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Abstract
Under 266-nm (deep ultraviolet, DUV) laser irradiation, an SrB4O7 (SBO) single crystal has been found to exhibit a surface laser-induced damage threshold (LIDT) of ∼ 16.4 J/cm2, which is higher than those of a synthetic silica glass (4.8 J/cm2) and a calcium fluoride (CaF2) crystal (11.4 J/cm2). By catalyst-referred etching (CARE), the LIDT of an SBO crystal can also be improved to around 24.1 J/cm2, which is 1.4 and 6.0 times higher compared to an unetched crystal and a silica glass, respectively. With high surface LIDTs, SBO single crystals can then be used as optical window materials for high-power DUV laser systems.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
With the advent of internet of things (IoT) and increased human mobility, glass composite substrates and carbon fiber-reinforced plastics (CFRPs) have found different applications in digital electronics, aviation, and automobile industries. To realize these important applications, high-power and short-wavelength laser systems have been developed to process these complex functional materials. However, conventional optical components used for such laser systems have become inadequate. For instance, calcium fluoride (CaF2) crystal, which is commonly used for lenses, prisms, and windows because of its UV region transparency and low nonlinear refractive index [1,2], has a low thermal and mechanical strength. Moreover, synthetic silica glass, which is also widely used for optical windows, has a low resistance to high-power deep ultraviolet (DUV) laser-induced damage due to two-photon absorption process [3]. Thus, there is a high demand and urgent necessity to consider new optical materials with high damage resistance or threshold especially in the DUV region (200 to 300 nm).
Among the candidate DUV optical materials, SrB4O7 (SBO) is a non-hygroscopic borate crystal which has high nonlinearity and transparency down to 130 nm [4–8]. Previously, we have reported the successful growth of high-quality SBO single crystals that exhibit resistance to laser-induced bulk damage under 266-nm laser irradiation [9]. SBO’s damage resistance is also higher compared to that of a synthetic silica glass under the same laser irradiation conditions. On the other hand, laser-induced damage occurs on the material surface more easily than on the bulk due to the presence of rough surfaces and/or residual abrasive materials [10]. The rough surfaces and residual abrasives come from the conventional polishing techniques, and they scatter and absorb the incident laser [11]. Surface finishing methods to obtain flat and clean surfaces, therefore, are important to further improve an optical material’s laser damage resistance, particularly the surface LIDT.
In this regard, we evaluate the surface LIDT of SBO single crystals as potential optical window materials in the DUV region. Because a comparison between SBO and CaF2 crystals has not yet been carried out, we also compare the two crystals in terms of laser-induced damage under 266-nm (DUV) laser irradiation. Furthermore, we examine the possible improvement of the surface LIDT of SBO crystal after chemical-mechanical polishing. Since surface finishing techniques such as optical near-field etching [12], float polishing [13], and hydrodynamic effect polishing [14] have already been studied, we implement the catalyst-referred etching (CARE) to process the SBO crystal surface similar to silicon carbide (4H-SiC) and gallium nitride (GaN) wafers [15–18]. CARE is an abrasive-free chemical-mechanical polishing or planarization method used to obtain atomically flat surfaces. During CARE, the catalyst generates reactive species that activate when only next to the catalyst surface and then deactivate when leaving the surface. The etching subsequently proceeds preferentially at the protrusions where the catalyst can come into contact with the surface more often, and a flat surface can be realized by chemical removal. As a whole, this study determines the viability of SBO single crystals as damage-resistant optical window materials for high-power DUV laser systems.
2. Experimental details
Surface LIDT measurements were performed at room temperature and in ambient atmosphere using the experimental setup illustrated in Fig. 1. As described previously in Ref. [19], the fourth harmonics (266 nm, 4ω) of a 1064-nm, 5-ns Nd:YAG laser (Continuum, Surelite) was used to irradiate the SBO crystal surface during the single-shot (1-on-1) tests wherein the irradiated spot was changed every after one shot. After passing through an attenuator which consists of a half-waveplate, a polarizer, and a beam splitter, the laser beam was focused towards the surface of a 15 mm (a) × 5.0 mm (b) × 15 mm (c) mechanically polished (020) SBO single crystal. This crystal was cut from a larger crystal grown by Kyropoulos method at an average rate of 1.1 mm/day along the a-axis. Taking into consideration the experimental setup and the SBO crystal’s orientation, the 266-nm laser was parallel to the crystal’s b-axis, i.e., incident on the (020) surface, while the laser’s direction of polarization was parallel to the crystal’s c-axis. The laser beam profile was Gaussian, and the laser beam diameter on the sample surface (where the intensity is at least 1/e2 of the maximum intensity) was 170 µm. Defined in accordance with the International Organization Standardization (ISO) standard 21254-2 [20] as the highest laser fluence with zero damage probability, the LIDT was then determined by the damage probability method similar to the one conducted in Ref. [19]. Due to the relatively small dimensions of the SBO crystal, measurements on the same crystal were performed for at most three times where each measurement consisted of an average of 40 irradiation spots with fluences ranging from 4.25 to 48.24 J/cm2. For comparison, surface LIDT measurements of conventional optical window materials – a commercial UV-grade synthetic silica glass (Tosoh, ESL-1) and a CaF2 crystal (Sigma Koki, OPCFU-30C02-P) – were also carried out under the same conditions.
