Bound band in the continuum from merging of topological edge states and magnetic interface states

The bandwidth of topological edge states (TESs) or other waveguides in photonic crystals (PCs) are always limited by the width of bandgap. In order to break this limitation, we propose a new concept of bound bands in the continuum. Through constructing a hexagonal lattice of gyromagnetic rods and optimizing the structure parameters, we find that the TESs and magnetic interface states (MISs) can be merged to form a unique bound band. The bound band can pass through the bulk bands from the bandgap and keep perfect localization field at the interface without hybridizing with bulk states even though they are degenerated at the same frequencies.