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Nanosecond single and multiple pulses laser damage studies on electro-optic transparent ceramic PLZT ceramic are performed. The evolution of damage morphology and damage probability threshold under multiple irradiations reveals that fatigue effects are affected by both laser fluence and shot numbers. The bulk damage is caused by themal explosion and self-focusing. Femtosecond single and multiple pulses are also employed to irradiate on the sample. Nonlinear absorption property of sample is studied to analyze different damage mechanism. Investigations on laser-induced damage in PLZT are of high practical importance for high-power laser applications.
© 2016 Optical Society of America
1. Introduction
Developing novel electro-optic (EO) modulators is a very key technology for raising current levels of laser technology, using the electrically controllable refractive index modulation effect. EO switches may realize high switch speeds (nanoseconds or even picoseconds), which will be very attractive for lasers applications [1]. EO crystals, such as KDP, LiNbO3, and BBO crystals are researched and used widely in the high power laser system. KDP crystals possess a large electro-optic coefficient, high laser induced damage threshold, and large size growth from solution. However the deliquescent properties of these crystals require very strict application environments. LiNbO3 crystals have wide applications in optical communication and sensors because of their quick response time and low half-wave voltage; their low laser induced damage threshold, however, restricts their usage in all-solid state high power laser modulation. Moreover, the polarization-dependent characteristic of the crystals is a significant destabilizing factor. BBO crystals have excellent damage thresholds in high power density systems, however tens of thousands volts of driven voltage is a harsh working condition [2,3].
The ABO3 ferroelectric transparent ceramics have been of great interest for their low production cost and unusually large EO response [4,5]. Recently, a giant EO effect has been observed in the relaxor ferroelectrics lanthanum-modified lead zirconate titanate/PLZT or lead magnesium niobate-lead titanite/PMNT) transparent ceramics [6–11], which attracts much attentions to develop EO devices based on their electro-controlled birefringence [12–15]. In general, the attractive features of PLZT or PMNT include a high electro-optic coefficient, good optical transparency, fast response time. Especially the large size by the mature hot-pressing technique is easily obtained. Therefore, these quadratic electro-optic ceramics may be adapted to high-power electro-optic components.
Laser-induced damage in optical materials is always a limiting factor in the development of high-power laser systems. This phenomenon, often called “fatigue” laser-induced damage, has been observed and studied in different transparent materials such as polymers, crystalline solids and glasses [16–21]. In the nanosecond pulse-width regime, damage is known to arise from defects, such as impurities or other imperfections. The study of laser damage of thin films has been an interesting subject for laser pulse widths from the nanosecond to femtosecond [22]. However, few reports deduced laser damage threshold in YAG ceramic [23,24], let alone a detailed damage mechanism in PLZT. Studies of PLZT’s laser damage threshold and damage mechanism irradiated by nanosecond and femtosecond optical pulses have not been previously reported. In this work, laser-induced damages in PLZT electro-optic ceramic are studied with femtosecond and nanosecond lasers. The damage thresholds and damage morphologies of all samples due to different laser irradiations are reported in the paper. Differences in the characteristic of damage morphologies for different laser pulse widths are described. In addition, different damage mechanisms are explored to explain PLZT under nanosecond laser pulses and femtosecond laser pulses. Therefore, the damage morphology, damage threshold, and damage mechanism are important points in the study.
2. Sample preparation and damage tests
2.1. Sample preparations
The PLZT transparent ceramic was fabricated by a two-stage sintering method using conventional powders. Different blend techniques were used to form pellets. The pellets were first sintered in an oxygen atmosphere at different temperatures. The sintered pellets were then hot pressed at a temperature >1000 oC for 16 h at pressure of 50–100 MPa. The sintered samples were polished to a thickness of 5 mm exhibiting less than 10 nm rms surface roughness.
