Mechanical Evidence of the Orbital Angular Momentum to Energy Ratio of Vortex Beams

Abstract

We measure, in a single experiment, both the radiation pressure and the torque due to a wide variety of propagating acoustic vortex beams. The results validate, for the first time directly, the theoretically predicted ratio of the orbital angular momentum to linear momentum in a propagating beam. We experimentally determine this ratio using simultaneous measurements of both the levitation force and the torque on an acoustic absorber exerted by a broad range of helical ultrasonic beams produced by a 1000-element matrix transducer array. In general, beams with helical phase fronts have been shown to contain orbital angular momentum as the result of the azimuthal component of the Poynting vector around the propagation axis. Theory predicts that for both optical and acoustic helical beams the ratio of the angular momentum current of the beam to the power should be given by the ratio of the beam's topological charge to its angular frequency. This direct experimental observation that the ratio of the torque to power does convincingly match the expected value (given by the topological charge to angular frequency ratio of the beam) is a fundamental result.

High-speed acoustic communication by multiplexing orbital angular momentum

“…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). However, information encoding through multiple OAM channels multiplexing/demultiplexing remains unexplored.…”

Dislocations in Magnetohydrodynamic Waves in a Stellar Atmosphere

“…It is tempting to think that edge dislocations in solar waves can also be a tracer of regions with high vorticity along the wave path and use them to determine the vorticity in the solar atmosphere. Finally, it is also known that vortex dislocations carry torque and that this torque can be transferred to an absorbing medium [16][17][18][19]. In the solar atmosphere the torque that might be transferred to the solar coronal plasma by upward propagating waves could have interesting implications in the dynamics of the corona.…”

A One-Sided View of Acoustic Traps

“…If the source is made of a set of independent transducers, electronic steering of the beam is feasible, such as spatial light modulators in optics [50]. Moreover, acoustical vortices carry a quantified orbital angular momentum [39,40] that can be transferred to matter [42][43][44][45][46]52,53], so controlled rotation of absorbing particles is expected to provide a 4th degree of freedom for contactless manipulation and forms the basis of an undergoing study.…”

“…Field synthesis.-These peculiar wave fields have been successfully generated in acoustics [39][40][41] and applied to investigate the transfer of orbital angular momentum to matter [42][43][44][45][46]. In Fig.…”