# Transport tuning of photonic topological edge states by optical cavities

## TL;DR

This paper explores how optical cavities can tune the transport properties of photonic topological edge states in waveguides, demonstrating control over reflection and transmission spectra through cavity design.

## Contribution

It introduces a theoretical framework for understanding PTES transport modulation via side optical cavities with different mode degeneracies.

## Key findings

- Single-mode cavities induce perfect reflection at resonance.
- Twofold degenerate cavities produce Fano-shaped transmission spectra.
- The coupling strength affects the width of the transmission dip.

## Abstract

Crystal-symmetry-protected photonic topological edge states (PTESs) based on air rods in conventional dielectric materials are designed as photonic topological waveguides (PTWs) coupled with side optical cavities. We demonstrate that the cavity coupled with the PTW can change the reflection-free transport of the PTESs, where the cavities with single mode and twofold degenerate modes are taken as examples. The single-mode cavities are able to perfectly reflect the PTESs at their resonant frequencies, forming a dip in the transmission spectra. The dip full width at half depth depends on the coupling strength between the cavity and PTW and thus on the cavity geometry and distance relative to the PTW. While the cavities with twofold degenerate modes lead to a more complex PTES transport whose transmission spectra can be in the Fano form. These effects well agree with the one-dimensional PTW-cavity transport theory we build, in which the coupling of the PTW with cavity is taken as $\delta$ or non-$\delta$ type. Such PTWs coupled with side cavities, combining topological properties and convenient tunability, have wide diversities for topological photonic devices.