2.2. Experimental setup
A Nd:YAG laser with pulse width of 12 ns operated at 1064nm in single longitudinal mode with up to a 5 Hz repetition rate was used in the test [25]. In our tests, the 1/e2 spot diameters in x and y axis were measured to be 425 μm and 160 μm with the knife-edge method. On the ceramic surface, the damage test was performed in a 1-on-1 regime; that is, each location on the sample was irradiated by only one laser pulse. Twenty sites for each energy density were tested, enough energy levels were test to obtain the LIDT. In order to probe the bulk material and avoid surface damage, a short focal length was employed in the laser system and the fluence was focused inside the PLZT ceramic to damage the sample. Twenty sites for each energy density were tested and 300 pulses were irradiated on each site. When no damages occurred on a site, the laser energy would increase at a new level. If a site damaged at early shot numbers, the laser irradiations would not stop unless the damage growth was very catastrophic. The on-line damage detection setup was comprised of a microscope focused on the tested area and a CCD to determine whether the radiation sites were damaged or not. The procedure was repeated for other fluence until the range of fluence was sufficiently broad to include points of zero damage probability and points of 100% damage probability to develop a plot of damage probability versus fluence. The relative error of damage probability was about ± 15%.
A Ti: sapphire laser system (Legend USP, Coherent Inc.) with an operation wavelength of 800 nm at pulse repetition rate of 1 kHz and a pulse width of 40 fs was also employed in the damage test. By using a time delay controller, single and multiple pulses could be obtained. The sample was set on a three-dimensional translation stage, and the pulse was focused with a spherical lens with NA of 0.15 on the front surface. The sample surface was monitored in situ with a charge-coupled device (CCD) and a cold light source. The cold light source could avoid heating the sample. Single shot and multiple shots are irradiated on the sample surface. Laser parameters in all damage tests are shown in Table 1.
Table 1. Laser Parameters in damage test
Damage mechanism such as nonlinear absorption was focused on the transparent ceramic under different fluence by using the traditional open aperture Z-scan system [26, 27]. The optical arrangement possesses advantages of simple, sensitive, rapid, and has been widely adopted to investigate the nonlinear absorption under the high density fluence. The Z-scan measurements were performed by employing a fiber laser of 340 fs pulses operating at 515 nm with the repetition rate of 100 Hz. The laser beams were tightly focused through a lens with the focal length of 10 cm. The transmittance varied as the sample moves toward the beam focus (z = 0), indicating a nonlinear absorption. In addition, the laser beam waist radius at the focus was estimated to be about 30 μm at 515nm.
2.3. Analysis methods
The damage morphologies were observed by a Leica microscope. A VHX-700F (Keyence Inc) camera and a scanning electron microscope (SEM) (Carl Zeiss) were used to obtain the details of the damage morphologies. An atomic force microscopy (Bruker Nano Inc) and a profilometer (Wyko NT9100,Bruker Nano Inc) were also employed in mapping damage depth.
3. Experimental results and discussions
3.1 Experimental results
The evolution of damage probability as a function of fluence was obtained for PLZT with the laser operating at 1064 nm with a pulse width of 12 ns. 1-on-1 damage probability curve is shown in Fig. 1, which obviously shows the laser induced damage threshold (LIDT) of 5.8 J/cm2 for the ceramic surface.
Fig. 1 1-on-1 damage threshold of ceramic surface with nanosecond laser irradiation, blue line is linear fitting result. A wavelength of 1064 nm, with a pulse length of 12 ns and a repetition rate of 5 kHz.
The experimental data in Table 2 shows that the damage threshold in the bulk is strongly dependent on the number of shots, as it is well known in multiple pulse damage studies. The LIDT is measure to be 6.5 J/cm2 for single shot test and 3.6 J/cm2 for 300 shots at a site. The low threshold of PLZT for damage within 300 shots is found to reduce about 40% to 50% compared with the single shot threshold. The LIDT decrease
Table 2. Damage probability in PLZT ceramic after i shots (Pi), with i = 1, 10, 100 or 300. A wavelength of 1064nm, with a pulse length of 12ns and a repetition rate of 5Hz.
Figure 2(a) demonstrates the curves of damage depth versus incident power with different shot numbers i = 1, 10, 100 and 1000. The value of damge diameter and depth is measured by profilemeter. When i = 1, the depth maintains stable with laser power growing. Whereas, the depth showed an upward trend with i = 10, 100 and 1000. Increasingly, damage depth depends on growth of incident energy and shot numbers. Damage diameter versus incident power with different shot numbers i = 1, 10, 100 and 1000 is shown in Fig. 2(b). From i = 1 to i = 10, the growth of diameter increases more and more gentle, while the diameters increase dramatically when i = 100 to i = 1000, with a inflection point of P = 15 mW. The closeness of curves of i = 100 and i = 1000 manifests that diameter would not increase when shot number approaches a very high value. Therefore, pulse accumulation acts on the damage depth if high power laser irradiated on the sample.
