Influence of incidence angle and polarization state on the damage site characteristics of fused silica Bin Ma,1,2 Yanyun Zhang,1,2 Hongping Ma,1,2 Hongfei Jiao,1,2 Xinbin Cheng,1,2 and Zhanshan Wang1,2,* 1
Institute of Precision Optical Engineering, Tongji University, Shanghai 200092, China
2
Key Laboratory of Advanced Micro-structure Materials, Ministry of Education, Shanghai 200092, China *Corresponding author: [email protected] Received 29 August 2013; accepted 21 October 2013; posted 25 October 2013 (Doc. ID 196594); published 19 November 2013
The influence of the incidence angle and polarization state on the damage site characteristics of fused silica under 355 nm laser irradiation was investigated. The initial damage morphologies and growth behaviors of the damage sites on the exit surface at incidence angles of 0° and 45° as well as in P and S states were compared to investigate the effects of various angles and polarization states. The relationships between the size of the initial damage sites and the laser fluence, as well as the growth threshold, were discussed. The damage morphologies of the craters and cracks at different incidence angles and polarization states were then investigated. Finally, the growth characteristics of the lateral size, crater depth, and crack depth were compared and analyzed. © 2013 Optical Society of America OCIS codes: (140.3330) Laser damage; (160.4670) Optical materials. http://dx.doi.org/10.1364/AO.53.000A96
1. Introduction
Laser damage events generally comprise damage initiation and growth. The discontinuous interface in the initial damage sites, which range from submicrometers to several micrometers, as well as material modification promote damage growth with subsequent laser irradiation, thereby resulting in further material modification and expansion of the lateral or vertical size to several hundred micrometers or even larger [1–5]. Previous studies have shown that the growth behavior is similar to the damage initiation process, in which both are strongly influenced by a large number of parameters, such as laser wavelength, pulse duration, beam shape, and fluence, as well as the size and morphology of the initial damage sites [6–9]. Moreover, the initiation of avalanche ionization caused by the defects at the damaged and modified material and the presence 1559-128X/14/040A96-07$15.00/0 © 2014 Optical Society of America A96
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of field intensification caused by cracks promote plasma formation, melting effect, stress release, and material modifications. These phenomena are proposed to demonstrate the growth mechanisms [10]. However, given the complexity of damage evolution [11,12], single-parameter and comparative studies are preferred in elucidating the growth mechanisms and establishing the growth laws [10,13]. With the increasing demands for optical transmission components with nonnormal incidence, understanding the damage growth at various incident laser irradiations is important. Previous experiments have shown the effects of incidence angle and polarizations on the growth damage sites at the output surface of fused silica [14,15]. These studies provide preliminary lateral growth trends that are dependent on the angle of incidence. However, details of the growth behaviors still need further investigation. The lateral growth of damage on the exit surface is best described by an exponential function of the laser shot number, whereas a linear dependence is found on the input surface, which can be
qualitatively explained by the directional expansion of plasma during the formation of craters and cracks [16–18]. However, evaluation of the relationship between plasma propagation and crater shape and crack morphology is important in elucidating the growth mechanism when the laser irradiates at a specified angle and before the emergence of drilling caused by the small beam [19]. Additionally, the difference between the near-Gaussian distribution of the small beam and the flat-top shape of the large beam results in difficulties in comparing the growth rules obtained at different conditions [9,15]. However, the offline small laser beam allows direct comparison at specified conditions, which is advantageous for monitoring damage growth evolutions and for showing the details of the damage growth events. In this paper, we focused on the effects of the incidence angle and polarization state on the characteristics of damage sites in fused silica. The damage growth behaviors of different initial damage sites initiated by 355 nm laser irradiation at various incidence angles and polarizations are preliminarily investigated and directly compared. Moreover, the crack morphology and damage growth trends characterized by lateral size, vertical crater depth, and crack depth are discussed in detail. 2. Experimental Procedures
Figure 1 shows the laser-induced damage threshold test platform. The Nd:YAG laser that radiates a 355 nm beam with 8 ns pulse was operated at 10 Hz. The sample was placed in a computerized three-dimensional motor-driven stage. The damage information of the testing sites was monitored online by using two optical microscopes. During the tests, the beam exhibited a near-Gaussian profile with a 1∕e2 diameter of 480 μm at the sample plane. The fused silica samples (50 mm × 50 mm × 10 mm) were super polished. Laser damage events are usually found on the exit surface; therefore, the laserinduced damage threshold of the samples was obtained prior to the experiments by using oneon-one tests. Higher initiating fluences were then employed to produce repeatable damage sites with various sizes and morphologies.
Two cameras were placed at the rear and side of the sample to record the lateral and vertical images. The damage site was characterized by its lateral diameter, crater depth, and crack depth. The lateral diameter was described in terms of an effective circular diameter calculated by the total measured area, whereas the crater depth was defined as the average maximum vertical extension at the leading edge of the crater. Occasional cracks that occur above the general level of the crater were not considered. When crack growth appears on the perimeter, the depth of the crack is defined as the distance from the surface to the bottom of the deepest crack. Prior to the tests, an ideal, clean, nonpolluted area on the output surface of the fused silica sample was selected to avoid the effects of impurities, defects, and other pollutants. The test area was irradiated in single-laser shots (starting from low energy and then gradually increasing) until the initial damage occurred. The values of the initial damage threshold F 0 , lateral equivalent diameter D0, vertical depth of the crater H 0, and vertical depth of the cracks L0 at the center of the broken spot were all recorded. In the growth tests, the initial damage sites were repeatedly irradiated. The growth threshold F 1 , lateral effective diameter Dn, vertical depth of the crater H n, vertical depth of the cracks Ln , and the laser shot number n were recorded. A widely used exponential model was adopted to characterize the growth trends, where α is the growth coefficient. However, the cracks sometimes tend to increase linearly. Thus, a linear model was also used to describe the growth trends of the cracks, where β is the linear growth coefficient: Dn D0 eαN n ;
(1)
H n H 0 eαN n ;
(2)
Ln L0 βN n :
(3)
Fig. 1. Schematic diagram of the test system for laser-induced damage threshold test system. 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
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3. Analysis and Discussion A.
