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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 1 — Jan. 2, 2012
  • pp: 698–705
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Laser conditioning on HfO2 film monitored by calorimeter

Liu Hao, Chen Songlin, Wei Yaowei, Zhang Zhe, Luo Jin, Zheng Nan, and Ma Ping  »View Author Affiliations


Optics Express, Vol. 20, Issue 1, pp. 698-705 (2012)
http://dx.doi.org/10.1364/OE.20.000698


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Abstract

Conditioning effect on HfO2 single-layer film by quasi-cw laser was investigated. The conditioning process was monitored with laser calorimeter. Experimental results revealed that the HfO2 film absorption decreased as a function of the irradiation dose. Higher laser power accelerated the conditioning process. The conditioning effect could not be explained by water annihilation. AFM pictures of the film surface showed that the structural information in the conditioned region was different from the unconditioned region. Monitoring the in situ absorption, laser calorimeter is a promising tool to investigate the laser conditioning process.

© 2011 OSA

1. Introduction

Electron-beam deposited multilayer coatings composed of silica and hafnia are frequently used on large-aperture mirrors for high power laser systems. However, the limited radiation resistance of the thin films has been a bottle neck for the development of high power laser. Over the past decades, many investigators have reported that laser conditioning was viable for increasing the laser induced damage threshold (LIDT) of thin films [1

1. C. J. Stolz, C. L. Weinzapfel, A. L. Rigatti, J. B. Oliver, J. Taniguchi, R. P. Bevis, and J. S. Rajasansi, “Fabrication of meter-scale laser-resistant mirrors for the National Ignition Facility, a fusion laser,” Proc. SPIE 5193, 50–58 (2004). [CrossRef]

4

4. Y. A. Zhao, G. H. Hu, J. D. Shao, X. F. Liu, H. B. He, and Z. X. Fan, “Laser conditioning process combining N/1 and S/1 programs to improve the damage resistance of KDP crystals,” Proc. SPIE 7504, 75041L (2009). [CrossRef]

]. The laser conditioning is a process of irradiating an optic by starting at a low fluence laser and gently ramping to higher fluences [5

5. A. B. Papandrew, C. J. Stolz, Z. L. Wu, G. E. Loomis, and S. Falabella, “Laser conditioning characterization and damage threshold prediction of hafnia/silica multilayer mirrors by photothermal microscopy,” Proc. SPIE 4347, 53–61 (2001). [CrossRef]

]. It is convinced that laser conditioning could permanently increase the LIDT of films by more than 2 times, especially for HfO2/SiO2 films [6

6. H. Bercegol, “What is laser conditioning? a review focused on dielectric multilayers,” Proc. SPIE 3578, 421–426 (1999). [CrossRef]

,7

7. M. R. Kozlowski, M. Staggs, and F. Rainer, “Laser conditioning and electronic defects of HfO2 and SiO2 thin films,” Proc. SPIE 1441, 269–282 (1991). [CrossRef]

]. However, the mechanism of laser conditioning is not fully understood yet, though nodular ejection was recognized and other mechanism has been presented [5

5. A. B. Papandrew, C. J. Stolz, Z. L. Wu, G. E. Loomis, and S. Falabella, “Laser conditioning characterization and damage threshold prediction of hafnia/silica multilayer mirrors by photothermal microscopy,” Proc. SPIE 4347, 53–61 (2001). [CrossRef]

7

7. M. R. Kozlowski, M. Staggs, and F. Rainer, “Laser conditioning and electronic defects of HfO2 and SiO2 thin films,” Proc. SPIE 1441, 269–282 (1991). [CrossRef]

]. The reason lies in that it was difficult to gain any insight into the conditioning process. If changes in optical materials by conditioning are identified, more about fundamental mechanisms of irradiation-induced damage may be learned, and the current laser conditioning technique may be optimized.

