SiO2-Based Hybrid Diffractive–Refractive Lenses Fabricated by Femtosecond Laser-Assisted Micromachining

In this study, we created nonplanar structures of inorganic optical materials by femtosecond (fs) laser lithography-assisted micromachining, which is a combined process of nonlinear lithography and plasma etching. Three-dimensional patterns, formed inside a positive-tone resist by fs laser exposure, were transferred to the underlying nonplanar substrates by plasma etching. This process enables fabrication of Si lines with a constant width of 5 µm, even on a 65-µm-high step. We demonstrated the fabrication of SiO2-based micro-Fresnel zone plates on convex lenses. This hybridization shifted the focal length of the lens by 200 µm, which was consistent with the theoretical value.