimetric thickness of a gas jet is adequate for protons acceleration. Since the pulse durations for a Ti:Sa and CO2 laser are 30 fs and 1 ps respectively, in order to have the same power the ratio of the energies must be 1/30. For the same power and the same value of *a*, the proton energy should be the same but their number would be higher for a CO2 pulse.
#### **3. The TNSA regime**
6 Will-be-set-by-IN-TECH
When *a* ∼ 1 the electron becomes relativistic since the energy acquired from the wave is comparable with its rest energy. The intensity is related to the electromagnetic energy by
<sup>8</sup>*<sup>π</sup>* <sup>=</sup> *<sup>π</sup>*
and consequently letting *rc* = *e*2/(*mc*2) = 3 10−<sup>13</sup> cm be the classical electron radius we have
*<sup>π</sup> mc*<sup>3</sup> *<sup>λ</sup>*<sup>2</sup> *<sup>I</sup>* <sup>=</sup> *<sup>I</sup>*
The most relevant plasma parameter is the electron density which determines the plasma
where *n* = *ρ*/*e* is the electron density, and *ρ* is the charge density. The plasma is an active
For *ω<sup>p</sup>* < *ω* the medium is transparent. The density *nc* at which the medium becomes opaque
When the plasma is overcritical the wave becomes evanescent and decays exponentially. The
If we assume the waist to be a fixed multiple of the wavelength, *w* = *κλ* then *a*<sup>2</sup> is proportional to the pulse power and does not depend on the wavelength. On the contrary the critical density in proportional to *λ*<sup>−</sup>2, and is *nc* = 10−<sup>19</sup> cm−<sup>3</sup> for a CO2 laser pulse. This means that a gas jet, from which a plasma density in the range of 1018 <sup>÷</sup> 1020 cm−<sup>3</sup> can be obtained, provides a medium with quasi critical electron density. Moreover since the pulse length is in the range of hundreds of microns, the millimetric thickness of a gas jet is adequate for protons acceleration. Since the pulse durations for a Ti:Sa and CO2 laser are 30 fs and 1 ps respectively, in order to have the same power the ratio of the energies must be 1/30. For the same power and the same value of *a*, the proton energy should be the same but their number would be
−1/2
<sup>1</sup> <sup>−</sup> *<sup>ω</sup>*<sup>2</sup> *p ω*2
� *<sup>π</sup> λ*2*rc* *A*2
*mc*3*nc*
1/2
<sup>=</sup> <sup>1021</sup>
<sup>=</sup> *<sup>λ</sup>* 2*π* *n nc* − 1
*<sup>λ</sup>*<sup>2</sup> (14)
1/2(W/cm2) *λ*(*μ*m) (16)
*<sup>m</sup>* <sup>=</sup> <sup>4</sup>*πrcc*2*<sup>n</sup>* (17)
*<sup>λ</sup>*2(*μ*m) cm−<sup>3</sup> (19)
−1/2
(15)
(18)
(20)
*I*
*<sup>a</sup>*<sup>2</sup> <sup>=</sup> *rc*
*a* = 0.85 10−<sup>9</sup> *I*
*ω*2
oscillations frequency *ω<sup>p</sup>*
optical medium and its refraction index is
decay length, called skin depth, is given by
higher for a CO2 pulse.
is called critical density and is given by *ω<sup>p</sup>* = *ω* namely
�*<sup>s</sup>* <sup>=</sup> *<sup>λ</sup>* 2*π*
*nc* <sup>=</sup> *<sup>ω</sup>*<sup>2</sup> 4*πc*2*rc*
> *ω*<sup>2</sup> *p <sup>ω</sup>*<sup>2</sup> <sup>−</sup> <sup>1</sup>
*<sup>c</sup>* <sup>=</sup> *<sup>B</sup>*<sup>2</sup> <sup>+</sup> *<sup>E</sup>*<sup>2</sup>
where *nc* is the critical density defined below (19). A frequently used formula follows
*<sup>p</sup>* <sup>=</sup> <sup>4</sup>*πe*2*<sup>n</sup>*
*n*refr =
This regime is observed when the laser beam interacts with a metallic foil whose electron density is largely overcritical *n* � *nc*, the thickness *h* of the foil is large with respect to the skin depth *h* � *<sup>s</sup>* and the polarization is linear. When the laser pulse interacts with the overcritical plasma it is reflected by the target. A sizable fraction of its energy can be absorbed by the electrons of the plasma by linear or nonlinear mechanisms. These electrons can travel through the target and expand around both the front and the rear side. The thickness of the electron cloud is estimated equal to some Debye lengths *λ<sup>D</sup>* where *λ*<sup>2</sup> *<sup>D</sup>* <sup>=</sup> *kB*Θ/(4*<sup>π</sup> <sup>e</sup>*2*n*) and Θ refers to the temperature of the electrons whose density is *n*. The protons which are present on both surfaces as part of the contaminants deposited on the target (hydrocarbons, water vapors) are accelerated by the electrostatic field built up by the expanding electron cloud preferentially along the normal to the surfaces. If the laser contrast is high enough the front side of the target may be preserved intact until the main part of the pulse interacts with the plasma and a substantially symmetric acceleration takes place in both forward direction (from the rear side) and backward direction (protons from the front side). On the other hand, if the front surface is destroyed by the prepulses or by the laser pedestal, only the forward acceleration is observed.
We can consider the motion of a single electron in a plane e.m. wave. Assuming that the propagation is along the *z* axis and the vector potential has only the *Ay* component, defining *<sup>a</sup>* <sup>=</sup> *eAy*/(*mc*2) the generalized momenta are *Px* <sup>=</sup> 0, *Py* <sup>=</sup> *py* <sup>−</sup> *mca*, *Pz* <sup>=</sup> *<sup>p</sup>*. If we have a particle in an external field then *Py* = 0 because it is so initially and it is conserved. When collective effects are present we consider a fluid approximation assuming that �*Py*� = 0 which implies �*py*� = *mca*. The longitudinal motion is a coherent one given by the ponderomotive force, whereas the transverse one is random and we may assume the temperature Θ to be given by
$$k\_B \Theta = T = mc^2[(1+a^2)^{1/2} - 1] \tag{21}$$
so that for *<sup>a</sup>* � 1 we have *kb*<sup>Θ</sup> � *a mc*2. As a consequence the Debye length *<sup>λ</sup><sup>D</sup>* is given by
$$
\lambda\_D^2 = \frac{k\_B \Theta}{mc^2} \frac{1}{4\pi r\_c n} = \frac{a}{4\pi r\_c n} \tag{22}
$$
where *n*<sup>0</sup> denotes the electron density. An estimate of the electrostatic field is obtained by supposing the electrons charge distribution obeys Boltzmann statistics so that *n* = *n*<sup>0</sup> *eeV*/(*kB*Θ) and *V* satisfies the Poisson-Boltzmann equation
$$\frac{d^2V}{dz^2} = 4\pi \, n\_0 \, e \, \exp\left(eV/(k\_B\Theta)\right) \tag{23}$$
which must be solved with the conditions *V*(*h*) = *V*� (*h*) = 0. The solution is given by
$$V = \frac{T}{e} \log\left(1 + \tan^2\left(\frac{h-z}{\lambda\_D \sqrt{2}}\right)\right) \tag{24}$$
The longitudinal electric field is given by *Ez* = −*V*� (*z*); the maximum protons energy *E*max is simply the potential energy at the origin and can be expressed as
The main feature of TNSA is that the energy spectrum has an exponential decay with a clear cut-off. Letting *N*(*E*) be the number of protons having energy in the range [0, *E*] the spectrum
Protons Acceleration by CO2 Laser Pulses and Perspectives for Medical Applications 423
where *E*max is the maximum energy where the exponential distribution is cut off. If *E*<sup>0</sup> � *E*max, so that the error in normalization while replacing the integration upper bound *E*max with ∞ is negligible, then *E*<sup>0</sup> is precisely the average energy and *N*<sup>0</sup> is the total protons number. For instance suppose we have *E*max = 20 MeV and and *E*<sup>0</sup> = 2 MeV with *N*<sup>0</sup> = 1012. Such a number would be obtained with a 6 J pulse if 0.3 J are transferred to the protons. We should notice that the number of protons with a narrow energy in the range [*E*, *E* + Δ*E*] would
and consequently choosing *E* = 10 MeV, Δ*E* = 0.1 MeV we would have *N* = 3 108 protons. The angular spread of the protons is important, so that after collimation with an iris in order to allow focusing with a quadrupole or a solenoid the number would be further reduced, possibly below 107, which is rather low (but might be acceptable) in view of possible
In figure 3 we show the energy spectrum for a metal foil and a foam layer whose parameters are the same as in figure 2. The maximum energy is *E*max � 14 MeV and the average energy
Fig. 3. Energy spectrum for metal foil with foam and the same parameters and the same laser
In figure 4 we show the scaling of the maximum energy and average energy at different *a*. In this case, the average *E* is not computed on all the particles, but instead only on a subset that linearly fits the logarithmic plot, where the beginning and the end of the spectrum are excluded. It turns out that the average energy, in 3D simulations, is ∼ 1/7 of the maximum
energy, both for the bare target and a target with a foam.
Δ*E E*0 *e*
*dE* <sup>=</sup> <sup>0</sup> *<sup>E</sup>* <sup>&</sup>gt; *<sup>E</sup>*max (26)
<sup>−</sup>*E*/*E*<sup>0</sup> (27)
<sup>−</sup>*E*/*E*<sup>0</sup> <sup>0</sup> <sup>≤</sup> *<sup>E</sup>* <sup>&</sup>lt; *<sup>E</sup>*max *dN*
*N*([*E*, *E* + Δ*E*]) � *N*<sup>0</sup>
is given by
be
applications.
is 1.8 MeV.
pulse as figure 2.
*dN dE* <sup>=</sup> *<sup>N</sup>*<sup>0</sup> *E*0 *e*
$$E\_{\text{max}}(\text{MeV}) = \frac{E\_{\text{max}}}{2mc^2} \simeq \frac{a}{2} \log\left(1 + \tan^2\sqrt{2}\right) \simeq 2a \tag{25}$$
This result is compatible with experiments which show a linear dependence of *E*max with *I*1/2. However for very short and very collimated laser pulses the experimentally observed scaling law is *E*max ∝ *I*0.8. More refined theoretical models agree with this scaling law [Zani et al. (2011)].
The efficiency of TNSA acceleration can be enhanced if the efficiency of the energy transfer from laser to the target can be increased. If the laser interacts with a near critical density plasma the energy coupling of the laser with the target is considerably increased, comparing with the case of highly overcritical plasma, and a higher number of "hot" electrons can be obtained. The pre-pulse induced ionization creates a pre-plasma and improves the energy transfer from the laser to the electrons letting the laser interact with a plasma at lower density. The characteristics of the preplasma are not easily controlled in a metallic target being the control of the laser-pedestal and pre-pulses very difficult. However a different design of the target may be considered where a foam layer is deposited on the thin metal foil [Nakamura, Tampo, Kodama, Bulanov & Kando (2010)]. Such a target leads to a considerably greater laser energy absorption and to possibly an improved control of the laser target interaction. Systematic 2D and 3D PIC simulations have shown that the presence of a foam increases the maximum protons energy..
In figure 2 we show a comparison of the results obtained from a fully 3D simulation of the interaction of a laser beam with *a* = 10 with a thin metal foil with and without the coating of a slightly overcritical foam layer. The saturation of the maximum energy in the considered time interval is quite evident and the gain with the foam layer is almost a factor 3.
Fig. 2. Comparison of the maximum proton energy rise with time for a metal foil (red curve) with a foil on which a foam is superimposed (blue). The laser pulse has *λ* = 0.8 *μ*m, its duration is 25 fs, the waist is 3 *μ*m and the power is *W* = 30 TW so that *a*<sup>0</sup> = 10. The foil is 0, 5 *μ*m thick with a *n* = 40 *nc* whereas the foam layer is 2*μ* thick and *n* = 2*nc*. The polarization is linear and the incidence is normal. The accelerated protons come form the contaminants layer which is modeled as an ultra-thin H layer of 50 nm at density *n* = 9*nc*.
8 Will-be-set-by-IN-TECH
This result is compatible with experiments which show a linear dependence of *E*max with *I*1/2. However for very short and very collimated laser pulses the experimentally observed scaling law is *E*max ∝ *I*0.8. More refined theoretical models agree with this scaling law [Zani
The efficiency of TNSA acceleration can be enhanced if the efficiency of the energy transfer from laser to the target can be increased. If the laser interacts with a near critical density plasma the energy coupling of the laser with the target is considerably increased, comparing with the case of highly overcritical plasma, and a higher number of "hot" electrons can be obtained. The pre-pulse induced ionization creates a pre-plasma and improves the energy transfer from the laser to the electrons letting the laser interact with a plasma at lower density. The characteristics of the preplasma are not easily controlled in a metallic target being the control of the laser-pedestal and pre-pulses very difficult. However a different design of the target may be considered where a foam layer is deposited on the thin metal foil [Nakamura, Tampo, Kodama, Bulanov & Kando (2010)]. Such a target leads to a considerably greater laser energy absorption and to possibly an improved control of the laser target interaction. Systematic 2D and 3D PIC simulations have shown that the presence of a foam increases the
In figure 2 we show a comparison of the results obtained from a fully 3D simulation of the interaction of a laser beam with *a* = 10 with a thin metal foil with and without the coating of a slightly overcritical foam layer. The saturation of the maximum energy in the considered time
Fig. 2. Comparison of the maximum proton energy rise with time for a metal foil (red curve) with a foil on which a foam is superimposed (blue). The laser pulse has *λ* = 0.8 *μ*m, its duration is 25 fs, the waist is 3 *μ*m and the power is *W* = 30 TW so that *a*<sup>0</sup> = 10. The foil is
0, 5 *μ*m thick with a *n* = 40 *nc* whereas the foam layer is 2*μ* thick and *n* = 2*nc*. The polarization is linear and the incidence is normal. The accelerated protons come form the contaminants layer which is modeled as an ultra-thin H layer of 50 nm at density *n* = 9*nc*.
interval is quite evident and the gain with the foam layer is almost a factor 3.
<sup>2</sup> log
1 + tan<sup>2</sup>
√ 2
� 2*a* (25)
<sup>2</sup>*mc*<sup>2</sup> � *<sup>a</sup>*
*<sup>E</sup>*max(MeV) = *<sup>E</sup>*max
et al. (2011)].
maximum protons energy..
The main feature of TNSA is that the energy spectrum has an exponential decay with a clear cut-off. Letting *N*(*E*) be the number of protons having energy in the range [0, *E*] the spectrum is given by
$$\frac{dN}{dE} = \frac{N\_0}{E\_0} e^{-E/E\_0} \quad 0 \le E < E\_{\text{max}} \qquad \frac{dN}{dE} = 0 \quad E > E\_{\text{max}} \tag{26}$$
where *E*max is the maximum energy where the exponential distribution is cut off. If *E*<sup>0</sup> � *E*max, so that the error in normalization while replacing the integration upper bound *E*max with ∞ is negligible, then *E*<sup>0</sup> is precisely the average energy and *N*<sup>0</sup> is the total protons number. For instance suppose we have *E*max = 20 MeV and and *E*<sup>0</sup> = 2 MeV with *N*<sup>0</sup> = 1012. Such a number would be obtained with a 6 J pulse if 0.3 J are transferred to the protons. We should notice that the number of protons with a narrow energy in the range [*E*, *E* + Δ*E*] would be
$$N([E\_\prime E + \Delta E]) \simeq N\_0 \frac{\Delta E}{E\_0} \ e^{-E/E\_0} \tag{27}$$
and consequently choosing *E* = 10 MeV, Δ*E* = 0.1 MeV we would have *N* = 3 108 protons. The angular spread of the protons is important, so that after collimation with an iris in order to allow focusing with a quadrupole or a solenoid the number would be further reduced, possibly below 107, which is rather low (but might be acceptable) in view of possible applications.
In figure 3 we show the energy spectrum for a metal foil and a foam layer whose parameters are the same as in figure 2. The maximum energy is *E*max � 14 MeV and the average energy is 1.8 MeV.
Fig. 3. Energy spectrum for metal foil with foam and the same parameters and the same laser pulse as figure 2.
In figure 4 we show the scaling of the maximum energy and average energy at different *a*. In this case, the average *E* is not computed on all the particles, but instead only on a subset that linearly fits the logarithmic plot, where the beginning and the end of the spectrum are excluded. It turns out that the average energy, in 3D simulations, is ∼ 1/7 of the maximum energy, both for the bare target and a target with a foam.
where **p** and **P** are the ordinary and generalized momenta. For a wave propagating on the *z* direction **A** = **A**(*z* − *ct*) an averaging procedure with respect to the explicit time dependence of the Hamiltonian can be carried out. For a plane wave the time average vanishes �**A**� = 0.
Protons Acceleration by CO2 Laser Pulses and Perspectives for Medical Applications 425
This is the case in the non relativistic limit since the Hamiltonian becomes quadratic or if the
*<sup>z</sup>* ) � 1 averaging after a first order expansion in this variable shows that (29) is still valid.
The averages of generalized and ordinary momentum are the same and the gradient of the
This force dominates when the laser pulse is circularly polarized, because the electrons heating is strongly suppressed. The features of the acceleration mechanism depend on the target
For targets of thickness comparable with the pulse wavelength, we have the *hole boring* regime. The electrons density wave brakes on a distance comparable with the skin depth and the fluid approximation is no longer applicable. A piecewise constant approximation leads to a linear electric fields and the protons maximum energy can be easily evaluated. The
and for an ion of charge *Z* the energy is multiplied by *Z*. The angular spread of the beam is significantly lower with respect to the TNSA acceleration but the factor *nc*/*n* strongly reduces the energy value. Even though the scaling with *a* is quadratic rather than linear, only at very high values of the intensity the energy overcomes the value reached with TNSA. The only solution is to avoid the use of solid targets and work with a near critical density. This condition is naturally met for a wavelength of 10 *μ*m since the critical density *nc* = 1019 cm−<sup>3</sup> is met on gas jets. An experiment recently performed confirms that such a regime can be met and protons with a narrow energy spectrum can be accelerated [Palmer et al. (2010); Pogorelsky
For ultra-thin targets of thickness comparable with the skin depth the radiation pressure is able to push all the electrons of the foil which create a huge charge separation and all the ions are promptly accelerated. The result is that the target is practically accelerated as a whole as a
**P**<sup>2</sup> + *<sup>e</sup>*<sup>2</sup>
*<sup>c</sup>*<sup>2</sup> �**A**2� *m*2*c*<sup>2</sup>
<sup>2</sup>*mc*2*<sup>γ</sup>* grad �**A**2� <sup>=</sup> <sup>−</sup> *mc*<sup>2</sup>
<sup>2</sup>*mc*<sup>2</sup> � *<sup>a</sup>*<sup>2</sup> *nc*
1/2
(29)
*y*/(*p*<sup>2</sup> *<sup>x</sup>* +
<sup>2</sup>*<sup>γ</sup>* grad �**a**2� (30)
*<sup>n</sup>* (31)
As a consequence under suitable conditions it can be easily shown that
1 +
vector potential has a single component **<sup>A</sup>** <sup>=</sup> *Ay*(*<sup>z</sup>* <sup>−</sup> *ct*) **<sup>e</sup>***y*. In this case supposing *<sup>p</sup>*<sup>2</sup>
*dt* <sup>=</sup> <sup>−</sup> *<sup>e</sup>*<sup>2</sup>
squared electromagnetic potential is the ponderomotive force in this approximation.
*<sup>E</sup>*max(MeV) = *<sup>E</sup>*max
�*H*� � *mc*<sup>2</sup>
*d***P**
As a consequence the equations of motion read
*d***r** *dt* <sup>=</sup> **<sup>P</sup>** *γ m*
*p*2
geometry.
(2010a)].
**4.2 Relativistic mirror**
**4.1 Hole Boring**
expression one obtains is
We noticed that a good linear fit holds in both cases and that for 2D simulations the maximum value of energy *E*max is about twice the energy obtained for 3D simulations.
Fig. 4. Scalings for the maximum and average energies computed from linear fit on a logarithmic plot for 2D and 3D simulations for a target with a metal foil 0.5 *μ*m thick and density *n* = 80*nc*, a foam layer of 2 *μ*m and density *n* = 2*nc*, a layer of contaminants of 50 nm and density of *n* = 9*nc*.
#### **3.1 CO**2 **results**
Experiments in Brookhaven with 1 TW CO2 laser pulses have shown that protons with maximum energy of 1 MeV can be obtained [Pogorelsky (2010b)]. The scaling laws obtained from both experiments and theoretical/numerical work does not offer significant perspectives to reach proton energies interesting for hadron therapy purposes unless powers in the PW range can be reached. As a consequence TNSA accelerated protons might be proposed for medical applications only coupled with a post acceleration device. Even in this case energies above 10 MeV should be reached. The use of targets with a coating of of 5-10 microns of a silicon foam with near critical density on the illuminated surface may increase the maximum energy by a significant factor (2-4) as simulations and experiments have shown [Nakamura, Tampo, Kodama, Bulanov & Kando (2010)]. As a consequence the injection energy may be reached with compact lasers. This improved type of targets may be used for CO2 laser pulses as well. The most promising scenarios however are met when quasi critical are used, typically provided by a gas jet. In this case however we face a different acceleration regime which is dominated by the radiation pressure if the electron heating is modest as in the case of a circular polarization.
#### **4. The RPA regime**
The radiation pressure becomes the dominant mechanism in the acceleration of protons when *I* > 1023 W/cm2. However the effect of radiation pressure prevails over electrostatic acceleration even at lower energies when the electrons heating is decreased using a circularly polarized light. Given a vector potential **A** the relativistic Hamiltonian of a charged particle is
$$H = mc^2(\gamma - 1) \qquad \qquad \gamma = \left(1 + \frac{\mathbf{p}^2}{m^2 c^2}\right)^{1/2} \qquad \mathbf{p} = \mathbf{P} - \frac{e}{c}\mathbf{A} \tag{28}$$
where **p** and **P** are the ordinary and generalized momenta. For a wave propagating on the *z* direction **A** = **A**(*z* − *ct*) an averaging procedure with respect to the explicit time dependence of the Hamiltonian can be carried out. For a plane wave the time average vanishes �**A**� = 0. As a consequence under suitable conditions it can be easily shown that
$$
\langle H \rangle \simeq mc^2 \left( 1 + \frac{\mathbf{P}^2 + \frac{\varepsilon^2}{c^2} \langle \mathbf{A}^2 \rangle}{m^2 c^2} \right)^{1/2} \tag{29}
$$
This is the case in the non relativistic limit since the Hamiltonian becomes quadratic or if the vector potential has a single component **<sup>A</sup>** <sup>=</sup> *Ay*(*<sup>z</sup>* <sup>−</sup> *ct*) **<sup>e</sup>***y*. In this case supposing *<sup>p</sup>*<sup>2</sup> *y*/(*p*<sup>2</sup> *<sup>x</sup>* + *p*2 *<sup>z</sup>* ) � 1 averaging after a first order expansion in this variable shows that (29) is still valid. As a consequence the equations of motion read
$$\frac{d\mathbf{r}}{dt} = \frac{\mathbf{P}}{\gamma \, m} \qquad \qquad \frac{d\mathbf{P}}{dt} = -\frac{e^2}{2mc^2\gamma} \operatorname{grad} \left< \mathbf{A}^2 \right> = -\frac{mc^2}{2\gamma} \operatorname{grad} \left< \mathbf{a}^2 \right> \tag{30}$$
The averages of generalized and ordinary momentum are the same and the gradient of the squared electromagnetic potential is the ponderomotive force in this approximation.
This force dominates when the laser pulse is circularly polarized, because the electrons heating is strongly suppressed. The features of the acceleration mechanism depend on the target geometry.
#### **4.1 Hole Boring**
10 Will-be-set-by-IN-TECH
We noticed that a good linear fit holds in both cases and that for 2D simulations the maximum
E (MeV)
Fig. 4. Scalings for the maximum and average energies computed from linear fit on a logarithmic plot for 2D and 3D simulations for a target with a metal foil 0.5 *μ*m thick and density *n* = 80*nc*, a foam layer of 2 *μ*m and density *n* = 2*nc*, a layer of contaminants of 50 nm
Experiments in Brookhaven with 1 TW CO2 laser pulses have shown that protons with maximum energy of 1 MeV can be obtained [Pogorelsky (2010b)]. The scaling laws obtained from both experiments and theoretical/numerical work does not offer significant perspectives to reach proton energies interesting for hadron therapy purposes unless powers in the PW range can be reached. As a consequence TNSA accelerated protons might be proposed for medical applications only coupled with a post acceleration device. Even in this case energies above 10 MeV should be reached. The use of targets with a coating of of 5-10 microns of a silicon foam with near critical density on the illuminated surface may increase the maximum energy by a significant factor (2-4) as simulations and experiments have shown [Nakamura, Tampo, Kodama, Bulanov & Kando (2010)]. As a consequence the injection energy may be reached with compact lasers. This improved type of targets may be used for CO2 laser pulses as well. The most promising scenarios however are met when quasi critical are used, typically provided by a gas jet. In this case however we face a different acceleration regime which is dominated by the radiation pressure if the electron heating is modest as in the case of a circular
The radiation pressure becomes the dominant mechanism in the acceleration of protons when *I* > 1023 W/cm2. However the effect of radiation pressure prevails over electrostatic acceleration even at lower energies when the electrons heating is decreased using a circularly polarized light. Given a vector potential **A** the relativistic Hamiltonian of a charged particle is
<sup>1</sup> <sup>+</sup> **<sup>p</sup>**<sup>2</sup> *m*2*c*<sup>2</sup> 1/2
**<sup>p</sup>** <sup>=</sup> **<sup>P</sup>** <sup>−</sup> *<sup>e</sup>*
*c*
**A** (28)
*<sup>H</sup>* <sup>=</sup> *mc*2(*<sup>γ</sup>* <sup>−</sup> <sup>1</sup>) *<sup>γ</sup>* <sup>=</sup>
Emax 7 x Eaverage
10 15 20 25 30
3D
a
value of energy *E*max is about twice the energy obtained for 3D simulations.
10 15 20 25 30
2D
a
**3.1 CO**2 **results**
polarization.
**4. The RPA regime**
and density of *n* = 9*nc*.
Emax 5 x Eaverage
E (MeV)
For targets of thickness comparable with the pulse wavelength, we have the *hole boring* regime. The electrons density wave brakes on a distance comparable with the skin depth and the fluid approximation is no longer applicable. A piecewise constant approximation leads to a linear electric fields and the protons maximum energy can be easily evaluated. The expression one obtains is
$$E\_{\text{max}}(\text{MeV}) = \frac{E\_{\text{max}}}{2mc^2} \simeq a^2 \, \frac{n\_c}{n} \tag{31}$$
and for an ion of charge *Z* the energy is multiplied by *Z*. The angular spread of the beam is significantly lower with respect to the TNSA acceleration but the factor *nc*/*n* strongly reduces the energy value. Even though the scaling with *a* is quadratic rather than linear, only at very high values of the intensity the energy overcomes the value reached with TNSA. The only solution is to avoid the use of solid targets and work with a near critical density. This condition is naturally met for a wavelength of 10 *μ*m since the critical density *nc* = 1019 cm−<sup>3</sup> is met on gas jets. An experiment recently performed confirms that such a regime can be met and protons with a narrow energy spectrum can be accelerated [Palmer et al. (2010); Pogorelsky (2010a)].
#### **4.2 Relativistic mirror**
For ultra-thin targets of thickness comparable with the skin depth the radiation pressure is able to push all the electrons of the foil which create a huge charge separation and all the ions are promptly accelerated. The result is that the target is practically accelerated as a whole as a
Fig. 5. Protons energy in MeV versus intensity for a pulse with *λ* = 10*μ*m. various foil thickness are considered: = 5*μ*m (red), = 10*μ*m (green), = 15*μ*m (Blue), = 30*μ*m (purple). The 1D PIC results are compared with the analytical results for the RPA hole boring
where *n* denotes the electrons density. The number *N* of ions in the target and the minimum number *N*<sup>∗</sup> below which transparency is induced and the corresponding minimum thickness
Protons Acceleration by CO2 Laser Pulses and Perspectives for Medical Applications 427
In this regime, keeping the ratio *h*/*λ* fixed, the volume of the accelerated ions increases as *λ*3, their number as *λ* if *n*/*nc* is also kept fixed. As a consequence, at the transparency limit the parameter *a* and the laser power have a fixed value. The parameter *α* is constant if the energy is kept proportional to *λ*. In this case the total energy and the proton number increase with *λ*, whereas the energy of each proton does not vary. If the laser energy and the pulse duration both increase with *λ* the power remains constant. In Figure 5 we compare the analytic scaling provided by equations (31) and (37) with 1D PIC simulations for a circularly polarized 10*μ*m pulse. The transition between these regimes is evident and the agreement between the analytical and numerical result is quite good. The RPA regime is well suited for CO2 lasers because the native polarization is circular. Slightly overcritical targets are provided by gas jets and in the hole boring regime the choice *n* � *nc* is necessary in order to achieve high energies, according to (31). In addition a quasi monochromatic spectrum is obtained at moderate intensities typical of *CO*<sup>2</sup> lasers. In this case the hole boring scenario is the most promising as recent experiments have shown [Palmer et al. (2010); Pogorelsky (2010a)].
The relativistic mirror model is very attractive on the basis on the analytical results obtained from the 1D model. However 2D and 3D simulations show that a deterioration occurs when the dimensionality is increased, due to the onset of Rayleigh-Taylor like instabilities. Considering also the difficulties met in the preparation of ultrathin targets in order to be close to the transparency limit and the requirements on the contrast, this regime is not likely to be
interesting for application, even using long wavelength pulses, in a near future.
*a λrc* *<sup>h</sup>*<sup>∗</sup> <sup>=</sup> *<sup>N</sup>*<sup>∗</sup>
*n S* <sup>=</sup> *<sup>a</sup>*
*Z λ rc ne*
(40)
*<sup>N</sup>*<sup>∗</sup> <sup>=</sup> *<sup>S</sup> Z*
(solid black line) and the relativistic mirror (dotted line).
*ζ λrc*
*h*<sup>∗</sup> of the target are given by
*<sup>N</sup>* <sup>=</sup> *<sup>n</sup>*
*<sup>Z</sup> h S* <sup>=</sup> *<sup>S</sup> Z*
rigid object behaving like a mirror whose equations of motion are
$$\frac{d\mathbf{x}}{dt} = c\boldsymbol{\mathfrak{E}} \qquad \qquad \frac{d\boldsymbol{\mathfrak{E}}}{dt} = \frac{2I}{\mu c^2} \frac{1-\boldsymbol{\mathfrak{E}}}{1+\boldsymbol{\mathfrak{E}}} (1-\boldsymbol{\mathfrak{E}}^2)^{3/2} \tag{32}$$
where *μ* is the surface density of the mirror and *I* = *I*<sup>0</sup> *f* ((*t* − *x*/*c*)/*τ*laser) is the laser pulse. The function *f*(*s*) vanishes except for |*s*| < 1 where it is positive. The equations of motion 30 have a first integral of motion. Setting
$$\mathbf{t}' = \frac{\mathbf{t}}{\tau\_{\text{laser}}}, \qquad \mathbf{x}' = \frac{\mathbf{x}}{c\tau\_{\text{laser}}}, \qquad w = t' - \mathbf{x}' \qquad \chi = \frac{2I\_0 \,\tau\_{\text{laser}}}{\mu c^2} \tag{33}$$
the equations of motion become
$$\frac{d\beta}{dt'} = \chi \ f(w) \ \frac{1-\beta}{1+\beta} \ (1-\beta^2)^{3/2} \qquad\qquad \frac{dw}{dt'} = 1-\beta \tag{34}$$
and introducing the integrating factor *<sup>C</sup>* = (<sup>1</sup> <sup>+</sup> *<sup>β</sup>*)(<sup>1</sup> <sup>−</sup> *<sup>β</sup>*)−1(<sup>1</sup> <sup>−</sup> *<sup>β</sup>*2)−3/2 the differential form *dH* = *C* (<sup>1</sup> <sup>−</sup> *<sup>β</sup>*)*d<sup>β</sup>* <sup>−</sup> *<sup>χ</sup> <sup>f</sup>*(*w*) (<sup>1</sup> <sup>−</sup> *<sup>β</sup>*2)3/2(<sup>1</sup> <sup>−</sup> *<sup>β</sup>*)/(<sup>1</sup> <sup>+</sup> *<sup>β</sup>*) *dw* becomes exact and the first integral is
$$H = \chi \int\_{-1}^{w} f(w') dw' - \left(\frac{1+\beta}{1-\beta}\right)^{1/2} \tag{35}$$
The initial condition corresponds to *w* = −1, *β* = 0 so that *H* = −1. At the end of the pulse we have *<sup>w</sup>* <sup>=</sup> 1 and *<sup>β</sup>* <sup>=</sup> *<sup>β</sup>*<sup>∗</sup> which is the highest speed value. Denoting by *<sup>F</sup>* <sup>=</sup> <sup>1</sup> <sup>−</sup><sup>1</sup> *<sup>f</sup>*(*w*)*dw* the fluence we have *<sup>χ</sup><sup>F</sup>* <sup>−</sup> [(<sup>1</sup> <sup>+</sup> *<sup>β</sup>*∗)/(<sup>1</sup> <sup>−</sup> *<sup>β</sup>*∗)]1/2 <sup>=</sup> <sup>−</sup>1. We obtain *<sup>β</sup>*<sup>∗</sup> and *<sup>γ</sup>*<sup>∗</sup> as a function of *α* = *χ F* which is given by
$$
\alpha = \chi F = \frac{2I\_0 \, F \, \tau\_{\text{laser S}} \, \text{S}}{\mu \, c^2 \, \text{S}} = \frac{2E\_{\text{laser}}}{E\_{\text{rest mirr}}} \tag{36}
$$
Expressing *<sup>γ</sup>*<sup>∗</sup> = (1<sup>−</sup> *<sup>β</sup>*<sup>2</sup> <sup>∗</sup>)−1/2 as a function of *<sup>α</sup>* we obtain the expression for the kinetic energy of the ion which is given by
$$E\_{\text{max}} = Am\_p c^2 (\gamma\_\* - 1) = Am\_p c^2 \frac{\alpha^2}{2 + 2\alpha} = \frac{E\_{\text{laser}}}{N} \frac{\alpha}{1 + \alpha} \tag{37}$$
where (36) has been used taking into account *<sup>E</sup>*rest mirr = *NAmpc*<sup>2</sup> where *<sup>N</sup>* is the number of ions in the mirror. As a consequence the efficiency of the acceleration process is given by
$$\eta = \frac{E\_{\text{mirr}}}{E\_{\text{laser}}} = \frac{\alpha}{1+\alpha} \tag{38}$$
From equation (36) it appears that the thinner is the mirror the higher is the efficiency and the protons energy because *μ* = *AZ*−<sup>1</sup> *mp h n*. However a limit is imposed by the transparency limit Macchi et al. (2009). The target remains opaque and is accelerated as a mirror provided that
$$a \le \zeta = \pi \frac{n}{n\_c} \left. \frac{h}{\lambda} \right| \tag{39}$$
12 Will-be-set-by-IN-TECH
*dt* <sup>=</sup> <sup>2</sup>*<sup>I</sup> μc*<sup>2</sup>
where *μ* is the surface density of the mirror and *I* = *I*<sup>0</sup> *f* ((*t* − *x*/*c*)/*τ*laser) is the laser pulse. The function *f*(*s*) vanishes except for |*s*| < 1 where it is positive. The equations of motion 30
, *w* = *t*
<sup>1</sup> <sup>+</sup> *<sup>β</sup>* (<sup>1</sup> <sup>−</sup> *<sup>β</sup>*2)3/2 *dw*
and introducing the integrating factor *<sup>C</sup>* = (<sup>1</sup> <sup>+</sup> *<sup>β</sup>*)(<sup>1</sup> <sup>−</sup> *<sup>β</sup>*)−1(<sup>1</sup> <sup>−</sup> *<sup>β</sup>*2)−3/2 the differential
)*dw*� −
The initial condition corresponds to *w* = −1, *β* = 0 so that *H* = −1. At the end of the pulse
the fluence we have *<sup>χ</sup><sup>F</sup>* <sup>−</sup> [(<sup>1</sup> <sup>+</sup> *<sup>β</sup>*∗)/(<sup>1</sup> <sup>−</sup> *<sup>β</sup>*∗)]1/2 <sup>=</sup> <sup>−</sup>1. We obtain *<sup>β</sup>*<sup>∗</sup> and *<sup>γ</sup>*<sup>∗</sup> as a function of
where (36) has been used taking into account *<sup>E</sup>*rest mirr = *NAmpc*<sup>2</sup> where *<sup>N</sup>* is the number of ions in the mirror. As a consequence the efficiency of the acceleration process is given by
From equation (36) it appears that the thinner is the mirror the higher is the efficiency and the protons energy because *μ* = *AZ*−<sup>1</sup> *mp h n*. However a limit is imposed by the transparency limit Macchi et al. (2009). The target remains opaque and is accelerated as a mirror provided
*a* ≤ *ζ* = *π*
<sup>=</sup> *<sup>α</sup>*
*n nc h*
*<sup>η</sup>* <sup>=</sup> *<sup>E</sup>*mirr *E*laser 1 + *β* 1 − *β*
*<sup>μ</sup> <sup>c</sup>*<sup>2</sup> *<sup>S</sup>* <sup>=</sup> <sup>2</sup>*E*laser
1/2
*E*rest mirr
<sup>2</sup> <sup>+</sup> <sup>2</sup>*<sup>α</sup>* <sup>=</sup> *<sup>E</sup>*laser
*N*
*α*
<sup>1</sup> <sup>+</sup> *<sup>α</sup>* (38)
*<sup>λ</sup>* (39)
<sup>∗</sup>)−1/2 as a function of *<sup>α</sup>* we obtain the expression for the kinetic energy
(<sup>1</sup> <sup>−</sup> *<sup>β</sup>*)*d<sup>β</sup>* <sup>−</sup> *<sup>χ</sup> <sup>f</sup>*(*w*) (<sup>1</sup> <sup>−</sup> *<sup>β</sup>*2)3/2(<sup>1</sup> <sup>−</sup> *<sup>β</sup>*)/(<sup>1</sup> <sup>+</sup> *<sup>β</sup>*) *dw*
we have *<sup>w</sup>* <sup>=</sup> 1 and *<sup>β</sup>* <sup>=</sup> *<sup>β</sup>*<sup>∗</sup> which is the highest speed value. Denoting by *<sup>F</sup>* <sup>=</sup> <sup>1</sup>
*<sup>α</sup>* <sup>=</sup> *<sup>χ</sup><sup>F</sup>* <sup>=</sup> <sup>2</sup>*I*<sup>0</sup> *<sup>F</sup> <sup>τ</sup>*laser *<sup>S</sup>*
*<sup>E</sup>*max <sup>=</sup> *Ampc*2(*γ*<sup>∗</sup> <sup>−</sup> <sup>1</sup>) = *Ampc*<sup>2</sup> *<sup>α</sup>*<sup>2</sup>
1 − *β*
<sup>1</sup> <sup>+</sup> *<sup>β</sup>* (<sup>1</sup> <sup>−</sup> *<sup>β</sup>*2)3/2 (32)
*<sup>μ</sup> <sup>c</sup>*<sup>2</sup> (33)
becomes exact and the
<sup>1</sup> <sup>+</sup> *<sup>α</sup>* (37)
(35)
(36)
<sup>−</sup><sup>1</sup> *<sup>f</sup>*(*w*)*dw*
*dt*� <sup>=</sup> <sup>1</sup> <sup>−</sup> *<sup>β</sup>* (34)
� <sup>−</sup> *<sup>x</sup>*� *<sup>χ</sup>* <sup>=</sup> <sup>2</sup>*I*<sup>0</sup> *<sup>τ</sup>*laser
rigid object behaving like a mirror whose equations of motion are
, *<sup>x</sup>*� <sup>=</sup> *<sup>x</sup>*
*dt*� <sup>=</sup> *<sup>χ</sup> <sup>f</sup>*(*w*) <sup>1</sup> <sup>−</sup> *<sup>β</sup>*
*H* = *χ*
*w* −1 *f*(*w*�
*cτ*laser
*dt* <sup>=</sup> *<sup>c</sup><sup>β</sup> <sup>d</sup><sup>β</sup>*
*dx*
have a first integral of motion. Setting
*dβ*
*t* � <sup>=</sup> *<sup>t</sup> τ*laser
the equations of motion become
*α* = *χ F* which is given by
Expressing *<sup>γ</sup>*<sup>∗</sup> = (1<sup>−</sup> *<sup>β</sup>*<sup>2</sup>
that
of the ion which is given by
form *dH* = *C*
first integral is
Fig. 5. Protons energy in MeV versus intensity for a pulse with *λ* = 10*μ*m. various foil thickness are considered: = 5*μ*m (red), = 10*μ*m (green), = 15*μ*m (Blue), = 30*μ*m (purple). The 1D PIC results are compared with the analytical results for the RPA hole boring (solid black line) and the relativistic mirror (dotted line).
where *n* denotes the electrons density. The number *N* of ions in the target and the minimum number *N*<sup>∗</sup> below which transparency is induced and the corresponding minimum thickness *h*<sup>∗</sup> of the target are given by
$$N = \frac{n}{Z}h \\ S = \frac{S}{Z} \begin{array}{c} \frac{S}{\lambda r\_{\odot}} \\ \end{array} \qquad \qquad N\_{\*} = \frac{S}{Z} \begin{array}{c} a \\ \frac{\lambda r\_{\odot}}{\lambda r\_{\odot}} \\ \end{array} \qquad \qquad h\_{\*} = \frac{N\_{\*}}{n \cdot S} = \frac{a}{Z \cdot \lambda \, r\_{\odot} \, n\_{\odot}} \tag{40}$$
In this regime, keeping the ratio *h*/*λ* fixed, the volume of the accelerated ions increases as *λ*3, their number as *λ* if *n*/*nc* is also kept fixed. As a consequence, at the transparency limit the parameter *a* and the laser power have a fixed value. The parameter *α* is constant if the energy is kept proportional to *λ*. In this case the total energy and the proton number increase with *λ*, whereas the energy of each proton does not vary. If the laser energy and the pulse duration both increase with *λ* the power remains constant. In Figure 5 we compare the analytic scaling provided by equations (31) and (37) with 1D PIC simulations for a circularly polarized 10*μ*m pulse. The transition between these regimes is evident and the agreement between the analytical and numerical result is quite good. The RPA regime is well suited for CO2 lasers because the native polarization is circular. Slightly overcritical targets are provided by gas jets and in the hole boring regime the choice *n* � *nc* is necessary in order to achieve high energies, according to (31). In addition a quasi monochromatic spectrum is obtained at moderate intensities typical of *CO*<sup>2</sup> lasers. In this case the hole boring scenario is the most promising as recent experiments have shown [Palmer et al. (2010); Pogorelsky (2010a)].
The relativistic mirror model is very attractive on the basis on the analytical results obtained from the 1D model. However 2D and 3D simulations show that a deterioration occurs when the dimensionality is increased, due to the onset of Rayleigh-Taylor like instabilities. Considering also the difficulties met in the preparation of ultrathin targets in order to be close to the transparency limit and the requirements on the contrast, this regime is not likely to be interesting for application, even using long wavelength pulses, in a near future.
and protons density at two different times exhibiting the formation of the channel are shown in figures 6 and 7 for a 3D simulation. The spectrum of the protons is still exponential and
Protons Acceleration by CO2 Laser Pulses and Perspectives for Medical Applications 429
Fig. 6. Electrons (left frame) and protons (right frame) density in the case of a laser pulse of 200 TW, with wavelength *λ* = 0.8 *μ*m, pulse duration *τ* = 25 fs and *a* = 32, incident on a target 30 *μ*m thick of density *n* = *nc* at time *t* = 40*μ*/*c* from the the beginning of the laser
the angular dispersion is better than for TNSA. In figure 8 we show the energy spectrum for a 200 TW laser pulse, the same as for figures 6 and 7. When a target is slightly overcritical it is transparent if the intensity is high enough. Indeed one has to take into account the self
As a consequence by increasing the intensity the medium becomes transparent and the laser drills a channel. If the intensities are moderate this effect is small and a medium at a few times the critical density remains opaque and the hole boring acceleration prevails. Using a *CO*<sup>2</sup> laser with a gas jet one can have both regimes. By increasing the density at a fixed intensity the medium looses its transparency and the highest protons energy decreases reaches a minimum
*<sup>γ</sup> <sup>π</sup> λ*2*rc*
*a* (45)
*nc* <sup>=</sup> *<sup>π</sup> λ*2*rc*
and then increases because the RPA hole boring regime sets in [Willingale (2009)].
target interaction. The simulation is 3D.
Fig. 7. The same as figure 6 at time *t* = 60*μ*/*c*
induced transparency since the critical density becomes
#### **5. Near critical targets**
The acceleration of protons on targets with a nearly critical electron density has been recently investigated on several experiments and PIC simulations. Positive features of this type of targets are a better efficiency in the energy transfer from the laser to the electrons and the absence of debris since the medium is transparent. The optimal length of the target is a few times the length of the laser pulse which drills a channel and accelerates longitudinally the electrons which create a magnetic field circulating around the beam axis. The magnetic field moves behind the laser pulse until it exits in the vacuum where it expands; the electrons, whose energy is dissipated, are displaced by the magnetic field and create a quasi static electric field. The ions coming from the filament around the axis are accelerated and collimated. More specifically the mechanism controlling the proton acceleration is provided by the formation of a slowly evolving magnetic dipole (a toroidal configuration in 3D geometry) behind the leading laser pulse. This structure is generated by the coherent return axial current due to the accelerated electron beam, which contains a large fraction of the laser pulse energy. The magnetic vortex, when exiting on a low density (or a vacuum) region, expands symmetrically thus creating a strong induction axial electric field. At higher electron density *nc* < *n* < 3*nc* this mechanism is the most effective in the acceleration process. At lower density *n* < *nc*, a significant contribution comes also from the electrostatic field due to charge separation at the channel rear side, much alike the TNSA regime.
The maximum energy of protons depends on the target thickness and density and a scaling law is obtained by equating the laser energy to the electrons energy, following the waveguide model, provided that the length of the plasma channel *h* is much larger than the length of the laser pulse *h* � *Lp* to insure that the depletion of the laser energy is complete. Using equation (15), *I* = *a*<sup>2</sup> *mc*3*nc*, the laser energy in a channel reads
$$E\_{\rm laser} = \pi \mathbf{R}^2 \,\mathrm{\tau}I = \pi \mathbf{R}^2 L\_{\rm laser} \, a^2 \, m c^2 \, n\_\odot \tag{41}$$
The electrons energy is given by
$$E\_{\rm el} = \pi R^2 \hbar \, n \, a \, m c^2 \tag{42}$$
and equating the energies we obtain
$$a \sim \frac{nh}{n\_{\text{c}} L\_{\text{laser}}} \tag{43}$$
Another scaling provides the optimal channel length which is given by
$$m \sim h^{3/2} \tag{44}$$
If the transition to the vacuum is not abrupt but the target with electron density *n* and length *h* continues with a decreasing density before reaching the vacuum, some improvements on the top energy and the collimation can be obtained. The main advantages with respect to the TNSA regime are that the energies reached are two or three times higher, the collimation is improved and the efficiency is higher. For wavelength in the micron range the problem is to find the right targets. This is solved naturally for pulses with wavelength in the 10 *μm* range, since gas jets can be used. Indeed promising results have been obtained in recent experiments. We have performed several 2D and 3D PIC simulations of quasi critical targets. The electrons 14 Will-be-set-by-IN-TECH
The acceleration of protons on targets with a nearly critical electron density has been recently investigated on several experiments and PIC simulations. Positive features of this type of targets are a better efficiency in the energy transfer from the laser to the electrons and the absence of debris since the medium is transparent. The optimal length of the target is a few times the length of the laser pulse which drills a channel and accelerates longitudinally the electrons which create a magnetic field circulating around the beam axis. The magnetic field moves behind the laser pulse until it exits in the vacuum where it expands; the electrons, whose energy is dissipated, are displaced by the magnetic field and create a quasi static electric field. The ions coming from the filament around the axis are accelerated and collimated. More specifically the mechanism controlling the proton acceleration is provided by the formation of a slowly evolving magnetic dipole (a toroidal configuration in 3D geometry) behind the leading laser pulse. This structure is generated by the coherent return axial current due to the accelerated electron beam, which contains a large fraction of the laser pulse energy. The magnetic vortex, when exiting on a low density (or a vacuum) region, expands symmetrically thus creating a strong induction axial electric field. At higher electron density *nc* < *n* < 3*nc* this mechanism is the most effective in the acceleration process. At lower density *n* < *nc*, a significant contribution comes also from the electrostatic field due to charge separation at the
The maximum energy of protons depends on the target thickness and density and a scaling law is obtained by equating the laser energy to the electrons energy, following the waveguide model, provided that the length of the plasma channel *h* is much larger than the length of the laser pulse *h* � *Lp* to insure that the depletion of the laser energy is complete. Using equation
> *<sup>a</sup>* <sup>∼</sup> *nh ncL*laser
If the transition to the vacuum is not abrupt but the target with electron density *n* and length *h* continues with a decreasing density before reaching the vacuum, some improvements on the top energy and the collimation can be obtained. The main advantages with respect to the TNSA regime are that the energies reached are two or three times higher, the collimation is improved and the efficiency is higher. For wavelength in the micron range the problem is to find the right targets. This is solved naturally for pulses with wavelength in the 10 *μm* range, since gas jets can be used. Indeed promising results have been obtained in recent experiments. We have performed several 2D and 3D PIC simulations of quasi critical targets. The electrons
Another scaling provides the optimal channel length which is given by
*<sup>E</sup>*laser <sup>=</sup> *<sup>π</sup>R*<sup>2</sup> *<sup>τ</sup><sup>I</sup>* <sup>=</sup> *<sup>π</sup>R*2*L*laser *<sup>a</sup>*<sup>2</sup> *mc*<sup>2</sup> *nc* (41)
*<sup>E</sup>*el <sup>=</sup> *<sup>π</sup>R*<sup>2</sup> *h n a mc*<sup>2</sup> (42)
*<sup>n</sup>* <sup>∼</sup> *<sup>h</sup>*3/2 (44)
(43)
**5. Near critical targets**
channel rear side, much alike the TNSA regime.
(15), *I* = *a*<sup>2</sup> *mc*3*nc*, the laser energy in a channel reads
The electrons energy is given by
and equating the energies we obtain
and protons density at two different times exhibiting the formation of the channel are shown in figures 6 and 7 for a 3D simulation. The spectrum of the protons is still exponential and
Fig. 6. Electrons (left frame) and protons (right frame) density in the case of a laser pulse of 200 TW, with wavelength *λ* = 0.8 *μ*m, pulse duration *τ* = 25 fs and *a* = 32, incident on a target 30 *μ*m thick of density *n* = *nc* at time *t* = 40*μ*/*c* from the the beginning of the laser target interaction. The simulation is 3D.
Fig. 7. The same as figure 6 at time *t* = 60*μ*/*c*
the angular dispersion is better than for TNSA. In figure 8 we show the energy spectrum for a 200 TW laser pulse, the same as for figures 6 and 7. When a target is slightly overcritical it is transparent if the intensity is high enough. Indeed one has to take into account the self induced transparency since the critical density becomes
$$m\_{\mathcal{C}} = \frac{\pi}{\lambda^2 r\_{\mathcal{C}}} \gamma \simeq \frac{\pi}{\lambda^2 r\_{\mathcal{C}}} a \tag{45}$$
As a consequence by increasing the intensity the medium becomes transparent and the laser drills a channel. If the intensities are moderate this effect is small and a medium at a few times the critical density remains opaque and the hole boring acceleration prevails. Using a *CO*<sup>2</sup> laser with a gas jet one can have both regimes. By increasing the density at a fixed intensity the medium looses its transparency and the highest protons energy decreases reaches a minimum and then increases because the RPA hole boring regime sets in [Willingale (2009)].
of transport was performed with a proton bunch accelerated by the laser PHELIX with an energy spectrum up to 30 MeV using quadrupoles or a solenoid, which proved to keep the emittance to a lower value [Hofmann (2009; 2011)]. A proposal was made for a 100 MeV device capable of delivering 10<sup>5</sup> protons per shot on the tissues starting from a 300 TW laser beam and *a* = 60 so that a dose of 40 Gy could be delivered on a target tissue of 0.03 g in 2 minutes at 10 Hz [Sakaki et al. (2009)] using a gantry with quadrupoles and bending magnets. Radiobiology experiments were carried out on cancer cells irradiating them with proton bunches of 0.8-2.4 MeV, obtained from laser accelerations, and the break-up of DNA double strands was observed [Yogo (2009)]. A design study for post-acceleration of a 10 MeV beam into a DTL was carried out showing that with a moderate power laser *W* ≤ 100 TW and the use of microlenses right after the interaction region an injectable beam with parameters
Protons Acceleration by CO2 Laser Pulses and Perspectives for Medical Applications 431
Fig. 9. Left frame: emittances *�<sup>x</sup>* (red), *�<sup>y</sup>* (blue) and *�<sup>z</sup>* (green) for a beam produced with a 30 TW laser pulse transported through an iris with *r* = 0.5 mm and a solenoid, starting right after the iris, of 11.7 cm, *B* = 10 Tesla and *λ* = 2 cm, where the *B* field is described by a function like *B*(*z*) = 1/(1 + *e*−*z*/*λ*). An energy cut is set at 5 < *E* < 5.5 MeV. Middle frame: envelopes *σ<sup>x</sup>* (red), *σ<sup>y</sup>* (blue) and *σ<sup>z</sup>* (green). Right frame: initial (blue) and final (red) energy
Fig. 10. Left frame: emittances *�<sup>x</sup>* (red), *�<sup>y</sup>* (blue) and *�<sup>z</sup>* (green) for a beam produced with a 30 TW laser pulse transported through an iris with *r* = 0.5 mm and a solenoid, starting right after the iris, of 15.6 cm, *B* = 10 Tesla and *λ* = 2 cm. An energy cut is set at 9.5 < *E* < 10.5 MeV. Middle frame: envelopes *σ<sup>x</sup>* (red), *σ<sup>y</sup>* (blue) and *σ<sup>z</sup>* (green). Right frame: initial (blue)
We have simulated the transport of a proton beam produced with a 30 TW laser pulse having a waist of 3 *<sup>μ</sup>*m so that *<sup>I</sup>* <sup>=</sup> 2 1020 W/cm<sup>2</sup> and *<sup>a</sup>* � 10. The simulated target was 0.5 *<sup>μ</sup>*m thick, orthogonal to the *z* propagation axis, with *n* = 40*nc*, with a 2 *μ*m coating with *n* = 2*nc* and a 50 nm layer of contaminants with *n* = 9*nc* on the opposite side. The maximum energy was ∼ 14 MeV. We have placed a collimator formed by a screen with a hole of 0.5 mm radius at 1
suitable for therapy could be obtained [Antici (2011)]
spectra. The laser has the same parameters as figure 2.
cm from the interaction region followed by a solenoid.
and final (red) energy spectra
Fig. 8. Energy spectrum for a target at critical density for a laser pulse with *W* = 200 TW for the same parameters as figures 6
#### **6. The transport**
The use of protons or ions beams for medical therapy has to face severe constraints, which are not yet met by present laser produced ion bunches. Indeed the energy range is 60 ≤ *E* ≤ 250 MeV, the average overall dose is 60 Gray (1 Gy corresponds to 1 mJ per gram) and the full dose delivery is reached in several treatments. Assuming that the dose for a single proton session is 10 Gy and that it is delivered in 2 minutes with 10 Hz pulses, the number of protons per shot to reach this dose on a 1 g tissue would be 106. For a TNSA beam with a maximum energy slightly above 60 MeV, such an intensity is not easily achievable because the energy spectrum is exponential and the average energy is much lower (1/7 in 3D simulations) than the maximum energy. The situation for beams obtained from the interaction with a quasi critical target is more favorable, since the collimation is better and the maximum energy is higher. The energy currently reached with compact Ti:Sa lasers are around 20 MeV. This value has been recently overcome with a CO2 laser working with a gas target, where a monochromatic protons bunch was produced. As a consequence, presently the conditions to use for therapy the proton beams accelerated by compact high repetition lasers are not yet met. The increase of power from 100-200 TW to 1 PW is likely to allow to reach the threshold of 60 MeV of maximum energy for therapeutic use even though the intensity might be lower than required.
A possible alternative consists in maintaining the energy in the 10-30 MeV range and post-accelerating the beam. Suitable devices have already been developed to accelerate a proton beam coming from a cyclotron in this energy range. The injection energy varies from 10 MeV for a rather large DTL device [Antici (2011)] to 30 MeV for compact high field linacs like ACLIP [Amaldi et al. (2009)]. Preliminary experiments and some simulations have been already carried out. The injection of a beam in a RF cavity at 1 Hz for monochromatizing it has been experimentally proved even though at low energy (2 MeV) [Nishiuchi (2010)]. Several experiments on beam transport have been performed using quadrupoles or solenoids to focus it. A beam with *E* ≤ 14 MeV has been transported through a line formed by two collimators and two permanent magnetic quadrupoles [Schollmeier (2008)]. An experiment 16 Will-be-set-by-IN-TECH
Fig. 8. Energy spectrum for a target at critical density for a laser pulse with *W* = 200 TW for
The use of protons or ions beams for medical therapy has to face severe constraints, which are not yet met by present laser produced ion bunches. Indeed the energy range is 60 ≤ *E* ≤ 250 MeV, the average overall dose is 60 Gray (1 Gy corresponds to 1 mJ per gram) and the full dose delivery is reached in several treatments. Assuming that the dose for a single proton session is 10 Gy and that it is delivered in 2 minutes with 10 Hz pulses, the number of protons per shot to reach this dose on a 1 g tissue would be 106. For a TNSA beam with a maximum energy slightly above 60 MeV, such an intensity is not easily achievable because the energy spectrum is exponential and the average energy is much lower (1/7 in 3D simulations) than the maximum energy. The situation for beams obtained from the interaction with a quasi critical target is more favorable, since the collimation is better and the maximum energy is higher. The energy currently reached with compact Ti:Sa lasers are around 20 MeV. This value has been recently overcome with a CO2 laser working with a gas target, where a monochromatic protons bunch was produced. As a consequence, presently the conditions to use for therapy the proton beams accelerated by compact high repetition lasers are not yet met. The increase of power from 100-200 TW to 1 PW is likely to allow to reach the threshold of 60 MeV of maximum energy for therapeutic use even though the intensity might be lower
A possible alternative consists in maintaining the energy in the 10-30 MeV range and post-accelerating the beam. Suitable devices have already been developed to accelerate a proton beam coming from a cyclotron in this energy range. The injection energy varies from 10 MeV for a rather large DTL device [Antici (2011)] to 30 MeV for compact high field linacs like ACLIP [Amaldi et al. (2009)]. Preliminary experiments and some simulations have been already carried out. The injection of a beam in a RF cavity at 1 Hz for monochromatizing it has been experimentally proved even though at low energy (2 MeV) [Nishiuchi (2010)]. Several experiments on beam transport have been performed using quadrupoles or solenoids to focus it. A beam with *E* ≤ 14 MeV has been transported through a line formed by two collimators and two permanent magnetic quadrupoles [Schollmeier (2008)]. An experiment
the same parameters as figures 6
**6. The transport**
than required.
of transport was performed with a proton bunch accelerated by the laser PHELIX with an energy spectrum up to 30 MeV using quadrupoles or a solenoid, which proved to keep the emittance to a lower value [Hofmann (2009; 2011)]. A proposal was made for a 100 MeV device capable of delivering 10<sup>5</sup> protons per shot on the tissues starting from a 300 TW laser beam and *a* = 60 so that a dose of 40 Gy could be delivered on a target tissue of 0.03 g in 2 minutes at 10 Hz [Sakaki et al. (2009)] using a gantry with quadrupoles and bending magnets. Radiobiology experiments were carried out on cancer cells irradiating them with proton bunches of 0.8-2.4 MeV, obtained from laser accelerations, and the break-up of DNA double strands was observed [Yogo (2009)]. A design study for post-acceleration of a 10 MeV beam into a DTL was carried out showing that with a moderate power laser *W* ≤ 100 TW and the use of microlenses right after the interaction region an injectable beam with parameters suitable for therapy could be obtained [Antici (2011)]
Fig. 9. Left frame: emittances *�<sup>x</sup>* (red), *�<sup>y</sup>* (blue) and *�<sup>z</sup>* (green) for a beam produced with a 30 TW laser pulse transported through an iris with *r* = 0.5 mm and a solenoid, starting right after the iris, of 11.7 cm, *B* = 10 Tesla and *λ* = 2 cm, where the *B* field is described by a function like *B*(*z*) = 1/(1 + *e*−*z*/*λ*). An energy cut is set at 5 < *E* < 5.5 MeV. Middle frame: envelopes *σ<sup>x</sup>* (red), *σ<sup>y</sup>* (blue) and *σ<sup>z</sup>* (green). Right frame: initial (blue) and final (red) energy spectra. The laser has the same parameters as figure 2.
