Pair-partitioned bulk localized states induced by topological band inversion

Abstract

Photonic topological insulators have recently received widespread attention mainly due to their ability to provide directions in the development of photonic integration platforms. The proposal for a topological bulk cavity with a single-mode expands upon previous research works on topological cavities; thus, interest in topological edge states and corner states is beginning to shift into analysis on bulk properties and their applications. However, there remains a gap in research on a multi-mode cavity of the topological photonic crystals (PCs). In this Letter, a cavity of the topological PCs is proposed involving pair-partitioned bulk localized states (BLSs) from a two-dimensional inner and outer nested square lattice (2D IONSL), which can enable a multi-mode cavity for the topological PCs. First, the topological characteristics are described in terms of a Zak phase, and band inversions are achieved by changing the size of scatterers in the inner and outer circles that reside within the unit cell. Afterwards, analogous to the tight-binding model for electronic systems, the Hamiltonian and topological phase transition conditions of 2D IONSL PCs are derived. Furthermore, it is proposed that the demonstrated optical field reflection and confinement mechanism induced by topological band inversions due to the opposite parities of wavefunctions may lead to the phenomenon of pair-partitioned BLSs. This research increases the research works of bulk topological effects, creating a route for photonic integration platforms for near-infrared.