Soft 3D acoustic metamaterial with negative index

Many efforts have been devoted to the design and achievement of negative-refractive-index metamaterials since the 2000s1-8. 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 newclass 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.