Initial Damage Characteristics
The laser-induced damage morphologies on the exit surface of fused silica have small scattered pits (5 to 30 μm) with partially adhesive qualities, which are related to the polishing powders that are residues in substrate processing and are strongly absorptive at the 355 nm laser. The different structures and sizes of initial damage morphologies may affect the subsequent growth features. Thus, determining the relationship between the initial damage properties and the initiating fluences is necessary. Five different fluences were selected to produce the initial damage sites. Higher initiating fluences starting at 60% damage probability from the one-on-one tests, from 19 to 57 J∕cm2 with several increments, were chosen to obtain damage sites with similar size and morphology. The horizontal ordinates in Fig. 2 represent the initial laser fluences, whereas the vertical coordinates represent the average lateral equivalent diameters and the crater depths of the initial damage sites irradiated at different laser fluences. In Figs. 2(a) and 2(b), the initial lateral sizes show obvious linear relationships with the fluences, and the straight line formulas of the data fit are y0D −89.6 8.49x0D and y45D −68.05 10.86x45D . Higher initial laser fluences result in larger initial damage sites. Moreover, the average sizes at an incidence angle of 45° are ∼1.6× larger than those at 0°, which is close to the angle reflection. In Figs. 2(c) and 2(d), the fitting lines between the average initial crater depths and fluences are y0H 14.34 0.166x0H and y45H 20.47 0.074x45H . Although the initiating fluences were much different, the values of the initial crater depths were close to one another (10 to 30 μm). No significant angle effect and no obvious cracks were observed. The ratios of the lateral diameters and vertical depths of the craters showed the structure characteristics of the damage. The horizontal ordinates in Fig. 3 represent the initial laser fluences, whereas the vertical coordinates represent the ratios of the average lateral sizes and the crater depths of the initial damage sites irradiated at different laser fluences. In Fig. 3(a), the average ratios at an incidence angle of 0° are 4.23, 6.89, 12.29, 14.77, and 16.48 with increasing fluences, whereas the values are about 1.4× higher at an incidence angle of 45°. Higher ratios clearly contribute to the larger damage area, but the crater depths were similar at an incidence angle of 45°. Furthermore, no obvious difference was observed between the results obtained at P and S polarization. In the subsequent growth process, the initial damage sites continuously grow into the depth direction. However, given the limitation of the fixed spot diameter, the growth rates of lateral sizes were smaller and eventually reached the maximum, whereas the ratios between the lateral sizes and crater depths gradually decreased. A98
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Fig. 2. Characteristics of the initial damage sites. The relationships between the lateral size, crater depth, and laser fluences at incidence angles of 0° and 45° are fitted using a linear formula. Twelve data points and the average values of the laser fluences are shown. (a) Lateral sizes at 0°. (b) Lateral sizes at 45°. (c) Crater depths at 0°. (d) Crater depths at 45°.
Fig. 4. Growth thresholds of the initial damage sites generated by different incidence angles and polarization states. The decreasing trends can be fitted in linear formulas, y0 10.7 − 0.054x0 , y45P 9.97 − 0.071x45P , and y45S 7.6.
structural damage. Moreover, a larger damage area had a greater number of individual pits, which can be described by a probability of growth. B. Features of the Craters and Cracks
Fig. 3. Lateral size/crater depth ratios at different conditions, which are described by linear fits, y0 −3.37 0.389x0 , y45P −1.83 0.444x45P , and y45S −0.90 0.365x45S . (a) 0° versus 45°and P polarization. (b) P versus S polarization at 45°.
Additionally, growth tests were conducted (from 3.8 J∕cm2 fluence with ∼0.5 J∕cm2 increment) to investigate the stabilization of the initial damage sites. Any visible changes in the lateral size and depth were predicted to occur during 50 pulses of each fluence. Otherwise, the fluence was increased until growth was observed. If the initial damage site is stable at subsequent laser shots, the performance of the optics is typically considered acceptable for practical applications. If the laser-initiated site is not stable and increases in size with increasing number of laser shots, the performance of the optic fails. The relationship between the initiating fluence and growth thresholds is given in Fig. 4. The results at different angles and polarization states were compared. The results show the average of 12 sets of measurements. At P polarization, the growth thresholds decreased with increasing initiating fluences, ranging from 9.5 to 7.6 J∕cm2 at an incidence angle of 0° and 8.6 to 6.7 J∕cm2 at an incidence angle of 45° at P polarization. By contrast, the growth threshold is constant (7.6 J∕cm2 ) at an incidence angle of 45° at S polarization. Higher fluence generated more severe
After the occurrence of the initial damages, the initial scattered damage sites (several micrometers) developed into craters (several hundred micrometers) during subsequent laser irradiation. A large number of cracks were also observed in the vertical direction because of the strong thermal mechanical effect. Given the three-dimensional crack structure, the top and side views captured during the experiments are shown in Fig. 5 for a more comprehensive display of the characteristics of the cracks. The crater morphologies have no significant difference at different conditions because the projection area is greater at an incidence angle of 45°, whereas the size in the top view is larger in Fig. 5(b). Additionally, three extending directions of typical cracks generally exist: one crack along the direction of incident laser, and the other two at about an incidence angle of 45°. However, the cracks in the different directions were not completely symmetrical. and large cracks may occur in one direction, thereby resulting in
