Three-dimensional SiO2 surface structures fabricated using femtosecond laser lithography
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We report the fabrication of three-dimensional (3-D) SiO2 surfaces using femtosecond-laser lithography-assisted micromachining, which is a combined process of nonlinear lithography and plasma etching. Using pattern transfer of photoresist structures written by femtosecond laser-induced nonlinear absorption, SiO2-based Fresnel lens arrays with 3-D surfaces were obtained for this study. Using the open-aperture z-scan method, the femtosecond laser two-photon absorption coefficient of the KMPR resist was estimated as 17–23 cm/TW, assuming that single-photon absorption was negligible. By adding O2 to the etching gas (CHF3) during pattern transfer, the surface roughness of the transferred structures was reduced to RMS 16.90 nm, which corresponds to one quarter of that without adding O2. When 632.8-nm-wavelength light was coupled to the lenses with 3-D surfaces, the focal length was measured as 2790 μm, which agreed well with the theoretical value.