Chiral quasi-bound states in the continuum for refractive-index sensing in metasurfaces

Chiral metasurfaces, driven by bound states in the continuum (BICs), provide an excellent platform for studying light-matter interactions. Currently, research on chirality is limited to enhancing the quality (Q) factor in low cyclic symmetry structures. Here we propose chiral metasurfaces consisting of a periodic arrangement of unit cells of skeleton fans with ${C}_{6}$ cyclic symmetry (SFCS) that are protected by the BIC topology. If the in-plane symmetry is broken, BIC polarization singularities (V points) with integer topological charges split into pairs of circular polarization points (C points) with half-integer topological charges. The C points simultaneously possess the high-Q and high circular dichroism (CD) responses, resulting from quasi-BICs based on toroidal dipole modes. In particular, the extrinsic and intrinsic chiralities based on BICs are achieved by breaking the out-of-plane and in-plane symmetries. The results also demonstrate good performance for high-Q chiral refractive-index sensing.