Experimental realization of a Weyl exceptional ring

TL;DR

This paper reports the first experimental observation of a Weyl exceptional ring in a non-Hermitian photonic system, demonstrating how non-Hermitian effects alter topological properties of Weyl points.

Contribution

It provides the first real-space measurement of a Weyl exceptional ring in an optical waveguide array, confirming theoretical predictions about non-Hermitian topological phenomena.

Findings

Observation of a Weyl exceptional ring in photonics

Altered Fermi arc surface states due to non-Hermitian effects

Changes in bulk diffraction and power ratios linked to the ring

Abstract

Weyl points are isolated degeneracies in reciprocal space that are monopoles of the Berry curvature. This topological charge makes them inherently robust to Hermitian perturbations of the system. However, non-Hermitian effects, usually inaccessible in condensed matter systems, are an important feature of photonics systems, and when added to an otherwise Hermitian Weyl material have been predicted to spread the Berry charge of the Weyl point out onto a ring of exceptional points, creating a Weyl exceptional ring and fundamentally altering its properties. Here, we observe the implications of the Weyl exceptional ring using real-space measurements of an evanescently-coupled bipartite optical waveguide array by probing its effects on the Fermi arc surface states, the bulk diffraction properties, and the output power ratio of the two constituent sublattices. This is the first realization of an object with topological Berry charge in a non-Hermitian system.