Soft 3D acoustic metamaterial with negative index

Abstract

Many efforts have been devoted to the design and achievement of negative-refractive-index metamaterials since the 2000s. One of the challenges at present is to extend that field beyond electromagnetism by realizing three-dimensional (3D) media with negative acoustic indices. 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.

Vibrant times for mechanical metamaterials

“…Corresponding experiments have used rubber spheres suspended in water [24] (taking advantage of the high-velocity contrast permitting to omit shear contributions) or silicone rubber embedded in a water-based gel host, leading to a negative index of refraction. [25] Other publications predict a negative bulk modulus transition induced by interplay of different force potentials. [26] Furthermore, a chiral route toward negative elastic refraction has lately been discussed.…”

Pillar-type acoustic metasurface

“…In both systems, the negative compressibility resulted from a monopolar resonance of the rubber spheres, whereas a dipolar resonance yielded to the negative mass density. Very recently, negative density in a liquid foam 4 and negative acoustic refractive index in Mie resonators made of random suspension of soft silicone rubber micro-beads 5,6 have been demonstrated. Both effective mass density and effective bulk modulus become negative when the resonators are in simultaneous dipolar and monopolar motions, out of phase with respect to the waves propagating in the background.…”

Microlattice Metamaterials for Tailoring Ultrasonic Transmission with Elastoacoustic Hybridization

“…Recent approaches to control the propagation of high-frequency ultrasound led to the development of soft acoustic metamaterials [16][17][18][19]. These materials exploit subwavelength resonators that are embedded in an acoustic host fluid [16,17,20,21].…”

“…These materials exploit subwavelength resonators that are embedded in an acoustic host fluid [16,17,20,21]. Resonators can be formed from bubbles that show Minnaert resonances [20,21], low-density particles exploiting low-frequency acoustic Mie resonances [16], or thin films in liquid foams [19].…”