Acoustical helicoidal waves and Laguerre–Gaussian beams: Applications to scattering and to angular momentum transport

Abstract

Some properties of acoustical traveling waves with helicoidal or "spiral-like" wavefronts are analyzed. These include orbital angular momentum flux and potentially useful imaging or scattering properties for the discrimination of axisymmetric objects from other objects. Measurements are summarized for a simple singleelement ultrasonic transducer for generating approximately helicoidal beam-waves in water.

Easy generation of airborne ultrasonic helical wavefronts

“…Basically, three different methods have been proposed for acoustic HW generation: I) multiple source synthesis [2], [5], [6], II) optoacoustic technique [7], and III) direct generation from a special shaped source [8]. Sometimes, a slightly different category, referred to as phased array system, is considered, in which a high number of multiple sources is used.…”

“…However, the helicoid is not required to be absorbent. In [8] (see also [6]), the generation of a helical beam in water is demonstrated using a ring of marine brass and a piezoelectric film of Polyvinylidene Fluoride (PVDF), which acts as active material, fixed to it. A similar approach is proposed in this work to easily generate airborne HW at ultrasonic frequencies using a different piezoelectreic polymer, i.e.…”

Generation of acoustic helical wavefronts using metasurfaces

“…The wave field singularity or a dislocation in the wavefront as a key feature of wave vortices was first studied in 1979 for acoustic waves [6]. Twenty years later, Hefner et al produced the first ultrasonic beam with helical phase and screw dislocation [7]. Moreover, it was shown using the concept of pseudomomentum that acoustical vortices have the same properties as their optical counterparts.…”

“…Hefner et al used an active polyvinylidene fluoride (PVDF) with a tangential helical surface supported by a twisted ring to create the first sonic helicoidal beam [7]. Later, Ealo et al improved this configuration using ferroelectrets [21].…”

Airborne ultrasonic vortex generation using flexible ferroelectrets

“…This can be attributed, in part, to the need for complex instrumentation to generate a helical wavefront. Thus far, different methods for AV generation have been proposed, namely, direct generation from a specially shaped source [7], [12], the optoacoustic technique [13], mode conversion from wave plane to AV [9], or use of a phased array system [11].…”