Chirality enhancement and topological dynamics of vectorial bound states in the continuum across structural parameter space

Optical bound states in the continuum (BICs) are widely studied in photonics and metasurfaces for their high-quality (<i>Q</i>) resonances, which underpin applications in sensing, lasing, and nonlinear optics. While much is known about symmetry-protected BICs, which arise from mismatches in profile symmetry with the continuum, Friedrich-Wintgen BICs (FWBICs)-formed by total destructive interference of multiple leaky modes-are inherently less predictable and remain understudied. Topologically, BICs are characterized as polarization singularities in momentum space, and their dynamic evolution (e.g., topological charge spawning, merging, and annihilation) has been extensively explored. However, most reported FWBICs are scalar, meaning only one linear polarization state is relevant due to the mirror symmetry of the system. In this study, we investigate vectorial FWBICs in a planar chiral metasurface lacking in-plane mirror symmetry, mapping their topological dynamics across a two-dimensional space of structural parameters. Additionally, we demonstrate that enhanced chirality at quasi-FWBICs enables asymmetric transmission of circularly polarized light, with calculated asymmetry values approaching the theoretical maximum of 0.25 at the exceptional point. These findings deepen the understanding of FWBICs and their associated chiral optical phenomena in planar metasurfaces, offering insights for advanced polarization control and chiral photonic devices.