BIC-Based Silicon Metasurfaces for Chiral Response and Tunable Chiral Absorption

Strong chiral responses in planar dielectric metasurfaces are important for polarization-selective nanophotonic devices, but achieving large and reversible circular dichroism (CD) in simple dielectric structures remains challenging. This work proposes a symmetry-broken silicon metasurface that realizes near-infrared chiral response based on bound states in the continuum (BICs). The unit cell consists of a silicon nanoblock with two through-air grooves. The in-plane displacement of the air grooves breaks the C₂ rotational symmetry and splits the BIC-related polarization singularity into two circularly polarized points (C points) with opposite handedness. By further introducing out-of-plane tilting, one of the C points is shifted to the Г point, enabling spin-selective coupling between normally incident circularly polarized light and the quasi-BIC mode. Reversing the out-of-plane tilt switches the sign of CD, with values reaching -0.98 and 0.98, approaching the theoretical limits of ±1. Under oblique incidence, the structure can also exhibit near-limit CD responses. Finally, by introducing graphene, the structure achieves tunable circular-polarization-selective absorption, with the absorption of CD approaching the theoretical limits of ±0.5 for the coupled system. This work provides a new design idea for compact chiral nanophotonic materials by using symmetry breaking to control spin-selective quasi-BIC coupling and tunable chiral absorption.