Soft 3D acoustic metamaterial with negative index



Many efforts have been devoted to the design and achievement of negative-refractive-index metamaterials since the 2000s1,2,3,4,5,6,7,8. One of the challenges at present is to extend that field beyond electromagnetism by realizing three-dimensional (3D) media with negative acoustic indices9. We report a new class of locally resonant ultrasonic metafluids consisting of a concentrated suspension of macroporous microbeads engineered using soft-matter techniques. The propagation of Gaussian pulses within these random distributions of ‘ultra-slow’ Mie resonators is investigated through in situ ultrasonic experiments. The real part of the acoustic index is shown to be negative (up to almost − 1) over broad frequency bandwidths, depending on the volume fraction of the microbeads as predicted by multiple-scattering calculations. These soft 3D acoustic metamaterials open the way for key applications such as sub-wavelength imaging and transformation acoustics, which require the production of acoustic devices with negative or zero-valued indices.

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Figure 1: Soft 3D acoustic metamaterials composed of ultra-slow Mie resonators.
Figure 2: Phase delays of the transmitted signals as a function of propagation distance.
Figure 3: Comparisons between acoustical experiments and theoretical predictions.


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This work was supported by the Agence Nationale pour la Recherche (Grant 2011-BS0902101 Metakoustik-Aerospace Valley) and the US Air Force European Office of Aerospace Research and Development (Grant FA8655-12-1-2067). This work was performed under the auspices of the Labex AMADEUS ANR-10-LABX-0042-AMADEUS with the help of French state Initiative d’Excellence IdEx ANR-10-IDEX-003-02. We thank G. Pibre from Bluestar Silicones for fruitful advice and providing us with the silicone rubber, and J. Valencony (Siltech Corp.) for the surfactants.

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T.B., O.M-M., J.L., O.P. and C.A. designed the project. A.M. synthesized the macroporous soft silicone rubber microbeads with the help of K.Z. Formulation aspects were supervised by O.M-M., A.M. set up the microfluidics with the advice of J.L., T.B. and B.M. conducted the acoustical measurements and performed the calculations under the guidance of C.A. and O.P. for the theoretical aspects. T.B. wrote the paper in collaboration with C.A., O.P., J.L. and O.M-M.

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Correspondence to Thomas Brunet.

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The authors declare no competing financial interests.

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Brunet, T., Merlin, A., Mascaro, B. et al. Soft 3D acoustic metamaterial with negative index. Nature Mater 14, 384–388 (2015).

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