By incorporating novel subwavelength structures into macroscopic materials, acousticians can create devices with exotic sound-altering properties.

Recent research has suggested the possibility of creating acoustic cloaks using metamaterial layers to eliminate the acoustic field scattered from an elastic object. This paper explores the possibility of applying the scattering cancellation cloaking technique to acoustic waves and the use of this method to investigate its effectiveness in cloaking elastic and fluid spheres using only a single isotropic elastic layer. Parametric studies showing the influence of cloak stiffness and geometry on the frequency dependent scattering cross-section of spheres have been developed to explore the design space of the cloaking layer. This analysis shows that an appropriately designed single isotropic elastic cloaking layer can provide up to 30 dB of scattering reduction for ka values up to 1.6. This work also illustrates the importance of accounting for the elasticity of the object and the relevant limitations of simplistic quasi-static analyses proposed in recent papers.

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Vortex waves, which carry orbital angular momentum, have found use in a range of fields from quantum communications to particle manipulation. Due to their widespread influence, significant attention has been paid to the methods by which vortex waves are generated. For example, active phased arrays generate diverse vortex modes at the cost of electronic complexity and power consumption [1][2][3][4] . Conversely, analog apertures, such as spiral phase plates 1,5 , metasurfaces 6 , and gratings 7 require separate apertures to generate each mode. Here we present a new class of metamaterial-based acoustic vortex generators, which are both geometrically and electronically simple, and topologically tunable. Our metamaterial approach generates vortex waves by wrapping an acoustic leaky wave antenna 8 back upon itself. Exploiting the antennas frequency-varying refractive index, we demonstrate experimentally and analytically that this analog structure generates both integer, and noninteger vortex modes. The metamaterial design makes the aperture compact and can thus be integrated into high-density systems.The total angular momentum of a system can be divided into two components, spin angular momentum, and orbital angular momentum (OAM). Although acoustic waves do not possess spin angular momentum they have been shown to carry OAM 1,2 . A drawing of a single mode helical wave with value L = −2 is shown in Fig. 1(a) where L is the OAM topological mode number. A wave with OAM index L = 0 describes a system with no helical phase front. The phase front of the propagating wave is a corkscrew-type phase advance with the sign of the topological charge positive, for clockwise rotation, or negative, for counter-clockwise rotation. These vortex waves have been found to be useful in an extremely diverse range of applications from communications 6,9-13 and imaging 14-16 to particle manipulation 17-19 over a wide range of length scales. In the most widely examined application, vortex waves have been harnessed for use in electromagnetic and quantum 2 communications.The importance of both topological diversity, and aperture simplicity in vortex mode generation becomes immediately evident when considering the applications of vortex waves. Inspired by recent research on acoustic leaky-wave antennas 8,23 , we present an air-acoustic vortex beam emitter which generates topologically diverse vortex waves using a single transducer coupled to a single analog metamaterial aperture. A leaky wave antenna is a device comprised of a one-or two-dimensional waveguide which leaks power along it's length with either a continuous leaking slot or sub-wavelength radiating shunts. Leaky wave antennas rely on geometry-controlled dispersion to tune the refractive index of the fluid inside the waveguide. The leaked energy then refracts from the antenna at an angle θ(ω), similar to the refraction mechanism of a prism. The value of θ(ω) is determined by the ratio of 3 wavenumber β(ω) inside the waveguide to the wavenumber in the surrounding area kAlthough not in...