Chiral quasi-bound states in the continuum for refractive-index sensing in metasurfaces
Jiangbin Li, Qilin Duan, Xinyong Dong, Zhou Yang, Zuoti Xie, Shan Zhu, Huanyang Chen
Abstract
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.