In this research, we used a subnanosecond Nd:YAG laser operating at 1.064 μ m wavelength, and inscribed diffraction gratings inside silicon substrate without damaging top or bottom surfaces. Thus it is worth investigating to apply one-photon absorption based fabrication in modification inside silicon. In addition, a Nd:YAG laser operating at 1.064 μ m is widely used. One-photon absorption based fabrication could be more efficient than that based on multi-photon absorption, whereas there are possible drawbacks that energy loss due to absorption in pre-focal region, and limitation in the inscribing depth due to damaging surface and/or pre-focal region. However, there have been no report on the use of one-photon absorption based fabrication for optical applications. Meanwhile, in cutting silicon substrate using internal modification, a Nd:YAG laser operating at 1.064 μ m wavelength is used 23, at this wavelength there is weak but non-negligible one-photon absorption (absorption coefficient of silicon at 1.064 μ m about 9.7 cm - 1 at 295 K) 24. ![]() Generally, one-photon absorption is not used for laser processing inside solid materials. Investigation on pulse-duration dependence was carried out by Das et al., and they reported that pulse duration of 5.4 ps or more is required for making modifications at NA = 0.85 22. inscribed waveguides that have symmetric cross-section in transverse geometry using 1.55 μ m wavelength and 3.5 ns duration pulses 21. fabricated waveguides 19 and gratings 20 using 1.55 μ m wavelength and 5 ns duration pulses, and estimated the degree of refractive index change 19, 20. made modifications inside silicon with 1.552 μ m wavelength and duration ranging from 800 fs to 10 ps, and reported that 10 ps pulses showed better reproducibility 18. ![]() They also reported fabrication of waveguide, and selective etching of modified region. In their method the length of modification along the optical axis was controlled by the number of pulses. used reflection on the back surface and fabricated modification inside silicon with 1.550 μ m and 5 ns pulses 17. reported modification inside silicon using laser pulse of 1.549 μ m wavelength and 3.5 ns duration 16. In contrast to ultrafast lasers, longer pulse lasers are effective for modification inside silicon. Investigation using THz-repetition-rate pulse bursts also showed the effectiveness of peak-suppressed consecutive pulses for laser modification inside silicon 15. Very recently, it was reported that temporal contrast (pre/post-pulse, pedestal), which is laser technology dependent, of ultrafast laser pulses has a significant effect on the modification threshold energy 14. 12, waveguides were inscribed in longitudinal geometry starting at the exit surface then moved upstream, in this case the waveguide inscription was significantly facilitated by an imperfectly flat exit surface 13. Thus, special methods have been executed, such as use of optical setup with extremely high numerical aperture of 2.97 9, double pulse 10, and use of high repetition rate laser 11– 13. However, unlike glasses, difficulty has been reported to process inside silicon with an ultrafast laser in the transparent wavelength region 8. Silicon is transparent in the near-infrared range (the band gap of silicon is 1.12 eV, and corresponding wavelength is 1.11 μ m), thus use of an ultrafast laser in this wavelength range seems promising. For such applications, 3D processing using lasers will be advantageous. Recently research on the use of silicon as a near-infrared (NIR) photonic platform material is active. In these, semiconductor technology based on photolithography, which is a two-dimensional technique, is mainly used for fabrication. Its application field includes large-scale integrated circuits (LSIs), micro-electro-mechanical systems (MEMS) devices, and multi-pixel photodetectors. Silicon is one of the most important material in modern technology. Now application fields of ultrafast laser processing inside glass is expanding such as microphotonics and microfluidics 1– 3. In these research, non-linear optical phenomena, such as multi-photon absorption, is utilized for localized modification of materials. For example, marking, inscribing waveguide, and selective etching inside glass have been reported 4– 7. In recent years, ultrafast laser processing inside transparent solid materials has been attracting interest as a tool of three-dimensional (3D) micro-nano processing technique 1– 3.
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