In this work, conditioning effect was found on HfO2 film by quasi-continuous-wave (quasi-cw) laser. Laser Calorimeter (LCA) was used to record the irradiation-induced temperature change and monitor the in situ absorption of the film. The absorption variance during laser conditioning was studied, and Atomic Force Microscope (AFM) was utilized to study the film structure after laser conditioning.

2. Experiment

HfO2 single-layer film was E-beam deposited on fused silica substrates (Φ25mm). The deposition rate was 0.2-0.4 nm/s, controlled by a quartz oscillator. The film thickness was about 300nm. The source material was high-purity HfO2 granules. The substrate temperature was 200°C. The film was deposited in O2 atmosphere, so as to avoid nonstoichiometric ratio. All samples were prepared at the same time.

The Quantronix Osprey laser system was employed to irradiate the samples. It was actually quasi-cw laser, with wavelength 532 nm, and highest output power 10W. The pulse frequency was 30 kHZ, and pulse width 26 ns. The beam had a Gaussian shape profile with diameter about 0.8 mm at 1/e2 measured at a normal incidence at the sample plane, as shown in Fig. 1(a)
Fig. 1 (a) Beam profile of the laser spot; (b) Shematic of the localizer.
. The maximum fluence of one pulse was about 16.6mJ/cm2. The pulse-to-pulse stability was less than 2% rms. This energy density was well below the damage threshold but thousands of pulses sufficed to cause decreases in absorption.

A localizer shown in Fig. 1(b) was used to mark the sample center. The localizer was Φ25mm in diameter with a Φ1mm pinhole in the center. The sample holder of LCA was designed to equip Φ25mm samples specifically and the laser should impinge on the sample center, as required by LCA [14

14. ISO 11551: “Optics and optical instruments-Lasers and laser-related equipment-Test method for absorptance of optical laser components” (2003).

,15

15. U. Willamowski, D. Ristau, and E. Welsch, “Measuring the absolute absorptance of optical laser components,” Appl. Opt. 37(36), 8362–8370 (1998). [CrossRef] [PubMed]

]. Equipping the sample on the localizer, the laser could be adjusted to irradiate the sample center. The sample center could also be found under microscope or Atomic Force Microscope (AFM) in this way.

Weak absorption was measured on Laser Calorimeter, set up according to ISO 11551 [14

14. ISO 11551: “Optics and optical instruments-Lasers and laser-related equipment-Test method for absorptance of optical laser components” (2003).

]. Laser Calorimeter was developed to characterize optical materials and dielectric coatings [15

15. U. Willamowski, D. Ristau, and E. Welsch, “Measuring the absolute absorptance of optical laser components,” Appl. Opt. 37(36), 8362–8370 (1998). [CrossRef] [PubMed]

,16

16. L. O. Jensen, I. Balasa, H. Blaschke, and D. Ristau, “Novel technique for the determination of hydroxyl distributions in fused silica,” Opt. Express 17(19), 17144–17149 (2009). [CrossRef] [PubMed]

]. The main mechanism was to measure the temperature precisely on the sample of interest. As indicated in Fig. 2
Fig. 2 Laser Calorimeter set up.
, a set of NTC thermistor was employed to perform an absolute temperature measurement. A reflecting mirror triggered by computer was used as a shutter to block the beam. With this configuration it is possible to detect increases in temperature well below 1 mK. The integral absorption A over the beam area was derived from the irradiation-induced temperature rise, the total energy of the burst, and the heat capacities of sample and mount.

The laser beam irradiated at nearly 0o to the central point of the sample. The NTCs were placed 7 mm away from the center, so as to avoid influence of thermal conductivity [14

14. ISO 11551: “Optics and optical instruments-Lasers and laser-related equipment-Test method for absorptance of optical laser components” (2003).