Fig. 10. Left frame: emittances *�<sup>x</sup>* (red), *�<sup>y</sup>* (blue) and *�<sup>z</sup>* (green) for a beam produced with a 30 TW laser pulse transported through an iris with *r* = 0.5 mm and a solenoid, starting right after the iris, of 15.6 cm, *B* = 10 Tesla and *λ* = 2 cm. An energy cut is set at 9.5 < *E* < 10.5 MeV. Middle frame: envelopes *σ<sup>x</sup>* (red), *σ<sup>y</sup>* (blue) and *σ<sup>z</sup>* (green). Right frame: initial (blue) and final (red) energy spectra
We have simulated the transport of a proton beam produced with a 30 TW laser pulse having a waist of 3 *<sup>μ</sup>*m so that *<sup>I</sup>* <sup>=</sup> 2 1020 W/cm<sup>2</sup> and *<sup>a</sup>* � 10. The simulated target was 0.5 *<sup>μ</sup>*m thick, orthogonal to the *z* propagation axis, with *n* = 40*nc*, with a 2 *μ*m coating with *n* = 2*nc* and a 50 nm layer of contaminants with *n* = 9*nc* on the opposite side. The maximum energy was ∼ 14 MeV. We have placed a collimator formed by a screen with a hole of 0.5 mm radius at 1 cm from the interaction region followed by a solenoid.
of depth exhibits a sharp peak, known as Bragg peak. Healthy tissues are spared and this therapy is applicable to the most severe cases which are not suitable for surgery. The protons are usually accelerated by cyclotrons and a large gantry is needed to rotate the beam around the patient. Carbon ions are accelerated by synchrotrons, which have a larger size, require heavy transport lines and a very large gantry. Even though the number of centers for protons and ions therapy is increasing they are up to now limited to national facilities. A reduction of size and cost of a protons accelerator for therapy would allow the creation of regional centers extending this treatment to a larger fraction of patients. Devices based on innovative techniques such as the superconducting cyclotron or the dielectric wall accelerator have been proposed but conclusive results have not yet been achieved. Laser acceleration of protons has entered this competition even though several years are needed before the feasibility is actually demonstrated. The typical dose for therapy is 60 Gy which means 60 mJ on a mass of 1 g. Split over 6 session this amounts to 10 mJ and corresponds to 10<sup>9</sup> protons of 60 MeV which is the threshold for very superficial tumors. Supposing each bunch contains 106 protons at 10 Hz repetition rate this dose is delivered in a couple of minutes. The major problem is that the energy and intensity required can hardly be reached with compact existing Ti:Sa lasers. Suppose that a bunch of 10<sup>12</sup> protons is accelerated by TNSA having an average energy of 10 MeV and maximum energy of 70 MeV, according to previous scaling, then the total proton energy is 1.6 J and supposing a 10% efficiency the laser energy would be 16 J. Since the spectrum is exponential the fraction of protons at 60 MeV with Δ*E*/*E* = 1% would be 1.5 108. This is a very demanding requirement on the laser. If we choose instead an average energy of 5 MeV we would have the same number of protons at an energy of 30 MeV, for the same
Protons Acceleration by CO2 Laser Pulses and Perspectives for Medical Applications 433
After post-acceleration one could reach not only the 60 MeV threshold but also higher
The real problem with TNSA or improved TNSA, achieved with a foam deposition at quasi critical density on the illuminated surface, is that the protons having the desired energy are a very small fraction of the total and carry out a very small fraction of the total energy. Increasing the repetition rate form 10 to 100 Hz would help but would not solve the problem. The way out is to produce a quasi monochromatic spectrum. This has been achieved with a CO2 laser on a gas jet and this result is of extreme interest. The use of ultrathin nanometric targets allows to obtain quasi monochromatic beams via RPA, but in spite of the scientific interest this regime is still very far out from possible applications due to the the extremely high contrast required on the laser beam. The use of gas targets and a suitably shaped beam pulse seem to be the corner stones in the production of a quasi monochromatic beam. In this respect the CO2 laser beams have an advantage with respect to pulses of shorter wavelength. For a quasi monochromatic beam the intensity is still rather low because only a small fraction of the laser energy is transferred to the beam. Once the energy and intensity requirements are satisfied other conditions have to be satisfied to render the proton beam suitable for therapy: the shot to shot stability must be kept within a narrow range and suitable dose control systems have to be developed [Linz & Alonso (2007)]. As a consequence it will take several years before laser accelerators can meet the requirements for clinical use. During this period the laser performance will be improved, new targets will be developed, transport and post acceleration systems will be tested and beam quality and stability will be pursued. Even though most of the research activity will be devoted to short wavelength lasers, the
Δ*E*/*E*.
energies, suitable for deep tumors.
Fig. 11. Left frame: emittances *�<sup>x</sup>* (red) and *�<sup>y</sup>* (blue) for a beam produced with a 30 TW laser pulse transported through a chicane (selecting around 5 MeV), an iris with *r* = 0.5 mm and a solenoid, starting right after the iris, of 11.7 cm, *B* = 10 Tesla and *λ* = 2 cm. Middle frame: envelopes *σ<sup>x</sup>* (red) and *σ<sup>y</sup>* (blue) Right frame: initial (blue) and final (red) energy spectra
In figure 9 we show the emittance and the envelope for a small fraction of the bunch, with a cut in the energy spectrum between 5 and 5.5 MeV. We have considered also the transport of the fraction of the beam with an energy between 9.5 MeV and 10.5 MeV. The results are comparable and the emittance and envelopes are shown in Figure 10. An energy selection can be made with a physical device such as a chicane or an RF cavity to achieve the rotation in the longitudinal phase space. The chicane applied to the full bunch leads to an emittance growth along the axis where the dipoles bend the beam. By applying suitable collimators the emittance and envelopes can be reduced to reasonable values but only a small fraction of the beam reaches the end of the transport line. In Figure 11, we show the full bunch propagating along a chicane and then focusing in a solenoid with field of *B* = 10 Tesla.
Other more effective methods based on selection of particles at the desired energy by putting collimator at the corresponding focus point of a solenoid are under investigation.
No transport experiments have been performed with CO2 laser accelerated proton beams, but the situation is certainly much more favorable if a quasi-monochromatic peak can be obtained with adequate intensity [Haberberger, Tochitsky, Fiuza, Gong, Fonseca, Silva, Mori & Joshi (2011); Haberberger, Tochitsky, Gong & Joshi (2011)]. In this case no energy selection is required and the transport on the beam becomes quite easy because tight focusing can be achieved preventing emittance increase and keeping the beam size to small values suitable for injection in a linac.
#### **7. Conclusions: Protons and ions therapy**
Cancer treatment, a priority for health care in advanced countries, is based on surgery, chemo-therapy and radiation therapy. Early detection of tumors increases the survival period but there are limits to massive and frequent screening. Unlike chemotherapy radiation therapy killing of malignant cells is quite homogeneous and can be effective even for massive tumors. The most common treatment is based on X rays, driven by electron accelerators. The reason is compactness of the accelerating device and moderate cost, affordable by medium size hospitals. However most of ionizing radiations, including X rays, exhibit a peak in dose deposition close to the entry point and a subsequent exponential decay. As a consequence the dose deposition in healthy tissues is important, even when this undesirable effect is minimized by irradiating from different directions and modulating the intensity (IMRT). The dose deposition mechanism for protons and ions is different and the dose curve as function 18 Will-be-set-by-IN-TECH
Fig. 11. Left frame: emittances *�<sup>x</sup>* (red) and *�<sup>y</sup>* (blue) for a beam produced with a 30 TW laser pulse transported through a chicane (selecting around 5 MeV), an iris with *r* = 0.5 mm and a solenoid, starting right after the iris, of 11.7 cm, *B* = 10 Tesla and *λ* = 2 cm. Middle frame: envelopes *σ<sup>x</sup>* (red) and *σ<sup>y</sup>* (blue) Right frame: initial (blue) and final (red) energy spectra
In figure 9 we show the emittance and the envelope for a small fraction of the bunch, with a cut in the energy spectrum between 5 and 5.5 MeV. We have considered also the transport of the fraction of the beam with an energy between 9.5 MeV and 10.5 MeV. The results are comparable and the emittance and envelopes are shown in Figure 10. An energy selection can be made with a physical device such as a chicane or an RF cavity to achieve the rotation in the longitudinal phase space. The chicane applied to the full bunch leads to an emittance growth along the axis where the dipoles bend the beam. By applying suitable collimators the emittance and envelopes can be reduced to reasonable values but only a small fraction of the beam reaches the end of the transport line. In Figure 11, we show the full bunch propagating
Other more effective methods based on selection of particles at the desired energy by putting
No transport experiments have been performed with CO2 laser accelerated proton beams, but the situation is certainly much more favorable if a quasi-monochromatic peak can be obtained with adequate intensity [Haberberger, Tochitsky, Fiuza, Gong, Fonseca, Silva, Mori & Joshi (2011); Haberberger, Tochitsky, Gong & Joshi (2011)]. In this case no energy selection is required and the transport on the beam becomes quite easy because tight focusing can be achieved preventing emittance increase and keeping the beam size to small values suitable for
Cancer treatment, a priority for health care in advanced countries, is based on surgery, chemo-therapy and radiation therapy. Early detection of tumors increases the survival period but there are limits to massive and frequent screening. Unlike chemotherapy radiation therapy killing of malignant cells is quite homogeneous and can be effective even for massive tumors. The most common treatment is based on X rays, driven by electron accelerators. The reason is compactness of the accelerating device and moderate cost, affordable by medium size hospitals. However most of ionizing radiations, including X rays, exhibit a peak in dose deposition close to the entry point and a subsequent exponential decay. As a consequence the dose deposition in healthy tissues is important, even when this undesirable effect is minimized by irradiating from different directions and modulating the intensity (IMRT). The dose deposition mechanism for protons and ions is different and the dose curve as function
along a chicane and then focusing in a solenoid with field of *B* = 10 Tesla.
injection in a linac.
**7. Conclusions: Protons and ions therapy**
collimator at the corresponding focus point of a solenoid are under investigation.
of depth exhibits a sharp peak, known as Bragg peak. Healthy tissues are spared and this therapy is applicable to the most severe cases which are not suitable for surgery. The protons are usually accelerated by cyclotrons and a large gantry is needed to rotate the beam around the patient. Carbon ions are accelerated by synchrotrons, which have a larger size, require heavy transport lines and a very large gantry. Even though the number of centers for protons and ions therapy is increasing they are up to now limited to national facilities. A reduction of size and cost of a protons accelerator for therapy would allow the creation of regional centers extending this treatment to a larger fraction of patients. Devices based on innovative techniques such as the superconducting cyclotron or the dielectric wall accelerator have been proposed but conclusive results have not yet been achieved. Laser acceleration of protons has entered this competition even though several years are needed before the feasibility is actually demonstrated. The typical dose for therapy is 60 Gy which means 60 mJ on a mass of 1 g. Split over 6 session this amounts to 10 mJ and corresponds to 10<sup>9</sup> protons of 60 MeV which is the threshold for very superficial tumors. Supposing each bunch contains 106 protons at 10 Hz repetition rate this dose is delivered in a couple of minutes. The major problem is that the energy and intensity required can hardly be reached with compact existing Ti:Sa lasers. Suppose that a bunch of 10<sup>12</sup> protons is accelerated by TNSA having an average energy of 10 MeV and maximum energy of 70 MeV, according to previous scaling, then the total proton energy is 1.6 J and supposing a 10% efficiency the laser energy would be 16 J. Since the spectrum is exponential the fraction of protons at 60 MeV with Δ*E*/*E* = 1% would be 1.5 108. This is a very demanding requirement on the laser. If we choose instead an average energy of 5 MeV we would have the same number of protons at an energy of 30 MeV, for the same Δ*E*/*E*.
After post-acceleration one could reach not only the 60 MeV threshold but also higher energies, suitable for deep tumors.
The real problem with TNSA or improved TNSA, achieved with a foam deposition at quasi critical density on the illuminated surface, is that the protons having the desired energy are a very small fraction of the total and carry out a very small fraction of the total energy. Increasing the repetition rate form 10 to 100 Hz would help but would not solve the problem. The way out is to produce a quasi monochromatic spectrum. This has been achieved with a CO2 laser on a gas jet and this result is of extreme interest. The use of ultrathin nanometric targets allows to obtain quasi monochromatic beams via RPA, but in spite of the scientific interest this regime is still very far out from possible applications due to the the extremely high contrast required on the laser beam. The use of gas targets and a suitably shaped beam pulse seem to be the corner stones in the production of a quasi monochromatic beam. In this respect the CO2 laser beams have an advantage with respect to pulses of shorter wavelength. For a quasi monochromatic beam the intensity is still rather low because only a small fraction of the laser energy is transferred to the beam. Once the energy and intensity requirements are satisfied other conditions have to be satisfied to render the proton beam suitable for therapy: the shot to shot stability must be kept within a narrow range and suitable dose control systems have to be developed [Linz & Alonso (2007)]. As a consequence it will take several years before laser accelerators can meet the requirements for clinical use. During this period the laser performance will be improved, new targets will be developed, transport and post acceleration systems will be tested and beam quality and stability will be pursued. Even though most of the research activity will be devoted to short wavelength lasers, the
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URL: *http://arxiv.org/abs/1006.3163*
miniature magnetic devices, *PRL* 101(055004).
Murakami, M. (2008). Radiotherapy using a laser proton accelerator.
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proton beams.
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of scaling laws with pic simulations for protons accelerated with long wavelength
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development of long wavelength system such as CO2 lasers will hopefully continue because they may provide a very valuable alternative and solve some of the most critical problems such as monochromaticity and proton beam quality.
#### **8. Acknowledgments**
We would like to thank the Italian Ministry of Foreign Affairs (MAE) for a grant we received for the scientific cooperation with Japan to develop the research project PROMETHEUS, devoted to a research infrastructure on laser driven proton sources for biomedical applications. We thank the Fondazione del Monte di Bologna e Ravenna for a grant devoted to the feasibility study of a hybrid accelerator devoted to biomedical Research within the framework of PROMETHEUS. We acknowledge the Alma Mater Foundation for the governance of the PROMETHEUS project.
#### **9. References**
URL: *http://arxiv.org/abs/1104.1932v1*
20 Will-be-set-by-IN-TECH
development of long wavelength system such as CO2 lasers will hopefully continue because they may provide a very valuable alternative and solve some of the most critical problems
We would like to thank the Italian Ministry of Foreign Affairs (MAE) for a grant we received for the scientific cooperation with Japan to develop the research project PROMETHEUS, devoted to a research infrastructure on laser driven proton sources for biomedical applications. We thank the Fondazione del Monte di Bologna e Ravenna for a grant devoted to the feasibility study of a hybrid accelerator devoted to biomedical Research within the framework of PROMETHEUS. We acknowledge the Alma Mater Foundation for
Amaldi, U., Braccini, S. & Puggioni, P. (2009). High frequency linacs for hadrontherapy,
Antici, P. (2011). A compact post-acceleration scheme for laser-generated protons, *Physics of*
Benedetti, C., Sgattoni, A., Turchetti, G. & Londrillo, P. (2008). Aladyn: A high accuracy pic code for the maxwell-vlasov equations, *IEEE Transactions on Plasma Science* 36(4). Bulanov, S. (2008). Accelerating protons to therapeutic energies with ultraintense, ultraclean,
Bulanov, S. S. (2010). Generation of gev protons from 1pw laser interaction with near critical
Fukuda, Y. & Bulanov, S. (2009). Energy increase in multi-mev ion acceleration in the
Haberberger, D., Tochitsky, S., Fiuza, F., Gong, C., Fonseca, R. A., Silva, L. O., Mori,
Haberberger, D., Tochitsky, S., Gong, C. & Joshi, C. (2011). Production of 25 mev protons in co2
Henig, A. (2009). Radiation-pressure acceleration of ion beams driven by circularly polarized
Hofmann, I. (2009). Laser accelerated ions and their potential for therapy, *Proceedings of*
Hofmann, I. (2011). Collection and focusing of laser accelerated ion beams for therapy
Leemans, W. P., Nagler, B., Gonsalves, A. J., Tóth, C., Nakamura, K., Geddes, C. G. R.,
Linz, U. & Alonso, J. (2007). What will it take for laser driven proton accelerators to be applied to tumor therapy, *Phys. Rev. Special Topics Accel. and Beams* 10(094801).
Esarey, E., Schroeder, C. B. & Hooker, S. M. (2006). Gev electron beams from a
interaction of short laser pulse with a cluster gas target, *Phys. Rev. Letters* 103(165002).
W. B. & Joshi, C. (2011). Collisionless shocks in laser-produced plasma generate
laser plasma interactions in a gas jet, *Proceedings of 2011 Particle Accelerator Conference,*
*Reviews of Accelerator Science and Technology* 2(111131).
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monoenergetic high-energy proton beams, *Nature Physics* .
applications, *Phys. Rev. Spec. Top. Acc. and Beams* 14(031304).
centimetre-scale accelerator, *Nature Physics* 2: 696–699.
such as monochromaticity and proton beam quality.
the governance of the PROMETHEUS project.
**8. Acknowledgments**
**9. References**
*Plasmas* 18.
*New York, NY, USA* .
*HIAT09, Venice, Italy* .
22 Will-be-set-by-IN-TECH
436 CO2 Laser – Optimisation and Application
Snavely, R. A. (2000). Intense high-energy proton beams from petawatt-laser irradiation of
Yogo, A. (2008). Laser ion acceleration via control of the near-critical density target, *Phys. Rev.*
Yogo, A. (2009). Application of laser-accelerated protons to the demonstration of dna double-strand breaks in human cancer cells, *Appl. Phys. Lett.* 94(181502). Zani, A., Sgattoni, A. & Passoni, M. (2011). Parametric investigations of target normal
Zeil, K., Kraft, S. D., Bock, S., Bussmann, M., Cowan, T. E., Kluge, T., Metzkes, J., Richter,
pulse laser plasma acceleration, *New Journal of Physics* 12(045015).
transport regime through measurements of energetic proton beams, *PRL* 102(125002).
sheath acceleration experiments, *Nuclear Instruments and Methods in Physics Research*
T., Sauerbrey, R. & Schramm, U. (2010). The scaling of proton energies in ultrashort
Tajima, T. & Dawson, J. M. (1979). Laser electron accelerator, *Phys. Rev. Lett.* 43(4): 267–270. Willingale, L. (2009). Characterization of high intensity laser propagation in the relativistic
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*E* 77(016401).
*A* 653: 94–97.
### *Edited by Dan C. Dumitras*
The present book includes several contributions aiming a deeper understanding of the basic processes in the operation of CO2 lasers (lasing on non-traditional bands, frequency stabilization, photoacoustic spectroscopy) and achievement of new systems (CO2 lasers generating ultrashort pulses or high average power, lasers based on diffusion cooled V-fold geometry, transmission of IR radiation through hollow core microstructured fibers). The second part of the book is dedicated to applications in material processing (heat treatment, welding, synthesis of new materials, micro fluidics) and in medicine (clinical applications, dentistry, non-ablative therapy, acceleration of protons for cancer treatment).
CO2 Laser - Optimisation and Application
CO2 Laser
Optimisation and Application
*Edited by Dan C. Dumitras*
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ffeea8d6-cc39-40d8-a97c-3ad92c128196.0 | *Edited by Jagannathan Thirumalai*
The book Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets presents topics on global advancements in theoretical and experimental facts, instrumentation and practical applications of thin-film material perspectives and its applications. The aspect of this book is associated with the thin-film physics, the methods of deposition, optimization parameters and its wide technological applications. This book is divided into three main sections: Thin Film Deposition Methods: A Synthesis Perspective; Optimization Parameters in the Thin Film Science and Application of Thin Films: A Synergistic Outlook. Collected chapters provide applicable knowledge for a wide range of readers: common men, students and researchers. It was constructed by experts in diverse fields of thin-film science and technology from over 15 research institutes across the globe.
ISBN 978-953-51-3067-3
Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
Photo by releon8211 / iStock
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ffeea8d6-cc39-40d8-a97c-3ad92c128196.1 | Thin Film Processes Artifacts on Surface Phenomena and Technological Facets
*Edited by Jagannathan Thirumalai*
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ffeea8d6-cc39-40d8-a97c-3ad92c128196.2 | **THIN FILM PROCESSES - ARTIFACTS ON SURFACE PHENOMENA AND TECHNOLOGICAL FACETS**
Edited by **Jagannathan Thirumalai**
#### **Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets**
http://dx.doi.org/10.5772/64990 Edited by Jagannathan Thirumalai
#### **Contributors**
Andre Slonopas, Zhuoqing Yang, Yi Zhang, Vanessa Rheinheimer, Husein Irzaman, Tetyana Torchynska, Brahim El Filali, Stephan Kozhukharov, Helena Castan Castan, Salvador Dueñas, Alberto Sardina, Hector Garcia, Tonis Arroval, Aile Tamm, Kaupo Kukli, Jaan Aarik, Taivo Jõgiaas, Sarkyt Kudaibergenov, Armando Rojas Hernandez, Santos Jesus Castillo, Jagannathan Thirumalai
#### **© The Editor(s) and the Author(s) 2017**
The moral rights of the and the author(s) have been asserted.
All rights to the book as a whole are reserved by INTECH. The book as a whole (compilation) cannot be reproduced, distributed or used for commercial or non-commercial purposes without INTECH's written permission. Enquiries concerning the use of the book should be directed to INTECH rights and permissions department ([email protected]).
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#### **Notice**
Statements and opinions expressed in the chapters are these of the individual contributors and not necessarily those of the editors or publisher. No responsibility is accepted for the accuracy of information contained in the published chapters. The publisher assumes no responsibility for any damage or injury to persons or property arising out of the use of any materials, instructions, methods or ideas contained in the book.
First published in Croatia, 2017 by INTECH d.o.o. eBook (PDF) Published by IN TECH d.o.o. Place and year of publication of eBook (PDF): Rijeka, 2019. IntechOpen is the global imprint of IN TECH d.o.o. Printed in Croatia
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Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets Edited by Jagannathan Thirumalai p. cm. Print ISBN 978-953-51-3067-3 Online ISBN 978-953-51-3068-0 eBook (PDF) ISBN 978-953-51-4874-6
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ffeea8d6-cc39-40d8-a97c-3ad92c128196.4 | **Meet the editor**
Dr. Jagannathan Thirumalai received his PhD from Alagappa University, Karaikudi in 2010. He was also awarded the Post-doctoral Fellowship from Pohang University of Science and Technology (POSTECH), Republic of Korea, in 2013. He worked as Assistant Professor of Physics, B.S. Abdur Rahman University, Chennai, India (2011 to 2016). Currently, he is working as Assistant
Professor of Physics, SASTRA University, Kumbakonam (T.N.), India. His research interests focus on luminescence, self-assembled nanomaterials and thin film opto-electronic devices. He has published more than 50 SCOPUS/ISI indexed papers and 8 book chapters and is a member of several national and international societies like RSC, OSA, etc. Currently, he is acting as a principal investigator for a funded project towards the application of luminescence-based thin film opto-electronic devices, funded by the Science and Engineering Research Board (SERB), India. As an expert in opto-electronics and nanotechnology area, he has been invited as an external and internal examiner to MSc and PhD theses and a reviewer for international and national journals.
## Contents
Pamela Norris and Ashok K. Sood
## Preface
Chapter 7 **Thin Films as a Tool for Nanoscale Studies of Cement Systems**
Vanessa Rheinheimer and Ignasi Casanova
**Section 3 Application of Thin Films: A Synergistic Outlook 145**
Chapter 8 **Layer-by-Layer Thin Films and Coatings Containing Metal**
Chapter 9 **RRAM Memories with ALD High-K Dielectrics: Electrical Characterization and Analytical Modeling 165**
Chapter 10 **Advanced Multifunctional Corrosion Protective Coating**
Stephan Vladymirov Kozhukharov
Sarkyt Kudaibergenov, Gulnur Tatykhanova, Nurlan Bakranov and
Helena Castán, Salvador Dueñas, Alberto Sardiña, Héctor García, Tõnis Arroval, Aile Tamm, Taivo Jõgiaas, Kaupo Kukli and Jaan Aarik
**Systems for Light-Weight Aircraft Alloys—Actual Trends and**
**and Building Materials 127**
**Nanoparticles in Catalysis 147**
Rosa Tursunova
**VI** Contents
**Challenges 179**
The book Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets aims to provide in-depth knowledge of classic topics on thin-film materials and it contains contributions of the researchers involved in different fields of research. The development of sophisticated thin-film technologies effectively depends on the manipulation of modernized physico-chemical materials. The framework of knowledge of thin-film materials provides a platform to innovate, which has the tendency to progress the life requirements rather than scientific pursuit. In modern times, thin-film technology is a developing and progressing sci‐ entific discipline at the front line of physical, chemical and biological sciences with remarka‐ ble international opportunities and relationships. Thin-film technology has diverse applications in daily life, starting from conventional lighting devices, and extending to solar cells and other electrical and electronic devices that pertain to thin- film materials. The chap‐ ters in the book have been written by established researchers in the area and cover the ad‐ vanced areas of research and developments in the field of materials science.
This book consists of ten chapters that have been categorized into three sections. Section 1 consists of four chapters on the thin-film deposition methods with synthesis aspects: Intro‐ ductory Chapter: The Prominence of Thin Film Science in Technological Scale; Synthesis of Thin Films of Sulfides of Cadmium Lead and Copper by Chemical Bath Deposition; Modi‐ fied Spin Coating Method for Coating and Fabricating Ferroelectric Thin Films as Sensors and Solar Cells and Emission, Defects, and Structure of ZnO Nanocrystal Films Obtained by Electrochemical Method. Section 2 contains of three chapters: Lab-on-a-Tube Surface Micro‐ machining Technology; Efficient Optimization of the Optoelectronic Performance in Chemi‐ cally Deposited Thin Films and Thin Films as a Tool for Nanoscale Studies of Cement Systems and Building Materials. Section 3 comprises of three chapters: Layer-by-Layer Thin Films and Coatings Containing Metal Nanoparticles in Catalysis; RRAM Memories with ALD High-K Dielectrics: Electrical Characterization and Analytical Modeling and Advanced Multifunctional Corrosion Protective Coating Systems for Light-Weight Aircraft Alloys-Ac‐ tual Trends and Challenges
Finally, I could never forget that my leap in the field of thin-film science was guided by Dr. R. Jagannathan and Professor R. Chandramohan. Our friendly collaboration has been going on for years and it's been very productive. I would like to thank all the authors in the book for their valuable contributions. A few words at the last, I would like to express my sincere gratitute to Ms. Andrea Koric, publishing process manager, for the effective support in the construction of this book.
**Assistant Professor Jagannathan Thirumalai**
Department of Physics, Srinivasa Ramanujan Center, SASTRA University, Tamil Nadu, India
**Thin Film Deposition Methods: A Synthesis Perspective**
#### **Introductory Chapter: The Prominence of Thin Film Science in Technological Scale Introductory Chapter: The Prominence of Thin Film Science in Technological Scale**
Jagannathan Thirumalai
Additional information is available at the end of the chapter Jagannathan Thirumalai
http://dx.doi.org/10.5772/67201 Additional information is available at the end of the chapter
## **1. A succinct testimony of thin film science**
Since antediluvian times, the term 'thin film coating technology' is more captivating towards mankind. More than 2000 eons ago, goldsmiths and silversmiths developed a variety of methods, including using mercury as an adhesive, to apply over thin films of metals to sculptures and other objects. The ancient mercury‐based processes like fire gilding and silvering techniques were used for the surface coating of less precious substrates having thin layers made up of gold or silver. They developed the technology of thin‐film coating that is unrivalled by today's process for manufacturing DVDs, electronic devices, solar cells and other relevant products and used it on statues, amulets, jewels and more common objects.
In reference to the technological aspect, these workmen over 2000 years ago manage to produce valuable metal coatings as thin and adherent as possible, which not only saved luxurious metals but also enriched resistance to wear that would cause from sustained usage and circulation. In ancient days, the craftsmen were methodically organized these metals to construct functional as well as decorative artistic objects, without having any fundamental knowledge about the physico‐chemical processes. The mercury‐based techniques were also deceitfully used in ancient times to create objects such as coins and jewels that looked like they would be made of gold or silver but actually had a less precious core. Ingo et al. [1, 2] set forth to apply the modern analytical methods to reveal the ancients' artistic secrets. By means of surface analytical methods, for example, selected area X‐ray photoelectron spectroscopy and scanning electron microscopy combined with energy dispersive X‐ray spectroscopy on Dark Ages objects such as St. Ambrogio's altar from 825 AD, they said that their discoveries endorse 'the high level of proficiency achieved by the craftsmen and artists of these primordial periods who created objects of an imaginative qualities that would not be ameliorated in ancient times and have not yet been technologically advanced in modern ones'.
A widespread responsiveness has found on thin film studies in many advanced new areas of research in the combination of chemical, physical and mechanical sciences, which are based on prodigies with unique features of the thickness, structure, geometry of the film, etc. [3]. Whereas bearing in mind, a thin film matter contains two surfaces that are as close to each other
© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
that they could have a conclusive impact on the internal physical properties and methods of the substance, which would differ, therefore, in a reflective way from that of a bulk material. A new phenomenon is arisen due to the diminution in distance flanked by the surfaces and its mutual interaction. At this juncture, the one‐dimensional structure of the material is abridged to an order of numerous atomic layers, which generate an intermediary scheme sandwiched between macro‐molecular systems, thus it offers us a technique of studying the microphysical nature of different phenomena. Thin films are precisely suitable for applications in the field of microelectronics, opto‐electronics, integrated optics, etc. Nonetheless, the physical properties of the films such as electrical resistivity do not considerably vary from the characteristics of the bulk material. The thickness is from a few tenths of nanometre to a few micrometres.
Albeit the erudition of thin film prodigies dates well back over an epoch, it is actually only over the last four decades, which they have been effectively used to a substantial extent in practical situations. The usages of thin and thick films are almost authoritative to the complete prerequisite of micro miniaturization. The growth of the computer technology would lead to an obligation for very high density systems of storage and it is this which has enthused utmost of the research on the opto‐electronics, magnetic and optical properties of the thin films. Sundry thin film devices had been industrialized which might found themselves looking for the applications or perhaps more prominently market.
A wide range of thin film materials, its fabrication techniques, deposition processing, spectroscopic and the optical characterization would probe which are adopted to create many novel devices. Thin film deposition is usually divided into two broad categories [3, 4].
• **Physical deposition process**
#### • **Chemical deposition process**
Widespread thin film techniques are summarized in the flowchart of **Figure 1** [5, 53]. The films are often capable of producing films around 1 µm or less and the thick films are naturally in the range of 1–20 µm, the range of resistivities are 10 Ω/square to 10 MΩ/square, there are significant possibilities for building multi‐layer structures. Though there are definite techniques that are only accomplished of producing thick films and these might include screen printing, electrophoretic deposition, flame spraying, glazing and painting.
Physical and chemical depositions are the two techniques that are used to create a very thin layer of material into a substrate. They are used greatly in the production of semiconductors where the very thin layers of p‐type and n‐type materials would create the necessary junctions. *Physical deposition* refers to a widespread range of technologies in that a material is released from the source and which would deposited on a substrate using mechanical, electromechanical or the thermodynamic processes. The two most general techniques of physical vapour deposition (PVD) are evaporation and sputtering. *Chemical deposition* is stated as when a volatile fluid precursor does a chemical change on a surface leaving a chemically deposited coating. When one tries to categorize deposition of films by chemical methods, one would find that they can be categorized into two classes. The first class is related to the chemical formation of the film from medium and typical methods included are chemical reduction plating, electroplating and vapour phase deposition. A second class is the formation of the respective film from Introductory Chapter: The Prominence of Thin Film Science in Technological Scale http://dx.doi.org/10.5772/67201 5
that they could have a conclusive impact on the internal physical properties and methods of the substance, which would differ, therefore, in a reflective way from that of a bulk material. A new phenomenon is arisen due to the diminution in distance flanked by the surfaces and its mutual interaction. At this juncture, the one‐dimensional structure of the material is abridged to an order of numerous atomic layers, which generate an intermediary scheme sandwiched between macro‐molecular systems, thus it offers us a technique of studying the microphysical nature of different phenomena. Thin films are precisely suitable for applications in the field of microelectronics, opto‐electronics, integrated optics, etc. Nonetheless, the physical properties of the films such as electrical resistivity do not considerably vary from the characteristics of the bulk material. The thickness is from a few tenths of nanometre to a few micrometres.
Albeit the erudition of thin film prodigies dates well back over an epoch, it is actually only over the last four decades, which they have been effectively used to a substantial extent in practical situations. The usages of thin and thick films are almost authoritative to the complete prerequisite of micro miniaturization. The growth of the computer technology would lead to an obligation for very high density systems of storage and it is this which has enthused utmost of the research on the opto‐electronics, magnetic and optical properties of the thin films. Sundry thin film devices had been industrialized which might found themselves look-
A wide range of thin film materials, its fabrication techniques, deposition processing, spectroscopic and the optical characterization would probe which are adopted to create many novel
Widespread thin film techniques are summarized in the flowchart of **Figure 1** [5, 53]. The films are often capable of producing films around 1 µm or less and the thick films are naturally in the range of 1–20 µm, the range of resistivities are 10 Ω/square to 10 MΩ/square, there are significant possibilities for building multi‐layer structures. Though there are definite techniques that are only accomplished of producing thick films and these might include screen
Physical and chemical depositions are the two techniques that are used to create a very thin layer of material into a substrate. They are used greatly in the production of semiconductors where the very thin layers of p‐type and n‐type materials would create the necessary junctions. *Physical deposition* refers to a widespread range of technologies in that a material is released from the source and which would deposited on a substrate using mechanical, electromechanical or the thermodynamic processes. The two most general techniques of physical vapour deposition (PVD) are evaporation and sputtering. *Chemical deposition* is stated as when a volatile fluid precursor does a chemical change on a surface leaving a chemically deposited coating. When one tries to categorize deposition of films by chemical methods, one would find that they can be categorized into two classes. The first class is related to the chemical formation of the film from medium and typical methods included are chemical reduction plating, electroplating and vapour phase deposition. A second class is the formation of the respective film from
devices. Thin film deposition is usually divided into two broad categories [3, 4].
printing, electrophoretic deposition, flame spraying, glazing and painting.
ing for the applications or perhaps more prominently market.
4 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
• **Physical deposition process** • **Chemical deposition process**
**Figure 1.** Flowchart illustrates the physical and chemical deposition process wide spread thin film techniques.
the precursor elements, e.g. iodization, gaseous iodization, sputtering ion beam implantation, thermal growth, CVD, MOCVD and vacuum evaporation that is used to produce the highest purity, reliable‐performance solid materials in the semiconductor industry nowadays.
Relationship between the structure and property of thin films is the characteristics of such devices and forms the basis of thin film technologies. For example, in PVD (physical vapour deposition), a pure source material is gasified through evaporation, the application of the high power electricity, laser ablation and other few techniques. The gasified material would then condense on the substrate material to form the desired layer. However, by CVD (chemical vapour deposition), the chemical reactions might depend on thermal effects, as in vapour phase deposition and also the thermal growth. However, in all of these cases (**Figure 1**), a definite chemical reaction is a requirement to obtain as the form of final film [5, 53].
## **2. Technological advancements in the science of thin films**
Thin film technology could be applied to various substrate materials, for example ceramics metals or polymers. The very common substrate materials are silicon, steel and glass. By appropriately cherry‐picking the deposition materials and the technology, properties of the substrate material could be upgraded, enriched and tailor‐made to meet the exceptional desires of a specific application. Furthermore, currently, thin film technologies are accessible that could be applicable to either flat substrates or objects with multifaceted geometrical silhouettes. Highlighting on device miniaturization and the technological parameters of alternate processes (such as thick film) are contributing to the expansion of the thin film industry and to the development of lower cost thin film equipment and processes. When the thin film is deposited, in many applications, it is obligatory to contour the film to a pre‐established pattern. This is usually accomplished by lithography and etching. The construction device process is accomplished by ultimate and packaging steps (such as assembly), which differ based on the type of device. Everyone owns a numerous astounding moments to have a high regard for the remarkable engage in regeneration of novel thin film devices, the consequence and the good organization of the assistance offered through thin film devices to extend our prospect, in addition to reward for its fascinated defects to make ourselves with recent technological illusions.
The well‐equipped novel thin film techniques have broad accessibility by means of ease procedure, sensitivity, selectivity, speed, accuracy and precision [6, 9, 34]. The novel applications of thin film devices have tendered innovative advancements in technology over few decades and these technological aspects were rapidly employed for cutting‐edge research mostly in all the field of science and technology. **Table 1** presents the some major innovative advancement in technology associated with the applications of thin films in a broad spectrum.
Thin‐film device fabrication technology has great advantages. Due to their characteristic features that they could be placed at virtually any wavelength in the broad region of transparency of their respective materials simply by varying the thicknesses of their layers, and, once a design had been established, the time for the production is exceptionally of short duration. In addition, a large field of application of thin film systems is that they act as laser mirrors, anti‐reflex coatings and other optically active surface modifications. In the optical industry, they have been coated on substrates which would ensure the stable mechanical and other specific properties. Thin films could similarly be present in opto‐electronic, magnetic and electronic apparatuses which could only be factory‐made due to the specific physical properties of thin films which might vary considerably in reference to the bulk material. A significant example for this case is hard disk read heads due to the giant magnetoresistance effect (GMR). These are having the special properties with a combination of insulating and magnetic thin flms.
The technological achievements in modern thin film synthesis over the past decade subsequently lead to the utilization of outstanding properties and development of a wide range of applications in various engineering fields. As a result, the current activity in the thin‐film device fabrication technology has been correlated and to expand our prospects based on the new ideas in the field of nanotechnology, LEDs and displays, photovoltaics/solar cells, environmental, biological science and so on. The current experimental standards for the assessment of environmental risk are the ones, which rely on the growth inhibition triggered by the chemical substance and would not include qualitative evaluation such as the process of enunciating
the substrate material could be upgraded, enriched and tailor‐made to meet the exceptional desires of a specific application. Furthermore, currently, thin film technologies are accessible that could be applicable to either flat substrates or objects with multifaceted geometrical silhouettes. Highlighting on device miniaturization and the technological parameters of alternate processes (such as thick film) are contributing to the expansion of the thin film industry and to the development of lower cost thin film equipment and processes. When the thin film is deposited, in many applications, it is obligatory to contour the film to a pre‐established pattern. This is usually accomplished by lithography and etching. The construction device process is accomplished by ultimate and packaging steps (such as assembly), which differ based on the type of device. Everyone owns a numerous astounding moments to have a high regard for the remarkable engage in regeneration of novel thin film devices, the consequence and the good organization of the assistance offered through thin film devices to extend our prospect, in addition to reward for its fascinated defects to make ourselves with recent technological
6 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
The well‐equipped novel thin film techniques have broad accessibility by means of ease procedure, sensitivity, selectivity, speed, accuracy and precision [6, 9, 34]. The novel applications of thin film devices have tendered innovative advancements in technology over few decades and these technological aspects were rapidly employed for cutting‐edge research mostly in all the field of science and technology. **Table 1** presents the some major innovative advancement in technology associated with the applications of thin films in a broad
Thin‐film device fabrication technology has great advantages. Due to their characteristic features that they could be placed at virtually any wavelength in the broad region of transparency of their respective materials simply by varying the thicknesses of their layers, and, once a design had been established, the time for the production is exceptionally of short duration. In addition, a large field of application of thin film systems is that they act as laser mirrors, anti‐reflex coatings and other optically active surface modifications. In the optical industry, they have been coated on substrates which would ensure the stable mechanical and other specific properties. Thin films could similarly be present in opto‐electronic, magnetic and electronic apparatuses which could only be factory‐made due to the specific physical properties of thin films which might vary considerably in reference to the bulk material. A significant example for this case is hard disk read heads due to the giant magnetoresistance effect (GMR). These are having the special properties with a combination of insulating and
The technological achievements in modern thin film synthesis over the past decade subsequently lead to the utilization of outstanding properties and development of a wide range of applications in various engineering fields. As a result, the current activity in the thin‐film device fabrication technology has been correlated and to expand our prospects based on the new ideas in the field of nanotechnology, LEDs and displays, photovoltaics/solar cells, environmental, biological science and so on. The current experimental standards for the assessment of environmental risk are the ones, which rely on the growth inhibition triggered by the chemical substance and would not include qualitative evaluation such as the process of enunciating
illusions.
spectrum.
magnetic thin flms.
**Table 1.** Innovative advancement with technological applications in thin films [53].
toxicity. Thus, it is figured out that this only evaluation is inadequate for building improvement, which leads to ecological preservation and to deep circumvention against human health.
## **3. Conclusion**
Persistent to the above discussion, thin film is not only well thought‐out a forerunner across the globe with highly novel scientific developments; however, facts also establish that it has been and would prolong to be imperious towards path‐breaking research against novel applications for the societal benefits. Amongst the major noteworthy developments in different fields of nanotechnology, LEDs and displays, photovoltaics/solar cells, environmental, and medical diagnostics are the most important worldwide challenges so far. Progress must continue in the novel thin film techniques, which is used in the field of spectral imaging, time‐correlated single‐photon counting, kinetic chemical reaction rates, non‐invasive optical biopsy and visual implants. Thus, research on unique thin film technological achievements might pave way for coating thin films in an atomic scale that may perhaps turn out to be the future signs of green energy in the upcoming scenario.
## **Acknowledgements**
Work incorporated in this chapter was partially supported by the Department of Science and Technology (SR/FTP/PS‐135/2011) Govt. of India. The authors apologize for inadvertent omission of any pertinent references.
## **Conflict of interest**
The authors declare that there is no conflict of interests regarding the publication of this paper.
## **Author details**
Jagannathan Thirumalai
Address all correspondence to: [email protected]
Srinivasa Ramanujan center, SASTRA University, Kumbakonam, Tamil Nadu, India
## **References**
toxicity. Thus, it is figured out that this only evaluation is inadequate for building improvement, which leads to ecological preservation and to deep circumvention against human health.
• Thermal management of architectural performance of ETFE foils (metal‐coated foils) [52]
• Spheroidization of high melting point materials (diameter 1–500 µm) [49]
Persistent to the above discussion, thin film is not only well thought‐out a forerunner across the globe with highly novel scientific developments; however, facts also establish that it has been and would prolong to be imperious towards path‐breaking research against novel applications for the societal benefits. Amongst the major noteworthy developments in different fields of nanotechnology, LEDs and displays, photovoltaics/solar cells, environmental, and medical diagnostics are the most important worldwide challenges so far. Progress must continue in the novel thin film techniques, which is used in the field of spectral imaging, time‐correlated single‐photon counting, kinetic chemical reaction rates, non‐invasive optical biopsy and visual implants. Thus, research on unique thin film technological achievements might pave way for coating thin films in an atomic scale that may perhaps turn out to be the future signs of green energy in the upcoming scenario.
Work incorporated in this chapter was partially supported by the Department of Science and Technology (SR/FTP/PS‐135/2011) Govt. of India. The authors apologize for inadvertent omis-
**3. Conclusion**
(Alternative) energies
**Field Application with examples**
Chemistry • Diffusion barriers [42]
• Video/audio tape [38]
• Biological sensorics [41]
• Neurological sensors [46] New materials • Metastable phases: metallic glasses [47, 48]
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Biomedicine • Biocompatible implant coating [45]
Sensorics • Data acquisition in aggressive environments and media [39]
8 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
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Provisional chapter
## **Synthesis of Thin Films of Sulfides of Cadmium, Lead and Copper by Chemical Bath Deposition** Synthesis of Thin Films of Sulfides of Cadmium, Lead
Armando Gregorio Rojas Hernández and S. Jesus Castillo Armando Gregorio Rojas Hernández and
and Copper by Chemical Bath Deposition
Additional information is available at the end of the chapter Additional information is available at the end of the chapter
http://dx.doi.org/10.5772/66751
#### Abstract
S. Jesus Castillo
The goal of this chapter is to present three kind of thin films for the materials, done by Chemical Bath Deposition technique, the materials are CdS, PbS and CuS. The characterization has been diversified, but consisting mainly X-ray Diffraction (XRD) giving hexagonal, cubic and amorphous structures of CdS, PbS and CuS respectively. The Raman dispersion let to found the characteristics peaks of vibration, one for the CdS located on 300.7 cm−<sup>1</sup> , three for PbS and two more for the CuS. We use X-rays Photoelectron Spectroscopy to formalize the chemical composition analysis, from this analysis we could to proof the high purity of the chemical bath deposition method in the materials preparation. We used UV-Vis Spectroscopy to determine simple optical responses, getting the biggest transmittances of 72% for CdS, 45% for PbS, and 80% for CuS, and direct energy band gaps of 2.47 eV for CdS, 1.78 eV for CuS as ground and with thermic annealing 2.45 eV which is believed result of amorphous to crystalline morphology changes, the indirect bandgap 0.94 eV is measured too. The AFM given information about the surface morphology and roughness, Scanning Electron Microscopy (SEM) micrography shows the polycrystallinity nature of the CuS including the smooth.
Keywords: thin films, semiconductors, solar cells, chalcogenides, CBD
## 1. Introduction
Lead sulfide (PbS) and cadmium sulfide (CdS) are two semiconductors studied since time ago, their combined research has around one century and the direct band gap for PbS is around 0.37 eV at 300 K [1–10]. The PbS is mainly used as an infrared detector in various fields has been used mainly as an infra-red detector in another diverse field [11–15]. On the other hand, the CdS material shows a direct band gap between 2.42 and 2.53 eV [16–25]. The CdS material was used as a pigment as well as for solar cells optical window; cadmium sulfide is a semiconductor
© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.
II–VI type which is mainly useful in optoelectronic devices and some researchers reported that it has low conductivity of 10.8 (Ω cm)−<sup>1</sup> .
Lead sulfide has a cubic crystallographic structure, while cadmium sulfide can be cubic or hexagonal, basically. Here we also discuss some features of copper sulfide (CuS) semiconductor film. These materials are mostly found in amorphous nature with poor crystallinity tending to nanocrystals. Some reports showed the possibility of converting CuS films into the chalcocite phase by mean copper atomic implanting; in reference [26] the authors reported an indirect band gap of 1.28 eV for CuS. CuS is used in various applications such as ion sensitive electrodes and photothermal conversion solar controllers [27, 28].
## 2. Synthesis of the thin films
CdS thin films were deposited on microscope glass substrates, immersed into a 100 ml beaker containing a solution mixture of 31 ml of deionized water, 4 ml of 0.1 M cadmium nitrate tetrahydrate (Cd(NO3)2 4H2O), 5ml of 0.5 M glycine (NH2CH2COOH), 2 ml of pH 11 buffer, 5 ml of 1 M thiourea ((NH2)2CS) and finally in the mixture solution of 60 ml of deionized water was added in order to increase the reaction volume. The mix of solutions was placed in a thermal reservoir maintained at 70°C for 10 min and a homogeneous CdS film with a direct band gap of 2.47 eV was obtained.
PbS thin films were obtained by sequentially adding 5 ml of lead acetate (0.5 M) and 5 ml of sodium hydroxide (2 M), 6 ml of thiourea (1 M) and 2 ml of triethanolamine (1 M) in mixture solution and finally in the solution, 82 ml of ionized water is added. After stirring the mixture solution, in order to homogenize the mixture, the reaction mixture was placed in a thermal source at 70 °C for 5 min.
CuS thin films were deposited in glass substrates obtained from a solution by adding 2 ml of dilute copper nitrate (0.1 M) into 31 ml of deionized water and then adding sequentially 2 ml of barium hydroxide (0.01 M), 2 ml of triethanolamine (1 M), 4 ml of thiourea (1 M) and finally 19 ml of deionized water. The determined reaction time was 20 min. Using the process, we are able to obtained CuS thin films of around 150 nm thickness, amorphous, weakly adhered and a direct energy band gap of 1.26 eV [9].
Rigaku Ultima III diffractometer with micro-Raman X'Plora BXT40 at 2400T resolution was used to collect the X-ray patterns. The chemical analysis was carried out using an XPS Perkin-Elmer Phi-5000 model. Transmission spectra were obtained using an Ocean Optics USB4000- UV-VIS spectrometer in the 280–850 nm wavelength range. The surface morphology of the samples was studied by atomic force microscopy (AFM), using a JSPM-4210 scanning probe microscope (JEOL Ltd.), SEM Zeiss SUPRA 40.
This section describes the chemical formulations used to obtain the selected thin films materials such as CdS, PbS and CuS. As can be observed, the used chemical compounds (precursors) are so easy to manipulate and the procedure just consists of adding the ordered aqueous solutions sequentially, heating and waiting for the deposition time.
The following are the chemical formulations to obtain cadmium sulfide (CdS) thin films:
II–VI type which is mainly useful in optoelectronic devices and some researchers reported that it
Lead sulfide has a cubic crystallographic structure, while cadmium sulfide can be cubic or hexagonal, basically. Here we also discuss some features of copper sulfide (CuS) semiconductor film. These materials are mostly found in amorphous nature with poor crystallinity tending to nanocrystals. Some reports showed the possibility of converting CuS films into the chalcocite phase by mean copper atomic implanting; in reference [26] the authors reported an indirect band gap of 1.28 eV for CuS. CuS is used in various applications such as ion sensitive
CdS thin films were deposited on microscope glass substrates, immersed into a 100 ml beaker containing a solution mixture of 31 ml of deionized water, 4 ml of 0.1 M cadmium nitrate tetrahydrate (Cd(NO3)2 4H2O), 5ml of 0.5 M glycine (NH2CH2COOH), 2 ml of pH 11 buffer, 5 ml of 1 M thiourea ((NH2)2CS) and finally in the mixture solution of 60 ml of deionized water was added in order to increase the reaction volume. The mix of solutions was placed in a thermal reservoir maintained at 70°C for 10 min and a homogeneous CdS film with a direct
PbS thin films were obtained by sequentially adding 5 ml of lead acetate (0.5 M) and 5 ml of sodium hydroxide (2 M), 6 ml of thiourea (1 M) and 2 ml of triethanolamine (1 M) in mixture solution and finally in the solution, 82 ml of ionized water is added. After stirring the mixture solution, in order to homogenize the mixture, the reaction mixture was placed in a thermal
CuS thin films were deposited in glass substrates obtained from a solution by adding 2 ml of dilute copper nitrate (0.1 M) into 31 ml of deionized water and then adding sequentially 2 ml of barium hydroxide (0.01 M), 2 ml of triethanolamine (1 M), 4 ml of thiourea (1 M) and finally 19 ml of deionized water. The determined reaction time was 20 min. Using the process, we are able to obtained CuS thin films of around 150 nm thickness, amorphous, weakly adhered and a
Rigaku Ultima III diffractometer with micro-Raman X'Plora BXT40 at 2400T resolution was used to collect the X-ray patterns. The chemical analysis was carried out using an XPS Perkin-Elmer Phi-5000 model. Transmission spectra were obtained using an Ocean Optics USB4000- UV-VIS spectrometer in the 280–850 nm wavelength range. The surface morphology of the samples was studied by atomic force microscopy (AFM), using a JSPM-4210 scanning probe
This section describes the chemical formulations used to obtain the selected thin films materials such as CdS, PbS and CuS. As can be observed, the used chemical compounds (precursors) are so easy to manipulate and the procedure just consists of adding the ordered
The following are the chemical formulations to obtain cadmium sulfide (CdS) thin films:
aqueous solutions sequentially, heating and waiting for the deposition time.
.
16 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
electrodes and photothermal conversion solar controllers [27, 28].
has low conductivity of 10.8 (Ω cm)−<sup>1</sup>
2. Synthesis of the thin films
band gap of 2.47 eV was obtained.
direct energy band gap of 1.26 eV [9].
microscope (JEOL Ltd.), SEM Zeiss SUPRA 40.
source at 70 °C for 5 min.
The following are the chemical formulations to obtain lead sulfide (PbS) thin films:
The following are the chemical formulations to obtain copper sulfide (CuS) thin films:
## 3. Results
The first characterizations to present are X-ray diffraction patterns for the thin films of materials (CdS, PbS and CuS) as ground and CuS thermal annealed (see Figure 1). Figure 1 shows the precise labels for each film. CdS PDF # 02-0563, PbS PDF # 65-9496 and CuS amorphous.
The Raman dispersion characterizations were carried out using a laser with a wavelength of 532 nm. Figure 2 shows a typical Raman signal for CdS [29], the Raman spectrum is noisy, but an adjustment was carried out in order to smooth.
Figure 1. XRD patterns for the synthesized CdS, PbS and CuS films, including a CuS film with thermal annealing.
Figure 2. Raman spectrum for CdS thin film prepared by chemical bath deposition at 70°C for 10 min.
For the PbS thin film, the Raman spectrum shows three more intense signals, located in 201.6, 319.9 and 449.07 cm−<sup>1</sup> (see Figure 3). Also a laser of 532 nm wavelength was used to obtain the Raman spectrum.
Figure 3. Raman spectrum for PbS thin film prepared by chemical bath deposition at 75°C for 5 min.
Figure 1. XRD patterns for the synthesized CdS, PbS and CuS films, including a CuS film with thermal annealing.
18 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
Figure 2. Raman spectrum for CdS thin film prepared by chemical bath deposition at 70°C for 10 min.
Figure 4. Raman spectrum for CuS thin film prepared by chemical bath deposition at 55°C for 20 min.
Table 1. Main chemical composition for three thin films elaborated by chemical bath deposition and their binding energies.
Figure 5. XPS spectra for our three compounds, PbS, CdS and CuS thin films. These plots confirm the chemical composition the obtained materials.
Raman spectrum for as ground CuS thin film (see Figure 4) shows two well-defined signals or dispersions at 263.5 and 471 cm−<sup>1</sup> .
XPS
energies.
sition the obtained materials.
Label CdS PbS CuS
20 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
Energy level eV Energy level eV Energy level eV
a Cd MNN (Auger) 882.32 O KLL (Auger) 749.31 Cu 2p1 954.6 b O KLL (Auger) 745.42 Pb 4p3 647.66 Cu 2p3 933.15 c Cd 3p1 655.44 O 1s 536.17 O KLL (Auger) 743.58 d Cd 3p3 620.19 Pb 4d3 438.41 O 1s 532.28 e O 1s 534.11 Pb 4d5 416.89 – 416.89 f Cd 3d3 414.83 C 1s 287.77 Cu LMM (Auger) 336.53 g Cd 3d5 407.05 S 2p3 164.6 C 1s 285.71 h C 1s 285.71 Pb 4f5 146.97 Cl 2s 264.19 i S 2s 227.1 Pb 4f7 139.19 S 2p 225.045 j S 2p 164.6 Pb 5d5 23.58 Si 2s 199.63 k Cd 4d5 13.96 – Cu 3s 162.54 l – – Si 2p 123.39 m – – Cu 3p3 76.46
Table 1. Main chemical composition for three thin films elaborated by chemical bath deposition and their binding
Figure 5. XPS spectra for our three compounds, PbS, CdS and CuS thin films. These plots confirm the chemical compo-
The next characterization is carried out by X-ray photoelectron spectroscopy; at this stage, it is possible to determine the chemical composition for the grown materials: CdS thin film, PbS
Figure 6. Optical absorption responses for the indicated thin films of CdS, PbS and CuS as ground and CuS thermal annealed.
Figure 7. The linear adjustment for the projected CdS thin film with a direct band gap of 2.47 eV.
Figure 8. Absorption coefficient and light penetration deep for the CdS, this graph can be used as a design tool to determine the thickness for the CdS layer for solar cells.
Figure 9. Band gap compute showing the region where is present the absorption edge for CuS as ground.
thin film and as ground CuS thin film and annealed CuS thin film (as with thermal annealing as without thermal annealing). The energetic levels located in each one of the thin films are shown in Table 1 and Figure 5. Table 1 also presents the name of each chemical compound and its location.
Figure 10. Band gap compute showing the region where is present the absorption edge for the CuS with thermic annealing.
Figure 11. Indirect band gap compute for CuS as ground.
Figure 8. Absorption coefficient and light penetration deep for the CdS, this graph can be used as a design tool to
Figure 9. Band gap compute showing the region where is present the absorption edge for CuS as ground.
determine the thickness for the CdS layer for solar cells.
22 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
Table 1 shows 11 peaks identified for the CdS, 10 for PbS and 13 for the CuS. All these peaks confirm the high purity of the material preparation.
On the other hand, Figure 6 depicts the absorption responses for one CdS, one PbS and two CuS thin films. The CuS thin films correspond one to as ground film and other with thermal annealing. Reaction conditions are as follows: for CdS: reaction temperature 70°C and reaction time, 10 min; for PbS: reaction temperature 75°C and reaction time 5 min; and for as ground CuS: reaction temperature 55°C and reaction time 20 min, while a replicate of CuS has been thermal annealed to 180°C for 20 min.
Figure 12. Images (a) and (b) show the surface profile corresponding to CdS thin film elaborated, (c) and (d) images show the corresponding PbS and (e) and (f) images show for CuS films [18].
Figure 7 shows the graphical calculation procedure which determines the optical direct band gap and this procedure is typically denominated by Tauc procedure. The intercept had a value of −3.67123 (a.u.), while the slope was 1.48674 (a.u./eV)
Table 1 shows 11 peaks identified for the CdS, 10 for PbS and 13 for the CuS. All these peaks
On the other hand, Figure 6 depicts the absorption responses for one CdS, one PbS and two CuS thin films. The CuS thin films correspond one to as ground film and other with thermal annealing. Reaction conditions are as follows: for CdS: reaction temperature 70°C and reaction time, 10 min; for PbS: reaction temperature 75°C and reaction time 5 min; and for as ground CuS: reaction temperature 55°C and reaction time 20 min, while a replicate of CuS has been
Figure 12. Images (a) and (b) show the surface profile corresponding to CdS thin film elaborated, (c) and (d) images show
the corresponding PbS and (e) and (f) images show for CuS films [18].
confirm the high purity of the material preparation.
24 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
thermal annealed to 180°C for 20 min.
A very interesting and useful analysis is the corresponding to comparison between the absorption coefficient (cm−<sup>1</sup> ) and the light penetration deep (nm), see Figure 8. The relationships among them are basically multiplicative inverses; for example, we choose the wavelength value of 595 nm and from there, the values for the absorption coefficient (α) and light penetration deep (Lpd) are 4 × 104 cm−<sup>1</sup> and 2.5 × 102 nm, respectively. This curve is important because is a good tool to solar cell designs. In this curve is possible chose the thickness to satisfy a quantity of absorption and penetration length deep.
As shown in Figure 9–Figure 11, the direct band gap value is computed for the CuS thin films obtained by chemical bath deposition, in the curve seen in Figure 9, the direct band gap is 1.78 eV for the CuS as ground, in Figure 10, the band gap is 2.74 eV for CuS which is subjected at thermal annealing. The indirect band gap of 0.94 eV for CuS is shown in Figure 11.
Figure 12 depicts the surface morphology of three sulfides CdS, PbS and CuS realized by AFM on square areas of 2.0 × 2.0 μm2 and 498 × 498 μm2 . (a) Image shows a top view for the CdS thin film, (b) image shows a perspective view corresponding to CdS material; (c) and (d) images show the PbS thin films and finally, the top and perspective views of the CuS thin film are shown in the images labeled (e) and (f). The cluster size of PbS is bigger than that of CdS, which are at the same scale, while the cluster size for CuS only was appreciable for a higher magnification; anyway, the higher profile heights were found for CuS films around six times bigger than CdS.
Figure 13 depicts an SEM micrograph of PbS and the reference scale is 200 nm and the superficial particles have a size of approximately 70 nm and are presented with less frequency. The morphology of the rest of the thin film is of particles more little and tight.
Figure 13. SEM micrograph of PbS thin film showing the superficial morphology for special conditions of 75°C for 5 min.
#### 4. Photoresponse
The CdS was a unique material that shows the interesting behavior with time. The response should be instantaneously in a conductor however due to charge effects this is retarded in CdS and it is a dielectric material therefore the response is similar to a capacitor, when the energy is increased above of the band gap this exponential behavior is increased. Figure 14 shows the behavior of the photoresponse at three different wavelengths, showing a greater need for stabilization time at a wavelength close to the bandwidth.
Figure 14. Response time for the CdS thin film synthetized by DBQ at 70°C for 10 min and studied at λ = 350.97, 498.9 and 510.02, respectively.
The graph of resistance vs. temperature for the CuS thin films with thermal annealing determines the semiconductor behavior from the slope of the curve of Figure 15. This curve is nearly linear and then it is possible fitting by a line. The minimal resistance is present at 112°C being 1047180 Ω. In this case, we can see that this curve is composed of three straight lines approximately all of these of semiconductor behavior but with different slopes (see Figure 16) [29].
Figure 15. Linear fitting from the resistance vs. temperature of the CuS thin film with thermal annealing.
Synthesis of Thin Films of Sulfides of Cadmium, Lead and Copper by Chemical Bath Deposition http://dx.doi.org/10.5772/66751 27
Figure 16. Nonlinear fitting from the resistance vs. temperature of the PbS thin film.
4. Photoresponse
and 510.02, respectively.
The CdS was a unique material that shows the interesting behavior with time. The response should be instantaneously in a conductor however due to charge effects this is retarded in CdS and it is a dielectric material therefore the response is similar to a capacitor, when the energy is increased above of the band gap this exponential behavior is increased. Figure 14 shows the behavior of the photoresponse at three different wavelengths, showing a greater need for
The graph of resistance vs. temperature for the CuS thin films with thermal annealing determines the semiconductor behavior from the slope of the curve of Figure 15. This curve is nearly linear and then it is possible fitting by a line. The minimal resistance is present at 112°C being 1047180 Ω. In this case, we can see that this curve is composed of three straight lines approximately all of these of semiconductor behavior but with different slopes (see Figure 16) [29].
Figure 15. Linear fitting from the resistance vs. temperature of the CuS thin film with thermal annealing.
Figure 14. Response time for the CdS thin film synthetized by DBQ at 70°C for 10 min and studied at λ = 350.97, 498.9
stabilization time at a wavelength close to the bandwidth.
26 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
Figure 17 shows the structure of a solar cell, where on a glass substrate covered by an ITO film is deposited CdS by the aforementioned procedure the PbS is then deposited following the formula of the section (Synthesis of the thin films) and finally are Deposited silver contacts to measure the complete structure, the contacts are periodically separated by 1 cm as shown in this figure.
The I–V curve in Figure 18 shows an on voltage that increase with the increase of the measure area because each measure is realized considering first E1 respect to ITO, after that E1 + E2 = E2 respect to ITO and so on. The measure result is shown in the curve I–V, which indicates that when the slope increases the resistance decreases, increasing therefore with current.
Figure 17. Three-dimensional solar cell structure showing details of front and rear contact arrangement.
Figure 18. I–V response for the example structure.
