Two-photon polymerization for fabrication of metalenses for diffraction-limited focusing and high-resolution imaging

Metalenses consisting of nanoscale unit cells provide new opportunities for miniaturization, light-weight, and diversification of conventional optical lenses. Fabrication of metalenses is conventionally by photolithography technique, which is complex involving multi-steps. In this study, two-photon polymerization (TPP) was used to directly write single-layer metalenses. The metalenses was designed according to Pancharatnam Berry (P-B) phase distribution model at the wavelengths of 515, 628, and 1030 nm, respectively. The focusing performance of the designed metalenses was simulated using a Finite Difference Time Domain (FDTD) software. Near optical diffraction limit focusing of the incident light was achieved on a low refractive index photoresist. The geometrical dimensions of the fabricated unit cells were compared with the designed parameters. The influence of laser power density on dimensions of the unit and the resultant the focusing effect were investigated. The metalenses prepared by TPP with optimum parameters show a minimum full width at half maximum (FWHM) of 978 nm and a focusing efficiency of 1.02% with 628 nm light source. A line width of 980 nm was obtained under a white light source. The research results demonstrate that TPP can be a feasible and simplified approach for fabrication of metalenses, but for higher focusing efficiency and imaging resolution metalenses, some high refractive index photoresist materials being suitable for the TPP should be developed.