Acoustic non-Hermitian skin effect from twisted winding topology

TL;DR

This paper experimentally demonstrates how twisted winding topology in non-Hermitian band structures leads to unique acoustic skin effects, revealing new physical phenomena in non-Hermitian topological systems.

Contribution

It reports the first experimental realization of NHSE with twisted winding topology in a 1D non-reciprocal acoustic crystal, uncovering novel features of NHSE.

Findings

Demonstrated bipolar localization of acoustic modes

Observed Bloch point phenomena in acoustic systems

Linked complex winding topology to physical NHSE features

Abstract

The recently discovered non-Hermitian skin effect (NHSE) manifests the breakdown of current classification of topological phases in energy-nonconservative systems, and necessitates the introduction of non-Hermitian band topology. So far, all NHSE observations are based on one type of non-Hermitian band topology, in which the complex energy spectrum winds along a closed loop. As recently characterized along a synthetic dimension on a photonic platform, non-Hermitian band topology can exhibit almost arbitrary windings in momentum space, but their actual phenomena in real physical systems remain unclear. Here, we report the experimental realization of NHSE in a one-dimensional (1D) non-reciprocal acoustic crystal. With direct acoustic measurement, we demonstrate that a twisted winding, whose topology consists of two oppositely oriented loops in contact rather than a single loop, will dramatically change the NHSE, following previous predictions of unique features such as the bipolar localization and the Bloch point for a Bloch-wave-like extended state. This work reveals previously unnoticed features of NHSE, and provides the observation of physical phenomena originating from complex non-Hermitian winding topology.