Topological local-valley interface in ordinary photonic crystal waveguide

Valley-contrasting physics is gaining growing attraction for its potentials as a promising information carrier in electric and classical systems. In this work, we present a new realization of topological edge states based on locally defined valley-Hall effect, which enables direction-locked polarizations and robust transmission through sharp corners. Unlike existing photonic topological insulators (PTIs), the proposed implementation does not strictly demand distinct claddings (domains) across the waveguide interface. In fact, the interfaced bulks (slabs) are identical -away from the interface- and have a triangular lattice of circular air holes as in common photonic crystal (PhC) waveguides. Here, the interface is defined along a one-dimensional line created by opposite shifts of the centers of the respective unit cells on the two sides, which result in a glide-symmetric holes arrangement. The region near the interface locally lacks mirror inversion symmetry, and electromagnetic fields exhibit opposite orbital-angular-momentum states on the two sides of the interface. These observations are known to be responsible for the distinct topological phases in typical valley PTIs. In our proposed structure, it is clear that the topological modes can be understood as a result of discontinuity in the fields at the interface. Intuitively, one may also regard the proposed structure as a typical valley PhC, which is based on graphene-like lattice but with either A or B site being vanishingly small (i.e. missing). Advantageously, our proposed approach can enable robust waveguiding in a simpler implementation than current PTI designs, thus benefiting practical applications and fabrication at optical frequencies.