# Topologically Protected Photonic Modes in Composite Quantum Hall/Quantum Spin Hall Waveguides

## TL;DR

This paper demonstrates an experimental photonic system combining quantum Hall and quantum spin-Hall topological phases, supporting reflection-free modes that enable novel, backscattering-immune optical devices with complex routing capabilities.

## Contribution

It introduces a composite topological photonic waveguide system that unifies QH and QSH phases, enabling reflectionless modes and novel optical device functionalities.

## Key findings

- Supported reflection-free edge modes in composite QH-QSH waveguides
- Demonstrated a 2-port optical isolator and a 4-port circulator
- Showed backscattering-immune light propagation around sharp corners

## Abstract

Photonic topological systems, the electromagnetic analog of the topological materials in condensed matter physics, create many opportunities to design optical devices with novel properties. We present an experimental realization of the bi-anisotropic meta waveguide photonic system replicating both quantum Hall (QH) and quantum spin-Hall (QSH) topological insulating phases. With careful design, a composite QH-QSH photonic topological material is created and experimentally shown to support reflection-free edgemodes, a heterogeneous topological structure that is unprecedented in condensed matter physics. The effective spin degree of freedom of such topologically protected modes determines their unique pathways through these systems, free from backscattering and able to travel around sharp corners. {As an example of their novel properties, we experimentally demonstrate reflection-less photonic devices including a 2-port isolator, a unique 3-port topological device, and a full 4-port circulator based on composite QH and QSH structures