Chiral Quasi‐BICs Stemming From a Γ‐point Degeneracy

ABSTRACT Band degeneracies representing symmetry properties of two‐dimensional (2D) periodic photonic structures are widely observed. In momentum space, exotic photonic phenomena have been demonstrated in the vicinity of such degeneracies, while studies on intrinsic properties of those isolated points remain limited. In this letter, we show that chiral quasi‐bound states in the continuum (quasi‐BICs) can be achieved through a Γ‐point degeneracy in resonant metasurfaces with C 4 symmetry. We demonstrate that, depending on their enantiomeric property, three‐dimensional (3D) chiral perturbations enable a ±π/2 phase difference between two degenerate and orthogonally orientated linear polarizations, resulting in the strong chiral selectivity at the Γ point. The proposed chiral quasi‐BIC is experimentally observed in a 3D‐printed dielectric metasurface. Our results reveal the potential of high‐symmetry degeneracy points as a means for empowering sophisticated light‐matter coupling.