Fig. 5. Typical top and side views of the damage sites at different incidence angles. The contrast of the images in the vertical directions shows that the cracks are not symmetrical and slightly change at different observation angles. No evidence of the directional expansion from the crater profiles is found. (a) 0° incidence angle. (b) 45° incidence angle. 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
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inconsistent images obtained from both sides. Moreover, no evidence shows that the plasma generated at an incidence angle of 45° affects the directionality of the craters and cracks. The lateral and vertical damage morphologies obtained at 0° and 45° S state and 45° P state conditions are compared in Fig. 6. The initial damage sites were all produced by the same initial laser fluence (28.5 J∕cm2 ), but the subsequent irradiation fluences and shot numbers were different. The first column pictures were obtained at the damage growth thresholds with multiple shot numbers, whereas the second column pictures were obtained at subsequent higher fluences with fewer shot numbers. The damage morphologies irradiated at 8.1 and 9.5 J∕cm2 and growth threshold of 6.7 J∕cm2 with 50 shots did not produce significant structural cracks, and the craters with smooth boundaries gradually extended to the interior. The ratios of lateral sizes and vertical depths (1.8, 2.3, and 1.9) were similar to one another. In the damage growth process at low-fluence irradiation, significant crack structures did not emerge but generated deeper crater structures even when the laser shot numbers were continuously increased because of the small thermal mechanical effect. Furthermore, the lateral damage
Fig. 6. Typical lateral and vertical views of the damage sites generated at different irradiation conditions. All the damage sites were initiated by 28.5 J∕cm2 but further irradiated at (a) 9.5 J∕cm2 with 50 shots and 28.5 J∕cm2 with 10 shots at an incidence angle of 0°, (b) 8.1 J∕cm2 with 50 shots and 28.5 J∕cm2 with 10 shots at an incidence angle of 45° at P polarization state and (c) 7.6 J∕cm2 with 50 shots and 28.5 J∕cm2 with 10 shots at an incidence angle of 45° at S polarization state. (a-0) 9.5 J∕cm2 with 50 shots at 0° (a-1) 28.5 J∕cm2 with 10 shots at 0°. (b-0) 8.1 J∕cm2 with 50 shots at 45° (b-1) 28.5 J∕cm2 with 10 shots at 45°. (c-0) 7.6 J∕cm2 with 50 shots at 45°-S (c-1) 28.5 J∕cm2 with 10 shots at 45°-S. A100
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morphologies were comparatively symmetrical, and the damages generally occur at the center of the highest fluence at an incidence angle of 0°. However, at an incidence angle of 45°, an obvious included angle between the phase plane and sample surface was observed, and the damage may preferentially occur at the position of the phase plane and continue to expand because of the follow-up energy shots at the same pulse [Figs. 6(b-0), 6(b-1), and 6(c-0)]. The cracks were generated by the 10 follow-up high-fluence shots at 28.5 J∕cm2 . The ratios between the crater equivalent diameters and crater depths were 4.5, 4.3, and 5.6, wherein the crack depths were generally 2× to 3× the crater depths. Additionally, the remaining crack structures were similar to one another, except for an obvious directional crack in (c-1), and the cluster-radial cracks were slightly apparent at 0° and 45° of the flank. C.
Characteristics of Laser-Induced Damage Growth
The growth characteristics of the initial damage sites generated by the same initial and subsequent fluences at different conditions were investigated. Typical results in terms of the lateral size, crater depth, and crack depth were chosen to describe the growth behaviors. The average values of all the results were calculated from three sets of measurements. In Fig. 7, given the initial damage sites generated at 19 J∕cm2 , the differences in the regularity of the lateral and vertical growths that were repeatedly irradiated at the same subsequent fluence of 14.25 J∕cm2 were determined. The horizontal ordinates indicate the shot numbers, whereas the vertical coordinates indicate the average lateral sizes, crater depths, and crack depths. In Fig. 7(a), the lateral sizes of the initial damage sites irradiated at an incidence angle of 0° are significantly less than those at an incidence angle of 45° at S or P state, but the growth coefficients are slightly higher than the latter. By contrast, the polarization states had no effect on the lateral sizes and growth trends. The obtained growth coefficients were 0.114, 0.069, and 0.064. Given the differences in the lateral sizes of the initial damage sites produced at different incidence angles, the results showed that smaller initial damage sites caused higher fluences at the edges of the damage sites, thereby resulting in smaller damage sites instead of growing rapidly. Similarly, larger initial damage sites resulted in smaller lateral growth coefficients irradiated at subsequent constant laser pulses. In Fig. 7(b), the crater depths of the initial damage sites irradiated at an incidence angle of 0° are also significantly less than those at an incidence angle of 45° at S or P state, but the growth coefficients are similar to each other. The polarization states also had no effect on the crater depths and growth trends. The growth coefficients were 0.124, 0.114, and 0.123. With subsequent constant laser irradiation, larger initial damage sites contained a larger area of material modification and localized damages, which
be fitted using linear equations, and the linear growth coefficients are 27.58, 41.09, and 31.71. The growth trends of crack depths at P or S state were not completely consistent, which was probably due to the relative complexity of the crack growth process. The growth trends were also closely related to the damage growth, and uncertainty is inevitable. D.
