Method of mitigation laser-damage growth on fused silica surface Zhou Fang,1,2 Yuan’an Zhao,1,* Shunli Chen,1,2 Wei Sun,1,2 and Jianda Shao1 1
Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China 2
University of Chinese Academy of Sciences, Beijing 100049, China *Corresponding author: [email protected]
Received 17 June 2013; revised 17 September 2013; accepted 20 September 2013; posted 23 September 2013 (Doc. ID 192443); published 10 October 2013
A reliable method, combining femtosecond (fs) laser mitigation and chemical (HF) etching, has been developed to mitigate laser-damage growth sites on a fused silica surface. A rectangular mitigation site was fabricated by an fs laser with a raster scan procedure; HF etching was then used to remove the redeposition material. The results show that the mitigation site exhibits good physical qualities with a smooth bottom and edge. The damage test results show that the growth threshold of the mitigation sites increases. Furthermore, the structural characteristic of samples was measured by a photoluminescence (PL) spectrometer, and the light intensification caused by damage and mitigation sites was numerically modeled by the finite-difference time-domain (FDTD). It revealed that the removal of damaged material and structure optimization contribute to the increase of the damage growth threshold of the mitigation site. © 2013 Optical Society of America OCIS codes: (140.3330) Laser damage; (140.3380) Laser materials; (140.7090) Ultrafast lasers. http://dx.doi.org/10.1364/AO.52.007186
1. Introduction
Fused silica is widely used in an inertial confinement fusion (ICF) laser system that often operates at the fluence near the laser-induced damage threshold (LIDT) of optics. The surface of fused silica is easily damaged under ultraviolet (351 nm) high-fluence laser irradiation, and the damage size grows exponentially with subsequent laser pulse exposures [1–3]. The growth of the damage sites, which is the key factor in determining the lifetime of the optics, increases the output power loss and enhances the beam obscuration. The method of CO2 laser mitigation is commonly used to resolve the problem of damage growth [4–12]. But the downstream intensification [5], generation of redeposition material [6], and thermal stress [7,10] are accompanied by the CO2 laser mitigation 1559-128X/13/297186-08$15.00/0 © 2013 Optical Society of America 7186
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process, which greatly limits the development of laser mitigation technology. The femtosecond (fs) laser exhibits extreme high peak power and very short pulse width. The characteristics of fs laser pulse interaction with material include basically negligible thermal damage, lower threshold fluence of ablation, higher precision of laser processing, and reduced redeposition material, which results from the extreme intensities and short time scale of fs pulse compared to the time scale of photon electron lattice interactions [13–16]. Therefore, fs laser micromachining may become a possible method of mitigation damage growth based on these unique performances. Recently, Wolfe and co-workers [17,18] successfully used the method of fs laser machining to create mitigation sites in multilayer dielectric mirror coatings. In our previous work [19], an fs laser was also used to mitigate laser damage growth sites on a fused silica surface; the effect of different fs laser machining energies and geometries of mitigation
sites were explored, but further research was needed to optimize the structure of mitigation sites and remove the redeposition material. In this work, fs laser micromachining is used to fabricate stable mitigation sites to replace the damage growth sites on a fused silica surface. HF etching is then used to remove the redeposition material. The laser damage growth test of the damage and mitigation sites is executed to compare the effect of mitigation. Furthermore, the possible reasons for an increased growth threshold of mitigation sites are proposed based on the analysis of structure characteristic and electric field distribution model. 2. Experimental Details
The experiment was performed on fused silica samples 10 mm in diameter and 5 mm thick, which exhibits less than 1 nm rms surface roughness after an ultrasonic bath. A.
Nanosecond Laser Damage Test of Fused Silica
Figure 1 shows the nanosecond laser damage test facility. An Nd:YAG laser that operated at 355 nm was used to produce the initial damage site and perform a laser damage growth test. The pulse duration and frequency of the laser were 8.0 ns and 10 Hz, respectively. An energy attenuator, composed of a λ∕2 wave plate and a polarizer, was used to adjust the laser energy. A specified number of laser pulses was selected by synchronizing a mechanical shutter to the output 10 Hz pulse train. The effective area of laser spot on the sample surface was 0.22 mm2, which was measured by a laser beam analyzer. The sample was illuminated by a white light, and the damage on the front and rear surfaces could be observed in situ with two image systems. In our experiment, all the initial damage sites on the fused silica surface were generated by irradiation with one shot at peak fluence of 45.4 J∕cm2. The laser damage growth test procedure was performed as follows. The damage and mitigation sites to be tested were located on the input surface of the sample. The site to be tested was first irradiated with 300 shots at lower laser fluence. If the site didn’t grow, increased fluence of about 2 J∕cm2 with 300 shots was then applied to the site until the phenomenon
Fig. 1. Schematic of nanosecond laser-damage test facility.
of damage growth occurred. And the maximum fluence at which the site didn’t grow was defined as the growth threshold. B. Mitigate Damage Sites on Fused Silica Surface
The fs laser mitigation facility is shown in Fig. 2. A chirped pulse Ti:sapphire fs laser system (Spectra Physics, Spitfire), with a pulse duration of 38 fs and central wavelength λ 800 nm, was used to mitigate damage growth sites. An energy attenuator was used to adjust the laser energy. The pulse energy was measured by an energy meter from a split-off portion of the beam. The lens with a 300 mm focal length focused the laser beam on the surface of sample. And the effective diameter (1∕e2 ) of laser spot on the focal plane was 62 μm, which was measured by a laser beam analyzer. The sample was illuminated by a white light, and the process of mitigation could be observed in situ by an image system. The sample was mounted on a motorized x and y translation sample stage. In this work, the optimal fs laser energy of 100 μJ and a raster scan procedure (Fig. 3) were chosen to mitigate damage sites based on our previous work [19]. This raster scan procedure (beam overlap at about 70%) was realized by a two-dimensional reentrant movement of sample stage, which is controlled by a computer. And the parameters of transverse distance x and vertical distance y of the mitigation site could be set up according to the actual size of the damage site in order to completely mitigate the damage site. After fs laser mitigation of the damage site, some redeposition material was found around the mitigation site. The HF etching procedure was used to remove the redeposition material. The fs laser mitigation samples were first submerged in a polypropylene tank filled with HF-based etchants (1% HF and 15% NH4F solution) for 1 h. Subsequently, the samples were submerged in a deionized water rinse tank with ultrasonic agitation. Finally, the samples were spray rinsed with deionized water and allowed to air dry. C.