## 5. Conclusions
The main conclusion establishes that the chemical bath deposition technique is a simple and low-cost process and that it is used to obtain thin films of CdS, PbS and CuS with very good homogeneity, pure enough and low cost, which can be used in wide range of applications.
CdS thin films obtained using glycine as a complexing agent presented hexagonal polycrystalline structure. The method used for PbS thin films in this work also produced a polycrystalline film but with cubic geometry. The CuS thin film was an amorphous material and weakly adhered to the substrate.
Their optical responses in the UV-vis range are according with some reported values. Some electrical and thermal tests were used on the obtained materials, In order to future applications.
## Acknowledgements
We would like to thank the facilities provided by the Laboratory XPS UNISON, M.C. Roberto Mora Monroy and XRD Laboratory CINVESTAV Qro. Q.A. Martin Adelaido Hernandez Landaverde.
## Author details
Armando Gregorio Rojas Hernández\* and S. Jesus Castillo
\*Address all correspondence to: [email protected]
Departamento de Investigación en Física, Universidad de Sonora, Hermosillo, Sonora, Mexico
## References
5. Conclusions
Figure 18. I–V response for the example structure.
28 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
adhered to the substrate.
Acknowledgements
tions.
Landaverde.
The main conclusion establishes that the chemical bath deposition technique is a simple and low-cost process and that it is used to obtain thin films of CdS, PbS and CuS with very good homogeneity, pure enough and low cost, which can be used in wide range of applications.
CdS thin films obtained using glycine as a complexing agent presented hexagonal polycrystalline structure. The method used for PbS thin films in this work also produced a polycrystalline film but with cubic geometry. The CuS thin film was an amorphous material and weakly
Their optical responses in the UV-vis range are according with some reported values. Some electrical and thermal tests were used on the obtained materials, In order to future applica-
We would like to thank the facilities provided by the Laboratory XPS UNISON, M.C. Roberto Mora Monroy and XRD Laboratory CINVESTAV Qro. Q.A. Martin Adelaido Hernandez
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30 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
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**Provisional chapter**
## **Modified Spin Coating Method for Coating and Fabricating Ferroelectric Thin Films as Sensors and Solar Cells and Fabricating Ferroelectric Thin Films as Sensors and Solar Cells**
**Modified Spin Coating Method for Coating**
Irzaman, Heriyanto Syafutra, Ridwan Siskandar, Aminullah and Husin Alatas Siskandar, Aminullah and Husin Alatas Additional information is available at the end of the chapter
Additional information is available at the end of the chapter
Irzaman, Heriyanto Syafutra, Ridwan
http://dx.doi.org/10.5772/66815
#### **Abstract**
Spin coating process with a modified spin coater is performed well, especially the second generation of modified spin coater, which has a maximum value of 18,000 rpm, is able for manufacturing/coating photonic crystal‐based ferroelectric thin films that require a high angular velocity (rpm). Ferroelectric thin films that use both 3000 and 6000 rpm have given good results in energy gap, electrical conductivity, etc. In addition, the modified spin coater has also produced several applications such as sensors in the device of blood sugar level noninvasively, sensors in the automatic drying system, sensors in the robotic system, and photovoltaic cells in the system of solar cells/panels which are being developed at present. These applications used ferroelectric material such as barium strontium titanate (BST), lithium niobate (LiNbO3 ), cuprous oxide (CuO), and lithium tantalate (LiTaO3 ).
**Keywords:** modified spin coating, ferroelectric thin films, sensors, solar cells
## **1. Introduction**
Thin film technology is one of the pillars of the current smart material technology due to its material and cost efficiencies. Industrial applications of thin films include electronic semi‐ conductors (especially solar cells), optical coatings, and sensors due to their dielectric con‐ stant, dielectric loss, pyroelectric coefficient, and dielectric tunability properties. Ferroelectric thin films such as barium strontium titanate, lithium tantalate, and lithium niobate can be manufactured by using CSD method and then performed by the spin coating process [1]. CSD method is one of the methods to create/develop thin films [2–12], which has advantages including the ability to control the film stoichiometry with good quality, easy procedure,
© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
require relatively low cost, and generate a good crystalline phase [13–15]. In addition, thin films can also be fabricated by other methods such as metal organic chemical vapor deposi‐ tion (MOCVD) [16–18], chemical vapor deposition [19], sol‐gel [20–23], atomic layer deposi‐ tion (ALD) [24], metal organic decomposition (MOD) [25], pulsed laser ablation deposition (PLAD) [26, 27], and RF sputtering [2, 21, 28].
Spin coating is a method for coating and fabricating uniform thin films by rotating substrate and solution of thin films with a certain angular velocity. Purwanto and Prajitno [29] stated that spin coating is a method to deposit a thin film by spreading the solution onto a substrate by utilizing the centripetal force, the substrate is rotated at a constant velocity and then thin film precipitate is obtained on the substrate. Spin coating process has several advantages: namely, it is a simple method that can be done at room temperature, and low cost, yet effec‐ tive enough for manufacturing thin films [30]. Coating technique with spin coating method is the best technique used to produce thin films with uniform thickness ranging from 0.3 to 5.0 μm on the substrate surfaces that are relatively small [31, 32]. The film thickness is determined by the flow rate and plating time [33]. A simple process of spin coating can be seen in **Figure 1**.
**Figure 1.** Simple process of spin coating [20, 21].
## **2. First generation of modified spin coater**
require relatively low cost, and generate a good crystalline phase [13–15]. In addition, thin films can also be fabricated by other methods such as metal organic chemical vapor deposi‐ tion (MOCVD) [16–18], chemical vapor deposition [19], sol‐gel [20–23], atomic layer deposi‐ tion (ALD) [24], metal organic decomposition (MOD) [25], pulsed laser ablation deposition
Spin coating is a method for coating and fabricating uniform thin films by rotating substrate and solution of thin films with a certain angular velocity. Purwanto and Prajitno [29] stated that spin coating is a method to deposit a thin film by spreading the solution onto a substrate by utilizing the centripetal force, the substrate is rotated at a constant velocity and then thin film precipitate is obtained on the substrate. Spin coating process has several advantages: namely, it is a simple method that can be done at room temperature, and low cost, yet effec‐ tive enough for manufacturing thin films [30]. Coating technique with spin coating method is the best technique used to produce thin films with uniform thickness ranging from 0.3 to 5.0 μm on the substrate surfaces that are relatively small [31, 32]. The film thickness is determined by the flow rate and plating time [33]. A simple process of spin coating can be seen in **Figure 1**.
(PLAD) [26, 27], and RF sputtering [2, 21, 28].
34 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
**Figure 1.** Simple process of spin coating [20, 21].
Proper design of the modified spin coating can reduce cost in the international market because its components can be purchased in Indonesia and adequate for our laboratory‐scale research. A schematic design of portable spin coating type 2004 can be seen in **Figure 2**. This device can be carried easily because it is very light and small as well as better than the traditional one in operational and production costs as well as in efficiency. For generating or rotating the disk in a spin coating, the step‐down transformer of 1 A is required with the output voltages based on the digital system. The output voltages of 7, 9, and 12 V are connected to the potentiometer, diode, and capacitor which resulting in the spin coating rotation of 3480, 4380, and 5840 rpms, respec‐ tively. The accuration test of the rotational velocity can be conducted by using a stroboscope.
The work mechanism of the device is as follows: the solution of thin films is dripped on a substrate which has been placed on the spin coating device. Then, the attached solution on a substrate is rotated at the desired rpm velocity. The voltage source used in the device is 220 V AC with current of 1 A, which is obtained through the step‐down transformer. The output voltage of 7, 9 or 12 V can be selected via potentiometer setting. A diode serves to rectify the AC current into DC and then the electrical charges stored in a capacitor. The rotations are about 3480, 4380, and 5840 rpms.
The modified spin coating device (**Figure 2**), which its patent has been registered in Indonesia with a number of P00201201122 in 2013, has a structural design that consists of four components.
**Figure 2.** Design of portable spin coating‐type 2004.
#### **2.1. Current source**
A current source of 220 V AC is connected to the switch and fuse safety systems.
#### **2.2. Step‐down transformer and potentiometer**
A step‐down transformer of 1A with input voltages of 7, 9 and 12 V. These are a set based on the resistance that changes in the potentiometer.
#### **2.3. Diode and capacitor**
A rectifier diode of standard currents is used to deliver a DC current of 50 V into a capacitor of 2200 μF and 50 V. The capacitor serves to store the charges and acts as a charge source for powering the disk spinner.
#### **2.4. Disk of spin coating**
The aluminum spinner device with a radius of 4 cm and thickness of 0.2 cm is used to rotate the substrate that has been dripped by solution of thin films. To generate rotation, a rotator machine is used to achieve the desired rpm velocity. **Figure 3** shows the exterior of the modified spin coating device. A disk spinner installed on a plastic container of PVC with a radius of 6 cm and thickness of 0.3 cm and a steel cantilever with a thickness of 2 cm and an area of 4 cm × 4 cm to stabilize the disk spinner. The electrical circuits are arranged in a container of 6.2 cm × 19 cm.
**Figure 3.** Exterior of the modified spin coating device.
## **3. Second generation of the modified spin coater**
Currently, the development of the spin coating device has started in which the angular veloc‐ ity of the device is increased up to 18,000 rpm. This is done because the manufacture of fer‐ roelectric‐based photonic crystals thin films requires a high‐angular velocity. There are five stages in the development of a high‐velocity spin coating device, namely designing, manu‐ facturing, testing, analyzing, and developing the device.
Modified Spin Coating Method for Coating and Fabricating Ferroelectric Thin Films as Sensors and Solar Cells http://dx.doi.org/10.5772/66815 37
**Figure 4.** Block diagram.
**2.1. Current source**
**2.3. Diode and capacitor**
powering the disk spinner.
**2.4. Disk of spin coating**
**2.2. Step‐down transformer and potentiometer**
36 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
the resistance that changes in the potentiometer.
**3. Second generation of the modified spin coater**
**Figure 3.** Exterior of the modified spin coating device.
facturing, testing, analyzing, and developing the device.
A current source of 220 V AC is connected to the switch and fuse safety systems.
A step‐down transformer of 1A with input voltages of 7, 9 and 12 V. These are a set based on
A rectifier diode of standard currents is used to deliver a DC current of 50 V into a capacitor of 2200 μF and 50 V. The capacitor serves to store the charges and acts as a charge source for
The aluminum spinner device with a radius of 4 cm and thickness of 0.2 cm is used to rotate the substrate that has been dripped by solution of thin films. To generate rotation, a rotator machine is used to achieve the desired rpm velocity. **Figure 3** shows the exterior of the modified spin coating device. A disk spinner installed on a plastic container of PVC with a radius of 6 cm and thickness of 0.3 cm and a steel cantilever with a thickness of 2 cm and an area of 4 cm × 4 cm to stabilize the disk spinner. The electrical circuits are arranged in a container of 6.2 cm × 19 cm.
Currently, the development of the spin coating device has started in which the angular veloc‐ ity of the device is increased up to 18,000 rpm. This is done because the manufacture of fer‐ roelectric‐based photonic crystals thin films requires a high‐angular velocity. There are five stages in the development of a high‐velocity spin coating device, namely designing, manu‐
**Figure 5.** Program flowchart.
#### **3.1. Designing the device**
In this stage, designing the workflow is conducted which is described in the block diagram and flowchart. The block diagram in **Figure 4** illustrates the Arduino Uno as a data processing center that received input from infrared sensors, controlled by the buttons, and the output as a signal to adjust the angular velocity of a brushless motor as the driving component via the ESC motor driver and the LCD will show the magnitude of the angular velocity in rotation per minute (RPM). **Figure 5** shows the program flowchart for a device that further will be integrated with the Arduino Uno.
#### **3.2. Manufacturing the device**
The schematic circuit diagram in **Figure 6** describes the use of Arduino Uno, buttons, infrared sensor, ESC, brushless motor, and LCD. There are eight Arduino pins, six digital, and two analog pins, which are used for the data path. Pin 4–7 on Arduino connected to four buttons. In connecting the ESC to Arduino, pins that support the pulse width modulation (PWM) are needed, so pin 3 was used on the Arduino. Infrared sensors require attach interrupt function on the program of pin 2. Then, to connect Arduino to LCD, I2C interface used to conserve the use of pins on the Arduino. From the circuit scheme, A4 pin on the Arduino is connected to the SLC in the I2C interface and A5 pin on the Arduino is connected to the SDA in I2C inter‐ face. The brushless motor has only three wires and those are connected to the ESC.
The used voltage source is an adapter of 12 V and 5 A. Most of the voltage source used to drive a brushless motor and ESC by using a voltage of 12 V and current of 5 A. Arduino gets voltage from the same source, but also need a resistor of 20 Ω on *V*in pin in order not to dam‐ age Arduino. While other components such as infrared sensors, I2C interface, and LCD can be run at a voltage of 5 V.
**Figure 6.** Circuit schematic.
After making a circuit scheme, the program code is conducted in accordance with the device workflow diagram that has been made at the designing stage. When the device circuit and the program code are completely made, it is necessary to merge the process of the device circuit with the program code so that the device can work in accordance with the flowchart. The incorporation using an application that matches to the Arduino device is Arduino IDE 1.6.8. Interface of Arduino IDE 1.6.8 application is shown in **Figure 7**.
#### **3.3. Testing the device**
**3.1. Designing the device**
38 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
integrated with the Arduino Uno.
**3.2. Manufacturing the device**
be run at a voltage of 5 V.
**Figure 6.** Circuit schematic.
In this stage, designing the workflow is conducted which is described in the block diagram and flowchart. The block diagram in **Figure 4** illustrates the Arduino Uno as a data processing center that received input from infrared sensors, controlled by the buttons, and the output as a signal to adjust the angular velocity of a brushless motor as the driving component via the ESC motor driver and the LCD will show the magnitude of the angular velocity in rotation per minute (RPM). **Figure 5** shows the program flowchart for a device that further will be
The schematic circuit diagram in **Figure 6** describes the use of Arduino Uno, buttons, infrared sensor, ESC, brushless motor, and LCD. There are eight Arduino pins, six digital, and two analog pins, which are used for the data path. Pin 4–7 on Arduino connected to four buttons. In connecting the ESC to Arduino, pins that support the pulse width modulation (PWM) are needed, so pin 3 was used on the Arduino. Infrared sensors require attach interrupt function on the program of pin 2. Then, to connect Arduino to LCD, I2C interface used to conserve the use of pins on the Arduino. From the circuit scheme, A4 pin on the Arduino is connected to the SLC in the I2C interface and A5 pin on the Arduino is connected to the SDA in I2C inter‐
The used voltage source is an adapter of 12 V and 5 A. Most of the voltage source used to drive a brushless motor and ESC by using a voltage of 12 V and current of 5 A. Arduino gets voltage from the same source, but also need a resistor of 20 Ω on *V*in pin in order not to dam‐ age Arduino. While other components such as infrared sensors, I2C interface, and LCD can
face. The brushless motor has only three wires and those are connected to the ESC.
LCD displays the measurement of angular velocity and value rpm. As can be seen from **Figure 8**, the first line of LCD includes the word "Speed" that shows the measurement of the angular velocity in units of rpm. The second line of LCD is"Set' that shows the rpm setting value that users want in units of rpm as well. **Table 1** shows the measured voltages and currents between circuit blocks of the spin coater which explains that an infrared sensor will produce an output voltage of 4.92 V if detects black color and will produce an output voltage of 0.11 Vif detects another colors. Arduino circuit blocks will produce different output voltage according to the type of the use of the IC.
The next is the accuracy of angular velocity of the spin coater using tachometer type DT‐2234C<sup>+</sup> as an angular velocity comparator (**Figure 9**). Angular velocities of the brushless motor are determined on the value from 1000 to 20,000 rpm.
**Figure 7.** Interface of Arduino IDE 1.6.8.
**Figure 8.** Test result.
**Table 1.** Measurements of voltage and current between circuit blocks.
From the experimental data, the brushless motor is only capable of producing the minimum angular velocity of 4000 rpm and the maximum of 18,000 rpm with no load. In addition, angular velocity on the spin coater has an average difference of 82.67 rpm obtained from the tachometer. The data can be seen in **Table 2** and **Figure 10**.
#### **3.4. Development analysis**
A function menu is added in the program of development. In this function, users can choose the mode of use, i.e., manual and automatic modes. In automatic mode, users need to set Modified Spin Coating Method for Coating and Fabricating Ferroelectric Thin Films as Sensors and Solar Cells http://dx.doi.org/10.5772/66815 41
**Figure 9.** Tachometer type DT‐2234C<sup>+</sup> .
the number of repetitions (step), the length of timer (timer), and the angular velocity (rpm set). The number of repetitions is only for a substrate. When the timer is up it is necessary to notify such as a sound, hence the buzzer circuit is added as a component to produce a sound as shown in **Figure 11(a)**. In addition, a button is also added, as shown in **Figure 11(b)**, as a supporter of the functions that will be created. The added button is useful for stop the button when the spin coater is started. The development of block diagram shown in **Figure 12** is not much different from the block diagram of before device development.
#### **3.5. Development**
From the experimental data, the brushless motor is only capable of producing the minimum angular velocity of 4000 rpm and the maximum of 18,000 rpm with no load. In addition, angular velocity on the spin coater has an average difference of 82.67 rpm obtained from the
Pin *V*cc 4.91 – 3.3 Pin SDA 2.13 – 0.12 Pin SLC 2.58 – 0.01
**Circuit blocks** *V***in (V)** *V***out (V)** *I* **(mA)** ESC 12.01 ∼ 5000
40 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
Detects black color 4.92 4.92 3.3 Detects another colors 4.92 0.11 3.3
Set of 0 rpm 11.07 0.21 0.11 Set of 4000 rpm 11.07 0.23 0.11 Set of 5000 rpm 11.07 0.24 0.11 Set of 6000 rpm 11.07 0.25 0.11 Set of 7000 rpm 11.07 0.25 0.11 Set of 8000 rpm 11.07 0.27 0.11 Set of 9000 rpm 11.07 0.28 0.11 Set of 10,000 rpm 11.07 0.29 0.11 Set of 11,000 rpm 11.07 0.3 0.11 Set of 12,000 rpm 11.07 0.33 0.11 Set of 13,000 rpm 11.07 0.35 0.11 Set of 14,000 rpm 11.07 0.41 0.11 Set of 15,000 rpm 11.07 0.51 0.11 Set of 16,000 rpm 11.07 0.53 0.11 Set of 17,000 rpm 11.07 0.58 0.11 Set of 18,000 rpm 11.07 0.6 0.11
A function menu is added in the program of development. In this function, users can choose the mode of use, i.e., manual and automatic modes. In automatic mode, users need to set
tachometer. The data can be seen in **Table 2** and **Figure 10**.
**Table 1.** Measurements of voltage and current between circuit blocks.
**3.4. Development analysis**
Infrared sensor
Arduino
LCD
The development of schematic circuit designing and flow of program are conducted in this stage. The developed schematic circuit shown in **Figure 13** is not much different from previ‐ ous schematic, only different in add buzzer and button components. A buzzer and a button are connected directly to the Arduino via pins 9 and 8, respectively. Overall device develop‐ ment and its power supply can be seen in **Figure 14**.
A flow diagram in **Figure 15** shows the flow of the program code. First, the user is prompted to select the mode to be used. The available modes are manual and automatic modes are shown in **Figure 16**. If users select the manual mode, it will display a condition before the device was developed. When choosing the automatic mode, the user is prompted to set the number
**Table 2.** The comparison results of angular velocity.
**Figure 10.** Angular velocity comparison.
of repetitions as shown in **Figure 17**, then adjust the angular velocity in rpm as shown in **Figure 18**, and set the length of time in seconds as shown in **Figure 19**. Once the setup process is complete, it will display a summary which is shown in **Figure 20** and then confirm to save the setting on Electrical Erasable Programmable Read‐only Memory (EEPROM) on Arduino Modified Spin Coating Method for Coating and Fabricating Ferroelectric Thin Films as Sensors and Solar Cells http://dx.doi.org/10.5772/66815 43
**Figure 11.** Buzzer (a) and button (b) component.
**Figure 12.** Development diagram.
**Figure 13.** Development circuit schematic.
of repetitions as shown in **Figure 17**, then adjust the angular velocity in rpm as shown in **Figure 18**, and set the length of time in seconds as shown in **Figure 19**. Once the setup process is complete, it will display a summary which is shown in **Figure 20** and then confirm to save the setting on Electrical Erasable Programmable Read‐only Memory (EEPROM) on Arduino
**Set value Spin coater (infrared sensor) Tachometer**
4000 4200 4200 5000 5280 5285 6000 6360 6342 7000 7380 7406 8000 8340 8470 9000 9540 9541 10,000 10,500 10,504 11,000 11,460 11,406 12,000 12,360 12,442 13,000 13,560 13,621 14,000 14,220 14,428 15,000 15,240 15,408 16,000 16,200 16,367 17,000 17,280 17,521 18,000 18,060 18,135
42 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
**Angular velocity (rpm)**
**Table 2.** The comparison results of angular velocity.
**Figure 10.** Angular velocity comparison.
**Figure 14.** Overall device development and its power supply.
semipermanently which is not lost when the power supply is not connected as in **Figure 21**. The save settings will display words of"Saving Data" as shown in **Figure 22**, after the save pro‐ cess is completed it will display the words of"Data saved" as shown in **Figure 23** and **a** buzzer will sound twice. If not saved then it will return to the setup process.
Once the data has been stored, the spin coater will begin the process of spin coating which displays some information such as repetition, duration, time remaining on the first line of the LCD, and the rpm setting value and measurement results of angular velocity on the sec‐ ond line of the LCD as in **Figure 24**. At each repetition process, the buzzer will sound once. If the spin coating process has been completed, it will display word of"Done" as shown in **Figure 25** and **a** buzzer will sound three times, after that confirmation of the spin coating reprocess will be also displayed as in **Figure 26**.
The aim of this stage is to improve the device accuracy by conducting device reconfiguration. The device reconfiguration will change the measurement of angular velocity, then retest must be done for the accuracy of device. The testing method as well as the testing method before development is compared to tachometer‐type DT‐2234C<sup>+</sup> .
In the measurement of angular velocity of the brushless motor, the rpm setting value on a spin coater is determined which starts from 4000 to 18,000 rpm. The brushless motor only produces the minimum angular velocity of 4000 rpm and the maximum angular velocity of 16,637 rpm under no‐load conditions. From data, the created spin coater has an accuracy rate of 98.9%. The results can be seen in **Figure 27** and **Table 3**.
Modified Spin Coating Method for Coating and Fabricating Ferroelectric Thin Films as Sensors and Solar Cells http://dx.doi.org/10.5772/66815 45
**Figure 15.** Program flowchart of a spin coater in the automatic mode.
semipermanently which is not lost when the power supply is not connected as in **Figure 21**. The save settings will display words of"Saving Data" as shown in **Figure 22**, after the save pro‐ cess is completed it will display the words of"Data saved" as shown in **Figure 23** and **a** buzzer
Once the data has been stored, the spin coater will begin the process of spin coating which displays some information such as repetition, duration, time remaining on the first line of the LCD, and the rpm setting value and measurement results of angular velocity on the sec‐ ond line of the LCD as in **Figure 24**. At each repetition process, the buzzer will sound once. If the spin coating process has been completed, it will display word of"Done" as shown in **Figure 25** and **a** buzzer will sound three times, after that confirmation of the spin coating
The aim of this stage is to improve the device accuracy by conducting device reconfiguration. The device reconfiguration will change the measurement of angular velocity, then retest must be done for the accuracy of device. The testing method as well as the testing method before
In the measurement of angular velocity of the brushless motor, the rpm setting value on a spin coater is determined which starts from 4000 to 18,000 rpm. The brushless motor only produces the minimum angular velocity of 4000 rpm and the maximum angular velocity of 16,637 rpm under no‐load conditions. From data, the created spin coater has an accuracy rate
.
will sound twice. If not saved then it will return to the setup process.
reprocess will be also displayed as in **Figure 26**.
**Figure 14.** Overall device development and its power supply.
44 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
development is compared to tachometer‐type DT‐2234C<sup>+</sup>
of 98.9%. The results can be seen in **Figure 27** and **Table 3**.
**Figure 16.** Display of mode select.
**Figure 17.** Display of repetition setting.
**Figure 18.** Display of velocity setting.
**Figure 19.** Display of time setting.
**Figure 20.** Display of setting summary.
**Figure 21.** Display of set saving.
Modified Spin Coating Method for Coating and Fabricating Ferroelectric Thin Films as Sensors and Solar Cells http://dx.doi.org/10.5772/66815 47
**Figure 22.** Display of data saving.
**Figure 16.** Display of mode select.
46 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
**Figure 17.** Display of repetition setting.
**Figure 18.** Display of velocity setting.
**Figure 19.** Display of time setting.
**Figure 20.** Display of setting summary.
**Figure 21.** Display of set saving.
**Figure 23.** Display of saved data.
**Figure 24.** Display of the spin coating process.
**Figure 25.** Display of the finished process.
**Figure 26.** Display of the spin coating reprocess.
**Figure 27. Comparison results of angular velocity after development.**
**Table 3.** Comparison data of angular velocity after development.
## **4. Results and applications**
**Figure 27. Comparison results of angular velocity after development.**
48 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
**Set value Spin coater Tachometer**
**Table 3.** Comparison data of angular velocity after development.
4000 3960 3916 99 5000 4980 4977 99.6 6000 5880 5887 98 7000 6960 6938 99.42 8000 7980 7998 99.75 9000 8940 9002 99.33 10,000 9960 9966 99.6 11,000 10,980 10,932 99.82 12,000 11,940 11,960 99.5 13,000 12,960 12,996 99.69 14,000 13,980 13,974 99.86 15,000 15,000 15,027 100 16,000 16,020 16,046 99.86 17,000 16,560 16,587 97.41 18,000 16,680 16,637 92.67 Mean 98.9
**Testing the angular velocity (RPM) Accuracy level of**
**spin coater (%)**
This method has been developed in our laboratory to fabricate thin films, especially fer‐ roelectric thin films such as barium strontium titanate (BST), lithium tantalate (LiTaO3 ), lithium niobate (LiNbO3 ), and copper oxide (CuO) thin films. Specifically for BST ferroelec‐ tric thin films, its patent has been registered in Indonesia with a number of P00201201119 in 2013.
BST thin films of the modified spin coating method resulting in a dielectric constant of 2–18 and conductivity values of 1.6 × 10–6 to 2.4 × 10–9 S/cm at the voltages of 1–4 V [34]. In addition, the particle distribution size of BST 0.45 is 134.93 nm which is smaller than BST 0.25 which gives 186.26 nm, BST 0.35 gives a value of 467.86 nm, and BST 0.55 is 407.49 nm [34].
BST thin films have been developed in a number of applications such as temperature and light sensors that are applied into the automatic drying system [35], light sensors into the luxme‐ ter system [36] and sensors in the blood sugar level system, its patent has been registered in Indonesia with a number of P00201508327 in 2015.
In the automatic drying system [35], light sensors have 0.176 mV/lux, which is the best sensitivity. While the best temperature sensor has a value range of 30–109°C, a sensitivity of 0.862 mV/°C, a film resolution of 1°C, an accuracy level of 92.2%, and small hysteresis [35]. These sensors, which are then integrated into microcontroller, have been successfully conducted on an ATMega8535 microcontroller based on our automatized drying system model.
In the luxmeter system [36], Ba0,25Sr0,75TiO3 (BST) thin films have been deposited on a Si (100) p‐type substrate by a chemical solution deposition (CSD) method followed by the spin coating technique (at 3000 rpm rotational speed for 30 seconds). This BST has an electrical conductiv‐ ity of 2.79 × 10–7 to 5.3 × 10–7 S/cm, which are in the range of semiconductor materials. The *I*‐*V* measurement on the films which was carried out under dark and bright conditions results in convincing conclusion that the films are photodiodes. The maximum optical absorbance found for green light with a wavelength of around 550 nm.
The blood sugar level system also uses the modified spin coating method. The test results showed that the light intensity that was received by photodiode sensor, represented by the output voltage, will change in line with changes in the value of blood sugar levels, follows the equation *y* = 0.0014*x*<sup>2</sup> − 0.6652*x* + 139.34. The coefficient of *R*<sup>2</sup> = 0.9544 indicates that *x* has a major effect on *y*, so that it can be concluded that the photodiode sensor can work well as a sensor of blood sugar level instrument. Accuracy and precision of the instrument are 98.92 and 97.41%, respectively.
BST thin films as solar cells enhanced by photonic crystals are currently being developed [37]. This enhancement needs higher angular velocity (*>*6000 rpm) than previous works in BST (3000 rpm). The second generation of modified spin coater is used for coating the thin films to fulfill the requirement. The optical characterization of BST (Ba*<sup>x</sup>* Sr1‐*<sup>x</sup>* TiO3 ) using a photonic crystal resulted in the average absorption percentages for mole fraction *x* = 0.25, 0.35, 0.45, and 0.55 were 92.04, 83.55, 91.16, and 80.12%, respectively [37]. In addition, the BST thin film with the embedded photonic crystal exhibited a relatively significant enhancement on photon absorption, with increasing values of 3.96, 7.07, 3.04, and 13.33%, respectively.
Furthermore, BST thin films are worked both as sensors and solar cells can be applied to the more sophisticated fields such as temperature and light sensors in the satellite technology [38, 39], photodiode in satellite technology [40], as well as solar cells for substituting conventional battery in satellite technology [41].
LiTaO3 thin films have also been developed by using the modified spin coating method as infrared sensors that are expected to be developed as an automatic switch in satellite technol‐ ogy [42]. These thin films have electrical conductivities of 10‐6–10–5 S/cm and the diffusion coefficient values of 57–391 nm<sup>2</sup> /s [43] as well as the energy gaps of 3.41–4.56 eV [42] while the LiTaO3 thin films have energy gaps of 2.43–2.80 eV [44]. The LiNbO3 thin films that were enhanced by a lanthanum dopant also use a velocity of 3000 rpm and have the energy gaps of 2.43–2.80 eV [45].
In addition to BST, LiTaO3 , and LiNbO3 , we are also develop other thin films, i.e., CuO thin films. This film is also developed as solar cells. CuO thin films were enhanced by photonic crystals that have absorbance in the visible region and energy gaps of 1.89–2.05 eV [46].
## **5. Conclusion**
The modified spin coaters both first and second generation have given very good results on ferroelectric films, which are BST, LiTaO3 , LiNbO3 , and CuO, as sensors and solar cells. In BST, the modified spin coater enhances the photon absorption percentages by using photonic crys‐ tals which need higher angular velocity (6000 rpm). Another ferroelectric thin film that use 3000 rpm also have given good results in energy gap, electrical conductivity, etc. From these data, they were succeed to be applied in an instrument or a device such as a blood sugar level, automatic drying, and luxmeter systems. In addition, these thin films also have potential to be applied in more sophisticated fields such as photodiode in satellite technology, temperature, and light sensors in the satellite technology, as well as solar cells for substituting conventional battery in satellite technology.
## **Acknowledgements**
This work was funded by Grant of International Research Collaboration and Scientific Publication from Ministry of Research, Technology and Higher Education, Republic of Indonesia under contract No. 082/SP2H/UPL/DIT.LITABMAS/II/2015 and Hibah Penelitian Institusi, Ministry of Research, Technology and Higher Education, Republic of Indonesia under contract No. 079/SP2H/LT/DRPM/II/2016.
## **Author details**
and 0.55 were 92.04, 83.55, 91.16, and 80.12%, respectively [37]. In addition, the BST thin film with the embedded photonic crystal exhibited a relatively significant enhancement on photon
Furthermore, BST thin films are worked both as sensors and solar cells can be applied to the more sophisticated fields such as temperature and light sensors in the satellite technology [38, 39], photodiode in satellite technology [40], as well as solar cells for substituting conventional
infrared sensors that are expected to be developed as an automatic switch in satellite technol‐ ogy [42]. These thin films have electrical conductivities of 10‐6–10–5 S/cm and the diffusion
enhanced by a lanthanum dopant also use a velocity of 3000 rpm and have the energy gaps
films. This film is also developed as solar cells. CuO thin films were enhanced by photonic crystals that have absorbance in the visible region and energy gaps of 1.89–2.05 eV [46].
The modified spin coaters both first and second generation have given very good results on
the modified spin coater enhances the photon absorption percentages by using photonic crys‐ tals which need higher angular velocity (6000 rpm). Another ferroelectric thin film that use 3000 rpm also have given good results in energy gap, electrical conductivity, etc. From these data, they were succeed to be applied in an instrument or a device such as a blood sugar level, automatic drying, and luxmeter systems. In addition, these thin films also have potential to be applied in more sophisticated fields such as photodiode in satellite technology, temperature, and light sensors in the satellite technology, as well as solar cells for substituting conventional
This work was funded by Grant of International Research Collaboration and Scientific Publication from Ministry of Research, Technology and Higher Education, Republic of Indonesia under contract No. 082/SP2H/UPL/DIT.LITABMAS/II/2015 and Hibah Penelitian Institusi, Ministry of Research, Technology and Higher Education, Republic of Indonesia
, LiNbO3
thin films have energy gaps of 2.43–2.80 eV [44]. The LiNbO3
, and LiNbO3
thin films have also been developed by using the modified spin coating method as
/s [43] as well as the energy gaps of 3.41–4.56 eV [42] while
, we are also develop other thin films, i.e., CuO thin
, and CuO, as sensors and solar cells. In BST,
thin films that were
absorption, with increasing values of 3.96, 7.07, 3.04, and 13.33%, respectively.
50 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
battery in satellite technology [41].
coefficient values of 57–391 nm<sup>2</sup>
LiTaO3
the LiTaO3
of 2.43–2.80 eV [45].
**5. Conclusion**
In addition to BST, LiTaO3
ferroelectric films, which are BST, LiTaO3
under contract No. 079/SP2H/LT/DRPM/II/2016.
battery in satellite technology.
**Acknowledgements**
Irzaman1 \*, Heriyanto Syafutra1 , Ridwan Siskandar<sup>2</sup> , Aminullah3 and Husin Alatas1
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#### **Emission, Defects, and Structure of ZnO Nanocrystal Films Obtained by Electrochemical Method Emission, Defects, and Structure of ZnO Nanocrystal Films Obtained by Electrochemical Method**
Tetyana V. Torchynska and Brahim El Filali Tetyana V. Torchynska and Brahim El Filali
Additional information is available at the end of the chapter Additional information is available at the end of the chapter
http://dx.doi.org/10.5772/66335
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ZnO nanocrystal (NC) films, prepared by electrochemical etching with varying the technological routines, have been studied by means of photoluminescence (PL), scanning electronic microscopy (SEM), energy dispersion spectroscopy (EDS), Raman scattering, and X ray diffraction (XRD) techniques. Raman and XRD studies have confirmed that annealing stimulates the ZnO oxidation and crystallization with the formation of wurtzite ZnO NCs. The ZnO NC size decreases from 250–300 nm down to 40–60 nm with increasing the etching time. Two PL bands connected with the near‐band edge (NBE) and defect‐related emissions have been detected. Their intensity stimulation with NC size decreasing has been detected. The NBE emission enhancement is attributed to the week quantum confinement and exciton‐light coupling with polariton formation in small ZnO NCs. The luminescence, morphology, and crystal structure of ZnO:Cu NCs versus Cu concentration have been investigated as well. The types of Cu‐related complexes are discussed using the correlation between the PL spectrum transformations and XRD parameters. It is shown that the plasmon generation in Cu nanoparticles leads to the surface enhanced Raman scattering (SERS) effect and to PL intensity increasing the defect‐related PL bands. The comparison of ZnO and ZnO:Cu NC emissions has been done and discussed.
**Keywords:** ZnO NCs, ZnO:Cu NCs, photoluminescence, XRD, weak quantum confinement
## **1. Introduction**
Porous semiconductor materials stimulate the scientific interest owing to the possibility for designing the properties not known in the bulk crystals [1]. The enormous attention in the last decades has been devoted to porous silicon (PSi) that was investigated for variety
and reproduction in any medium, provided the original work is properly cited.
© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
of applications in electronics, as well as for chemical, biological, and medical sensing [1]. Recently, a great interest has been shown to the ZnO nanocrystals (NCs) obtained using electrochemical technology, earlier elaborated for PSi [1–3].
The wide band gap semiconductor, such as zinc oxide, which has a direct energy band gap (3.37 eV) and a great exciton binding energy (60 meV) at 300 K, promises a lot of applications in optoelectronic devices [4–8]. Actually ZnO nanowalls and wires can be applied in white light‐emitting structures [5] and UV lasers [6]. ZnO nanoneedles demonstrate the field emission characteristics [7]. Field‐effect transistors on the base of ZnO nanorods were presented in Ref. [8]. ZnO nanostructures have provoked great attention recently owing to their perspectives for low voltage and short‐wavelength (368 nm) electro‐optical devices, as well as for protection films with high UV transparency, in different gas sensors and structures for spintronic devices [9], for a room temperature ferromagnetism, a huge magneto‐optic effect, and chemical sensing [5, 9, 10].
In addition, the ZnO NC structures are interesting for phosphor applications owing to the excellent emission in orange, yellow, green, and blue ranges of PL spectra. The high concentration of radiative defects obtained in ZnO NC films permits to expect a wide spectrum of luminescence bands that is important for "white" light emitting structures [11–14]. However, the relations between the defects and structural properties of ZnO NCs (nanosheets, nanorods, etc.) are not clear yet.
For growing ZnO NCs, the following methods were used: thermal evaporation technique [15], sol‐gel deposition [16], metal organic chemical vapor deposition (MOCVD) [17], molecular beam epitaxy (MBE) [18], pulse laser preparation (PLD) [19], or spray pyrolysis [20]. All mentioned methods require very expensive equipment and do not produce ZnO NCs with bright PL bands that were demonstrated recently for the electrochemical technology [21, 22]. The anodization method permits to control the ZnO NC size by varying an electrolyte content, etching times, and voltages. Additionally, this method permits simple doping of ZnO NC films by different elements.
In this chapter, ZnO and ZnO:Cu NCs were created by the electrochemical method at varying etching times or etching voltages with film annealing at high temperature (400°C) in ambient air. Photoluminescence, scanning electronic microscopy (SEM), energy dispersion spectroscopy (EDS), Raman scattering, and X‐ray diffraction (XRD) have been applied for the study of ZnO and ZnO:Cu NC films.
## **2. ZnO NC preparation and investigations**
The electrochemical anodization of Zn foils was performed in an electrolyte using two Zn electrode systems with the distance between the electrodes being 10 mm. The electrolyte was a 1:10 volume mixture of HF acid (Aldrich) and deionized water. Ultrasonic cleaning of Zn foil pieces (Aldrich 99.99%) of 6 mm radius was performed in acetone and ethanol for 15 min before etching. To investigate the impact of etching times on ZnO NC parameters, the applied voltage was 5 V and the varied times were 1, 3, 6, and 10 min. Then ZnO films were cleaned in deionized water and annealed at 400°C for 2 h in ambient air.
To investigate the influence of voltage, the etching time was kept at 6 min and the applied voltages varied as 1, 5, 10, 15, and 20 V. Obtained ZnO films were washed in deionized water and annealed at 400°C for 2 h in ambient air.
To investigate the effect of Cu doping on the structure and optical properties of ZnO:Cu NCs, and to compare it with those in ZnO NCs, the electrochemical anodization was performed with: (i) two Zn electrodes at the creation of ZnO NCs or (ii) cathode Zn and anode Cu electrodes at the growth of ZnO:Cu NCs. Zn (Aldrich 99.99%) and Cu (Aldrich 99.99%) foils were used. At etching, the applied voltage was 5 V and the times used were 1, 3, or 6 min. Then the films were annealed at 400°C for 2 h in ambient air.
SEM and EDS studies were done in JSM7800F‐JEOL with an additional detector Apollo X 10 mark EDAX. The XRD equipment XPERT MRD, with a pixel detector, three‐axis goniometry, a parallel collimator, and a resolution of 0.0001°, was applied to the crystal structure investigation. The Cu source with *K*α<sup>1</sup> line *λ* = 1.5406 Å was used. XRD was performed for the angle range 20°–80° with a 0.05° step and a step duration of 10 s.
PL spectra, excited by a He‐Cd laser with a wavelength of 325 nm and a beam power of 80 mW, were measured at 10–300 K using a PL setup based on a spectrometer SPEX500 described in references [23, 24]. Raman scattering spectra were studied in Jobin‐Yvon LabRAM HR 800UV micro‐Raman system using an excitation by a solid‐state light‐emitting diode with a light wavelength of 785 nm [25, 26].
## **3. The etching time impact on parameters of ZnO NC films**
## **3.1. SEM and XRD studies**
of applications in electronics, as well as for chemical, biological, and medical sensing [1]. Recently, a great interest has been shown to the ZnO nanocrystals (NCs) obtained using elec-
The wide band gap semiconductor, such as zinc oxide, which has a direct energy band gap (3.37 eV) and a great exciton binding energy (60 meV) at 300 K, promises a lot of applications in optoelectronic devices [4–8]. Actually ZnO nanowalls and wires can be applied in white light‐emitting structures [5] and UV lasers [6]. ZnO nanoneedles demonstrate the field emission characteristics [7]. Field‐effect transistors on the base of ZnO nanorods were presented in Ref. [8]. ZnO nanostructures have provoked great attention recently owing to their perspectives for low voltage and short‐wavelength (368 nm) electro‐optical devices, as well as for protection films with high UV transparency, in different gas sensors and structures for spintronic devices [9], for a room temperature ferromagnetism, a huge magneto‐optic effect,
In addition, the ZnO NC structures are interesting for phosphor applications owing to the excellent emission in orange, yellow, green, and blue ranges of PL spectra. The high concentration of radiative defects obtained in ZnO NC films permits to expect a wide spectrum of luminescence bands that is important for "white" light emitting structures [11–14]. However, the relations between the defects and structural properties of ZnO NCs (nanosheets, nanorods,
For growing ZnO NCs, the following methods were used: thermal evaporation technique [15], sol‐gel deposition [16], metal organic chemical vapor deposition (MOCVD) [17], molecular beam epitaxy (MBE) [18], pulse laser preparation (PLD) [19], or spray pyrolysis [20]. All mentioned methods require very expensive equipment and do not produce ZnO NCs with bright PL bands that were demonstrated recently for the electrochemical technology [21, 22]. The anodization method permits to control the ZnO NC size by varying an electrolyte content, etching times, and voltages. Additionally, this method permits simple doping of ZnO
In this chapter, ZnO and ZnO:Cu NCs were created by the electrochemical method at varying etching times or etching voltages with film annealing at high temperature (400°C) in ambient air. Photoluminescence, scanning electronic microscopy (SEM), energy dispersion spectroscopy (EDS), Raman scattering, and X‐ray diffraction (XRD) have been applied for the study of
The electrochemical anodization of Zn foils was performed in an electrolyte using two Zn electrode systems with the distance between the electrodes being 10 mm. The electrolyte was a 1:10 volume mixture of HF acid (Aldrich) and deionized water. Ultrasonic cleaning of Zn foil pieces (Aldrich 99.99%) of 6 mm radius was performed in acetone and ethanol for 15 min before etching. To investigate the impact of etching times on ZnO NC parameters, the applied
trochemical technology, earlier elaborated for PSi [1–3].
56 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
and chemical sensing [5, 9, 10].
NC films by different elements.
ZnO and ZnO:Cu NC films.
**2. ZnO NC preparation and investigations**
etc.) are not clear yet.
**Figure 1** presents the SEM images of ZnO NC films grown at different etching times after thermal annealing. It is clear that the size of ZnO NCs decreases versus etching times (**Figure 1d**): from 200–360 nm (for 1 min) down to 30–60 nm (for 10 min).
XRD results are summarized in **Figure 2**. As‐grown ZnO films are characterized by the amorphous phase (**Figure 2a**) and the Zn substrate XRD peaks at the 2*θ* angles of 38.993, 43.233, and 70.058° have been seen (**Figure 2a**). These peaks owe to the diffraction from the (100), (101), and (103) crystal planes in the wurtzite Zn crystal lattice [27].
Thermal annealing at 400°C stimulates the ZnO oxidation and crystallization. A set of XRD peaks appears at the 2*θ* angles equaling to 31.770, 34.422, 36.253, 47.540, 56.604, and 62.865° after ZnO film annealing (**Figure 2b** and **c**). These XRD peaks correspond to the diffraction from the (100), (002), (101), (102), (110), and (103) crystal planes in the wurtzite ZnO crystal structure [27]. At first (1–6 min etching), the volume of crystalline ZnO phase enlarges that manifests itself in increasing the XRD peak intensities (**Figure 2d**). Then at higher etching time
**Figure 1.** SEM images of ZnO NCs after thermal annealing (a, b, c), obtained at the voltage of 5V and times of 1 min (a), 6 min (b), and 10 min (c). Widths and lengths of annealed ZnO NCs (d) for the etching durations of 1 min (A), 6 min (B), and 10 min (C) [22].
(10 min), ZnO NC films are characterized by smaller XRD peak intensities (**Figure 2d**) owing to, apparently, the material dissolution at the high anodization duration and increasing the volume of pores in the films.
#### **3.2. Raman scattering study**
Raman scattering spectra of ZnO NC films are presented in **Figure 3**. Raman spectra of as‐ grown ZnO films do not demonstrate any Raman peaks (**Figure 3a**). The small Raman band
Emission, Defects, and Structure of ZnO Nanocrystal Films Obtained by Electrochemical Method http://dx.doi.org/10.5772/66335 59
**Figure 2.** XRD results for as‐grown (a) and annealed (b, c) ZnO NCs obtained at etching durations of 1 (b) and 10 (a, c) min. Dependences (d) of (100) XRD peak intensity (1) and integrated PL intensity (2) for the PL band peaked at 3.1 eV in annealed films versus etching times [22].
(10 min), ZnO NC films are characterized by smaller XRD peak intensities (**Figure 2d**) owing to, apparently, the material dissolution at the high anodization duration and increasing the
**Figure 1.** SEM images of ZnO NCs after thermal annealing (a, b, c), obtained at the voltage of 5V and times of 1 min (a), 6 min (b), and 10 min (c). Widths and lengths of annealed ZnO NCs (d) for the etching durations of 1 min (A), 6 min (B),
58 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
Raman scattering spectra of ZnO NC films are presented in **Figure 3**. Raman spectra of as‐ grown ZnO films do not demonstrate any Raman peaks (**Figure 3a**). The small Raman band
volume of pores in the films.
and 10 min (C) [22].
**3.2. Raman scattering study**
**Figure 3.** Raman scattering spectra of as‐grown (a) and annealed (b) ZnO films obtained at etching times: 1 min (curve 1), 6 min (curve 2), and 10 min (a) and (b), curve 3.
in the range of 400–500 cm−1 is related to Raman scattering in the ZnO amorphous phase. The Raman study confirms that as‐grown ZnO films are characterized by an amorphous phase mainly and it is consistent with the XRD data.
Annealing at 400°C stimulates the ZnO crystallization and four peaks at 327, 379, 434, and 549–556 cm−1 appear in Raman spectra (**Table 1**). The Raman peak intensity rises versus etching time owing to the volume enlargement of the crystalline ZnO phase (**Figure 3b**).
The group theory predicts for the wurzite ZnO crystal structure the Raman active phonons in Brillouin zone center as: A<sup>1</sup> and E<sup>1</sup> symmetry polar phonons with two frequencies for the transverse (TO) and longitudinal (LO) optic phonons, and E2 symmetry nonpolar phonon mode with two frequencies E2 (low) and E2 (high). E2 (low) and E2 (high) modes are attributed to oxygen and zinc sublattices, respectively, in ZnO [28, 29].
Raman peaks at 327 and 437 cm−1 are attributed, as a rule, to second‐order Raman peaks arising from the zone boundary phonons 3E2H‐E2L and E2H modes in ZnO NCs [30]. Raman peaks at 379 and 434 cm−1 can be attributed to the A<sup>1</sup> (TO) and E2 (high) phonon modes in ZnO NCs (**Table 1**). The nature of the Raman peak at 549–556 cm −1 is not clear. Its variable position in different samples (**Figure 3**, curves 2 and 3) and the location between TO and LO optic phonons permit to assign this Raman peak to the surface phonon (SP). The SP frequency (*ωSP*) in ZnO NCs can be calculated using the formula [30, 31]:
$$
\omega\_{sp} = \omega\_{TO} \sqrt{\frac{\varepsilon\_o l + \varepsilon\_M (l+1)}{\varepsilon\_u l + \varepsilon\_M (l+1)}} \tag{1}
$$
where *ωTO* is a frequincy of TO phonon, *ε*<sup>0</sup> and *ε*∞ are the static and high‐frequency dielectric constants in a bulk ZnO crystal, *ε*M is a static dielectric constant of surrounding medium (air). Assuming *ε*M = 1 for air in pores of ZnO NCs and using *ε*<sup>0</sup> and *ε*∞ equal to 8.36 [32] and 3.77 [32], respectively, the SP frequency of 550 cm−1 for the lowest (l = 1) mode has been estimated that is in a good agreement with detected values of 549–556 cm−1 (**Table 1**).
#### **3.3. ZnO NC emission study**
PL spectra of ZnO films obtained at different etching times in as‐grown states are shown in **Figure 4**. PL spectra are presented as the superposition of three PL bands centered at 1.90– 2.03, 2.49–2.51, and 2.80–2.85 eV (**Figure 4**, curves a, b, and c), which are attributed to the defect‐related emission in an amorphous ZnO films. The PL intensity of the above‐mentioned peaks increases with increasing anodization duration up to 6 min due to increasing the volume of ZnO amorphous phase. In samples prepared at 10 min (**Figure 4**, curve 3), the PL
**Table 1.** Raman peaks in ZnO crystals.
Emission, Defects, and Structure of ZnO Nanocrystal Films Obtained by Electrochemical Method http://dx.doi.org/10.5772/66335 61
in the range of 400–500 cm−1 is related to Raman scattering in the ZnO amorphous phase. The Raman study confirms that as‐grown ZnO films are characterized by an amorphous phase
Annealing at 400°C stimulates the ZnO crystallization and four peaks at 327, 379, 434, and 549–556 cm−1 appear in Raman spectra (**Table 1**). The Raman peak intensity rises versus etch-
The group theory predicts for the wurzite ZnO crystal structure the Raman active phonons
(high). E2
Raman peaks at 327 and 437 cm−1 are attributed, as a rule, to second‐order Raman peaks arising from the zone boundary phonons 3E2H‐E2L and E2H modes in ZnO NCs [30]. Raman peaks
(**Table 1**). The nature of the Raman peak at 549–556 cm −1 is not clear. Its variable position in different samples (**Figure 3**, curves 2 and 3) and the location between TO and LO optic phonons permit to assign this Raman peak to the surface phonon (SP). The SP frequency (*ωSP*) in
constants in a bulk ZnO crystal, *ε*M is a static dielectric constant of surrounding medium (air).
[32], respectively, the SP frequency of 550 cm−1 for the lowest (l = 1) mode has been estimated
PL spectra of ZnO films obtained at different etching times in as‐grown states are shown in **Figure 4**. PL spectra are presented as the superposition of three PL bands centered at 1.90– 2.03, 2.49–2.51, and 2.80–2.85 eV (**Figure 4**, curves a, b, and c), which are attributed to the defect‐related emission in an amorphous ZnO films. The PL intensity of the above‐mentioned peaks increases with increasing anodization duration up to 6 min due to increasing the volume of ZnO amorphous phase. In samples prepared at 10 min (**Figure 4**, curve 3), the PL
> **E1 (TO) cm-1**
**E2 (high) cm-1**
102 327 379 410 434–439 549–556 574 591
**SP cm-1** **A1 (LO) cm-1**
**E1 (LO) cm-1**
(TO) and E2
\_\_\_\_\_\_\_\_\_\_ *<sup>ε</sup>*<sup>0</sup> *<sup>l</sup>* <sup>+</sup> *<sup>ε</sup>M*(*<sup>l</sup>* <sup>+</sup> <sup>1</sup> ) \_\_\_\_\_\_\_\_\_\_ *ε*<sup>∞</sup> *l* + *εM*(*l* + 1 )
symmetry polar phonons with two frequencies for the
(low) and E2
symmetry nonpolar phonon
(high) phonon modes in ZnO NCs
, (1)
and *ε*∞ equal to 8.36 [32] and 3.77
and *ε*∞ are the static and high‐frequency dielectric
(high) modes are attributed
ing time owing to the volume enlargement of the crystalline ZnO phase (**Figure 3b**).
and E<sup>1</sup>
(low) and E2
transverse (TO) and longitudinal (LO) optic phonons, and E2
to oxygen and zinc sublattices, respectively, in ZnO [28, 29].
mainly and it is consistent with the XRD data.
60 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
at 379 and 434 cm−1 can be attributed to the A<sup>1</sup>
*<sup>ω</sup>SP* <sup>=</sup> *<sup>ω</sup>TO* <sup>√</sup>
where *ωTO* is a frequincy of TO phonon, *ε*<sup>0</sup>
**3.3. ZnO NC emission study**
**(low) cm-1**
**Table 1.** Raman peaks in ZnO crystals.
**3E2H-E2L cm-1**
**A1 (TO) cm-1**
**Samples E2**
Bulk ZnO [29] and ZnO NCs
ZnO NCs can be calculated using the formula [30, 31]:
Assuming *ε*M = 1 for air in pores of ZnO NCs and using *ε*<sup>0</sup>
that is in a good agreement with detected values of 549–556 cm−1 (**Table 1**).
in Brillouin zone center as: A<sup>1</sup>
mode with two frequencies E2
**Figure 4.** PL spectra of as‐grown ZnO NCs obtained at durations of 1 (1), 6 (2), and 10 (3) min. Dashed curves represent the deconvolution result on elementary PL bands (a, b, c).
intensity of the mentioned PL peaks decreases owing to decreasing the ZnO NC volume that is consistent with the XRD data (**Figure 2**).
Annealing at 400°C stimulates the ZnO crystallization, which is accompanied by the PL spectrum transformation (**Figure 5**). PL spectra of ZnO NCs are complex as well (**Figure 5**, curves 1, 2, and 3) and can be represented by a set of elementary PL bands (**Figure 5**, curves a, b, and c) with the peaks at 2.06–2.10, 2.52, and 3.10 eV. The defect‐related PL band with the peak at 2.80 eV disappeared completely at the ZnO oxidation and crystallization, and the new PL band at 3.10 eV appeared in PL spectra of ZnO NCs. PL intensities of the mentioned PL bands (**Figure 5**) vary as XRD peak intensity changes, which is related to the changing crystalline ZnO volume versus anodization times (**Figure 2**d).
**Figure 5.** PL spectra of annealed ZnO NCs obtained at durations of 1 (1), 6 (2), and 10 (3) min. Dashed curves represent the deconvolution on elementary PL bands (a, b, c) [22].
The bulk ZnO crystals are characterized by the variety of luminescence bands in UV and visible spectral ranges [33, 34]. The origin of these emissions has not been conclusively established, and a number of hypotheses have been proposed for each emission band. NBE band at 3.1 eV is attributed to the optical transition between the shallow donor and valence band, to the phonon replicas of bound exciton, or free exciton (FE) emissions [34, 35]. The high intensity of NBE emission at 300 K and a small band half width in ZnO NCs permits to attribute the 3.1 eV PL band to a LO phonon replica of free exciton.
The blue PL band with the peak at 2.80 eV, which disappeared completely after the oxidation at annealing in ambient air, can be assigned to emission via the native defects in ZnO films [36, 37]. The defect‐related green PL band in the range 2.40–2.50 eV is assigned to oxygen vacancies [36], Cu impurities [38], or surface defects [39] in ZnO. The PL intensity of 2.49–2.52 eV PL band does not change in the processes of oxidation and crystallization (**Figure 5**) that permits to assign this PL to some surface defects.
The PL band centered at 2.00–2.10 eV was assigned earlier to interstitial oxygen atoms (2.02 eV) [40] or hydroxyl groups (2.10 eV) [41, 42]. The PL intensity of 2.06–2.10 eV PL band increases at ZnO oxidation (**Figure 5**) and the assumption that corresponding defects are related to oxygen interstitials looks very reliable.
It is essential that ZnO NCs obtained by other methods, such as Zn powder thermal evaporation [43], sol‐gel ZnO films [44], or MOCVD growth ZnO films [45], do not permit to obtain great variety of PL bands that have been demonstrated in the studied ZnO NCs.
## **4. Anodization voltage impact on the structure and emission of ZnO NC films**
#### **4.1. XRD and SEM study**
The XRD study has shown that as‐grown ZnO films are characterized by Zn substrate‐related XRD peaks with highest XRD peak intensities at the angles 2*θ* equal to 38.993, 43.233, and 70.058° (**Figure 6a**). These peaks correspond to the diffraction from the (100), (101), and (103) crystal planes, respectively, in the hexagonal Zn crystal lattice with the lattice parameters of *a* = 2.6650 Å and *c* = 4.9470 Å [27]. Annealing at 400°C stimulates the crystallization of ZnO NCs and a set of XRD peaks appear at the angles 2*θ* equal to 31.770, 34.422, 36.253, 47.540, 56.604, and 62.865° (**Figure 6b**). These XRD peaks correspond to the X‐ray diffraction from the (100), (002), (101), (102), (110), and (103) crystal planes in the wurtzite ZnO crystal structure [27].
The volume of crystalline ZnO phase enlarges versus anodization voltages up to 15 V that manifests itself by increasing the XRD peak intensities. However, ZnO NCs obtained at the voltage of 20 V are characterized by smaller intensity of XRD peaks that, apparently, connects with the ZnO dissolution in an electrolyte at higher anodization voltages and increases the volume of the pores.
SEM images after thermal annealing of ZnO NCs, obtained at voltages of 1, 5, and 15 V, are presented in **Figure 7(a)**–**(c)**. The dimension of ZnO NCs does not change essentially versus
Emission, Defects, and Structure of ZnO Nanocrystal Films Obtained by Electrochemical Method http://dx.doi.org/10.5772/66335 63
**Figure 6.** XRD results for as‐grown film (a) and annealed (b) ZnO NCs prepared at etching voltage 15 V and etching time 6 min [24].
voltages at the anodization (**Figure 7d**). The sizes estimated from SEM images vary in the range 45–60 nm up to 85–105 nm. Anodization voltage increase leads to a raise in the thickness of ZnO NC layers mainly.
#### **4.2. Emission study of ZnO NCs**
The bulk ZnO crystals are characterized by the variety of luminescence bands in UV and visible spectral ranges [33, 34]. The origin of these emissions has not been conclusively established, and a number of hypotheses have been proposed for each emission band. NBE band at 3.1 eV is attributed to the optical transition between the shallow donor and valence band, to the phonon replicas of bound exciton, or free exciton (FE) emissions [34, 35]. The high intensity of NBE emission at 300 K and a small band half width in ZnO NCs permits to attribute
The blue PL band with the peak at 2.80 eV, which disappeared completely after the oxidation at annealing in ambient air, can be assigned to emission via the native defects in ZnO films [36, 37]. The defect‐related green PL band in the range 2.40–2.50 eV is assigned to oxygen vacancies [36], Cu impurities [38], or surface defects [39] in ZnO. The PL intensity of 2.49–2.52 eV PL band does not change in the processes of oxidation and crystallization (**Figure 5**) that
The PL band centered at 2.00–2.10 eV was assigned earlier to interstitial oxygen atoms (2.02 eV) [40] or hydroxyl groups (2.10 eV) [41, 42]. The PL intensity of 2.06–2.10 eV PL band increases at ZnO oxidation (**Figure 5**) and the assumption that corresponding defects are
It is essential that ZnO NCs obtained by other methods, such as Zn powder thermal evaporation [43], sol‐gel ZnO films [44], or MOCVD growth ZnO films [45], do not permit to obtain
**4. Anodization voltage impact on the structure and emission of ZnO NC**
The XRD study has shown that as‐grown ZnO films are characterized by Zn substrate‐related XRD peaks with highest XRD peak intensities at the angles 2*θ* equal to 38.993, 43.233, and 70.058° (**Figure 6a**). These peaks correspond to the diffraction from the (100), (101), and (103) crystal planes, respectively, in the hexagonal Zn crystal lattice with the lattice parameters of *a* = 2.6650 Å and *c* = 4.9470 Å [27]. Annealing at 400°C stimulates the crystallization of ZnO NCs and a set of XRD peaks appear at the angles 2*θ* equal to 31.770, 34.422, 36.253, 47.540, 56.604, and 62.865° (**Figure 6b**). These XRD peaks correspond to the X‐ray diffraction from the (100), (002), (101), (102), (110), and (103) crystal planes in the wurtzite ZnO crystal structure [27].
The volume of crystalline ZnO phase enlarges versus anodization voltages up to 15 V that manifests itself by increasing the XRD peak intensities. However, ZnO NCs obtained at the voltage of 20 V are characterized by smaller intensity of XRD peaks that, apparently, connects with the ZnO dissolution in an electrolyte at higher anodization voltages and increases the
SEM images after thermal annealing of ZnO NCs, obtained at voltages of 1, 5, and 15 V, are presented in **Figure 7(a)**–**(c)**. The dimension of ZnO NCs does not change essentially versus
great variety of PL bands that have been demonstrated in the studied ZnO NCs.
the 3.1 eV PL band to a LO phonon replica of free exciton.
62 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
permits to assign this PL to some surface defects.
related to oxygen interstitials looks very reliable.
**films**
**4.1. XRD and SEM study**
volume of the pores.
PL spectra of ZnO NCs prep ared at different anodization voltages and annealed at 400°C are complex and can be presented as a set of PL bands. The deconvolution procedure permits to obtain the PL bands centered at: 3.18, 3.02, 2.94, 2.55, and 1.98 eV (**Figure 8**).
Elementary PL bands with the peaks at 2.55 and 1.98 eV were attributed earlier to the defect‐related emissions in ZnO NCs [33–36]. The intensity of defect‐related PL bands enlarges with increase in the anodization voltages due to increase in the volume of ZnO NC layers. The high energy PL bands (3.18, 3.02, and 2.94 eV) were assigned to NBE emission in ZnO [31].
