Observation of acoustic spin

TL;DR

This paper experimentally demonstrates the existence of acoustic spin in fluids, revealing spin angular momentum in scalar pressure fields and its effects on wave propagation and particle rotation, opening new avenues in acoustic control.

Contribution

It provides the first experimental evidence of acoustic spin and spin-momentum locking in fluids, a phenomenon previously thought impossible due to scalar wave nature.

Findings

Measured acoustic spin and spin-induced torque on meta-atoms

Observed spin in evanescent guided modes in metamaterials

Identified spin-momentum locking in acoustic waves

Abstract

Unlike optical waves, acoustic waves in fluids are described by scalar pressure fields, and therefore are considered spinless. Here, we demonstrate experimentally the existence of spin in acoustics. In the interference of two acoustic waves propagating perpendicularly to each other, we observed the spin angular momentum in free space as a result of the rotation of local particle velocity. We successfully measured the acoustic spin, and spin induced torque acting on a lossy acoustic meta-atom that results from absorption of the spin angular momentum. The acoustic spin is also observed in the evanescent field of a guided mode traveling along a metamaterial waveguide. We found spin-momentum locking in acoustic waves whose propagation direction is determined by the sign of spin. The observed acoustic spin could open a new door in acoustics and their applications for the control of wave propagation and particle rotation.