Analysis Techniques
The morphologies of damage sites and mitigation sites were characterized by a field emission scanning electron microscope (FE-SEM, Zeiss Auriga S40). A step profiler (DektakXT, Bruker) was used to measure the depth profile of sample surface. The
Fig. 2. Schematic of fs laser mitigation bench. 10 October 2013 / Vol. 52, No. 29 / APPLIED OPTICS
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and fibers that result from a thermal effect after high-energy absorption. The edges of the damage site are surrounded by a circle of fractured structure, which shows the mechanical damage accompanied by the removal of material. It’s also found that the bottom of the damage site is very rough, and the maximum depth reaches to about 8 μm [Fig. 4(d)]. In addition, our previous work [20] proved that the longitudinal and lateral cracks were generated below and around the damage site after laser damage. Fig. 3. Schematic of raster scan procedure.
structural characteristics of samples were measured by photoluminescence (PL) spectrometer (R928, PTI). Finite-difference time-domain (FDTD) simulations were performed to model the local electric field distribution around the damage and mitigation sites. 3. Results A.
Characteristics of Initial Damage Site
Figure 4 shows the morphologies and horizontal depth profile of the initial damage site induced by one laser pulse of 355 nm light with peak fluences 45.4 J∕cm2 . Figure 4(a) illustrates the damage morphology of the whole damage site, which appears to be an ellipsoidal type with 200–300 μm diameter. SEM images at higher magnification focusing on the center and edge of the damage site reveal that the damage site involves two different regions: molten region [Fig. 4(b)] and fractured region [Fig. 4(c)]. The central molten region contains molten nodules
B. Characteristics of Mitigation Site
Figure 5 shows the morphology, structure, and depth profile images of the fs laser mitigation site [Fig. 5(a)] and the mitigation site treated with fs laser mitigation and HF etching [Fig. 5(b)]. Comparing these images [Figs. 5(a1), 5(b1), 5(a2), and 5(b2)], one can notice that the redeposition material around the fs laser mitigation site is completely removed after HF etching, the fs laser mitigation site composed of the flocculent microstructure, whereas the mitigation site exhibits porous microstructure after HF etching. Figure 5(a3) shows the longitudinal cutting morphology of the fs laser mitigation site, which is shown in Fig. 5(a1), obtained by focused ion beam (FIB) technology. It’s found that the crack below the damage site is removed after fs laser mitigation. The vertical crosssectional depth profile of mitigation site [Fig. 5(b1)] is shown in Fig. 5(b3). After fs laser mitigation and HF etching, the mitigation site has basically a smooth bottom, and the maximum depth is nearly 30 μm. It’s concluded that the mitigation site exhibits good physical qualities with smooth geometry.
Fig. 4. SEM images and horizontal depth profile of damage site induced by one shot with peak fluences 45.4 J∕cm2 . (a) The whole damage site. (b) The molten region of the damage site. (c) The fractured region at the edge of the damage site. (d) The horizontal depth profile of the damage site. 7188
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Fig. 5. (a1) SEM image of fs laser mitigation site. (a2) Amplified image. (a3) FIB cross-sectional image below the fs laser mitigation site shown in Fig. 5(a1). The SEM images of mitigation site [(b1) and (b2)] treated with fs laser mitigation and HF etching. (b3) Vertical crosssectional depth profile of mitigation site shown in (b1).
C.
Results of Laser Damage Growth Test
In this study, we measured the damage growth threshold of five damage sites, 10 fs laser mitigation sites, and 10 mitigation sites treated with fs laser and HF etching. The threshold results are shown in Fig. 6. The average damage growth threshold of damage sites is 6.2 J∕cm2, and the threshold of different damage sites changes is very small. After fs laser mitigation, the average growth threshold is 10.4 J∕cm2 , and the maximum threshold increases to 13.8 J∕cm2 . The average and maximum growth threshold of mitigation sites, which is treated with fs laser mitigation and HF etching, are 9.5 J∕cm2 and 12.1 J∕cm2 , respectively. In summary, the average growth threshold of fs laser mitigation sites is significantly increased, and the maximum growth threshold is almost two times more than that of the damage sites. It’s worth noting that the growth thresholds of mitigation sites decrease slightly after HF etching in comparison with that of fs laser mitigation sites, which can be explained by the
redeposition of the HF etching reaction products. The aqueous phase etching reaction product SiF2− 6 has limited solubility in the HF etching solution, which will become a localized optical absorption center and ultimately lead to laser damage of material when exposed to sufficient laser fluence [21]. Figure 7 shows the morphology of a mitigation site after a laser damage growth test. It’s found that the damage growth starts at the edge of the mitigation site. D.