PL spectra of ZnO NCs measured in the temperature range 10–300 K are presented in **Figure 9**. The intensities of defect‐related PL bands (1.98 and 2.55 eV) decrease significantly in this temperature range. Integrated PL intensities versus temperature have been presented in Arrhenius coordinates in **Figures 10** and **11** with the aim to estimate the activation energies of PL thermal decays.
**Figure 7.** SEM images of annealed ZnO NCs obtained at anodization voltages 1 (a), 5 (b), and 15 V(c) for etching time 6 min. Widths and lengths of annealed ZnO NCs (d) obtained at different etching voltages of 10, 15, and 20 V [24].
**Figure 8.** PL spectra of ZnO NCs prepared at voltages: 1 (1), 15 (2), and 20 (3) V.
Emission, Defects, and Structure of ZnO Nanocrystal Films Obtained by Electrochemical Method http://dx.doi.org/10.5772/66335 65
**Figure 9.** PL spectra of ZnO NCs obtained at 15 V and measured at 10–300 K with the step of 30 K.
**Figure 10.** Dependences of integrated PL intensities versus temperature presented in Arrhenius coordinates for defect‐ related PL bands.
**Figure 7.** SEM images of annealed ZnO NCs obtained at anodization voltages 1 (a), 5 (b), and 15 V(c) for etching time 6 min. Widths and lengths of annealed ZnO NCs (d) obtained at different etching voltages of 10, 15, and 20 V [24].
**Figure 8.** PL spectra of ZnO NCs prepared at voltages: 1 (1), 15 (2), and 20 (3) V.
64 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
**Figure 11.** Dependences of integrated PL intensities versus temperature presented in Arrhenius coordinates for NBE emission bands.
PL thermal decays in the range 10–150 K for all PL bands realize with the same activation energy (8.6–8.8 meV) due to the activation, apparently, of some nonradiative recombination centers. Thermal quenching of all PL bands starts at higher temperatures (150–300 K). The 2.94 and 3.06 eV PL bands, which demonstrate the PL thermal decay with the activation energy of 18meV, are related, apparently, to the LO phonon replicas of bond‐exciton emission. The 3.18 eV PL band, with highest PL decay activation energy (50 meV), is connected, probably, with the LO phonon replicas of FE emission.
## **5. Size‐dependent effects in emission of ZnO NC films**
The impact of NC sizes on PL spectra has been studied using ZnO NCs prepared at a voltage of 5 V, times: 1 (1), 3 (2), 6 (3), and 10 (4) min and annealed at 400°C. These ZnO NCs have been discussed in Section 3 and their sizes are summarized in **Table 2**. PL spectra of these ZnO NCs are a superposition of PL bands with the peaks at 2.05, 2.45, and 3.11 eV (**Figure 12**, dashed lines).
**Figure 13** shows the variation of NBE emission in ZnO NCs of different sizes. The 3.10 eV PL band at 300 K belongs to the FE phonon‐assisted replica in ZnO NCs [21, 22]. The deconvolution procedure has been applied to PL spectra and its result is presented by dashed curves in **Figure 13**. PL bands centered at 3.010, 3.082, 3.154, and 3.226 eV were chosen for the deconvolution, which can be attributed to the LO phonon replicas (FE‐5LO, −4LO, −3LO, and −2LO) of A exciton (3.373 eV). The energy difference between these PL transitions is close to some numbers of LO phonons (72 meV) in ZnO [46].
Integrated PL intensities of 2.45, 3.010, 3.082, 3.154, and 3.226 eV PL bands increase significantly with diminishing the ZnO NC size and with enlarging the surface‐to‐volume ratio (**Figure 14a** and **b**).
Additionally, the main PL peak of NBE emission shifts to higher energy owing to the PL intensity enlargement of 3LO and 2LO phonon replicas mainly (**Figure 13**). Note that PL spectra of ZnO NCs, typically, do not reveal any FE peaks or phonon replicas owing to poor material quality and high concentrations of structural defects.
**Table 2.** The size of ZnO NCs from SEM images and hexagonal crystal lattice parameters estimated from XRD data.
Emission, Defects, and Structure of ZnO Nanocrystal Films Obtained by Electrochemical Method http://dx.doi.org/10.5772/66335 67
**Figure 12.** Normalized PL spectra of ZnO NCs obtained at times of 1 (1), 3 (2), 6 (3), and 10 (4) min. Dashed curves present the deconvolution of the PL spectrum (4) [26].
The defect‐related green PL band, detected traditionally in the spectral range of 2.40–2.50 eV in ZnO, has been assigned to the oxygen vacancies [36], Cu impurities [38], or surface defects [22, 39]. In our experiments, the intensity of 2.45 eV PL band increases significantly (**Figure 14a**) in PL spectra at decreasing the ZnO NC size and increasing the surface‐to‐volume ratio in NCs. Thus, it can be supposed that emission centers, responsible for the 2.45 eV PL band, relate to the surface defects and their concentration enlarges with increasing the surface‐to‐volume ration in ZnO NCs.
#### **5.1. NBE intensity stimulation at ZnO NC size decreasing**
PL thermal decays in the range 10–150 K for all PL bands realize with the same activation energy (8.6–8.8 meV) due to the activation, apparently, of some nonradiative recombination centers. Thermal quenching of all PL bands starts at higher temperatures (150–300 K). The 2.94 and 3.06 eV PL bands, which demonstrate the PL thermal decay with the activation energy of 18meV, are related, apparently, to the LO phonon replicas of bond‐exciton emission. The 3.18 eV PL band, with highest PL decay activation energy (50 meV), is connected, probably, with
The impact of NC sizes on PL spectra has been studied using ZnO NCs prepared at a voltage of 5 V, times: 1 (1), 3 (2), 6 (3), and 10 (4) min and annealed at 400°C. These ZnO NCs have been discussed in Section 3 and their sizes are summarized in **Table 2**. PL spectra of these ZnO NCs are a superposition of PL bands with the peaks at 2.05, 2.45, and 3.11 eV (**Figure 12**,
**Figure 13** shows the variation of NBE emission in ZnO NCs of different sizes. The 3.10 eV PL band at 300 K belongs to the FE phonon‐assisted replica in ZnO NCs [21, 22]. The deconvolution procedure has been applied to PL spectra and its result is presented by dashed curves in **Figure 13**. PL bands centered at 3.010, 3.082, 3.154, and 3.226 eV were chosen for the deconvolution, which can be attributed to the LO phonon replicas (FE‐5LO, −4LO, −3LO, and −2LO) of A exciton (3.373 eV). The energy difference between these PL transitions is close to some
Integrated PL intensities of 2.45, 3.010, 3.082, 3.154, and 3.226 eV PL bands increase significantly with diminishing the ZnO NC size and with enlarging the surface‐to‐volume ratio
Additionally, the main PL peak of NBE emission shifts to higher energy owing to the PL intensity enlargement of 3LO and 2LO phonon replicas mainly (**Figure 13**). Note that PL spectra of ZnO NCs, typically, do not reveal any FE peaks or phonon replicas owing to poor mate-
**Lattice parameter,**
**Lattice parameter,**
**Porosity,** *c***<sup>0</sup>**
**(%)**
**"***c***" (Å)**
**"***a***" (Å)**
the LO phonon replicas of FE emission.
numbers of LO phonons (72 meV) in ZnO [46].
rial quality and high concentrations of structural defects.
**Average NC length (nm)**
308 600 3.2504 5.2071 25 253 459 3.2534 5.2119 28 170 316 3.2564 5.2167 37 67 131 3.2584 5.2199 50
**Table 2.** The size of ZnO NCs from SEM images and hexagonal crystal lattice parameters estimated from XRD data.
Bulk ZnO [27] 3.2495 5.2069
dashed lines).
(**Figure 14a** and **b**).
**Sample numbers Average NC**
**width (nm)**
**5. Size‐dependent effects in emission of ZnO NC films**
66 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
The intensity of 3.082–3.226 eV PL bands, related to FE phonon‐assisted replicas, increases significantly with decreasing the ZnO NC size (**Figure 14b**). Since the process of reduction in size is accompanied by increasing the interplanar distances in ZnO NCs (**Table 2**), the FE‐related PL bands have to shift to lower energy in small NCs (4). But the main PL peak of NBE emission shifts to high energy (**Figure 13**) from 3.08 eV (1) to 3.14 eV (4). It means that some other physical mechanism is responsible for the PL intensity enlargement and spectral transformation of NBE emission band.
The PL intensity *W*PL of ZnO NCs can be represented by the formula [47]:
$$\mathcal{W}\_{\rm pl} = I\_0 \left( 1 - R \right) \left( 1 - c\_0 \right) \left[ 1 - \exp(-ad \,) \right] \eta \tag{2}$$
where *I*<sup>0</sup> is the excitation light intensity, *d* is the ZnO NC layer thickness, *R* and *α* are the reflection and absorption coefficients, and *η* is the internal quantum emission efficiency that is (*<sup>η</sup>* <sup>=</sup> *<sup>τ</sup><sup>R</sup>* −1 \_\_\_\_\_\_ *τR* <sup>−</sup><sup>1</sup> + *τNR* <sup>−</sup><sup>1</sup> ), *τ*R and *τ*NR are radiative and nonradiative recombination times, respectively,
**Figure 13.** NBE emission bands in annealed ZnO NCs obtained at the durations of 1 (1), 3 (2), 6 (3), and 10 min (4). Dashed curves present the deconvolution of PL spectra on LO phonon‐assisted PL bands [26].
and *c*<sup>0</sup> is the porosity of ZnO layers (**Table 2**). At high porosity, the ZnO volume, which absorbs the excitation light, decreases.
Let us consider the coefficients of relative varying the integrated PL intensities in the studied structures, in comparison with the structure 1, at permanent parameters of the excitation light intensity (*I* 0 ) and *R* coefficient. Actually, the excitation intensity *I*<sup>0</sup> was the same in our experiments and the reflection coefficient decreases a little versus NC sizes in the range 60–600 nm. The relation of PL intensities can be presented as:
The relation of PL intensities can be presented as:
$$K\_{\alpha} = \frac{W\_{\mathcal{H}}^{\text{Ni}}}{W\_{\mathcal{H}}^{\text{Ti}}} = \frac{(1 - c\_{\alpha})}{(1 - c\_{\alpha})} \frac{[1 - \exp(-\alpha \, d\_{\circ})] \eta\_{i}}{[1 - \exp(-\alpha \, d\_{\circ})] \eta\_{1}} \tag{3}$$
Emission, Defects, and Structure of ZnO Nanocrystal Films Obtained by Electrochemical Method http://dx.doi.org/10.5772/66335 69
**Figure 14.** The variation of integrated PL intensities versus sizes of ZnO NCs for PL bands: (a) 2.05 (1) and 2.45 (2) eV and (b) 3.226 (1), 3.154 (2), 3.082 (3), and 3.010 (4) eV [26].
The values of *K*ex parameters are summarized in **Table 3**. Some rising in exciton PL intensity in ZnO NCs at the NC size variation from 308 × 600 nm (1) down to 253 × 459 nm (2) can be explained by increasing the NC layer thickness and excitation light absorption. However, the PL intensities of all PL bands enlarge significantly (**Figure 14**, **Table 3**) when the size of ZnO NCs decreases down to 67 × 131 nm (4). To explain the reasons of such PL stimulation, the factors that influent on the internal quantum efficiency and radiative recombination rates have to be discussed.
The stimulation of internal quantum efficiency in small ZnO NCs (3, 4) can be attributed to exciton recombination rate increasing owing to the realization of the weak exciton confinement. The theoretical consideration of this effect was presented in references [48, 49] and two regimes of weak exciton confinement are discussed. The first regime deals with NCs of the size "bigger" than the Bohr exciton radius, but "smaller" than a wavelength of emitted light in ZnO. Oscillator strength increase in the first case is a result of oscillator strength enhancement for localized excitons in proportion to spreading their wave functions, predicted for the bound exciton early [50].
**Table 3.** Experimental rations *K*ex for integrated PL intensities Wi /W<sup>1</sup> of studied PL bands.
and *c*<sup>0</sup>
intensity (*I*
0
absorbs the excitation light, decreases.
*Kex* <sup>=</sup> *WPL*
The relation of PL intensities can be presented as:
is the porosity of ZnO layers (**Table 2**). At high porosity, the ZnO volume, which
was the same in our experi-
(3)
Let us consider the coefficients of relative varying the integrated PL intensities in the studied structures, in comparison with the structure 1, at permanent parameters of the excitation light
**Figure 13.** NBE emission bands in annealed ZnO NCs obtained at the durations of 1 (1), 3 (2), 6 (3), and 10 min (4).
ments and the reflection coefficient decreases a little versus NC sizes in the range 60–600 nm.
[ 1 − exp(−*α di* ) ]*η<sup>i</sup>* \_\_\_\_\_\_\_\_\_\_\_\_\_ [ <sup>1</sup> <sup>−</sup> exp(−*<sup>α</sup> <sup>d</sup>*<sup>1</sup> ) ]*η*<sup>1</sup>
) and *R* coefficient. Actually, the excitation intensity *I*<sup>0</sup>
Dashed curves present the deconvolution of PL spectra on LO phonon‐assisted PL bands [26].
68 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
#*i* \_\_\_\_*WPL*
#1 <sup>=</sup> (1 <sup>−</sup> *coi* ) \_\_\_\_\_ (1 − *co*<sup>1</sup> )
The second weak confinement regime is connected with exciton‐light coupling with the formation of polaritons, which becomes strong if the size of NCs approaches to light wavelength in ZnO. The exciton recombination rate *G*, at the assumption of exiton‐light coupling was considered as the product of photon and exciton eigenstates early in [48, 49]:
$$G \quad = K \left[ \left[ dr \, \cos(kr) \, \Phi(r) \right] \right]^2, \text{ with } k = \sqrt{\varepsilon\_\omega} \, k\_\circ \tag{4}$$
where *k* is the wave vector of light in a material with the high frequency dielectric constant, *ε*∞, that equals to *ε*∞ = 3.77 [32] in ZnO, *k*<sup>0</sup> = 2*π*/*λ*<sup>0</sup> is the wave vector of light at an exciton resonance frequency and *K* is a characteristic of long‐range exchange energy splitting for exciton in NCs [48]. The Ф(*r*) in Eq. (4) was taken as a Gaussian function to get an analytical formula and formulas for the exciton recombination rate *G* and exciton recombination time *τR* were obtained as [48, 49]:
$$G = \frac{\sqrt[4]{2\pi}}{12} \omega\_{\perp \Gamma} \left(\frac{2\pi}{\lambda\_{\circ}}\right)^3 \langle r \rangle^3 \exp\{-8\,\varepsilon\_{\omega}\frac{\pi^2 \left\langle r \right\rangle^2}{\lambda\_{\circ}^2}\} \tag{5}$$
$$
\pi\_{\mathbb{R}} = \frac{1}{G} \tag{6}
$$
The radiative recombination rates (**Figure 15**) and corresponding radiative lifetimes (**Figure 16**) have been numerically estimated using Eqs. (5) and (6) for the quant energy of FE phonon‐assisted replicas (3.226, 3.154, 3.082, and 3.010 eV) in ZnO NCs of different sizes.
The recombination rates approach to maximum in ZnO NCs with diameters of 59–64 nm for the exciton‐light coupling model (**Figure 15**) and starting from 59 to 64 nm the radiative recombination rate decreases (radiative lifetime increases) versus NC sizes (**Figures 15** and **16**).
The estimation of exciton recombination rates (**Figure 15**) has been done for the spherical shape of NCs. In the studied films, the ZnO NCs have a rhomb shape (**Figure 1**). The exciton
**Figure 15.** Numerically simulated exciton recombination rates for LO phonon‐assisted PL bands in ZnO NCs of different sizes [26].
Emission, Defects, and Structure of ZnO Nanocrystal Films Obtained by Electrochemical Method http://dx.doi.org/10.5772/66335 71
The second weak confinement regime is connected with exciton‐light coupling with the formation of polaritons, which becomes strong if the size of NCs approaches to light wavelength in ZnO. The exciton recombination rate *G*, at the assumption of exiton‐light coupling was
where *k* is the wave vector of light in a material with the high frequency dielectric constant,
nance frequency and *K* is a characteristic of long‐range exchange energy splitting for exciton in NCs [48]. The Ф(*r*) in Eq. (4) was taken as a Gaussian function to get an analytical formula and formulas for the exciton recombination rate *G* and exciton recombination time *τR* were
The radiative recombination rates (**Figure 15**) and corresponding radiative lifetimes (**Figure 16**) have been numerically estimated using Eqs. (5) and (6) for the quant energy of FE phonon‐assisted replicas (3.226, 3.154, 3.082, and 3.010 eV) in ZnO NCs of different sizes. The recombination rates approach to maximum in ZnO NCs with diameters of 59–64 nm for the exciton‐light coupling model (**Figure 15**) and starting from 59 to 64 nm the radiative recombination rate decreases (radiative lifetime increases) versus NC sizes (**Figures 15** and **16**).
The estimation of exciton recombination rates (**Figure 15**) has been done for the spherical shape of NCs. In the studied films, the ZnO NCs have a rhomb shape (**Figure 1**). The exciton
**Figure 15.** Numerically simulated exciton recombination rates for LO phonon‐assisted PL bands in ZnO NCs of different
〈*r*〉<sup>3</sup> exp(−8 *ε*<sup>∞</sup>
= 2*π*/*λ*<sup>0</sup>
\_\_\_ 2*π λo* ) 3 , with *k* = √
\_\_\_
*π*<sup>2</sup> 〈*r*〉 \_\_\_\_\_\_ *λo* 2 2
is the wave vector of light at an exciton reso-
*<sup>G</sup>* (6)
*ε*<sup>∞</sup> *ko* (4)
) (5)
considered as the product of photon and exciton eigenstates early in [48, 49]:
\_\_\_ \_\_\_2*π* <sup>12</sup> *ωLT* (
*G* = *K* [∫*dr* cos(*kr* ) *Φ*(*r* )]<sup>2</sup>
70 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
*<sup>τ</sup><sup>R</sup>* <sup>=</sup> \_\_\_ <sup>1</sup>
*ε*∞, that equals to *ε*∞ = 3.77 [32] in ZnO, *k*<sup>0</sup>
*G* = <sup>√</sup>
obtained as [48, 49]:
sizes [26].
**Figure 16.** Numerically simulated exciton radiative lifetimes for LO phonon‐assisted PL bands in ZnO NCs of different sizes [26].
recombination rate decreases fast versus sizes (**Figure 15**) and due to this is reasonable to consider only the smallest NC parameter (width), because the recombination rate along the largest NC sizes (length) will be some orders smaller. Thus, the variation of PL intensity in ZnO NCs is reasonable to present versus NC widths, as shown in **Figure 14**.
The exciton‐light coupling model predicts well the ZnO NC size (59–64 nm), where the maximum of PL intensity can be expected. This size correlates with the NC width (67 nm) in the structure 4 with highest detected PL intensity. However, the PL intensities of FE phonon‐ assisted PL bands enlarge 15‐ or 10‐fold with decreasing NC widths from 250 down to 67 nm (**Table 3**). At the same time, the estimated recombination rate has to increase 90‐fold or even more in the proposed model (**Figure 15**). This difference can be attributed to the following reasons: (i) the enlargement of nonradiative recombination rate in smallest ZnO NCs due to increasing significantly the concentration of surface defects and (ii) the exciton‐light coupling and polariton orientation partially along the length of ZnO NCs, where the exciton recombination rate is smaller. Thus, the stimulation of both recombination parameters *τ*<sup>R</sup> −1 and *τ*NR −1 with decreasing ZnO NC sizes and random polariton orientation in ZnO NCs lead to smaller internal quantum efficiency, *η*, and integrated PL intensity, *WPL* , than it is predicted by the exciton‐light coupling model.
We need to discuss as well increasing the integrated PL intensity of FE‐2LO and FE‐3LO replicas in comparison with the intensity of FE‐4LO and FE‐5LO bands (**Figure 13**, **Table 3**) in small ZnO NCs. This effect, probably, deals with decreasing the exciton‐phonon coupling strength in the smallest ZnO NCs that were predicted early for ZnO QDs [51]. The effect was attributed to diminishing exciton polarity in small NCs and to decreasing Frohlich polar intraband scattering that induces phonon‐assisted emission bands [52–54].
## **6. Impact of Cu-doping on the structure and emission properties of ZnO NC films**
To the adjustment of ZnO NC characteristics the doping by different metals Al [55], Co [56], Ni [57], Cu [58, 59], or Ag [60] can be used. Cu atoms are most impotent impurities due to the low toxicity and large source content. ZnO:Cu NCs have demonstrated excellent electrical, magnetic, and photoelectrical characteristics and gas sensing [58, 59]. The Cu atoms are well known as emission activators for semiconductors that can change essentially the emission intensity in ZnO NCs [58, 59].
A lot of papers related to doping ZnO films by Cu in different concentrations were published recently [61–64]. A systematic change of XRD parameters with Cu content increasing in the ZnO crystal lattice was observed in Ref. [63], together with decreasing the ZnO energy band gap versus Cu contents [63]. NBE intensity enhancement in ZnO films with Cu doping at 2.0 at% and emission quenching at Cu doping 4.4 at% was reported in Ref. [56].
A set of published papers are devoted to the defect study if the ZnO:Cu crystals [64, 65]. The green PL band at 2.45 eV with the LO phonon‐related structure and zero‐phonon line was assigned to the optical transition via CuZn acceptors [64, 65]. The structure‐less green emission was assigned to the recombination of electron from a shallow donor with hole bounds to Cu+ ions [65].
The assumptions concerning the Cu defect structure have been presented, but only some of them look as reliable. The purpose of our work is connected with the investigation of correlated varying the XRD parameters and PL spectra of ZnO Cu NCs versus Cu contents with the aim to analyze the Cu‐related defects. The parameters of the studied ZnO and ZnO Cu NCs are summarized in **Table 4**.
#### **6.1. SEM and XRD studies**
SEM images of ZnO and ZnO Cu NCs obtained at the adonization duration of 3 min with thermal treatment at 400°C are presented in **Figure 17(a)** and **(b)**. ZnO NCs have a rhomb
**Table 4.** The average NC sizes obtained from SEM images.
Emission, Defects, and Structure of ZnO Nanocrystal Films Obtained by Electrochemical Method http://dx.doi.org/10.5772/66335 73
**6. Impact of Cu-doping on the structure and emission properties of ZnO**
72 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
To the adjustment of ZnO NC characteristics the doping by different metals Al [55], Co [56], Ni [57], Cu [58, 59], or Ag [60] can be used. Cu atoms are most impotent impurities due to the low toxicity and large source content. ZnO:Cu NCs have demonstrated excellent electrical, magnetic, and photoelectrical characteristics and gas sensing [58, 59]. The Cu atoms are well known as emission activators for semiconductors that can change essentially the emission
A lot of papers related to doping ZnO films by Cu in different concentrations were published recently [61–64]. A systematic change of XRD parameters with Cu content increasing in the ZnO crystal lattice was observed in Ref. [63], together with decreasing the ZnO energy band gap versus Cu contents [63]. NBE intensity enhancement in ZnO films with Cu doping at 2.0
A set of published papers are devoted to the defect study if the ZnO:Cu crystals [64, 65]. The green PL band at 2.45 eV with the LO phonon‐related structure and zero‐phonon line was assigned to the optical transition via CuZn acceptors [64, 65]. The structure‐less green emission was assigned to the recombination of electron from a shallow donor with hole bounds to Cu+
The assumptions concerning the Cu defect structure have been presented, but only some of them look as reliable. The purpose of our work is connected with the investigation of correlated varying the XRD parameters and PL spectra of ZnO Cu NCs versus Cu contents with the aim to analyze the Cu‐related defects. The parameters of the studied ZnO and ZnO Cu NCs
SEM images of ZnO and ZnO Cu NCs obtained at the adonization duration of 3 min with thermal treatment at 400°C are presented in **Figure 17(a)** and **(b)**. ZnO NCs have a rhomb
**(V)**
ZnO 1 5 308 × 548 1.0 × 105 ZnO 3 5 273 × 459 2.0 × 105 ZnO 6 5 170 × 316 2.4 × 105 ZnO Cu 1 5 300 × 540 1.3 × 105 ZnO Cu 3 5 282 × 510 3.3 × 105 ZnO Cu 6 5 200 × 320 1.7 × 105
**Etching voltage**
**NC size (nm) Integrated PL**
**un.)**
**intensity of all bands at 10 K (arb.**
at% and emission quenching at Cu doping 4.4 at% was reported in Ref. [56].
**NC films**
ions [65].
intensity in ZnO NCs [58, 59].
are summarized in **Table 4**.
**6.1. SEM and XRD studies**
**Sample number Type of NCs Etching duration**
**Table 4.** The average NC sizes obtained from SEM images.
**(min)**
**Figure 17.** SEM images of N2 ZnO NCs (a) and N5 ZnO Cu (b) NCs obtained at an anodization time of 3 min after thermal annealing.
shape and we used width and length for the size characterization (**Table 4**). The size of ZnO NCs decreases versus etching times from 308 × 548 nm to 170 × 316 nm (**Table 4**).
At first, the XRD investigation of ZnO and ZnO Cu NCs has been done with the aim to confirm that Cu atoms are incorporated in ZnO NCs. The five XRD peaks have been detected in both types of NCs (**Figures 18** and **19**), related to the diffraction from the (100), (002), (101), (110), and (103) crystal planes in the wurtzite ZnO crystal lattice [27].
**Figure 18.** XRD results for the N2 (a) and N3 (b) ZnO NCs (Table 4) after thermal annealing [66].
**Figure 19.** XRD results for the N5 (a) and N6 (b) ZnO Cu NCs (Table 4) after thermal annealing [66].
The decreasing ZnO NC size stimulates the shift of XRD peaks to lower angles (**Table 5**) that testify on larger interplane distances in small ZnO NCs. It was supposed early that this effect is related to compressive strain decreasing in small ZnO NCs [26].
At the XRD study of ZnO:Cu NCs with small Cu concentration of ≤2.28 at% [67] (samples N4 and N5), the XRD peaks shift to bigger diffraction angles (2 theta) in comparison with ZnO NCs (samples N1, N2), together with decreasing the NC sizes (**Table 5**). This effect testifies on smaller interplanar distances in ZnO:Cu NCs. At the Cu concentration higher than 2.28 at% (sample N6), XRD peaks shift to smaller 2 theta values even more essential than those are detected in ZnO NCs (sample N3). The last effect corresponds to increasing the interplanar space in ZnO Cu NCs of N6.
**Table 5.** XRD peaks for studied ZnO NCs and ZnO Cu NCs [66].
The valence of Cu could be +1 or +2 and the radius of Cu<sup>+</sup> , Cu+2, and Zn+2 ions are 0.096, 0.072, and 0.074 nm, respectively, in ZnO [68–72]. Substitution ions Cu<sup>+</sup> and Cu+2 and interstitial Cu+2 ions can be incorporated in ZnO. Lattice constants in ZnO:Cu NCs increase, in comparison with those in undoped ZnO NCs, when Cu<sup>+</sup> substituted Zn+2 ions, together with compressive strain enlarging [68, 69]. In ZnO:Cu nanowires (NWs), a decrease in lattice parameter was detected and interpreted as Cu+2 ions substituted of Zn+2 ions in ZnO [70]. In addition, Cu atoms can form the complex defects [CuZn Zn<sup>i</sup> ]x as well [70].
In our study of ZnO and ZnO:Cu NCs three XRD effects have been revealed:
In ZnO:Cu NCs the XRD peak intensity increases versus etching durations (**Figure 19**) due to thickness increasing of the ZnO Cu NC layers. Simultaneously, the new XRD peak (2θ = 43.2963°) related to the diffraction from (111) crystal planes in metallic Cu nanoparticles with a cubic crystal lattice [73] has been revealed (**Table 5**, **Figure 19**).
#### **6.2. Comparative PL study of ZnO and ZnO Cu NCs**
The decreasing ZnO NC size stimulates the shift of XRD peaks to lower angles (**Table 5**) that testify on larger interplane distances in small ZnO NCs. It was supposed early that this effect
At the XRD study of ZnO:Cu NCs with small Cu concentration of ≤2.28 at% [67] (samples N4 and N5), the XRD peaks shift to bigger diffraction angles (2 theta) in comparison with ZnO NCs (samples N1, N2), together with decreasing the NC sizes (**Table 5**). This effect testifies on smaller interplanar distances in ZnO:Cu NCs. At the Cu concentration higher than 2.28 at% (sample N6), XRD peaks shift to smaller 2 theta values even more essential than those are detected in ZnO NCs (sample N3). The last effect corresponds to increasing the interplanar
> **ZnO (101)**
31.7622 34.4022 36.2322 56.5422 62.8922 31.7322 34.4022 36.2321 56.5422 62.8922 31.7022 34.3722 36.2022 56.5222 62.8822 31.7713 34.4663 36.2793 56.6143 62.8473 31.7813 34.4763 36.2893 43.2963 56.6733 62.8963 31.6891 34.3351 36.1481 43.2531 56.4831 62.8041
**Cu (111)** **ZnO (110)** **ZnO (103)**
is related to compressive strain decreasing in small ZnO NCs [26].
74 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
**ZnO (002)**
**Table 5.** XRD peaks for studied ZnO NCs and ZnO Cu NCs [66].
**Figure 19.** XRD results for the N5 (a) and N6 (b) ZnO Cu NCs (Table 4) after thermal annealing [66].
space in ZnO Cu NCs of N6.
**ZnO (100)**
**Sample number**
PL spectra of ZnO and ZnO Cu NCs obtained at different etching times (3 and 6 min) and measured at 10 K are presented in **Figure 20(a)** and **(b)**. PL spectra are complex and include two wide PL bands well known in ZnO:NBE emission in the spectral range 2.80–3.37 eV [34] and defect‐ related emission in the range 1.70–2.80 eV [37–39]. The intensity of NBE luminescence rises versus anodization time owing to enlarging the thickness of ZnO NC layers (**Figure 20a** and **b**).
Simultaneously, the NBE band shifts to lower energy in PL spectra (**Figure 20a** and **b**) that is caused by decreasing the NC sizes, compressive strains, and the energy band gap of ZnO NCs [26].
The variation of Cu concentrations in ZnO:Cu NCs influent mainly on the intensity and shape of defect‐related PL bands in the range 1.7–2.8 eV. PL band intensity increases when the Cu concentration approaches to 2.28 at% and then decreases at higher Cu contents (**Figure 20a** and **b**). At high Cu concentration, the new PL band centered at 2.61–2.70 eV at 10 K has been detected in PL spectra of ZnO Cu NCs (**Figure 20b**, curve 2).
To make the conclusion concerning the light‐emitting mechanisms of visible PL bands in ZnO:Cu NCs, PL spectra have been studied in the range of 10–300 K (**Figure 21**).
Normalized PL spectra of ZnO and ZnO:Cu NCs for the visible spectral range measured at different temperatures are presented in **Figure 22(a)** and **(b)**. It is clear that four PL bands (A, B, C, D) have composed PL spectra in the orange‐yellow‐green‐blue ranges of ZnO Cu NCs, which are characterized by different kinetics of PL intensity thermal decays.
**Figure 20.** PL spectra of N2 (curve 1) and N3 (curve 2) ZnO NCs (a) and N5 (curve 1) and N6 (curve 2) ZnO Cu NCs (b).
The deconvolution procedure was applied to PL spectra of **Figure 22(a)** and **(b)** that permits to obtain three‐ and four‐elementary PL bands in ZnO and ZnO Cu NCs, respectively, peaked at 10 K: 1.95–2.00 eV(A), 2.15–2.23 eV(B), 2.43–2.50 eV(C), and 2.61–2.69 eV(D).
The PL intensity of bands A, B, and C decreases faster (N6) versus etching duration in comparison with the PL intensity of band D (**Figure 20b**, curve 2). The variation of integrated PL intensities of all PL bands versus temperatures in ZnO and ZnO Cu NCs is presented in **Figure 23(a)** and **(b)**. The band D intensity decreases at low temperatures (starting from 70 K). PL thermal decays of the bands A, B, and C are similar in ZnO and ZnO Cu NCs. PL intensities of bands (A, B, C) fall down slowly in the range 10–100 K, and only at higher temperatures (100–300 K) their PL thermal decays become faster.
To estimate the activation energies of PL thermal decay in different temperature ranges, the Arrhenius plots have been designed (**Figure 24**). At low temperatures, the activation energies of PL intensity decays are estimated as: 9 meV in ZnO or 16 meV in ZnO Cu NCs. These small activation energies of PL decay are related to the thermal activation of some nonradiative recombination centers (NRC). At higher temperatures PL intensities of A, B, and C bands decay with activation energies: 44meV (A) and 35 meV (B and C) in ZnO NCs (**Figure 24a**) as well as 37 meV (A) and 27 meV (B and C) in ZnO Cu NCs (**Figure 24b**). The PL band D intensity decreases with the activation energy of 20 meV (**Figure 24b**, curve 4).
Emission, Defects, and Structure of ZnO Nanocrystal Films Obtained by Electrochemical Method http://dx.doi.org/10.5772/66335 77
**Figure 21.** PL spectra of ZnO:Cu NCs measured in the range 10–300K.
The deconvolution procedure was applied to PL spectra of **Figure 22(a)** and **(b)** that permits to obtain three‐ and four‐elementary PL bands in ZnO and ZnO Cu NCs, respectively, peaked
**Figure 20.** PL spectra of N2 (curve 1) and N3 (curve 2) ZnO NCs (a) and N5 (curve 1) and N6 (curve 2) ZnO Cu NCs (b).
The PL intensity of bands A, B, and C decreases faster (N6) versus etching duration in comparison with the PL intensity of band D (**Figure 20b**, curve 2). The variation of integrated PL intensities of all PL bands versus temperatures in ZnO and ZnO Cu NCs is presented in **Figure 23(a)** and **(b)**. The band D intensity decreases at low temperatures (starting from 70 K). PL thermal decays of the bands A, B, and C are similar in ZnO and ZnO Cu NCs. PL intensities of bands (A, B, C) fall down slowly in the range 10–100 K, and only at higher temperatures
To estimate the activation energies of PL thermal decay in different temperature ranges, the Arrhenius plots have been designed (**Figure 24**). At low temperatures, the activation energies of PL intensity decays are estimated as: 9 meV in ZnO or 16 meV in ZnO Cu NCs. These small activation energies of PL decay are related to the thermal activation of some nonradiative recombination centers (NRC). At higher temperatures PL intensities of A, B, and C bands decay with activation energies: 44meV (A) and 35 meV (B and C) in ZnO NCs (**Figure 24a**) as well as 37 meV (A) and 27 meV (B and C) in ZnO Cu NCs (**Figure 24b**). The PL band D inten-
at 10 K: 1.95–2.00 eV(A), 2.15–2.23 eV(B), 2.43–2.50 eV(C), and 2.61–2.69 eV(D).
sity decreases with the activation energy of 20 meV (**Figure 24b**, curve 4).
(100–300 K) their PL thermal decays become faster.
76 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
**Figure 22.** Normalized PL spectra in the range of defect‐related PL bands for ZnO NCs (a) and ZnO Cu NCs (b).
**Figure 23.** Thermal dependences of integrated PL intensities in ZnO (a) and ZnO Cu NCs (b) [66].
**Figure 24.** Arrhenius plots obtained for different PL bands for ZnO (a) and ZnO Cu NCs (b) [67].
#### **6.3. Orange‐yellow‐green PL bands**
The orange PL band (1.95–2.05 eV) in ZnO was assigned to oxygen interstitials, Oi (2.02 eV) [40], to shallow donor‐deep acceptor pairs [11], or to hydroxyl groups (2.10 eV) [41]. The yellow band (2.15–2.23 eV) in undoped and doped (with N or Ga) ZnO layers [11, 74] was attributed to the electron transitions from shallow donors to deep acceptors with an energy level of 0.7 eV above the valence band, connected with the zinc vacancy, VZn, or VZn‐shallow donor complex [11]. The green PL band (2.43–2.50 eV) is complex and includes five PL bands connected with different defects in undoped ZnO NCs [75–78]. The types of corresponding defects and optical transitions are not clear yet.
The ZnO and ZnO:Cu NC layers were annealed at 400°C in air that is the O‐ and N‐rich condition. The defects favored for this condition are VZn, Oi , and OZn [79] that act as deep acceptors in ZnO [79]. However, the probability of forming OZn defects is low due to the high value of its formation energy in ZnO. The PL intensity of 2.40–2.50 eV band increases with NC size decreasing and surface‐to‐volume ratio rising [26], which permits to attribute the green radiative centers to native defects (zinc vacancy or surface defects) in ZnO NCs (**Figure 20a**).
Cu doping with the concentration ≤2.28 at% stimulates significantly the intensity of orange‐ yellow‐green PL bands in ZnO:Cu NCs (**Figure 20b** and **Table 4**). Simultaneously, the XRD study has detected a high angle shift of all XRD peaks in ZnO Cu NCs owing to, apparently, the substitution of Zn+2 ions by Cu+2 ions in ZnO. Thus, the radiative centers connected with structureless green PL band (2.43–2.50 eV) in ZnO:Cu NCs can be attributed to CuZn +2 defects [64, 65].
The high PL intensity of green band is detected together with the high intensities of orange and yellow PL bands in the ZnO Cu NCs (**Figure 20b**). It can be supposed that the process of Zn+2 ion substitution by Cu+2 ions at the thermal treatment (400°C) is accompanied by appearing other native acceptors: zinc vacancies, oxygen interstitials, or their complexes. Zinc vacancies, VZn, are the deep acceptors with the transition energy levels *E*(0/−) = 0.18 eV and *E*(−/2−) = 0.87 eV above the valence band according to the calculations reported in [79]. Oxygen interstitials are characterized by deep acceptor transition levels *E*(0/−) = 0.72 and *E*(−/2−) = 1.59 eV above the valence band [79]. Cu atoms act as deep acceptors in ZnO as well [80]. Thus, all these deep acceptors can be responsible for the stimulation of orange‐yellow‐green PL bands in ZnO:Cu NCs.
Small activation energies of emission thermal decays (**Figure 24**) for bands (A, B, C) permit to attribute the corresponding centers to the shallow donor‐deep acceptor pars (DAPs). Note that PL thermal decays of B and C bands are characterized by the same activation energies owing to, apparently, the formation of corresponding DAPs from the same shallow donors, for example, Zni , and different deep acceptors.
Note that the defect concentration in ZnO:Cu NCs is higher than its value in ZnO NCs. Defect concentration increasing leads to the stimulation of the PL intensities of A, B, and C bands. Simultaneously, the activation energies of PL decays decrease in ZnO Cu NCs that is a result of distance decreasing between donors and acceptors in DAPs. The last effect provokes attractive interaction increasing in DAPs and, as a result, the shift of donor and acceptor energy levels closer to the conduction (valence) bands. In this case, the PL thermal decays in ZnO:Cu NCs are characterized by smaller activation energies that actually has been revealed: 37 meV (A) and 27 meV (B and C) (**Figure 24**).
The energy position of Zni +1/0 donor levels was estimated early as 50 meV [81]. A shallow donor with ionization energy of 30 meV was detected at high‐energy electron irradiation experiments [82]. The authors suggested that these shallow donors owe to Zn‐sublattice
**6.3. Orange‐yellow‐green PL bands**
The orange PL band (1.95–2.05 eV) in ZnO was assigned to oxygen interstitials, Oi
**Figure 24.** Arrhenius plots obtained for different PL bands for ZnO (a) and ZnO Cu NCs (b) [67].
**Figure 23.** Thermal dependences of integrated PL intensities in ZnO (a) and ZnO Cu NCs (b) [66].
78 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
[40], to shallow donor‐deep acceptor pairs [11], or to hydroxyl groups (2.10 eV) [41]. The yellow band (2.15–2.23 eV) in undoped and doped (with N or Ga) ZnO layers [11, 74] was
(2.02 eV)
defects: Zn interstitialses or Zn‐interstitial‐related complex [83]. In addition, under the N‐rich ambient condition the shallow donors, Zni –NO, can be obtained as it was supposed in [84]. Thus, it can be assumed that in ZnO NCs the orange (A), yellow (B), and green (C) emissions are related to DAPs, which include the deep acceptors, such as oxygen interstitials and zinc vacancies, and shallow donors, like zinc interstitials or their complexes. In ZnO Cu NCs the substitutional CuZn atoms form the DAPs with shallow donors that are responsible on the structureless green PL band.
#### **6.4. Blue PL band**
The blue PL band D (2.61–2.70 eV) appears in PL spectra of ZnO:Cu NCs (sample N6, **Figure 20b**) at higher Cu contents (≥2.28 at%). Simultaneously, the intensities of other PL bands decrease (**Table 4**) and the band D dominates in the PL spectrum (**Figure 20b**, curve 2). At higher Cu concentrations in ZnO:Cu NCs, the nonradiative recombination centers (NRC), probably, appear that provokes decreasing the PL intensity of the bands (A, B, C). At the same time, the D band intensity decreases slowly that testifies on concentration increasing of D band emitting defects. In this case, a low angle shift of XRD peaks has been revealed at XRD study. This fact testifies on increasing the lattice parameters and interplanar distances owing to the formation of some Cu‐related complexes in the ZnO matrix. Cu‐complex defects can be attributed to [CuZn Zn<sup>i</sup> ] x complexes proposed in [65].
## **7. Plasmon‐related effects in ZnO Cu NC films with metallic Cu nanoparticles**
The purpose of this part deals with the study of another effect related to Cu doping of the ZnO NCs. In the earlier mentioned papers [11, 58–63], the ZnO NCs were doped by Cu in low concentrations, when Cu atoms substituted Zn atoms in the ZnO crystal lattice. However, the method of Zn etching technology with thermal treatment permits to prepare ZnO:Cu NCs with metallic Cu nanoparticles located at the ZnO NC surface (**Figure 19**, **Table 5**). In this ZnO:Cu films, as expected, it is possible to generate plasmon by light in Cu nanoparticles that permit to study its impact on optical properties [80].
At first, the XRD (**Figure 19**) and EDS (**Figure 25**) methods have been used for the confirmation that Cu atoms and Cu nanoparticles exist in ZnO Cu NC films (**Table 6**).
**Figure 25(b)** clearly demonstrates the CuK line in the high resolution insertion for ZnO:Cu NCs. The EDS analysis and estimated element concentrations are presented in **Table 6**.
XRD comparative investigations of ZnO and ZnO Cu NCs have been presented early in **Figures 18** and **19** and analyzed in Section 6. In addition to the above discussed Cu peak at 2 theta equal to 43.2963° (**Figure 19**), which corresponds to the diffraction from the (111) crystal planes in the cubic crystal lattice of metallic Cu nanoparticles [73], the second Cu (200) peak at the 2 theta equaling to 50.4308° has been revealed in ZnO Cu NCs as well (**Figure 26a** and **b**) [85].
Emission, Defects, and Structure of ZnO Nanocrystal Films Obtained by Electrochemical Method http://dx.doi.org/10.5772/66335 81
**Figure 25.** EDS spectra of N3‐ZnO (a) and N6‐ZnO Cu (b) NC samples obtained at an anodization time 6 min after thermal annealing [85]. The insertions present the high resolution EDS spectra in the range 7.8‐8.3 keV for ZnO (a) and ZnO:Cu (b) NCs.
In ZnO:Cu NCs obtained with the etching time of 6 min, the concentration of Cu NCs enlarges, which manifests itself by increasing the intensity of the Cu (111) and Cu (200) XRD peaks (**Figures 19b** and **26b**). The oxidation of Cu nanoparticles can be realized, partially as well, at annealing in ambient air. The small peculiarities in XRD diagrams (**Figure 19**) for the sample M6 in the range 38–40° can be related to the CuO or Cu2 O phases.
#### **7.1. Raman scattering study**
defects: Zn interstitialses or Zn‐interstitial‐related complex [83]. In addition, under the N‐rich
Thus, it can be assumed that in ZnO NCs the orange (A), yellow (B), and green (C) emissions are related to DAPs, which include the deep acceptors, such as oxygen interstitials and zinc vacancies, and shallow donors, like zinc interstitials or their complexes. In ZnO Cu NCs the substitutional CuZn atoms form the DAPs with shallow donors that are responsible on the
The blue PL band D (2.61–2.70 eV) appears in PL spectra of ZnO:Cu NCs (sample N6, **Figure 20b**) at higher Cu contents (≥2.28 at%). Simultaneously, the intensities of other PL bands decrease (**Table 4**) and the band D dominates in the PL spectrum (**Figure 20b**, curve 2). At higher Cu concentrations in ZnO:Cu NCs, the nonradiative recombination centers (NRC), probably, appear that provokes decreasing the PL intensity of the bands (A, B, C). At the same time, the D band intensity decreases slowly that testifies on concentration increasing of D band emitting defects. In this case, a low angle shift of XRD peaks has been revealed at XRD study. This fact testifies on increasing the lattice parameters and interplanar distances owing to the formation of some Cu‐related complexes in the ZnO matrix. Cu‐complex defects can be
complexes proposed in [65].
**7. Plasmon‐related effects in ZnO Cu NC films with metallic Cu**
The purpose of this part deals with the study of another effect related to Cu doping of the ZnO NCs. In the earlier mentioned papers [11, 58–63], the ZnO NCs were doped by Cu in low concentrations, when Cu atoms substituted Zn atoms in the ZnO crystal lattice. However, the method of Zn etching technology with thermal treatment permits to prepare ZnO:Cu NCs with metallic Cu nanoparticles located at the ZnO NC surface (**Figure 19**, **Table 5**). In this ZnO:Cu films, as expected, it is possible to generate plasmon by light in Cu nanoparticles that
At first, the XRD (**Figure 19**) and EDS (**Figure 25**) methods have been used for the confirma-
**Figure 25(b)** clearly demonstrates the CuK line in the high resolution insertion for ZnO:Cu NCs. The EDS analysis and estimated element concentrations are presented in **Table 6**.
XRD comparative investigations of ZnO and ZnO Cu NCs have been presented early in **Figures 18** and **19** and analyzed in Section 6. In addition to the above discussed Cu peak at 2 theta equal to 43.2963° (**Figure 19**), which corresponds to the diffraction from the (111) crystal planes in the cubic crystal lattice of metallic Cu nanoparticles [73], the second Cu (200) peak at the 2 theta equaling to 50.4308° has been revealed in ZnO Cu NCs as well
tion that Cu atoms and Cu nanoparticles exist in ZnO Cu NC films (**Table 6**).
–NO, can be obtained as it was supposed in [84].
ambient condition the shallow donors, Zni
80 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
structureless green PL band.
**6.4. Blue PL band**
attributed to [CuZn Zn<sup>i</sup>
(**Figure 26a** and **b**) [85].
**nanoparticles**
]x
permit to study its impact on optical properties [80].
Raman peaks at 331, 379, 437, and 572–575 cm−1 have been detected in Raman scattering spectra measured in the 100–650 cm−1 range in ZnO and ZnO:Cu NCs (**Figure 27**). The nature of these Raman peaks has been discussed in Section 3.2. The increase in etching time leads to the enlargement of Raman peak intensities owing to the growth of ZnO NC volume and varying the geometry of Raman scattering measurement in small ZnO NC films (**Figure 27**). The intensity of all Raman peaks in ZnO Cu NCs is threefold higher than its value in ZnO NCs (**Figure 27a** and **b**). The studied NCs are characterized by the identical crystal structure and NC sizes (**Table 4**). The difference in the Raman scattering intensity can be attributed to the surface‐enhanced Raman scattering (SERS) effect in ZnO Cu NCs [72].
**Table 6.** Analysis of K‐lines in EDS results for N5 ZnO Cu NCs.
**Figure 26.** XRD results for the ZnO Cu NCs obtained at an anodization times of 3 min (a) and 6 min (b) after thermal annealing in the XRD range 50–51° [85].
The light‐enhanced electric field and the SERS effect are attributed to the plasmon resonance at the material interface with metallic nanoparticles. In the studied case, apparently, the excitation light used at Raman scattering study stimulates the plasmon generation in metallic Cu nanoparticles at the surface of ZnO:Cu NCs with corresponding wavelength for plasmon‐ polariton resonance that is needed for SERS effect.
#### **7.2. ZnO emission study**
PL spectra of ZnO and ZnO Cu NCs are presented in **Figure 28** for the comparison. The PL intensity of defect‐related PL bands (2.08 and 2.50 eV) increases, in comparison with the NBE emission intensity, when the NC size falls down in ZnO and ZnO Cu NCs together with the surface‐to‐volume ratio rising (**Figure 28**, **Table 4**). It is known that 2.02–2.08 eV PL band
**Figure 27.** Raman scattering spectra of thermal annealed ZnO NCs (a) and ZnO Cu NCs (b) obtained at the etching times: 1 (1), 3 (2), and 6 min (3) [85].
Emission, Defects, and Structure of ZnO Nanocrystal Films Obtained by Electrochemical Method http://dx.doi.org/10.5772/66335 83
**Figure 28.** PL spectra of ZnO NCs (a) and ZnO Cu NCs (b) obtained at the anodization durations of 1 (1), 3 (2), and 6 min (3). Dashed curves represent the deconvolution of experimental PL spectrum on the elementary PL bands (curves 4, 5, 6) [85].
increased significantly its intensity after oxidation at annealing in ambient air [21, 22] that is accompanied by the oxygen interstitial content enlargement in ZnO. The detection of identical orange emission peaks in ZnO and ZnO Cu NCs permits to attribute the PL band (2.08 eV) to radiative defects included the oxygen interstitials.
The influence of metallic Cu nanoparticles on the PL band intensities in PL spectra of ZnO:Cu NCs is different significantly.
The influence of Cu nanoparticles on the intensities of PL bands in ZnO NCs is different. **Figure 28** shows that the PL intensity of defect‐related PL bands is higher by twofold in ZnO Cu NCs than it value in ZnO NCs. This fact can be assigned to the plasmon‐enhancing recombination via these defects [1]. However, NBE emission in ZnO Cu NCs is less effective in comparison with ZnO NCs (**Figure 28**). The last effect can be explained by the destruction of excitons by locally enhanced electric field in Cu nanoparticles at the ZnO Cu NC surface and due to this diminish the exciton‐related emission.
## **8. Conclusion**
The light‐enhanced electric field and the SERS effect are attributed to the plasmon resonance at the material interface with metallic nanoparticles. In the studied case, apparently, the excitation light used at Raman scattering study stimulates the plasmon generation in metallic Cu nanoparticles at the surface of ZnO:Cu NCs with corresponding wavelength for plasmon‐
**Figure 26.** XRD results for the ZnO Cu NCs obtained at an anodization times of 3 min (a) and 6 min (b) after thermal
PL spectra of ZnO and ZnO Cu NCs are presented in **Figure 28** for the comparison. The PL intensity of defect‐related PL bands (2.08 and 2.50 eV) increases, in comparison with the NBE emission intensity, when the NC size falls down in ZnO and ZnO Cu NCs together with the surface‐to‐volume ratio rising (**Figure 28**, **Table 4**). It is known that 2.02–2.08 eV PL band
**Figure 27.** Raman scattering spectra of thermal annealed ZnO NCs (a) and ZnO Cu NCs (b) obtained at the etching times:
polariton resonance that is needed for SERS effect.
82 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
**7.2. ZnO emission study**
1 (1), 3 (2), and 6 min (3) [85].
annealing in the XRD range 50–51° [85].
The morphology, crystal structure, Raman scattering, and multicolor emission have been comparatively studied in ZnO and ZnO:Cu NCs. XRD study confirms the wurtzite structure of ZnO NCs obtained by electrochemical method. The PL intensity enhancement of exciton emission is detected in NCs with the size of 67–170 nm and attributed to the week quantum confinement and exciton light coupling with the formation of polaritons. It is shown that metallic Cu nanoparticles on the surface of ZnO:Cu NCs stimulate the SERS effect and PL intensity rising the visible PL bands owing to, apparently, the plasmon generation in Cu NCs. Simultaneously, NBE emission decreases in ZnO:Cu NCs due to the exciton destruction by plasmon‐enhanced electric field.
## **Acknowledgements**
The authors would like to thank the SIP‐IPN, Mexico (projects 20160285) and CONACYT, Mexico (project 258224) for the financial support.
## **Author details**
Tetyana V. Torchynska<sup>1</sup> \* and Brahim El Filali2
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**Acknowledgements**
**Author details**
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Tetyana V. Torchynska<sup>1</sup>
Mexico (project 258224) for the financial support.
84 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
\*Address all correspondence to: [email protected]
1 Instituto Politecnico Nacional, ESFM, Mexico City, Mexico 2 Instituto Politecnico Nacional, UPIITA, Mexico City, Mexico
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**Optimization Parameters in the Thin Film Science**
## **Lab-on-a-Tube Surface Micromachining Technology Lab-on-a-Tube Surface Micromachining Technology**
Zhuoqing Yang and Yi Zhang Zhuoqing Yang and Yi Zhang
Additional information is available at the end of the chapter Additional information is available at the end of the chapter
http://dx.doi.org/10.5772/66664
#### **Abstract**
In this chapter, the lab-on-a-tube surface micromachining technology will be used to fabricate a flexible implantable microtemperature sensor for hyperthermia application and a three-electrode system on a polymer tube surface for glucose monitoring application. This micromachining process is based on two homemade equipments: a spray coating equipment and a programmable UV 3D projection lithography system with alignment. In the spray coating system, there is a heater nozzle next to the spray nozzle for real-time heating. Pure nitrogen is flowed through the heater nozzle, warmed up and sprayed onto the tube substrate surface. The programmable UV lithography equipment for cylindrical substrate mainly consists of four parts: a uniform illumination system, a reduced projection lithography system, a synchronized motion stage system, and a Charge-Coupled Device (CCD) multilayer alignment system which is used to observe simultaneously the projected mask's patterns and those ever fabricated on the tube. Using the developed labon-a-tube surface micromachining technology, an implantable flexible microtemperature sensor and a three-electrode microstructure are successfully fabricated on the flexible polymer tube with 330 μm outer diameter, respectively. The test temperature coefficient of resistance (TCR) of the temperature sensor is 0.0034/°C. The measured cyclic voltammetry curve shows that the three-electrode system has a good redox property.
**Keywords:** micromachining, lab-on-a-tube, thin film, flexible cylindrical substrate, 3D lithography
## **1. Introduction**
With the development of the Internet of Things (IoT), wearable devices, and implantable biomedical components, the flexible sensors, actuators, and electrical circuits have been demanded more and more widely. However, the traditional silicon-based surface micromachining technologies are difficult to meet this requirement due to natural brittle structure. With regard to this, we developed a novel micromachining method that mainly includes
© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
spray coating, lithography pattering, and multilayer alignment on the flexible cylindrical substrate, such as fiber, polymer tube, capillary, and other tubes. It can realize the integrated fabrication of many sensors and actuators with different functional materials on an ultrathin (hundreds of micrometers) flexible cylindrical substrate, which is very promising to wearable devices and biomedical applications in the future. Here, we called it "lab-on-a-tube surface micromachining technology."
In this chapter, the equipments used for lab-on-a-tube surface micromachining will be described in detail. Firstly, a spray coating system for cylindrical substrate will be introduced, and the effect of process parameters on the quality of the coated photoresist (PR) film on tube surface will be also discussed. Then, the UV projection lithography system for cylindrical substrate is developed, and its working principle is described in detail. Finally, two application examples of the developed lab-on-a-tube surface micromachining technology will be shown, one is the flexible implantable microtemperature sensor on a polymer tube surface, another is flexible implantable microneedle with three-electrode system for glucose sensor.
## **2. Equipments for lab-on-a-tube surface micromachining**
#### **2.1. Spray coating system for cylindrical substrate**
A homemade equipment was built, as shown in **Figure 1**, which can realize the spray coating of PR film on the tube surface. In order to complete real-time heating for the tube with coated film, a heater nozzle was used, where nitrogen gas (N<sup>2</sup> ) will be flowed. The temperature of N2 gas was controlled by using two temperature sensors at the inside and outlet of the heater nozzle. The scan speed of the spraying nozzle, the rotation speed of the tube, and the distance between spray nozzle and the tube can be all independently controlled. They are main process parameters in the current system [1].
In the present work, the photoresist (PR) solution including a positive photoresist S1830 and a thinner AZ 5200 was used as the PR film coated on the tube surface. In the solution, their weight ratio is 1:1. **Figure 2** shows the effect of the nozzle/tube distance on the thickness of coated PR film when using the real-time heating or not. **Figure 3** is the measured variations of the PR film thickness versus the rotation speed when using different weight ratios and
**Figure 1.** Homemade spray coating system for tube substrate and its schematic setup [3].
spray coating, lithography pattering, and multilayer alignment on the flexible cylindrical substrate, such as fiber, polymer tube, capillary, and other tubes. It can realize the integrated fabrication of many sensors and actuators with different functional materials on an ultrathin (hundreds of micrometers) flexible cylindrical substrate, which is very promising to wearable devices and biomedical applications in the future. Here, we called it "lab-on-a-tube surface
In this chapter, the equipments used for lab-on-a-tube surface micromachining will be described in detail. Firstly, a spray coating system for cylindrical substrate will be introduced, and the effect of process parameters on the quality of the coated photoresist (PR) film on tube surface will be also discussed. Then, the UV projection lithography system for cylindrical substrate is developed, and its working principle is described in detail. Finally, two application examples of the developed lab-on-a-tube surface micromachining technology will be shown, one is the flexible implantable microtemperature sensor on a polymer tube surface, another is flexible implantable microneedle with three-electrode system for
A homemade equipment was built, as shown in **Figure 1**, which can realize the spray coating of PR film on the tube surface. In order to complete real-time heating for the tube with coated
gas was controlled by using two temperature sensors at the inside and outlet of the heater nozzle. The scan speed of the spraying nozzle, the rotation speed of the tube, and the distance between spray nozzle and the tube can be all independently controlled. They are main pro-
In the present work, the photoresist (PR) solution including a positive photoresist S1830 and a thinner AZ 5200 was used as the PR film coated on the tube surface. In the solution, their weight ratio is 1:1. **Figure 2** shows the effect of the nozzle/tube distance on the thickness of coated PR film when using the real-time heating or not. **Figure 3** is the measured variations of the PR film thickness versus the rotation speed when using different weight ratios and
) will be flowed. The temperature of
**2. Equipments for lab-on-a-tube surface micromachining**
**Figure 1.** Homemade spray coating system for tube substrate and its schematic setup [3].
**2.1. Spray coating system for cylindrical substrate**
cess parameters in the current system [1].
film, a heater nozzle was used, where nitrogen gas (N<sup>2</sup>
92 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
micromachining technology."
glucose sensor.
N2
**Figure 2.** Measured effects of the real-time heating process on the variation of the PR film thickness versus the nozzle tube distance.
**Figure 3.** Measured variations of the PR film thickness versus the rotation speed under different weight ratios and cycle scanning speeds.
cycle scanning speeds. It can be found that the quality of the coated PR films is not good if there is no real-time heating during spray coating. The thinner PR film would be obtained if increasing the rotation speed and thinner concentration. Because that the centrifugal force will become larger when the rotation speed of the tube increases. So PR solution particles will be thrown away more from the tube surface. Finally, the PR films become thinner when increasing the rotation speed.
These experimental results about the spraying PR film on the tube surface are generally in accordance with the previous reports [1, 2]. However, considering the size of tube is very small compared to those of the spray jet and the nozzle/tube distance. So its basic principle is different from the traditional coating process, where the size of wafer is usually planar shape with several inches. A so-called impinging region existing below the nozzle will cover the tube spraying area. Moreover, considering the spraying PR particles (about 10–20 μm) is usually only one-tenth of the tube diameter, the influence of the rotation speed is more obvious than the real-time heating during the spray coating. Especially for that using low-volatility thinner, the real-time heating is more important. Otherwise, it is difficult to obtain continuous and smooth PR film surface. In addition, the effect of process parameters in the spray coating on the quality of PR film on tube surface has been fully studied in our previous work [3, 4].
#### **2.2. UV projection lithography system for cylindrical substrate**
**Figure 4** shows the sketch of the developed UV lithography system for tube surface. The whole system mainly consists of four units: a synchronized motion stage, a reduced projection exposure unit, a CCD multilayer alignment part, and a uniform illumination unit. After through a reduced mask image, the UV light used as exposure source will be focused onto the bottom surface of the tube with coated PR film. The wavelength of the used UV light is 250–600 nm. With regard to this, a 436 ± 10 nm interference filter was used to eliminate the aberration. Finally, the magnifying power of the objective lens will determine the amplification factor of the whole lithography system. In the present work, the overall reduced factor is 0.5 when the pattern of mask is transferred to the surface of the tube. This is reasonable and enough for our current research. By the abovementioned setting, a focal depth of ±45 μm can be realized in the developed UV lithography system that will be used to exposure and patterning the tube surface with coated PR film. Two chucks in the rotation stages are used to fix the tube in order to make sure the coaxial rotation. In addition, for adjusting the coaxiality conveniently, a laser was also used as a reference.
The final whole lithography system that includes five degrees of freedom (DOFs) is shown in **Figure 5(a)**. The patterns on mask and tube can be seen simultaneously by using two CCD, which can help to complete the secondary or multilayer alignment in the exposure. The side
**Figure 4.** Schematic illustration of programmable UV lithography system with alignment for cylindrical substrates [4].
**Figure 5.** Optical photo of (a) final developed lithography equipment and CCD for alignment and (b) side view and (c) top view in the programmable operation window.
view and top view in the programmable operation window are shown in **Figure 5(b)** and **(c)**, respectively. The programmable software can automatically control motorized XYZ-θ stages and exposure time. The movement and the rotation accuracies are 0.1 μm and 0.01° in the axial direction and the θ-direction, respectively. By using PC operation window and doing those programmable sentences, the PR film on the tube surface can be patterned according to the design. Finally, the PR patterns with expected microstructures will be obtained on the tube surface after the development. This equipment can be used not only for tube exposure but also for those similar cylindrical substrates, such as optical fibers, capillary, metal or fabric wires, etc. The working principle of the developed lithography system can be found in our previous works [4, 5].
thinner, the real-time heating is more important. Otherwise, it is difficult to obtain continuous and smooth PR film surface. In addition, the effect of process parameters in the spray coating on the quality of PR film on tube surface has been fully studied in our previous work [3, 4].
