Topological nature of non-Hermitian degenerate bands in structural parameter space

Band degeneracies at high-symmetry points in wavevector space have been shown to exhibit rich physical phenomena in photonics, but obtaining degenerate bands away from such points is highly nontrivial. This work demonstrates complex band degeneracy in a photonic crystal structure over a region of momentum space. The authors show that this band degeneracy corresponds to polarization-independent transmission and manifests as a topological singularity in the structural parameter space of the system. Such degeneracy can be harnessed for the design of nonlocal polarization-independent photonic structures.