Acoustic flat lensing using an indefinite medium

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New routes for studying, designing and characterizing future acoustic metamaterials

Acoustic metamaterials enhance the control of sound wave propagation thanks to specific effective properties (density and compressibility for acoustics). Alike their electromagnetic counterparts, they gain these macroscopic properties from the periodic arrangement of sub-wavelength inclusions. In this work, we present the experimental demonstration of 3D focusing of an acoustic source thanks to the strong anisotropic effective properties of a phononic crystal. These properties are obtained after a careful study of wave propagation in a periodic medium approximated by a spring-mass model. The structure obtained after the theoretical study is a three-dimensions network of four hundred 3D-printed polymer spheres.
The experimental setup developed uses acoustic bursts with a central frequency of 10.6 kHz emitted by a small source located at different locations. For each of these locations, the phononic crystal is able to produce distinct focal points that are imaged by a non-invasive probing of the acoustic pressure based on optical feedback interferometry. These unique observations validate the ability of the spring-mass model to predict accurately the propagation of the acoustic wave-front through the designed structure. These findings open new routes for studying, designing and characterizing future acoustic metamaterials.

Figure Caption : (left) Equifrequency surfaces obtained at 10.6 kHz with discrete and continuous modeling of the structure. (right) Experimental measurements of the acoustic wave focusing for different source positions

These results have been obtained by a collaboration between researchers from Institut Fresnel, Imperial College of London, LAAS (CNRS, Université de Toulouse, INP, Toulouse, France), FEMTO-ST (CNRS/Université de Technologie Belfort-Montbéliard/Université de Franche-Comté/ENSMM), IRPHE (CNRS/École Centrale de Marseille/Aix-Marseille Université), University of Queensland (Australia) and Multiwave Technologies (Start-up company).

 The results have been published and highlighted by editors in the Physical Review B – Rapid Communication journal.

 They have also been highlighted on the Institut des Sciences et de l’Ingénierie des Systems (CNRS) website.

Reference : M. Dubois, J. Perchoux, A. L. Vanel, C. Tronche, Y. Achaoui, G. Dupont, K. Bertling, A. D. Rakić, T. Antonakakis, S. Enoch, R. Abdeddaim, R. V. Craster, and S. Guenneau, Acoustic flat lensing using an indefinite medium, Phys. Rev. B 99, 100301(R) (2019)

Contact : Marc Dubois