**Figure 4** shows the sketch of the developed UV lithography system for tube surface. The whole system mainly consists of four units: a synchronized motion stage, a reduced projection exposure unit, a CCD multilayer alignment part, and a uniform illumination unit. After through a reduced mask image, the UV light used as exposure source will be focused onto the bottom surface of the tube with coated PR film. The wavelength of the used UV light is 250–600 nm. With regard to this, a 436 ± 10 nm interference filter was used to eliminate the aberration. Finally, the magnifying power of the objective lens will determine the amplification factor of the whole lithography system. In the present work, the overall reduced factor is 0.5 when the pattern of mask is transferred to the surface of the tube. This is reasonable and enough for our current research. By the abovementioned setting, a focal depth of ±45 μm can be realized in the developed UV lithography system that will be used to exposure and patterning the tube surface with coated PR film. Two chucks in the rotation stages are used to fix the tube in order to make sure the coaxial rotation. In addition, for adjusting the coaxiality
The final whole lithography system that includes five degrees of freedom (DOFs) is shown in **Figure 5(a)**. The patterns on mask and tube can be seen simultaneously by using two CCD, which can help to complete the secondary or multilayer alignment in the exposure. The side
**Figure 4.** Schematic illustration of programmable UV lithography system with alignment for cylindrical substrates [4].
**Figure 5.** Optical photo of (a) final developed lithography equipment and CCD for alignment and (b) side view and
**2.2. UV projection lithography system for cylindrical substrate**
94 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
conveniently, a laser was also used as a reference.
(c) top view in the programmable operation window.
Micropatterning on the tube surface can be easily realized by using the developed programmable lithography equipment. Firstly, you can decompose the designed pattern into sequential programmable sentences in the PC operation window. Then, the software will automatically control the movements of stage, mask, and tube; the UV light will complete the exposure to the tube surface with coated PR film according to the programmable sentences. For example, you can complete an oblique line pattern on the tube by controlling the tube to parallel translation and rotate simultaneously. Of course, the final angle of the oblique line will be determined together by their two speed values in the respective directions. In the exposure, we can conveniently tune the intensity by changing the quantity of projection light and movement speed of the tube. **Figure 6** is the preview patterns of some microstructures after completing the programmable sentences, such as (a) micro-heater and (b) resonator. These micropatterns will be firstly drawn by programmable operation window in our developed lithography system shown in **Figure 5**. Then, they will be fabricated on the silica glass tube surface. After the exposure of these cylindrical tubes with coated PR film by the above programmable lithography system, the development was carried out subsequently. As a result, the PR films on the tube surfaces have been successfully patterned as those expected program patterns in **Figure 6(a)** and **(b)**, which are as shown in **Figure 6**(**a**′) and (**b**′).
After sputtering Pt film and subsequent lift-off process, the two kinds of microstructures are clearly seen, including zigzag micro-heater and partial patterns of resonator, which are
**Figure 6.** Planar previews of the programmed (a) micro-heater and (b) resonator patterns and (a′–b′) obtained micropatterns on silica glass tube after exposure and development according to corresponding programmed patterns.
**Figure 7.** SEM of fabricated (a) zigzag structures of micro-heater and (b) resonator.
shown in **Figure 6** (**a**′) and (**b**′). **Figure 7** shows the Scanning Electron Microscope (SEMs) of these microstructures. The PR film thickness and the line width are ~2 and 20 μm, respectively. As so far, the line width of 10 μm has been obtained by using special fabrication process in our developed lithography system.
## **3. Application examples**
#### **3.1. Flexible implantable microtemperature sensor**
The hyperthermia is still considered as an effective way for the cancer treatment, which has been proven in many clinical studies. In the treatment, the microwave was used to heat the lesion to above 42°C in order to kill tumors. At the same time, we have to make sure the normal tissue not to be damaged [6]. So, in the hyperthermia it is necessary to develop a microtemperature sensor for measuring the temperature precisely. Although many researchers have fabricated some microtemperature sensors, they cannot be used as a flexible device to implant into the objects due to its fabrication based on silicon process [6–8]. Its natural brittle feature is not beneficial. Therefore, some researchers try to develop thinner sensor based on the cylindrical substrate in order to implant into the tissue. For example, a microcoil on the capillary surface for magnetic resonance imaging (MRI) interventional treatment has been reported in Ref. [9]. Similarly, in Refs. [10, 11], a radio frequency (RF) coil has also been developed on the cylindrical surface for portable nuclear magnetic resonance (NMR) diagnosis. In addition, the soft lithography technology was used to realize patterning on the curved surface in Ref. [12]. Even, in order to fabricate microstructures on the thin cylindrical substrate, an automatic wire bonder was also utilized in the work [13, 14]. However, in these methods the devices are usually fabricated on glass capillary or metal stick surface. The whole flexibility of the sensor is poor, and corresponding fabrication resolution and sensitivity are neither not good. In addition, in these methods only one kind of material can be used and fabricated because the multilayer alignment cannot be realized. As a result, their applications have been subjected to a lot of limitations.
In the present work, using the developed programmable UV lithography system, a flexible implantable microtemperature sensor for hyperthermia application will be designed and fabricated. Finally, the fabricated microtemperature sensor will be also evaluated in detail.
In this work, we design a flexible implantable microtemperature sensor on a polymer tube with only 330 μm outer diameter for hyperthermia application. This sensor will be fabricated using the above developed lithography system. The design sketch of the temperature sensor and its corresponding general working principle is shown in **Figure 8(a)**. In the future practical application, you can implant the part of the sensor into the tumor and monitor its temperature. The doctor can make a right decision and precise operation by referring the measured result from the microtemperature sensor. The material of the sensing element in the sensor is platinum (Pt) considering its good resistance-temperature effect. The sensing element and its geometric parameters are shown in **Figure 8(b)** and **(c)**, respectively.
ANSYS was used to do the simulation of the microtemperature sensor. **Figure 9** is the 2D model of the sensor, which only contains one Pt line and pitch unit because of its symmetry. The polymer tube from Furukawa Electric Co., Ltd. was used as the substrate because of its excellent physical and chemical stabilities. It is substantially a kind of commercial polyimide (PI) material. Especially, this tube can withstand the temperature up to 200°C. Before simulation the related boundary conditions need to be determined according to application environment. The temperature of tube surface was set as 37°C, which was considered as the normal tissue. Here, 42°C is set as the highest temperature in the hyperthermia. The transient simulation was carried out for dynamic response of the designed microtemperature sensor. **Figure 9(b)**–**(g)** shows the simulated results. It can be seen that the temperature didn't continue to spread along the vertical direction of the tube surface until the moment about 2–4 ms, which means the response time of the sensor.
shown in **Figure 6** (**a**′) and (**b**′). **Figure 7** shows the Scanning Electron Microscope (SEMs) of these microstructures. The PR film thickness and the line width are ~2 and 20 μm, respectively. As so far, the line width of 10 μm has been obtained by using special fabrication
The hyperthermia is still considered as an effective way for the cancer treatment, which has been proven in many clinical studies. In the treatment, the microwave was used to heat the lesion to above 42°C in order to kill tumors. At the same time, we have to make sure the normal tissue not to be damaged [6]. So, in the hyperthermia it is necessary to develop a microtemperature sensor for measuring the temperature precisely. Although many researchers have fabricated some microtemperature sensors, they cannot be used as a flexible device to implant into the objects due to its fabrication based on silicon process [6–8]. Its natural brittle feature is not beneficial. Therefore, some researchers try to develop thinner sensor based on the cylindrical substrate in order to implant into the tissue. For example, a microcoil on the capillary surface for magnetic resonance imaging (MRI) interventional treatment has been reported in Ref. [9]. Similarly, in Refs. [10, 11], a radio frequency (RF) coil has also been developed on the cylindrical surface for portable nuclear magnetic resonance (NMR) diagnosis. In addition, the soft lithography technology was used to realize patterning on the curved surface in Ref. [12]. Even, in order to fabricate microstructures on the thin cylindrical substrate, an automatic wire bonder was also utilized in the work [13, 14]. However, in these methods the devices are usually fabricated on glass capillary or metal stick surface. The whole flexibility of the sensor is poor, and corresponding fabrication resolution and sensitivity are neither not good. In addition, in these methods only one kind of material can be used and fabricated because the multilayer alignment cannot be realized. As a result, their applications have been
In the present work, using the developed programmable UV lithography system, a flexible implantable microtemperature sensor for hyperthermia application will be designed and fabricated. Finally, the fabricated microtemperature sensor will be also evaluated in detail. In this work, we design a flexible implantable microtemperature sensor on a polymer tube with only 330 μm outer diameter for hyperthermia application. This sensor will be fabricated
process in our developed lithography system.
**Figure 7.** SEM of fabricated (a) zigzag structures of micro-heater and (b) resonator.
96 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
**3.1. Flexible implantable microtemperature sensor**
**3. Application examples**
subjected to a lot of limitations.
**Figure 8.** (a) Sketch of the flexible microtemperature sensor on the polymer tube for hyperthermia, (b) sensing element, and (c) its structural parameters [4].
**Figure 9.** Dynamic simulation of microtemperature sensor used in hyperthermia. (a) FE model and (b)–(g) transient temperature field distributions [4].
The general fabrication process of the temperature sensor is described in **Figure 10**, as follows:
The polymer tube substrates used in this work were all cleaned by immersing in the H2 SiO4 / H2 O2 solution at 115°C for 15 min and then rinsed with purified water. The PR film was depositing on the tube surface by homemade spray coating setup. During the spray coating, the real-time heating was used always. The thickness of coated PR film can be controlled by cycle spray coating number and tube rotation speed. In the fabrication of Pt sensing element in the
**Figure 10.** Main fabrication processes of flexible microtemperature sensor on the polymer tube.
sensor, the quality of the Pt film is very important considering it is used as the critical sensitive material. The sputtering condition is the vacuum 7.0 × 10−4 Pa, the purity of Ar gas 99.9%, the purity of Pt target 99.99%, and sputtering power 100 W. The fabrication process steps of the microtemperature sensor are described in our previous work [15].
The general fabrication process of the temperature sensor is described in **Figure 10**, as follows: **a.** Cleaning the polymer tube. Using a UV ozone treatment unit (VX-0200HK-002, ACingTec,
**b.** Spray coating ~2.5 μm PR on the tube substrate by developed homemade coating
**c.** Exposure and patterning of the polymer tube with coated PR film by developed cylindrical
**i.** Obtaining the microtemperature sensor after removing the residual PR film by acetone solution. The detailed fabrication process steps of the sensor can be found in our previous
solution at 115°C for 15 min and then rinsed with purified water. The PR film was depositing on the tube surface by homemade spray coating setup. During the spray coating, the real-time heating was used always. The thickness of coated PR film can be controlled by cycle spray coating number and tube rotation speed. In the fabrication of Pt sensing element in the
SiO4 /
The polymer tube substrates used in this work were all cleaned by immersing in the H2
**d.** Magnetron sputtering Pt film (~0.13 μm) onto the patterned tube surface.
**f.** Again spray coating PR film (~2 μm) on the tube with patterned Pt microstructures.
**g.** Secondary exposure and development were done to pattern the PR film again.
layer ~0.3 μm was deposited on the Pt film for electric isolation.
**Figure 10.** Main fabrication processes of flexible microtemperature sensor on the polymer tube.
Japan) to clean the surface and modification.
98 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
system.
**h.** SiO2
H2 O2
report [15].
lithography method.
**e.** PR film was removed by lift-off.
The optical photo of the final fabricated flexible microtemperature sensor is shown in **Figure 11**. It can be seen that the sensor has a good flexibility. The Pt sensing element in the sensor can be also seen clearly in the right picture in the figure. Also, the line patterns of the fabricated sensing element on the polymer tube are clearly shown in **Figure 12**.
Platinum (Pt) film temperature sensing will experience a change in resistance with environmental temperature according to the following formula:
$$R\_i = R\_0[1 + \alpha(T - T\_o)]\tag{1}$$
where *Rt* is the resistance at the working temperature *T* and *R0* is the resistance at the reference temperature *T0* . And, *α* is the temperature coefficient of resistance (TCR). By slowly increasing and reducing the temperature from room temperature to 90°C, we can obtain the TCR, as shown in **Figure 13**. It is generally 0.0034/°C. This measured value is lower than the theoretical value of bulk pure Pt (0.0039/°C) because the electron scattering would be cause grain not be density during the film sputtering [16]. Of course, the deviation will also be caused by fabrication parameters, testing method, etc.
**Figure 11.** Optical photo of the fabricated flexible microtemperature sensor next to a coin for reference and its sensing element micropattern.
**Figure 12.** SEM photograph of the fabricated flexible microtemperature sensor on polymer tube surface.
#### **3.2. Flexible implantable microneedle with three-electrode system for glucose sensor**
As another example of the developed programmable UV lithography system, a threeelectrode system will be designed and fabricated on a polymer tube surface. As a result, the tube with three-electrode microstructure can be used as a flexible microneedle, which could be promising in the implantable glucose sensor for human body in the future. **Figure 14(a)** is the sketch of the designed system and its configuration. The one end of the microneedle can be used as an implantable sensor considering its flexibility and thin size. In addition, the hollow structure of the tube may be as a convenient path for potential drug delivery. The three-electrode structure is as follows: counter electrode (CE), reference electrode (RE), and working electrode (WE). **Figure 14(b)** and **(c)** shows the corresponding geometric parameters and configurations. Some external measuring and controlling systems are also necessary if the microneedle would be used in the glucose monitoring and insulin injecting.
**Figure 15** generally describes fabrication steps of the three-electrode structure on the polymer tube, as follows:
**Figure 13.** Test TCR of the flexible microtemperature sensor fabricated on the polymer tube surface.
**Figure 14.** (a) Sketch of the interventional flexible implantable microneedle with three-electrode system for continuous glucose monitoring and drug delivery, (b) distributions, and (c) main structural parameters [17].
**3.2. Flexible implantable microneedle with three-electrode system for glucose sensor**
100 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
insulin injecting.
tube, as follows:
system.
As another example of the developed programmable UV lithography system, a threeelectrode system will be designed and fabricated on a polymer tube surface. As a result, the tube with three-electrode microstructure can be used as a flexible microneedle, which could be promising in the implantable glucose sensor for human body in the future. **Figure 14(a)** is the sketch of the designed system and its configuration. The one end of the microneedle can be used as an implantable sensor considering its flexibility and thin size. In addition, the hollow structure of the tube may be as a convenient path for potential drug delivery. The three-electrode structure is as follows: counter electrode (CE), reference electrode (RE), and working electrode (WE). **Figure 14(b)** and **(c)** shows the corresponding geometric parameters and configurations. Some external measuring and controlling systems are also necessary if the microneedle would be used in the glucose monitoring and
**Figure 15** generally describes fabrication steps of the three-electrode structure on the polymer
**b.** Spray coating PR film ~2 μm on the polymer tube surface by developed spray coating
**Figure 14.** (a) Sketch of the interventional flexible implantable microneedle with three-electrode system for continuous
glucose monitoring and drug delivery, (b) distributions, and (c) main structural parameters [17].
**Figure 13.** Test TCR of the flexible microtemperature sensor fabricated on the polymer tube surface.
**a.** Polymer tube substrate ready and cleaning the surface of the tube using plasma.
**f.** Sputtering Ag layer and AgCl layer preparation; remove residual PR film by lift-off process. Finally, the three-electrode pattern was completed. The detailed fabrication process steps of the microneedle can be found in our previous reports [17, 18].
In the above fabrication of the three-electrode structure, the Ag/AgCl electrode must be completed according to programmable patterns as shown in **Figure 16**. In addition, the second lithography with alignment in the experiment is very critical. The related operation can be found in our previous work [17]. **Figure 16** shows the programmable micropattern of the three-electrode system (left) and the three-electrode structure obtained by programmable lithography equipment with multilayer alignment: PR boundaries in −40° and −10° views after the development by secondary alignment (right). So far, we have realized the ±1 μm alignment precision using the developed lithography system.
**Figure 17** is the final fabricated three-electrode structure on the polymer tube surface. The whole structure can be used as an implantable flexible microneedle because of its good flexibility property. Generally, this proposed device shows better overall property than other similar reports [19–24]. The novel design including a three-electrode structure on a thin hollow tube will be very useful for some applications in the micro-total analysis systems (μTAS) in the future.
**Figure 15.** Main fabrication process of the three-electrode structure in flexible microneedle on polymer tube surface.
**Figure 16.** Programmable micropattern of the three-electrode system (left) and three-electrode structure obtained by programmable lithography equipment with multilayer alignment: PR boundaries in −40° and −10° views after the development by secondary alignment (right).
**Figure 17.** SEM and optical photo of the fabricated three-electrode system on a polymer tube surface as aflexible microneedle.
**Figure 18.** Measured cyclic voltammetry curve of the three-electrode pattern on the fabricated prototype microneedle on polymer tube surface.
Next, the fundamental electrochemical property of the fabricated three-electrode structure was measured. The detailed measuring method, conditions, and related equipment used can be found in our previous work [17]. The measured cyclic voltammetry (CV) curve is shown in **Figure 18**. It indicates the device has a good redox property. But the peak of the test current is not very evident. Very small reaction area in the three-electrode pattern is considered as a main reason because of thin tube diameter. In addition, the quality of the sputtering Pt film is another affected factor. We can obtain an estimated peak current density according to the above measured result, about 0.8 mA/dm2 , which is sufficient for subsequent circuit signal processing in the glucose sensor application. In addition, our fabricated flexible microneedle device can be more easily implanted into the objects compared to other reported ones [23–26].
#### **4. Conclusion**
In this chapter, a lab-on-a-tube surface micromachining technology for cylindrical substrates has been built for the first time based on the developed programmable UV lithography system with alignment. The related equipments used in the lab-on-a-tube surface micromachining, including a homemade spray coating system and a projection exposure system for cylindrical substrate, have been introduced, and corresponding working principle and process parameters have been also explained. Then, as the application examples, an implantable flexible microtemperature sensor and an implantable microneedle with integrated three-electrode system on the tube surfaces have been proposed, fabricated, and characterized. The magnetron sputtering Pt film is used as the sensing material in the temperature sensor. The test TCR of the fabricated sensor is 0.0034/°C. The fabricated three-electrode structure on the polymer tube in CV measurement shows a good performance. The developed microneedle with the integrated three-electrode pattern will be very promising in the implantable glucose sensor for the human body in the future.
## **Acknowledgements**
The authors would like to express their gratitude for the support from the National Natural Science Foundation of China (No. 61571287), Hi-Tech Research and Development Program of China (2015AA042701), Shanghai Pujiang Program (14PJ1405800), and Japan Society for the Promotion of Science (JSPS).
## **Author details**
Zhuoqing Yang1 and Yi Zhang2 \*
## **References**
Next, the fundamental electrochemical property of the fabricated three-electrode structure was measured. The detailed measuring method, conditions, and related equipment used can be found in our previous work [17]. The measured cyclic voltammetry (CV) curve is shown in **Figure 18**. It indicates the device has a good redox property. But the peak of the test current is not very evident. Very small reaction area in the three-electrode pattern is considered as a main reason because of thin tube diameter. In addition, the quality of the sputtering Pt film is another affected factor. We can obtain an estimated peak current density according to the above mea-
**Figure 18.** Measured cyclic voltammetry curve of the three-electrode pattern on the fabricated prototype microneedle
**Figure 17.** SEM and optical photo of the fabricated three-electrode system on a polymer tube surface as aflexible microneedle.
102 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
glucose sensor application. In addition, our fabricated flexible microneedle device can be more
In this chapter, a lab-on-a-tube surface micromachining technology for cylindrical substrates has been built for the first time based on the developed programmable UV lithography system with alignment. The related equipments used in the lab-on-a-tube surface micromachining,
easily implanted into the objects compared to other reported ones [23–26].
, which is sufficient for subsequent circuit signal processing in the
sured result, about 0.8 mA/dm2
**4. Conclusion**
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104 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
**Provisional chapter**
## **Efficient Optimization of the Optoelectronic Performance in Chemically Deposited Thin Films Efficient Optimization of the Optoelectronic**
**Performance in Chemically Deposited Thin Films**
Andre Slonopas, Nibir K. Dhar, Herbert Ryan,
Jerome P. Ferrance, Pamela Norris and Andre Slonopas, Nibir K. Dhar, Herbert Ryan, Jerome P. Ferrance, Pamela Norris and Ashok
Ashok K. Sood K. Sood
Additional information is available at the end of the chapter Additional information is available at the end of the chapter
http://dx.doi.org/10.5772/67315
#### **Abstract**
Chemical deposition methodology is a well-understood and highly documented category of deposition techniques. In recent years, chemical bath deposition (CBD) and chemical vapor deposition (CVD) have garnered considerable attention as an effective alternative to other deposition methods. The applicability of CVD and CBD for industrial-sized operations is perhaps the most attractive aspect, in that thin-film deposition costs inversely scale with the processing batch size without loss of desirable optoelectronic properties in the materials. A downside of the method is that the optoelectronic characteristics of these films are highly susceptible to spurious deposition growth mechanisms. For example, increasing the temperature of the chemical deposition bath can shift the deposition mechanisms from ion-by-ion (two dimensional) precipitation to bulk solution cluster-by-cluster (three dimensional) formation which then deposit. This drastically changes the structural, optical, and electrical characteristics of CBD-deposited thin films. A similar phenomenon is observed in CVD deposited materials. Thus, it is of great interest to study the coupling between the deposition parameters and subsequent effects on film performance. Such studies have been conducted to elucidate the correlation between growth mechanisms and film performance. Here, we present a review of the current literature demonstrating that simple changes can be made in processing conditions to optimize the characteristics of these films for optoelectronic applications.
**Keywords:** chemical bath deposition, chemical vapor deposition, performance optimization, perovskites, optoelectronic performance
© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,
and reproduction in any medium, provided the original work is properly cited.
## **1. Introduction**
The intent of this chapter is to cover the most recent advances achieved in optimization of chemically deposited thin films. The most prominent and widely used chemical deposition processes are chemical bath deposition (CBD) and chemical vapor deposition (CVD) methods. Recent breakthroughs in film optimizations relating to these methods will be the focus of this monograph.
Advanced thin films are the key enabler of the modern high-tech explosion. These materials find uses in military, defense, private, and commercial products with a truly limitless potential. It can be argued that every piece of electrical equipment, tool, hardware, and device commercially available relies, to some extent, on advanced processing of thin-film materials and our ability to scale those processes for cost-effective mass production.
Although CBD and CVD methods have been known since the 1940s and 1950s, respectively, the pursuit of cost reductions has driven recent research efforts in these fields [1, 2]. Specifically, the past decade has seen developments in the performance optimization of thin films by altering the deposition parameters. This has allowed for improved film characteristics without additional cost and spurred research into the growth and optimization of organic semiconductors [3–6]. Further, CVD and CBD have facilitated efficient device manufacturing by enabling systematic film layer depositions. Additional layers of a single material or of multiple materials may be deposited as necessary to grow monolithic structures for advanced applications. Such mesoscopic assemblies opened new doors for device manufacturing and optimization. These novel approaches are expected to further expand the capabilities of the current technology in a wide range of applications.
The recent advances in this particular field of science are far-reaching and extend beyond the scope of this chapter. This chapter reviews only the mainstream and high-impact achievements in chemically deposited thin films. Some of the current challenges and limitations of these methods are also addressed. This chapter is divided into two sections: the first section addresses the chemical bath deposition technique, structural and optoelectronic characterization, and performance of CBD deposited thin films; the second section deals with the chemical vapor deposition and performance and tunability of CVD deposited thin films. Tunability and deposition parameter optimization of film performance are addressed in both sections.
## **2. Chemical bath deposition (CBD)**
The roots of chemical bath deposition extend back over a century [7]. It was initially shown that high-quality chalcogenides and oxides could be deposited using this simple cost-effective technique [8]. At the time, however, the semiconductor theory was decades away. Thus, only a few enthusiasts exhibited initial interest in the method. As the power of thin-film semiconductors was realized, the demand for such high-performance materials exploded. The market demand required a cost-effective method, which could sustain the production of high-quality thin films. By this time, CBD was already a well-established process shown to produce highly crystalline structures and, thus, was a natural choice for such an application [9]. Research at that time showed that the method could also be applied for the deposition of metals, metallic alloys, chalcogenides, oxides, carbonates, and halides, all of which are an inherent part of next-generation organic semiconductors [10–12]. In more recent years, the process has been extended to deposit electron and hole transport layers (ETL and HTL, respectively), transparent conducting oxides (TCO), nanotubes, copper indium gallium selenide (CIGS) devices, and numerous other applications [13–17]. Further, the CBD method is inexpensive, easy to implement, convenient for large area depositions, and associated with highly favorable optoelectronic and structural properties [18, 19]. The power and the usefulness of the method cannot be overstated. Because of its broad applicability, CBD became a focus of numerous research groups.
#### **2.1. CBD theoretical considerations**
**1. Introduction**
of this monograph.
The intent of this chapter is to cover the most recent advances achieved in optimization of chemically deposited thin films. The most prominent and widely used chemical deposition processes are chemical bath deposition (CBD) and chemical vapor deposition (CVD) methods. Recent breakthroughs in film optimizations relating to these methods will be the focus
Advanced thin films are the key enabler of the modern high-tech explosion. These materials find uses in military, defense, private, and commercial products with a truly limitless potential. It can be argued that every piece of electrical equipment, tool, hardware, and device commercially available relies, to some extent, on advanced processing of thin-film materials and
Although CBD and CVD methods have been known since the 1940s and 1950s, respectively, the pursuit of cost reductions has driven recent research efforts in these fields [1, 2]. Specifically, the past decade has seen developments in the performance optimization of thin films by altering the deposition parameters. This has allowed for improved film characteristics without additional cost and spurred research into the growth and optimization of organic semiconductors [3–6]. Further, CVD and CBD have facilitated efficient device manufacturing by enabling systematic film layer depositions. Additional layers of a single material or of multiple materials may be deposited as necessary to grow monolithic structures for advanced applications. Such mesoscopic assemblies opened new doors for device manufacturing and optimization. These novel approaches are expected to further expand the capabilities of the
The recent advances in this particular field of science are far-reaching and extend beyond the scope of this chapter. This chapter reviews only the mainstream and high-impact achievements in chemically deposited thin films. Some of the current challenges and limitations of these methods are also addressed. This chapter is divided into two sections: the first section addresses the chemical bath deposition technique, structural and optoelectronic characterization, and performance of CBD deposited thin films; the second section deals with the chemical vapor deposition and performance and tunability of CVD deposited thin films. Tunability and deposition parameter optimization of film performance are addressed in both sections.
The roots of chemical bath deposition extend back over a century [7]. It was initially shown that high-quality chalcogenides and oxides could be deposited using this simple cost-effective technique [8]. At the time, however, the semiconductor theory was decades away. Thus, only a few enthusiasts exhibited initial interest in the method. As the power of thin-film semiconductors was realized, the demand for such high-performance materials exploded. The market demand required a cost-effective method, which could sustain the production of high-quality
our ability to scale those processes for cost-effective mass production.
108 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
current technology in a wide range of applications.
**2. Chemical bath deposition (CBD)**
The underlying concept behind CBD is the rearrangement of the chemical constituents in the bath or already deposited on a substrate into functional crystalline structures during the chemical reactions. The original size of the chemical constituents can range from subatomic particles to microscale molecules. CBD finds a broad range of applications because it can be applied to a wide variety of chemicals. This concept is demonstrated further in the following examples.
(a) Electron exchange (redox)
Changes in oxidation numbers in elements during a chemical reaction indicate a redox reaction taking place. Such reactions take place in the CBD formations of metal, nonmetallic compounds, and oxide films, through direct oxidation or oxidative redux in which the electron exchange takes place among more than two elements. Insoluble lead dioxide [PbO2 ], for example, forms from the oxidative redux of peroxydisulfate ion [S2 O8 2−] in water, as shown in Eq. (1) [20]:
$$\rm Pb^{2+} + \rm S\_2O\_8^{2-} + 2\, H\_2O \to PbO\_2 + 2\, SO\_4^{2-} + 4\, H^+ \tag{1}$$
#### (b) Ligand exchange
This type of reaction involves the exchange of ligands in a complex ion. Highly desired silicon dioxide [SiO2 ] is precipitated using reaction 2 through the exchange of oxygen and fluorine ions. In general, this reaction holds an advantage in that it is generally more specific, allowing high selectivity in the produced compounds to be achieved [20]:
$$\rm{SiF\_4^{2-}} + 4\,\rm{H^+} + \rm{BO\_3^{3-}} \rightarrow \rm{SiO\_2} + \rm{H\_2O} + \rm{BF\_4^-} \tag{2}$$
#### (c) Complex reaction
This type of reaction occurs by a coordinated exchange of complexes in a chemical bath, as illustrated in Eq. (3). Because of the potential for additional reactions, complex exchange reactions can have drawbacks that require advanced consideration. For example, due to its availability and low cost, thiourea [SC(NH2 )2 ] is a common sulfur source used in CBD. Exchange of sulfur from the thiourea requires complex decomposition and reaction. Although the stable molecule cyanamide [CN2 H2 ] is a common by-product of the thiourea decomposition, formation of reactive and toxic cyanide [CN− ] has also been reported [21]. Such undesirable decompositions can have adverse effects on the overall deposition process:
$$\text{Cd} \text{ (NH}\_3\text{)}\_4^{2+} + \text{SC (NH}\_2\text{)}\_2 + 2\text{OH}^- \rightarrow \text{CdS} + \text{CN}\_2\text{H}\_2 + 2\text{H}\_2\text{O} \tag{3}$$
The existence of numerous approaches to growing films in solutions is illustrated in the basic reactions discussed in Eqs. (1)–(3). It should also be inferred that careful consideration of each chemical reaction by-product is necessary to prevent contamination of the film or undesired waste products. The structural properties, precipitation rates, crystallinities, and—of greater interest—optical and electrical performance of materials can thus be controlled with careful planning. Perhaps, the most promising category of materials derived from a theoretical understanding of CBD has been organic semiconductors. Recently, CBD was used to crystallize CH3 NH3 PbI3 perovskite powders for use in highly efficient organic-inorganic perovskite solar cells [22]. This would not have been possible without a deep theoretical understanding of the chemical reaction.
Reliably predicting film performance and controlling the deposition mechanisms is a formidable challenge. Charge carrier mobilities, for example, depend on the grain size, layer composition, porosity, interstitial trapping, doping, contaminants, diffusion lengths, substrate, bond lengths, and a plethora of other factors [23–25]. Recent deviations between continuum and atomistic level simulations stress this point further [26, 27].
However, it has been shown that it is possible to conduct an in situ study of the deposition parameters, deduce growth mechanisms, and reproduce the film performance from such simulations [28]. Accumulated experimental data are then utilized to converge atomistic models for accurate computational predictions [29]. The remainder of this section will discuss the accumulated experimental data on the corporeal control of the CBD process and its effects on film performance.
#### **2.2. CBD experimental data**
Two main deposition mechanisms dominate thin-film growth during CBD. The first, ion-by-ion (two dimensional) mechanism is the sequential reaction between ions to form clusters, shown in **Figure 1(a)**. Typically, this method produces highly stoichiometric crystals and can be finely controlled by the bath pH, temperature, and constituent concentrations. The second mechanism utilizing precipitation taking place in the bulk of the solution and known as cluster-by-cluster (three dimensional) growth is shown in **Figure 1(b)**. In general, there is less control over this latter mechanism, with the resulting structures deviating from stoichiometry calculations, often containing interstitial traps and producing unique optical and electrical material properties.
It is widely reported that the optoelectronic performance of chemically grown thin films is strongly dependent on the deposition mechanisms [30]. To evaluate this, the nonintrusive method of spectroscopic ellipsometry (SE) could be used to analyze the role of bath parameters on the deposition mechanisms. The films could then be subsequently analyzed using more intrusive methods to better understand the full scope of the relationship.
Efficient Optimization of the Optoelectronic Performance in Chemically Deposited Thin Films http://dx.doi.org/10.5772/67315 111
of sulfur from the thiourea requires complex decomposition and reaction. Although the stable
The existence of numerous approaches to growing films in solutions is illustrated in the basic reactions discussed in Eqs. (1)–(3). It should also be inferred that careful consideration of each chemical reaction by-product is necessary to prevent contamination of the film or undesired waste products. The structural properties, precipitation rates, crystallinities, and—of greater interest—optical and electrical performance of materials can thus be controlled with careful planning. Perhaps, the most promising category of materials derived from a theoretical understand-
perovskite powders for use in highly efficient organic-inorganic perovskite solar cells [22]. This would not have been possible without a deep theoretical understanding of the chemical reaction. Reliably predicting film performance and controlling the deposition mechanisms is a formidable challenge. Charge carrier mobilities, for example, depend on the grain size, layer composition, porosity, interstitial trapping, doping, contaminants, diffusion lengths, substrate, bond lengths, and a plethora of other factors [23–25]. Recent deviations between continuum
However, it has been shown that it is possible to conduct an in situ study of the deposition parameters, deduce growth mechanisms, and reproduce the film performance from such simulations [28]. Accumulated experimental data are then utilized to converge atomistic models for accurate computational predictions [29]. The remainder of this section will discuss the accumulated experimental data on the corporeal control of the CBD process and its effects on
Two main deposition mechanisms dominate thin-film growth during CBD. The first, ion-by-ion (two dimensional) mechanism is the sequential reaction between ions to form clusters, shown in **Figure 1(a)**. Typically, this method produces highly stoichiometric crystals and can be finely controlled by the bath pH, temperature, and constituent concentrations. The second mechanism utilizing precipitation taking place in the bulk of the solution and known as cluster-by-cluster (three dimensional) growth is shown in **Figure 1(b)**. In general, there is less control over this latter mechanism, with the resulting structures deviating from stoichiometry calculations, often containing interstitial traps and producing unique optical and electrical material properties.
It is widely reported that the optoelectronic performance of chemically grown thin films is strongly dependent on the deposition mechanisms [30]. To evaluate this, the nonintrusive method of spectroscopic ellipsometry (SE) could be used to analyze the role of bath parameters on the deposition mechanisms. The films could then be subsequently analyzed using
more intrusive methods to better understand the full scope of the relationship.
] is a common by-product of the thiourea decomposition, forma-
] has also been reported [21]. Such undesirable decom-
+ 2 OH− → CdS + CN2 H2 + 2 H2 O (3)
NH3 PbI3
molecule cyanamide [CN2
Cd (NH3)
film performance.
**2.2. CBD experimental data**
tion of reactive and toxic cyanide [CN−
H2
110 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
4
positions can have adverse effects on the overall deposition process:
<sup>2</sup><sup>+</sup> + SC (NH2)
and atomistic level simulations stress this point further [26, 27].
2
ing of CBD has been organic semiconductors. Recently, CBD was used to crystallize CH3
**Figure 1.** (a) Ion by ion and (b) cluster by cluster are the two dominating deposition mechanisms during chemical bath deposition. Ions permeate the solution precipitating on top of the substrate during growth in the ion-by-ion mechanism. As opposed to the cluster agglomerations forming in the bulk of the solution prior to attaching themselves to the substrate in the cluster-by-cluster mechanism.
SE analysis revealed three growth stages. The first stage was a short induction time, during which time little or no observable growth took place. Some of the material was observed to precipitate in the solution (although negligible amount showed adhesion to the substrate). The masses forming in the solution at this stage have not reached the critical diameter for crystallization. The second stage was a fairly linear growth period, dominated by the ion-by-ion mechanism.
This steady epitaxial growth produced highly compact stoichiometric films. The third stage was then dominated by the cluster-by-cluster mechanism in which clusters formed in the solution reached a critical size and could precipitate, producing a porous layer on top of the film. The porous layer typically established the surface roughness of the film. In some instances, this layer could be minimized by immediate washing following the chemical bath deposition [30]. Three distinct layers deposited by CBD are visible in the scanning electron microscope (SEM) images shown in **Figure 2**.
The nonintrusive SE measurements were used to study the effects of bath temperatures on the thickness of each layer. Data were collected on the films deposited at bath temperatures ranging from 55°C to 95°C. Physical models representative of the film structures were constructed consisting of three layers of quartz glass. The porosity and chemical composition of each layer were adjusted until a high correlation between the experimental and measured data was achieved [28].
It was found that at low bath temperature (i.e., 55°C) nearly 85% of the structure consisted of a highly stoichiometric and compact thin film. The porous surface layers in such films were ~1% of the entire structure thickness. This is indicative of substantial growth through the ionby-ion deposition mechanism, which overwhelmingly dominates at low bath temperatures. Increase in the bath temperature was accompanied by a steady transition to the cluster-bycluster mechanism. At a bath temperature of 95°C, the compact layer constituted ~60% of the entire structure thickness.
**Figure 2.** (a) Planar and (b) cross-sectional SEM reveal tight crystalline structure with three distinct layers.
The porous layer made up the majority of the rest of the film assembly [28]. These results indicate the coexistence of the two deposition mechanisms taking place in the bath during film formation. At lower temperatures, the ions are less likely to saturate the solution allowing the two-dimensional mechanism to dominate the film growth. As the temperature rises, the ion concentrations saturate and begin to form particulates in solution, causing the ion-by-ion mechanism to be supplemented by cluster-by-cluster growth.
It was realized that if a clear correlation between the optoelectronic performance of films and their growth mechanisms could be established, optimization of film performance through efficient means in the bath could be achieved. The high-impact nature of establishing this correlation prompted a significant amount of research on the topic.
One research topic of interest was the oxidation of the dangling bonds on the surface of stoichiometric crystalline films, which could be studied using X-ray photoelectron spectroscopy (XPS). XPS spectra of a CdS thin film as deposited on an n-InP substrate and a corresponding film after a 1-minute treatment with a buffer oxide etch (BOE) are shown in **Figure 3(a)** and **(b),** respectively. The ratio of S:Cd prior to the 1 min BOE etch is found to be ~0.6 and increases to ~0.85 after the etch. These results reveal a thin (~30 nm) passivation CdO layer forming on top of the films [31]. Similar results have been widely reported in various materials deposited by CBD method [19]. Such stoichiometric structures are explained by the dangling bonds on the surface of the materials.
In general, the valence electrons deep in the bulk of the material are committed to the covalent bonds between the elements. Electrons near the film surface are less constrained, resulting in the dangling bonds. This allows for the ambient oxygen to oxidize the materials at or near the surface, resulting in a thin layer of oxide being formed [19]. This thin oxidation layer prevents degradation of the films and introduces passivation properties in high-speed field-effect transistors. During subsequent depositions the layer acts as a buffer [31]. Oxidation removal from nanocrystalline thin films becomes an important step in efficiency enhancement of the monolithic structures, such as in CIGS. Similar passivation layer formations are not observed under three-dimensional (cluster by cluster) deposition mechanisms [28].
~1% of the entire structure thickness. This is indicative of substantial growth through the ionby-ion deposition mechanism, which overwhelmingly dominates at low bath temperatures. Increase in the bath temperature was accompanied by a steady transition to the cluster-bycluster mechanism. At a bath temperature of 95°C, the compact layer constituted ~60% of the
The porous layer made up the majority of the rest of the film assembly [28]. These results indicate the coexistence of the two deposition mechanisms taking place in the bath during film formation. At lower temperatures, the ions are less likely to saturate the solution allowing the two-dimensional mechanism to dominate the film growth. As the temperature rises, the ion concentrations saturate and begin to form particulates in solution, causing the ion-by-ion
**Figure 2.** (a) Planar and (b) cross-sectional SEM reveal tight crystalline structure with three distinct layers.
It was realized that if a clear correlation between the optoelectronic performance of films and their growth mechanisms could be established, optimization of film performance through efficient means in the bath could be achieved. The high-impact nature of establishing this cor-
One research topic of interest was the oxidation of the dangling bonds on the surface of stoichiometric crystalline films, which could be studied using X-ray photoelectron spectroscopy (XPS). XPS spectra of a CdS thin film as deposited on an n-InP substrate and a corresponding film after a 1-minute treatment with a buffer oxide etch (BOE) are shown in **Figure 3(a)** and **(b),** respectively. The ratio of S:Cd prior to the 1 min BOE etch is found to be ~0.6 and increases to ~0.85 after the etch. These results reveal a thin (~30 nm) passivation CdO layer forming on top of the films [31]. Similar results have been widely reported in various materials deposited by CBD method [19]. Such stoichiometric structures are explained by the dangling bonds on
In general, the valence electrons deep in the bulk of the material are committed to the covalent bonds between the elements. Electrons near the film surface are less constrained, resulting in the dangling bonds. This allows for the ambient oxygen to oxidize the materials at or near
mechanism to be supplemented by cluster-by-cluster growth.
relation prompted a significant amount of research on the topic.
entire structure thickness.
112 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
the surface of the materials.
**Figure 3.** XPS spectra of an n-InP sample with (a) 2 min CdS deposition at 75°C and (b) CdO removal with 1 min of BOE immersion.
Optoelectronic performance of films is known to be heavily dependent on the crystalline structure of the materials [32, 33]. Crystallinity is of great interest for lattice matching in thinfilm devices such as CIGS. Extensive X-ray diffraction (XRD) studies of film structures have been conducted and revealed that crystallinities are highly dependent on deposition temperatures. As an example, XRD data of CdS films deposited at various bath temperatures are shown in **Figure 4**. Lower deposition temperatures tend to deposit highly symmetric crystals—zinc blende (cubic) in the case of the CdS. As the bath temperatures are increased, a noticeable shift from symmetric to asymmetric structures is observed—wurtzite (hexagonal) structures in the case of CdS.
**Figure 4.** XRD patterns for CdS samples deposited at various bath temperatures. Spectra reveal a shift towards the hexagonal structures at higher deposition temperatures.
Under the three-dimensional deposition mechanism, much of the cluster agglomeration takes place in the bulk of the solution. These irregularly shaped clusters form the asymmetrical structures that attach themselves to the substrate. This is in contrast to the two-dimensional deposition mechanism which creates a uniform lateral expansion of the crystals. This contrast in film growth causes the structural differences produced by the two methods.
The cluster agglomeration, i.e., three-dimensional deposition mechanism, is expected to force interstitial trapping of the large cations in the bath. These trapped ions then act as dopants in the semiconductor, either releasing free electrons or introducing holes to the material. This drastically changes the optoelectronic performance of the films. Several groups pursued solid-state nuclear magnetic resonance (NMR) studies that substantiate this hypothesis. NMR analysis of three-dimensional deposited films reveals an increase in the peak intensity corresponding to cations intrinsic in the solution [28, 34].
These studies validate that the typically large cations of the inorganic thin films are trapped during the three-dimensional cluster agglomeration. Careful consideration of this phenomenon may be used to optimize film performance without the use of extrinsic dopants. Conversion efficiencies have been shown to improve nearly 5% by the careful use of interstitial trappings [34]. There is an additional challenge, however, that needs to be considered in order to achieve effective doping. Transition metals with large numbers of valence electrons are widely used as dopants in various inorganic thin films [35, 36].
However, work in this field demonstrated a limit in the doping efficiency for these metals [37, 38]. This is mainly due to formation of polyoxometalates between the transition metals and other ions in the chemical bath. The valence electrons that otherwise would be donated to the material are instead localized into formed complexes [39], which limit the doping efficiencies to ~1 donated electron per ion. This phenomenon is observed even in the materials with upward of seven valence electrons [37, 38]. Unpublished results from an ongoing study at the University of Virginia use interstitial iridium trapping to prove the possibility of overcoming theoretical doping efficiency limits.
Building on the aforementioned research, it is of great interest to tie the optoelectronic performance of the films to the growth mechanisms in the bath. Highly stoichiometric films grown by the ion-by-ion deposition mechanisms possess properties typical of intrinsic materials. Deviation from stoichiometry (i.e., introduction of the cluster-by-cluster mechanisms) noticeably changes the film characteristics. A case study of CdS is presented below.
Thin films of CdS fabricated under various deposition mechanisms were widely studied to show that the cluster-by-cluster growth mechanism produces a blend of crystalline structures. As previously mentioned, the XRD data revealed the formation of a blended cubic/ hexagonal structures as temperature increased (**Figure 4**) [40, 41].
The refractive index (*n*) and extinction coefficient (*k*) over the range of deposition temperatures were obtained utilizing multiwavelength ellipsometer and are shown in **Figure 5(a)** and **(b)**, respectively. Two maxima in the refractive index located at ~280 and ~410 nm are visible in the films deposited at 55°C bath temperatures. There is a noticeable shift in the location of the maxima at higher bath temperatures. The two maxima in the latter cases are found at ~475 and ~275 nm wavelengths. These maxima are well studied and understood to be the fundamental absorption peaks in the transition along Γ→A Brillouin zone (BZ) boundaries in the CdS structure [42]. The shift in the locations of the maxima, however, testifies to the structural changes taking place in the crystals. At low bath temperatures, the location of the maxima is found to match expectations for the cubic structured CdS.
Under the three-dimensional deposition mechanism, much of the cluster agglomeration takes place in the bulk of the solution. These irregularly shaped clusters form the asymmetrical structures that attach themselves to the substrate. This is in contrast to the two-dimensional deposition mechanism which creates a uniform lateral expansion of the crystals. This contrast
**Figure 4.** XRD patterns for CdS samples deposited at various bath temperatures. Spectra reveal a shift towards the
The cluster agglomeration, i.e., three-dimensional deposition mechanism, is expected to force interstitial trapping of the large cations in the bath. These trapped ions then act as dopants in the semiconductor, either releasing free electrons or introducing holes to the material. This drastically changes the optoelectronic performance of the films. Several groups pursued solid-state nuclear magnetic resonance (NMR) studies that substantiate this hypothesis. NMR analysis of three-dimensional deposited films reveals an increase in the peak intensity cor-
These studies validate that the typically large cations of the inorganic thin films are trapped during the three-dimensional cluster agglomeration. Careful consideration of this phenomenon may be used to optimize film performance without the use of extrinsic dopants. Conversion efficiencies have been shown to improve nearly 5% by the careful use of interstitial trappings [34]. There is an additional challenge, however, that needs to be considered in order to achieve effective doping. Transition metals with large numbers of valence electrons
in film growth causes the structural differences produced by the two methods.
responding to cations intrinsic in the solution [28, 34].
hexagonal structures at higher deposition temperatures.
114 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
are widely used as dopants in various inorganic thin films [35, 36].
At higher deposition temperatures, however, the maxima are located slightly below the expected locations for the hexagonal structures [43, 44]. A noticeable variation in the change of the onset of the extinction coefficient is also observed. This is suggestive of a change in the optical band gap. Such a shift has been widely reported and is attributed to the cubic-hexagonal transitions taking place in the CdS film structure [45]. In a greater context, the multiwavelength analysis testifies that the ion-by-ion deposition mechanism favors the crystallization of highly symmetric structures, whereas the cluster-by-cluster deposition mechanism tends to produce asymmetry in crystallization and deviation from stoichiometry. However, the resulting structures are rarely of a single phase, and even at higher bath temperatures, the two-dimensional deposition mechanism contributes significantly to the overall structure of the film.
Changes in the crystallinity of the materials have a significant impact on the overall optical properties. Optical performance of the aforementioned CdS films was studied in an effort to understand the growth mechanism/optical performance relationship. Significant changes in the reflectance and absorbance spectra are observed with the rise of the hexagonal phase in the film, as shown in **Figure 5(c)** and **(d)**, respectively. The three observed dips in the reflectance and absorbance data coincide with the valence band splits in Γ<sup>9</sup> , Γ7 , and Γ<sup>5</sup> previously reported for CdS [46]. Reflectance is significantly higher in three-dimensionally grown films. The absorbance curve shows a steeper slope for the low deposition temperatures and a fairly flat absorption tail. These results suggest lower reflectance and higher absorbance of the symmetric structures deposited by two-dimensional mechanisms. Such results would be expected in the epitaxially grown films. The random distribution of the clusters produced by the threedimensional mechanisms scatters light resulting in less favorable optical properties.
**Figure 5.** (a) Refractive index (*n*), (b) extinction coefficient (*k*), (c) reflectance, and (d) absorbance over a range of deposition temperatures.
Analysis of the electrical performance of CdS films deposited under different conditions was also conducted. Summary of the film's electrical properties is shown in **Table 1**. Cd:S ratios were computed from the XPS spectra and validated using energy-dispersive spectroscopy (EDS). Cd and S constituted the majority of the film composition. Traces of C, Ca, and Na contaminants were also observed but in negligible amounts. As can be seen from the data, the films deposited at low bath temperatures are highly stoichiometric.
This is evident from the 1:1 ratio between the Cd2+ and S2− ions. Films deviate significantly from stoichiometry at higher bath temperatures, reaching ~1.67 Cd2+ ions per one S2−. This is understood from the previous discussion of the interstitial trapping. The large Cd2+ ions are caught in the lattice, offsetting the Cd:S ratio; correspondingly, the electrical performance was observed to be enhanced.
**Table 1.** Deposition temperatures and film parameter summary [28].
For example, the carrier concentrations (*n*) increase over sevenfold. This is achieved without sacrificing the carrier mobility (*μ*) which also rises nearly four times. Resistivity (*ρ*) decreases over threefold as well. This phenomenon is contrary to the common empirical relationship between carrier concentrations and mobility [47, 48]. This is partially due to the larger grain sizes in the cluster-by-cluster grown lattices. More importantly, however, this is suggestive that unlike extrinsic doping, the interstitially trapped dopants contribute carriers without following the inverse proportional relationship between carrier concentrations and mobility. This is a significant result, allowing for semiconducting thin films to overcome current limitations.
#### **2.3. CBD conclusions**
**Figure 5.** (a) Refractive index (*n*), (b) extinction coefficient (*k*), (c) reflectance, and (d) absorbance over a range of
Analysis of the electrical performance of CdS films deposited under different conditions was also conducted. Summary of the film's electrical properties is shown in **Table 1**. Cd:S ratios were computed from the XPS spectra and validated using energy-dispersive spectroscopy (EDS). Cd and S constituted the majority of the film composition. Traces of C, Ca, and Na contaminants were also observed but in negligible amounts. As can be seen from the data, the
This is evident from the 1:1 ratio between the Cd2+ and S2− ions. Films deviate significantly from stoichiometry at higher bath temperatures, reaching ~1.67 Cd2+ ions per one S2−. This is
films deposited at low bath temperatures are highly stoichiometric.
the reflectance and absorbance spectra are observed with the rise of the hexagonal phase in the film, as shown in **Figure 5(c)** and **(d)**, respectively. The three observed dips in the reflec-
reported for CdS [46]. Reflectance is significantly higher in three-dimensionally grown films. The absorbance curve shows a steeper slope for the low deposition temperatures and a fairly flat absorption tail. These results suggest lower reflectance and higher absorbance of the symmetric structures deposited by two-dimensional mechanisms. Such results would be expected in the epitaxially grown films. The random distribution of the clusters produced by the three-
dimensional mechanisms scatters light resulting in less favorable optical properties.
, Γ7
, and Γ<sup>5</sup>
previously
tance and absorbance data coincide with the valence band splits in Γ<sup>9</sup>
116 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
deposition temperatures.
From the presented research, it is evident that the growth mechanism during chemical bath deposition can follow several routes during the fabrication of thin film, which can affect their performance. Careful study of the deposition mechanisms allows for a controlled deposition of the film with the desired optoelectronic properties. Furthermore, control of the growth mechanism can allow for the theoretical doping limits to be overcome and for the exploitation of novel film regimes. All of this can be achieved simply by controlling the deposition temperature with negligible changes in the deposition costs.
## **3. Chemical vapor deposition (CVD)**
CVD is another promising chemical deposition method for the production of high-quality thin films. Although this method was developed and studied since the 1960s, the recent interest in organic and two-dimensional semiconductors has reinvigorated the field. Perhaps, the most attractive characteristic of CVD is just how effective and versatile it can be [49]. Nanostructured graphene, carbon nanotubes, CH3 NH3 PbI3 perovskites with applications in optoelectronics, solid oxide fuel cells, batteries, sensors, and high-performance organic photovoltaics are all manufactured using CVD [50–52]. Moreover, this method has been shown to be adaptable for the deposition of a single-layer material and scalable for mass production [53, 54]. Another advantage of CVD is the conformity of the deposited films (i.e., the thicknesses and grain sizes near the substrate edges are comparable across the sample) [55]. Hence, such films can be deposited on elaborate shapes, inside underlying features, and in high aspect ratio holes. Thus, there are virtually no limits to the types of films that may be deposited using this method. CVD does not discriminate between transition metals, heavy metals, organics, and inorganics and has the ability to deposit all of these without distorting the structures of the films. Additionally, cost-effective distillation of the precursors allows for deposition of high-purity films. The power of this method cannot be overstated. Chemical vapor methods yield to chemical bath only in the areas of deposition surface size and the equipment cost.
#### **3.1. CVD theoretical considerations**
CVD is influenced by numerous factors. For example, the type, shape, size of the reactor, gas flow, flow rates, flow order, arrangements coating, and substrates all affect the overall deposition results and mechanisms. The deposition reactions themselves may require complicated reaction schemes, involving pyrolysis, reduction, oxidation, disproportionation, hydrolysis, or some combination of each [56]. Despite the various approaches to CVD, in general, the results are achieved in a linear sequence. First, reagents are applied to the substrate to create an initial kinetic barrier. This barrier needs to be permeated by the gaseous diffusion prior to the preliminary reactions. Initial absorption then takes place on the substrate surface followed by reactions among the chemical constituents which results in nucleation. As with the CBD study, careful consideration of the CVD experiments allows for an effective tunability of the film characteristics. Here, we present a short theoretical discussion.
In general, the flow of the gases is assumed to be laminar, with zero velocity near the surface of the substrate, and increasing linearly to a constant value at some distance from the substrate. In such an approximated case, the boundary layer theory (BLT) is used for the study of the reaction dynamics [57]. This approach couples the chemical and mass transport processes on the heated substrate surface with a gas flow.
The free energy of the chemical reaction is analogous to Gibbs free energy and may be easily shown to be
$$
\Delta G = \sum\_{\text{full}}^{\text{products}} \Delta \, G\_{\text{products}} - \sum\_{\text{full}}^{\text{reactants}} \Delta \, G\_{\text{reactants}} \tag{4}
$$
where ∆*G* is related to the equilibrium constant *kp* :
$$
\Delta G = \text{2.3RT } \log \left( k\_p \right) \tag{5}
$$
Due to vapors utilized in the CVD process, the equilibrium constant is related to the partial pressures of the reactants and products. It is of greater interest, however, to relate the equilibrium constant in terms of concentrations. This is achieved using the ideal gas law [58]. The resulting free energy of the system consisting of *g* gaseous and *s* solid phases is thus
Efficient Optimization of the Optoelectronic Performance in Chemically Deposited Thin Films http://dx.doi.org/10.5772/67315 119
$$
\Delta G = \sum\_{i=t\_s}^{\mathcal{S}} \left[ \eta\_{\mathcal{S}} \Delta \, G\_{\mathcal{S}} + RT \ln(P) + 2T \ln \left( \frac{\eta\_{\mathcal{S}}}{N\_{\mathcal{S}}} \right) \right] + \sum\_{i=1}^{\mathcal{S}} \eta\_{\mathcal{S}} \Delta \, G\_{\mathcal{S}} \tag{6}
$$
where *ng* and *ns* are the number of moles of a particular reagent in the gaseous and solid state, respectively, and *Ng* is the total number of moles of all gaseous components. *P* and *T* are the total pressure and temperature, respectively. The Δ*Gg* and *Gs* are the free energy of formation at specific temperatures for the gaseous and solid species, respectively. Thus, the equation can be solved iteratively for the free energy minima, i.e., the point at which nucleation will commence. An analysis of the equation also reveals that the reagent concentrations can be offset by the pressure and temperature in the CVD chamber.
Hypothetically, it should be possible to conduct thin-film growth at low pressures and temperatures with high reagent concentrations; although such an approach would not be most efficient from the chemical perspective, it would, however, help alleviate the requirement for sophisticated equipment. Further, the low-temperature approach allows for the deposition of highly sought organic materials. Of course, there is also a limit to the operability temperatures, since at some pressure and temperature the molecular gases will liquefy.
The analogousness between the CBD and CVD methods should be obvious from the brief theoretical introduction. The free energy of the chemical reaction, Eq. (6), shows that reagent concentrations, pressure, and temperature are the control parameters of interest in CVD. Thus, similar to CBD method, the optoelectronic performance of the CVD grown films will depend heavily on the deposition parameters and mechanisms.
#### **3.2. CVD experimental data**
graphene, carbon nanotubes, CH3
**3.1. CVD theoretical considerations**
NH3 PbI3
118 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
ical bath only in the areas of deposition surface size and the equipment cost.
film characteristics. Here, we present a short theoretical discussion.
*i*=1 #products
on the heated substrate surface with a gas flow.
where ∆*G* is related to the equilibrium constant *kp*
Δ*G* = ∑
shown to be
solid oxide fuel cells, batteries, sensors, and high-performance organic photovoltaics are all manufactured using CVD [50–52]. Moreover, this method has been shown to be adaptable for the deposition of a single-layer material and scalable for mass production [53, 54]. Another advantage of CVD is the conformity of the deposited films (i.e., the thicknesses and grain sizes near the substrate edges are comparable across the sample) [55]. Hence, such films can be deposited on elaborate shapes, inside underlying features, and in high aspect ratio holes. Thus, there are virtually no limits to the types of films that may be deposited using this method. CVD does not discriminate between transition metals, heavy metals, organics, and inorganics and has the ability to deposit all of these without distorting the structures of the films. Additionally, cost-effective distillation of the precursors allows for deposition of high-purity films. The power of this method cannot be overstated. Chemical vapor methods yield to chem-
CVD is influenced by numerous factors. For example, the type, shape, size of the reactor, gas flow, flow rates, flow order, arrangements coating, and substrates all affect the overall deposition results and mechanisms. The deposition reactions themselves may require complicated reaction schemes, involving pyrolysis, reduction, oxidation, disproportionation, hydrolysis, or some combination of each [56]. Despite the various approaches to CVD, in general, the results are achieved in a linear sequence. First, reagents are applied to the substrate to create an initial kinetic barrier. This barrier needs to be permeated by the gaseous diffusion prior to the preliminary reactions. Initial absorption then takes place on the substrate surface followed by reactions among the chemical constituents which results in nucleation. As with the CBD study, careful consideration of the CVD experiments allows for an effective tunability of the
In general, the flow of the gases is assumed to be laminar, with zero velocity near the surface of the substrate, and increasing linearly to a constant value at some distance from the substrate. In such an approximated case, the boundary layer theory (BLT) is used for the study of the reaction dynamics [57]. This approach couples the chemical and mass transport processes
The free energy of the chemical reaction is analogous to Gibbs free energy and may be easily
Δ *G*products − ∑
Δ*G* = 2.3*RT* log(*kp*) (5) Due to vapors utilized in the CVD process, the equilibrium constant is related to the partial pressures of the reactants and products. It is of greater interest, however, to relate the equilibrium constant in terms of concentrations. This is achieved using the ideal gas law [58]. The
resulting free energy of the system consisting of *g* gaseous and *s* solid phases is thus
:
*i*=1 #reactants
Δ *G*reactants (4)
perovskites with applications in optoelectronics,
Low vapor temperatures result in less random scattering of reagents; thus, clusters, ~10 μm in diameter, form in the flowing gas prior to reaching the substrate. These clusters then coalesce on the surface of the substrate. The formation of CH<sup>4</sup> /H2 clusters is shown in **Figure 6(a)** and **(b)** [59]. The semisolid state of the clusters allows for coagulation with other clusters as they hit the substrate. In this case thermal diffusivity of the materials will determine the final structure of the materials [60]. In materials with low thermal diffusivity, e.g., organics, the structure resulting from the cluster-by-cluster deposition is similar to that of epitaxial growth. Consequently, materials with low thermal conductivity can be deposited at low temperatures. Thus, it follows that theoretically it may be possible to achieve firm crystalline structures and optimal optoelectronic performance of the films without the need for high deposition temperatures.
**Figure 6.** SEM images of carbon cluster formations on mirror-polished substrates.
Reagent concentrations also greatly affect the structural quality of the materials and their optoelectronic performance. For example, graphene deposited under high reagent concentrations showed high disorder, requiring synthesis of additional layers [61]. Raman analysis of graphene deposited under various methane concentrations partially reveals the causes behind the phenomenon, as shown in **Figure 7**. The data reveal an upshift of ~5 cm−1 and a downshift of ~6 cm−1 in the D and 2D peaks, respectively. Extensive research on such peak dispersions was shown to be caused by the formation of additional graphene layers [62].
**Figure 7.** Raman spectra of CVD synthesized graphene with a Cu catalyst and SiO2 substrates.
Furthermore, a significant increase in the D peak intensity is observed. This is typical of an increase in disorder [63]. The types of the disorders, whether layer or defect related, remain to be determined. Cumulatively, the results show that an increase in reagent concentrations produces mismatched layers and film defects. This causes anharmonic interactions between the phonons and electron-hole pairs [64], having a significant impact on the performance of such films. It is of great interest to conduct further research in minimizing layer mismatches and film defects. At present, this work is ongoing.
Preliminary results of the CVD flow rates and gas purity studies also show an effect on the optoelectronic performance of chemically deposited films. This research, however, is in the infancy stage and requires further analysis and effort.
There is a great potential for the efficient optimization of the optoelectronic performance of CVD deposited materials. Significant additional research into the deposition parameters and their effects on the growth mechanisms and optoelectronic performance will be required to fully understand the effects of each deposition parameter. It is expected that these issues will be resolved in the near future, allowing for effective optimization of these types of advanced materials.
#### **3.3. CVD conclusions**
In the optimization of CVD materials, thus far, the growth mechanisms have not been completely elucidated. There is, however, a strong correlation between the optoelectronic performance and the deposition parameters. Much more research in the field of CVD modern-advanced materials is required but, once solved, will allow for an efficient optimization of such films.
