Non-Reciprocal Willis Coupling in Zero-Index Moving Media

TL;DR

This paper demonstrates that combining zero-index acoustic media with modest mechanical motion induces extreme non-reciprocity and bianisotropy, enabling novel non-reciprocal acoustic devices like directional lenses.

Contribution

The study introduces a metamaterial with asymmetric Willis coupling that achieves significant non-reciprocity at low speeds, a novel approach in acoustic wave control.

Findings

Achieved opposite signs of refractive index for opposite directions.

Demonstrated a non-reciprocal acoustic lens with directional focusing.

Revealed the role of asymmetric Willis coupling in non-reciprocal media.

Abstract

Mechanical motion can break the symmetry in which sound travels in a medium, but significant non-reciprocity is typically achieved only for very large motion speeds. Here we combine moving media with zero-index acoustic propagation, yielding extreme non-reciprocity and induced bianisotropy for modest applied speeds. The metamaterial is formed by an array of waveguides loaded by Helmholtz resonators, and it exhibits opposite signs of the refractive index sustained by asymmetric Willis coupling for propagation in opposite directions. We use this response to design a non-reciprocal acoustic lens focusing only when excitation from one side, with applications for imaging and ultrasound technology.