Description
We propose a new class of chiral quasi-bound states in the continuum (q-BICs) operating at large oblique angles, overcoming the conventional limitation of near-Γ, normal-incidence resonances. The approach exploits the enhanced radiative density of states near the light cone to realize high-Q resonances with nearly pure circular polarization in momentum regions approaching grazing emission. Using dielectric metasurfaces with a monoclinic lattice and broken in-plane symmetries, we achieve tunable control over the momentum-space positions of these states through lattice-angle deformation.
Unlike previously demonstrated chiral q-BICs derived directly from symmetry-protected Γ-point BICs, our mechanism first splits a high-order Γ-point BIC into a pair of off-Γ accidental BICs. Controlled breaking of both lattice and unit-cell symmetries then transforms these accidental BICs into q-BICs accompanied by pairs of C-points. Near the light cone, lattice deformation shifts the q-BICs and C-points at different rates due to the rapid increase in radiative density of states. This enables alignment of a Q-factor maximum with one C-point at oblique incidence, while the other crosses the light cone and becomes non-radiative, forming a light-cone-proximal chiral q-BIC.
These states support highly directional circularly polarized emission at angles exceeding 50°, with Q-factors and polarization purity comparable to conventional Γ-point resonances. Full-wave simulations confirm their persistence in finite metasurfaces and demonstrate single-mode dual-beam emission with opposite helicities, establishing a route toward angularly tunable multi-beam chiral nanolasers and integrated polarization-selective light sources.
| I am the presenting author | Yes |
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