Fig. 1. Unlocking the height of dielectric resonators for photonic qBIC engineering.

a Established qBIC geometries use in-plane symmetry breaking to couple otherwise dark BIC states to the radiation continuum. In contrast, we introduce out-of-plane symmetry breaking for qBICs of optical metasurfaces. b Simulated transmittance spectra for various height differences starting from Δh = 0 nm (black dashed line, top) up to Δh = 40 nm (dark brown curve, bottom), with the corresponding electric near field shown in (d). c Resonance Q factors extracted from the spectra in (b) follow an inverse quadratic dependence (fitted as grey dashed line) typical for symmetry-broken qBIC metasurfaces. e Tailoring the height-driven qBIC by an opening angle $\theta$ creates metasurfaces with maximum chirality. f Spectral response of a left-handed structure for different incident circular polarizations exhibiting a pronounced qBIC resonance only for LCP light. g Dependence of the maximum ΔT on the opening angle $\theta$ showing a maximally chiral response at $\theta =8.5°$. h Corresponding near-field of the chiral qBIC resonance