Polarization-Insensitive Concentric Circular Grating Filters Featuring a Couple of Resonant Peaks

We demonstrate here guided mode resonance (GMR) concentric circular grating filters (CCGFs) on an HfO₂-on-silicon platform. The fabricated devices are realized with a membrane structure by completely removing the silicon substrate beneath the HfO₂ device layer and, thus, can operate across the entire visible wavelength band. Polarization-insensitive properties under normal incidence are experimentally achieved resulting from their rotational symmetry. Substantially differing from 1-D linear grating filters, CCGFs typically feature a couple of resonant peaks for normal incident arbitrary linear polarization, which, from a local point of view, can be physically attributed to the coexistence of azimuthal and radial components of the incident light electric field. The CCGFs here have potential applications, such as polarization-insensitive two-color mixing devices and polarization-insensitive color filtering devices.