Nonplanar surface structures of inorganic materials fabricated by femtosecond laser lithography

Silica-based nonplanar surface structures were fabricated by use of femtosecond laser lithography-assisted micromachining (FLAM), which is a combined process of femtosecond laser lithography and plasma etching. Diffractive optical elements (DOEs) are widely used for photonic applications such as optical pickup, interconnection and so forth. Most DOEs have been produced by semiconductor fabrication process. Although this process is useful to form complicated fine structures, there exist two problems. First, it is rather difficult to fabricate nonplanar surfaces including slopes and curves, which is effective to enhance diffraction efficiencies of DOEs. Second, microstructures cannot be fabricated onto nonplanar substrates. In the FLAM, nonplanar patterns are directly written inside resists by use of femtosecond laser-induced nonlinear optical absorption. Then, the patterns are transferred to underlying silica glasses by CHF3 plasma. By use of FLAM, we successfully fabricated silica-based microFresnel lenses including curves and slopes on planar substrates and cross-grating structures onto a convex microlens. In particular, as for the latter, uniform grating structures with smooth surfaces were observed even at top and curved regions of the lenses. The FLAM is expected to be useful for the fabrication of highly functional DOEs such as diffractive/refractive hybrid microlenses.