PL Spectra
The normalized PL spectra of the damage site and the fs laser mitigation site are shown in Fig. 8. It’s found that two emission peaks are obtained for a damage site with 351 nm laser excitation. The peak ∼470 nm correlates with the emission from mechanically damage, and the peak at ∼550 nm may be due to Si /O substoichiometry produced by stripping oxygen from silica [22,23]. The peak at ∼550 nm almost disappears after fs laser mitigation. It’s concluded that the Si/O substoichiometry is generated in the 10 October 2013 / Vol. 52, No. 29 / APPLIED OPTICS
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Fig. 6. Results of damage-growth threshold of damage and mitigation sites.
damaged area because of oxygen loss, and the disappearance peak at ∼550 nm indicates that the oxygen content of fs laser mitigation site returns to a normal level, which is in agreement with our previous analysis results of chemical composition [19]. 4. FDTD Model and Results
The rough geometry of the damage site affects the propagation of the laser beam, and results in the enhancement of the local light field. The mitigation site exhibits regular geometry with a relative smooth edge and bottom, which can reduce the impact of light field modulation. In another aspect, the light intensity is proportional to the square of electric field. So the enhancement value of the light field can be obtained by calculation of electric field distribution. The method of FDTD is used to calculate and compare the electric field distribution around the damage and mitigation sites. Figure 9 shows the two-dimensional model of the damage and mitigation sites located on the surface of fused silica. The simulation domain is in the symmetrical x (70 μm) and y (20 μm) space with uniform grids of 20 nm spacing. The depth and lateral size of the site are marked with alphabet D and L, respectively. The zigzag structure that has periodic spacing l, and height d is used to model the rough bottom of the damage and mitigation sites. The crack below and around the damage site isn’t considered in our
Fig. 7. Morphology of a mitigation site after laser-damage growth test. 7190
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Fig. 8. Normalized PL spectra of the damage site and fs laser mitigation site. The 351 nm excitation laser is focused on the center of the damage site and the fs laser mitigation site.
model. A polarized TE 351 nm light normally incidents to the surface of fused silica. The index of refraction of air and fused silica are 1.0 and 1.5, respectively. The perfected matched layer-absorbing boundary condition is applied in the vertical direction (y), and the periodic boundary condition is applied in the horizontal direction (x). All the electric field amplitudes (EFA) that we discuss here are normalized to the EFA of incident intensity. Figure 10 shows the calculated electric field distribution around four sites with a different depth d of the zigzag structure. The depths d of the zigzag structure are (a) 3 μm; (b) 1 μm; (c) 0.5 μm; (d) 0 μm, respectively. It’s found that the electric field distributions are very different. The normalize peak EFA of the four sites are (a) 2.15; (b) 1.76; (c) 1.55; (d) 1.54, respectively. The peak EFA of Figs. 10(c) and 10(d) located at the edge of the site are marked with a black ring. When the depth d of the zigzag structure decreases from 3 to 0.5 μm, the peak EFA decreases 28%. The peak EFA of Figs. 10(c) and 10(d) are almost same, and both are located at the edge of the site, which shows that the mitigation process is successful after the depth d of zigzag structure decreases to 0.5 μm.
Fig. 9. Calculation model of damage and mitigation sites. The different depth d and periodic spacing l of the zigzag structure are used to model the bottom of the mitigation site and damage site.
For a more comprehensive comparison of the electric field modulation caused by different size of the zigzag structure, Fig. 11 shows the dependencies of normalized peak EFA on periodic spacing l and depth d of the zigzag structure. It’s concluded that the normalized peak EFA increases with the decrease of spacing l, and increases with the depth d. When the spacing l is increased to 12 μm, the peak EFA are lower than 2.1, whereas the peak EFA are lower than 1.85 when the depth d is decreased to 0.5 μm. The peak EFA of the sites are almost the same with the smooth structure (d 0 μm) when the periodic spacing l exceeds 8 μm, and the depth d decreases to 0.5 μm. So the periodic spacing l and the depth d of the zigzag structure will both affect the peak EFA around the site, but the depth d plays the major role. On the other hand, some scaling studies based on the dimension of the site are carried out because of the limitation of the calculation area. Figure 12 shows the dependencies of normalized peak EFA on lateral size L of the site, and two kinds of zigzag structure with different depths are chosen. For depth d 0.5 μm, the peak EFA are less than 1.85 and basically not changed with increased L. As for the depth d 3 μm, the peak EFA are more than 2.3. We consider that the depth d of the zigzag structure of the damage site and mitigation site are 3 and 0.5 μm according to our experimental results. So we can reach the conclusion that the change of the lateral size L of the site has little impact on the effect of the electric field optimization after mitigation process. 5. Discussion
In the region of nanosecond, thermal effect plays a key role in determining the occurrence of damage. When the surface temperature of material reaches the critical temperature, the damage behavior occurs. This critical temperature may be the melting temperature of material or the thermal stress damage temperature. Here we use the maximum temperature of material T max as a criterion of damage growth.
Fig. 10. Electric field distribution around the sites with different depths d of the zigzag structure. The lateral size L, depth D of the site, and the periodic spacing l of the zigzag structure are fixed to 48, 5, and 8 μm. The depths d of the zigzag structure are (a) 3 μm; (b) 1 μm; (c) 0.5 μm; (d) 0 μm, respectively. The locations of the peak electric field are marked with a black ring.
Fig. 11. Dependencies of normalized peak EFA on the periodic spacing l and depth d of the zigzag structure. The lateral size L and depth D of the site are fixed to 48 and 5 μm. 10 October 2013 / Vol. 52, No. 29 / APPLIED OPTICS
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effect result in the generation of damage site. During the damage process, the network of glass is destroyed, and some point defects, such as oxygen deficiency defect (ODC: ≡ Si–Si ≡) and nonbridging oxygen hole centers (NBOHC: ≡ Si–O•), are generated accompanied by the damage process [26–28]: ≡ Si–O–Si ≡ → ≡ Si–Si ≡ Oint PORs and O2 ; (3) nhυ
≡ Si–O–Si ≡ → ≡ Si–O••Si ≡ :
Fig. 12. Dependencies of normalized peak EFA on the lateral size L of the site. The depth D of the site and the periodic spacing l of the zigzag structure are fixed to 5 and 4 μm.