Fig. 7. Comparison of the growth trends of lateral sizes, crater depths, and cracks depths. The average values were obtained from three sets of results. The initial damage sites were initiated by 19 J∕cm2 and grown at 14.25 J∕cm2 . (a) Growth trends comparison of lateral sizes. (b) Growth trends comparison of crater depths. (c) Growth trends comparison of crack depths.
lead to greater laser absorption and scattering. Thus, more severe damages are more likely to occur in the depth direction. Given that the fitting curves of P and S states were deviant at low fluences, the growth rates were decreased to a particular extent. Furthermore, the growth characteristics of the crack depths at different conditions were investigated. As shown in Fig. 7(c), the growth trends can
Discussion
The morphologies of the damage sites generated by the 355 nm laser beam were characterized using smaller but relatively uniform pits with shallower depth. The morphology of the pits was mainly due to the significantly higher absorption coefficient (at 355 nm) of the impurities and defects introduced during the pregrinding and polishing processes. During laser irradiation, the localized defects absorb more laser energy and suddenly cause a local increase in temperature, which thereby produces significant temperature and stress differences. Thus, thermal melting damage and stress rupture occur. After the appearance of the damage sites, the discontinuous interface of the damaged region as well as the material modification promote damage growth, which results in plasma re-ignition and causes further expansion in the lateral and vertical directions. By comparison, the differences in the damage morphologies and growth characteristics at different incidence angles were mainly due to the projection areas of the incident laser. However, the growth characteristics were less affected by the polarization states. When significant structural damages were generated, material modifications, nonsmooth interfaces, and stress areas occurred. The absorbency of the localized area then increases remarkably and the structure stability is greatly reduced. Particularly, the localized damage is not sensitive to the incidence angles and polarization states. Therefore, similar severe damage was generated by the subsequent laser shots at any incidence angle and polarization state, along with relatively great thermal mechanical effects. This phenomenon caused the emergence and rapid extension of the cracks to the vertical direction. When the laser is constantly irradiated at an incidence angle of 45°, the craters or cracks reach a specific depth, and the direction of crack extension is consistent with the laser incidence angle. 4. Conclusions
The effects of the incidence angle and polarization state on the characteristics of damage sites in fused silica at 355 nm small beam laser irradiation were investigated. Preliminary statistical results showed an increasing trend of the lateral size and crater depth with the increase in initiating fluences at different incidence angles and polarizations. The ratios of the crater diameters and depths depend on the projection area of the laser spot on the sample surface, and the average growth thresholds of the initial damage sites slowly decreased. The morphologies of 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
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the craters and cracks caused the same laser fluence and shot numbers but were similar at different incidence angles and polarization states. The typical cracks generally had three directions of extension— one extended along the direction of the incident laser, and the other two were at about the incidence angle 45°—whereas the crack structures were not completely symmetrical. Additionally, the growth coefficients of the lateral sizes and crater depths irradiated at an incidence angle of 0° were slightly higher, but the growth coefficients were less than those at an incidence angle of 45° at the P or S states. However, no obvious differences in the damage morphologies and growth characteristics between the P and S states were observed. This work is sponsored by the National Natural Science Foundation of China (Grant Nos. 61205124, 61235011, 61008030, 61108014, and 11105099). References 1. P. E. Miller, T. I. Suratwala, J. D. Bude, T. A. Laurence, N. Shen, W. A. Steele, M. D. Feit, J. A. Menapace, and L. L. Wong, “Laser damage precursors in fused silica,” Proc. SPIE 7504, 75040X (2009). 2. X. B. Cheng, J. L. Zhang, T. Ding, Z. Y. Wei, H. Q. Li, and Z. S. Wang, “The effect of an electric field on the thermomechanical damage of nodular defects in dielectric multilayer coatings irradiated by nanosecond laser pulses,” Light Sci. Appl. 2, e80 (2013). 3. J. Neauport, L. Lamaignere, H. Bercegol, F. Pilon, and J.-C. Birolleau, “Polishing-induced contamination of fused silica optics and laser induced damage density at 351 nm,” Opt. Express 13, 10163–10171 (2005). 4. M. A. Norton, J. J. Adams, C. W. Carr, E. E. Donohue, M. D. Feit, R. P. Hackel, W. G. Hollingsworth, J. A. Jarboe, M. J. Matthews, A. M. Rubenchik, and M. L. Spaeth, “Growth of laser damage in fused silica: diameter to depth ratio,” Proc. SPIE 6720, 67200H (2007). 5. S. G. Demos and M. Staggs, “Characterization of laser induced damage sites in optical components,” Opt. Express 10, 1444– 1450 (2002). 6. M. A. Norton, L. W. Hrubesh, Z. Wu, E. E. Donohue, M. D. Feit, M. R. Kozlowski, D. Milam, K. P. Neeb, W. A. Molander, A. M. Rubenchik, W. D. Sell, and P. Wegner, “Growth of laser initiated damage in fused silica at 351 nm,” Proc. SPIE 4347, 468–473 (2001).
