Chiral Quasi-Bound States in the Continuum on the Verge of the Light Cone

Chiral quasi-bound states in the continuum (q-BICs) have recently emerged in metaphotonics as resonances that combine ultra-high-quality factors with near-unity circular polarization in the far field. However, these states are typically confined to the Γ-point (normal incidence) due to their symmetry-protected origins. We propose a new mechanism for realizing light-cone-proximal chiral q-BICs at large oblique angles, enabled by the divergence of the radiative density of states near the light cone. Using dielectric metasurfaces with a monoclinic lattice and broken in-plane mirror symmetry, we demonstrate that tuning the lattice angle allows for robust control of these resonances. The resulting chiral q-BICs exhibit near-unity circular dichroism in transmission and fully circularly polarized emission at angles exceeding 50° from normal. Our results establish a general route to off-normal and grazing-angle chiral q-BICs, enabling directional chiral lasing and providing a versatile platform for quantum and nonlinear photonics.