Microlens arrays of high-refractive-index glass fabricated by femtosecond laser lithography

Microlens arrays of high-refractive-index glass GeO2–SiO2 were fabricated by femtosecond laser lithography assisted micromachining. GeO2–SiO2 thin glass films, which were deposited by plasma-enhanced chemical vapor deposition, have a refractive index of 1.4902 and exhibit high transparency at wavelengths longer than 320 nm. Using a femtosecond laser, three-dimensional patterns were written inside resists on GeO2–SiO2 films, and then the patterns were transferred to the underlying films by CHF3 and O2 plasma treatments. This combined process enabled us to obtain uniform microlens structures with a diameter of 38 μm. The heights of the transferred lenses were approximately one-quarter the height of the resist patterns, due to differences in the plasma etching rates between GeO2–SiO2 and the resist. The lens surfaces were smooth. When 632.8-nm-wavelength He–Ne laser light was normally coupled to the lenses, focal spots with a diameter of 3.0 μm were uniformly observed. The combined process was effective in fabricating three-dimensional surfaces of inorganic optical materials.