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7. M. A. Norton, E. E. Donohue, W. G. Hollingsworth, J. N. McElroy, and R. P. Hackel, “Growth of laser initiated damage in fused silica at 527 nm,” Proc. SPIE 5273, 236–243 (2004). 8. C. W. Carr, D. A. Cross, M. A. Norton, and R. A. Negres, “The effect of laser pulse shape and duration on the size at which damage sites initiate and the implications to subsequent repair,” Opt. Express 19, A859–A864 (2011). 9. R. Courchinoux, G. Razé, C. Sudre, M. Josse, A. Boscheron, C. Lepage, E. Mazataud, E. Bordenave, L. Lamaignère, M. Loiseau, T. Donval, and H. Bercegol, “Laser-induced damage growth with small and large beams: Comparison between laboratory experiments and large-scale laser data,” Proc. SPIE 5273, 99–106 (2004). 10. R. A. Negres, M. A. Norton, D. A. Cross, and C. W. Carr, “Growth behavior of laser-induced damage on fused silica optics under UV, ns laser irradiation,” Opt. Express 18, 19966–19976 (2010). 11. R. N. Raman, R. A. Negres, and S. G. Demos, “Kinetics of ejected particles during breakdown in fused silica by nanosecond laser pulses,” Appl. Phys. Lett. 98, 051901 (2011). 12. C. W. Carr, H. B. Radousky, A. M. Rubenchik, M. D. Feit, and S. G. Demos, “Localized dynamics during laser induced damage in optical materials,” Phys. Rev. Lett. 92, 087401 (2004). 13. S. G. Demos, M. R. Kozlowski, M. Staggs, L. L. Chase, A. Bumham, and H. B. Radousky, “Mechanisms to explain damage growth in optical materials,” Proc. SPIE 4347, 277–284 (2001). 14. G. Razé, J. M. Morchain, M. Loiseau, L. Lamaignère, M. Josse, and H. Bercegol, “Parametric study of the growth of damage sites on the rear surface of fused silica windows,” Proc. SPIE 4932, 127–135 (2003). 15. M. Loiseau, L. Lamaignère, R. Courchinoux, G. Razé, C. Sudre, M. Josse, T. Donval, and H. Bercegol, “Automatic damage test benches: from samples to large-aperture optical components,” Proc. SPIE 5252, 412–422 (2004). 16. S. Papernov and A. W. Schmid, “Testing asymmetry in plasma-ball growth seeded by a nanoscale absorbing defect embedded in a SiO2 thin-film matrix subjected to UV pulsed-laser radiation,” J. Appl. Phys. 104, 063101 (2008). 17. S. Papernov and A. W. Schmid, “Laser-induced surface damage of optical materials: absorption sources, initiation, growth, and mitigation,” Proc. SPIE 7132, 71321J (2008). 18. M. A. Norton, E. E. Donohue, M. D. Feit, R. P. Hackel, W. G. Hollingsworth, A. M. Rubenchik, and M. L. Spaeth, “Growth of laser damage on the input surface of SiO2 at 351 nm,” Proc. SPIE 6403, 64030L (2007). 19. A. Salleo, R. Chinsio, and F. Y. Génin, “Crack propagation in fused silica during UV and IR ns-laser illumination,” Proc. SPIE 3578, 456–471 (1999).
Institute of Precision Optical Engineering, Tongji University, Shanghai 200092, China
2
Key Laboratory of Advanced Micro-structure Materials, Ministry of Education, Shanghai 200092, China *Corresponding author: [email protected] Received 29 August 2013; accepted 21 October 2013; posted 25 October 2013 (Doc. ID 196594); published 19 November 2013
The influence of the incidence angle and polarization state on the damage site characteristics of fused silica under 355 nm laser irradiation was investigated. The initial damage morphologies and growth behaviors of the damage sites on the exit surface at incidence angles of 0° and 45° as well as in P and S states were compared to investigate the effects of various angles and polarization states. The relationships between the size of the initial damage sites and the laser fluence, as well as the growth threshold, were discussed. The damage morphologies of the craters and cracks at different incidence angles and polarization states were then investigated. Finally, the growth characteristics of the lateral size, crater depth, and crack depth were compared and analyzed. © 2013 Optical Society of America OCIS codes: (140.3330) Laser damage; (160.4670) Optical materials. http://dx.doi.org/10.1364/AO.53.000A96
1. Introduction
Laser damage events generally comprise damage initiation and growth. The discontinuous interface in the initial damage sites, which range from submicrometers to several micrometers, as well as material modification promote damage growth with subsequent laser irradiation, thereby resulting in further material modification and expansion of the lateral or vertical size to several hundred micrometers or even larger [1–5]. Previous studies have shown that the growth behavior is similar to the damage initiation process, in which both are strongly influenced by a large number of parameters, such as laser wavelength, pulse duration, beam shape, and fluence, as well as the size and morphology of the initial damage sites [6–9]. Moreover, the initiation of avalanche ionization caused by the defects at the damaged and modified material and the presence 1559-128X/14/040A96-07$15.00/0 © 2014 Optical Society of America A96
APPLIED OPTICS / Vol. 53, No. 4 / 1 February 2014
of field intensification caused by cracks promote plasma formation, melting effect, stress release, and material modifications. These phenomena are proposed to demonstrate the growth mechanisms [10]. However, given the complexity of damage evolution [11,12], single-parameter and comparative studies are preferred in elucidating the growth mechanisms and establishing the growth laws [10,13]. With the increasing demands for optical transmission components with nonnormal incidence, understanding the damage growth at various incident laser irradiations is important. Previous experiments have shown the effects of incidence angle and polarizations on the growth damage sites at the output surface of fused silica [14,15]. These studies provide preliminary lateral growth trends that are dependent on the angle of incidence. However, details of the growth behaviors still need further investigation. The lateral growth of damage on the exit surface is best described by an exponential function of the laser shot number, whereas a linear dependence is found on the input surface, which can be
qualitatively explained by the directional expansion of plasma during the formation of craters and cracks [16–18]. However, evaluation of the relationship between plasma propagation and crater shape and crack morphology is important in elucidating the growth mechanism when the laser irradiates at a specified angle and before the emergence of drilling caused by the small beam [19]. Additionally, the difference between the near-Gaussian distribution of the small beam and the flat-top shape of the large beam results in difficulties in comparing the growth rules obtained at different conditions [9,15]. However, the offline small laser beam allows direct comparison at specified conditions, which is advantageous for monitoring damage growth evolutions and for showing the details of the damage growth events. In this paper, we focused on the effects of the incidence angle and polarization state on the characteristics of damage sites in fused silica. The damage growth behaviors of different initial damage sites initiated by 355 nm laser irradiation at various incidence angles and polarizations are preliminarily investigated and directly compared. Moreover, the crack morphology and damage growth trends characterized by lateral size, vertical crater depth, and crack depth are discussed in detail. 2. Experimental Procedures
Figure 1 shows the laser-induced damage threshold test platform. The Nd:YAG laser that radiates a 355 nm beam with 8 ns pulse was operated at 10 Hz. The sample was placed in a computerized three-dimensional motor-driven stage. The damage information of the testing sites was monitored online by using two optical microscopes. During the tests, the beam exhibited a near-Gaussian profile with a 1∕e2 diameter of 480 μm at the sample plane. The fused silica samples (50 mm × 50 mm × 10 mm) were super polished. Laser damage events are usually found on the exit surface; therefore, the laserinduced damage threshold of the samples was obtained prior to the experiments by using oneon-one tests. Higher initiating fluences were then employed to produce repeatable damage sites with various sizes and morphologies.