After laser irradiation, the temperature of the sample T can be calculated by thermal transfer equation [24,25]: ρc
∂T dI ∇ • k∇T αhv e−αhvx ; ∂t dt
(1)
where ρ is the density of material, c is the thermal capacity, k is the thermal conductivity, α is the absorption coefficient of the material, hv is the single photon energy, I is the laser energy density, and t is the pulse duration. The ρ, c, and k are considered as constants here. The maximum temperature T max is [24,25] 2αhvI (2) T max ≈ p : kπρct It is found from Eq. (2) that the maximum temperature T max of material is related to the absorption coefficient of material α and the incident laser energy density I. The results of Section 3.C reveal that the damage growth threshold of the mitigation sites is significantly greater than that of the damage sites. In the following discussion, two kinds of reasons are considered to play roles in the increase of the growth threshold of the mitigation site. A.
Removal of Damaged Material
The damage site consists of a molten region and fractured area, and the bottom of the damage site is very rough (Fig. 4). The central molten region contains molten nodules and fibers; the edge of the damage site is surrounded by a circle of fractured materials. The longitudinal and lateral cracks are also found below and around the damage site. After mitigation with fs laser and HF etching, the damage site is completely removed, and the mitigation site exhibits good physical qualities with basically a smooth bottom, and no redeposition material or cracks are found around the mitigation site (Fig. 5). After the interaction between ultraviolet laser and material, the thermal effect and the mechanical 7192
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(4)
The interstitial O may escape from the glass surface and result in the decrease of oxygen content. The characteristic absorption peaks of these point defects are much lower than that of undamaged fused silica, which will result in the increase of the absorption coefficient of material α. So these defects can strongly absorb ultraviolet light and lead to the increase of maximum temperature T max of material according to Eq. (2). As mentioned above, the PL emission peak at ∼550 nm is due to Si/O substoichiometry produced by stripping oxygen from silica. After mitigation with the fs laser, the PL peak at ∼550 nm almost disappears. It’s concluded that the damaged material with point defects is removed by mitigation process, which will increase the laser damage resistance of the fs laser mitigation site. The crack below and around the damage site is generated after initial laser damage. The crack, which will reduce the mechanical performance of material and result in the local enhancement of light electric, is also a precursor of damage growth. As discussed above, the crack isn’t generated below the fs laser mitigation site, and the maximum depth of mitigation site is 30 μm. Such a deep mitigation site can certainly remove the crack below the damage site. The removal of damaged material, including point defects and cracks, is an attribute of the laserresistant mitigation site. B. Structure Optimization
On the other hand, the optimized geometry of the mitigation site also plays a role in the increase of the damage growth threshold. The rough damage site will affect the propagation of the laser beam and result in the enhancement of the local light field around the damage site. After mitigation with the fs laser and HF etching, the bottom and edge of the mitigation site are almost smooth, and the crack is removed. Such optimized mitigation geometry can reduce the enhancement of light field; this is confirmed by the electric field model with the FDTD method in Section 4. The FDTD model results reveal that the periodic spacing l and the depth d of the zigzag structure will affect the peak EFA around the site, whereas the lateral size L of the site has little impact on the effect of the electric field optimization after mitigation. By considering that the depth d of the zigzag structure of the damage site
and mitigation site are 3 and 0.5 μm, we can calculate that the maximum light intensity of the mitigation site decreases at least 1.7 times compared to the damage site, according to the relationship that the light intensity is proportional to the square of electric field. So the T max will also at least decrease 1.7 times after mitigation according to Eq. (2), which is in agreement with our experiment results: that the damage growth threshold of the mitigation sites is almost two times more than that of the damage sites. Simultaneously, damage growth starts at the edge of the mitigation site, which is also in agreement with our model result that the peak EFA locates on the edge bottom of the mitigation site. 6. Conclusion
The rectangular mitigation sites, which exhibit good physical qualities with a smooth bottom and edge, were fabricated by fs laser mitigation and HF etching. Laser damage growth measurement results showed that the growth threshold of the mitigation sites was almost two times larger than that of damage sites. The PL test and morphology analysis revealed that the damaged material, including point defects and cracks, was removed after the mitigation process. The FDTD model illustrated that the regular geometry of the mitigation site can reduce the local light intensity. In conclusion, the removal of damaged material and structure optimization were considered to play a role in the increase of growth threshold of the mitigation sites, and the structure optimization may play a major role, according to the experimental and simulation results. Continued work to optimize the shape of the mitigation site and HF etching process is ongoing. This work was supported by the Chinese National Natural Science Foundation (Grant 11104293). References 1. 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). 2. 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 (2008). 3. M. A. Norton, L. W. Hrubesh, Z. L. 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 (2001). 4. I. L. Bass, G. M. Guss, M. J. Nostrand, and P. J. Wegner, “An improved method of mitigating laser induced surface damage growth in fused silica using a rastered, pulsed CO2 laser,” Proc. SPIE 7842, 784220 (2010). 5. M. J. Matthews, I. L. Bass, G. M. Guss, C. C. Widmayer, and F. L. Ravizza, “Downstream intensification effects associated with CO2 laser mitigation of fused silica,” Proc. SPIE 6720, 67200A (2007). 6. I. L. Bass, G. M. Guss, and R. P. Hackel, “Mitigation of laser damage growth in fused silica with a galvanometer scanned CO2 laser,” Proc. SPIE 5991, 59910C (2005). 7. L. Gallais, P. Cormont, and J. L. Rullier, “Investigation of stress induced by CO2 laser processing of fused silica optics
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Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China 2
University of Chinese Academy of Sciences, Beijing 100049, China *Corresponding author: [email protected]
Received 17 June 2013; revised 17 September 2013; accepted 20 September 2013; posted 23 September 2013 (Doc. ID 192443); published 10 October 2013
