Acoustic beams with modulated helicity and a simple helicity-selective acoustic reciever.

Abstract

Acoustic helicoidal beams have an azimuthal phase gradient and an axial null in amplitude. The sign and magnitude of the azimuthal phase gradient determine the helicity and topological charge of the beam. Beams with a unit-magnitude topological charge have been generated with a four-quadrant sectored array, adjacent quadrants being driven with a 90 deg phase offset [B. T. Hefner and P. L. Marston, J. Acoust. Soc. Am. 106, 3313–3316 (1999)]. The scattering by symmetric targets placed on the axis of such beams preserves the helicity and the axial null [P. L. Marston, J. Acoust. Soc. Am. 124, 2905–2910 (2008); F. G. Mitri, Ann. Phys. 323, 2840–2850 (2008)]. The present work shows how to rapidly alter (or modulate) the helicity with an appropriately timed reversal of the excitation of a single pair of opposite quadrants. A simple four-element helicity-selective detector (using appropriately offset elements) was also demonstrated. These beams may be useful for digital acoustic communication and they have a modulated axial angular momentum flux. They should also generate a modulated acoustic torque on targets. [Work partially supported by ONR.]

A study of nondiffracting Lommel beams propagating in a medium containing spherical scatterers

“…For instance, we cite: combustion, remote sensing, communications, biology, medicine and light-matter interactions [12][13][14][15][16]. In this context, a lot of studies of the scattering of acoustical waves by a sphere have been elaborated by Marston [17][18][19]. This author has obtained the expression of radiation force on a sphere centered on an acoustic Bessel beam in an inviscid fluid.…”

Linear axial scattering of an acoustical high-order Bessel trigonometric beam by compressible soft fluid spheres

“…This approach was used to model the electromagnetic 26,27 and acoustic 12 field of a zero-order Bessel beam as well as in the description of an acoustical Bessel vortex beam of unit topological charge 14 that is a particular case of a generalized solution given initially in Refs. 13, 15, and 25.…”

“…With 0 = m =0, ͑2͒ reduces to the expression of a zero-order Bessel beam. 12 The ͑complex͒ pressure field scattered by a spherical object placed along the axis of wave propagation of a HOBTB is expressed as…”

“…Appropriate selection of the beam's parameters, such as the focus, waist, divergence, directionality, collimation, etc., or more generally the beam's type ͓i.e., diffracting, ͑i.e., Gaussian, Bessel-Gaussian, LaguerreGaussian, fractional Bessel, etc.͒ versus nondiffracting, ͑i.e., Bessel of integer order, high-order Bessel Beam of fractional type ␣, etc.͔͒, may be used to advantage to enhance or suppress the scattering. [12][13][14][15][16][17] Ideal nondiffracting beams are proper solutions of the homogeneous ͑source-free͒ Helmholtz equation, 18 for which the transverse pressure ͑or intensity͒ distribution profile remains unchanged during wave propagation. In practice, a very close approximation to an ideal nondiffracting beam can be achieved experimentally by a Gaussian transmittance apodization to produce a "limited-diffracting"-Gauss beam.…”

Broadband and stable acoustic vortex emitter with multi-arm coiling slits

“…[1][2][3] One typical characteristic of the acoustic vortices is the helical dislocation of the wave front which is described by an azimuthal h phase dependence expðimhÞ. 4,5 The integer number m is called the topological charge, corresponding to the order of the vortex, which determines the total phase accumulated in one full annular loop around the propagation axis. Another property of the acoustic vortices is the indeterminate phase and null pressure magnitude at the core of vortices, which is useful for trapping and manipulating particles smaller than wavelength, through controlling and moving position of the central vortex null.…”