Fig. 2 (a)Damage depth and (b)diameter versus incident power and pulse number. A wavelength of 800 nm, with a pulse length of 40 fs and a repetition rate of 1 kHz.
To research the mechanism of laser induced damage, the nonlinear abosorption was focused on the transparent ceramic to illustrate the interaction between femtosecond laser and PLZT ceramic. The band gap energy of sample is 3.1 ev while incident beam works at 515 nm. Figure 3(a) shows the typical Z-scan results for PLZT. The normalized transmittance decreases as the sample moves toward the beam focus (z = 0), indicating a clear multi-photon (two-photon and multi-photon included) absorption. The absorption becomes much more pronounced as the incident energy increases (before damage). The solid lines are the fitting results. Figure 3(b) demonstrates the normalized transmittance variation with incident laser fluence. The experiment agrees with the theoretical mode based on electron production via multiphoton ionization, Joule heating, and collisional(avalanche) ionization proposed by B. C. Stuart et al [22].
Fig. 3 (a): Z-scan curves at different incident pulse energies of the sample. The solid lines are the fitting results. (b): Fitting results of normalized transmission as a function of input laser intensity. A wavelength of 515 nm, with a pulse length of 340 fs and a repetition rate of 100 Hz
3.2 Morphology of damage sites and discussion
A. 1-on-1 test on the sample surface
The damage occurs over the front surface, shown in Fig. 4. Three representative damage morphologies (a) Gray Hazelet damage, (b) polished layer damage and (c) crater damage are observed on the suface. Gray Hazelet damage is the subsurface damage from grinding and polishing of the conventional surface roughness [21], which lead to the LIDT decrease(5.8 J/cm2 for the surface and 6.5 J/cm2 for the bulk). A higher fluence irradiation may cause a thermal explosion in the bulk and cracks on the surface and retroflexion of polished layers emerges, shown in Fig. 4(b). When the laser energy reach a threshold, big bulk is peeled off due to stress-cracking from thermal expansion, exhibited in Fig. 4(c). White dots in circle around the damage sites shows the refractive index changes due to the self-focusing effect. There are few melting damages in PLZT ceramic.
Figure 5(a) and 5(b) shows the damage morphologies and three dimension profile at the fluence of 15.6 J/cm2. The suface cracks along grain boundary with intact grain in the site. Damage depth increases in the center of irradiated laser spots. The lattice structure and grain boundary defect are observed with SEM, shown in Fig. 5(c) and 5(d). Pb2+ depositon and air bubble in the boundary influence the compactness of the ceramic. Therefore, grain boundary as the main damage source appears unlikely. This may result from the fact that that grain boundaries arising from the non-press vacuum sintering method, are deprived of any secondary phase and can be as thin as one atomic layer, as can be seen in transmission electron microscope photographs in Fig. 5(c). Self-focusing is another phenomenon that may reduce LIDT for large beam diameters. Selffocusing depends mainly on the total power. As the beam diameter increases, the required power approaches the critical self-focusing power.
Fig. 5 (a)Damage morphology on PLZT surface of 15.6 J/cm2 ; (b)3-dimension figure of (a); (c)SEM picture of grain structure of PLZT; (d)SEM picture of grain boundary.
To verify the mechanism above that compactness of ceramic has a considerable affect on the LIDT, different preparation of raw material and hot press and different hot press techniques are adjusted to obtain samples of different compactness. Figure 6(a) shows damage morphology of sample with LIDT of 5.8 J/cm2 at the fluence of 20.8 J/cm2. For laser pulses with Gaussian spatial beam profile, center region carried the higher flux of photons than any other regions. So the ablation threshold was reached in the center region first and the surface materials was easier to remove. Subsurface damge is the mainly mechanism. In the red circle “A” indicates a defect in the irradiation site. The damage depth of sample is 150 nm with 3 μm of “A”. Therefore defect may cause a deep crack along longitudinal direction rather than transverse extension. when the sample is irradiated by fluencies of 7.2 J/cm2, 14 J/cm2 and 20.8 J/cm2, the corresponding damage diameter is measured to be 41.5μm, 129.3μm and 170.5 μm. With increasing laser fluence, laser intensity in other regions reached ablation threshold successively. Therefore, the diameter increased, and then showed a faint fluctuation due to the deposition of molten materials on grain wall. Grain structure exhibits clealy after irradiation by 15.6 J/cm2 on the sample with LIDT of 3.9 J/cm2, shown in Fig. 6(b). Figure 6(c) demonstrates the picture of a damage site irradiated by 8.1 J/cm2 on sample with LIDT of 2.1 J/cm2. High temperature leads to element component decomposition with volatilization of PbO and the lattice structure would be destroyed. The comparison of Figs. 6(b) and 6(c) indicates that, smaller grain size and higher compactness may raise the LIDT. The grain structure maintains primary morphology under the test laser energy.
