PHOTOCHEMICAL FABRICATION OF AMORPHOUS SEMICONDUCTOR BY FEMTOSECOND LASER EXPOSURE

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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.