## **4. Conclusion**
Reagent concentrations also greatly affect the structural quality of the materials and their optoelectronic performance. For example, graphene deposited under high reagent concentrations showed high disorder, requiring synthesis of additional layers [61]. Raman analysis of graphene deposited under various methane concentrations partially reveals the causes behind the phenomenon, as shown in **Figure 7**. The data reveal an upshift of ~5 cm−1 and a downshift of ~6 cm−1 in the D and 2D peaks, respectively. Extensive research on such peak dispersions was shown to be caused by the formation of additional graphene layers [62].
Furthermore, a significant increase in the D peak intensity is observed. This is typical of an increase in disorder [63]. The types of the disorders, whether layer or defect related, remain to be determined. Cumulatively, the results show that an increase in reagent concentrations produces mismatched layers and film defects. This causes anharmonic interactions between the phonons and electron-hole pairs [64], having a significant impact on the performance of such films. It is of great interest to conduct further research in minimizing layer mismatches
substrates.
Preliminary results of the CVD flow rates and gas purity studies also show an effect on the optoelectronic performance of chemically deposited films. This research, however, is in the
There is a great potential for the efficient optimization of the optoelectronic performance of CVD deposited materials. Significant additional research into the deposition parameters and their effects on the growth mechanisms and optoelectronic performance will be required to fully understand the effects of each deposition parameter. It is expected that these issues will be resolved in
In the optimization of CVD materials, thus far, the growth mechanisms have not been completely elucidated. There is, however, a strong correlation between the optoelectronic performance and
the near future, allowing for effective optimization of these types of advanced materials.
and film defects. At present, this work is ongoing.
**Figure 7.** Raman spectra of CVD synthesized graphene with a Cu catalyst and SiO2
120 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
infancy stage and requires further analysis and effort.
**3.3. CVD conclusions**
This chapter discussed the growth mechanics, characteristics, and optimization of the optoelectronic performance in the chemically deposited materials. Two methods of interest are the chemical bath and chemical vapor depositions. Much more work has been completed in the field of CBD, but CVD is showing great promise in the deposition of novel advanced materials. The growth mechanisms are well understood for the chemical bath but remain to be elucidated for chemical vapor.
Once the growth mechanics are firmly established, it is possible to manipulate the chemical composition and other deposition parameters to efficiently optimize the optoelectronic film performance. Such results are abundant for the chemical bath, as is evident from the CdS case presented above, while for chemical vapor deposition, the research is ongoing. Each of these technologies is continuing to find uses in increasingly complicated manufacturing applications. It is expected that the ongoing research will enable new technologies for a wide variety of applications.
## **Acknowledgements**
The authors are grateful to the contributions of many researchers at the University of Virginia and the NVESD, who graciously provided their results and experimental data for the publication. These contributions are paramount in advancing the development of novel thin films and manufacturing methods.
## **Author details**
Andre Slonopas<sup>1</sup> \*, Nibir K. Dhar<sup>1</sup> , Herbert Ryan2 , Jerome P. Ferrance3 , Pamela Norris<sup>4</sup> and Ashok K. Sood<sup>5</sup>
\*Address all correspondence to: [email protected]
1 U.S. Army RDECOM CERDEC Night Vision and Electronic Sensors Directorate, Fort Belvoir, VA, USA
2 Bitome Inc., Boston, MA, USA
3 Department of Chemistry, University of Virginia, Charlottesville, VA, USA
4 Department of Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, VA, USA
5 Magnolia Optical Technologies, Inc., Woburn, MA, USA
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#### **Thin Films as a Tool for Nanoscale Studies of Cement Systems and Building Materials Thin Films as a Tool for Nanoscale Studies of Cement Systems and Building Materials**
Vanessa Rheinheimer and Ignasi Casanova Vanessa Rheinheimer and Ignasi Casanova
Additional information is available at the end of the chapter Additional information is available at the end of the chapter
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#### **Abstract**
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Many efforts have been made over the last decades to improve and develop new technologies for cement and chemical industries that can provide materials that are more durable and cost efficient, stronger and less environmentally harmful. Studies at small scale in cementitious materials usually require special sample preparation, which can damage the material and mislead the analysis. In nanoscale experiments, several techniques require samples to be extremely thin, while others need the samples to be very flat. The possibility of using thin films of clinker phases in cement research opens far-reaching opportunities for the development of this material and the materials associated to this. Testing different evaporation parameters, the deposition of films with a few tens of nanometers in thickness was achieved for all the clinker phases individually. This chapter will present the attempts for synthesizing thin films of all main clinker phases by the use of electron beam evaporation technique, as well as data on the hydration of the calcium silicate thin, flat and homogeneous samples. Changes are tracked chemically and mineralogically. This study redirects cement science to new perspectives of understanding the nanostructure of cement products. This leads to basis for developing stronger and more durable cement-based materials.
**Keywords:** cement, clinker phases, thin films, electron beam evaporation
## **1. Introduction**
The Portland cement is an inorganic material, which results from burning and grounding of a raw material containing CaO, SiO2, Al2O3 and small quantities of other materials. Its mixture with water results in a cement paste that sets and hardens due to a reaction called hydration. The result is a strong material that is stable even under aqueous conditions.
© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Cement hydration reactions take place simultaneously and in a very complex way, and understanding the behavior of such systems, with the ultimate purpose of improving durability, strength, cost and environmental impact, often requires studies at nanoscale [1].
These experiments in cementitious materials normally involve special sample preparation that can damage the material, change its properties or make it difficult to run analysis due to certain morphological characteristics of the material.
X-ray photoelectron spectroscopy (XPS) requires no special sample preparation for measurements, allowing progressive ion milling and analysis of flat surfaces to follow the progress of surface reactions [2]. In addition, in situ analyses of samples provide information not only about chemical composition but also changes in the coordination state of the elements; this has been successfully used to follow the progress of cement hydration reactions and subsequent polymerization of the materials [3–11].
Clinker is composed mainly of calcium silicates, calcium aluminate and calcium aluminoferrite. Its hydration is a very complex process, involving several phases and the formation of secondary products. Subsequently, properties of concrete are governed by processes at a molecular level with single-crystal formation. Thus, the durability and behavior of concrete during its service life strongly depend on its nanostructure.
Here, electron beam evaporation methods have been used to evaporate different clinker phases individually to synthesize thin films in an effort for producing samples suitable for nanoscale studies of cementitious materials and avoiding typical problems of sample preparation when studying these materials. This kind of sample permits elemental studies intending to design the behavior of each phase individually and the cement itself, at nanoscale, providing tools for understanding and changing the materials characteristics, looking forward to obtain better performance and durability.
Thin films techniques are widely used to produce ceramic films in the semiconductor, aerospace and optics industry, but thus far have not been applied to clinker phases. This study is an effort to develop integrated tools that allow improving the knowledge of early stage clinker phases' hydration at the molecular level and with this to better understand the behavior of these materials. With this, this work is aimed at producing clinker phase thin films suitable for hydration studies using electron beam evaporation techniques. Evaporation of each phase was repeated several times in order to optimize the most favorable experimental conditions and assess the reproducibility of the process.
## **2. Synthesis of clinker phases' thin films**
This session describes the attempt to synthesize thin films of the four main clinker phases by means of electron beam evaporation and the outcomes. Clinker is the product from the sintering of limestone and clay during the cement manufacturing, composed mainly of four components: tricalcium silicate (C3S), β-dicalcium silicate (C2S), tricalcium aluminate (C3A) and ferrite (C4AF). Notation is the standard used in cement chemistry. Commercially available clinker phases were used here as bulk material. All the components were 3600 cm2 /g, with free lime content below 0.5%.
Thin films were prepared with electron beam evaporation by sending a current through a tungsten filament outside the deposition zone to avoid any contamination. The filament is heated until the start of the electrons thermionic emission. Magnets focus and direct the electrons toward the bulk material that is in a constantly cooled crucible. When the electron beam hits the evaporant's surface, kinetic energy is converted into heat, releasing high energy. The evaporant boils and evaporates, condensing on the substrate and all surfaces inside the vacuum chamber. Thicknesses of the films are a function of irradiation time, estimated by an in situ quartz deposition controller.
The bulk powder phases were placed in crucibles previously cleaned with isopropyl alcohol and mounted in a vacuum chamber Univex 450B Oerlikon Leybold in a clean room. A power supply controller Telemark with a beam generating system and beam deflection unit with electromagnetic deflection for the x- and y-axis was used in the electron beam evaporator.
**Table 1.** Parameters for each evaporation.
Cement hydration reactions take place simultaneously and in a very complex way, and understanding the behavior of such systems, with the ultimate purpose of improving durability, strength, cost and environmental impact, often requires studies at nanoscale [1].
These experiments in cementitious materials normally involve special sample preparation that can damage the material, change its properties or make it difficult to run analysis due to certain
X-ray photoelectron spectroscopy (XPS) requires no special sample preparation for measurements, allowing progressive ion milling and analysis of flat surfaces to follow the progress of surface reactions [2]. In addition, in situ analyses of samples provide information not only about chemical composition but also changes in the coordination state of the elements; this has been successfully used to follow the progress of cement hydration reactions and subsequent
Clinker is composed mainly of calcium silicates, calcium aluminate and calcium aluminoferrite. Its hydration is a very complex process, involving several phases and the formation of secondary products. Subsequently, properties of concrete are governed by processes at a molecular level with single-crystal formation. Thus, the durability and behavior of concrete
Here, electron beam evaporation methods have been used to evaporate different clinker phases individually to synthesize thin films in an effort for producing samples suitable for nanoscale studies of cementitious materials and avoiding typical problems of sample preparation when studying these materials. This kind of sample permits elemental studies intending to design the behavior of each phase individually and the cement itself, at nanoscale, providing tools for understanding and changing the materials characteristics, looking forward to obtain better
Thin films techniques are widely used to produce ceramic films in the semiconductor, aerospace and optics industry, but thus far have not been applied to clinker phases. This study is an effort to develop integrated tools that allow improving the knowledge of early stage clinker phases' hydration at the molecular level and with this to better understand the behavior of these materials. With this, this work is aimed at producing clinker phase thin films suitable for hydration studies using electron beam evaporation techniques. Evaporation of each phase was repeated several times in order to optimize the most favorable experimental conditions
This session describes the attempt to synthesize thin films of the four main clinker phases by means of electron beam evaporation and the outcomes. Clinker is the product from the sintering of limestone and clay during the cement manufacturing, composed mainly of four components: tricalcium silicate (C3S), β-dicalcium silicate (C2S), tricalcium aluminate (C3A) and ferrite (C4AF). Notation is the standard used in cement chemistry. Commercially available
morphological characteristics of the material.
128 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
polymerization of the materials [3–11].
performance and durability.
and assess the reproducibility of the process.
**2. Synthesis of clinker phases' thin films**
during its service life strongly depend on its nanostructure.
Silicon wafers with a 100 crystallographic orientation were used as deposition substrate, placed in a plate 30 cm above the crucible and held by metallic clips, and kept at ambient temperature. At a stable pressure, 4 sccm of a mix of 50% oxygen and 50% argon was inserted into the chamber to compensate the possible oxygen lost. Parameters used for each evaporation are listed in **Table 1**. The temperature in the silicon wafers was monitored and showed to be constant at room temperature during all the process; however, it is not possible to confirm the temperature achieved in the bulk sample during the evaporation.
**Table 2.** Thicknesses of the thin films.
Each sample behaved differently when bombarded by the electron beam: calcium silicate powders sublimated and did not liquefy, while during the evaporation of calcium aluminates, the powder seemed to liquefy and higher intensities caused blasts. The resulting thin films had the thickness described in **Table 2**, measured with a profilometer.
X-ray diffraction (XRD) investigations of the bulk material were carried out in a Bruker D83 Advanced diffractometer operated at an accelerating voltage of 40 keV on a CuKα anode, irradiation intensity of 40 mA and 40 scans in steps of 0.02°/s, and results are described in the next section.
**Table 3.** Peak position and (FWHM) for all samples.
On the other hand, due to the small diffracting volumes, which result in low intensities compared to the substrate and background, mineralogical phases on thin films are difficult to be identified. GIXRD allows increasing the intensity of the signal produced from the film by increasing the path length of the incident X-ray beam through it, so that conventional phase identification analysis can be run [12–14]. GIXRD experiments were performed using a Bragg-Brentano Siemens D-500 X-ray diffractometer at 45 kV accelerating voltage, on a CuKα anode, with 40 mA irradiation intensity and 0.05°/s steps, with an incident angle of w = 0.4°. No monochromator was used to increase the signal. Different detector apertures were used for each sample, depending on their size.
Chemical composition of a material's surface was assessed by XPS, which works by irradiating the sample with a X-rays beam and measuring both the kinetic energy and the number of electrons escaping from its surface.
Chemical composition of the thin films was verified using a SPECS™ X-ray photoelectron spectroscopy system equipped with an XR50 Al anode source operating at 150 W and a Phoibos MCD-9 detector. Spectra were recorded with pass energy of 25 eV at 0.1 eV steps at a pressure below 10-9 mbar.
For analysis of the bulk material, powder was pressed into pellets and fixed onto holders with a copper tape, same fixing as the silicon wafers with the thin films. For each sample, general scan was repeated three times, and specific high-resolution scans were carried out for elements of interest, such as calcium, silicon, carbon, oxygen, iron and aluminum. Data were extracted from the spectra via peak fitting using CasaXPS™ software. Shirley background was assumed in all cases. The adventitious carbon peak at 284.8 eV was used for correction of the charging effects. Three scans with a passing energy of 5 eV were carried out for the acquisition of each pattern.
XPS binding energies (BEs) and full width half maximum (FWHM) peak parameters are shown in **Table 3**.
#### **2.1. Calcium silicates**
the powder seemed to liquefy and higher intensities caused blasts. The resulting thin films had
X-ray diffraction (XRD) investigations of the bulk material were carried out in a Bruker D83 Advanced diffractometer operated at an accelerating voltage of 40 keV on a CuKα anode, irradiation intensity of 40 mA and 40 scans in steps of 0.02°/s, and results are described in the
**Ca 2p 1/2 Ca 2p 3/2 Si 2p O 1s Al 2p Fe 2p**
531.6 (2.80)
531.6 (2.75)
533.4 (2.19) –
530.6 (2.92) 74.1 (2.25)
531.1 (2.68) 723.8 (2.29)
530.7 (2.82) 723.9 (2.57)
531.4 (2.12) 723.0 (3.41)
711.8 (3.33)
725.2 (3.43)
712.6 (2.92)
725.8 (3.80)
101.7 (2.70) 531.3 (3.02)
the thickness described in **Table 2**, measured with a profilometer.
130 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
C3S bulk 350.2 (2.5) 346.8 (2.46) 101.0 (2.83) 530.1 (2.35)
C3S thin film 350.2 (1.85) 346.8 (2.07) 98.2 (1.81) 529.8 (1.69)
C2S bulk 350.3 (2.41) 346.7 (2.28) 101.0 (2.68) 530.1 (2.23)
C2S thin film 350.4 (2.24) 346.8 (2.13) 100.8 (2.08) 530.0 (1.59)
351.1 (1.85) 347.5 (2.41) 100.6 (1.83) 531.2 (2.57)
C3A bulk 350.3 (2.43) 346.8 (2.28) 531.4 (2.15) 73.2 (2.48)
C3A crucible 350.4 (2.03) 346.9 (2.05) 529.7 (1.5) 73.6 (1.72)
C4AF bulk 350.4 (2.25) 346.9 (2.11) 529.0 (1.95) 73.1 (2.32) 710.2 (2.60)
C4AF thin film 351.1 (2.24) 347.6 (2.44) 529.8 (1.45) – 710.4 (2.91)
C4AF crucible 349.9 (2.51) 346.4 (2.51) 529.4 (2.13) 73.2 (1.92) 709.8 (3.70)
C3A thin film 350.6 (2.17) 347.0 (2.05) 531.5 (2.24) –
102.4 (2.48)
**Sample Peak position (eV) and (FWHM)**
**Table 3.** Peak position and (FWHM) for all samples.
next section.
The similarity between the GIXRD patterns of the bulk material and the XRD of the thin film (**Figure 1a**) strongly suggests that the evaporated and recondensed material keeps the mineralogical composition of the original bulk material. Even with the relatively faint signal observed due to the thickness of the film and the high noise level, the presence of the most intense peaks of C3S (at 32.07°, 34.29° and 32.33°) is clearly noticed as a broad peak in the pattern. This broadening in the peaks has been observed in several other researches when working with thin films, specially using Bragg-Brentano configuration [15–17]. This is likely due to the thin film's poor crystallinity.
Similarly to C3S, the C2S samples show a strong match between the diffraction patterns of bulk material and thin film (**Figure 1b**). The same low signal and high noise are observed, but the presence of the most intense peaks of C2S (at 31.98°, 32.38° and 40.55°) is coincident with the broad peak in the thin film spectrum, suggesting that the evaporated material keeps the bulk mineralogical composition. Even the broad peak at low degrees is observed in both spectra, which can be either due to the presence of amorphous material or an artifact of the technique due to the small size of the sample.
**Figure 1.** (a) C3S powder (top) and grazing incident angle (bottom) XRD from the bulk and thin film samples, respectively. (b) C2S powder (top) and grazing incident angle (bottom) XRD from the bulk and thin film samples, respectively.
**Figure 2.** Top: XPS spectra of the C3S (top) and C2S (bottom) bulk material and thin film. Thick lines are the spectra collected, while the thin lines are the background and the result of peak fitting.
On the other hand, a comparison between XPS patterns from bulk material and the thin film shows that the elements are present in the same elemental coordination state. Some changes are noticed though, as an additional calcium peak at 351.1 eV, which can be related to prehydration on the surface due to the contact with the atmosphere. The same behavior was observed in previous researches [3, 4]. This is confirmed by the silicon peak, which presents a split into two peaks at 102.4 and 100.6 eV. This shift to higher BE is related to pre-hydration and the formation of C-S-H [18]. At the same time, there is the presence of a peak related to the metallic silicon from the thin film (98.2 eV), which is associated with the substrate and is probably due to the porosity of the layer, despite its thickness (**Figure 2**); hence, reaction with the substrate cannot be totally discarded [19]. Changes in the oxygen peak positions are also related to this pre-hydration effect.
Likewise, XPS results assure that the evaporated C2S chemical material is very similar to the bulk material, with the peaks located at the same BE and with similar FWHM values (**Figure 2**), and the appearance of a new silicon peak, together with the slight shifts in the oxygen peaks energy, can also be attributed to the substrate as well as to pre-hydration. On the other hand, magnesium (a minor component in the original bulk material) is also detected in the thin film spectrum.
#### **2.2. Calcium aluminates**
which can be either due to the presence of amorphous material or an artifact of the technique
**Figure 1.** (a) C3S powder (top) and grazing incident angle (bottom) XRD from the bulk and thin film samples, respectively. (b) C2S powder (top) and grazing incident angle (bottom) XRD from the bulk and thin film samples, respectively.
**Figure 2.** Top: XPS spectra of the C3S (top) and C2S (bottom) bulk material and thin film. Thick lines are the spectra
collected, while the thin lines are the background and the result of peak fitting.
due to the small size of the sample.
132 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
C3A films exhibit better-defined GIXRD peaks (**Figure 3a**), less noise and higher intensity than the silicate samples. This is due to the larger sample used for the test, which provides larger area for analysis, since the detector aperture can be larger, allowing more signal to be collected. In this case, the film produced from the evaporation of C3A was formed by other phases (including calcite, lime and portlandite; **Figure 4a**), but no phases with aluminum are identified.
The evaporated C4AF has a similar behavior to C3A. In this case, the GIXRD spectrum is not conclusive as it is extremely noisy and indicates the formation of amorphous materials (**Figure 3b**). For the bulk material, the peaks correspond to Brownmillerite, or C4AF, except for one peak at 18.11°, which may be related to the presence of Fe3O4, which cannot be discarded, as discussed later in the XPS analysis.
Diffraction data are confirmed by the XPS analyses, which show that the calcium is deposited in similar chemical state as in the bulk material (shifts of 0.2–0.3 eV), but the aluminum peak does not appear in the thin film spectrum, meaning that it did not recondense on the substrate (**Figure 4**).
C4AF bulk and thin film present Fe 2p peaks with two components, indicating the presence of two compounds (**Table 3**, **Figure 4**). The BE themselves are not conclusive of what compounds are present, as the observed BEs are similar to the ones of Fe2O3, Fe3O4 and FeO. While the Fe 2p components at lower BE present negligible changes from the bulk to the thin film, the second component at higher energies presents significant changes (about +0.7 eV); this indicates strong bonding changes in iron after the evaporation. Since differences in the O 1s BE are observed as well by shifts of +0.4 and +0.8 eV, differences between this two samples are clear. Finally, Ca 2p peaks shift to higher energies by +0.7 eV, indicating again differences in the thin film material.
**Figure 3.** (a) C3A powder (top) and grazing incident angle (bottom) XRD from the bulk and thin lm samples, respectively. (b) C4AF powder (top) and grazing incident angle (bottom) XRD from the bulk and thin lm samples, respectively.
**Figure 4.** XPS spectra of the C3A (top) and C4AF (bottom), for their respective bulk material and thin film. Thick lines are the spectra collected, while the thin lines are the background and the result of peak fitting. XPS spectra for Ca 2p, O 1s, Al 2p and Fe 2p (for C4AF only) are shown.
Analysis of the material left in the crucible after the evaporation of C3A shows the presence of tricalcium aluminate and mayenite (Ca12Al14O33 or C12A7; **Figure 5a**). C12A7 is formed from the phase that liquefied during the evaporation, indicating at least partially incongruent evaporation of C3A, yielding to an evaporate/condensate that is substantially richer in Ca (note that the Ca/Al ratio of mayenite is 0.86, about 3.5 times lower than that of C3A, and hence, the Ca enrichment of the evaporate can be inferred). The presence of portlandite and calcite in the thin films can therefore be interpreted as partial hydration and carbonation of a Ca-rich phase upon atmospheric exposure (although limited) of the thin films.
2p peaks shift to higher energies by +0.7 eV, indicating again differences in the thin film
134 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
**Figure 3.** (a) C3A powder (top) and grazing incident angle (bottom) XRD from the bulk and thin lm samples, respectively. (b) C4AF powder (top) and grazing incident angle (bottom) XRD from the bulk and thin lm samples, respective-
**Figure 4.** XPS spectra of the C3A (top) and C4AF (bottom), for their respective bulk material and thin film. Thick lines are the spectra collected, while the thin lines are the background and the result of peak fitting. XPS spectra for Ca 2p, O
1s, Al 2p and Fe 2p (for C4AF only) are shown.
material.
ly.
**Figure 5.** XRD of the residue left in the crucible after the evaporation of (a) C3A and (b) C4AF.
**Figure 6.** XPS spectra of the material left in the crucible after evaporation of C3A (top) and C4AF (bottom). Thick lines are the spectra collected, while the thin lines are the background and the result of peak fitting.
The absence of Al in the XPS spectra (**Figure 6**) could be due to some reaction scavenging from the silicon substrate, leading to aluminosilicates, similarly to what has been observed by Toda et al. [19] during production of C12A7 thin films. However, XPS data do not show any evidence of silicon, while reaction at deeper regions cannot be excluded, as on XPS the X-rays only penetrate a few nanometers on the surface.
Before completion of C3A evaporation, a liquid phase formed while, probably, the bulk material was being converted into a mix of solid CaO and liquid calcium aluminate, with higher aluminum content than the initial C3A. Possibly, only the solid phase evaporated and became the precursor of the generated thin film, since no aluminum is observed there.
The BE of the calcium peaks does not present relevant shifts from those in the bulk material. However, oxygen and aluminum peaks in the melt residue in the crucible present significant changes in both BE and FWHM (**Table 3**, **Figure 6**), which indicates a different coordination state.
Electron beam bombardment of C4AF led, as in the case of C3A, to the formation of a liquid phase, with a higher concentration of aluminum than in the bulk material. The phase diagram of CaO-Fe2O3-Al2O3 [20] shows that, when there is an incomplete fusion, the equilibrium is reached with a Ca2(Al,Fe+3)2O5 phase with higher content of Fe (solid) and one liquid phase mix of CaO.Al2O3, without Fe. Analogous formation happens in the C3A at up to 1542°C, when the equilibrium is reached for CaO solid + liquid (∼40%wt of Al2O3 + CaO).
XRD patterns of the bulk material (**Figure 3a**) and the residue left in the crucible after the evaporation (**Figure 5a**) are remarkably similar. The main component in both materials is brownmillerite (Ca2(Al,Fe+3)2O5); however, the presence of Fe3O4 or Ca2Al1.38Fe0.62O5 cannot be discarded.
In fact, Taylor [20] stated that "iron-rich mixes tend to lose oxygen when heated in air above 1200–1300°C, with consequent replacement of hematite by magnetite (Fe304)," which is in accordance with the observations here, even though it was not possible to verify the temperature in the crucible during the evaporation.
Likewise in the C3A sample, during the evaporation of C4AF, the material in the crucible melted and there were eruptions, as well as there was no aluminum in the film. That means the residue left in the crucible has to be richer in aluminum. Again, as observed for the C3A sample, reaction with the substrate at deeper regions, as observed by Toda et al. [19], cannot be discarded. However, no XPS signal of silicon can be detected, suggesting that such a reaction does not occur.
In the same way, the XPS analyses for the material left in the crucible after the evaporation of the C4AF (**Figure 5b**) present a strong difference when compared to the initial bulk material: Fe 2p peak presents only one component at 709.8 eV (Fe 2p3/2) and 723.0 eV (Fe 2p1/2), meaning important changes happened in the chemical/electronic state of this element. O 1s presents BE shifts of +0.3 eV and Ca 2p of -0.5 eV, while BE of Al 2p is similar to the starting material (**Table 3**).
The XPS 2p spectra of Fe, as other transition metals, present complex lineshapes due to the electrons exchange interaction effects, as well as electron correlation effects [21]. Normally, Fe 2p peaks are broad, asymmetric and may contain satellites, which are observed here as well.
On the other hand, for iron oxides, the O 1s BE is expected to increase with a decrease in the oxidation number of the cation of the same metal; it is observed here an increase in O 1s BE by +0.3 eV from the bulk to the material left in the crucible. At the same time, Graat and Somers [22] describe an increase in BE for increase in oxidation state of iron; here, this is very much evident as with the existence of only one iron component, which still presents a BE lower by -0.4 eV than the Fe 2p peak with the lowest BE in the bulk material (709.8 vs 710.2 eV), suggesting a decreasing in the oxidation state for the material left in the crucible (**Table 3**).
This may explain the possible existence of Ca2Al1.38Fe0.62O5, besides the brownmillerite, in the material in the crucible, as XRD could suggest. This compound was also observed by Vázquez-Acosta et al. [23].
The phase diagram for the C4AF partial fusion relates to a solid phase C2F (2CaO.Fe2O3) and a liquid with composition similar to the Ca2(AlxFe1-x)O5, in direction to phases with higher aluminum concentration than the C4AF itself. As underlined by Taylor [20] "for bulk compositions in the Ca2(AlxFe1-x)O5 series, the liquid is of higher Al/Fe ratio than the ferrite phase with which it is in equilibrium."
## **3. Hydration of calcium silicate thin films**
The absence of Al in the XPS spectra (**Figure 6**) could be due to some reaction scavenging from the silicon substrate, leading to aluminosilicates, similarly to what has been observed by Toda et al. [19] during production of C12A7 thin films. However, XPS data do not show any evidence of silicon, while reaction at deeper regions cannot be excluded, as on XPS the X-rays only
Before completion of C3A evaporation, a liquid phase formed while, probably, the bulk material was being converted into a mix of solid CaO and liquid calcium aluminate, with higher aluminum content than the initial C3A. Possibly, only the solid phase evaporated and became
The BE of the calcium peaks does not present relevant shifts from those in the bulk material. However, oxygen and aluminum peaks in the melt residue in the crucible present significant changes in both BE and FWHM (**Table 3**, **Figure 6**), which indicates a different coordination
Electron beam bombardment of C4AF led, as in the case of C3A, to the formation of a liquid phase, with a higher concentration of aluminum than in the bulk material. The phase diagram of CaO-Fe2O3-Al2O3 [20] shows that, when there is an incomplete fusion, the equilibrium is reached with a Ca2(Al,Fe+3)2O5 phase with higher content of Fe (solid) and one liquid phase mix of CaO.Al2O3, without Fe. Analogous formation happens in the C3A at up to 1542°C, when
XRD patterns of the bulk material (**Figure 3a**) and the residue left in the crucible after the evaporation (**Figure 5a**) are remarkably similar. The main component in both materials is brownmillerite (Ca2(Al,Fe+3)2O5); however, the presence of Fe3O4 or Ca2Al1.38Fe0.62O5 cannot be
In fact, Taylor [20] stated that "iron-rich mixes tend to lose oxygen when heated in air above 1200–1300°C, with consequent replacement of hematite by magnetite (Fe304)," which is in accordance with the observations here, even though it was not possible to verify the temper-
Likewise in the C3A sample, during the evaporation of C4AF, the material in the crucible melted and there were eruptions, as well as there was no aluminum in the film. That means the residue left in the crucible has to be richer in aluminum. Again, as observed for the C3A sample, reaction with the substrate at deeper regions, as observed by Toda et al. [19], cannot be discarded. However, no XPS signal of silicon can be detected, suggesting that such a reaction does not
In the same way, the XPS analyses for the material left in the crucible after the evaporation of the C4AF (**Figure 5b**) present a strong difference when compared to the initial bulk material: Fe 2p peak presents only one component at 709.8 eV (Fe 2p3/2) and 723.0 eV (Fe 2p1/2), meaning important changes happened in the chemical/electronic state of this element. O 1s presents BE shifts of +0.3 eV and Ca 2p of -0.5 eV, while BE of Al 2p is similar to the starting
the precursor of the generated thin film, since no aluminum is observed there.
the equilibrium is reached for CaO solid + liquid (∼40%wt of Al2O3 + CaO).
penetrate a few nanometers on the surface.
136 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
ature in the crucible during the evaporation.
state.
discarded.
occur.
material (**Table 3**).
Calcium silicates' hydration happens over a dissolution-precipitation process that may occur through the formation of etch pits, where the calcium silicate hydrate (C-S-H) preferentially precipitates, together with calcium hydroxide (portlandite or CH), the two hydration products. C-S-H is the main product formed by the hydration of the calcium silicates and responsible for the strength of the cement paste. It is mostly amorphous, and the dashes indicate a disordered structure. Its particle density was found by Gauffinet et al. [18] to be in the order of 2500 kg/m3 .
At the molecular level, C-S-H has a structure similar to the tobermorite (Ca5Si6O16(OH)2∙8H2O) and jennite (Ca9Si6O18(OH)6∙8H2O), rare minerals found in nature. Most of the models predicting the C-S-H nanostructure involve elements of jennite-like or even tobermorite-jennite structures [26].
After running the initial pattern presented previously (**Figures 2** and **3**), C3S and C2S dry thin film samples were hydrated separately in a reaction chamber that was located in the same vacuum line as the XPS, hence avoiding any hydration and/or carbonation from the contact with the atmosphere. Saturated water vapor with argon as a carrier was inserted at 20 mL/min. Prior to hydration, the system was purged for 5 min to avoid any contamination.
XPS analyses of the hydrated 110-nm-thick C3S thin film are presented in detail in Ref. [3]. Results show, after 3 h of exposure to water vapor, a shift in the Si 2p peak of about 0.7 eV (from 101.9 to 102.6 eV) (**Table 4**), while the literature reports lower BE for this peak (100.57 eV in fresh C3S, [6]). This indicates a progressive disordering of the silicate structure, which denotes the C-S-H formation.
**Table 4.** Evolution of binding energies and peak width (FWHM) of Si and Ca, Ca-Si peak distance and Ca/Si ratio during vapor hydration of C3S.
The measurement of the energy separation δCa 2p-Si 2p between the Ca 2p 3/2 and Si 2p peaks minimizes errors due to the charge correction and provides reliable information on chemical changes [8, 24]: Shifts on Si peaks to higher BE due to the progressive hydration of C3S lead to reduction in the distance between the peaks of calcium and silicon, as there is a polymerization of the isolated silicate tetrahedra upon formation of C-S-H, and/or carbonation [9]. However, the formation of calcium carbonate is unlikely to occur here due to the nature of the experiments. Initial δCa 2p-Si 2p values found here (245.3 eV) are lower than those from the literature [6, 24, 25]; however, the decrease in the Ca-Si distance after 3 h of hydration of C3S (δCa 2p 3/2 - Si 2p = 0.6) is identical to that found by [24] after 4 h.
In the same way, the molar Ca/Si ratio of the newly formed hydrates Ca/Si ratio remains the same, as the expected isochemical conditions of the experiment. The excess Ca content (Ca/Si of about 3.5 vs. 3.0 of stoichiometric tricalcium silicate, **Table 4**) during the early hydration can be related to fast partial hydration and carbonation of the upper few nanometers of the sample during manipulation under atmospheric conditions prior to the experiments.
However, peak deconvolution of the Ca 2p 1/2 peaks shows contributions from carbonates and silicates: Calcium silicates have Ca 2p BE slightly higher than those of calcium carbonates. Considering only the calcium related to silicates in this equation, the Ca/Si ratio drops from the initial 1.6 to 0.5, as hydration progresses. Such low values indicate a progressive polymerization of silicate tetrahedra and subsequent increase in C-S-H chain length, equivalent to an increase in SiO2 content [9, 26]. These results agree with Taylor [26], who suggests that the first precipitate is a jennite-like material, with a Ca/Si ratio of about 1.5, evolving to a tobermorite-like material as the hydration progresses. Detailed information on the tobermorite and jennite structure can be found elsewhere [20, 26].
101.9 to 102.6 eV) (**Table 4**), while the literature reports lower BE for this peak (100.57 eV in fresh C3S, [6]). This indicates a progressive disordering of the silicate structure, which denotes
> **Peak positon (eV) Ca/Si ratio Si 2p δCa 2p‐Si 2p Ca 2p 3/2 Ca 2p/Si 2p**
**Table 4.** Evolution of binding energies and peak width (FWHM) of Si and Ca, Ca-Si peak distance and Ca/Si ratio
2p 3/2 - Si 2p = 0.6) is identical to that found by [24] after 4 h.
The measurement of the energy separation δCa 2p-Si 2p between the Ca 2p 3/2 and Si 2p peaks minimizes errors due to the charge correction and provides reliable information on chemical changes [8, 24]: Shifts on Si peaks to higher BE due to the progressive hydration of C3S lead to reduction in the distance between the peaks of calcium and silicon, as there is a polymerization of the isolated silicate tetrahedra upon formation of C-S-H, and/or carbonation [9]. However, the formation of calcium carbonate is unlikely to occur here due to the nature of the experiments. Initial δCa 2p-Si 2p values found here (245.3 eV) are lower than those from the literature [6, 24, 25]; however, the decrease in the Ca-Si distance after 3 h of hydration of C3S (δCa
In the same way, the molar Ca/Si ratio of the newly formed hydrates Ca/Si ratio remains the same, as the expected isochemical conditions of the experiment. The excess Ca content (Ca/Si of about 3.5 vs. 3.0 of stoichiometric tricalcium silicate, **Table 4**) during the early hydration can be related to fast partial hydration and carbonation of the upper few nanometers of the sample
However, peak deconvolution of the Ca 2p 1/2 peaks shows contributions from carbonates and silicates: Calcium silicates have Ca 2p BE slightly higher than those of calcium carbonates. Considering only the calcium related to silicates in this equation, the Ca/Si ratio drops from the initial 1.6 to 0.5, as hydration progresses. Such low values indicate a progressive polymerization of silicate tetrahedra and subsequent increase in C-S-H chain
during manipulation under atmospheric conditions prior to the experiments.
101.9 (2.5) 245.2 346.8 (2.7) 3.4 101.9 (2.4) 245.3 347.0 (2.7) 3.4 102.2 (2.4) 245.1 347.1 (2.6) 3.4 102.4 (2.5) 245.0 347.3 (2.6) 3.4 102.4 (2.5) 244.9 347.3 (2.6) 3.4 102.6 (2.6) 245.0 347.4 (2.5) 3.8 102.5 (2.5) 244.9 347.4 (2.5) 3.6 102.6 (2.5) 244.9 347.4 (2.5) 3.6 102.6 (2.5) 244.6 347.2 (2.5) 3.5 102.6 (2.5) 244.6 347.2 (2.5) 3.6
138 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
the C-S-H formation.
during vapor hydration of C3S.
**Time (mins)** The same experimental procedure was used for the hydration of a 200-nm-thick C2S thin film (presented in detail in [4]), which, with exposure to water vapor, shows a similar drift of the main Si 2p peak, from 101.2 to 102.6 eV, denoting the disordering of the silicate structure related to progressive C-S-H formation (**Table 5**). This shift is accompanied by a peak broadening at higher ages, forming an additional peak at around 100.2 range, same as described by Black et al. [6], where a Si 2p 3/2 BE of 100.8 eV is observed for the fresh C2S and followed by two peaks at 102.44 and 100.51 eV, which are seen for the aged sample. The peak at lower energies is related to unaltered β-C2S, while the peak at higher binding energy is assigned to a calciumdepleted C-S-H phase.
**Table 5.** Evolution of binding energies of Si, Ca and O (peak width, FWHM), Ca-Si peak distance and NBO-BO peak distance during vapor hydration of C2S.
The Ca BE, on the other hand, shows a small progressive shift (0.7 eV) when hydration goes on. The Ca 2p3/2 peak is centered at around 347.0 eV (**Table 5**) for fresh C2S, a value slightly lower than the one previously reported by Black et al. [6] (347.23 eV).
The initial energy separation between Ca 2p 3/2 and Si 2p peaks found in this work (245.8 eV) is fairly lower than those reported by Black et al. [6], (246.72 eV). The progressive hydration of C2S results in shifting of the Si 2p peak to higher BE, therefore reducing the δCa 2p-Si 2p, due to the polymerization of the isolated silicate tetrahedra upon formation of C-S-H.
The molar Ca/Si ratio of the newly formed hydrates in C2S thin film starts with a lower value than expected from the bulk material (initial Ca/Si of about 1.6 vs. 2.0 of stoichiometric dicalcium silicate), indicating a partial hydration of the upper few nanometers of the sample during manipulation under atmospheric conditions, and decreases over time, reaching Ca/Si = 1.0. The same values are described by Regourd et al. [24], who found this value after 4 h of hydration. While small contributions from portandite and/or unhydrated C2S cannot be excluded, such low Ca/Si values clearly evidence the formation of C-S-H. Here, different peaks related to the contributions from carbonates and silicates are not observed.
In this sample, it was possible to study the O 1s spectrum, which provides important information on the structure of minerals and glasses but is complex to be assessed. O 1s can be present in different components, which have different BE: non-bridging oxygen (530–530.5 eV), bridging oxygen (531.5–532.7 eV), hydroxide species (533–533.5 eV), bound water (534 eV) and portlandite at calcium-rich samples (531.6 eV), while amorphous silica does not add any extra component [27].
Changes in the peaks separation between bridging and non-bridging oxygen atoms (δNBO-BO) are related to silicon polymerization: Peak distance grows with the falling of the Ca/Si ratio that happens together with hydration by water vapor, since the contribution from nonbridging oxygen decreases as the hydration progresses (the number of Ca-O-Si units decreases and Si-O-Si units increases), as observed in **Table 5**.
## **4. Final remarks**
The fact that the chemical and mineralogical composition of the calcium silicate thin films is in accordance with the respective bulk materials proves that the electron beam evaporation is a useful and powerful way for synthesizing thin films of calcium silicates. However, not the same was observed for the calcium aluminate phases: The electron beam evaporation conditions used in this work are not suitable for producing thin films of these materials. Chemical and mineralogical analysis of the residue left in the crucible after the evaporation shows that the aluminum present on these phases does not evaporate, and for this reason, there is no signal of this element in the thin films. Aluminum reacts abnormally when submitted to the extreme conditions during the sample preparation.
This behavior is clearly observed with the GIXRD and XPS data. Even as the electron beam evaporation fails to provide the expected results for the evaporation of phases containing aluminum, other standard techniques for producing ceramic thin films can be applied. Presumably, sputtering may allow synthesizing thin films of these phases as in this technique the sample is not heated. Instead, atoms are ejected through the bombardment of the target material by energetic particles.
The difficulty on using sputtering techniques arises from the fact that it is challenging to have suitable targets as it demands unusual preparation and generally companies that manufacture them refuse to produce targets with special specifications. In this case, additional manufacturing effort is needed for producing targets so that this technique can be applied for the synthesis of thin films of clinker phases.
This type of sample has been proven to be useful in attaining information related to dissolution, hydration and carbonation in ways never before explored. Research using thin films of clinker phases for hydration analysis is found elsewhere [3, 4].
XPS provides information on the upper few nanometers of the sample only, being suitable for thin films studies, offering accurate chemical composition and coordination state data. Regarding the XPS data for all the C3S and β-C2S samples, the peak positions, peak distances and peak widths are typically equivalent to the bulk material, proving that they have the same chemical composition. However, pre-hydration is observed due to the contact of the sample with the atmosphere.
Hydration of C3S presented shifts on Si 2p peak to higher BE, related to silicon polymerization by the formation of C-S-H. The δCa 2p-Si 2p distance decreased with time, indicating that the kinetics of early C-S-H formation is not significantly altered when vapor is used instead of liquid water.
The molar Ca/Si ratio in both C2S and (from carbonate contribution) C3S decreases as the hydration proceeds, due to the progressive polymerization with an increase in chain length of the silicate hydrates formed. Initial Ca/Si values in C3S correspond to a jennite-like material and evolve to a tobermorite-like component after 3 h of exposure to water.
Some possible pre-hydration is observed in both calcium carbonates, by the contact with the atmosphere during the sample manipulation or due to the preferential deposition of the silicon on the sample's surface after the evaporation. Besides that, the peak positions of the bulk material are maintained and so the chemical state.
The C2S samples provided clearly distinguished O 1s spectra, allowing to identify the δNBO-BO peak separation, which increases with the silicon polymerization, describing the C-S-H formation.
## **Acknowledgements**
The Ca BE, on the other hand, shows a small progressive shift (0.7 eV) when hydration goes on. The Ca 2p3/2 peak is centered at around 347.0 eV (**Table 5**) for fresh C2S, a value slightly
The initial energy separation between Ca 2p 3/2 and Si 2p peaks found in this work (245.8 eV) is fairly lower than those reported by Black et al. [6], (246.72 eV). The progressive hydration of C2S results in shifting of the Si 2p peak to higher BE, therefore reducing the δCa 2p-Si 2p, due
The molar Ca/Si ratio of the newly formed hydrates in C2S thin film starts with a lower value than expected from the bulk material (initial Ca/Si of about 1.6 vs. 2.0 of stoichiometric dicalcium silicate), indicating a partial hydration of the upper few nanometers of the sample during manipulation under atmospheric conditions, and decreases over time, reaching Ca/Si = 1.0. The same values are described by Regourd et al. [24], who found this value after 4 h of hydration. While small contributions from portandite and/or unhydrated C2S cannot be excluded, such low Ca/Si values clearly evidence the formation of C-S-H. Here, different peaks
In this sample, it was possible to study the O 1s spectrum, which provides important information on the structure of minerals and glasses but is complex to be assessed. O 1s can be present in different components, which have different BE: non-bridging oxygen (530–530.5 eV), bridging oxygen (531.5–532.7 eV), hydroxide species (533–533.5 eV), bound water (534 eV) and portlandite at calcium-rich samples (531.6 eV), while amorphous silica does not add any
Changes in the peaks separation between bridging and non-bridging oxygen atoms (δNBO-BO) are related to silicon polymerization: Peak distance grows with the falling of the Ca/Si ratio that happens together with hydration by water vapor, since the contribution from nonbridging oxygen decreases as the hydration progresses (the number of Ca-O-Si units decreases
The fact that the chemical and mineralogical composition of the calcium silicate thin films is in accordance with the respective bulk materials proves that the electron beam evaporation is a useful and powerful way for synthesizing thin films of calcium silicates. However, not the same was observed for the calcium aluminate phases: The electron beam evaporation conditions used in this work are not suitable for producing thin films of these materials. Chemical and mineralogical analysis of the residue left in the crucible after the evaporation shows that the aluminum present on these phases does not evaporate, and for this reason, there is no signal of this element in the thin films. Aluminum reacts abnormally when submitted to the extreme
This behavior is clearly observed with the GIXRD and XPS data. Even as the electron beam evaporation fails to provide the expected results for the evaporation of phases containing
to the polymerization of the isolated silicate tetrahedra upon formation of C-S-H.
related to the contributions from carbonates and silicates are not observed.
extra component [27].
**4. Final remarks**
and Si-O-Si units increases), as observed in **Table 5**.
conditions during the sample preparation.
lower than the one previously reported by Black et al. [6] (347.23 eV).
140 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
Vanessa Rheinheimer had the financial support of the CUR of the DIUE of the Generalitat de Catalunya. This work is partially funded by Grant CTQ2009-12520 from the Spanish Ministry of Science and Innovation. The authors also thank Diogo Topolski for the help with the XRD analysis.
## **Author details**
Vanessa Rheinheimer1\* and Ignasi Casanova2
\*Address all correspondence to: [email protected]
1 Berkeley Education Alliance for Research in Singapore—BEARS, Create Way, Singapore
2 Department of Construction Engineering and Center for Research in Nanoengineering, Polytechnic University of Catalonia—Barcelona Tech., Barcelona, Spain
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of Science and Innovation. The authors also thank Diogo Topolski for the help with the XRD
1 Berkeley Education Alliance for Research in Singapore—BEARS, Create Way, Singapore
2 Department of Construction Engineering and Center for Research in Nanoengineering,
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[8] L. Black, K. Garbev, G. Beuchle, P. Stemmermann, and D. Schild. X-ray photoelectron spectroscopic investigation of nanocrystalline calcium silicate hydrates synthesised by
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**Application of Thin Films: A Synergistic Outlook**
[24] M. Regourd, J. H. Thomassin, P. Baillif, and J. C. Touray. Study of the early hydration of Ca3SiO5 by X-ray photoelectron spectrometry. Cem. Concr. Res. 1980;10(2):223–230.
[25] S. Long, C. Liu, and Y. Wu. ESCA study on the early C3S hydration in NaOH solution
[26] H. F. W. Taylor. Proposed structure for calcium silicate hydrate gel. J. Am. Ceram. Soc.
[27] M. Regourd, C. Defosse, S. A. Jefferis, and J. Bensted. Microanalytical studies (X-ray photoelectron spectrometry) of surface hydration reactions of cement compounds [and discussion]. Philos. Trans. R. Soc. London. Ser. A, Math. Phys. Sci.. 1983;310(1511):
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**Provisional chapter**
## **Layer-by-Layer Thin Films and Coatings Containing Metal Nanoparticles in Catalysis Layer-by-Layer Thin Films and Coatings Containing Metal Nanoparticles in Catalysis**
Sarkyt Kudaibergenov, Gulnur Tatykhanova, Nurlan Bakranov and Rosa Tursunova Nurlan Bakranov and Rosa Tursunova Additional information is available at the end of the chapter
Sarkyt Kudaibergenov, Gulnur Tatykhanova,
Additional information is available at the end of the chapter
http://dx.doi.org/10.5772/67215
#### **Abstract**
The layer-by-layer (LbL) technique is one of the most promising ways of fabricating multilayer thin films and coatings with precisely controlled composition, thickness, and architecture on a nanometer scale. This chapter considers the multilayer thin films and coatings containing metal nanoparticles. The main attention was paid to LbL films containing metal nanoparticles assembled by convenient methods based on the different intermolecular interactions, such as hydrogen bonding, charge transfer interaction, molecular recognition, coordination interactions, as driving force for the multilayer buildup. Much attention has paid to the LbL films containing metal nanocomposites for multifunctional catalytic applications, in particular, photocatalysis, thermal catalysis, and electrocatalysis. The preparation protocol of LbL-assembled multilayer thin films containing metal nanoparticles (such as Au, Ag, Pd, Pt), metal oxides (Fe<sup>3</sup> O4 ), and sulfides (CdS) that are supported on the various surfaces of nanotubes of TiO<sup>2</sup> , Al<sup>2</sup> O3 membranes, graphene nanosheets, graphene oxide and further applications as catalysts with respect to photocatalytic, electrocatalytic performances is discussed. The systematization and analysis of literature data on synthesis, characterization, and application of multilayer thin films and coatings containing metal nanoparticles on the diverse supports may open new directions and perspectives in this unique and exciting subject.
**Keywords:** layer-by-layer assembling, thin films and coatings, polyelectrolytes, metal nanoparticles, immobilization, semiconductors, catalysts
## **1. Introduction**
One of the main purposes of nanotechnology is fabrication of highly functional low-dimensional materials and systems [1]. Most of such systems are produced by assembling of nano
© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
objects in thin films or coatings [2]. The utilization of high functional materials is steadily growing and covers many areas of human activity such as fabrication of drug delivery systems [3], antibacterial coatings [4, 5], electronics [6], chemical sensors [7], and even in dentist applications. Some assembling procedures need to immobilize metal nanoparticles such as gold, silver, copper, palladium, and platinum to preserve them in origin state and to avoid the aggregation. Such immobilization can be proceeded by the incorporating of nanoclusters into polymer scaffold. The scaffold plays three main roles [8]:
Polymer science and technology that spreads polymer-based biomaterials, catalysts bounded by polymer, nanofibers fabricated by polymer assisted electrospun and attracts tremendous attention [9]. The interlacing of polymer technology with nanoscience allows expanding both of them and solving the existing challenges in the fabrication on a nanoscale. Because of plenty of binding region in polymer, it can effectively immobilize the nanoparticles and at the same time act as a mediator between the substrate and nano objects [2]. Wide exploitation of polymers as supporting agents during formation of the coating is applied in the LbL assembling technique. The LbL assembling approach can be qualified as a good alternative approach to the well-known deposition methods, because it is versatile, inexpensive, and comfortable for use. Moreover, the large advantage of LbL method is that, in contrast to widespread strategies of fabrication of nanoscale structures on top of planar substrate such as chemical vapor deposition [10], atomic layer deposition [11], molecular beam epitaxy [12], hydrothermal deposition [13] and so forth, it is suitable for the formation of a uniform coating on curved surfaces [14]. The whole process of multilayer structures buildup by LbL assembling, whose driving force, despite hydrogen bonding, covalent bonding, etc., is mainly electrostatic interaction between the oppositely charged species [14, 15], usually consists of four steps [16] (**Figure 1a**):
By repetition of these cycles [17], a desired number of layers can be achieved on curved planes, as shown in **Figure 1b**, therefore the whole thickness of multilayer is easy controlled. Generally, to create a difference in surface charges, using the LbL procedure, various polyelectrolytes are employed. The mostly applied positively charged polyelectrolytes are: poly(allylamine hydrochloride) (PAH), polyethyleneimine (PEI), or poly(diallyldimethylammonium chloride) (PDDA) and negatively charged polyelectrolytes are: poly(vinyl sulfate) (PVS), poly(acrylic acid) (PAA), or poly(styrene sulfonate) (PSS). It should be also mentioned that the LbL method expands the possibilities of obtaining organic/inorganic films with high accuracy just by changing the number of multilayers, concentration and pH of the solution. The application area of nanosystems obtained by LbL assembly is very wide. It includes fabrication of layers with magnetic, fluorescent, catalytic and various electronic properties. For example, the magnetic multilayers can be used in medicine as well as other technical applications. Fluorescent properties are widely benefited in optical devices. LbL-assembled catalysts can be applied for hydrogenation, oxidation of various substrates and water splitting. Nobel metal particles incorporated into the multilayer structures are adopted as coatings for light absorption enhancing by the surface plasmonic effect. In the following subsection we consider the LbL thin films and coatings containing metal nanoparticles, metal oxides, and sulfides together with their application of in catalysis.
objects in thin films or coatings [2]. The utilization of high functional materials is steadily growing and covers many areas of human activity such as fabrication of drug delivery systems [3], antibacterial coatings [4, 5], electronics [6], chemical sensors [7], and even in dentist applications. Some assembling procedures need to immobilize metal nanoparticles such as gold, silver, copper, palladium, and platinum to preserve them in origin state and to avoid the aggregation. Such immobilization can be proceeded by the incorporating of nanoclusters
(2) It ministers like a matrix for ordering and homogeneous orientation of systems.
(3) Due to its some properties, such electronic properties, it acts as a functional element.
Polymer science and technology that spreads polymer-based biomaterials, catalysts bounded by polymer, nanofibers fabricated by polymer assisted electrospun and attracts tremendous attention [9]. The interlacing of polymer technology with nanoscience allows expanding both of them and solving the existing challenges in the fabrication on a nanoscale. Because of plenty of binding region in polymer, it can effectively immobilize the nanoparticles and at the same time act as a mediator between the substrate and nano objects [2]. Wide exploitation of polymers as supporting agents during formation of the coating is applied in the LbL assembling technique. The LbL assembling approach can be qualified as a good alternative approach to the well-known deposition methods, because it is versatile, inexpensive, and comfortable for use. Moreover, the large advantage of LbL method is that, in contrast to widespread strategies of fabrication of nanoscale structures on top of planar substrate such as chemical vapor deposition [10], atomic layer deposition [11], molecular beam epitaxy [12], hydrothermal deposition [13] and so forth, it is suitable for the formation of a uniform coating on curved surfaces [14]. The whole process of multilayer structures buildup by LbL assembling, whose driving force, despite hydrogen bonding, covalent bonding, etc., is mainly electrostatic interaction between the oppositely charged species [14, 15], usually consists of four
(1) Immersing of a cleaned, positively charged solid substrate into the solution of an anionic polyelectrolyte. Electrostatic force, collectively with adsorption, builds the first layer of
(2) Removal of the excess and weak adsorbed polyelectrolyte from the surface is carried out
(3) Bilayer structure is achieved by immersing of the substrate into the solution of the cati-
By repetition of these cycles [17], a desired number of layers can be achieved on curved planes, as shown in **Figure 1b**, therefore the whole thickness of multilayer is easy controlled. Generally, to create a difference in surface charges, using the LbL procedure,
onic polyelectrolyte. This step restores the original surface charge.
(4) Final rinsing removes the excess of cationic polyelectrolyte.
into polymer scaffold. The scaffold plays three main roles [8]:
148 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
(1) It provides assistance to particle assembling.
steps [16] (**Figure 1a**):
multilayer construction.
by substrate rinsing with deionized water.
**Figure 1.** Schematic illustration of the LbL assembling process by alternately dipping of a positively charged substrate into the solutions of a oppositely charged polyelectrolyte (a) and formation of shell structure on curved planes (b).
## **2. LbL thin films and coatings containing metal nanoparticles, metal oxides, and sulfides**
#### **2.1. LbL thin film and coatings containing gold (AuNPs) and silver (AgNPs) nanoparticles**
The metal nanoparticles with attractive optical, electronic, and catalytic properties are used in a broad range of applications ranging from physics to medicine [18]. For assembling such nanoscaled particles with controlled parameters, it is necessary for scientists to develop new methods which can allow to obtain constructions with required properties [19]. The charge transfer properties of metal particles incorporated in a thin layer depend on particle size and distance between them in vertical and space distributions [20]. At the same time, the dependence between the particle size reflects to a high reactivity/selectivity for the hydrogenation of unsaturated alcohols [21, 22]. Recently, noticeable research works were conducted with respect to immobilization of mono- and bimetallic nanoparticles into the matrices of ultrathin films to obtain the effective nanocatalysts [23]. Such, LbL-assembled layers of gold nanoparticles within interpolyelectrolyte complexes can be formed either by interaction of poly(ethyleneimine)-gold nanoparticles (PEI-AuNPs) with poly(acrylic acid) (PAA) or by interaction of poly(acrylic acid)-gold nanoparticles (PAA-AuNPs) with PEI [24]. It is also well known that the noble metal nanoparticles, in particular Au, Ag, and Pt possess strongly marked plasmonic properties, which can be controlled by changing fabrication parameters. Thus, changing of volume, dipolar coupling, or a type of solvent alters the wavelength of plasmonic resonance. For instance, dipolar coupling of Au can be controlled by changing the distance between the particles, such distance among the particles is easily adjusted by modification of dendrimers, highly branched monodisperse molecules. The dendrimers have the series of chemical modifications and cavities which act as templates for nanoparticle growth. For example, polyamidoamine (PAMAM) or carboxyl-terminated PAMAM [25] dendrimers are utilized as a matrix with effective nanoparticle stabilization [26]; therefore, Au, Cu, Pt, and Pd nanoparticles can be formed and stabilized therein [27]. The wavelength of surface plasmonic resonance of nanoscaled gold nano objects incorporated within PAMAM depends on the number of the layers. Increasing of the number of LbL-assembled PAMAM enlarges the distance between the Au nano objects, which leads to an ultraviolet shifting of plasmonic glow. Besides the tuning of plasmonic properties, it is also possible to adjust the fluorescent properties of gold nano objects only by magnification the thickness of multilayer or by the increasing the number of LbL cycles. For example, fluorescence of Au, covered in a core-shell manner by organic multilayers, can be easily tuned by varying the number of nonfluorescent layers [28].
Silver/gold coatings formed onto a commercial anion exchange resin via LbL [5] are very appropriate bimetallic composition for catalytic reduction of nitroaromatic compounds. Such core-shell heterostructures can be prepared by using of electrostatic force of the charged resin beads. Such resin beads support immobilization of anionic metal precursors of silver/gold nanoparticles onto the solid resin matrix and reduce 2-nitrobenzoic acid to obtain the corresponding amines through the effective catalysts.
**Figure 2** demonstrates a simple method of integrating the electroactive gold nanoparticles (AuNPs) with graphene oxide (GO) nanosheets. Such LbL structures composed of threedimensional electrocatalytic thin films are active toward methanol oxidation [9]. This approach involves the electrostatic interaction of negatively charged graphene oxide nanosheet with positively charged AuNPs. The distribution of gold nanoparticles on the surface of GO can be controlled using the LbL method, the latter enhances the stability keeping from aggregation during the electrocatalytic cycles. Due to high versatile and tunable properties of LBLassembled thin films, a hybrid electrocatalyst can be easily designed for direct methanol fuel cell (DMFC). Such LbL assembly allows for the fabrication of the nanoparticle/graphene hybrid multilayer structure, which exhibits a wide range of functionalities. **Figure 3** shows a schematic representation of the LbL film made of poly-N-vinylpyrrolidone-stabilized AuNPs (PVP-AuNPs) and single-walled CNTs deposited on a fluorine-doped tin oxide (FTO) glass. Such structures may be used in the field of catalysis, fuel cells, and sensing.
**2. LbL thin films and coatings containing metal nanoparticles, metal**
150 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
**2.1. LbL thin film and coatings containing gold (AuNPs) and silver (AgNPs) nanoparticles**
The metal nanoparticles with attractive optical, electronic, and catalytic properties are used in a broad range of applications ranging from physics to medicine [18]. For assembling such nanoscaled particles with controlled parameters, it is necessary for scientists to develop new methods which can allow to obtain constructions with required properties [19]. The charge transfer properties of metal particles incorporated in a thin layer depend on particle size and distance between them in vertical and space distributions [20]. At the same time, the dependence between the particle size reflects to a high reactivity/selectivity for the hydrogenation of unsaturated alcohols [21, 22]. Recently, noticeable research works were conducted with respect to immobilization of mono- and bimetallic nanoparticles into the matrices of ultrathin films to obtain the effective nanocatalysts [23]. Such, LbL-assembled layers of gold nanoparticles within interpolyelectrolyte complexes can be formed either by interaction of poly(ethyleneimine)-gold nanoparticles (PEI-AuNPs) with poly(acrylic acid) (PAA) or by interaction of poly(acrylic acid)-gold nanoparticles (PAA-AuNPs) with PEI [24]. It is also well known that the noble metal nanoparticles, in particular Au, Ag, and Pt possess strongly marked plasmonic properties, which can be controlled by changing fabrication parameters. Thus, changing of volume, dipolar coupling, or a type of solvent alters the wavelength of plasmonic resonance. For instance, dipolar coupling of Au can be controlled by changing the distance between the particles, such distance among the particles is easily adjusted by modification of dendrimers, highly branched monodisperse molecules. The dendrimers have the series of chemical modifications and cavities which act as templates for nanoparticle growth. For example, polyamidoamine (PAMAM) or carboxyl-terminated PAMAM [25] dendrimers are utilized as a matrix with effective nanoparticle stabilization [26]; therefore, Au, Cu, Pt, and Pd nanoparticles can be formed and stabilized therein [27]. The wavelength of surface plasmonic resonance of nanoscaled gold nano objects incorporated within PAMAM depends on the number of the layers. Increasing of the number of LbL-assembled PAMAM enlarges the distance between the Au nano objects, which leads to an ultraviolet shifting of plasmonic glow. Besides the tuning of plasmonic properties, it is also possible to adjust the fluorescent properties of gold nano objects only by magnification the thickness of multilayer or by the increasing the number of LbL cycles. For example, fluorescence of Au, covered in a core-shell manner by organic multilayers, can be easily tuned by varying the number of nonfluorescent layers [28]. Silver/gold coatings formed onto a commercial anion exchange resin via LbL [5] are very appropriate bimetallic composition for catalytic reduction of nitroaromatic compounds. Such core-shell heterostructures can be prepared by using of electrostatic force of the charged resin beads. Such resin beads support immobilization of anionic metal precursors of silver/gold nanoparticles onto the solid resin matrix and reduce 2-nitrobenzoic acid to obtain the corre-
**Figure 2** demonstrates a simple method of integrating the electroactive gold nanoparticles (AuNPs) with graphene oxide (GO) nanosheets. Such LbL structures composed of threedimensional electrocatalytic thin films are active toward methanol oxidation [9]. This approach
**oxides, and sulfides**
sponding amines through the effective catalysts.
