Second‐Order Photonic Topological Corner States in Square Lattices with Low Symmetry

Abstract

AbstractIn most cases, higher‐order topological insulating phases are observed in crystalline materials and photonic/phononic crystals with high crystalline symmetries. Few exceptions include amorphous and quasi‐crystalline matters, which have recently been demonstrated to exhibit such topological phases as well. This report reveals analytically and numerically that the photonic crystals with asymmetric sublattices can also exhibit second‐order topological insulating phases. The appearance of band inversion for the change of unit cell parameters, the quantized 2D polarizations and topological corner charges, and the generation of corner states explicitly reveal the existence of such second‐order photonic topological insulating phases. Quite interestingly, for arbitrarily shaped asymmetric unit cells, second‐order topological corner states are also observed, provided that the intercell coupling dominates over the intracell one.