Two cameras were placed at the rear and side of the sample to record the lateral and vertical images. The damage site was characterized by its lateral diameter, crater depth, and crack depth. The lateral diameter was described in terms of an effective circular diameter calculated by the total measured area, whereas the crater depth was defined as the average maximum vertical extension at the leading edge of the crater. Occasional cracks that occur above the general level of the crater were not considered. When crack growth appears on the perimeter, the depth of the crack is defined as the distance from the surface to the bottom of the deepest crack. Prior to the tests, an ideal, clean, nonpolluted area on the output surface of the fused silica sample was selected to avoid the effects of impurities, defects, and other pollutants. The test area was irradiated in single-laser shots (starting from low energy and then gradually increasing) until the initial damage occurred. The values of the initial damage threshold F 0 , lateral equivalent diameter D0, vertical depth of the crater H 0, and vertical depth of the cracks L0 at the center of the broken spot were all recorded. In the growth tests, the initial damage sites were repeatedly irradiated. The growth threshold F 1 , lateral effective diameter Dn, vertical depth of the crater H n, vertical depth of the cracks Ln , and the laser shot number n were recorded. A widely used exponential model was adopted to characterize the growth trends, where α is the growth coefficient. However, the cracks sometimes tend to increase linearly. Thus, a linear model was also used to describe the growth trends of the cracks, where β is the linear growth coefficient: Dn D0 eαN n ;
(1)
H n H 0 eαN n ;
(2)
Ln L0 βN n :
(3)
Fig. 1. Schematic diagram of the test system for laser-induced damage threshold test system. 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
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3. Analysis and Discussion A.
Initial Damage Characteristics
The laser-induced damage morphologies on the exit surface of fused silica have small scattered pits (5 to 30 μm) with partially adhesive qualities, which are related to the polishing powders that are residues in substrate processing and are strongly absorptive at the 355 nm laser. The different structures and sizes of initial damage morphologies may affect the subsequent growth features. Thus, determining the relationship between the initial damage properties and the initiating fluences is necessary. Five different fluences were selected to produce the initial damage sites. Higher initiating fluences starting at 60% damage probability from the one-on-one tests, from 19 to 57 J∕cm2 with several increments, were chosen to obtain damage sites with similar size and morphology. The horizontal ordinates in Fig. 2 represent the initial laser fluences, whereas the vertical coordinates represent the average lateral equivalent diameters and the crater depths of the initial damage sites irradiated at different laser fluences. In Figs. 2(a) and 2(b), the initial lateral sizes show obvious linear relationships with the fluences, and the straight line formulas of the data fit are y0D −89.6 8.49x0D and y45D −68.05 10.86x45D . Higher initial laser fluences result in larger initial damage sites. Moreover, the average sizes at an incidence angle of 45° are ∼1.6× larger than those at 0°, which is close to the angle reflection. In Figs. 2(c) and 2(d), the fitting lines between the average initial crater depths and fluences are y0H 14.34 0.166x0H and y45H 20.47 0.074x45H . Although the initiating fluences were much different, the values of the initial crater depths were close to one another (10 to 30 μm). No significant angle effect and no obvious cracks were observed. The ratios of the lateral diameters and vertical depths of the craters showed the structure characteristics of the damage. The horizontal ordinates in Fig. 3 represent the initial laser fluences, whereas the vertical coordinates represent the ratios of the average lateral sizes and the crater depths of the initial damage sites irradiated at different laser fluences. In Fig. 3(a), the average ratios at an incidence angle of 0° are 4.23, 6.89, 12.29, 14.77, and 16.48 with increasing fluences, whereas the values are about 1.4× higher at an incidence angle of 45°. Higher ratios clearly contribute to the larger damage area, but the crater depths were similar at an incidence angle of 45°. Furthermore, no obvious difference was observed between the results obtained at P and S polarization. In the subsequent growth process, the initial damage sites continuously grow into the depth direction. However, given the limitation of the fixed spot diameter, the growth rates of lateral sizes were smaller and eventually reached the maximum, whereas the ratios between the lateral sizes and crater depths gradually decreased. A98
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Fig. 2. Characteristics of the initial damage sites. The relationships between the lateral size, crater depth, and laser fluences at incidence angles of 0° and 45° are fitted using a linear formula. Twelve data points and the average values of the laser fluences are shown. (a) Lateral sizes at 0°. (b) Lateral sizes at 45°. (c) Crater depths at 0°. (d) Crater depths at 45°.
Fig. 4. Growth thresholds of the initial damage sites generated by different incidence angles and polarization states. The decreasing trends can be fitted in linear formulas, y0 10.7 − 0.054x0 , y45P 9.97 − 0.071x45P , and y45S 7.6.
structural damage. Moreover, a larger damage area had a greater number of individual pits, which can be described by a probability of growth. B. Features of the Craters and Cracks
Fig. 3. Lateral size/crater depth ratios at different conditions, which are described by linear fits, y0 −3.37 0.389x0 , y45P −1.83 0.444x45P , and y45S −0.90 0.365x45S . (a) 0° versus 45°and P polarization. (b) P versus S polarization at 45°.
