Valley Polarized Edge States beyond Inversion Symmetry Breaking

Valley-contrast physics has gained considerable attention, particularly for realizing photonic topological insulators (PTIs) that support reflection-free valley-polarized edge modes (VPEMs) in the absence of inter-valley scattering. It is an open question whether similar robust states can exist in the absence of topological valley phase, i.e., nonvanishing Berry curvature at the valleys. It is shown that a C6υ-symmetric triangular photonic crystal (PhC) inherently exhibits uniform distribution of spatially varying phase vortices, which support a local (limited) version of valley Hall effect (LVHE), where the valley polarization is location defined as opposed to being fixed throughout the bulk. Then it is demonstrated that defect regions with sublattice asymmetry in otherwise a symmetric PhC lead to wave localization and splitting of photons according to their valley index, thus enabling VPEMs along a line defect waveguide. The device is fabricated on a silicon-on-insulator (SOI) slab, and it is characterized at near-infrared frequencies showing robust transmission through sharp bends comparable to valley PTIs. The results present a new perspective to creating valley edge states and outline a new waveguiding mechanism applicable to electromagnetics as well as plasmonics, mechanics, and acoustics.