# Corner states of light in photonic waveguides

## TL;DR

This paper demonstrates the experimental realization of corner states of light in 3D photonic structures, showcasing their robustness and controllability, and highlighting potential for advanced photonic devices.

## Contribution

It introduces a novel method using femtosecond laser inscription to create and analyze corner states in 3D photonic lattices, advancing topological photonics.

## Key findings

- Corner states are localized at the corners of photonic structures.
- Light can be fractionalized, localizing at multiple corners simultaneously.
- The method allows for flexible and controlled realization of topological states.

## Abstract

The recently established paradigm of higher-order topological states of matter has shown that not only, as previously thought, edge and surface states but also states localised to corners can have robust and exotic properties. Here we report on the experimental realisation of novel corner states made out of classical light in three-dimensional photonic structures inscribed in glass samples using femtosecond (fs) laser technology. By creating and analysing waveguide arrays forming two-dimensional breathing kagome lattices in various sample geometries, we establish this as a platform for corner states exhibiting a remarkable degree of flexibility and control. In each sample geometry we measure eigenmodes that are localised at the corners in a finite frequency range in complete analogy with a theoretical model of the breathing kagome. Here, the measurements reveal that light can be "fractionalised", corresponding to simultaneous localisation to each corner of a triangular sample, even in the presence of defects. The fabrication method applied in this work exposes the advantage of using fs-laser writing for producing compact three-dimensional devices thus paving the way for technological applications by simulating novel higher-order states of matter.