Additionally, growth tests were conducted (from 3.8 J∕cm2 fluence with ∼0.5 J∕cm2 increment) to investigate the stabilization of the initial damage sites. Any visible changes in the lateral size and depth were predicted to occur during 50 pulses of each fluence. Otherwise, the fluence was increased until growth was observed. If the initial damage site is stable at subsequent laser shots, the performance of the optics is typically considered acceptable for practical applications. If the laser-initiated site is not stable and increases in size with increasing number of laser shots, the performance of the optic fails. The relationship between the initiating fluence and growth thresholds is given in Fig. 4. The results at different angles and polarization states were compared. The results show the average of 12 sets of measurements. At P polarization, the growth thresholds decreased with increasing initiating fluences, ranging from 9.5 to 7.6 J∕cm2 at an incidence angle of 0° and 8.6 to 6.7 J∕cm2 at an incidence angle of 45° at P polarization. By contrast, the growth threshold is constant (7.6 J∕cm2 ) at an incidence angle of 45° at S polarization. Higher fluence generated more severe
After the occurrence of the initial damages, the initial scattered damage sites (several micrometers) developed into craters (several hundred micrometers) during subsequent laser irradiation. A large number of cracks were also observed in the vertical direction because of the strong thermal mechanical effect. Given the three-dimensional crack structure, the top and side views captured during the experiments are shown in Fig. 5 for a more comprehensive display of the characteristics of the cracks. The crater morphologies have no significant difference at different conditions because the projection area is greater at an incidence angle of 45°, whereas the size in the top view is larger in Fig. 5(b). Additionally, three extending directions of typical cracks generally exist: one crack along the direction of incident laser, and the other two at about an incidence angle of 45°. However, the cracks in the different directions were not completely symmetrical. and large cracks may occur in one direction, thereby resulting in
Fig. 5. Typical top and side views of the damage sites at different incidence angles. The contrast of the images in the vertical directions shows that the cracks are not symmetrical and slightly change at different observation angles. No evidence of the directional expansion from the crater profiles is found. (a) 0° incidence angle. (b) 45° incidence angle. 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
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inconsistent images obtained from both sides. Moreover, no evidence shows that the plasma generated at an incidence angle of 45° affects the directionality of the craters and cracks. The lateral and vertical damage morphologies obtained at 0° and 45° S state and 45° P state conditions are compared in Fig. 6. The initial damage sites were all produced by the same initial laser fluence (28.5 J∕cm2 ), but the subsequent irradiation fluences and shot numbers were different. The first column pictures were obtained at the damage growth thresholds with multiple shot numbers, whereas the second column pictures were obtained at subsequent higher fluences with fewer shot numbers. The damage morphologies irradiated at 8.1 and 9.5 J∕cm2 and growth threshold of 6.7 J∕cm2 with 50 shots did not produce significant structural cracks, and the craters with smooth boundaries gradually extended to the interior. The ratios of lateral sizes and vertical depths (1.8, 2.3, and 1.9) were similar to one another. In the damage growth process at low-fluence irradiation, significant crack structures did not emerge but generated deeper crater structures even when the laser shot numbers were continuously increased because of the small thermal mechanical effect. Furthermore, the lateral damage
Fig. 6. Typical lateral and vertical views of the damage sites generated at different irradiation conditions. All the damage sites were initiated by 28.5 J∕cm2 but further irradiated at (a) 9.5 J∕cm2 with 50 shots and 28.5 J∕cm2 with 10 shots at an incidence angle of 0°, (b) 8.1 J∕cm2 with 50 shots and 28.5 J∕cm2 with 10 shots at an incidence angle of 45° at P polarization state and (c) 7.6 J∕cm2 with 50 shots and 28.5 J∕cm2 with 10 shots at an incidence angle of 45° at S polarization state. (a-0) 9.5 J∕cm2 with 50 shots at 0° (a-1) 28.5 J∕cm2 with 10 shots at 0°. (b-0) 8.1 J∕cm2 with 50 shots at 45° (b-1) 28.5 J∕cm2 with 10 shots at 45°. (c-0) 7.6 J∕cm2 with 50 shots at 45°-S (c-1) 28.5 J∕cm2 with 10 shots at 45°-S. A100
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morphologies were comparatively symmetrical, and the damages generally occur at the center of the highest fluence at an incidence angle of 0°. However, at an incidence angle of 45°, an obvious included angle between the phase plane and sample surface was observed, and the damage may preferentially occur at the position of the phase plane and continue to expand because of the follow-up energy shots at the same pulse [Figs. 6(b-0), 6(b-1), and 6(c-0)]. The cracks were generated by the 10 follow-up high-fluence shots at 28.5 J∕cm2 . The ratios between the crater equivalent diameters and crater depths were 4.5, 4.3, and 5.6, wherein the crack depths were generally 2× to 3× the crater depths. Additionally, the remaining crack structures were similar to one another, except for an obvious directional crack in (c-1), and the cluster-radial cracks were slightly apparent at 0° and 45° of the flank. C.
Characteristics of Laser-Induced Damage Growth
The growth characteristics of the initial damage sites generated by the same initial and subsequent fluences at different conditions were investigated. Typical results in terms of the lateral size, crater depth, and crack depth were chosen to describe the growth behaviors. The average values of all the results were calculated from three sets of measurements. In Fig. 7, given the initial damage sites generated at 19 J∕cm2 , the differences in the regularity of the lateral and vertical growths that were repeatedly irradiated at the same subsequent fluence of 14.25 J∕cm2 were determined. The horizontal ordinates indicate the shot numbers, whereas the vertical coordinates indicate the average lateral sizes, crater depths, and crack depths. In Fig. 7(a), the lateral sizes of the initial damage sites irradiated at an incidence angle of 0° are significantly less than those at an incidence angle of 45° at S or P state, but the growth coefficients are slightly higher than the latter. By contrast, the polarization states had no effect on the lateral sizes and growth trends. The obtained growth coefficients were 0.114, 0.069, and 0.064. Given the differences in the lateral sizes of the initial damage sites produced at different incidence angles, the results showed that smaller initial damage sites caused higher fluences at the edges of the damage sites, thereby resulting in smaller damage sites instead of growing rapidly. Similarly, larger initial damage sites resulted in smaller lateral growth coefficients irradiated at subsequent constant laser pulses. In Fig. 7(b), the crater depths of the initial damage sites irradiated at an incidence angle of 0° are also significantly less than those at an incidence angle of 45° at S or P state, but the growth coefficients are similar to each other. The polarization states also had no effect on the crater depths and growth trends. The growth coefficients were 0.124, 0.114, and 0.123. With subsequent constant laser irradiation, larger initial damage sites contained a larger area of material modification and localized damages, which
be fitted using linear equations, and the linear growth coefficients are 27.58, 41.09, and 31.71. The growth trends of crack depths at P or S state were not completely consistent, which was probably due to the relative complexity of the crack growth process. The growth trends were also closely related to the damage growth, and uncertainty is inevitable. D.
