Near-perfect chiral absorption enabled by coupled chiral bound states in the continuum.

Abstract

Near-perfect chiral absorption in open dielectric systems remains challenging because a single resonance in a two-port structure is fundamentally limited to 50% absorption under critical coupling, while existing approaches typically rely on reflective substrates to suppress transmission. Here, we demonstrate a substrate-free strategy for achieving near-perfect chiral absorption by exploiting the coherent coupling of two chiral quasi-bound states in the continuum (quasi-BICs). The proposed metasurface supports two symmetry-protected BICs associated with polarization vortices carrying integer topological charges in momentum space. Breaking the structural symmetry transforms these vortices into circularly polarized point pairs with opposite handedness, giving rise to two high-Q chiral quasi-BICs under oblique incidence. By tuning the geometric parameters, the two resonances are spectrally brought together and coherently coupled, simultaneously suppressing reflection and transmission for the selected spin channel and enabling near-perfect chiral absorption without metallic mirrors or Bragg reflectors. Our work establishes a general topological mechanism for overcoming the absorption limit of open photonic systems and provides a promising platform for high-Q chiral photodetection, polarization-selective optics, and spin-controlled nanophotonic devices.