Non-zero Integral Spin of Acoustic Vortices and Spin-orbit Interaction in Longitudinal Acoustics

TL;DR

This paper demonstrates that guided acoustic vortices can carry non-zero integral spin angular momentum and exhibit spin-orbit interaction, challenging previous assumptions about longitudinal acoustic waves and opening new avenues for applications.

Contribution

It provides the first analytical calculations showing non-zero spin AM in acoustic vortices and experimentally observes spin-orbit interaction in longitudinal acoustics.

Findings

Guided acoustic vortices carry non-zero integral spin AM.

Spin-orbit interaction observed in longitudinal acoustic waves.

Canonical-Minkowski AM is conserved, unlike kinetic-Abraham AM.

Abstract

Spin and orbital angular momenta (AM) are of fundamental interest in wave physics. Acoustic wave, as a typical longitudinal wave, has been well studied in terms of orbital AM, but still considered unable to carry non-zero integral spin AM or spin-orbital interaction in homogeneous media due to its spin-0 nature. Here we give the first self-consistent analytical calculations of spin, orbital and total AM of guided vortices under different boundary conditions, revealing that vortex field can carry non-zero integral spin AM. We also introduce for acoustic waves the canonical-Minkowski and kinetic-Abraham AM, which has aroused long-lasting debate in optics, and prove that only the former is conserved with the corresponding symmetries. Furthermore, we present the theoretical and experimental observation of the spin-orbit interaction of vortices in longitudinal acoustics, which is thought beyond attainable in longitudinal waves in the absence of spin degree of freedom. Our work provides a solid platform for future studies of the spin and orbital AM of guided acoustic waves and may open up a new dimension for acoustic vortex-based applications such as underwater communications and object manipulations.