Bound States in the Continuum Underpin Near-Lossless Maximum Chirality in Dielectric Metasurfaces

Metasurfaces without a mirror symmetry may exhibit chiral electromagnetic response that differs substantially from any type of polarization transformation. A typical design of chiral metasurfaces is based on a complex arrangement of meta-atoms with chiral shapes assembled into rotationally symmetric arrays. Here it is demonstrated that, in a sharp contrast to our intuition, metasurfaces that break all point symmetries can outperform their rotationally symmetric counterparts and exhibit near-lossless maximum chirality. The authors employ the special type of high-quality-factor resonances-bound states in the continuum (BICs)-that are manifested in physical systems as quasi-BICs, and allow engineering the coupling of light with resonant metasurfaces to achieve maximum chirality. A dielectric metasurface composed of pairs of rectangular bars is designed that fully transmits one circular polarization of light and resonantly reflects the other circular polarization without any polarization conversion. Proof-of-concept experimental results that confirm directly the prediction of maximum chiral response of the BIC-empowered asymmetric resonant dielectric metasurfaces are presented.