A reliable method, combining femtosecond (fs) laser mitigation and chemical (HF) etching, has been developed to mitigate laser-damage growth sites on a fused silica surface. A rectangular mitigation site was fabricated by an fs laser with a raster scan procedure; HF etching was then used to remove the redeposition material. The results show that the mitigation site exhibits good physical qualities with a smooth bottom and edge. The damage test results show that the growth threshold of the mitigation sites increases. Furthermore, the structural characteristic of samples was measured by a photoluminescence (PL) spectrometer, and the light intensification caused by damage and mitigation sites was numerically modeled by the finite-difference time-domain (FDTD). It revealed that the removal of damaged material and structure optimization contribute to the increase of the damage growth threshold of the mitigation site. © 2013 Optical Society of America OCIS codes: (140.3330) Laser damage; (140.3380) Laser materials; (140.7090) Ultrafast lasers. http://dx.doi.org/10.1364/AO.52.007186
1. Introduction
Fused silica is widely used in an inertial confinement fusion (ICF) laser system that often operates at the fluence near the laser-induced damage threshold (LIDT) of optics. The surface of fused silica is easily damaged under ultraviolet (351 nm) high-fluence laser irradiation, and the damage size grows exponentially with subsequent laser pulse exposures [1–3]. The growth of the damage sites, which is the key factor in determining the lifetime of the optics, increases the output power loss and enhances the beam obscuration. The method of CO2 laser mitigation is commonly used to resolve the problem of damage growth [4–12]. But the downstream intensification [5], generation of redeposition material [6], and thermal stress [7,10] are accompanied by the CO2 laser mitigation 1559-128X/13/297186-08$15.00/0 © 2013 Optical Society of America 7186
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process, which greatly limits the development of laser mitigation technology. The femtosecond (fs) laser exhibits extreme high peak power and very short pulse width. The characteristics of fs laser pulse interaction with material include basically negligible thermal damage, lower threshold fluence of ablation, higher precision of laser processing, and reduced redeposition material, which results from the extreme intensities and short time scale of fs pulse compared to the time scale of photon electron lattice interactions [13–16]. Therefore, fs laser micromachining may become a possible method of mitigation damage growth based on these unique performances. Recently, Wolfe and co-workers [17,18] successfully used the method of fs laser machining to create mitigation sites in multilayer dielectric mirror coatings. In our previous work [19], an fs laser was also used to mitigate laser damage growth sites on a fused silica surface; the effect of different fs laser machining energies and geometries of mitigation
sites were explored, but further research was needed to optimize the structure of mitigation sites and remove the redeposition material. In this work, fs laser micromachining is used to fabricate stable mitigation sites to replace the damage growth sites on a fused silica surface. HF etching is then used to remove the redeposition material. The laser damage growth test of the damage and mitigation sites is executed to compare the effect of mitigation. Furthermore, the possible reasons for an increased growth threshold of mitigation sites are proposed based on the analysis of structure characteristic and electric field distribution model. 2. Experimental Details
The experiment was performed on fused silica samples 10 mm in diameter and 5 mm thick, which exhibits less than 1 nm rms surface roughness after an ultrasonic bath. A.
Nanosecond Laser Damage Test of Fused Silica
Figure 1 shows the nanosecond laser damage test facility. An Nd:YAG laser that operated at 355 nm was used to produce the initial damage site and perform a laser damage growth test. The pulse duration and frequency of the laser were 8.0 ns and 10 Hz, respectively. An energy attenuator, composed of a λ∕2 wave plate and a polarizer, was used to adjust the laser energy. A specified number of laser pulses was selected by synchronizing a mechanical shutter to the output 10 Hz pulse train. The effective area of laser spot on the sample surface was 0.22 mm2, which was measured by a laser beam analyzer. The sample was illuminated by a white light, and the damage on the front and rear surfaces could be observed in situ with two image systems. In our experiment, all the initial damage sites on the fused silica surface were generated by irradiation with one shot at peak fluence of 45.4 J∕cm2. The laser damage growth test procedure was performed as follows. The damage and mitigation sites to be tested were located on the input surface of the sample. The site to be tested was first irradiated with 300 shots at lower laser fluence. If the site didn’t grow, increased fluence of about 2 J∕cm2 with 300 shots was then applied to the site until the phenomenon
Fig. 1. Schematic of nanosecond laser-damage test facility.
of damage growth occurred. And the maximum fluence at which the site didn’t grow was defined as the growth threshold. B. Mitigate Damage Sites on Fused Silica Surface
The fs laser mitigation facility is shown in Fig. 2. A chirped pulse Ti:sapphire fs laser system (Spectra Physics, Spitfire), with a pulse duration of 38 fs and central wavelength λ 800 nm, was used to mitigate damage growth sites. An energy attenuator was used to adjust the laser energy. The pulse energy was measured by an energy meter from a split-off portion of the beam. The lens with a 300 mm focal length focused the laser beam on the surface of sample. And the effective diameter (1∕e2 ) of laser spot on the focal plane was 62 μm, which was measured by a laser beam analyzer. The sample was illuminated by a white light, and the process of mitigation could be observed in situ by an image system. The sample was mounted on a motorized x and y translation sample stage. In this work, the optimal fs laser energy of 100 μJ and a raster scan procedure (Fig. 3) were chosen to mitigate damage sites based on our previous work [19]. This raster scan procedure (beam overlap at about 70%) was realized by a two-dimensional reentrant movement of sample stage, which is controlled by a computer. And the parameters of transverse distance x and vertical distance y of the mitigation site could be set up according to the actual size of the damage site in order to completely mitigate the damage site. After fs laser mitigation of the damage site, some redeposition material was found around the mitigation site. The HF etching procedure was used to remove the redeposition material. The fs laser mitigation samples were first submerged in a polypropylene tank filled with HF-based etchants (1% HF and 15% NH4F solution) for 1 h. Subsequently, the samples were submerged in a deionized water rinse tank with ultrasonic agitation. Finally, the samples were spray rinsed with deionized water and allowed to air dry. C.
