Soft 3D acoustic metamaterial with negative index

• Published in: Nature materials Vol. 14; no. 4; pp. 384 - 388
• Main Authors: Brunet, Thomas, Merlin, Aurore, Mascaro, Benoit, Zimny, Kevin, Leng, Jacques, Poncelet, Olivier, Aristégui, Christophe, Mondain-Monval, Olivier
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.04.2015; Nature Publishing Group
• Subjects: 639/301/923; Acoustics; Biomaterials; Condensed Matter Physics; Design engineering; Electromagnetism; Gaussian; letter; Materials Science; Mathematical analysis; Metamaterials; Nanoparticles; Nanotechnology; Optical and Electronic Materials; Physics; Porous materials; Three dimensional; Transformations; Acoustic Metamaterials
• ISSN: 1476-1122; 1476-4660
• DOI: 10.1038/nmat4164

Soft acoustic metamaterials that consist of a concentrated suspension of macroporous microbeads and that show a broadband negative acoustic refractive index are now demonstrated. Many efforts have been devoted to the design and achievement of negative-refractive-index metamaterials since the 2000s 1 , 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 indices 9 . 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.