,15

15. U. Willamowski, D. Ristau, and E. Welsch, “Measuring the absolute absorptance of optical laser components,” Appl. Opt. 37(36), 8362–8370 (1998). [CrossRef] [PubMed]

]. The temperature flow was about 55 μK in the insulated chamber. A hollow sample made of blackened aluminum was used to check noise. The absorption of 0.64 ppm (shown in Fig. 3
Fig. 3 Absorption measurement of hollow sample.
) indicated a rather slight impact of scattering light. The fused silica substrates were measured before coating. The absorption ranged from 7.7 ppm to 8.6 ppm. Compared to the absorption of HfO2 film, the influence of substrates could be ignored. The laser power used in these measurements was 9 W.

3. Results and discussion

3.1 Laser conditioning effect

Three pieces of HfO2 samples were picked out randomly. Each one was irradiated for over 15 cycles, until the absorption became rather stable. The value of 1st and 15th cycle of sample one was shown in Fig. 4
Fig. 4 Substrate and HfO2 absorption.
, respectively. All absorption results were illustrated in Fig. 5
Fig. 5 Absorption towards irradiating cycles of three HfO2 samples.
.

The absorption of each sample decreased markedly with the cumulative irradiating cycles and became close to a saturation level. The initial absorption of the 3 samples were different, this was probably caused by the influence of film surfaces. The laser power was kept at 9W. The three curves had similar tendency. The following relation yielded excellent fit of the experimental data (Eq. (1)):

A=a+bTc
(1)

In which parameter A represented the absorption value, and T denoted the irradiating cycles. The fitted parameters a、b、and c were shown in Table 1

Table 1. Parameters of the Fit Curve

table-icon
View This Table
. The R2 in the Table denoted the correlation coefficient, representing the quality of fitting.

It could be inferred that the absorption value of each sample was divided into two parts. One was the saturation absorption, represented by parameter a, which might be regarded as the intrinsic absorption that cannot be further conditioned. The other was the conditionable absorption, which was proportional to parameter b. Both parameter b and c was correlated to the decreasing rate. The error of parameter c was about 0.2.

3.2 Mechanism study

The morphology of the sample center was inspected under Leica microscope (100 × ) both before and after conditioning. No significant difference was observed. The samples were then investigated by AFM. The localizer illustrated in Fig. 2 was utilized to grasp the sample so that the irradiated area was at the pinhole. A number of spots either at the center of pinhole or 3-4mm away from the pinhole were scanned after laser conditioning. The sample before laser conditioning was not scanned by AFM so as to avoid any impact on the samples’ absorption. One pair of image was shown in Fig. 6
Fig. 6 3-D AFM image after laser conditioning. (a).unconditioned region; (b).conditioned region.
.

The 3-D AFM images offered a direct view of the film structure. Compared to the unconditioned region (Fig. 6(a)), the grains in the conditioned region (Fig. 6(b)) were arranged more orderly, though the appearance of each grain was almost not changed. The power spectral density (PSD) curves conveyed more information. The PSD curves of 4 spots within the conditioned region and 4 spots out of the conditioned region were illustrated in Fig. 7
Fig. 7 2-D PSD curves of the AFM images.
. In the conditioned region, signal was relatively stronger at higher frequency (8-20/μm), while weaker at lower frequency (1-5/μm). It indicated that roughness components of larger lateral extension were less in the conditioned region. Regarding to the 3-D images, the higher frequency power spectrum agreed to the more orderly arranged grains. It could be inferred that some structural information was lost in the conditioned region, which might be defects or contaminants. As a result, the absorption coefficient became smaller.

Additional experiments were carried out. One piece of sample was cleaned by ultrasonic technique first and then irradiated. The absorption decreased as well, though the initial value became much larger because of ultrasonic induced relaxation of films. Another sample was baked at 150°C for one hour and then irradiated. The decreasing tendency of absorption was not improved. Besides, a sample was measured 24 hours later after laser conditioning, the absorption was not elevated. It indicated that water annihilation was not the mechanism, and that the conditioning effect was irreversible.

