  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, we demonstrate the existence of
second-order topological insulators 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. Our
results provide an alternative approach to explore higher-order topological
physics with significant potential for applications in integrated and quantum
photonics.
