# Topologically-protected refraction of robust kink states in valley photonic crystals

## TL;DR

This paper experimentally demonstrates topologically protected kink states in valley photonic crystals that are robust against scattering, support polarization multiplexing, and efficiently couple to free space, advancing topological photonics for optical communications.

## Contribution

It presents the first experimental demonstration of meron-like valley-projected topological phases with high out-coupling efficiency in valley photonic crystals.

## Key findings

- Kink states are robust and support polarization multiplexing.
- Propagation direction is locked to valley indices, preventing reversal without inter-valley scattering.
- Achieved >97% out-coupling efficiency into free-space beams.

## Abstract

Recently discovered valley photonic crystals (VPCs) mimic many of the unusual properties of two-dimensional gapped valleytronic materials such as bilayer graphene or MoS2. Of the utmost interest to optical communications is their ability to support topologically protected chiral edge (kink) states at the internal domain wall between two VPCs with spectrally overlapping bandgap zones and opposite half-integer valley-Chern indices. We experimentally demonstrate the robustness of the kink states in VPCs that support degenerate transverse-electric-like (TE) and transverse-magnetic-like (TM) topological phases, thus enabling polarization multiplexing in a single topological waveguide. The propagation direction of the kink states is locked to the valleys of the reverse Brave lattice and, therefore, cannot be reversed in the absence of inter-valley scattering. At the intersection between the internal domain wall and the external edge separating the VPCs from free space, the kink states are shown to exhibit >97% out-coupling efficiency into directional free-space beams. This constitutes the first experimental demonstration of meron-like valley-projected topological phases with half-integer valley-Chern indices.

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Source: https://tomesphere.com/paper/1706.04731