3.3 Influence of laser power and irradiating time

Another HfO2 sample was conditioned for 3 cycles and then irradiated by laser incessantly for 14 minutes, followed by several more cycles. The fluence of 14 minutes’ irradiation was equal to 7 conditioning cycles. Thus, the 14 minutes’ irradiation was converted to 7 cycles’ conditioning intentionally. The result was shown in Fig. 9
Fig. 9 Absorption V.S. Measuring rounds curve, in which 14 min irradiation was equivalent to 7 irradiating rounds.
. The curve could be well fit, with correlation coefficient above 99.8%. It proved that the laser conditioning effect was determined by the cumulative dose that impinged on the sample.

As for the empirical formula (1), it could be inferred that laser power P was included in parameter b. Equation (1) might be rewritten as following:

A={a+b(PT)c,T1a+b,T=0
(2)

In which bcould be regarded as the conditionable absorption. P was the laser power. However, the role of parameter c was still not clear, and the precise form of this formula needed further investigation.

Based on this study, the one step conditioning process using proper fluence and proper number of pulses is suggested in formal laser conditioning on high power laser optics. It seems to be unnecessary to use many steps of ramping fluences to do laser conditioning, unless the LIDT was not known before. Besides, as to the measurement of weak absorption precisely, a small laser power is suggested in order to avoid the laser conditioning effect.

4. Summary

Conditioning effect on HfO2 film by quasi-cw laser at 532 nm was observed. The conditioning process was monitored by Laser Calorimeter. Combining conditioning and absorption measuring, Laser Calorimeter is a promising tool to investigate the laser conditioning process.

The absorption of HfO2 film was found to be a function against irradiating time. The conditionable absorption or saturation absorption was unrelated to laser power. However, the laser power was correlated to conditioning rate, while the cumulative irradiating dose determined the reduced absorption. Nevertheless, more investigation is needed to make clear the formula of laser conditioning process.

Laser Calorimeter is also nondestructive to samples, making it possible to inspect the film structure after conditioning process. AFM analyses gave evidence of structural change induced by laser conditioning. By experiments of ultrasonic cleaning and baking, the mechanism of laser cleaning was proved unrelated to the conditioning effect.

References and links

1.

C. J. Stolz, C. L. Weinzapfel, A. L. Rigatti, J. B. Oliver, J. Taniguchi, R. P. Bevis, and J. S. Rajasansi, “Fabrication of meter-scale laser-resistant mirrors for the National Ignition Facility, a fusion laser,” Proc. SPIE 5193, 50–58 (2004). [CrossRef]

2.

L. Sheehan, M. Kozlowski, F. Rainer, and M. Staggs, “Large-area conditioning of optics for high-power laser systems,” Proc. SPIE 2114, 559–568 (1994). [CrossRef]

3.

C. J. Stolz, L. M. Sheehan, S. M. Maricle, S. Schwartz, M. R. Kozlowski, R. T. Jennings, and J. Hue, “Laser conditioning methods in hafnia silica multilayer mirrors,” Proc. SPIE 3264, 105–112 (1998). [CrossRef]

4.

Y. A. Zhao, G. H. Hu, J. D. Shao, X. F. Liu, H. B. He, and Z. X. Fan, “Laser conditioning process combining N/1 and S/1 programs to improve the damage resistance of KDP crystals,” Proc. SPIE 7504, 75041L (2009). [CrossRef]

5.

A. B. Papandrew, C. J. Stolz, Z. L. Wu, G. E. Loomis, and S. Falabella, “Laser conditioning characterization and damage threshold prediction of hafnia/silica multilayer mirrors by photothermal microscopy,” Proc. SPIE 4347, 53–61 (2001). [CrossRef]

6.

H. Bercegol, “What is laser conditioning? a review focused on dielectric multilayers,” Proc. SPIE 3578, 421–426 (1999). [CrossRef]

7.

M. R. Kozlowski, M. Staggs, and F. Rainer, “Laser conditioning and electronic defects of HfO2 and SiO2 thin films,” Proc. SPIE 1441, 269–282 (1991). [CrossRef]

8.

