# Observation of Photonic Topological Valley-Hall Edge States

## TL;DR

This paper experimentally demonstrates topological edge states in 2D honeycomb photonic lattices caused by the valley-Hall effect, revealing how domain wall orientation affects edge state properties and introducing a new approach for photonic topological insulators.

## Contribution

The study provides the first experimental observation of valley-Hall topological edge states in photonic systems with broken inversion symmetry, highlighting the impact of domain wall orientation.

## Key findings

- Armchair domain walls become gapped with detuning.
- Zig-zag domain walls remain ungapped until a cutoff.
- Valley-Hall effect enables time-reversal invariant photonic topological insulators.

## Abstract

We experimentally demonstrate topological edge states arising from the valley-Hall effect in twodimensional honeycomb photonic lattices with broken inversion symmetry. We break inversion symmetry by detuning the refractive indices of the two honeycomb sublattices, giving rise to a boron nitride-like band structure. The edge states therefore exist along the domain walls between regions of opposite valley Chern numbers. We probe both the armchair and zig-zag domain walls and show that the former become gapped for any detuning, whereas the latter remain ungapped until a cutoff is reached. The valley-Hall effect provides a new mechanism for the realization of time-reversal invariant photonic topological insulators.