Flat Pathways to Maximum Optical Chirality

Dielectric metasurfaces with eigenstates designed to selectively couple to circularly polarized light offer a game-changing platform for advanced chiral metaphotonics. Current designs rely on breaking the mirror symmetry by variable height, elevation or tilt of subwavelength meta-atoms, which complicates their fabrication. We discuss novel strategies for achieving maximum optical chirality with metasurfaces composed of flat layers processed with conventional two-dimensional lithography. Apart from extrinsic-chiral arrangements, which owe their broken mirror symmetry to a sample tilt, various single- and multi-layer designs are capable of maximizing intrinsic chirality at the normal incidence. We reveal specific conditions, when even weak symmetry-breaking effect of a transparent substrate can lead to maximum chirality.