Analysis Techniques
The morphologies of damage sites and mitigation sites were characterized by a field emission scanning electron microscope (FE-SEM, Zeiss Auriga S40). A step profiler (DektakXT, Bruker) was used to measure the depth profile of sample surface. The
Fig. 2. Schematic of fs laser mitigation bench. 10 October 2013 / Vol. 52, No. 29 / APPLIED OPTICS
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and fibers that result from a thermal effect after high-energy absorption. The edges of the damage site are surrounded by a circle of fractured structure, which shows the mechanical damage accompanied by the removal of material. It’s also found that the bottom of the damage site is very rough, and the maximum depth reaches to about 8 μm [Fig. 4(d)]. In addition, our previous work [20] proved that the longitudinal and lateral cracks were generated below and around the damage site after laser damage. Fig. 3. Schematic of raster scan procedure.
structural characteristics of samples were measured by photoluminescence (PL) spectrometer (R928, PTI). Finite-difference time-domain (FDTD) simulations were performed to model the local electric field distribution around the damage and mitigation sites. 3. Results A.
Characteristics of Initial Damage Site
Figure 4 shows the morphologies and horizontal depth profile of the initial damage site induced by one laser pulse of 355 nm light with peak fluences 45.4 J∕cm2 . Figure 4(a) illustrates the damage morphology of the whole damage site, which appears to be an ellipsoidal type with 200–300 μm diameter. SEM images at higher magnification focusing on the center and edge of the damage site reveal that the damage site involves two different regions: molten region [Fig. 4(b)] and fractured region [Fig. 4(c)]. The central molten region contains molten nodules
B. Characteristics of Mitigation Site
Figure 5 shows the morphology, structure, and depth profile images of the fs laser mitigation site [Fig. 5(a)] and the mitigation site treated with fs laser mitigation and HF etching [Fig. 5(b)]. Comparing these images [Figs. 5(a1), 5(b1), 5(a2), and 5(b2)], one can notice that the redeposition material around the fs laser mitigation site is completely removed after HF etching, the fs laser mitigation site composed of the flocculent microstructure, whereas the mitigation site exhibits porous microstructure after HF etching. Figure 5(a3) shows the longitudinal cutting morphology of the fs laser mitigation site, which is shown in Fig. 5(a1), obtained by focused ion beam (FIB) technology. It’s found that the crack below the damage site is removed after fs laser mitigation. The vertical crosssectional depth profile of mitigation site [Fig. 5(b1)] is shown in Fig. 5(b3). After fs laser mitigation and HF etching, the mitigation site has basically a smooth bottom, and the maximum depth is nearly 30 μm. It’s concluded that the mitigation site exhibits good physical qualities with smooth geometry.
Fig. 4. SEM images and horizontal depth profile of damage site induced by one shot with peak fluences 45.4 J∕cm2 . (a) The whole damage site. (b) The molten region of the damage site. (c) The fractured region at the edge of the damage site. (d) The horizontal depth profile of the damage site. 7188
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Fig. 5. (a1) SEM image of fs laser mitigation site. (a2) Amplified image. (a3) FIB cross-sectional image below the fs laser mitigation site shown in Fig. 5(a1). The SEM images of mitigation site [(b1) and (b2)] treated with fs laser mitigation and HF etching. (b3) Vertical crosssectional depth profile of mitigation site shown in (b1).
C.
Results of Laser Damage Growth Test
In this study, we measured the damage growth threshold of five damage sites, 10 fs laser mitigation sites, and 10 mitigation sites treated with fs laser and HF etching. The threshold results are shown in Fig. 6. The average damage growth threshold of damage sites is 6.2 J∕cm2, and the threshold of different damage sites changes is very small. After fs laser mitigation, the average growth threshold is 10.4 J∕cm2 , and the maximum threshold increases to 13.8 J∕cm2 . The average and maximum growth threshold of mitigation sites, which is treated with fs laser mitigation and HF etching, are 9.5 J∕cm2 and 12.1 J∕cm2 , respectively. In summary, the average growth threshold of fs laser mitigation sites is significantly increased, and the maximum growth threshold is almost two times more than that of the damage sites. It’s worth noting that the growth thresholds of mitigation sites decrease slightly after HF etching in comparison with that of fs laser mitigation sites, which can be explained by the
redeposition of the HF etching reaction products. The aqueous phase etching reaction product SiF2− 6 has limited solubility in the HF etching solution, which will become a localized optical absorption center and ultimately lead to laser damage of material when exposed to sufficient laser fluence [21]. Figure 7 shows the morphology of a mitigation site after a laser damage growth test. It’s found that the damage growth starts at the edge of the mitigation site. D.