**Figure 2.** LbL integration of gold nanoparticles (AuNPs) with graphene oxide (GO) nanosheet.
**Figure 3.** LbL films of PVP-AuNPs and single-walled CNTs supported on a fluorine-doped tin oxide (FTO) glass.
#### **2.2. Immobilization of Pd and Pt nanoparticles into the LbL matrix**
One of the main important catalytic properties of Pt and Pd containing films is electrocatalytic oxidation of methanol [12, 29, 30]. Immobilization of Pd nanoparticles (PdNPs) usually proceeds on solid supports, such as carbon, graphene, metal oxides, and zeolites [26]. Assembling of PdNPs onto carbon allows for obtaining a nanocomposite possessing chemosensitive properties. The electrocatalytically active graphene-palladium composites can be utilized as hydrogen detectors [31]. The PdNPs are stabilized by capping with ligands, ranging from small organic molecules to large polymers [22]. Immobilization of PdNPs in the form of spherical aggregates takes place by using of dendritic molecules such as amine-terminated PAMAM dendrimers (G1.0 PAMAM) or POSS-NH<sup>3</sup> + [32]. The process of self-organization of spherical templates is carried out in solution at room temperature, which allows for obtaining the dendrimers with an average size of about 70 nm.
The PdNPs synthesized by the reduction of Pd(II) to Pd(0) by using NaBH<sup>4</sup> can be incorporated onto magnetic nanoparticles (MNPs). Such incorporation is carried out by the LbL technique, which is suitable to poly(acrylic acid)-poly(ethyleneimine)/Pd(II) multilayers formation in a core-shell manner [33]. Such hybrid structures are considered to employ for the hydrogenation of various olefin alcohols. Besides using MNP as a substrate, it is also possible to assemble nanosized Pd-polyelectrolyte multilayer onto aluminum powder [34]. Diversity of PdNPs diameters, within multilayers onto aluminum powder, can be regulated by changing the ratio between poly(acrylic acid) and Pd(II). Such changing allows to obtain the ranging of particles with diameters from 2.2 to 3.4 nm. Consequently, by this way, it is easy to tune the catalytic selectivity of such a hybrid system.
The Pt nanoparticles (PtNPs) with good optical and catalytic properties can also be incorporated within PAMAM dendrimers. The simple way of PAMAM dendrimers with incorporated Pt nanoclusters deposition is LbL assembling them onto the solid substrates [28]. For instance, the Pt-PAMAM structures are obtained through the chemical reduction of H<sup>2</sup> PtCl6, in the presence of PAMAM, using formic acid as a reducing agent. Then, by alternating immersions of the substrate into the polyelectrolyte solutions consisting of poly(vinylsulfonic acid) and PAMAM dendrimers the multilayer structures are produced. Time duration for each layer formation is about 5 min [35].
Using PAMAM dendrimers/PtNPs allows to obtain the nonvolatile memory (NVM) devices [36]. However, the process of NVM assembling is slightly differ from the above-described process and involves the formation of PtNPs within a ultrathin film matrix, formed by covalent LbL assembly of pyromellitic dianhydride (PMDA) and second generation of PAMAM dendrimer in supercritical carbon dioxide (SCCO<sup>2</sup> ). To design such a structure, nanoparticles' precursor is sequestered within a dendrimer matrix by using SCCO<sup>2</sup> as a processing medium. This technique of preparation nanostructured films, with assistance of SCCO<sup>2</sup> , at room temperature is a comparable clean process.
Aside from NVM, the metal-insulator semiconducting (MIS) devices can be formed using dendrimer-encapsulated nanoparticles. MIS installation proceeds by the covalent molecular assembly of dendrimers with incorporated agents. The MIS device configuration is shown in **Figure 4**.
**Figure 4.** MIS device configuration consisting of Au bottom electrode, dendrimer-encapsulated nanoparticle layer, Al<sup>2</sup> O3 layer, and Au top electrode.
#### **2.3. Assembling of LbL films and coatings containing Fe<sup>3</sup> O4 and CdS**
**2.2. Immobilization of Pd and Pt nanoparticles into the LbL matrix**
152 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
PAMAM dendrimers (G1.0 PAMAM) or POSS-NH<sup>3</sup>
the dendrimers with an average size of about 70 nm.
the catalytic selectivity of such a hybrid system.
each layer formation is about 5 min [35].
perature is a comparable clean process.
dendrimer in supercritical carbon dioxide (SCCO<sup>2</sup>
precursor is sequestered within a dendrimer matrix by using SCCO<sup>2</sup>
This technique of preparation nanostructured films, with assistance of SCCO<sup>2</sup>
One of the main important catalytic properties of Pt and Pd containing films is electrocatalytic oxidation of methanol [12, 29, 30]. Immobilization of Pd nanoparticles (PdNPs) usually proceeds on solid supports, such as carbon, graphene, metal oxides, and zeolites [26]. Assembling of PdNPs onto carbon allows for obtaining a nanocomposite possessing chemosensitive properties. The electrocatalytically active graphene-palladium composites can be utilized as hydrogen detectors [31]. The PdNPs are stabilized by capping with ligands, ranging from small organic molecules to large polymers [22]. Immobilization of PdNPs in the form of spherical aggregates takes place by using of dendritic molecules such as amine-terminated
spherical templates is carried out in solution at room temperature, which allows for obtaining
porated onto magnetic nanoparticles (MNPs). Such incorporation is carried out by the LbL technique, which is suitable to poly(acrylic acid)-poly(ethyleneimine)/Pd(II) multilayers formation in a core-shell manner [33]. Such hybrid structures are considered to employ for the hydrogenation of various olefin alcohols. Besides using MNP as a substrate, it is also possible to assemble nanosized Pd-polyelectrolyte multilayer onto aluminum powder [34]. Diversity of PdNPs diameters, within multilayers onto aluminum powder, can be regulated by changing the ratio between poly(acrylic acid) and Pd(II). Such changing allows to obtain the ranging of particles with diameters from 2.2 to 3.4 nm. Consequently, by this way, it is easy to tune
The Pt nanoparticles (PtNPs) with good optical and catalytic properties can also be incorporated within PAMAM dendrimers. The simple way of PAMAM dendrimers with incorporated Pt nanoclusters deposition is LbL assembling them onto the solid substrates [28]. For instance, the Pt-PAMAM structures are obtained through the chemical reduction of H<sup>2</sup>
in the presence of PAMAM, using formic acid as a reducing agent. Then, by alternating immersions of the substrate into the polyelectrolyte solutions consisting of poly(vinylsulfonic acid) and PAMAM dendrimers the multilayer structures are produced. Time duration for
Using PAMAM dendrimers/PtNPs allows to obtain the nonvolatile memory (NVM) devices [36]. However, the process of NVM assembling is slightly differ from the above-described process and involves the formation of PtNPs within a ultrathin film matrix, formed by covalent LbL assembly of pyromellitic dianhydride (PMDA) and second generation of PAMAM
Aside from NVM, the metal-insulator semiconducting (MIS) devices can be formed using dendrimer-encapsulated nanoparticles. MIS installation proceeds by the covalent molecular assembly of dendrimers with incorporated agents. The MIS device configuration is shown in **Figure 4**.
The PdNPs synthesized by the reduction of Pd(II) to Pd(0) by using NaBH<sup>4</sup>
+
[32]. The process of self-organization of
). To design such a structure, nanoparticles'
as a processing medium.
, at room tem-
can be incor-
PtCl6,
In this section, we describe LbL immobilized Fe<sup>3</sup> O4 and CdS nanoparticles that possess magnetic, semiconducting, optic, and other properties. Since metal oxides are widely used in gas sensing application [38], electrochemical capacitors [39], lithium-ion batteries [40], photocatalytic materials [41]; it is important to develop their installation direct on the electrodes. Therefore, the process of immobilization of metal oxides such as TiO<sup>2</sup> , Fe<sup>3</sup> O4 , and ZnO by using the LbL method, where the general assembling of metal oxides proceeds with assistance of polyelectrolytes [42], has been tremendously studied [43]. In the past two decades, a great attention has been paid to the synthesis of Fe<sup>3</sup> O4 magnetic nanoparticles due to their cheapness, nontoxicity and readily producing. Direct deposition of Fe<sup>3</sup> O4 onto an electron conductive material, such as indium tin oxide (ITO)-coated glass, allows using it in electrochemical capacitor application [44] and biomedicine. Combination of various fabrication methods of these particles with the LbL deposition technique allows to obtain high quality core-shell architectures. The Fe<sup>3</sup> O4 preparation method in general involves the dissolution of the mixture of FeCl<sup>3</sup> and FeCl<sup>2</sup> in aqueous solution (chemical coprecipitation). The obtained particles of Fe<sup>3</sup> O4 , modified by polymers, can be coated onto quantum dots (Qds), such as CdTe, in a core-shell manner via the LbL technique. Such strategy allows to fabricate the magnetic luminescence Fe<sup>3</sup> O4 nanocomposites [44]. The medical application of magnetic structures, incorporated via LbL has also been developed. In particular, the Fe<sup>3</sup> O4 and Pt nanoparticles, incorporated into hemoglobin, improves the biosensitivity of the protein [45]. Employment of Fe<sup>3</sup> O4 for magnetic separations of protein is possible by coating it onto SiO<sup>2</sup> particles [46]. Another good example of Fe<sup>3</sup> O4 particles involved in the high functional core-shell formation is Fe<sup>3</sup> O4 /Au structure. Such composition is widely applied in biomedical and technological fields due to their unique optical, magnetic, and catalytic properties [47].
Since the Fe<sup>3</sup> O4 particles possess magnetic properties, it is possible to assemble them with the help of magnetic field. Combination of the LbL technique and magnetic field leads to compaction of particle packing without increasing the total thickness of the obtained film [48].
CdS is one of the most interesting semiconducting materials due to its band structure, luminescent aptitude, etc. Nanoscaled CdS particles with a size range up to 10 nm can be considered as Qds and currently attract a large number of researchers due to their unique optical and electrical properties [49]. There are several CdS Qds preparation methods, but among them the LbL derivation, also known as successive ionic layer adsorption SILAR [50] deposition, is a very promising approach due to its versatility and simplicity to obtain high controlled objects. The method is as follows: in separate beakers, the dissolved ions of Cd<sup>+</sup> and S− are deposited onto a substrate, forming CdS structure. It is obvious that the structure-building force is Columbic attraction. This technique proceeds without participation of polymers, consequently it is not necessary to remove organics by the calcination of structure for increasing the attaching surface between the CdS layer and the active materials. The theoretical description of SILAR (LbL derivation) [50] can be described as following, the sequential immersion of the substrate into oppositely charged liquid solution results the reaction between the substrate and dissolved species. Heterogeneity of layer structure enriches by rinsing in water after each dipping into electrolyte solution. The principle of film growing can be explained by the following equation:
$$(p\,X\_{aq}^{a\*} + q\,X\_{aq}^{b\*}) + (b\,\,^{\prime}Y\_{aq}^{q+} + a\,A^{q-}) \to \,\,\,\text{Kp}\,A\_{s\downarrow} + q\,X\_{aq}^{b\*} + b^{\prime\prime}i\,;ap=bq=b^{\prime}q^{\prime},\tag{1}$$
where,
K is cation (Cd2+, Fe3+, Cu<sup>+</sup> , etc.)
A represents the anions (O, S, or Se)
*p* is the number of cations
*a* is the numerical value of charge on cation
Y is the ion which is attached to chalcogen ion
X represents an ion in cationic precursors
*q* represents the number of ions
*b* represents the charge value of ions
*b*' represents the number of ions attached to chalcogens
*q*' represents the charge value of ions attached to chalcogens
Schematic representation of the SILAR process is shown in **Figure 5**. The CdS Qds obtained by the LbL (SILAR) method can easily be used in the fabrication of Qds sensitized solar cells, which are very well suited for the creation of alternatives to silicon-based photovoltaic devices. A simple example of preparation of CdS-based Qds-sensitized solar energy converting construct is given by Chen and co. [51]. They sensitized ZnO nanosheets, obtained through the three-electrode electrodeposition method, with further coating them by CdS Qds. Such coating can be assembled through alternating dipping of ZnO nanosheets comprise a glass substrate into the liquid solutions. The whole process of LbL CdS assembling can be described as:
**Figure 5.** General scheme of the SILAR method for the fabrication of Qds.
## **3. Application of NPs immobilized via the LbL assembly in catalysis**
#### **3.1. LbL-assembled layers for water splitting application**
Hydrogen production by solar driving water splitting is a promising energy generation way. The development of hydrogen production is based on the fact that it is an ideal fuel for the future [52]. Among the different approaches to release hydrogen gas, photoelectrochemical (PEC) water splitting is the most promising. Overall water splitting reaction can be written as:
$$\mathbf{H}\_{\text{2}}\mathbf{O}(\text{liquid}) = \mathbf{H}\_{\text{2}}(\text{gas}) + \mathbb{W}\_{\text{2}}(\text{gas})\,. \tag{2}$$
A PEC cell consists of two electrodes: one is called anode/photoanode and another is cathode/ photocathode. On the surface of cathode, H<sup>2</sup> O molecule is reduced:
$$2\mathbf{H} + 2\,\mathbf{e}^- = \mathbf{H}\_{2(ga)}\tag{3}$$
whereas on the anodes, it oxidized.
Qds and currently attract a large number of researchers due to their unique optical and electrical properties [49]. There are several CdS Qds preparation methods, but among them the LbL derivation, also known as successive ionic layer adsorption SILAR [50] deposition, is a very promising approach due to its versatility and simplicity to obtain high controlled objects. The method
forming CdS structure. It is obvious that the structure-building force is Columbic attraction. This technique proceeds without participation of polymers, consequently it is not necessary to remove organics by the calcination of structure for increasing the attaching surface between the CdS layer and the active materials. The theoretical description of SILAR (LbL derivation) [50] can be described as following, the sequential immersion of the substrate into oppositely charged liquid solution results the reaction between the substrate and dissolved species. Heterogeneity of layer structure enriches by rinsing in water after each dipping into electrolyte solution. The
<sup>+</sup> + *a Ap*<sup>−</sup> ) → *KpA as*↓ + *q Xaq*
Schematic representation of the SILAR process is shown in **Figure 5**. The CdS Qds obtained by the LbL (SILAR) method can easily be used in the fabrication of Qds sensitized solar cells, which are very well suited for the creation of alternatives to silicon-based photovoltaic devices. A simple example of preparation of CdS-based Qds-sensitized solar energy converting construct is given by Chen and co. [51]. They sensitized ZnO nanosheets, obtained through the three-electrode electrodeposition method, with further coating them by CdS Qds. Such coating can be assembled through alternating dipping of ZnO nanosheets comprise a glass substrate into the liquid solutions. The whole process of LbL CdS assembling
(1) Immersing of negatively charged ZnO deposited glass substrate in a beaker containing
to adsorb Cd2+. Resulting charge of the glass surface becomes positive.
and S−
*<sup>b</sup>*<sup>−</sup> + *b*′*Yaq q* ′ +
; *ap* = *bq* = *b*′
are deposited onto a substrate,
*q*′ , (1)
S to deposit S2−.
is as follows: in separate beakers, the dissolved ions of Cd<sup>+</sup>
154 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
(*p Kaq*
K is cation (Cd2+, Fe3+, Cu<sup>+</sup>
*p* is the number of cations
can be described as:
0.05 M Cd(NO<sup>3</sup>
) 2
(2) Rinsing surface with deionized water to remove excess of ions.
(3) Dipping of a positively charged substrate into dissolved 0.05 M Na<sup>2</sup>
(4) Final cleaning of surface is carried out by rinsing with clean water.
where,
*<sup>a</sup>*<sup>+</sup> + *q Xaq*
A represents the anions (O, S, or Se)
*q* represents the number of ions *b* represents the charge value of ions
*a* is the numerical value of charge on cation Y is the ion which is attached to chalcogen ion X represents an ion in cationic precursors
*<sup>b</sup>*<sup>−</sup> ) +(*b*′
, etc.)
*b*' represents the number of ions attached to chalcogens *q*' represents the charge value of ions attached to chalcogens
principle of film growing can be explained by the following equation:
*Yaq q*′
$$\text{H}\_2\text{O}\_{\text{(l/quadd)}} + 2\,\text{h}^+ = 2\text{H} + \text{O}\_{2(gas)}\tag{4}$$
where e<sup>−</sup> is an electron and h<sup>+</sup> is a hole. This process must take place under solar irradiation. Efficiency of this process depends on the following factors:
To overcome these affecting factors on the efficiency of the cell, scientists need to develop more ideal electrodes. It means the control of all technical parameters including thickness of semiconducting layer, morphology, and density of particles, which forms active layer and so on. A simple way to control the distance between the electroactive species inside of multilayer structure is offered by LbL assembling [53]. Absorption of light by semiconducting materials results in creation an exciton, electron-hole pair. To use an exciton in the water splitting process, it must be separated. The separated electron and hole act as reduction and oxidation centers. Coupling of two different types of active materials allows the faster separation of exciton. For example, a couple of anatase and rutile phases of TiO<sup>2</sup> improves the separation of exciton, created in the anatase structure [43]. In order to obtain a homogeneously distributed anatase/rutile, a heterostructure LbL approach is applied. The LbL assembling to building of anatase/rutile heterostructure is carried out in the presence of poly(sodium-4-styrenesulfonate) (PSS). This polymer serves as an adsorption layer. The rutile phase of TiO2 coated by PSS adsorbs an anatase structure. To remove polymer interlayer and form good attachment between rutile and anatase the obtained material is heated up to 500o C (**Figure 6**).
**Figure 6.** Formation of rutile/anatase heterostructure by LbL assembling.
Improvement of solar energy conversion can be achieved using an aligned structure, such as ZnO nanorods, because of reducing the charge pathway (**Figure 7**). The ZnO is a broadly used as a wide band gap semiconductor [37], and it plays a great role in gas sensing, optical and electrical devices. In spite of similarity of band structures between ZnO and TiO<sup>2</sup> , electron mobility on ZnO is much higher. But negligible instability of ZnO in an aqueous solution makes it unfavorable for this material in the water splitting process. Recently, protection of ZnO was offered by building a core/shell structure, where a core is ZnO and a shell is represented by narrow band gap semiconductors. Covering of ZnO nanorods by narrow band gap CdS was studied a lot. The most attractive approach for it is using of SILAR technique [20, 54, 55]. The covering of free-standing ZnO nanorods by CeO<sup>2</sup> , CdS, and Ag nanoparticles proceeds, as shown in **Figure 8**. The negative charges on the ZnO surface are formed by immersing the substrate into PAH and PSS. Then, the oppositely charged ions (Ce3+, Cd2+, Ag<sup>+</sup> ) are adsorbed on the surface of negatively charged ZnO nanorods due to strong electrostatic attraction. Reduction of metal ions is performed by NaBH<sup>4</sup> . To use such composition for PEC water splitting or in dye-sensitized solar cells they should have a good contact to the electrode.
This process must take place under solar irradiation. Efficiency of this process depends on the
To overcome these affecting factors on the efficiency of the cell, scientists need to develop more ideal electrodes. It means the control of all technical parameters including thickness of semiconducting layer, morphology, and density of particles, which forms active layer and so on. A simple way to control the distance between the electroactive species inside of multilayer structure is offered by LbL assembling [53]. Absorption of light by semiconducting materials results in creation an exciton, electron-hole pair. To use an exciton in the water splitting process, it must be separated. The separated electron and hole act as reduction and oxidation centers. Coupling of two different types of active materials allows the faster separation of exciton. For example, a couple of anatase and rutile phases of TiO<sup>2</sup> improves the separation of exciton, created in the anatase structure [43]. In order to obtain a homogeneously distributed anatase/rutile, a heterostructure LbL approach is applied. The LbL assembling to building of anatase/rutile heterostructure is carried out in the presence of poly(sodium-4-styrenesulfonate) (PSS). This polymer serves as an adsorption layer. The
layer and form good attachment between rutile and anatase the obtained material is heated
coated by PSS adsorbs an anatase structure. To remove polymer inter-
following factors:
rutile phase of TiO2
C (**Figure 6**).
**Figure 6.** Formation of rutile/anatase heterostructure by LbL assembling.
up to 500o
(1) The ability of photoelectrodes to adsorb a photon
156 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
(3) The efficacy of charge carriers in the water splitting process.
(2) The performance of creation charge carries
**Figure 7.** Schematic representation of the electron transport process through nanoparticles (a) and nanorods (b).
The immersing of the conductive substrate coated by aligned ZnO nanorods in two different aqueous solutions of Cd2+ cations and S2− anions for 20 second allows to obtain a ZnO/ CdS core-shell structure, which is a suitable heteromaterial for water splitting application. Effectiveness of such PEC water splitting of the ZnO/CdS core-shell composition has been showed an increased open voltage of around −1.55 V vs. bare ZnO arrays whose open voltage is – 0.8 V [56].
#### **3.2. Reduction and oxidation of organic substrates by metal nanoparticles immobilized within LbL films and membranes**
In the past few years, LbL films and membranes are more commonly used in catalytic processes. Films with incorporated metal nanoparticles synthesized by the LbL technique are commonly used for multifunctional catalytic applications, including photocatalysis, thermal catalysis, and electrocatalysis, which means the reduction [57] and oxidation of various organic substrates [18, 58]. Authors [59] showed that LbL covalently stacked multilayer structure of immobilized metal nanoparticles ensure the stability of particles against aggregation. Such multilayer structure can be achieved by embedding the nanoparticles into a porous polymer membrane. The porous structure of polymer membrane is utility to creation of concentration gradient between the aqueous medium and reaction centers, which accelerate the reaction rate. The catalytic activity of immobilized metal nanoparticles within the polymer scaffold depends on the number of bilayers as well as the size of the particles. For instance, 10 bilayers show higher catalytic activity than the higher numbers of layers. While the high number of bilayers exhibits good stability. The optimization of such criteria is possible by modulating the number of layers in the LBL structure, which allows to prepare highly catalytic active and stable films using this simple and versatile approach.
**Figure 8.** Using the LbL technique for covering free-standing ZnO nanorods by CeO<sup>2</sup> , CdS, and Ag nanoparticles.
The multilayered metal nanoparticles deposited onto TiO<sup>2</sup> nanotubes demonstrate efficient thermal catalytic activities toward reduction of nitrophenol to nitroaniline under ambient conditions [60]. The catalytic properties of metal nanoparticles/TiO<sup>2</sup> nanotubes (M/TNT) nanocomposites are achieved mainly due to the distribution of monodispersed metal nanoparticles on TNT [61, 62]. It is evident that distribution of nano objects onto a substrate affects the catalytic performance of multilayer films. Therefore, surface modification of 1D semiconductors through the LbL assemble strategy can be used as an effective way to achieve a uniform deposition of metal nanoparticles for various catalytic applications. The electrocatalytic performances of LbL-assembled multilayers are also extensively exploit for selective hydrogenation of a series of unsaturated alcohols [34] and methanol oxidation. For example, a strong synergistic catalytic behavior exhibits polyaniline-Pt (PANI/Pt) nanocomposites fabricated by the modification of LbL assembly by electrodeposition [42]. Such synergic catalytic activity is used for methanol oxidation. But the catalytic activity of such composites depends not only on a number of layers (and hence the Pt loading) but mostly on nature of the outermost layer. The catalytic activity of such multilayers can be enhanced when they end by the PANI layer as the oxidation of methanol by Pt particles facilitated by the formation of hydrogen bonds with the outer PANI layer.
## **4. Concluding remarks**
catalysis, and electrocatalysis, which means the reduction [57] and oxidation of various organic substrates [18, 58]. Authors [59] showed that LbL covalently stacked multilayer structure of immobilized metal nanoparticles ensure the stability of particles against aggregation. Such multilayer structure can be achieved by embedding the nanoparticles into a porous polymer membrane. The porous structure of polymer membrane is utility to creation of concentration gradient between the aqueous medium and reaction centers, which accelerate the reaction rate. The catalytic activity of immobilized metal nanoparticles within the polymer scaffold depends on the number of bilayers as well as the size of the particles. For instance, 10 bilayers show higher catalytic activity than the higher numbers of layers. While the high number of bilayers exhibits good stability. The optimization of such criteria is possible by modulating the number of layers in the LBL structure, which allows to prepare highly catalytic active and stable films
using this simple and versatile approach.
158 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
The multilayered metal nanoparticles deposited onto TiO<sup>2</sup>
**Figure 8.** Using the LbL technique for covering free-standing ZnO nanorods by CeO<sup>2</sup>
[60]. The catalytic properties of metal nanoparticles/TiO<sup>2</sup>
mal catalytic activities toward reduction of nitrophenol to nitroaniline under ambient conditions
nanotubes demonstrate efficient ther-
, CdS, and Ag nanoparticles.
nanotubes (M/TNT) nanocomposites are
Immobilization of metal and semiconducting nanoparticles within the multilayer structure by the LbL self-assembling technique is an effective process to design drug delivery systems, capacitors, sensors, solar and fuel cells, quantum dots, catalysts with unique properties. Gold, silver, palladium, and platinum nanoparticles incorporated into the polymer thin films leads to enhancement of plasmonic and catalytic properties, which can be easily tuned by changing multilayer structure. Preparation of nanocatalysts using the LbL assembly technique represents a comparatively simple, robust, efficient, and highly versatile method and demonstrates significant advantages over routine methods. The multilayer thin films containing metal nanoparticles demonstrate efficient catalytic activities toward reduction of nitrophenol, oxidation of methanol, and selective hydrogenation of unsaturated alcohols under ambient conditions. One of the promising areas is to use the LbL technology for photocatalytic decomposition of water. Photoelectrochemical cells can be develop by alternately applying of organic and inorganic semiconducting materials and dyes on transparent conductive substrates such as indium tin oxide glass, aluminum-doped zinc oxide glass, and so forth. It is anticipated that the LbL methods and technologies will definitely expand its horizon toward practical applications in the commercial realm and to new discoveries in the fields of polymer and materials science and engineering.
## **Author details**
Sarkyt Kudaibergenov1,2\*, Gulnur Tatykhanova1,<sup>2</sup> , Nurlan Bakranov<sup>1</sup> and Rosa Tursunova<sup>1</sup>
\*Address all correspondence to: [email protected]
1 Laboratory of Engineering Profile, K.I Satpayev Kazakh National Research Technical University, Almaty, Kazakhstan
2 Private Institution "Institute of Polymer Materials and Technology", Almaty, Kazakhstan
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#### **RRAM Memories with ALD High-K Dielectrics: Electrical Characterization and Analytical Modeling RRAM Memories with ALD High-K Dielectrics: Electrical Characterization and Analytical Modeling**
Helena Castán, Salvador Dueñas, Alberto Sardiña, Helena Castán, Salvador Dueñas, Alberto Sardiña, Héctor García, Tõnis Arroval, Aile
Héctor García, Tõnis Arroval, Aile Tamm, Tamm, Taivo Jõgiaas, Kaupo Kukli and Jaan
Taivo Jõgiaas, Kaupo Kukli and Jaan Aarik Aarik
Additional information is available at the end of the chapter Additional information is available at the end of the chapter
http://dx.doi.org/10.5772/66666
#### **Abstract**
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Reduction of H<sup>2</sup>
Resistive switching phenomena with adequate repetitiveness on Ta2 O5 -TiO2 -Ta2 O5 and TiO2 -Ta2 O5 -TiO2 stacks are reported. In particular, 5–nm-thick TiO2 films embedding a monolayer of Ta2 O5 show the best behavior in terms of bipolar cycles loop width, with separate low and high resistive states up to two orders of magnitude. Tantalum oxide layer increases the defect density in titania that becomes less leaky, and thus, resistive switching effects appear. Small signal ac parameters measured at low and medium frequencies, namely capacitance and conductance, also show hysteretic behavior during a whole bipolar switching cycle. This means that the memory state can be read at 0 V, without any power consumption. High-frequency measurements provide information about dipole relaxation frequency values in the dielectric bulk, and this can be connected with resistive switching behavior. Finally, a double tunneling barrier model fits I-V curves at the low-resistance state even at the bias range where reset occurs and a sharp fall takes place.
**Keywords:** resistive switching, tantalum oxide, titanium oxide, RF impedance, modeling
## **1. Introduction**
Nowadays, resistive random access memories (RRAMs) have been considered as adequate candidates to replace the current nonvolatile memories, because of their good characteristics
and reproduction in any medium, provided the original work is properly cited.
© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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in terms of integration density, speed, power dissipation, and endurance [1, 2]. RRAMs modify the resistivity of metal-insulator-metal (MIM) or metal-insulator-semiconductor (MIS) structures. These devices exhibit a resistive switching (RS) behavior, due to the creation after an initial step (electroforming), of one or several nano-conductive filaments that can connect the two electrodes [3]. Filaments can be broken and formed again by means of an external bias; hence, there are two different resistive states, low resistance (LR) and high resistance (HR). The device can remain in one of the two resistive states for a long time. The RS behavior depends on the dielectric material. Also, top and bottom electrodes play an important role. It has been reported that atomic layer deposited (ALD) transition metal oxides, such as HfO<sup>2</sup> , ZrO2 , TiO2 , and so on, exhibit RS behavior [4]. Usually, RS is classified into unipolar and bipolar: The first one depends only on the amplitude of the applied voltage, whereas the second one depends also on the polarity of the applied voltage. Three different mechanisms are considered as responsible for the RS phenomena [5, 6]: the conducting bridge random access memories (CBRAM), in which the conductive filament is formed from the atoms of one of the two metallic electrodes; the valence change mechanism that is attributed to the migration of oxygen anions and a subsequent redox reaction; and finally, the thermochemical mechanism that consists of a change of stoichiometry related to the temperature increment. Despite the great amount of work done, the physic mechanisms of RS are not fully understood; therefore, a great deal of research must be still carried out.
This chapter consists of three parts: In the first one, constituting the main body of this work, resistive switching phenomena on Ta2 O5 -TiO2 -based metal-insulator-metal (MIM) structures are reported. Ta2 O5 -TiO2 films were grown to target thickness of 6 nm. The films were grown either as nanolaminate-like stacks consisting of Ta2 O5 and TiO2 constituent layers, each grown to nominal thickness of 2 nm, or 5- to 6–nm-thick TiO2 films embedding a monolayer of Ta<sup>2</sup> O5 grown using only 1–3 ALD cycles of Ta<sup>2</sup> O5 . The stacks were grown in order to increase the defect densities in titania by inserting otherwise more insulating tantalum oxide and examine the possible effect of the controlled, artificial, layering of different metal oxides to the appearance of resistive switching effect. In the second part, some RF impedance measurement results in more standard metal-insulator-semiconductor (MIS) samples (Ni/HfO<sup>2</sup> /Si and W/ HfO2 /Si) are shown. The response at frequencies up to 3 GHz is analyzed. The most remarkable fact is that both the relaxation frequency in capacitance curves and the conductance maximum position can vary with the bias voltage depending on the top electrode material. Finally, in the third part of this chapter, two analytical models that fit well the current values in the low-resistance state for Ni/HfO<sup>2</sup> /Si structures are described. The first one considers a single tunneling barrier, whereas the second one uses the double tunneling barrier model. A comparison between the two models is carried out.
#### **2. Resistive switching on Ta2 O5 -TiO2 -based MIM structures**
MIM samples investigated were obtained by depositing the films on 15 nm-RuO<sup>2</sup> /10 nm-TiN/ Si substrates. Thin solid titanium tantalum oxide films were grown in an in-house built lowpressure flow-type ALD reactor [7] as stacks formed as TiO2 -Ta2 O5 -TiO2 or Ta2 O5 -TiO2 -Ta2 O5 triple layers [8] at the substrate temperature of 350°C. Constituent TiO<sup>2</sup> and Ta2 O5 layers were grown by using TiCl<sup>4</sup> [7] and TaCl5 [9], respectively, as metal precursors. In both cases, ozone, O3 , was applied as oxygen precursor. TiO2 layers were grown using cycle times 2–2–5–5 s, denoting sequence of TiCl4 pulse length—purge time—ozone pulse length—purge time, respectively. For Ta<sup>2</sup> O5 , the corresponding cycle times were 3–2–5–5 s. In all cases studied, these process time parameters allowed reliable self-limited ALD-type growth of constituent layers in this reactor, as evaluated by quartz crystal microbalances in real time prior to the growth of stacked layers. For the growth of Ta<sup>2</sup> O5 -TiO2 -Ta2 O5 stacks, 60 × Ta<sup>2</sup> O5 + 50 × TiO<sup>2</sup> + 60 × Ta<sup>2</sup> O5 ALD cycles were applied, denoting the consequent numbers of the constituent oxide growth cycles. Analogously, for the growth of TiO2 -Ta2 O5 -TiO2 stacks, 50 × TiO<sup>2</sup> + 70 × Ta<sup>2</sup> O5 + 50 × TiO<sup>2</sup> ALD cycles were applied. Additional TiO<sup>2</sup> -Ta2 O5 -TiO2 stacks were also grown after 75 × TiO<sup>2</sup> + 1 × Ta<sup>2</sup> O5 + 75 × TiO<sup>2</sup> and 75 × TiO<sup>2</sup> + 3 × Ta<sup>2</sup> O5 + 75 × TiO<sup>2</sup> ALD cycles. The abovementioned stacked films will hereafter be denoted as samples (60-50-60), (50-70-50), (75-1-75), and (75-3-75), respectively. Top electrodes were Pt dots with two different areas (0.52 × 10−3 cm2 and 2.04 × 10−3 cm2 ).
in terms of integration density, speed, power dissipation, and endurance [1, 2]. RRAMs modify the resistivity of metal-insulator-metal (MIM) or metal-insulator-semiconductor (MIS) structures. These devices exhibit a resistive switching (RS) behavior, due to the creation after an initial step (electroforming), of one or several nano-conductive filaments that can connect the two electrodes [3]. Filaments can be broken and formed again by means of an external bias; hence, there are two different resistive states, low resistance (LR) and high resistance (HR). The device can remain in one of the two resistive states for a long time. The RS behavior depends on the dielectric material. Also, top and bottom electrodes play an important role. It has been reported that atomic layer deposited (ALD) transition metal oxides, such as HfO<sup>2</sup>
bipolar: The first one depends only on the amplitude of the applied voltage, whereas the second one depends also on the polarity of the applied voltage. Three different mechanisms are considered as responsible for the RS phenomena [5, 6]: the conducting bridge random access memories (CBRAM), in which the conductive filament is formed from the atoms of one of the two metallic electrodes; the valence change mechanism that is attributed to the migration of oxygen anions and a subsequent redox reaction; and finally, the thermochemical mechanism that consists of a change of stoichiometry related to the temperature increment. Despite the great amount of work done, the physic mechanisms of RS are not fully understood; therefore,
This chapter consists of three parts: In the first one, constituting the main body of this work,
defect densities in titania by inserting otherwise more insulating tantalum oxide and examine the possible effect of the controlled, artificial, layering of different metal oxides to the appearance of resistive switching effect. In the second part, some RF impedance measurement
/Si) are shown. The response at frequencies up to 3 GHz is analyzed. The most remarkable fact is that both the relaxation frequency in capacitance curves and the conductance maximum position can vary with the bias voltage depending on the top electrode material. Finally, in the third part of this chapter, two analytical models that fit well the current values
single tunneling barrier, whereas the second one uses the double tunneling barrier model. A
Si substrates. Thin solid titanium tantalum oxide films were grown in an in-house built low-
O5
. The stacks were grown in order to increase the
/Si structures are described. The first one considers a
**-based MIM structures**
O5 -TiO2 -Ta2 O5
films embedding a monolayer of Ta<sup>2</sup>
constituent layers, each grown
films were grown to target thickness of 6 nm. The films were grown
and TiO2
O5 -TiO2
O5
results in more standard metal-insulator-semiconductor (MIS) samples (Ni/HfO<sup>2</sup>
**O5**
pressure flow-type ALD reactor [7] as stacks formed as TiO2
**-TiO2**
MIM samples investigated were obtained by depositing the films on 15 nm-RuO<sup>2</sup>
, and so on, exhibit RS behavior [4]. Usually, RS is classified into unipolar and
ZrO2
, TiO2
are reported. Ta2
HfO2
a great deal of research must be still carried out.
166 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
either as nanolaminate-like stacks consisting of Ta2
to nominal thickness of 2 nm, or 5- to 6–nm-thick TiO2
resistive switching phenomena on Ta2
grown using only 1–3 ALD cycles of Ta<sup>2</sup>
in the low-resistance state for Ni/HfO<sup>2</sup>
**2. Resistive switching on Ta2**
comparison between the two models is carried out.
O5 -TiO2 ,
O5
/Si and W/
/10 nm-TiN/
TiCl4 was kept at room temperature (22 ± 3°C). TaCl<sup>5</sup> was evaporated at 80-85°C from a fused silica boat inside the reactor. O<sup>3</sup> was generated from O2 (99.999%) in a BMT Messtechnik 802N ozone generator. Ozone concentration measured with BMT Messtechnik 964 analyzer at the reactor inlet was 200–230 g/m<sup>3</sup> at the normal pressure. N2 (99.999%) was used as the carrier and purging gas. No post-deposition heat treatment was applied on the samples.
The mass thickness and elemental composition of the films were measured by X-ray fluorescence (XRF) spectroscopy method using ZSX400 (Rigaku) spectrometer, recording Kα lines for Ti, Cl, and O, and Lα for Ta. For the calibration of the XRF measurement procedure, binary TiO2 and Ta2 O5 films earlier grown to known thicknesses and densities determined by the X-ray reflection (XRR) method were used. Grazing incidence X-ray diffraction (GIXRD) was applied for the examination of the film structure using Smartlab (Rigaku) X-ray diffractometer with CuKα radiation. In accord with the XRF analysis, the (60-50-60) stacks contained 54.5 wt.% Ta, 12.1 wt.% Ti, 32.8 wt.% O, and 0.55 wt.% residual Cl, whereas the (50-70-50) ones contained 48.8 wt.% Ta, 21.0 wt.% Ti, 30.0 wt.% O, and 0.24 wt.% residual Cl. On the other hand, the (75-3-75) stacks contained 10.9 wt.% Ta, 48.7 wt.% Ti, 40.0 wt.% O, and 0.28 wt.% residual Cl, whereas the (75-1-75) ones contained 4.6 wt.% Ta, 53.8 wt.% Ti, 41.4 wt.% O, and 0.23 wt.% residual Cl.
Considering the results of the compositional analysis, the relative amounts of titanium and tantalum (oxides) in the films appreciably correlated with the amounts of cycles applied for either constituent oxide. Certain residual contamination with chlorine was expected due to the presence of chlorine as ligand atoms in both metal precursors and their incomplete removal during surface reactions with ozone. The films deposited to rather low thicknesses of 6 nm and below that were essentially amorphous as revealed by their featureless XRD patterns (**Figure 1**). Short-range order was recognized, however, in the TiO2 -rich films deposited using the cycle sequence 75-1-75, that is, in the film containing markedly less than one monolayer of Ta2 O5 between two TiO2 layers both grown using 75 ALD cycles. The amorphicity of the films can be explained taking into account that layers are too thin to become ordered
**Figure 1.** Grazing incidence X-ray diffraction (GIXRD) patterns from nanolaminate TiO<sup>2</sup> -Ta2 O5 -TiO2 and Ta2 O5 -TiO2 - Ta2 O5 stacks. The numbers of both constituent oxide growth cycles in sequence is indicated by labels.
**Figure 2.** Raman spectra from bare ruthenium oxide electrode (bottom curve), and TiO<sup>2</sup> -Ta2 O5 -TiO2 stacks grown applying one (middle curve) and three (top curve) ALD cycles of Ta<sup>2</sup> O5 between TiO<sup>2</sup> layers. The total film thicknesses were 5.0 and 5.5 nm, respectively.
crystallographically. Moreover, the layers consist of mixed materials foreign to each other both chemically and structurally. This, as any other doping, essentially increases the disorder in the materials, both in terms of long-range and short-range periodicity. In the Raman spectra of the 75-1-75 sample (**Figure 2**), a peak typical for anatase phase was detected at 143–145 cm−1 [10, 11] and also seen earlier in TiO2 films grown by ALD from TiCl<sup>4</sup> to H2 O [12]. In the 75-3- 75 sample, three Ta2 O5 growth cycles were applied between the halves of the TiO<sup>2</sup> host layer, and the structural disorder was evidently increased. Consequently, anatase phase could not be recognized any more. Instead, broad Raman bands appeared at 300 and 800 cm−1, which could not be clearly attributed to any known TiO2 phase. However, the bands follow those obtained from the bare reference RuO<sup>x</sup> electrode substrate. RuO<sup>2</sup> [13] is known as the material possessing rutile structure, and the bands in **Figure 2** are thus denoted with R, to guide the readers eye. In this connection, these 5- to 7-nm-thick films studied are to be characterized as crystallographically very weakly ordered and highly defective.
Electrical measurements of MIM structures were carried out, putting the sample in a lighttight and electrically shielded box. I-V curves were measured using a HP-4155B semiconductor parameter analyzer. C-V and G-V measurement setup was based on a Keithley 4200SCS semiconductor analyzer. After the study of pristine samples, the filaments in MIM devices were electroformed by DC bias sweeping from 0 to 0.7 V with a current compliance of 10 mA. Then, successive I-V cycles showing low-resistance state (LRS) to high-resistance state (HRS) transitions were recorded with current compliance of 100 mA.
Current measurements were carried out by varying the applied voltage in two modes: DC sweep and pulse modes. It is mandatory to carry out measurements using bias pulses, because in the final high-end applications of RRAM, devices are operated in the pulse mode [14]. **Figure 3** shows filament electroforming and the first resistive switching cycles of a Pt/Ta<sup>2</sup> O5 - TiO2 -Ta2 O5 /RuO<sup>x</sup> MIM sample at room temperature. Voltage bias applied was progressively varied as it is indicated by arrows. After the first forming cycle at 0.7 V, subsequent voltage ramps were applied showing the two different resistance states. Positive voltages produce the high-resistance state to the low-resistance state transition (set). On the back sweep, the lowresistance state is maintained. Using a negative polarity, when voltage reaches the values of about −1 V, the device is switched back to the high-resistance state (reset). Thus, this sample exhibits bipolar resistance switching at low-voltage values. RS parameters are independent on the electrode area; therefore, the switching mechanism is governed by filamentary conduction. The most likely hypothesis is the generation of oxygen vacancies under the applied electric field during positive forming sweep [15]. Oxygen vacancies tend to cluster and generally form filamentary shapes under an electric field. When such clusters are formed, the resistance of the local region becomes much lower than that of the surrounding oxide, and the low-resistance and high-resistance states will therefore be determined by the creation and
crystallographically. Moreover, the layers consist of mixed materials foreign to each other both chemically and structurally. This, as any other doping, essentially increases the disorder in the materials, both in terms of long-range and short-range periodicity. In the Raman spectra of the 75-1-75 sample (**Figure 2**), a peak typical for anatase phase was detected at 143–145 cm−1
**Figure 1.** Grazing incidence X-ray diffraction (GIXRD) patterns from nanolaminate TiO<sup>2</sup>
168 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
**Figure 2.** Raman spectra from bare ruthenium oxide electrode (bottom curve), and TiO<sup>2</sup>
applying one (middle curve) and three (top curve) ALD cycles of Ta<sup>2</sup>
stacks. The numbers of both constituent oxide growth cycles in sequence is indicated by labels.
and the structural disorder was evidently increased. Consequently, anatase phase could not be recognized any more. Instead, broad Raman bands appeared at 300 and 800 cm−1, which could not be clearly attributed to any known TiO2 phase. However, the bands follow those
possessing rutile structure, and the bands in **Figure 2** are thus denoted with R, to guide the
films grown by ALD from TiCl<sup>4</sup>
O5
between TiO<sup>2</sup>
growth cycles were applied between the halves of the TiO<sup>2</sup>
electrode substrate. RuO<sup>2</sup>
to H2
layers. The total film thicknesses
and Ta2 O5 -TiO2 -
O [12]. In the 75-3-
[13] is known as the material
host layer,
stacks grown
[10, 11] and also seen earlier in TiO2
obtained from the bare reference RuO<sup>x</sup>
O5
75 sample, three Ta2
were 5.0 and 5.5 nm, respectively.
Ta2 O5
**Figure 3.** CF electroforming and the first bipolar switching cycles of Pt/Ta<sup>2</sup> O5 -TiO2 -Ta2 O5 /RuO<sup>x</sup> MIM samples at room temperature.
rupture of the filaments, respectively, which percolate through the sample. In general, set and reset loops are asymmetrical, as it is seen in **Figure 3**. Electroforming in bipolar switching may be a process of introducing asymmetric interfaces in a two-terminal switching cell, which are beyond the asymmetry due to asymmetric electrodes, that is, different top and bottom electrodes. Thus, the electroforming may take one of the two interfaces as an active interface, depending on the polarity of the electroforming voltage, so that reactions taking place in the vicinity of the active interface are responsible for the subsequent bipolar switching [6].
Pulsed biasing comprises positive and negative pulses which lead the samples to the lowresistance and high-resistance states, respectively. To illustrate this technique, we have included the example for a Pt/Ta<sup>2</sup> O5 -TiO2 -Ta2 O5 /RuO<sup>x</sup> MIM capacitor in **Figure 4**. Using stairshaped voltage instead of a voltage ramp allows us to record current transients at different states. In this case, we can see that current transients appear when switching to negative voltages, that is, in the reset process (low-resistance to high-resistance transition). Interestingly, resistive switching affects not only the dc currents, but also the small signal ac parameters measured at low and medium frequencies. Indeed, capacitance and conductance also show hysteretic behavior during a whole bipolar switching cycle, as we can see in **Figure 5**. Both magnitudes varied in great extent when the sample was driven from the low-resistance state to the high-resistance state or *vice versa*, even at 0 V bias. This fact indicates that the memory state can be read at 0 V by sensing the admittance at 0 V, without any power consumption.
In order to study the influence of set voltage values on the RS cycles shape, the following experiment was carried out. After the initial electroforming step, some RS cycles under the same condition of current compliance were made in order to stabilize the process. Once repetitive RS curves were obtained, some cycles were recorded by varying the set voltage values regardless of the current compliance values (see **Figure 6**). By increasing the set voltage values, wider loops were obtained. It can be seen that current increases gradually in the
**Figure 4.** Bipolar switching response to stair-shaped voltage of Pt/Ta<sup>2</sup> O5 -TiO2 -Ta2 O5 /RuO<sup>x</sup> MIM samples.
RRAM Memories with ALD High-K Dielectrics: Electrical Characterization and Analytical Modeling http://dx.doi.org/10.5772/66666 171
rupture of the filaments, respectively, which percolate through the sample. In general, set and reset loops are asymmetrical, as it is seen in **Figure 3**. Electroforming in bipolar switching may be a process of introducing asymmetric interfaces in a two-terminal switching cell, which are beyond the asymmetry due to asymmetric electrodes, that is, different top and bottom electrodes. Thus, the electroforming may take one of the two interfaces as an active interface, depending on the polarity of the electroforming voltage, so that reactions taking place in the vicinity of the active interface are responsible for the subsequent bipolar switching [6].
Pulsed biasing comprises positive and negative pulses which lead the samples to the lowresistance and high-resistance states, respectively. To illustrate this technique, we have
shaped voltage instead of a voltage ramp allows us to record current transients at different states. In this case, we can see that current transients appear when switching to negative voltages, that is, in the reset process (low-resistance to high-resistance transition). Interestingly, resistive switching affects not only the dc currents, but also the small signal ac parameters measured at low and medium frequencies. Indeed, capacitance and conductance also show hysteretic behavior during a whole bipolar switching cycle, as we can see in **Figure 5**. Both magnitudes varied in great extent when the sample was driven from the low-resistance state to the high-resistance state or *vice versa*, even at 0 V bias. This fact indicates that the memory state can be read at 0 V by sensing the admittance at 0 V, without any power consumption.
In order to study the influence of set voltage values on the RS cycles shape, the following experiment was carried out. After the initial electroforming step, some RS cycles under the same condition of current compliance were made in order to stabilize the process. Once repetitive RS curves were obtained, some cycles were recorded by varying the set voltage values regardless of the current compliance values (see **Figure 6**). By increasing the set voltage values, wider loops were obtained. It can be seen that current increases gradually in the
> O5 -TiO2 -Ta2 O5 /RuO<sup>x</sup>
MIM samples.
MIM capacitor in **Figure 4**. Using stair-
O5 -TiO2 -Ta2 O5 /RuO<sup>x</sup>
170 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
**Figure 4.** Bipolar switching response to stair-shaped voltage of Pt/Ta<sup>2</sup>
included the example for a Pt/Ta<sup>2</sup>
**Figure 5.** Capacitance (a) and conductance (b) hysteresis during a whole bipolar switching cycle of Pt/Ta<sup>2</sup> O5 -TiO2 -Ta2 O5 / RuOx MIM samples at room temperature.
**Figure 6.** Bipolar switching cycles of Pt/Ta<sup>2</sup> O5 -TiO2 -Ta2 O5 /RuO<sup>x</sup> and Pt/TiO<sup>2</sup> -Ta2 O5 -TiO2 -/RuO<sup>x</sup> MIM samples at room temperature, obtained by varying the SET voltage.
set processes, whereas it abruptly falls in the reset processes. From **Figure 6a** and **6b**, it seems apparent that RS cycles are wider in the TiO<sup>2</sup> -Ta2 O5 -TiO2 stacks than in the Ta2 O5 -TiO2 -Ta2 O5 ones, indicating much more marked differences between high- and low-resistance states when the inner layer is Ta2 O5 . Making very much thinner the Ta2 O5 layer with respect to the two TiO2 surrounding layers (**Figure 6c** and **6d**), the RS cycles become wider, but in the (75-1- 75) sample, the cycles are not enough stable, providing a mixed picture. On the contrary, the (75-3-75) sample maintains adequate repetitiveness conditions since the very first cycles, and therefore, the two resistive states are clearly distinguished.
In **Figure 7**, the linear correlation between set and reset voltages is depicted for the (75-3-75) sample (blue line). The rise of set voltage values induces a consequent increase in the absolute values of reset voltage. In the same figure, current difference values in the two resistance states measured at a fixed reset value (−0.5 V) for different set voltage values are shown (red line). It is clear that the current window of high- and low-resistance states opens as set voltage value increases. Both tendencies of **Figure 7**, although in minor extent, were also observed in all samples.
**Figure 7.** Variation of VReset with VSet of Pt/TiO<sup>2</sup> -Ta2O5 -TiO2 -/RuOx MIM samples. The current variation values and VSET relationship are also shown.
#### **3. RF impedance measurements**
A deep knowledge of dielectric properties could provide a wider insight of the RRAM behavior. In particular, RF impedance spectroscopy measurements allow detection of the dipolar relaxation of the dielectrics. In a dielectric material, the bound charges are polarized under the influence of an external electric field. Also, surfaces, grain boundaries, and interphase boundaries into the dielectric material contain dipoles that are oriented in an external field and thus contribute to the polarization of the material. When dipole relaxation occurs, the real part of the permittivity, ε′, shows an inflection point, whereas the imaginary part, ε″, has a maximum. In a capacitor, ε′ and ε″ are proportional to the capacitance and conductance signals, respectively. So, admittance measurements at high frequencies provide information about the permittivity relaxation [16].
set processes, whereas it abruptly falls in the reset processes. From **Figure 6a** and **6b**, it seems
ones, indicating much more marked differences between high- and low-resistance states
. Making very much thinner the Ta2
75) sample, the cycles are not enough stable, providing a mixed picture. On the contrary, the (75-3-75) sample maintains adequate repetitiveness conditions since the very first cycles, and
In **Figure 7**, the linear correlation between set and reset voltages is depicted for the (75-3-75) sample (blue line). The rise of set voltage values induces a consequent increase in the absolute values of reset voltage. In the same figure, current difference values in the two resistance states measured at a fixed reset value (−0.5 V) for different set voltage values are shown (red line). It is clear that the current window of high- and low-resistance states opens as set voltage value increases. Both tendencies of **Figure 7**, although in minor extent, were also observed in
A deep knowledge of dielectric properties could provide a wider insight of the RRAM behavior. In particular, RF impedance spectroscopy measurements allow detection of the dipolar relaxation of the dielectrics. In a dielectric material, the bound charges are polarized under the influence of an external electric field. Also, surfaces, grain boundaries, and interphase boundaries into the dielectric material contain dipoles that are oriented in an external field and thus contribute to the polarization of the material. When dipole relaxation occurs, the real part of the permittivity, ε′, shows an inflection point, whereas the imaginary part, ε″, has a maximum. In a capacitor, ε′
surrounding layers (**Figure 6c** and **6d**), the RS cycles become wider, but in the (75-1-
stacks than in the Ta2
O5
O5 -TiO2 -Ta2 O5
layer with respect to the
apparent that RS cycles are wider in the TiO<sup>2</sup>
**3. RF impedance measurements**
**Figure 7.** Variation of VReset with VSet of Pt/TiO<sup>2</sup>
VSET relationship are also shown.
O5
172 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
therefore, the two resistive states are clearly distinguished.
when the inner layer is Ta2
two TiO2
all samples.
This study was carried out by using a Keysight E4991B RF Impedance Analyzer, which allowed to carry out measurements in a frequency range of 1 MHz - 3 GHz. Capacitance and conductance of the samples were obtained by scanning the frequency of the ac signal while keeping the gate voltage at a given value. The gate voltage is applied by the voltage source that is built in the E4991B analyzer. A whole RF characterization is obtained by varying the voltage from accumulation to inversion regime. The influence of the gate voltage on the RF characteristics is obtained in this way. In **Figure 8**, we plot RF admittance curves of a W/HfO<sup>2</sup> / Si MIS structure. The most noticeable point is the fact that the frequency of the inflection point of the capacitance signal and the maximum of the conductance signal depends on the bias voltage: More positive voltages yield to higher relaxation frequencies. In this case, MIS capacitors are in the inversion regime for positive bias and in accumulation for negative ones. The main conclusion is that the inversion layer at the interface channel affects to the dipole relaxation in such a way that it occurs at higher frequencies. In accumulation, the voltage drop in the oxide is equal to the applied gate voltage, whereas in depletion or inversion regime, part of the applied voltage drops in the semiconductor layer close to the interface. Hence, higher electric field exists on the accumulation regime, dipole orientation is more effective in this regime, and dipoles could not respond to so high frequencies as in the inversion regime. **Figure 9** shows this effect from a three-dimensional point of view.
In order to check the influence of top electrode material on this effect, the same measurements were carried out on similar samples with nickel instead tungsten as top electrode (**Figure 10**). In this case, relaxation occurs at lower frequencies (15 MHz) and no influence of voltage bias on the dipole relaxation frequency values was observed. This can be due to some Fermi level pinning effect in the nickel samples. Also, it can be related to the fact that nickel ions diffuse inside the insulator. These charged ions create local electric fields that interact with insulator dipoles in such a way that relaxation occurs at lower frequencies. Local electric field
**Figure 8.** Frequency variations of capacitance (a) and conductance (b) for a W/HfO<sup>2</sup> (10 nm)/Si MIS capacitor at different voltage values.
**Figure 9.** Three-dimensional plots showing frequency and voltage variations of capacitance (a) and conductance (b) for a W/HfO<sup>2</sup> (20 nm)/Si MIS capacitor.
**Figure 10.** Three-dimensional plots showing frequency and voltage variations of capacitance (a) and conductance (b) for a Ni/HfO<sup>2</sup> (20 nm)/Si MIS capacitor.
dominates over the external applied field, and the resonance frequency results independent of the externally applied voltage.
From the resistive switching point of view, it can be worth to point out here that W/HfO<sup>2</sup> /Si MIS samples do not show any RS behavior, whereas Ni/HfO<sup>2</sup> /Si MIS samples exhibit unipolar RS due to the CBRAM mechanism [8, 17]. The set mechanism is controlled by the thermally enhanced diffusion of Ni ions induced by local Joule heating, forming a connected nanofilament path. In the reset process, the previously formed filament is partially broken, limiting the current flow. These results agree well with the RF results described before. When no local electric field is detected, there is not resistive switching, as in the W top electrode case. However, in the Ni top electrode case, a local electrical field is detected as a consequence of the diffusion of Ni ions that, in the end, form the conductive filaments thus provoking the RS phenomena.
More detailed studies in this matter should be done in order to connect in more extent the resistive switching behavior with the dielectric properties.
## **4. Resistive switching modeling**
dominates over the external applied field, and the resonance frequency results independent
**Figure 10.** Three-dimensional plots showing frequency and voltage variations of capacitance (a) and conductance (b) for
**Figure 9.** Three-dimensional plots showing frequency and voltage variations of capacitance (a) and conductance (b) for
/Si
/Si MIS samples exhibit unipolar
From the resistive switching point of view, it can be worth to point out here that W/HfO<sup>2</sup>
RS due to the CBRAM mechanism [8, 17]. The set mechanism is controlled by the thermally
of the externally applied voltage.
(20 nm)/Si MIS capacitor.
a W/HfO<sup>2</sup>
a Ni/HfO<sup>2</sup>
(20 nm)/Si MIS capacitor.
174 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
MIS samples do not show any RS behavior, whereas Ni/HfO<sup>2</sup>
A better knowledge of the nature of the switching behavior requires extending the previous experiments in a wide range of temperatures [18]. We reported an analytical model which fit well with the experimental results for temperatures ranging from 77 to 400 K [17]. This model assumes that the conductive filament does not entirely extend from the top to bottom electrode, and it is interrupted in a region close to one of the electrodes, as is drawn in the inset of **Figure 11**. This gap region behaves as a barrier for the conduction. When the barrier is narrow enough, a current can flow through it by tunneling, and the device is at the low-resistance state. On the contrary, when part of the filament closer to the gap is dissolved, the barrier
**Figure 11.** Fitting of a unipolar LRS cycle with the single tunneling barrier analytical model for a Ni/HfO<sup>2</sup> /Si MIS capacitor.
becomes thicker and tunneling can-not take place, so producing the low-resistance to highresistance switching. Very good fitting is obtained with this model as is plotted in **Figure 11**, where the red line is the experimental I-V curves, and the blue line is the best fitting obtained with the following transcendental equation, which is the basis of this model:
$$V\_1 = V - \frac{R\_{-}}{R\_0} \cdot V\_1 \cdot e^{\mu V\_1} \tag{1}$$
where *V1* is the voltage drop in the barrier, V is the applied bias voltage, and *R*<sup>0</sup> and *R∞* are the resistance of the conductive filament at 0 V, and when reset occurs, respectively, α is a parameter very closely related to the barrier tunneling probability, that is, with the filament gap thickness.
An improvement in the previous model assumes a double barrier instead of a single one, as it is illustrated in **Figure 12**. In this model, the current through the barriers is described by a Fowler-Nordheim law as follows:
$$I = \frac{1}{R} \cdot E^2 \cdot e^{-a/E} \tag{2}$$
and the barrier thickness is related to the voltage according a potential law:
$$t(\mathsf{V}) = \mathsf{V}^{\mathsf{I}} \tag{3}$$
## **5. Conclusions**
becomes thicker and tunneling can-not take place, so producing the low-resistance to highresistance switching. Very good fitting is obtained with this model as is plotted in **Figure 11**, where the red line is the experimental I-V curves, and the blue line is the best fitting obtained
*R*0
the resistance of the conductive filament at 0 V, and when reset occurs, respectively, α is a parameter very closely related to the barrier tunneling probability, that is, with the filament
An improvement in the previous model assumes a double barrier instead of a single one, as it is illustrated in **Figure 12**. In this model, the current through the barriers is described by a
*t*(*V*) = *V<sup>λ</sup>* (3)
**Figure 12.** Fitting of a unipolar LRS cycle with the double tunneling barrier analytical model for a Ni/HfO<sup>2</sup>
is the voltage drop in the barrier, V is the applied bias voltage, and *R*<sup>0</sup>
· *V*<sup>1</sup> · *e <sup>α</sup>*·*V*<sup>1</sup> (1)
*<sup>R</sup>* · *E*<sup>2</sup> · *e* <sup>−</sup>*α*/*<sup>E</sup>* (2)
and *R∞* are
/Si MIS
with the following transcendental equation, which is the basis of this model:
and the barrier thickness is related to the voltage according a potential law:
*<sup>V</sup>*<sup>1</sup> <sup>=</sup> *<sup>V</sup>* <sup>−</sup> *<sup>R</sup>*\_\_\_<sup>∞</sup>
176 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
where *V1*
capacitor.
gap thickness.
Fowler-Nordheim law as follows:
*I* = \_\_1
Resistive switching behavior in Ta<sup>2</sup> O5 -TiO2 -Ta2 O5 and TiO2 -Ta2 O5 -TiO2 stacks-based MIM structures was reported. Dielectric layers were grown by ALD. The best results were obtained for 75 × TiO<sup>2</sup> + 3 × Ta<sup>2</sup> O5 + 75 × TiO<sup>2</sup> ALD cycles, yielding around 1 monolayer of Ta2 O5 in a structure with the total thickness of 5 nm. In this sample, wide RS loops were obtained. Moreover, current window of high- and low-resistance states opens as set voltage value increases, with very adequate repetitiveness. In order to try to connect the dielectric properties with the RS behavior, some high-frequency impedance measurements were carried out in W/HfO<sup>2</sup> /Si and Ni/HfO<sup>2</sup> /Si MIS samples. The detection of a local electric field in the last ones can be related to the diffusion of ions from the top electrode that creates the conductive filament and provoke the resistive switching effect. Finally, an analytical model based on double tunneling barrier was applied to low-resistance cycles of Ni/HfO<sup>2</sup> /Si MIS structures, with very good fitting. With respect to the single tunneling barrier, the sharp fall of current at the reset transition (low to high resistance) is also fitted.
## **Acknowledgements**
This work was funded by the Spanish Ministry of Economy and Competitiveness through project TEC2014-52152-C3-3-R, with support of Feder funds, Finnish Centre of Excellence in Atomic Layer Deposition, and Estonian Research Agency (PUT170, IUT2–24), and by the European Regional Development Fund projects TK134 "Emerging orders in quantum and nanomaterials" and TK141 "Advanced materials and high-technology devices for energy recuperation systems". Authors would like to acknowledge Dr. M. B. González and Prof. F. Campabadal (IMB-CNM, Barcelona, Spain) for providing some samples of this study.