3. Results and discussion
Figure 2 shows the surface LIDTs of the synthetic silica glass, CaF2 crystal, and SBO crystal under 266-nm laser irradiation. The synthetic silica glass has the lowest threshold of 4.8 ± 0.1 cm2 followed by the CaF2 crystal with a threshold of 11.4 ± 0.2 J/cm2. On the other hand, the SBO crystal has the highest LIDT of 16.4 ± 0.1 J/cm2 which is 3.4 times higher that of synthetic silica and 1.4 times higher than that of CaF2. Moreover, Fig. 3 shows the 266-nm laser-induced damage on the surfaces of the synthetic silica glass, CaF2 crystal, and SBO crystal above their respective LIDTs. The optical micrographs were taken using a digital microscope (Keyence, VHX-950F). Before laser irradiation, the surfaces of the synthetic silica glass, CaF2 crystal, and SBO crystal are featureless and have root-mean-square (RMS) roughness values of 0.84, 0.84, and 0.35 nm, respectively. After laser-induced damage, the surface of the synthetic silica glass consists of a high density of shallow pits [Fig. 3(a)] which are referred to as “gray haze” due to the optical absorption of contaminants in the near-surface region due to polishing [21,22]. In addition, the laser-induced damage on the CaF2 crystal surface looks like a crack [Fig. 3(b)] which can be attributed to CaF2’s thermal cleavability. On the contrary, the SBO crystal exhibits an almost featureless surface [Fig. 3(c)] and does not seem to be heavily damaged by the laser irradiation. These results reveal that the SBO crystal has superior resistance to DUV laser-induced damage compared to synthetic silica glass and CaF2 crystal.
Fig. 2. Surface LIDTs of synthetic silica glass, CaF2 crystal, and SBO crystal under 266-nm (DUV) laser irradiation.
Fig. 3. Optical micrographs of the 266-nm (DUV) laser-induced damage on the surfaces of (a) synthetic silica glass, (b) CaF2 crystal, and (c) SBO crystal above their respective LIDTs.
Figure 4 shows the surface topographies of the SBO crystal before and after CARE. The topographies within a 2.0 µm x 2.0 µm surface area were taken using an atomic force microscope (AFM, Shimadzu SPM-9700HT) in contact mode. The (020) SBO crystal has a 15 mm (a) × 1.0 mm (b) × 15 mm (c) dimension and was also mechanically polished on both sides to an optical finish. For the CARE process, platinum (Pt) and deionized (DI) water were used as the catalyst and the etching solution, respectively. The SBO crystal was etched for 3.0 h with a removal rate of 364 nm/h. Before CARE, the SBO crystal has a featureless surface [Fig. 4(a)] with an RMS roughness of 0.46 nm. After CARE, the surface morphology of the SBO crystal has changed drastically, and the RMS roughness is reduced to 0.33 nm. As shown in Figs. 4(b) and 4(c), the surface of the etched SBO crystal exhibits a uniform step-and-terrace structure with a step height of approximately 0.20 nm. Similar surface morphologies are also observed from CARE-processed 4H-SiC [15,17] and GaN [16] wafers. The 0.20-nm step height corresponds to around one-fifth of the b-axis lattice constant of SBO (1.0709 nm [4]). These results show that an atomically flat and well-ordered surface, similar to semiconductor materials, can be successfully obtained for an SBO crystal through CARE.
Fig. 4. Surfaces topographies of an SBO crystal (a) before and after (b) CARE. The CARE-processed (etched) SBO crystal has a step-and-terrace structure with step height (c) of ∼ 0.20 nm.
