Dual-band polarization converter with antireflection-assisted all-dielectric metasurface

In this study, we demonstrate an all-dielectric metasurface that can efficiently manipulate polarization for two broadband polarization control in the infrared band. By breaking the correlation between phase delay and frequency, independent polarization control over two wavelengths (2 mu m and 4 mu m) can be achieved. The phase delay can be freely controlled by controlling the number of waveguide modes in the grating. When operating in single mode, the phase delay increases in a linear fashion with the thickness of the grating. While at two modes, the phase delay is periodically perturbed with increasing thickness, which is caused by the mutual interference between the two modes. Therefore, the phase delay of two independent bands can be freely controlled. Here, a free combination of a dual-band polarized half-wave plate and quarter-wave plate are designed. Half-quarter wave plate and quarter-quarter wave plate, are fabricated and experimentally demonstrated to have efficient broadband polarization conversion efficiency with relative bandwidths exceeding 42.8 %. Since only is the device made of pure silicon, we utilize micro-spheres structure on the back side as an anti-reflection layer, and the highest transmittance can reach about 87 %.