PL Spectra
The normalized PL spectra of the damage site and the fs laser mitigation site are shown in Fig. 8. It’s found that two emission peaks are obtained for a damage site with 351 nm laser excitation. The peak ∼470 nm correlates with the emission from mechanically damage, and the peak at ∼550 nm may be due to Si /O substoichiometry produced by stripping oxygen from silica [22,23]. The peak at ∼550 nm almost disappears after fs laser mitigation. It’s concluded that the Si/O substoichiometry is generated in the 10 October 2013 / Vol. 52, No. 29 / APPLIED OPTICS
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Fig. 6. Results of damage-growth threshold of damage and mitigation sites.
damaged area because of oxygen loss, and the disappearance peak at ∼550 nm indicates that the oxygen content of fs laser mitigation site returns to a normal level, which is in agreement with our previous analysis results of chemical composition [19]. 4. FDTD Model and Results
The rough geometry of the damage site affects the propagation of the laser beam, and results in the enhancement of the local light field. The mitigation site exhibits regular geometry with a relative smooth edge and bottom, which can reduce the impact of light field modulation. In another aspect, the light intensity is proportional to the square of electric field. So the enhancement value of the light field can be obtained by calculation of electric field distribution. The method of FDTD is used to calculate and compare the electric field distribution around the damage and mitigation sites. Figure 9 shows the two-dimensional model of the damage and mitigation sites located on the surface of fused silica. The simulation domain is in the symmetrical x (70 μm) and y (20 μm) space with uniform grids of 20 nm spacing. The depth and lateral size of the site are marked with alphabet D and L, respectively. The zigzag structure that has periodic spacing l, and height d is used to model the rough bottom of the damage and mitigation sites. The crack below and around the damage site isn’t considered in our
Fig. 7. Morphology of a mitigation site after laser-damage growth test. 7190
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Fig. 8. Normalized PL spectra of the damage site and fs laser mitigation site. The 351 nm excitation laser is focused on the center of the damage site and the fs laser mitigation site.
model. A polarized TE 351 nm light normally incidents to the surface of fused silica. The index of refraction of air and fused silica are 1.0 and 1.5, respectively. The perfected matched layer-absorbing boundary condition is applied in the vertical direction (y), and the periodic boundary condition is applied in the horizontal direction (x). All the electric field amplitudes (EFA) that we discuss here are normalized to the EFA of incident intensity. Figure 10 shows the calculated electric field distribution around four sites with a different depth d of the zigzag structure. The depths d of the zigzag structure are (a) 3 μm; (b) 1 μm; (c) 0.5 μm; (d) 0 μm, respectively. It’s found that the electric field distributions are very different. The normalize peak EFA of the four sites are (a) 2.15; (b) 1.76; (c) 1.55; (d) 1.54, respectively. The peak EFA of Figs. 10(c) and 10(d) located at the edge of the site are marked with a black ring. When the depth d of the zigzag structure decreases from 3 to 0.5 μm, the peak EFA decreases 28%. The peak EFA of Figs. 10(c) and 10(d) are almost same, and both are located at the edge of the site, which shows that the mitigation process is successful after the depth d of zigzag structure decreases to 0.5 μm.
Fig. 9. Calculation model of damage and mitigation sites. The different depth d and periodic spacing l of the zigzag structure are used to model the bottom of the mitigation site and damage site.
For a more comprehensive comparison of the electric field modulation caused by different size of the zigzag structure, Fig. 11 shows the dependencies of normalized peak EFA on periodic spacing l and depth d of the zigzag structure. It’s concluded that the normalized peak EFA increases with the decrease of spacing l, and increases with the depth d. When the spacing l is increased to 12 μm, the peak EFA are lower than 2.1, whereas the peak EFA are lower than 1.85 when the depth d is decreased to 0.5 μm. The peak EFA of the sites are almost the same with the smooth structure (d 0 μm) when the periodic spacing l exceeds 8 μm, and the depth d decreases to 0.5 μm. So the periodic spacing l and the depth d of the zigzag structure will both affect the peak EFA around the site, but the depth d plays the major role. On the other hand, some scaling studies based on the dimension of the site are carried out because of the limitation of the calculation area. Figure 12 shows the dependencies of normalized peak EFA on lateral size L of the site, and two kinds of zigzag structure with different depths are chosen. For depth d 0.5 μm, the peak EFA are less than 1.85 and basically not changed with increased L. As for the depth d 3 μm, the peak EFA are more than 2.3. We consider that the depth d of the zigzag structure of the damage site and mitigation site are 3 and 0.5 μm according to our experimental results. So we can reach the conclusion that the change of the lateral size L of the site has little impact on the effect of the electric field optimization after mitigation process. 5. Discussion
In the region of nanosecond, thermal effect plays a key role in determining the occurrence of damage. When the surface temperature of material reaches the critical temperature, the damage behavior occurs. This critical temperature may be the melting temperature of material or the thermal stress damage temperature. Here we use the maximum temperature of material T max as a criterion of damage growth.
Fig. 10. Electric field distribution around the sites with different depths d of the zigzag structure. The lateral size L, depth D of the site, and the periodic spacing l of the zigzag structure are fixed to 48, 5, and 8 μm. The depths d of the zigzag structure are (a) 3 μm; (b) 1 μm; (c) 0.5 μm; (d) 0 μm, respectively. The locations of the peak electric field are marked with a black ring.
Fig. 11. Dependencies of normalized peak EFA on the periodic spacing l and depth d of the zigzag structure. The lateral size L and depth D of the site are fixed to 48 and 5 μm. 10 October 2013 / Vol. 52, No. 29 / APPLIED OPTICS
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effect result in the generation of damage site. During the damage process, the network of glass is destroyed, and some point defects, such as oxygen deficiency defect (ODC: ≡ Si–Si ≡) and nonbridging oxygen hole centers (NBOHC: ≡ Si–O•), are generated accompanied by the damage process [26–28]: ≡ Si–O–Si ≡ → ≡ Si–Si ≡ Oint PORs and O2 ; (3) nhυ
≡ Si–O–Si ≡ → ≡ Si–O••Si ≡ :
Fig. 12. Dependencies of normalized peak EFA on the lateral size L of the site. The depth D of the site and the periodic spacing l of the zigzag structure are fixed to 5 and 4 μm.
