Evolution of circularly polarized states spawned from off-Γ point BICs

Bound states in the continuum (BICs) can manifest as vortex polarization singularities in momentum space.The BICs at Γ point are proved to be protected by the rotational symmetry, and previous studies have explored the at-Γ BICs split into pairs of circularly polarized states with opposite chirality after breaking the in-plane inversion symmetry.As another type of conventional BICs, off-Γ BICs, have been less explored in their evolution and modulation relations compared to their at-Γ counterparts.Here, we investigate the evolution of circularly polarized states spawned from off-Γ BICs by selectively breaking in-plane and out-of-plane symmetry.The circularly polarized states exhibit distinct evolutionary trajectories under different symmetry-breaking conditions.This study offers an effective approach for manipulating the evolution of circularly polarized states in momentum space from off-Γ BICs, holding promise for potential applications in the field of polarization modulation and topological photonics.