Spin and orbital angular momenta of acoustic beams

TL;DR

This paper investigates the spin and orbital angular momenta in monochromatic acoustic beams, revealing that despite being longitudinal, these waves can have nonzero local spin density and complex polarization structures, with implications for observable phenomena.

Contribution

It demonstrates that inhomogeneous acoustic waves can possess local spin angular momentum and describes the polarization structure of acoustic Bessel beams, a novel insight into acoustic wave properties.

Findings

Localized acoustic waves can have nonzero spin density.

Acoustic Bessel beams exhibit complex polarization structures.

Helicity density vanishes in acoustic fields.

Abstract

We analyze spin and orbital angular momenta in monochromatic acoustic wave fields in a homogeneous medium. Despite being purely longitudinal (curl-free), inhomogeneous acoustic waves generically possess nonzero spin angular momentum density caused by the local rotation of the vector velocity field. We show that the integral spin of a localized acoustic wave vanishes in agreement with the spin-0 nature of longitudinal phonons. We also show that the helicity or chirality density vanishes identically in acoustic fields. As an example, we consider nonparaxial acoustic Bessel beams carrying well-defined integer orbital angular momentum, as well as nonzero local spin density, with both transverse and longitudinal components. We describe the nontrivial polarization structure in acoustic Bessel beams and indicate a number of observable phenomena, such as nonzero energy density and purely-circular transverse polarization in the center of the first-order vortex beams.