Controllable perfect chiral optical response in planar metasurfaces empowered by quasi-bound states in the continuum.

Chiral metasurfaces with strong chirality and high quality factors (Q-factors) have become essential components for achieving strong light-matter interactions and have a wide range of applications in chiral lasers, detectors, etc. However, current schemes primarily focus on enhancing the chiral response and Q-factor, with limited consideration of their modulability and flexibility. In this paper, we present a chiral a-Si metasurface that can support multiple symmetry-protected bound states in the continuum (BIC). The perfect extrinsic and intrinsic chiral responses (circular dichroism exceeding 0.99), with ultra-high Q-factors, are achieved by utilizing quasi-BICs induced by illumination symmetry and in-plane symmetry breaking. The circular dichroism value and the transmittance of the two circular polarization states can be arbitrarily controlled by adjusting the structural parameters. Furthermore, the feasibility of achieving dynamic modulation of chiral response is demonstrated based on the a-Si-graphene hybrid metasurface. Our research offers an approach to the design of controllable planar optical chirality, which also provides promising avenues for applications in spin-selective bio-detection, electrically tunable chiral switching, and chiral lasers.