Figure 5 shows the surface LIDT of the CARE-processed (i.e., etched) SBO crystal under 266-nm laser irradiation in comparison with those of a synthetic silica glass and an unetched SBO crystal. The etched SBO crystal has a threshold of 24.1 ± 0.8 J/cm2 which is 6.0 times higher than the synthetic silica glass (4.0 ± 0.3 J/cm2) and 1.4 times higher than the unetched SBO crystal (17.3 ± 0.9 J/cm2). Moreover, Fig. 6 shows the 266-nm laser-induced damage on the surfaces of the unetched and etched SBO crystal at a laser fluence of 33.15 J/cm2 which is above their respective LIDTs. The surfaces of both crystals differ significantly wherein the laser-induced damage is more obvious on the unetched crystal [Fig. 6(a)] than on the etched crystal [Fig. 6(b)]. This observation can be explained by the presence of residual abrasives from mechanical polishing which absorb the incident laser resulting in the unetched crystal’s more obvious laser-induced damage and lower LIDT [11]. On the other hand, these abrasives or surface contaminants are removed from the SBO crystal surface by CARE, so a more obvious laser-induced damage and higher LIDT are observed from the etched crystal. These results indicate that CARE successfully removes the surface contaminants due to the mechanical polishing of the SBO crystal leading to an improved surface LIDT under 266-nm laser irradiation
Fig. 5. Surface LIDTs of synthetic silica glass, unetched SBO crystal, and CARE-processed (etched) SBO crystal under 266-nm (DUV) laser irradiation.
Fig. 6. Optical micrographs of the 266-nm laser-induced damage on the surfaces of (a) unetched and (b) CARE-processed (etched) SBO crystal under the laser fluence of 33.15 J/cm2.
4. Conclusion
We have confirmed that SBO single crystals are more resistant to optical damage compared to conventional optical window materials. Under 266-nm laser irradiation, the surface LIDT of a SBO crystal is 3.4 and 1.4 times higher compared to those of a synthetic silica glass and a CaF2 crystal, respectively. Furthermore, SBO’s damage threshold was further improved by implementing the abrasive-free CARE planarization method. Aside from an atomically flat and well-ordered surface with a RMS roughness of 0.33 nm, the etched crystal exhibits a surface LIDT of around 24.1 J/cm2 which is 1.4 times higher than that of an unetched crystal and even 6.0 times higher than that of a synthetic silica glass. Since we have successfully demonstrated that CARE improves the surface LIDT of an SBO crystal, additional investigations are anticipated in the future to further analyze the damage morphology and understand the damage mechanism of SBO crystals under DUV laser irradiation. Our results reveal the viability of SBO single crystals as laser-induced damage resistant optical window materials for high-power DUV laser systems.
Disclosures
The authors declare no conflicts of interest.
References
1. S. Sakuragi, Y. Taguchi, H. Sato, A. Kasai, H. Nanba, T. Kawai, and S. Hashimoto, “Evaluation of high-quality CaF2 single crystals for ultraviolet laser applications,” Proc. SPIE 5647, 314–321 (2005). [CrossRef]
2. J. T. Mouchovski, K. A. Temelkov, N. K. Vuchkov, and N. V. Sabotinov, “Laser grade CaF2 with controllable properties: growing conditions and structural imperfection,” J. Phys. D: Appl. Phys. 40(24), 7682–7686 (2007). [CrossRef]
3. N. Kuzuu, K. Yoshida, H. Yoshida, T. Kamimura, and N. Kamisugi, “Laser-induced bulk damage in various types of vitreous silica at 1064, 532, 355, and 266 nm: evidence of different damage mechanisms between 266-nm and longer wavelengths,” Appl. Opt. 38(12), 2510–2515 (1999). [CrossRef]
4. Y. S. Oseledchik, A. L. Prosvirnin, V. V. Starshenko, V. V. Osadchuk, A. I. Pisarevskiy, S. P. Belokrys, A. S. Korol, N. V. Svitanko, A. F. Selevich, and S. A. Krikunov, “Crystal growth and properties of strontium tetraborate,” J. Cryst. Growth 135(1-2), 373–376 (1994). [CrossRef]
