# Light-cone-proximal quasi-BICs for chiral lasing at grazing angles

## TL;DR

This paper introduces a novel mechanism for creating chiral quasi-bound states in the continuum at large oblique angles using dielectric metasurfaces, enabling directional chiral lasing and efficient off-normal photonic devices.

## Contribution

It proposes a new method to realize light-cone-proximal chiral q-BICs at large angles by tuning lattice geometry, expanding applications in directional chiral lasing.

## Key findings

- Achieved near-unity circular dichroism at angles >50°
- Demonstrated robust control of chiral resonances via lattice tuning
- Enabled fully circularly polarized emission at grazing angles

## Abstract

Chiral quasi-bound states in the continuum (q-BICs) have recently emerged in metaphotonics as resonances that combine ultrahigh quality factors with near-unity circular polarization. However, these states are typically confined to the Gamma-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 local 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 degrees from normal. This approach paves the way for directional chiral lasing at grazing angles and for photonic devices operating efficiently in off-normal geometries.