## **Author details**
Helena Castán\*<sup>1</sup> , Salvador Dueñas<sup>1</sup> , Alberto Sardiña<sup>1</sup> , Héctor García<sup>1</sup> , Tõnis Arroval2 , Aile Tamm2 , Taivo Jõgiaas<sup>2</sup> , Kaupo Kukli2,3 and Jaan Aarik2
\*Address all correspondence to: [email protected]
1 Electronic Devices and Materials Characterization Group, Department of Electronics, University of Valladolid, Valladolid, Spain
2 Institute of Physics, University of Tartu, Tartu, Estonia
3 Department of Chemistry, University of Helsinki, Helsinki, Finland
## **References**
Provisional chapter
## **Advanced Multifunctional Corrosion Protective Coating Systems for Light-Weight Aircraft Alloys— Actual Trends and Challenges** Advanced Multifunctional Corrosion Protective Coating Systems for Light-Weight Aircraft Alloys—Actual Trends and Challenges
Stephan Vladymirov Kozhukharov Stephan Vladymirov Kozhukharov
Additional information is available at the end of the chapter Additional information is available at the end of the chapter
http://dx.doi.org/10.5772/67237
#### Abstract
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178 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
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The present chapter is devoted to the recent trends in the field of the advanced corrosion protective layers elaboration. The chapter begins with brief classification of the standard aluminum alloys, remarking their importance for the transport sector, as well as the basic corrosion forms, typical for these alloys. It continues with the basic requirements regarding the elaboration of durable and reliable coating systems and the factors of detrimental effect during the service life time. The concept for passive and active corrosion protection capabilities is remarked as well. After description of the need for multilayered coating systems elaboration, the function of each layer is described beginning from (i) UV light–absorbing exterior layers, (ii) self-repairing reinforced intermediate barrier layers, and (iii) cerium oxide primer layers (CeOPL). The importance and the basic approaches for metallic alloy preliminary treatment are remarked, as well. Finally, the basic concepts and the function of each layer in advanced multilayered coating system are summarized in a special section. The chapter finishes with brief conceptual description of two advanced versatile technological synthesis methods, which enable elaboration of organic/inorganic hybrid polymers and reinforcing nanoparticles.
Keywords: aircraft alloys, corrosion protection, cerium conversion coatings, technological aspects, hybrid and nanocomposite materials, corrosion inhibitors, multifunctionality
## 1. Aluminum alloys as basic constructional material for the aircraft and transport industry
Aluminum (Al) is a lightweight relatively easily treatable metal that possesses an aptitude for passivation by formation of a natural oxide layer. Nevertheless, the pure Al is inapplicable in the industrial practice, since it does not present satisfying mechanical properties. The Al-based and
© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.
magnesium (Mg)-based materials used for construction of various transport devices and vehicles must correspond to industrial standards, to construct transport equipment with reliable and predictable properties and performance. Recently, besides in the aircraft industry [1–3], the aluminum alloys have found continuously increasing applications for car body panels [4] and even in the modern shipbuilding [5, 6]. Besides, it is a common practice to apply multilayered coating systems for corrosion protection of metallic details, assemblies, and entire vehicle constrictions [7] (Figure 1).
According to the worldwide nomenclature, the Al alloys are denominated as "AA" with four digit numbers that reveal their chemical compositions as follows: AA1XXX—almost pure Al; AA2XXX with 1.9%–6.8% of copper (Cu); AA3XXX with 0.3%–1.5% of manganese; AA4XXX with silicon (Si) addition between 3.6% and 13.5%, AA5XXX with Mg content between 0.5% and 5.5%; AA6XXX prepared by both Mg 0.4%–1.5% and Si 0.2%–1.7%; AA7XXX with zinc (1%–8.2%); and finally, AA8XXX with other additives [14]. The rest three digits reveal the lower content elements and the acceptable contaminant concentrations. The compositions of the most widely used Al–Cu alloys, according to ISO 3522-2007, are summarized in Table 1 [14].
The aluminum alloys are ranked second following the steels for industrial and household applications. Their mechanical properties—strength, stiffness, and durability—combined with their low weight and relatively low price make them preferable constructive materials. For instance, about 70% of all metal details in the nowadays airplanes are composed of aluminum alloys [3]. Irrespective of their excellent mechanical properties, these alloys exhibit a serious disadvantage—they are susceptible to corrosion. It is attributed to the additive components (Cu, Fe, Mn, Mg, Si, and so forth), which form intermetallics of a various composition dispersed throughout the aluminum matrix during the alloys hot rolling. In aggressive media, such as chloride ions containing ones, these inclusions become centers of initiation and further proliferation of localized corrosion [15].
Figure 1. Various kinds of vehicles composed by aluminum alloys. (a) Airbus A380 [8], (b) Boeing 747 [9], (c) Antonov 225 [10], (d, e) aluminum car bodies [11], (f) aluminum ship type Littoral Combat Ships [12], (g) aluminum sport boat [13].
Table 1. Nominal standard compositions of copper-containing aluminum alloys.
## 2. Corrosion processes nature and impact
magnesium (Mg)-based materials used for construction of various transport devices and vehicles must correspond to industrial standards, to construct transport equipment with reliable and predictable properties and performance. Recently, besides in the aircraft industry [1–3], the aluminum alloys have found continuously increasing applications for car body panels [4] and even in the modern shipbuilding [5, 6]. Besides, it is a common practice to apply multilayered coating systems for corrosion protection of metallic details, assemblies, and entire vehicle con-
180 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
According to the worldwide nomenclature, the Al alloys are denominated as "AA" with four digit numbers that reveal their chemical compositions as follows: AA1XXX—almost pure Al; AA2XXX with 1.9%–6.8% of copper (Cu); AA3XXX with 0.3%–1.5% of manganese; AA4XXX with silicon (Si) addition between 3.6% and 13.5%, AA5XXX with Mg content between 0.5% and 5.5%; AA6XXX prepared by both Mg 0.4%–1.5% and Si 0.2%–1.7%; AA7XXX with zinc (1%–8.2%); and finally, AA8XXX with other additives [14]. The rest three digits reveal the lower content elements and the acceptable contaminant concentrations. The compositions of the most widely used Al–Cu alloys, according to ISO 3522-2007, are summarized in Table 1
The aluminum alloys are ranked second following the steels for industrial and household applications. Their mechanical properties—strength, stiffness, and durability—combined with their low weight and relatively low price make them preferable constructive materials. For instance, about 70% of all metal details in the nowadays airplanes are composed of aluminum alloys [3]. Irrespective of their excellent mechanical properties, these alloys exhibit a serious disadvantage—they are susceptible to corrosion. It is attributed to the additive components (Cu, Fe, Mn, Mg, Si, and so forth), which form intermetallics of a various composition dispersed throughout the aluminum matrix during the alloys hot rolling. In aggressive media, such as chloride ions containing ones, these inclusions become centers of initiation and further
Figure 1. Various kinds of vehicles composed by aluminum alloys. (a) Airbus A380 [8], (b) Boeing 747 [9], (c) Antonov 225 [10], (d, e) aluminum car bodies [11], (f) aluminum ship type Littoral Combat Ships [12], (g) aluminum sport boat [13].
strictions [7] (Figure 1).
proliferation of localized corrosion [15].
[14].
According to the exact definition, the corrosion, according to IUPAC, is a physical-chemical interaction between a metal and its environment, which results in changes in the properties of the metal and which may often lead to impairment of the function of the metal, the environment, or the technical system of which these form a part [16].
Nowadays, the term "corrosion of the materials" comprises a large variety of detrimental processes which lead to lose of material integrity and mechanical properties of the corresponding details. Thus, recently, concepts for "corrosion of the plastics," formulated by Hojo et al. [17], as "degradation of polymeric materials with chemical reaction in which the molecular chains is cut" or "glass materials corrosion" [18], described as "fast and structure-dependent proton/cation(s) exchange and associated volume contraction which mechanically ruin the parts." In addition, the term "corrosion" already includes "microbially induced corrosion (MIC)" [19–21] caused by the metabolism of various acidogenic microorganisms or alcaligenes. The anaerobic microorganisms, such as iron-reducing bacteria (IRB) and sulfate reduction bacteria (SRB), also cause considerable corrosion damages [19]. The former reduces the insoluble ferric corrosion products to the soluble ferrous ones, whereas the latter reduces sulfate to sulfide compounds, contributing for additional cathodic activity enhancement. In general terms, all the corrosion processes result in partial or complete destruction of the metallic details.
Particularly, the metal corrosion processes possess electrochemical nature, and their appearance, rate, and impact are strongly dependent on the metal part features (i.e., structure and composition) and the environmental conditions (i.e., temperatures, pH, oxidant concentrations, and so forth).
Davis [22] has summarized all the basic corrosion impact forms, classifying them in uniform and localized corrosion phenomena (Figure 2).
Figure 2. Basic types of corrosion phenomena, according to Davis [22].
Finally, it is worth to remark that the corrosion process can alter its form pitting to intergranular [23]. The occurrence of especially cyclic mechanical loading enhances the corrosion impact, resulting in complete assembly or equipment failure. In general terms, the corrosion processes, related to the industrial alloys, usually begins as galvanic corrosion on the alloys' surface and continue subsequently as complete selective dissolution of the more active composing metals. Afterward, the complete dissolution of these alloy's components, resulting in pitting corrosion, continues in depth on the grain boundaries inside the metallic alloy, converting to intergranular corrosion. This already severe form of corrosion can lead to large domains exfoliation, due to thermal expansion of the heaped corrosion products and the entrapped moisture freezing. All these localized corrosion forms decrease the efficient cross-section of the metallic details, composing whatever assembly or construction. Finally, all these processes together lead to complete constructional failure, being sometimes even potential danger for the human life. One of the most famous cases is the accident with flight no. 243 at 1988 [24] (Figure 3).
After this accident, all the worldwide aircraft industry has accepted regular inspections, and the exploitation lifetime of the used commercial airplanes has been strongly restricted, in terms of maximal permitted flight hours, before decommission.
Thus, from all statements mentioned above, it can be considered that the term "corrosion" relays to whatever physical, chemical, or biological process which causes gradual geometrical shape altering and mechanical properties deterioration of given solid state object of industrial origin, due to interactions with its surrounding environment.
To prevent all these phenomena, the metallic surfaces should be insulated from the surrounding environment by coating. However, it should be mentioned that even the protective coatings suffer destructive ageing processes and consequently should be created advanced coating systems with extended durability and capabilities for active protection even after any damage of their integrity.
Figure 3. Photography of the flight accident happened at 28th of April 1988 due to corrosion fatigue [24].
## 3. Protective coatings and layers: basic requirements
Finally, it is worth to remark that the corrosion process can alter its form pitting to intergranular [23]. The occurrence of especially cyclic mechanical loading enhances the corrosion impact, resulting in complete assembly or equipment failure. In general terms, the corrosion processes, related to the industrial alloys, usually begins as galvanic corrosion on the alloys' surface and continue subsequently as complete selective dissolution of the more active composing metals. Afterward, the complete dissolution of these alloy's components, resulting in pitting corrosion, continues in depth on the grain boundaries inside the metallic alloy, converting to intergranular corrosion. This already severe form of corrosion can lead to large domains exfoliation, due to thermal expansion of the heaped corrosion products and the entrapped moisture freezing. All these localized corrosion forms decrease the efficient cross-section of the metallic details, composing whatever assembly or construction. Finally, all these processes together lead to complete constructional failure, being sometimes even potential danger for the human life. One of the
After this accident, all the worldwide aircraft industry has accepted regular inspections, and the exploitation lifetime of the used commercial airplanes has been strongly restricted, in terms
Thus, from all statements mentioned above, it can be considered that the term "corrosion" relays to whatever physical, chemical, or biological process which causes gradual geometrical shape altering and mechanical properties deterioration of given solid state object of industrial
To prevent all these phenomena, the metallic surfaces should be insulated from the surrounding environment by coating. However, it should be mentioned that even the protective coatings
most famous cases is the accident with flight no. 243 at 1988 [24] (Figure 3).
of maximal permitted flight hours, before decommission.
Figure 2. Basic types of corrosion phenomena, according to Davis [22].
182 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
origin, due to interactions with its surrounding environment.
The efficient corrosion protection is only achievable when dense, uniform, and adherent layers are deposited. These protective layers should serve as efficient barriers against corrosive species ingress toward the protected metallic surface. However, these protective layers suffer the surrounding environment impact, as well. Consequently, the protective layers are susceptible to deterioration until barrier layer failure. Such failure can appear due to various reasons, like:
Undoubtedly, the real exploitation conditions usually include combinations of the above mentioned detrimental factors. Thus, the coating systems should form durable adhesive barrier layers to execute efficiently their function of passive corrosion protection. Hence, in the industrial practice, the deposition of multilayered coating systems is commonly accepted practice (Figure 4). This
Figure 4. Schematic presentation of multilayer coating system according [7]. (1, 2) finishing double layer of polyurethane, (3) intermediate adhesive layer; (4) primer hybrid coating; (5) metal substrate.
approach enables deposition of advanced barrier coatings, where each composing layer has its own function.
However, recently new requirements arisen, related to further extension of the corrosion protective capabilities after barrier layer integrity disruption. Consequently, nowadays, coating systems should be capable for active corrosion protection, after damaging of their integrity. There are three basic roads to achieve active corrosion protective abilities:
According to the application of the coating system, other more specific requirements should appear. Montemor [26] proposes more complete classification of the beneficial properties and the respective requirements, related to the coating system application (Figure 5).
Other actual aspects related to the elaboration of advanced corrosion protective layers are rather related to the technological approaches used for their synthesis and deposition and the respective coating ingredients. In other words, every new coating system elaboration should comply the environmental restrictions, related to the use of volatile organic compounds (VOCs), [27, 28] and toxic metals, such as Pb, Cr, As, and so forth [29, 30].
Advanced Multifunctional Corrosion Protective Coating Systems for Light-Weight Aircraft Alloys—Actual… http://dx.doi.org/10.5772/67237 185
Figure 5. Overview of surface properties that can be developed or enhanced through functionalization by organic, inorganic or hybrid coatings, according to Montemor [26].
This considerable variety of requirements, related to the elaboration of durable and reliable, environmentally compliant coating systems with active corrosion protective capability impose the need for multilayered systems, where each layer has its own function. Thus, the coatings have at least exterior films, intermediate layers, and coating primers. Hence, the large number of requirements can be distributed to each one of the coating layers to obtain advanced multilayered coating system.
## 4. Exterior protective coatings and layers: basic requirements
approach enables deposition of advanced barrier coatings, where each composing layer has its
Figure 4. Schematic presentation of multilayer coating system according [7]. (1, 2) finishing double layer of polyurethane,
However, recently new requirements arisen, related to further extension of the corrosion protective capabilities after barrier layer integrity disruption. Consequently, nowadays, coating systems should be capable for active corrosion protection, after damaging of their integrity.
According to the application of the coating system, other more specific requirements should appear. Montemor [26] proposes more complete classification of the beneficial properties and
Other actual aspects related to the elaboration of advanced corrosion protective layers are rather related to the technological approaches used for their synthesis and deposition and the respective coating ingredients. In other words, every new coating system elaboration should comply the environmental restrictions, related to the use of volatile organic compounds (VOCs), [27, 28] and
the respective requirements, related to the coating system application (Figure 5).
There are three basic roads to achieve active corrosion protective abilities:
(3) intermediate adhesive layer; (4) primer hybrid coating; (5) metal substrate.
184 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
• Incorporation of corrosion inhibitors
• Addition of UV absorbers
• Encapsulation of polymerizable compounds
toxic metals, such as Pb, Cr, As, and so forth [29, 30].
• Introduction of hydrophobic compounds
own function.
The main function of the exterior layers is to protect the coating intermediate and primer layers against the impact of the surrounding environment. These exterior layers should possess mechanical strength against abrasion with high-speed solid particles, like dust or ice fogs, which spoil the fuselage metallic surface with the same speed as of the flight (i.e., up to 850– 900 km/h). Other properties necessary for extension of the coating service lifetime are UV light absorption ability and hydrophobicity. The former property can be rendered by addition of UV absorbers into the external layers' composition. In this sense, Guillet [25] proposes several biphenyl, carbonyl-based compounds (Table 2).
Because the direct inclusion of these compounds to polymeric chain matrix is related to considerable difficulties, their preliminary incorporation in porous nanoparticles looks more appropriate approach. In this sense, ZnO, TiO2, and titanates-based porous nanoparticles are much appropriated, since these oxides possess photochemical properties, as well. Besides, the photosensitizing of TiO2 particles is well-known method used for other applications such as photocatalysis [31], UV-spectrum photosensors [32, 33], elaboration of alternative energy sources [34, 35], and so forth.
Nevertheless, the approach of involvement of TiO2 with incorporated photoabsorbers should be applied very attentively because the TiO2 particles possess photodecomposition activity [36–39], and the organic UV light absorbers can terminate the polymerization processes of the basic coating layer matrix synthesis. Consequently, the elaboration of such coating compositions by addition of TiO2 particles loaded by photoabsorbers should be combined by systematic comparative investigations with long-term UV-illumination, to detect whatever incompatibility effects on the basic polymer matrix. On the other hand, the successful introduction of preliminary impregnated titania nanoparticles into the basic polymer matrix converts the final coating as a composite material, where the TiO2 particles serve as reinforcing phase, enhancing the mechanical strength, whereas the organic UV absorbers protect the polymer matrix against photochemical degradation. On the other hand, since ZrO2 occupies the second place of the Mohs hardness scale after the diamond. That is why, zirconia-based composite protective coatings are already proposed for epoxide [40] and hybrid [41] polymer matrix.
Another aspect for barrier properties enhancement and service lifetime extension is to render hydrophobicity of the coating surface. Undoubtedly, one of the most efficient manners to avoid any corrosion phenomena is to render hydrophobic properties to the metallic surface. The surface hydrophobicity remarkably decreases the contact area with water condense. Another,
Table 2. UV light–absorbing compounds proposed by Guillet [25].
much more important benefit is that the water drops have low cohesion to the metallic surface and can be leached from it by the spoiling air streams during flight. The hydrophobic surfaces do not allow water film formation on the metallic surface, preventing continuous contact between the metallic surface and any corrosive electrolyte.
much appropriated, since these oxides possess photochemical properties, as well. Besides, the photosensitizing of TiO2 particles is well-known method used for other applications such as photocatalysis [31], UV-spectrum photosensors [32, 33], elaboration of alternative energy
Nevertheless, the approach of involvement of TiO2 with incorporated photoabsorbers should be applied very attentively because the TiO2 particles possess photodecomposition activity [36–39], and the organic UV light absorbers can terminate the polymerization processes of the basic coating layer matrix synthesis. Consequently, the elaboration of such coating compositions by addition of TiO2 particles loaded by photoabsorbers should be combined by systematic comparative investigations with long-term UV-illumination, to detect whatever incompatibility effects on the basic polymer matrix. On the other hand, the successful introduction of preliminary impregnated titania nanoparticles into the basic polymer matrix converts the final coating as a composite material, where the TiO2 particles serve as reinforcing phase, enhancing the mechanical strength, whereas the organic UV absorbers protect the polymer matrix against photochemical degradation. On the other hand, since ZrO2 occupies the second place of the Mohs hardness scale after the diamond. That is why, zirconia-based composite protective coatings are already
Another aspect for barrier properties enhancement and service lifetime extension is to render hydrophobicity of the coating surface. Undoubtedly, one of the most efficient manners to avoid any corrosion phenomena is to render hydrophobic properties to the metallic surface. The surface hydrophobicity remarkably decreases the contact area with water condense. Another,
2-Hydroxy-4-methoxy benzophenone 326 nm 42.4
Phenyl salicylate 310 nm 23.6
Wavelength of maximal
327 nm 41.2
343 nm 50.2
340 nm 73.0
Specific absorptivity
absorbance
sources [34, 35], and so forth.
proposed for epoxide [40] and hybrid [41] polymer matrix.
186 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
Name Structural formula
2,2<sup>0</sup>
2,2<sup>0</sup>
2-(2<sup>0</sup>
benzophenone
dimethoxybenzophenone
Table 2. UV light–absorbing compounds proposed by Guillet [25].
The actual trends for hydrophobic and super-hydrophobic coating layers development are based on the formation of highly textured cerium oxide primers with micro and nanoflower [42], Gecko footprint [43] morphologies. This approach enables to avoid also the snow and ice heaping on the wings, which was the suspected reason for the aircraft crash at 7th of February 1958 in Munich, which was appeared to be a great disaster for the "Manchester United" football team during large years [44].
Nowadays, the de-icing procedures accepted as a common practice for the winter time exploitation in the commercial aircraft services appears the main reason for inconveniences, often related to schedule delays. In addition, these operations increase the economical spends, related to the needs for deicing solutions, staff, and equipment. In this sense, the hybrid polyfluorinated hybrid coatings [45] appear to be rather attractive alternative, since these coating materials combine the hydrophobicity with the beneficial features of the hybrid materials, discussed in the previous sections.
An interesting approach appears to be the proposed one by Arellanes-Lozada et al. [46]. This author's work-team proposes poly(1-vinyl-3-alkyl-imidazolium hexafluorophosphate, as corrosion inhibitor for aluminum corrosion in acidic media. This polymer seems really interesting, since it is able to be used as an alternative hydrophobic coating system, because it combines the beneficial effects of the presence of phosphate groups, long aliphatic chains, and hydrophobic fluorine moieties (Figure 6).
Figure 6. Schematic model of the corrosion protective action of poly(1-vinyl-3-alkyl-imidazolium Hexafluorophosphate toward AA 6061, proposed in [46].
Finally, it is important to mention that the exterior finishing coating layers should possess good adhesion to the intermediate layers. This indispensable property can be achieved easily when the basic polymer matrix is based on the same ingredients. Thus, some recent trends for development of the basic matrix of the exterior layers are described in the next paragraph.
## 5. Intermediate coating layers: barrier properties and self-reparation abilities
The polymer matrixes can possess organic nature, like polyepoxides, polyurethanes, polymethylmethacrilates, and so forth, or to have inorganic composition, being in form of glasses (i.e. silicates). Recently, various intermediate classes of materials have been introduced in the industrial practice. Hence, the use of organically modified inorganic polymers or hybrid materials opens entire new directions for advanced polymer matrix development. Haas and Rose [47] have done a versatile classification, including namely these intermediate classes of materials (Figure 7).
Figure 7. Classification of the basic types of polymer materials, according to Haas and Rose [47].
These intermediate groups of materials enable specific combination of the beneficial features of both the organic and inorganic composing moieties as follows [48, 49]:
Finally, it is important to mention that the exterior finishing coating layers should possess good adhesion to the intermediate layers. This indispensable property can be achieved easily when the basic polymer matrix is based on the same ingredients. Thus, some recent trends for development of the basic matrix of the exterior layers are described in the next paragraph.
The polymer matrixes can possess organic nature, like polyepoxides, polyurethanes, polymethylmethacrilates, and so forth, or to have inorganic composition, being in form of glasses (i.e. silicates). Recently, various intermediate classes of materials have been introduced in the industrial practice. Hence, the use of organically modified inorganic polymers or hybrid materials opens entire new directions for advanced polymer matrix development. Haas and Rose [47] have done a versatile classification, including namely these intermediate classes of
5. Intermediate coating layers: barrier properties and self-reparation
188 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
Figure 7. Classification of the basic types of polymer materials, according to Haas and Rose [47].
abilities
materials (Figure 7).
In this sense, Frignani and coauthors have done [50] comparative assessment of primer coatings, obtained from alkoxides with different aliphatic chains, as follows: c n-propyl
Figure 8. Schematic view of self-assembled barrier protective film on aluminum surface according to Frignani et al. [50].
trimethoxysilane-C3H7-Si-(OCH3)3; n-octyl trimetoxisilane-C8H17-Si-(OCH3)3; n-octadecyl trimethoxysilane-C18H37-Si-(OCH3)3 and bis-trioximethyl-silyl-ethane – (CH3O)3-Si-C2H7-Si- (OCH3)3. As conclusion, the authors have established that the larger aliphatic chains enhance the obtaining of thicker layers. The aptitude for self-healing provided by the hydrophobic intermolecular attraction (Figure 8), with simultaneous repulsion of entrapped water drops appear additional beneficial properties
These layers reveal aptitude for self-assembling, and this kind of hybrid materials is also known as (SAM—self-assembled monolayers). They have significantly lower number and size of defects in their structures and thus enable more efficient protection via formation of dense barrier layers [51, 52].
Another strategy is to encapsulate polymerizable substances. In this case, polymerizable compounds are enclosed inside polymer or glass capsules to polymerase when are exposed to air, by mixing each other or by toughing ingredients of the basic matrix. Following this concept, several authors [53, 54] have characterized an epoxy resin loaded by urea-formaldehyde submicrometer-sized capsules, filled with dicyclopentadiene as an active healing agent. Recently, three review works have been done over the variety of possible self-healing organic coatings [55–57] (Table 3).
The protective properties of the intermediate layers can be further improved, by addition of ceramic micro-sized and nanosized particles as reinforcing phase. In this sense, CeO2 [58, 59] and titanium dioxide (TiO2) [60, 61] have been recently obtained by precipitation from colloidal systems. Both these oxides possess significant mechanical and thermal strength and can be successfully used as reinforcing phase of advanced nanocomposite coating systems with
Table 3. Possible self-healing mechanism and active reparation agents according to Wu et al. [55].
extended durability. In addition, when these particles are preliminary filled by corrosion inhibitors, the final nanocomposite coatings obtain additional capability for active corrosion protection by gradual inhibitor release in the damaged zones, as it is already proposed by Zheludkevich et al. [48, 62] (Figure 9).
trimethoxysilane-C3H7-Si-(OCH3)3; n-octyl trimetoxisilane-C8H17-Si-(OCH3)3; n-octadecyl trimethoxysilane-C18H37-Si-(OCH3)3 and bis-trioximethyl-silyl-ethane – (CH3O)3-Si-C2H7-Si- (OCH3)3. As conclusion, the authors have established that the larger aliphatic chains enhance the obtaining of thicker layers. The aptitude for self-healing provided by the hydrophobic intermolecular attraction (Figure 8), with simultaneous repulsion of entrapped water drops
These layers reveal aptitude for self-assembling, and this kind of hybrid materials is also known as (SAM—self-assembled monolayers). They have significantly lower number and size of defects in their structures and thus enable more efficient protection via formation of dense
Another strategy is to encapsulate polymerizable substances. In this case, polymerizable compounds are enclosed inside polymer or glass capsules to polymerase when are exposed to air, by mixing each other or by toughing ingredients of the basic matrix. Following this concept, several authors [53, 54] have characterized an epoxy resin loaded by urea-formaldehyde submicrometer-sized capsules, filled with dicyclopentadiene as an active healing agent. Recently, three review works have been done over the variety of possible self-healing organic
The protective properties of the intermediate layers can be further improved, by addition of ceramic micro-sized and nanosized particles as reinforcing phase. In this sense, CeO2 [58, 59] and titanium dioxide (TiO2) [60, 61] have been recently obtained by precipitation from colloidal systems. Both these oxides possess significant mechanical and thermal strength and can be successfully used as reinforcing phase of advanced nanocomposite coating systems with
triethyleneglycol dimethylacrylate (TEGDMA)–based monomers
(PEK), and PEEK, polyphenylene ether (PPE)
Living ring-opening metathesis polymerization (ROMP)
Organosiloxanes, ionomers
Described above
Described above
dimethylamine (BDMA)
Table 3. Possible self-healing mechanism and active reparation agents according to Wu et al. [55].
Poly(methyl methacrylate) (PMMA) and PMMA–poly(methoxyethyl acrylate) (PMEA)
1,1,1-Tris-(cinnamoyloxymethyl) ethane (TCE) with urethane dimethacrylate (UDME),
Diglycidyl ether of bisphenol-A (DGEBA), nadic methyl anhydride (NMA), benzyl
Reactions such as polycarbonate (PC), polybutylene terephthalate (PBT), polyetherketone
appear additional beneficial properties
190 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
barrier layers [51, 52].
coatings [55–57] (Table 3).
Self-reparation
2. Photo-induced selfhealing
3. Recombination of chain ends
4. Reversible bond formation
5. Living polymer approach
6. Self-healing by nanoparticles
7. Hollow fiber approach
8. Microencapsulation approach
1. Molecular interdiffusion
mechanism Examples for self-reparation agents
copolymers
No
A promising alternative to the use of ceramic nanoparticles as inhibitor containers reinforcing the basic polymer matrix is the involvement of carbon nanoparticles (CNP) [63, 64]. Especially, the hollow carbon nanotubes (CNT) allow to be filled by inhibitors via intercalation. Indeed, the remarkable adsorption capability of the carbon-based materials was demonstrated by Lavrova [65] and Jumayeva et al. [66].
Various corrosion inhibitors are recently tested to substitute the already banned Cr(VI) compounds. Hence, the corrosion inhibition capability of benzotriazole and tolyltriazole [67–70] was examined, evincing the possibility for the use of these compounds as corrosion inhibitors. The inhibition mechanism of these compounds is based on film formation via adsorption on the metallic surface or by precipitation of derivative complex compounds. In this sense, the UV light absorbers, proposed by Guillet [25] (Table 2), should also possess potential corrosion inhibitive effect, protecting simultaneously both the coating polymer matrix against UV radiation, metallic surface against corrosion.
Alternatively, Cotting and Aoki [71] have proposed to encapsulate cerium-based corrosion inhibitor inside polystyrene capsules before their inclusion in an epoxide matrix. Indeed, the lanthanides have also shown high inhibition efficiency, due to their aptitude to form insoluble
Figure 9. Schematic view of self-healing effect by inhibitor gradual release from reinforcing nanoparticles.
hydroxides with the free OH ions in the corrosive medium. The compounds of these elements are particularly efficient for corrosion prevention of Al corrosion, since this metal and its alloys are very susceptible to corrosion even in weakly alkaline media. Among the lanthanides, the cerium compounds have shown the highest inhibition efficiency [72]. In this sense, various comparative investigations have been performed, to determine the influence of Ce(III) or Ce (IV) compounds [73–75], the optimal Ce-inhibitor concentration [73, 75, 76], as well as the impact of the anionic moiety of the respective Ce-salt [76]. As general results of these research activities, it was evinced that the Ce(III) compounds are much more efficient inhibitors for AA2024-T3, whereas the Ce(IV) ones can even enhance the corrosion attack at higher concentrations. Furthermore, for each Ce(III) compound, an optimal concentration threshold exists, depending on the alloy to be protected, the corrosive medium properties, and so forth. Thus, at higher concentrations, the Ce-compounds enhance the corrosion instead to inhibit it. Here, the term "catalyst" is not appropriate, because the Ce-compounds also suffer chemical conversions, being activators of the corrosion processes. As was already mentioned above, the Cecompounds and the organic inhibitors follow different mechanisms of inhibition. Besides the former decelerate the cathodic reactions, affecting predominantly the cathodic zones, whereas the latter form protective films on the entire metallic surface. Consequently, a question has appeared about whether a synergistic effect could occur between these distinguishable kinds of inhibitors. Hence, recently, the potential synergism between both kinds of inhibitors was examined by various authors on Al/Cu couples [77], AA5052 alloy [78], and even carbon steel [79]. Finally, the Ce-compounds have shown capabilities to form uniform adherent primer layers at defined conditions.
## 6. Cerium oxide primer layers deposition: basic concepts and requirements
The basic function of the primer coating layers (i.e., the coating primers) is to improve the adhesion between the native Al-oxide layer of the metallic substrate and the upper (i.e., intermediate and finishing) layers, commented in the previous sections. Some authors even report for self-healing properties, possessed by these films [80].
Undoubtedly, among the basic advantages of this group of coatings is the possibility to deposit them by physical [81, 82], electrochemical [83–87], or chemical methods [88–94]. Regardless the method applied for cerium oxide primer layer (CeOPL) deposition, the respective technological regime includes three basic stages: (i) preliminary treatment of the metallic substrate, (ii) the CeOPL deposition itself, and (iii) final sealing of the already deposited CeOPL film [94, 95]. In this sense, a large field of possible combinations among the conditions for execution of each technological procedure exists.
As was mentioned in the previous sections, the inhibitor effect of the cerium compounds is based on the precipitation of Ce(OH)3 and/or Ce(OH)4, predominantly on the cathodic areas of the Al alloys (composed by nobler metals), due to the cathodic reduction of the dissolved oxygen. However, the real mechanism is much more complicated and includes participation of intermediate peroxo-complexes [93, 94].
Scholes et al. [93] have made a simplified scheme (Figure 10) of the processes proceeding during the CeOPL deposition, after addition of H2O2 as deposition activator.
hydroxides with the free OH ions in the corrosive medium. The compounds of these elements are particularly efficient for corrosion prevention of Al corrosion, since this metal and its alloys are very susceptible to corrosion even in weakly alkaline media. Among the lanthanides, the cerium compounds have shown the highest inhibition efficiency [72]. In this sense, various comparative investigations have been performed, to determine the influence of Ce(III) or Ce (IV) compounds [73–75], the optimal Ce-inhibitor concentration [73, 75, 76], as well as the impact of the anionic moiety of the respective Ce-salt [76]. As general results of these research activities, it was evinced that the Ce(III) compounds are much more efficient inhibitors for AA2024-T3, whereas the Ce(IV) ones can even enhance the corrosion attack at higher concentrations. Furthermore, for each Ce(III) compound, an optimal concentration threshold exists, depending on the alloy to be protected, the corrosive medium properties, and so forth. Thus, at higher concentrations, the Ce-compounds enhance the corrosion instead to inhibit it. Here, the term "catalyst" is not appropriate, because the Ce-compounds also suffer chemical conversions, being activators of the corrosion processes. As was already mentioned above, the Cecompounds and the organic inhibitors follow different mechanisms of inhibition. Besides the former decelerate the cathodic reactions, affecting predominantly the cathodic zones, whereas the latter form protective films on the entire metallic surface. Consequently, a question has appeared about whether a synergistic effect could occur between these distinguishable kinds of inhibitors. Hence, recently, the potential synergism between both kinds of inhibitors was examined by various authors on Al/Cu couples [77], AA5052 alloy [78], and even carbon steel [79]. Finally, the Ce-compounds have shown capabilities to form uniform adherent primer
192 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
6. Cerium oxide primer layers deposition: basic concepts and requirements
The basic function of the primer coating layers (i.e., the coating primers) is to improve the adhesion between the native Al-oxide layer of the metallic substrate and the upper (i.e., intermediate and finishing) layers, commented in the previous sections. Some authors even
Undoubtedly, among the basic advantages of this group of coatings is the possibility to deposit them by physical [81, 82], electrochemical [83–87], or chemical methods [88–94]. Regardless the method applied for cerium oxide primer layer (CeOPL) deposition, the respective technological regime includes three basic stages: (i) preliminary treatment of the metallic substrate, (ii) the CeOPL deposition itself, and (iii) final sealing of the already deposited CeOPL film [94, 95]. In this sense, a large field of possible combinations among the conditions for execution of each
As was mentioned in the previous sections, the inhibitor effect of the cerium compounds is based on the precipitation of Ce(OH)3 and/or Ce(OH)4, predominantly on the cathodic areas of the Al alloys (composed by nobler metals), due to the cathodic reduction of the dissolved oxygen. However, the real mechanism is much more complicated and includes participation
report for self-healing properties, possessed by these films [80].
layers at defined conditions.
technological procedure exists.
of intermediate peroxo-complexes [93, 94].
The deposition of uniform, dense, and adherent CeOPL films instead of obtaining of discrete Ce precipitates appears a great challenge namely because of the complicated chemical mechanism. Besides the influence of the Ce-compound type and concentration, and, of course, the H2O2 activator content, the deposition mechanism and rate can be driven by regulation of various factors, like pH, temperature, occurrence of additives, and so forth. In this sense, the beneficial effect of various additives has been evinced, like black cuprous oxide "smut" [94], Al3+, and Cl ions [96], pH-buffers [90, 97], and so forth. The recent work of Jiang et al. [98] has proposed combined Ce-V conversion layers for protection of magnesium alloys, discovering entire new direction for elaboration of combined conversion coatings. Finally, it should be mentioned that the metallic surface roughness and composition are of key importance for the quality and performance of the CeOPL, predetermining its adherence, structure, and density. The metallic surface characteristics can be easily modified by suitable preliminary surface treatment.
Figure 10. Schematic summarizing of the proposed mechanism of the cerium oxide layer deposition by Scholes [93].
## 7. Preliminary treatment procedures: basic concepts
The purpose of the preliminary treatment procedure is to modify the metallic substrate surface to be suitable for deposition of uniform and adherent primer layers. The impact of the preliminary surface treatment procedures comprises both the metallic surface roughness (by mechanical grinding in laboratory's conditions, or sand blasting in industrial scale) and the superficial chemical composition (by selective dissolution of the intermetallic inclusions, or affecting the surface oxide layer [92]).
The commonly accepted preliminary treatment procedures include four basic steps:
In addition, the surface oxide layers of the highly doped Al alloys are always interrupted by the intermetallic inclusions on the alloy's surface.
• Acidic desmutting—The purpose of this procedure is to remove the black smut of the oxides of the intermetallic inclusions (like Cu2O, CuO, MnO2, and so forth). Usually, this process is being performed by dissolution in diluted HNO3.
Each one of these operations should be performed very attentively to obtain desirable surface conditioning. Besides, the optimal technological prescription for given aluminum alloy is not always appropriate for other alloys.
In this aspect, a comparative systematical investigation [100] was recently performed to evaluate the impact of each one of the above mentioned procedures. The authors have compared four groups of AA2024-T3 plates:
• Group G1: degreasing with an equal ratio of ethanol/ether mixture at room temperature for 10 minutes subjected to continuous stirring and subsequent abundant washing with distilled water;
Advanced Multifunctional Corrosion Protective Coating Systems for Light-Weight Aircraft Alloys—Actual… http://dx.doi.org/10.5772/67237 195
7. Preliminary treatment procedures: basic concepts
194 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
surface oxide layer [92]).
attack.
the intermetallic inclusions on the alloy's surface.
always appropriate for other alloys.
four groups of AA2024-T3 plates:
distilled water;
The purpose of the preliminary treatment procedure is to modify the metallic substrate surface to be suitable for deposition of uniform and adherent primer layers. The impact of the preliminary surface treatment procedures comprises both the metallic surface roughness (by mechanical grinding in laboratory's conditions, or sand blasting in industrial scale) and the superficial chemical composition (by selective dissolution of the intermetallic inclusions, or affecting the
• Degreasing—by organic solvents, for removal of the temporal anticorrosion protective layer, deposited by the corresponding Al-producer, for protection during the alloy storage
• Mechanical grinding—This procedure is being performed by emery papers in laboratory's conditions and sand blasting, in real industrial scale. The basic purpose of this procedure is to smooth the metallic surface and to remove the thicker oxide layer, formed during the finishing metallurgical thermal posttreatment. Of course, immediately after this procedure, a new oxide layer is being formed on the bare metallic surface (even underwater
• Alkaline etching—This procedure results in dissolution of the surface aluminum layer, penetrating through the thin oxide layer, due to its defective structure and nonuniform composition. Usually, this film consists of boehmite γ-AlO(OH) domains with about 5 nm of thickness [99]. Even the corundum Al2O3 fraction has strongly defective surface, due to the incompleteness of the superficial crystalline lattice cells. That is why, the surface oxide layer cannot protect the underlying metal against the aggressive OH
In addition, the surface oxide layers of the highly doped Al alloys are always interrupted by
• Acidic desmutting—The purpose of this procedure is to remove the black smut of the oxides of the intermetallic inclusions (like Cu2O, CuO, MnO2, and so forth). Usually, this
Each one of these operations should be performed very attentively to obtain desirable surface conditioning. Besides, the optimal technological prescription for given aluminum alloy is not
In this aspect, a comparative systematical investigation [100] was recently performed to evaluate the impact of each one of the above mentioned procedures. The authors have compared
• Group G1: degreasing with an equal ratio of ethanol/ether mixture at room temperature for 10 minutes subjected to continuous stirring and subsequent abundant washing with
The commonly accepted preliminary treatment procedures include four basic steps:
and transportation before its use for production of Al-details and tools.
film) because the exceptional passivation aptitude of this metal.
process is being performed by dissolution in diluted HNO3.
Figure 11. AFM images of samples underwent different approaches of superficial treatment; (a) degreasing (G1), (b) grinding (G2), (c) grinding and etching with a weak alkaline solution (G3) and (d) grinding and etching with a strong alkaline solution at high temperature (G4).
As main conclusion, the authors have established that the highest rate of reproducibility belongs to the mechanically polished (G2) and the softly etched (G3) groups, whereas the only degreased (G1) and the hard etched (G4) samples are more distinguishable among themselves. Besides, the different preliminary treatment procedures have rather distinguishable impacts on the resulting morphology (Figure 11).
The surface morphology of given Al alloy after any preliminary surface treatment procedure depends on both its chemical composition and the thermal treatment regime, applied for its metallurgical production. Thus, although the same chemical composition, the obtained alloys could possess completely different mechanical properties, predetermined by the finishing metallurgical thermal treatment. Indeed, in another comparative investigation, involving CeOPL
Figure 12. SEM (a, b) and EDX (c, d) images of the boundary between the bare and the coated areas of AA2024-T3 (a, c) and D16 AM (b, d) [100].
deposition at the same regime on AA2024-T3, and its Russian analogue—D16 AM tempered clad alloy [101], it was established that the CeOPL is much more uniform and fine grained in the former case, whereas in the latter case, the film is less uniform, because of the tempered shielding layers (Figure 12).
## 8. Conceptual summary
Following all the statements and the concepts, described in the present chapter, it can be inferred that a continuous need exists for elaboration of reinforced multilayered durable coating systems capable to provide active corrosion protection even after already appeared mechanical damages. Namely, the providing of durable and reliable corrosion protection enables extension of the service life time of the engineering construction and equipment and particularly—transport vehicles and aircraft. Thus, the reliable corrosion protection is the right way to save economical spends related to the shortening of the service life time before decommission of the used transport and especially aircraft equipment. In this sense, the UV light–protected, hydrophobic-reinforced self-reparable coating systems form reliable barrier against access of corrosive species to the constructive element metallic surfaces.
In this sense, the recent trends related to the elaboration of durable and reliable corrosion protective systems include several basic directions:
As a result, advanced, environmentally compliant multilayered systems are under elaboration. Such system is well illustrated by Figueira et al. [102] (Figure 13).
Advanced Multifunctional Corrosion Protective Coating Systems for Light-Weight Aircraft Alloys—Actual… http://dx.doi.org/10.5772/67237 197
Figure 13. Schematic view of advanced multilayered coating system capable for active corrosion protection, according to Figueira et al. [102].
## 9. Advanced technological approaches
Besides the VOC-emission removal, the rest general trend in the technological aspect regarding the elaboration of advanced corrosion protective coating systems is to decrease the number of the intermediate technological states and the related energetic and material spends.
#### 9.1. Sol-gel road
deposition at the same regime on AA2024-T3, and its Russian analogue—D16 AM tempered clad alloy [101], it was established that the CeOPL is much more uniform and fine grained in the former case, whereas in the latter case, the film is less uniform, because of the tempered shielding
Following all the statements and the concepts, described in the present chapter, it can be inferred that a continuous need exists for elaboration of reinforced multilayered durable coating systems capable to provide active corrosion protection even after already appeared mechanical damages. Namely, the providing of durable and reliable corrosion protection enables extension of the service life time of the engineering construction and equipment and particularly—transport vehicles and aircraft. Thus, the reliable corrosion protection is the right way to save economical spends related to the shortening of the service life time before decommission of the used transport and especially aircraft equipment. In this sense, the UV light–protected, hydrophobic-reinforced self-reparable coating systems form reliable barrier against access of corrosive species to the constructive element metallic sur-
In this sense, the recent trends related to the elaboration of durable and reliable corrosion
• Introduction of UV-radiation absorbers—to extend the service lifetime of the external
• Involvement of reinforcing phases—to enhance the mechanical strength of the coating exterior finishes and the intermediate interlayers. This phase can serve even for UV-
• Use of hydrophobic and super hydrophobic finishes—to repel the water drops, preventing water film formation. Besides the hydrophobic intermolecular attraction forces among the coating ingredients enable additional repulsion of the already penetrated humidity in the
• Encapsulation of active polymerizable agents—to achieve self-healing effect via coating
• Addition of environmentally friendly synergistic corrosion inhibitor mixtures—to protect
• Deposition of reliable environmentally friendly coating primers—to substitute the widely
As a result, advanced, environmentally compliant multilayered systems are under elaboration.
used but already banned chromium conversion coatings (CCC).
Such system is well illustrated by Figueira et al. [102] (Figure 13).
layers (Figure 12).
faces.
8. Conceptual summary
protective systems include several basic directions:
196 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
absorbent and/or corrosion inhibitor carriers.
coating bulk, achieving active corrosion protection.
the metal surface even after coating damage.
coating layers polymeric matrixes.
self-recuperation.
The literature analysis shows that recently the sol-gel approach enlarges its application, being more attractive for ease synthesis of hybrid polymer matrixes, able to combine the benefits of the organic and inorganic moieties (discussed above). As a result, a miscellaneous organic/ inorganic polymer matrix is being obtained, as is illustrated by Wang and Bierwagen [103] (Figure 14).
The sol-gel method, with hybrid precursors, like the proposed by Frignani and coauthors [50], is based on hydrolysis of the metal alkoxides, with coincident polymerization of the hydrolyzed radicals. By this manner, the initial precursor mixture (sol) gradually converts to gel.
Figure 14. Simplified schematic of bonding mechanism between silane molecules and metal surface hydroxide layer (a) before condensation: hydrogen-bonded interface; (b) after condensation: covalent-bonded interface [103].
After deposition and posterior appropriated thermal treatment, the already deposited gel film converts to solid state protective coating layer.
According to Kozhukharov [104], there are seven basic important conditions, which have to be driven, to obtain desirable gel product (i.e., dense, uniform film):
The basic technological stages of the sol-gel synthesis process are illustrated in Figure 15.
As was mentioned in the present chapter, the basic manner for further improvement of the solgel–derived hybrid polymer matrices is to add micro-/or nanosized solid state fine dispersed phase directly into the sol-gel liquid system at intensive stirring, to obtain equally distribution. Among the most efficient methods for production nanoparticle production is the so-called
Advanced Multifunctional Corrosion Protective Coating Systems for Light-Weight Aircraft Alloys—Actual… http://dx.doi.org/10.5772/67237 199
Figure 15. Schematic presentation of the basic stages of the sol-gel synthesis process [104]: (1) initial gel formation, (2) gel drying, (3) finishing high temperature treatment.
spray pyrolysis synthesis (SPS). Besides, thin ceramic protective films able to work at very high temperatures can be produced by so-called spray pyrolysis deposition (SPD). This method is applicable for protective layer deposition for turbine blades in jet turbines, compressors, and so forth
#### 9.2. Spray pyrolysis
After deposition and posterior appropriated thermal treatment, the already deposited gel film
Figure 14. Simplified schematic of bonding mechanism between silane molecules and metal surface hydroxide layer (a)
before condensation: hydrogen-bonded interface; (b) after condensation: covalent-bonded interface [103].
According to Kozhukharov [104], there are seven basic important conditions, which have to be
• Molar factor (ratio between the alcohol as medium and the alkoxides as precursors)
• Pressure and chemical composition of the gaseous medium over the gelling system during
As was mentioned in the present chapter, the basic manner for further improvement of the solgel–derived hybrid polymer matrices is to add micro-/or nanosized solid state fine dispersed phase directly into the sol-gel liquid system at intensive stirring, to obtain equally distribution. Among the most efficient methods for production nanoparticle production is the so-called
The basic technological stages of the sol-gel synthesis process are illustrated in Figure 15.
converts to solid state protective coating layer.
• Chemical composition of the liquid medium
• pH of the medium
• Temperature
• Presence of additives
the drying (annealing) process
• Chemical composition of the precursors (alkoxides)
198 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
driven, to obtain desirable gel product (i.e., dense, uniform film):
According to [105], when a liquid drop of solution is already sprayed, it undergoes several processes in the high temperature space, until its conversion to a solid particle. These processes are illustrated in Figure 16 and described, in brief, below.
Initially, each drop suffers heating, and evaporation of the solvent (1), until it achieves the stage A. It could be described as a liquid particle, represented by saturated solution, surrounded by vapors of the solvent. Afterward, due to the evaporation, an additive solid shell forms (2), leading to intermediated three phases system (B). It is represented by vapors, solid porous shell, and still liquid core. That process passes simultaneously with initiation of chemical conversion of the precursors to the desired product. During this stage, an additive interaction between components of the vapors and the solid shell are possible, as well. After completion of the evaporation,
Figure 16. Thermal processes related to SPS [105].
because of expense of the entire liquid, a spherical solid particle of the product appears (C). The process could be finished until this stage. That approach is also known as "one drop–one particle." It enables production of powder materials with one fraction size of particles. If these particles are submitted to further calcinations, then they could split up to form even smaller particles (E). That approach permits production of ultra dispersive nanoparticles. The transition from stage C to E passes through intermediate stage D. This stage could be reached because of appearance of cracks and ruptures (4). Their appearance is consequence of mechanical tensions, due to difference of the temperatures, and the volume expansions between the core and the surface of the respective particle. Another reason for the crumbling of the particles is that the processes described above could pass accompanied by polymorphic transitions in the solid phase.
Among the basic advantages of this method is that it enables large scale production of fine dispersion solid particles by use of rather simple equipment [106].
Figure 17. Basic constructions of spray pyrolysis installations [106, 107] a- vertical chambers; b- horizontal chambers; cchambers for film deposition; 1- vessel for precursor solution; 2- gas bottle for carrier gas; 3- spray nozzle; 4- spray burner; 5- gas bottle for combustible; 6- gas bottle for oxidizer; 7- vertical chamber; 8- horizontal chamber; 9- chamber for spray deposition; 10- electric heaters; 11- powder collectors; 12- substrates for film deposition; 13- filters.
In general, all the possible modifications of the spray pyrolysis installations are consisted on several basic operation units, which function is assisted by additional devices. The main equipment components for synthesis via spray pyrolysis method are: 1—spraying nozzle, 2—High temperature work space (i.e., furnace), 3—product collector for the fine dispersed powder-like products or film deposition substrate. To insure the regular function of these basic operation units, additional units are necessary, such as: initial precursor solution containers, nozzle feeding pumps, carrier gas compressors, thermal energy sources, powder fraction separators, and so forth. The basic types of SPS/SPD installations are illustrated in Figure 17 [107].
Of course, the spray drops and the respective obtained particles size formation are almost entirely predetermined by the nozzle construction. There are specially designed nebulizers and atomizers, which enable nanosized drops formation. Alternatively, nozzle-less spray pyrolysis equipment is also proposed in the literature [108].
## 10. General conclusions
because of expense of the entire liquid, a spherical solid particle of the product appears (C). The process could be finished until this stage. That approach is also known as "one drop–one particle." It enables production of powder materials with one fraction size of particles. If these particles are submitted to further calcinations, then they could split up to form even smaller particles (E). That approach permits production of ultra dispersive nanoparticles. The transition from stage C to E passes through intermediate stage D. This stage could be reached because of appearance of cracks and ruptures (4). Their appearance is consequence of mechanical tensions, due to difference of the temperatures, and the volume expansions between the core and the surface of the respective particle. Another reason for the crumbling of the particles is that the processes described above could pass accompanied by polymorphic transitions in the solid phase. Among the basic advantages of this method is that it enables large scale production of fine
Figure 17. Basic constructions of spray pyrolysis installations [106, 107] a- vertical chambers; b- horizontal chambers; cchambers for film deposition; 1- vessel for precursor solution; 2- gas bottle for carrier gas; 3- spray nozzle; 4- spray burner; 5- gas bottle for combustible; 6- gas bottle for oxidizer; 7- vertical chamber; 8- horizontal chamber; 9- chamber for spray
deposition; 10- electric heaters; 11- powder collectors; 12- substrates for film deposition; 13- filters.
dispersion solid particles by use of rather simple equipment [106].
Figure 16. Thermal processes related to SPS [105].
200 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
Aluminum (Al) is a lightweight, relatively easily treatable metal that possesses an aptitude for passivation by formation of a natural oxide layer. Nevertheless, the pure Al is inapplicable in the industrial practice, since it does not present satisfying mechanical properties. Irrespective of their excellent mechanical properties, these alloys are very susceptible to corrosion, due to occurrence of alloying phases, which become centers of initiation and further proliferation of localized corrosion
Nowadays, the term "corrosion of the materials" comprises a large variety of detrimental processes which lead to lose of material integrity and mechanical properties of the corresponding details. In addition, the term "corrosion," already includes "microbially induced corrosion (MIC)". Particularly, the metal corrosion processes possess electrochemical nature, and their appearance, rate and impact are strongly dependent on the metal part features (i.e. structure and composition) and the environmental conditions (i.e. temperatures, pH, oxidant concentrations, and so forth).
Thus, the term corrosion relays to whatever physical, chemical or biological process which causes gradual geometrical shape altering and mechanical properties deterioration of given solid state object of industrial origin, due to interactions with its surrounding environment.
To prevent all these phenomena, the metallic surfaces should be insulated from the surrounding environment by coating. However, it should be mentioned that even the protective coatings suffer destructive ageing processes and consequently, should be created advanced coating systems with extended durability and capabilities for active protection even after any damage of their integrity.
Other actual aspects, related to the elaboration of advanced corrosion protective layers, are rather related to the technological approaches used for their synthesis and deposition and the respective coating ingredients. In other words, every new coating system elaboration should comply the environmental restrictions, related to the use of volatile organic compounds (VOCs), and toxic metals, such as Pb, Cr, As, and so forth.
This considerable variety of requirements, related to the elaboration of durable and reliable, environmentally compliant coating systems with active corrosion protective capability imposes the need for multilayered systems, where each layer has its own function. The main function of the exterior layers is to protect the coating underlayers against the impact of the surrounding environment, like UV light absorption ability and hydrophobicity. The intermediate coating layers should possess extended barrier properties and self-repairing ability. The actual trend in this sense is to use hybrid matrix-based polymeric materials, able to combine the benefits of the organic and inorganic materials. Besides, this approach enables to deposit self-assembled monolayers (SAM), composed by siloxanes with large aliphatic chains. They have significantly lower number and size of defects in their structures, and thus enable more efficient protection via formation of dense barrier layers. Another strategy is to encapsulate polymerizable substances. In this case, polymerizable compounds are enclosed inside polymer or glass capsules to polymerase when are exposed to air, by mixing each other or by toughing ingredients of the basic matrix. A promising alternative to the use of ceramic nanoparticles as inhibitor containers reinforcing the basic polymer matrix is the involvement of carbon nanoparticles (CNP). Especially, the hollow carbon nanotubes (CNTs) allow to be filled by inhibitors via intercalation.
The basic function of the primer coating layers (i.e., the coating primers) is to improve the adhesion between the native Al-oxide layer of the metallic substrate and the upper (i.e., intermediate and finishing) layers, commented in the previous sections. Some authors even report for self-healing properties, possessed by these films. Undoubtedly, among the basic advantages of this group of coatings is the possibility to deposit them by physical, electrochemical, or chemical methods. Regardless the method applied for cerium oxide primer layer (CeOPL) deposition, the respective technological regime includes three basic stages: (i) preliminary treatment of the metallic substrate, (ii) the CeOPL deposition itself, and (iii) final sealing of the already deposited CeOPL film.
The purpose of the preliminary treatment procedure is to modify the metallic substrate surface, to be suitable for deposition of uniform and adherent primer layers. The impact of the preliminary surface treatment procedures comprises both the metallic surface roughness (by mechanical grinding in laboratory's conditions, or sand blasting in industrial scale) and the superficial chemical composition (by selective dissolution of the intermetallic inclusions or affecting the surface oxide layer). The commonly accepted preliminary treatment procedures include four basic steps: degreasing, mechanical grinding, alkaline etching, and acidic desmutting.
Finally, it should be remarked that among the various technological approaches, the sol-gel route and the spray-based techniques have shown to provide the synthesis of various nanosized materials and thin layer deposition.
## Acknowledgements
The present research work has been performed by the financial support of the Bulgarian National Scientific Fund, Project T 02 – 27. The author acknowledges the Bulgarian National Scientific Research Fund, contract DFNI-T02-27.
## Author details
This considerable variety of requirements, related to the elaboration of durable and reliable, environmentally compliant coating systems with active corrosion protective capability imposes the need for multilayered systems, where each layer has its own function. The main function of the exterior layers is to protect the coating underlayers against the impact of the surrounding environment, like UV light absorption ability and hydrophobicity. The intermediate coating layers should possess extended barrier properties and self-repairing ability. The actual trend in this sense is to use hybrid matrix-based polymeric materials, able to combine the benefits of the organic and inorganic materials. Besides, this approach enables to deposit self-assembled monolayers (SAM), composed by siloxanes with large aliphatic chains. They have significantly lower number and size of defects in their structures, and thus enable more efficient protection via formation of dense barrier layers. Another strategy is to encapsulate polymerizable substances. In this case, polymerizable compounds are enclosed inside polymer or glass capsules to polymerase when are exposed to air, by mixing each other or by toughing ingredients of the basic matrix. A promising alternative to the use of ceramic nanoparticles as inhibitor containers reinforcing the basic polymer matrix is the involvement of carbon nanoparticles (CNP). Especially, the hollow carbon nanotubes (CNTs) allow to be filled by inhibitors via intercalation.
202 Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
The basic function of the primer coating layers (i.e., the coating primers) is to improve the adhesion between the native Al-oxide layer of the metallic substrate and the upper (i.e., intermediate and finishing) layers, commented in the previous sections. Some authors even report for self-healing properties, possessed by these films. Undoubtedly, among the basic advantages of this group of coatings is the possibility to deposit them by physical, electrochemical, or chemical methods. Regardless the method applied for cerium oxide primer layer (CeOPL) deposition, the respective technological regime includes three basic stages: (i) preliminary treatment of the metallic substrate, (ii) the CeOPL deposition itself, and (iii) final sealing
The purpose of the preliminary treatment procedure is to modify the metallic substrate surface, to be suitable for deposition of uniform and adherent primer layers. The impact of the preliminary surface treatment procedures comprises both the metallic surface roughness (by mechanical grinding in laboratory's conditions, or sand blasting in industrial scale) and the superficial chemical composition (by selective dissolution of the intermetallic inclusions or affecting the surface oxide layer). The commonly accepted preliminary treatment procedures include four
Finally, it should be remarked that among the various technological approaches, the sol-gel route and the spray-based techniques have shown to provide the synthesis of various
The present research work has been performed by the financial support of the Bulgarian National Scientific Fund, Project T 02 – 27. The author acknowledges the Bulgarian National
basic steps: degreasing, mechanical grinding, alkaline etching, and acidic desmutting.
of the already deposited CeOPL film.
nanosized materials and thin layer deposition.
Scientific Research Fund, contract DFNI-T02-27.
Acknowledgements
Stephan Vladymirov Kozhukharov
Address all correspondence to: [email protected]
University of Chemical Technology and Metallurgy, Sofia, Bulgaria
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## *Edited by Jagannathan Thirumalai*
The book Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets presents topics on global advancements in theoretical and experimental facts, instrumentation and practical applications of thin-film material perspectives and its applications. The aspect of this book is associated with the thin-film physics, the methods of deposition, optimization parameters and its wide technological applications. This book is divided into three main sections: Thin Film Deposition Methods: A Synthesis Perspective; Optimization Parameters in the Thin Film Science and Application of Thin Films: A Synergistic Outlook. Collected chapters provide applicable knowledge for a wide range of readers: common men, students and researchers. It was constructed by experts in diverse fields of thin-film science and technology from over 15 research institutes across the globe.
Thin Film Processes - Artifacts on Surface Phenomena and Technological Facets
Thin Film Processes
Artifacts on Surface Phenomena and
Technological Facets
*Edited by Jagannathan Thirumalai*
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• Ermina Begovic and Simone Mancini
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ffefa398-de0d-4481-a8ce-0276e7d09a9c.1 | **Stability and Seakeeping of Marine Vessels**
Ermina Begovic and Simone Mancini Edited by
Printed Edition of the Special Issue Published in *Journal of Marine Science and Engineering*
www.mdpi.com/journal/jmse
| doab | 2025-04-07T04:13:04.611082 | 11-1-2022 14:43 | {
"license": "Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/",
"book_id": "ffefa398-de0d-4481-a8ce-0276e7d09a9c",
"url": "https://mdpi.com/books/pdfview/book/4264",
"author": "",
"title": "Stability and Seakeeping of Marine Vessels",
"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
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ffefa398-de0d-4481-a8ce-0276e7d09a9c.3 | **Stability and Seakeeping of Marine Vessels**
Editors
**Ermina Begovic Simone Mancini**
MDPI • Basel • Beijing • Wuhan • Barcelona • Belgrade • Manchester • Tokyo • Cluj • Tianjin
*Editors* Ermina Begovic Department of Industrial Engineering University of Naples "Federico II" Italy
Simone Mancini Hydro and Aerodynamics Department FORCE Technology Denmark
*Editorial Office* MDPI St. Alban-Anlage 66 4052 Basel, Switzerland
This is a reprint of articles from the Special Issue published online in the open access journal *Journal of Marine Science and Engineering* (ISSN 2077-1312) (available at: https://www.mdpi.com/ journal/jmse/special issues/bz stability seakeeping marine vessels).
For citation purposes, cite each article independently as indicated on the article page online and as indicated below:
LastName, A.A.; LastName, B.B.; LastName, C.C. Article Title. *Journal Name* **Year**, *Volume Number*, Page Range.
**ISBN 978-3-0365-0970-9 (Hbk) ISBN 978-3-0365-0971-6 (PDF)**
Cover image courtesy of Cameron Venti
© 2021 by the authors. Articles in this book are Open Access and distributed under the Creative Commons Attribution (CC BY) license, which allows users to download, copy and build upon published articles, as long as the author and publisher are properly credited, which ensures maximum dissemination and a wider impact of our publications.
The book as a whole is distributed by MDPI under the terms and conditions of the Creative Commons license CC BY-NC-ND.
| doab | 2025-04-07T04:13:04.611128 | 11-1-2022 14:43 | {
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"publisher": "MDPI - Multidisciplinary Digital Publishing Institute",
"isbn": "9783036509709",
"section_idx": 3
} |
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