Simultaneous Appearance of Different Topological Phases in a Single Photonic System: Coexisting Phases Characterized by Bulk Polarization and Valley‐Chern Number Enabling Dual‐Band Second‐Order Topological States

Abstract

Topological phases appearing in photonic systems have recently attracted much attention in the community of photonics due to the fundamental and practical significances. Despite extensive investigations, the preceding results show that the photonic systems exhibit single topological phases with few exceptions. This work presents the simultaneous appearance of different topological phases in a single photonic crystal structure, which are characterized by bulk polarization and valley‐Chern number. Square‐lattice photonic crystals with unit cells consisting of four dielectric rods, one of which has the size different from the other three ones, exhibit the topological phase characterized by bulk polarization in the first bandgap, depending on the distance between the dielectric rods. Due to the broken inversion symmetry, in the meantime, the photonic systems show the valley‐Hall topological phase in the fourth bandgap. The proposed structures have the advantage of flexible design for generating the dual‐band topological edge and corner states compared with other photonic systems exhibiting single topological phases. From the fundamental and practical significances, the presented results will trigger the extensive investigations for exploring the photonic systems with multiple topological phases and find important practical applications.