B. morphology of 300-on-1 test in the bulk
Figure 7(a) shows the morphology of top view at an 30° angle, where the black tail behind the surface damage indicating the grain expansion and refractive change. A short focal lens was employed to focus the pulse in the bulk, the explosion damage morphology in the case of multiple shots observed of side view in Figs. 7(b), 7(c) and 7(d). The light propagation direction is along the white arrow, and the focal waist is located at the center of the fracture zone. Self-focusing in the sample is responsible for the hairline scratch in Fig. 7(c). In addition, the catastrophic breakdown occurs at the top end of the damage trail, which implies that the damage moves quickly after the initial damage at the focus, so most of the energy in the pulse is absorbed at the upstream end of the trail. If the laser energy is strong enough, big burst would happen along the propagation direction, shown in Fig. 7(d). The multi-pulse laser damage studies in the bulk modification regime is to identify which is the light-induced material modification and by which physical process this modification lowers the laser damage threshold (from 6.5 J/cm2 to 3.6 J/cm2). Thermal self-focusing or formation of an index gradient ‘lens’ due to material compaction induce by early shots could modulate the following pulses. The modification might also influence directly the absorption coefficient of the sample, which acts on the mechanical yield strength of the material by introducing strain in the vicinity of the material modifications [29,30]. Therefore, a large number of possible fatigue mechanisms take place.
Fig. 7 Images after successive shots in the bulk of PLZT ceramic. Laser focused on surface of (a); inside the sample of (b), (c) and (d).
C. Morphology of damage sites irradiated by femtosecond pulses
Different from the nanosecond laser irradiation, femtosecond laser induced damage acts on the electrons of elements rather than grain structure, illustrated in Fig. 8. Figures 8(a), 8(b), 8(c), 8(d), 8(e) and 8(f) demonstrate the damage profile when P = 2 mW, 10 mW, 15 mW, 20mW, 25mW, 30mW with single shot on the suface, indicating the damage from the polishing layer. The corresponding damage diameter equals to 2.6 μm, 9 μm, 11.5 μm, 12.5 μm, 13.5 μm, 14 μm. The same powers P = 2 mW, 10mW, 15 mW, 20 mW, 25 mW, 30 mW were employed with shot numbers i = 10 overlying one site, shown in Figs. 8(g), 8(h), 8(i), 8(j), 8(k) and 8(l). Ten shots overlaying irradiation on one site lead to dramastic damage. The diameter could be measured to 8.5 μm, 11.5 μm, 14 μm, 15 μm, 15.5 μm, 16 μm. With high fluence over 20 mW, cracks appear at damage edge due to mechanical stress effect, similar with damage mechanism in the nanosecond laser irradiation. Because of the short pulswidth, there are little thermal expansion in the action process.
Fig. 8 Femtosecond laser induced damge morphologis with single pulse and 10 pulses under different energy levels.
The damage morphology is also strongly dependent on the number of femtosecond laser shots, as shown in Figs. 9(a), 9(b), 9(c) and 9(d), with the irradiation power of 25 mW. As a result, from i = 1 to i = 100, the damage area becomes expansion sharply. The damage would not increase from i = 100 to i = 1000, if the shot number is over 100. The laser enegy accumulation is reflected by the depth of damage site.
This damage morphology is also reproducible from pulse to pulse. The high degree of reproducibility of damage morphology for each measurement condition strongly refutes initiation by randomly dispersed impurity inclusions. In femtosecond regime, the nonlinear multiphoton absorption grows increasingly versus the incident laser energy. Intensities corresponding to breakdown produce electrons via photoionization soon afterwards. And these electrons initiate the avalanche. As long as intensities approach the limit in which multiphoton ionization alone is capable of producing electron densities high enough, that is plasma critical density, more and more energy accumulating, the damage ocurrs.
