Valley polarized edge states beyond inversion symmetry breaking

TL;DR

This paper demonstrates that valley-polarized edge states can exist without topological valley phases, using a symmetric photonic crystal with defect-induced waveguides, enabling robust, reflection-free light propagation.

Contribution

It introduces a new mechanism for valley edge states based on local valley Hall effects in symmetric photonic crystals, independent of topological valley phases.

Findings

Demonstrated wave localization and photon splitting by valley index in defect regions.

Fabricated silicon photonic devices showing robust transmission through sharp bends.

Established a new waveguiding mechanism applicable across various wave-based systems.

Abstract

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. We show that a C6\u{psion}-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. We then demonstrate 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. We fabricate our device on a silicon-on-insulator (SOI) slab and characterize it at near-infrared frequencies showing robust transmission through sharp bends comparable to valley PTIs. Our 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.