Generation of topologically diverse acoustic vortex beams using a compact metamaterial aperture

Abstract

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...

High-speed acoustic communication by multiplexing orbital angular momentum

“…For acoustics, the underwater propagation of vortex beams with single topological charge was demonstrated with active phase arrays (20,21). Passive acoustic phase modulation structures were proposed to generate single-charge vortex beams (22)(23)(24)(25). These acoustic vortex beams were used to develop acoustic tweezers, and screwdrivers for particle trapping, levitation, and manipulations (26)(27)(28)(29)(30).…”

Superresolution using an acoustic vortex wave antenna

“…In this case, the waveguide is curved into an annulus shape, with the sound entering into the annulus from a source, propagating around the circular path, and then exiting the annulus, similar to the arrangement examined in Ref. 61. Although the physical structure consists of circular annulus, a spacer at θ = 0 prevents the interaction of sound within the waveguide between the signals entering and exiting the annulus.…”

“…While extensively studied for microwaves 46,47 and more recently using metamaterials [48][49][50][51][52][53][54][55][56] , optical LWAs present various challenges which are the subject of ongoing research 47,57 . In addition to electromagnetic (EM) waves, acoustic LWAs have recently been demonstrated to generate one-dimensional (1D) axisymmetric planar acoustic waves 58,59 , two-dimensional (2D) planar waves 60 and topologically diverse circular acoustic vortex waves 61 . A detailed theoretical formulation for an acoustic antenna utilizing these recent advances with acoustic leaky-wave antennas is presented in Sec.…”

Highly Efficient Generation of Angular Momentum with Cylindrical Bianisotropic Metasurfaces

“…Recent research has also demonstrated the manipulation of beams for particle trapping [23,24] and boosting communication efficiency [25,26] with acoustic angular momentum. Passive generation of wave fields with non-zero angular momentum is typically imple-mented by leaky wave antennas or metasurfaces based on generalized Snell's law (GSL) [27,28] for acoustic waves and inhomogeneous anisotropic media [29], spatial light modulator or spiral phase plates [30,31] for electromagnetic waves. However, the recent advances in metasurfaces for perfect wavefront manipulation show that if only the transmission phase profile is controlled, parasitic scattering inevitably appears, which reduces the efficiency, or even cause the failure to the structures, especially for large angular momentum.…”