Topological Corner Modes by Composite Wannier States in Glide-Symmetric Photonic Crystal

Second-order topological insulators can be characterized by their bulk polarization, which is believed to be intrinsically connected to the center of the Wannier function. In this study, the existence of second-order topological insulators is demonstrated that feature a pair of partially degenerate photonic bands. These arise from the nonsymmorphic glide symmetry in an all-dielectric photonic crystal. The center of the maximally localized Wannier function (MLWF) is consistently located at the origin but is not equivalent with respect to the sum of constituent polarizations. As a result, topological corner modes can be identified by the distinctly hybridized MLWFs that truncate at the sample boundary. Through full-wave numerical simulations paired with microwave experiments, the second-order topology is clearly confirmed and characterized. These topological corner states exhibit notably unique modal symmetries, which are made possible by the inversion of the Wannier bands. These results provide an alternative approach to explore higher-order topological physics with significant potential for applications in integrated and quantum photonics. For entangled bands induced by nonsymmorphic glide symmetry, the scheme by exploring the hybridization pattern of the maximally localized Wannier function, i.e., the Wannier band-resolved polarization components, serves as a finer second-order topological identifications and classifications that captures the observable and distinct corner states that arise from the domain interface. image