Optical seminar | 13 November 2020

Over the past years, it has been repeatedly demonstrated that chiral metasurfaces – planar arrays of subwavelength elements of broken mirror symmetry – are capable of unprecedentedly high optical chirality. While zero chirality is attributed to achiral objects which equally strongly interact with light of both circular polarizations, the maximum chirality is assigned to those that are uncoupled from one circular polarization and extremely strongly interact with the opposite one. We demonstrate how precise shaping of the coupling of metasurface quasi bound states in the continuum (BICs) to free-space waves can empower and maximize the optical chirality. Based on the phenomenological coupled-mode theory, we elucidate the general properties of the maximum chiral BIC resonances that manifest themselves as narrow peaks of unit height in the circular dichroism spectrum. A particular realization of maximum chiral BIC metasurface can be based on rotationally symmetric arrays of pairs of dielectric bars. We present a step-by-step design, starting from arrays with fully uncoupled BICs and carefully enabling their chiral selective coupling by symmetry breaking perturbations.