4. Conclusion
In conclusion, the damage characteristics and damage mechanism for PLZT induced by nanosecond and femtosecond pulse lasers under single-pulse and multiple-pulse irradiations are discussed. The damage morphology by femtosecond pulses is considerably different from which by nanosecond pulses. The laser-induced damage thresholds of all samples are given. In the nanosecond regime, the LIDT of the sample pulse is lower on the surface than bulk due to the polishing defect. Low grain compactness, defects in grain boundary self-focusing in the bulk and thermal expansion lead to serious damage. In the femtosecond regime, multi-photon abosorption and collisional ionization is used to explain the experimental result.
Improved preparation methods to realize high-density sample with few structure defects, which lead to a high LIDT of PLZT will be the further work. Samples with voltages applied will be tested to highlight different behaviors and mechanisms in the EO application.
Acknowledgments
This project is supported by the National Natural Science Foundation of China (61137004, 61405218, 61535014), Shanghai Natural Science Foundation (14ZR1445100) and the Key Basic Project of Science and Technology Commission of Shanghai Municipality (Grant No. 11JC1413500). We acknowledge Professor Yuanan Zhao and Dr. Dawei Li at Shanghai Institute of Optics and Fine Mechnics for their support in the damage test. We also acknowledge Professor Xiyun He for her discussion.
References and links
1. R. Li, C. Wang, G. Z. Su, K. Zhang, L. Tang, and C. Li, “Development and applications of spaceborne LiDAR,” Science & Technology Review 25, 58–63 (2007).
2. W. Y. Anthony, A. K. Michael, J. H. David, B. A. James, and X. Sun, “Spaceborne laser instruments for high-resolution mapping,” Proc. SPIE 7578, 757802 (2010). [CrossRef]
3. Y. H. Wang, J. F. Wang, Y. E. Jiang, Y. Bao, X. C. Li, and Z. Q. Lin, “Laser pulse spectral shaping based on electro-optic modulation,” Chin. Opt. Lett. 6(11), 841–844 (2008). [CrossRef]
4. M. Veithen, X. Gonze, and P. Ghosez, “First-Principles Study of the Electro-Optic Effect in Ferroelectric Oxides,” Phys. Rev. Lett. 93(18), 187401 (2004). [CrossRef] [PubMed]
5. K. Uchino, “Electro-optic ceramics and their display applications,” Ceram. Int. 21(5), 309–315 (1995). [CrossRef]
6. G. H. Haertling, “PLZT electrooptic materials and applications—a review,” Ferroelectrics 75(1), 25–55 (1987). [CrossRef]
7. H. Jiang, Y. K. Zou, Q. Chen, K. K. Li, R. Zhang, Y. Wang, H. Ming, and Z. Zheng, “Transparent electro-optic ceramics and devices,” Proc. SPIE 5644, 380–394 (2005). [CrossRef]
8. G. H. Haertling and C. E. Land, “Hot-Pressed (Pb, La) (Zr, Ti) O3 Ferroelectric Ceramics for Electrooptic Applications,” J. Am. Ceram. Soc. 54(1), 1–11 (1971). [CrossRef]
9. L. S. Kamzina, R. Wei, G. Li, J. Zeng, and A. Ding, “Electro-optical properties of PMN-xPT compounds: single crystrals and transparent ferroelectric ceramic,” Magnetism and Ferroelectricity 52, 2142–2146 (2010).
10. Q. Lei, Y. Qing, J. Gan, H. Cai, and R. Qu, “Optical characteristics of transparent PMNT ceramic and its application at high speed electro-optic switch,” Opt. Commun. 284(16-17), 3886–3890 (2011). [CrossRef]
