Unidirectional exceptional point of reflectionless states in a magnonic mirror array

TL;DR

This paper demonstrates a unidirectional reflectionless exceptional point in a magnonic mirror array, enabling control over collective coherence and broadband unidirectional devices through engineered non-Hermitian physics.

Contribution

It is the first experimental realization of a unidirectional reflectionless exceptional point in a magnonic system by breaking inversion symmetry with a spin ensemble.

Findings

Reflection spectrum flattens and broadens at the RL EP

Dark-state behaviors are observed in spectral valleys

Unidirectional reflectionless states enable broadband control

Abstract

Exceptional points (EPs) in non-Hermitian systems are singularities where both eigenvalues and eigenvectors coalesce. In scattering systems, EPs correspond to the merging of scattering states, leading to reflectionless (RL) behavior. A reflectionless exceptional point (RL EP) arises when two RL states further coalesce, yielding an anomalous quartic spectral response. While RL EPs have been explored in bidirectional systems, their unidirectional realization remains elusive. Here, we experimentally demonstrate a unidirectional RL EP by engineering collective states in an anti-Bragg magnonic mirror array. Inversion symmetry is broken using a giant spin ensemble that couples to a waveguide at three spatially separated points, enabling unidirectional reflectionless. At the RL EP, the reflection spectrum flattens and broadens significantly beyond the Lorentzian profile. The observed spectral valleys also expose dark-state behaviors that are typically inaccessible through conventional measurements. Our results provide a route toward controlling collective coherence in open systems and developing broadband unidirectional devices.