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

Abstract

Microlens arrays of high-refractive-index glass GeO 2–SiO 2 were fabricated by femtosecond laser lithography assisted micromachining. GeO 2–SiO 2 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 GeO 2–SiO 2 films, and then the patterns were transferred to the underlying films by CHF 3 and O 2 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 GeO 2–SiO 2 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.