Observation of acoustic spin.

Chengzhi Shi,Sui Yang,Jie Ren,Yang Long,Yuan Wang,Hong Chen,Rongkuo Zhao,Xiang Zhang

doi:10.1093/nsr/nwz059

Abstract

ABSTRACTUnlike 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 designed lossy acoustic probe 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 its applications for the control of wave propagation and particle rotation.

Highlights

The spin angular momentum describes the rotation of a vector field [1, 2]
One may consider acoustic spin as the rotation of the wave polarization given by its local particle velocity, but an acoustic plane wave propagating in free space is a longitudinal wave whose particle velocity always oscillates along the propagation direction and does not rotates [14]
Note that the spin angular momentum is different from the orbital angular momentum observed in acoustic vortices representing the circulation of energy flux [15,16,17] or helical shaped acoustic or optical beams associated with the twisted wavefront [18,19,20,21,22,23,24]

Summary

Introduction

The spin angular momentum describes the rotation of a vector field [1, 2]. It provides an extra degree of freedom for the control of wave propagation and wave matter interactions. We report the existence of spin angular momentum in airborne acoustics characterized by the rotation of local particle velocity. The local particle velocity rotates clockwise or counterclockwise circularly depending on the relative phase difference (Figure 1a).

Results

Conclusion