# Broadband localization of light at the termination of a topological photonic waveguide

## TL;DR

Researchers showed how light can be strongly localized at the end of a special waveguide, enabling better control and sensing of light.

## Contribution

A new mechanism for broadband light localization using symmetry-protected valley states in topological photonic waveguides.

## Key findings

- Localized light at waveguide termination through suppressed backscattering.
- Symmetry conservation prevents intervalley scattering at discontinuities.
- Broadband effect confirmed via near-field microscopy and geometry comparisons.

## Abstract

Localized optical field enhancement enables strong light-matter interactions necessary for efficient manipulation and sensing of light. Specifically, tunable broadband energy localization in nanoscale hotspots offers many applications in nanophotonics and quantum optics. We experimentally demonstrate a mechanism for the local enhancement of electromagnetic fields based on strong suppression of backscattering. This is achieved at a designed termination of a topologically nontrivial waveguide that nearly preserves the valley degree of freedom. The symmetry origin of the valley degree of freedom prevents edge states to undergo intervalley scattering at waveguide discontinuities that obey the symmetry of the crystal. Using near-field microscopy, we reveal that this leads to strong confinement of light at the termination of a topological photonic waveguide, even without breaking reciprocity. We emphasize the importance of symmetry conservation by comparing different waveguide termination geometries, confirming that the origin of suppressed backscattering lies with the near conservation of the valley degree of freedom, and show the broad bandwidth of the effect.

Light is observed to localize and enhance at the termination of a topological photonic waveguide due to suppressed backscattering.