# Extremely asymmetrical acoustic metasurface mirror at the exceptional point

## TL;DR

This paper introduces a non-Hermitian acoustic metasurface mirror that exploits loss to achieve highly asymmetrical reflection, including retro-reflection from one side and absorption from the other, opening new avenues for wave control.

## Contribution

It demonstrates, both theoretically and experimentally, a novel non-Hermitian acoustic metasurface at the exceptional point with asymmetric reflection properties.

## Key findings

- Achieves high-efficiency retro-reflection from one incident side.
- Realizes complete absorption when waves come from the opposite side.
- Unveils new wave manipulation mechanisms using loss in metasurfaces.

## Abstract

Previous research has attempted to minimize the influence of loss in reflection- and transmission-type acoustic metasurfaces. This letter shows that, by treating the acoustic metasurface as a non-Hermitian system and by harnessing loss, unconventional wave behaviors that do not exist in lossless metasurfaces can be uncovered. Specifically, we theoretically and experimentally demonstrate a non-Hermitian acoustic metasurface mirror featuring extremely asymmetrical reflection at the exception point. As an example, the metasurface mirror is designed to have high-efficiency retro-reflection when the wave incidents from one side and complete absorption when the wave incidents from the other side. This work marries conventional gradient index metasurfaces with the exception point from non-Hermitian systems, and paves the way for identifying new mechanisms and functionalities for wave manipulation.