Chiral quasi-bound states in the continuum via Brillouin zone folding

Chiral quasi-bound states in the continuum (q-BICs) provide a way to investigate light–matter interactions. Currently, the chiral response is fundamentally constrained by the rapid decay of quality (Q-) factors with symmetry breaking, making the robustness of higher-Q particularly crucial. In this paper, we propose the metasurfaces composed of the double-skeleton-fan unit cells with sixfold rotational symmetry. Moreover, by introducing gap perturbation and scale perturbation, the guided modes originally below the light cone are folded above the light cone through Brillouin zone folding (BZF), enabling the q-BZF-BICs with enhanced Q-factors (Q > 106). Furthermore, by tuning the tilted extrusion parameters, near-perfect circularly polarized light (ellipticity χ = 0.99) is achieved in the parameter space. Notably, the extrinsic (circular dichroism CD = 0.98) and intrinsic chirality (CD = 0.97) are realized through simultaneous breaking of in-plane and out-of-plane symmetries. Chiral metasurfaces enable sensing, lasing, and pharmaceutical applications.