Design of pure and hybrid photonic corner states through sublattice symmetry breaking

Topological corner states have significantly deepened our understanding of intriguing higher-order topological phases of matter. However, the subsymmetry protection mechanism of topological corner states has rarely been explored, and their spectral degeneracies are commonly attributed to conventional point-group symmetry. Here, we explore the sublattice symmetry protection of topological corner states in a C 4 -symmetric dielectric photonic crystal and introduce another sublattice symmetry-breaking scheme for designing both pure and hybrid topological corner states, along with their on-demand hybridization. We also find that the corner states always exhibit asymmetric spectra when the sublattice symmetries are locally broken in one dimension or two dimensions. The complete topological phase diagram and the corner index of our photonic crystals are also presented. This work further reveals the subsymmetry protection mechanisms of topological corner states in photonic crystals and provides an alternative pathway for multifunctional photonic devices that harness the unique properties of topological corner states.