R. Wolf, G. Zscherpe, E. Welsch, V. Goepner, and D. Schafer, “Absorption influenced laser damage resistance of Ta2O5 coatings,” Opt. Acta (Lond.) 33(7), 919–924 (1986). [CrossRef]

9.

R. Wolf, G. Zscherpe, E. Welsch, V. Goepner, and D. Schafer, “Ageing influence on the absorption and laser damage resistance of Ta2O5 thin films,” J. Mod. Opt. 34(12), 1585–1588 (1987). [CrossRef]

10.

A. During, M. Commandre, C. Fossati, B. Bertussi, J. Y. Natoli, J. L. Rullier, H. Bercegol, and P. Bouchut, “Integrated photothermal microscope and laser damage test facility for in-situ investigation of nanodefect induced damage,” Opt. Express 11(20), 2497–2501 (2003). [CrossRef] [PubMed]

11.

Z. L. Wu, C. J. Stolz, S. C. Weakley, J. D. Hughes, and Q. Zhao, “Damage threshold prediction of hafnia-silica multilayer coatings by nondestructive evaluation of fluence-limiting defects,” Appl. Opt. 40(12), 1897–1906 (2001). [CrossRef] [PubMed]

12.

S. Papernov, A. Tait, W. Bittle, A. W. Schmid, J. B. Oliver, and P. Kupinski, “Near-ultraviolet absorption and nanosecond-pulse-laser damage in HfO2 monolayers studied by submicrometer-resolution photothermal heterodyne imaging and atomic force microscopy,” J. Appl. Phys. 109(11), 113106 (2011). [CrossRef]

13.

E. Eva, K. Mann, N. Kaiser, B. Anton, R. Henking, D. Ristau, P. Weissbrodt, D. Mademann, L. Raupach, and E. Hacker, “Laser conditioning of LaF3/MgF2 dielectric coatings at 248 nm,” Appl. Opt. 35(28), 5613–5619 (1996). [CrossRef] [PubMed]

14.

ISO 11551: “Optics and optical instruments-Lasers and laser-related equipment-Test method for absorptance of optical laser components” (2003).

15.

U. Willamowski, D. Ristau, and E. Welsch, “Measuring the absolute absorptance of optical laser components,” Appl. Opt. 37(36), 8362–8370 (1998). [CrossRef] [PubMed]

16.

L. O. Jensen, I. Balasa, H. Blaschke, and D. Ristau, “Novel technique for the determination of hydroxyl distributions in fused silica,” Opt. Express 17(19), 17144–17149 (2009). [CrossRef] [PubMed]

17.

G. Duchateau, “Modeling laser conditioning of KDP crystals,” Proc. SPIE 7504, 75041K (2009).

OCIS Codes
(140.3330) Lasers and laser optics : Laser damage
(140.3440) Lasers and laser optics : Laser-induced breakdown

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: October 14, 2011
Revised Manuscript: December 9, 2011
Manuscript Accepted: December 12, 2011
Published: December 23, 2011

Citation
Liu Hao, Chen Songlin, Wei Yaowei, Zhang Zhe, Luo Jin, Zheng Nan, and Ma Ping, "Laser conditioning on HfO2 film monitored by calorimeter," Opt. Express 20, 698-705 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-1-698


