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

Abstract 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.