Enhanced optical activity in hyperbolic metasurfaces

The optical activity of achiral nonmagnetic uniaxial metasurfaces caused by the extrinsic chirality arising from the mutual orientation of their anisotropy axis and the light plane of incidence is studied. The hyperbolic regime of the metasurfaces manifests in the amplification of the polarization rotation in transmitted light and in the giant enhancement of the effect in reflected light. The transition to this regime is frequency dependent and has a topological nature. The key role in the optical activity enhancement belongs to the $\ensuremath{\sigma}$-near-pole and hyperbolic $\ensuremath{\sigma}$-near-zero regimes of the metasurfaces. The hyperbolic metasurfaces based on graphene strips or metal disks, and a black phosphorus thin film as a natural hyperbolic layer are considered. The efficiency of the predicted effects depends on the metasurface implementation and the role of a substrate. By varying the metasurface parameters and choosing the physical realizations it is possible to design such a hyperbolic metasurface polarizer for the THz, infrared, and visible working ranges. Our findings may help to improve metasurface polarizers by using them in the hyperbolic regime.