PHOTOCHEMICAL FABRICATION OF AMORPHOUS SEMICONDUCTOR BY FEMTOSECOND LASER EXPOSURE

硫系玻璃在光照射下的光膨胀和体积增加从玻璃制造的应用角度得到了新的关注。光膨胀不是通过热过程发生的，而是通过光化学过程发生的，并且膨胀仅限于玻璃的被照射部分。利用这些特性，成功地演示了微光学器件的形成。然而，由于以往的研究大多采用单光子过程和w光进行电子激发，暴露光与玻璃吸收系数的波长匹配限制了玻璃的性能，限制了玻璃的种类、尺寸和制造时间。为了扩大光扩展在玻璃制造中的适用性，我们在本研究中提出使用透明波长的脉冲激光进行曝光。期望通过脉冲激光的多光子过程实现带间激发。由于多光子过程的吸收系数取决于光子密度，因此可以通过激光通量来控制曝光时间和穿透深度。由于多光子过程只有当它们的光子能量总和超过玻璃的光学间隙时才能实现带到带激发，因此可以用相同的激光器制造各种类型的玻璃。用飞秒钛蓝宝石激光照射As_2S_3玻璃，用激光显微镜观察玻璃表面的变化。在0.41mW激光照射128s后，可检测到高达0.9μm的膨胀。膨胀的增长率与激光透过率的关系表明，光膨胀主要是由激光的双光子吸收过程引起的。

The photo-expansion an volume increase by light exposure in chalcogenide glasses has received renewed interests from viewpoints of application to glass fabrication. The photo-expansion occurs through not thermal but photochemical processes, and the expansion is restricted to illuminated fractions of glasses. Utilizing these characteristics, formation of micro-optical devices was successfully demonstrated. However, since one-photon processes with cw-light were used for the electronic excitation in most previous studies, the wavelength matching of exposing light to the absorption coefficient of glasses restricts their performance applicable glass species, sizes, and fabrication times. To extend the applicability of photoexpension to glass fabrication, we have proposed in this study to use pulsed laser light with transparent wavelengths for exposure. The band to band excitation is expected through multi-photon processes with the pulsed laser light. Since the absorption coefficient for the multi-photon process depends on the photon density, the exposure time and the penetration depth can be controlled by the laser flux. Since the multiphoton processes enable band to band excitation only if their sum photon energy exceeds the optical gap of the glass, various species of glasses can be fabricated with an identical laser. We exposed As_2S_3 glasses with femtosecond Ti-sapphire laser light and observed the transformation of the glass surface with a laser microscope. Expansion as high as 0.9μm was detected after exposure with 0.41mW laser light for 128s. Laser power dependence of the growth rate of the expansion and the laser transmittance showed that the photo-expansion is induced mainly by a two-photon absorption process of the laser light.