After laser irradiation, the temperature of the sample T can be calculated by thermal transfer equation [24,25]: ρc
∂T dI ∇ • k∇T αhv e−αhvx ; ∂t dt
(1)
where ρ is the density of material, c is the thermal capacity, k is the thermal conductivity, α is the absorption coefficient of the material, hv is the single photon energy, I is the laser energy density, and t is the pulse duration. The ρ, c, and k are considered as constants here. The maximum temperature T max is [24,25] 2αhvI (2) T max ≈ p : kπρct It is found from Eq. (2) that the maximum temperature T max of material is related to the absorption coefficient of material α and the incident laser energy density I. The results of Section 3.C reveal that the damage growth threshold of the mitigation sites is significantly greater than that of the damage sites. In the following discussion, two kinds of reasons are considered to play roles in the increase of the growth threshold of the mitigation site. A.
Removal of Damaged Material
The damage site consists of a molten region and fractured area, and the bottom of the damage site is very rough (Fig. 4). The central molten region contains molten nodules and fibers; the edge of the damage site is surrounded by a circle of fractured materials. The longitudinal and lateral cracks are also found below and around the damage site. After mitigation with fs laser and HF etching, the damage site is completely removed, and the mitigation site exhibits good physical qualities with basically a smooth bottom, and no redeposition material or cracks are found around the mitigation site (Fig. 5). After the interaction between ultraviolet laser and material, the thermal effect and the mechanical 7192
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(4)
The interstitial O may escape from the glass surface and result in the decrease of oxygen content. The characteristic absorption peaks of these point defects are much lower than that of undamaged fused silica, which will result in the increase of the absorption coefficient of material α. So these defects can strongly absorb ultraviolet light and lead to the increase of maximum temperature T max of material according to Eq. (2). As mentioned above, the PL emission peak at ∼550 nm is due to Si/O substoichiometry produced by stripping oxygen from silica. After mitigation with the fs laser, the PL peak at ∼550 nm almost disappears. It’s concluded that the damaged material with point defects is removed by mitigation process, which will increase the laser damage resistance of the fs laser mitigation site. The crack below and around the damage site is generated after initial laser damage. The crack, which will reduce the mechanical performance of material and result in the local enhancement of light electric, is also a precursor of damage growth. As discussed above, the crack isn’t generated below the fs laser mitigation site, and the maximum depth of mitigation site is 30 μm. Such a deep mitigation site can certainly remove the crack below the damage site. The removal of damaged material, including point defects and cracks, is an attribute of the laserresistant mitigation site. B. Structure Optimization
On the other hand, the optimized geometry of the mitigation site also plays a role in the increase of the damage growth threshold. The rough damage site will affect the propagation of the laser beam and result in the enhancement of the local light field around the damage site. After mitigation with the fs laser and HF etching, the bottom and edge of the mitigation site are almost smooth, and the crack is removed. Such optimized mitigation geometry can reduce the enhancement of light field; this is confirmed by the electric field model with the FDTD method in Section 4. The FDTD model results reveal that the periodic spacing l and the depth d of the zigzag structure will affect the peak EFA around the site, whereas the lateral size L of the site has little impact on the effect of the electric field optimization after mitigation. By considering that the depth d of the zigzag structure of the damage site
and mitigation site are 3 and 0.5 μm, we can calculate that the maximum light intensity of the mitigation site decreases at least 1.7 times compared to the damage site, according to the relationship that the light intensity is proportional to the square of electric field. So the T max will also at least decrease 1.7 times after mitigation according to Eq. (2), which is in agreement with our experiment results: that the damage growth threshold of the mitigation sites is almost two times more than that of the damage sites. Simultaneously, damage growth starts at the edge of the mitigation site, which is also in agreement with our model result that the peak EFA locates on the edge bottom of the mitigation site. 6. Conclusion
The rectangular mitigation sites, which exhibit good physical qualities with a smooth bottom and edge, were fabricated by fs laser mitigation and HF etching. Laser damage growth measurement results showed that the growth threshold of the mitigation sites was almost two times larger than that of damage sites. The PL test and morphology analysis revealed that the damaged material, including point defects and cracks, was removed after the mitigation process. The FDTD model illustrated that the regular geometry of the mitigation site can reduce the local light intensity. In conclusion, the removal of damaged material and structure optimization were considered to play a role in the increase of growth threshold of the mitigation sites, and the structure optimization may play a major role, according to the experimental and simulation results. Continued work to optimize the shape of the mitigation site and HF etching process is ongoing. This work was supported by the Chinese National Natural Science Foundation (Grant 11104293). References 1. 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). 2. 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 (2008). 3. M. A. Norton, L. W. Hrubesh, Z. L. 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 (2001). 4. I. L. Bass, G. M. Guss, M. J. Nostrand, and P. J. Wegner, “An improved method of mitigating laser induced surface damage growth in fused silica using a rastered, pulsed CO2 laser,” Proc. SPIE 7842, 784220 (2010). 5. M. J. Matthews, I. L. Bass, G. M. Guss, C. C. Widmayer, and F. L. Ravizza, “Downstream intensification effects associated with CO2 laser mitigation of fused silica,” Proc. SPIE 6720, 67200A (2007). 6. I. L. Bass, G. M. Guss, and R. P. Hackel, “Mitigation of laser damage growth in fused silica with a galvanometer scanned CO2 laser,” Proc. SPIE 5991, 59910C (2005). 7. L. Gallais, P. Cormont, and J. L. Rullier, “Investigation of stress induced by CO2 laser processing of fused silica optics
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