Observation of acoustic spin

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

doi:10.1093/nsr/nwz059

Observation of acoustic spin

Natl Sci Rev. 2019 Jul;6(4):707-712. doi: 10.1093/nsr/nwz059. Epub 2019 May 11.

Authors

Chengzhi Shi^{1

2}, Rongkuo Zhao¹, Yang Long³, Sui Yang¹, Yuan Wang¹, Hong Chen³, Jie Ren³, Xiang Zhang^{1

4}

Affiliations

¹ NSF Nano-scale Science and Engineering Center (NSEC), University of California, Berkeley, Berkeley, CA 94720, USA.
² George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA.
³ Center for Phononics and Thermal Energy Science, China-EU Joint Center for Nanophononics, Shanghai Key Laboratory of Special Artificial Microstructure Materials and Technology, School of Physics Sciences and Engineering, Tongji University, Shanghai 200092, China.
⁴ Faculties of Science and Engineering, University of Hong Kong, Hong Kong, China.

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 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.

Keywords: acoustic spin; spin-induced torque; spin–momentum locking.