11. K. K. Li, “Electro-optic ceramics and devices, PMNT,” US Patent Application. 10/139857, 5/6/2002.
12. Q. Ye, Z. Dong, Z. Fang, and R. Qu, “Experimental investigation of optical beam deflection based on PLZT electro-optic ceramic,” Opt. Express 15(25), 16933–16944 (2007). [CrossRef] [PubMed]
13. Q. Ye, L. Qiao, H. Cai, and R. Qu, “High-efficiency electrically tunable phase diffraction grating based on a transparent lead magnesium niobate-lead titanite electro-optic ceramic,” Opt. Lett. 36(13), 2453–2455 (2011). [CrossRef] [PubMed]
14. Q. Ye, L. Qiao, J. Gan, H. Cai, and R. Qu, “Fiber Sagnac π-shifted interferometer for a polarization-independent PMNT high-speed electro-optic switch,” Opt. Lett. 35(24), 4187–4189 (2010). [CrossRef] [PubMed]
15. X. J. Zhang, Q. Ye, H. W. Cai, and R. H. Qu, “Polarization-independent electro-optic modulator based on PMNT electrically-controlled birefringence effect and Sagnac interferometer,” Opt. Laser Technol. 57, 5–8 (2014). [CrossRef]
16. R. M. O’Connell, T. F. Deaton, and T. T. Saito, “Single- and multiple-shot laser-damaged properties of commercial grade PMMA,” Appl. Opt. 23(5), 682–688 (1984). [CrossRef] [PubMed]
17. P. K. Bandyopadhyay and L. D. Merkle, “Laser-induced damage in quartz: A study of the influence of impurities and defects,” J. Appl. Phys. 63(5), 1392–1398 (1988). [CrossRef]
18. L. D. Merkle, N. Koumvakalis, and M. Bass, “Laser-induced bulk damage in SiO2 at 1.064, 0.532 and 0.355 μm,” J. Appl. Phys. 55(3), 772–775 (1984). [CrossRef]
19. D. W. Camp, M. R. Kozlowski, L. M. Sheehan, M. Nichols, M. Dovik, R. Raether, and I. Thomas, “Subsurface damage and polishing compound affect the 355-nm laser damage threshold of fused silica surfaces,” Proc. SPIE 3244, 356–364 (1998). [CrossRef]
20. M. D. Feit and A. M. Rubenchik, “Influence of subsurface cracks on laser induced surface damage,” Proc. SPIE 5273, 264–272 (2004). [CrossRef]
21. L. Gallais, J. Natoli, and C. Amra, “Statistical study of single and multiple pulse laser-induced damage in glasses,” Opt. Express 10(25), 1465–1474 (2002). [CrossRef] [PubMed]
22. B. C. Stuart, M. D. Feit, S. Herman, A. M. Rubenchik, B. W. Shore, and M. D. Perry, “Nanosecond-to-femtosecond laser-induced breakdown in dielectrics,” Phys. Rev. B Condens. Matter 53(4), 1749–1761 (1996). [CrossRef] [PubMed]
23. W. Pompe, H.-A. Bahr, I. Pflugbeil, G. Kirchhoff, P. Langmeier, and H.-J. Weiss, “Laser induced creep and fracture in ceramics,” Mater. Sci. Eng. A 233(1-2), 167–175 (1997). [CrossRef]
24. J.-F. Bisson, Y. Feng, A. Shirakawa, H. Yoneda, J. Lu, H. Yagi, T. Yanaitani, and K.-I. Ueda, “Laser Damage Threshold of Ceramic YAG,” Jpn. J. Appl. Phys. 42(Part 2, No. 8B), 1025–1027 (2003). [CrossRef]
25. L. Yan, C. Wei, D. Li, G. Hu, K. Yi, and Z. Fan, “Coupling effect of multi-wavelength lasers in damage performance of beam splitters at 355 nm and 1064 nm,” Appl. Opt. 51(16), 3243–3249 (2012). [CrossRef] [PubMed]
26. M. S. Bahae, A. A. Said, T. H. Wei, D. J. Hagan, and E. W. Van Stryland, “Sensitive measurement of optical nonlinearities using a single beam,” IEEE J. Quantum Electron. 26(4), 760–769 (1990). [CrossRef]
27. H. Yan and J. S. Wei, “False nonlinear effect in z-scan measurement based on semiconductor laser devices: theory and experiments,” Photon. Res. 2(2), 51–58 (2014). [CrossRef]
28. F. R. Wagner, C. Gouldieff, and J. Y. Natoli, “Contrasted material responses to nanosecond multiple-pulse laser damage: from statistical behavior to material modification,” Opt. Lett. 38(11), 1869–1871 (2013). [CrossRef] [PubMed]
29. S. C. Jones, P. Braunlich, R. T. Casper, X. A. Shen, and P. Kelly, “Recent progress on laser-induced modifications and intrinsic bulk damage of wide-gap optical-materials,” Opt. Eng. 28(10), 281039 (1989). [CrossRef]
30. C. Gouldieff, F. Wagner, and J. Y. Natoli, “Nanosecond UV laser-induced fatigue effects in the bulk of synthetic fused silica: a multi-parameter study,” Opt. Express 23(3), 2962–2972 (2015). [CrossRef] [PubMed]