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References

  1. C. J. Stolz, C. L. Weinzapfel, A. L. Rigatti, J. B. Oliver, J. Taniguchi, R. P. Bevis, and J. S. Rajasansi, “Fabrication of meter-scale laser-resistant mirrors for the National Ignition Facility, a fusion laser,” Proc. SPIE5193, 50–58 (2004). [CrossRef]
  2. L. Sheehan, M. Kozlowski, F. Rainer, and M. Staggs, “Large-area conditioning of optics for high-power laser systems,” Proc. SPIE2114, 559–568 (1994). [CrossRef]
  3. C. J. Stolz, L. M. Sheehan, S. M. Maricle, S. Schwartz, M. R. Kozlowski, R. T. Jennings, and J. Hue, “Laser conditioning methods in hafnia silica multilayer mirrors,” Proc. SPIE3264, 105–112 (1998). [CrossRef]
  4. Y. A. Zhao, G. H. Hu, J. D. Shao, X. F. Liu, H. B. He, and Z. X. Fan, “Laser conditioning process combining N/1 and S/1 programs to improve the damage resistance of KDP crystals,” Proc. SPIE7504, 75041L (2009). [CrossRef]
  5. A. B. Papandrew, C. J. Stolz, Z. L. Wu, G. E. Loomis, and S. Falabella, “Laser conditioning characterization and damage threshold prediction of hafnia/silica multilayer mirrors by photothermal microscopy,” Proc. SPIE4347, 53–61 (2001). [CrossRef]
  6. H. Bercegol, “What is laser conditioning? a review focused on dielectric multilayers,” Proc. SPIE3578, 421–426 (1999). [CrossRef]
  7. M. R. Kozlowski, M. Staggs, and F. Rainer, “Laser conditioning and electronic defects of HfO2 and SiO2 thin films,” Proc. SPIE1441, 269–282 (1991). [CrossRef]
  8. R. Wolf, G. Zscherpe, E. Welsch, V. Goepner, and D. Schafer, “Absorption influenced laser damage resistance of Ta2O5 coatings,” Opt. Acta (Lond.)33(7), 919–924 (1986). [CrossRef]
  9. R. Wolf, G. Zscherpe, E. Welsch, V. Goepner, and D. Schafer, “Ageing influence on the absorption and laser damage resistance of Ta2O5 thin films,” J. Mod. Opt.34(12), 1585–1588 (1987). [CrossRef]
  10. A. During, M. Commandre, C. Fossati, B. Bertussi, J. Y. Natoli, J. L. Rullier, H. Bercegol, and P. Bouchut, “Integrated photothermal microscope and laser damage test facility for in-situ investigation of nanodefect induced damage,” Opt. Express11(20), 2497–2501 (2003). [CrossRef] [PubMed]
  11. Z. L. Wu, C. J. Stolz, S. C. Weakley, J. D. Hughes, and Q. Zhao, “Damage threshold prediction of hafnia-silica multilayer coatings by nondestructive evaluation of fluence-limiting defects,” Appl. Opt.40(12), 1897–1906 (2001). [CrossRef] [PubMed]
  12. S. Papernov, A. Tait, W. Bittle, A. W. Schmid, J. B. Oliver, and P. Kupinski, “Near-ultraviolet absorption and nanosecond-pulse-laser damage in HfO2 monolayers studied by submicrometer-resolution photothermal heterodyne imaging and atomic force microscopy,” J. Appl. Phys.109(11), 113106 (2011). [CrossRef]
  13. E. Eva, K. Mann, N. Kaiser, B. Anton, R. Henking, D. Ristau, P. Weissbrodt, D. Mademann, L. Raupach, and E. Hacker, “Laser conditioning of LaF3/MgF2 dielectric coatings at 248 nm,” Appl. Opt.35(28), 5613–5619 (1996). [CrossRef] [PubMed]
  14. ISO 11551: “Optics and optical instruments-Lasers and laser-related equipment-Test method for absorptance of optical laser components” (2003).
  15. U. Willamowski, D. Ristau, and E. Welsch, “Measuring the absolute absorptance of optical laser components,” Appl. Opt.37(36), 8362–8370 (1998). [CrossRef] [PubMed]
  16. L. O. Jensen, I. Balasa, H. Blaschke, and D. Ristau, “Novel technique for the determination of hydroxyl distributions in fused silica,” Opt. Express17(19), 17144–17149 (2009). [CrossRef] [PubMed]
  17. G. Duchateau, “Modeling laser conditioning of KDP crystals,” Proc. SPIE7504, 75041K (2009).

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