Fig. 7. Comparison of the growth trends of lateral sizes, crater depths, and cracks depths. The average values were obtained from three sets of results. The initial damage sites were initiated by 19 J∕cm2 and grown at 14.25 J∕cm2 . (a) Growth trends comparison of lateral sizes. (b) Growth trends comparison of crater depths. (c) Growth trends comparison of crack depths.
lead to greater laser absorption and scattering. Thus, more severe damages are more likely to occur in the depth direction. Given that the fitting curves of P and S states were deviant at low fluences, the growth rates were decreased to a particular extent. Furthermore, the growth characteristics of the crack depths at different conditions were investigated. As shown in Fig. 7(c), the growth trends can
Discussion
The morphologies of the damage sites generated by the 355 nm laser beam were characterized using smaller but relatively uniform pits with shallower depth. The morphology of the pits was mainly due to the significantly higher absorption coefficient (at 355 nm) of the impurities and defects introduced during the pregrinding and polishing processes. During laser irradiation, the localized defects absorb more laser energy and suddenly cause a local increase in temperature, which thereby produces significant temperature and stress differences. Thus, thermal melting damage and stress rupture occur. After the appearance of the damage sites, the discontinuous interface of the damaged region as well as the material modification promote damage growth, which results in plasma re-ignition and causes further expansion in the lateral and vertical directions. By comparison, the differences in the damage morphologies and growth characteristics at different incidence angles were mainly due to the projection areas of the incident laser. However, the growth characteristics were less affected by the polarization states. When significant structural damages were generated, material modifications, nonsmooth interfaces, and stress areas occurred. The absorbency of the localized area then increases remarkably and the structure stability is greatly reduced. Particularly, the localized damage is not sensitive to the incidence angles and polarization states. Therefore, similar severe damage was generated by the subsequent laser shots at any incidence angle and polarization state, along with relatively great thermal mechanical effects. This phenomenon caused the emergence and rapid extension of the cracks to the vertical direction. When the laser is constantly irradiated at an incidence angle of 45°, the craters or cracks reach a specific depth, and the direction of crack extension is consistent with the laser incidence angle. 4. Conclusions
The effects of the incidence angle and polarization state on the characteristics of damage sites in fused silica at 355 nm small beam laser irradiation were investigated. Preliminary statistical results showed an increasing trend of the lateral size and crater depth with the increase in initiating fluences at different incidence angles and polarizations. The ratios of the crater diameters and depths depend on the projection area of the laser spot on the sample surface, and the average growth thresholds of the initial damage sites slowly decreased. The morphologies of 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
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the craters and cracks caused the same laser fluence and shot numbers but were similar at different incidence angles and polarization states. The typical cracks generally had three directions of extension— one extended along the direction of the incident laser, and the other two were at about the incidence angle 45°—whereas the crack structures were not completely symmetrical. Additionally, the growth coefficients of the lateral sizes and crater depths irradiated at an incidence angle of 0° were slightly higher, but the growth coefficients were less than those at an incidence angle of 45° at the P or S states. However, no obvious differences in the damage morphologies and growth characteristics between the P and S states were observed. This work is sponsored by the National Natural Science Foundation of China (Grant Nos. 61205124, 61235011, 61008030, 61108014, and 11105099). References 1. P. E. Miller, T. I. Suratwala, J. D. Bude, T. A. Laurence, N. Shen, W. A. Steele, M. D. Feit, J. A. Menapace, and L. L. Wong, “Laser damage precursors in fused silica,” Proc. SPIE 7504, 75040X (2009). 2. X. B. Cheng, J. L. Zhang, T. Ding, Z. Y. Wei, H. Q. Li, and Z. S. Wang, “The effect of an electric field on the thermomechanical damage of nodular defects in dielectric multilayer coatings irradiated by nanosecond laser pulses,” Light Sci. Appl. 2, e80 (2013). 3. J. Neauport, L. Lamaignere, H. Bercegol, F. Pilon, and J.-C. Birolleau, “Polishing-induced contamination of fused silica optics and laser induced damage density at 351 nm,” Opt. Express 13, 10163–10171 (2005). 4. M. A. Norton, J. J. Adams, C. W. Carr, E. E. Donohue, M. D. Feit, R. P. Hackel, W. G. Hollingsworth, J. A. Jarboe, M. J. Matthews, A. M. Rubenchik, and M. L. Spaeth, “Growth of laser damage in fused silica: diameter to depth ratio,” Proc. SPIE 6720, 67200H (2007). 5. S. G. Demos and M. Staggs, “Characterization of laser induced damage sites in optical components,” Opt. Express 10, 1444– 1450 (2002). 6. M. A. Norton, L. W. Hrubesh, Z. Wu, E. E. Donohue, M. D. Feit, M. R. Kozlowski, D. Milam, K. P. Neeb, W. A. Molander, A. M. Rubenchik, W. D. Sell, and P. Wegner, “Growth of laser initiated damage in fused silica at 351 nm,” Proc. SPIE 4347, 468–473 (2001).
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