Fig. 1. Schematics of different BIC states and planar chiral q-BIC metasurface with a polarization singularity enclosed by vortex elliptical eigen-polarizations with non-vanishing helicities.

a–c Comparison of q-BICs (a) in planar metasurfaces without polarization effect, (b) in stereo-nanostructures with 3D chiral effect in reflection, and (c) in planar metasurfaces with 2D chiral effect in transmission. d Schematic of the unit cell of the planar chiral q-BIC metasurface formed by the amorphous silicon (α-Si) DSS structure on a SiO₂ substrate with parameters P = 850 nm, R = 280 nm, L₁ = L₂ = 220 nm, W₁ = W₂ = 191 nm, and H = 350 nm. e Simulated band structure with real parts of the eigen-wavelength of the DSS metasurface near the Г point. Insets show the field patterns (color: magnetic distributions, arrow: electric vectors) of the three typical modes. f Simulated Q-factors of the eigenmode in the k-space in the vicinity of the Г point. Insets show the first Brillouin zone and eigen-polarization profiles. On the eigen-polarization profile, the polarization states are represented by ellipses of which the blue and red represent the eigen left-handed and right-handed states, respectively, and the black dot represents the V point (BIC).