Design of pure and hybrid corner states in all-dielectric second-order photonic topological insulators.

This article focuses on the design and manipulation of photonic corner states in two-dimensional all-dielectric second-order topological photonic insulators (SOTIs). It demonstrates that, beyond conventional time-reversal and point-group (C2) symmetries, local sublattice symmetry plays a crucial role in protecting the degeneracy of corner states. By locally breaking this sublattice symmetry at the corners, the study achieves tunable control over the degeneracy levels, frequencies, and spectral separations of both pure and hybrid photonic corner states within the proposed photonic crystal structure. The work also identifies various topological phases—including topological metallic, half topological metallic, and half topological insulator phases—and suggests potential applications in highly tunable zero-dimensional photonic devices such as corner-mode lasers.