5. Y. S. Oseledchik, A. L. Prosvirnin, A. I. Pisarevskiy, V. V. Starshenko, V. V. Osadchuk, S. P. Belokrys, N. V. Svitanko, A. S. Korol, S. A. Krikunov, and A. F. Selevich, “New nonlinear optical crystals: strontium and lead tetraborates,” Opt. Mater. 4(6), 669–674 (1995). [CrossRef]
6. F. Pan, G. Shen, R. Wang, X. Wang, and D. Shen, “Growth, characterization and nonlinear optical properties of SrB4O7 crystals,” J. Cryst. Growth 241(1-2), 108–114 (2002). [CrossRef]
7. A. I. Zaitzev, A. S. Aleksandrovskii, A. V. Zamkov, and A. M. Sysoev, “Nonlinear Optical, Piezoelectric, and Acoustic Properties of SrB4O7,” Inorg. Mater. 42(12), 1360–1362 (2006). [CrossRef]
8. R. Komatsu, H. Kawano, Z. Oumaru, K. Shinoda, and V. Petrov, “Growth of transparent SrB4O7 single crystal and its new applications,” J. Cryst. Growth 275(1-2), e843–e847 (2005). [CrossRef]
9. Y. Tanaka, K. Shikata, R. Murai, Y. Takahashi, M. Imanishi, T. Sugita, Y. Mori, and M. Yoshimura, “Growth of high-quality transparent SrB4O7 single crystals with high degradation resistance for DUV laser application,” Appl. Phys. Express 11(12), 125501 (2018). [CrossRef]
10. A. A. Manenkov, “Fundamental mechanisms of laser-induced damage in optical materials: understanding after a 40-years research,” Proc. SPIE 7132, 713202 (2008). [CrossRef]
11. M. R. Kozlowski, J. Carr, I. Hutcheon, R. Torres, L. Sheehan, D. Camp, and M. Yan, “Depth profiling of polishing-induced contamination on fused silica surfaces,” Proc. SPIE 3244, 365–375 (1998). [CrossRef]
12. T. Yatsui, K. Hirata, W. Nomura, Y. Tabata, and M. Ohtsu, “Realization of an ultra-flat silica surface with angstrom-scale average roughness using nonadiabatic optical near-field etching,” Appl. Phys. B 93(1), 55–57 (2008). [CrossRef]
13. S. F. Soares, D. R. Baselt, J. P. Black, K. C. Jungling, and W. K. Stowell, “Float-polishing process and analysis of float-polished quartz,” Appl. Opt. 33(1), 89–95 (1994). [CrossRef]
14. W. Peng, C. Guan, and S. Li, “Ultrasmooth surface polishing based on the hydrodynamic effect,” Appl. Opt. 52(25), 6411–6416 (2013). [CrossRef]
15. H. Hara, Y. Sano, H. Mimura, K. Arima, A. Kubota, K. Yagi, J. Murata, and K. Yamauchi, “Novel Abrasive-Free Planarization of 4H-SiC (2006) Using Catalyst,” J. Electron. Mater. 35(8), L11–L14 (2006). [CrossRef]
16. J. Murata, T. Okamoto, S. Sadakuni, A. N. Hattori, K. Yagi, Y. Sano, K. Arima, and K. Yamauchi, “Atomically Smooth Gallium Nitride Surfaces Prepared by Chemical Etching with Platinum Catalyst in Water,” J. Electrochem. Soc. 159(4), H417–H420 (2012). [CrossRef]
17. A. Isohashi, P. V. Bui, D. Toh, S. Matsuyama, Y. Sano, K. Inagaki, Y. Morikawa, and K. Yamauchi, “Chemical etching of silicon carbide in pure water by using platinum catalyst,” Appl. Phys. Lett. 110(20), 201601 (2017). [CrossRef]
18. P. V. Bui, D. Toh, A. Isohashi, S. Matsuyama, K. Inagaki, Y. Sano, K. Yamauchi, and Y. Morikawa, “Platinum-catalyzed hydrolysis etching of SiC in water: A density functional theory study,” Jpn. J. Appl. Phys. 57(5), 055703 (2018). [CrossRef]
19. J. Long, D. Ross, E. Tastepe, M. Lamb, Y. Funamoto, D. Shima, T. Kamimura, and H. Yamaguchi, “Fused silica contamination layer removal using magnetic field-assisted finishing,” J. Am. Ceram. Soc. 103(5), 3008–3019 (2020). [CrossRef]
20. . “Lasers and laser-related equipment—Test methods for laser induced damage threshold—Part 2: Threshold determination,” ISO 21254-2:2011(E).
21. 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(25), 10163–10171 (2005). [CrossRef]
22. J. Wong, J. L. Ferriera, E. F. Lindsey, D. L. Haupt, I. D. Hutcheon, and J. H. Kinney, “Morphology and microstructure in fused silica induced by high fluence ultraviolet 3ω (355 nm) laser pulses,” J. Non-Cryst. Solids 352(3), 255–272 (2006). [CrossRef]
