Visible-light chiral absorption via quasi-BIC: from intrinsic to extrinsic chirality

Achieving simultaneous narrowband chiral absorption, near-unity circular dichroism (CD), and high-quality factor ( Q ) in the visible regime remains a major challenge for polarization-selective photonic devices, narrowband filtering, and chiral sensing. Here, we propose a visible-light metasurface absorber based on symmetry-protected quasi-bound states in the continuum (quasi-BICs) for ultranarrowband chiral absorption. The proposed metasurface enables near-perfect absorption for one circular polarization while strongly suppressing the opposite handedness. To elucidate the physical origin of the chiroptical response, we investigate two complementary symmetry-breaking mechanisms. In the intrinsically chiral configuration, in-plane displacement of dielectric nanodimers induces strong chiral absorption under normal incidence, yielding CD≈0.99 and Q factors as high as 4.2×10 4 . In the extrinsically chiral configuration, a geometrically symmetric dimer array acquires chirality under oblique illumination, where symmetry breaking arises from the incident wavevector and excites a chiral resonance. Furthermore, we show that reducing structural asymmetry simultaneously enhances the Q factor and improves the angular robustness of the chiral response. The resonance behavior is accurately captured by temporal coupled-mode theory (TCMT) and corroborated by strongly confined electromagnetic fields within the dielectric resonators at resonance. These results establish a practical platform for ultra-high- Q chiral photonic devices operating in the visible regime.