Acoustic meta-atom with experimentally verified maximum Willis coupling

Anton Melnikov,Steffen Marburg,Yan Kei Chiang,Li Quan,David Powell,Sebastian Oberst,Andrea Alù

doi:10.1038/s41467-019-10915-5

Abstract

Acoustic metamaterials are structures with exotic acoustic properties, with promising applications in acoustic beam steering, focusing, impedance matching, absorption and isolation. Recent work has shown that the efficiency of many acoustic metamaterials can be enhanced by controlling an additional parameter known as Willis coupling, which is analogous to bianisotropy in electromagnetic metamaterials. The magnitude of Willis coupling in a passive acoustic meta-atom has been shown theoretically to have an upper limit, however the feasibility of reaching this limit has not been experimentally investigated. Here we introduce a meta-atom with Willis coupling which closely approaches this theoretical limit, that is much simpler and less prone to thermo-viscous losses than previously reported structures. We perform two-dimensional experiments to measure the strong Willis coupling, supported by numerical calculations. Our meta-atom geometry is readily modeled analytically, enabling the strength of Willis coupling and its peak frequency to be easily controlled.

Highlights

Acoustic metamaterials are structures with exotic acoustic properties, with promising applications in acoustic beam steering, focusing, impedance matching, absorption and isolation
Recent work has demonstrated that the incorporation of Willis coupling or bianisotropy into metamaterial structures of subwavelength thickness, known as metasurfaces, can improve their efficiency when refracting at large angles[20,21,22]
To resolve this question we propose an acoustic meta-atom, which is designed to realize maximum Willis coupling and to minimize thermo-viscous losses

Summary

Introduction

Acoustic metamaterials are structures with exotic acoustic properties, with promising applications in acoustic beam steering, focusing, impedance matching, absorption and isolation. A bound on the maximum value of the Willis coupling parameter was derived, based on the conservation of energy[21] It was shown how meta-atoms can be designed to reach this theoretical bound, using space-coiling structures with long meander-line channels[21,24,25,26]. The theoretical limit on the strength of Willis coupling has not been tested experimentally, and it remains unknown how closely this limit may be approached in practice To resolve this question we propose an acoustic meta-atom, which is designed to realize maximum Willis coupling and to minimize thermo-viscous losses. The simplicity of the structure enables us to present an analytical model for its polarizability, showing how the Willis coupling can be tailored to have any value between zero and the theoretical bound

Methods

Results

Conclusion