Acoustic meta-atom with experimentally verified maximum Willis coupling

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.

Cloaking In-Plane Elastic Waves with Swiss Rolls

“…Our observation opens interesting avenues for the design of cylindrical elastodynamic cloaks via homogenization approaches combining recent findings in metamaterials displaying strong Willis coupling [12,13] and chiral elasticity features [8,11], as we shall see in the sequel. To exemplify the usefulness of the transformed Willis equation with non-fully symmetric elasticity tensors, we propose to design a microstructured cloak consisting of swiss-rolls displaying the usual features encountered in both Cosserat and Willis media in the low frequency limit.…”

Symmetry breaking creates electro-momentum coupling in piezoelectric metamaterials

“…Therefore, they capture a new mechanism that can be employed for rectifying mechanical waves by modulation of external stimuli. We expect that future studies will show that extraordinary wave response such as asymmetric reflections and unidirectional transmission are modeled by the new couplings [42], analogously to Willis coupling [28,[32][33][34]. These kind of results are to be guided by theoretical analyses based upon proper homogenization schemes [25].…”

“…A series of theoretical studies were carried out to characterize Willis coupling and understand its physical origins [18][19][20][21][22][23][24][25][26]. Guided by accompanying predictions that Willis coupling is connected with unusual phenomena such as asymmetric reflections and unidirectional transmission, recent experimental realizations of Willis metamaterials that demonstrate these phenomena were reported [27][28][29][30][31][32][33][34][35]. To date, these investigations were limited to metamaterials that are deformable only by mechanical forces.…”

Recent Advances in Non-Traditional Elastic Wave Manipulation by Macroscopic Artificial Structures

“…Several approximations and simplifications to the general approach of Willis media have been published [117][118][119][120][121][122][123]. In fact, most studies of Willis coupling are more active in the acoustics [124][125][126][127][128][129][130] than in elastodynamics, but these developments in acoustics indicate that Willis metamaterial for elastic waves in solid may become practical. Recently, Willis metamaterial for flexural waves has been reported [131].…”