Mutual circular polarization conversions in asymmetric transmission and reflection modes by three-layer metasurface with gold split-rings.

Plasmonic metasurfaces can be used to replace traditional polarization devices for various integrated optical devices because of their novel polarization control ability on a subwavelength scale. In particular, the asymmetric transmission of circularly polarized light by anisotropic metasurface has attracted much attention recently. Here, a simple and effective circular polarization converter composed of three layers of rotated gold split-rings and a SiN substrate is proposed, which is appropriate for both transmission and reflection modes. For transmission mode, it can convert left- and right- handed circularly polarized light into orthogonally polarized light in two adjacent bands with conversion efficiencies of 65% and 75%, respectively. For the reflection mode, the mutual conversion efficiencies can be up to 58% and 64%, respectively. Meanwhile, the structure has moderate asymmetric transmission and reflection efficiencies. The operating band of the metasurface can be adjusted continuously and linearly by changing the refractive index of the substrate. The dual-band asymmetric effects may contribute to information encoding and decoding for communication applications. As an ultra-thin planar optical element, the proposed metasurface can be used in integrated photonics, optical sensing, and other fields.