Direct Mechanical Detection and Measurement of Wave-Matter Orbital Angular Momentum Transfer by Nondissipative Vortex Mode Conversion

Abstract

We quantitatively report on the rotational mechanical effect of wave orbital angular momentum on matter by nondissipative vortex mode conversion. Our experiments consist of ultrasonic waves reflected off freely spinning helical acoustic mirrors that are capillary trapped at a curved air-water interface. Considering helical mirrors with integer topological charges these results represent the demonstration of the experiment proposed by Allen et al. originally introduced in the optical domain [Phys. Rev. A 45, 8185 (1992)], whose quantitative implementation remains elusive to date whatever the nature of the wave. The study is further generalized to helical mirrors with fractional charges.

Enhanced Fractional Acoustic Vortices by an Annulus Acoustic Metasurface with Multi‐Layered Rings

“…Some groups reported that the torque Γ of the AV on an object is proportional to the energy flow P absorbed by the object with a scaling factor ( m /ω). [ 45,46 ] Here, m is the TC and ω is the frequency of the incident acoustic waves. When P and ω are fixed, the magnitude of Γ is dependent on the m‐ value.…”

Torsional Mechanical Oscillator Driven by the Orbital Angular Momentum of Sound

“…This choice implies that the reflected ("r") pressure field is of the form p r ∝ exp(−iωt + iκz − iℓφ), where ω = 2π f and κ = 2π f /c + iα is the complex wave vector, with c = 343 m s −1 and α ≃ 0.38 m −1 at 100 kHz frequency [17] being the sound wave celerity and attenuation in air, respectively. The reflected sound wave thus corresponds to an acoustic vortex beam with topological charge −ℓ, whose generation is associated (within the paraxial approximation) with an acoustic radiation torque z = ℓP/ω exerted on the helical mirror where P is the acoustic power intercepted by the helical mirror [18].…”

“…These robust experimental observations also allow one to address the challenge of quantitative tests of the acoustic radiation torque formulation in the paraxial regime, so far based on the observation of objects immersed in fluids set into spin owing to dissipative [23,24] or nondissipative [18,25] transfer of orbital angular momentum between wave and matter. Here, the acoustic torque contribution of the harmonic at frequency F is expressed as (F) dyn = 2ϵℓP/ω = K (F) dyn .…”

“…Here we justify from acoustic propagation simulations that the cavity effects between transducer and pendulum can be neglected for d = 100 mm. The field propagation method used is a generic one that applies to all kinds of scalar propagating waves, which is based on Fourier transforms (see [18]). The latter claim is supported by the results shown in Fig.…”

Orbital Angular Momentum Multiplexing in Space–Time Thermoacoustic Metasurfaces

“…[10][11][12][13][14] The OAM mode characterized by the topological charge l, describes the azimuthal eigenmodes of vortex beams with a phase singularity that are theoretically unbound in free space. [15][16][17] The pioneering design to experimentally implement OAM modes into acoustic communications is based on transducer arrays, which are digitally controlled to generate multiplexed OAMs and the data information contained in each channel is extracted by using mode orthogonality relation. [7] Using this scheme, a high spectral efficiency of 8.0 ± 0.4 bit s −1 Hz −1 has been obtained based on multiplexing eight different vortex beams.…”