Acoustic non-Hermitian skin effect from twisted winding topology (2104.08844v2)

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.

• The paper experimentally realizes the acoustic non-Hermitian skin effect in a 1D crystal with a novel twisted winding configuration.
• It uses non-reciprocal nearest- and next-nearest neighbor couplings to induce bipolar localization of bulk eigenstates.
• The study challenges traditional bulk-boundary correspondence and paves the way for applications in sensitive acoustic sensors and one-way waveguides.

Acoustic Non-Hermitian Skin Effect from Twisted Winding Topology

The work presented in "Acoustic non-Hermitian skin effect from twisted winding topology" addresses the experimental realization of the Non-Hermitian Skin Effect (NHSE) in a physical system, specifically within a one-dimensional (1D) non-reciprocal acoustic crystal. The research offers valuable insights into the implications of non-Hermitian band topology, particularly highlighting an instance of a twisted winding configuration.

Non-Hermitian Band Topology and Skin Effects

Within the field of non-Hermitian physics, NHSE denotes the localization of bulk eigenstates at a boundary upon the introduction of non-reciprocity or gain/loss mechanisms in the system, challenging the conventional bulk-boundary correspondence principle in Hermitian scenarios. The paper advances the understanding of NHSE by moving beyond the singular closed-loop topology observed thus far, to a more complex topology featuring twisted windings.

The twisted winding topology, as implemented in this work, comprises two oppositely oriented loops in contact, differing significantly from the traditional single-loop topology. This configuration leads to bipolar localization, where bulk states localize towards two directions simultaneously—a phenomenon previously uncharacterized in finite physical systems.

Experimental Implementation and Results

The researchers constructed a 1D acoustic crystal consisting of 20 resonators with non-reciprocal couplings. In establishing the baseline, non-Hermitian effects were initially demonstrated through nearest-neighbor couplings, which led to all eigenstates localizing at one boundary of the system. Moving to more intricate configurations, they introduced non-reciprocal couplings to the next-nearest neighbors, thus realizing the anticipated twisted topology.

Upon this structural alteration, the NHSE exhibited properties such as bipolar localization and the manifestation of a Bloch-wave-like extended state at the Bloch point. These phenomena were substantiated experimentally through field distribution measurements. The evidence indicated that the eigenstates bifurcated towards both boundaries depending on the operating frequency, conforming closely to theoretical expectations.

Implications and Future Directions

The implications of this work resonate both theoretically and practically. The realization of complex non-Hermitian winding topologies paves the way for exploring generalized topological principles in non-Hermitian systems, including potential extensions beyond 1D spaces. The framework set by this research could lead to practical applications like sensitive acoustic sensors and one-way waveguides, leveraging the unique wave propagation properties associated with NHSE.

In terms of future developments, the extension of this research to higher dimensions appears feasible and promising. By integrating the NHSE into higher-dimensional systems, researchers could uncover exotic physical phenomena characteristic of higher-order NHSE. Additionally, the universal nature of feedback mechanisms in non-Hermitian systems raises the potential for broader applications across both acoustics and electromagnetics.

In conclusion, the experimental observations of twisted non-Hermitian band topologies underscore a significant advancement in understanding NHSE. This work provides a tangible platform to explore unconventional topological windings and encourages further research into the theoretical frameworks governing non-Hermitian physics.