The discovery of acoustic spin

Spin is a fundamental physical quantity that describes a specific type of angular momentum. Take the motion of the Earth as an example, the spin of the Earth characterizes the angular momentum associated with the rotation about the Earth axis. This classification distinguishes the Earth self-rotation from the angular momentum resulted from the orbiting around the Sun known as orbital angular momentum. In particle physics, the spin carried by the fundamental particles helps categorize them as Fermions and Bosons. Electrons with spin ½ are Fermions that follows the Pauli Exclusion Principle. On the other hand, photons with spin 1 following Boson-Einstein distribution are Bosons. The spins of photons (optical waves) are characterized by the rotation of their polarization, which usually exists in circular polarized light. For acoustic waves propagating in fluids, it is commonly considered as spinless due to the longitudinal wave nature. Without the existence of acoustic spin, many interesting spin-related phenomena such as quantum spin Hall effect and spin-momentum locking are difficult to be realized for sound waves that normally require enormous design effort to create pseudospin modes. Recently, Chengzhi Shi (now at Georgia Tech), Rongkuo Zhao, Sui Yang, Yuan Wang, and Xiang Zhang from the University of California, Berkeley and Long Yang, Hong Chen, and Jie Ren from Tongji University discover and experimentally observe the existence of acoustic spin in airborne sound waves. This breakthrough is recently published in National Science Review (NSR).

The discovered acoustic spin was observed in the interference of two perpendicular propagating acoustic beams and evanescent sound waves propagating along a groove waveguide. For two perpendicular acoustic beams, each of them contributes to a component of the local particle velocity field that characterizes the polarization of the sound wave. When these two local particle velocity components are 90 degrees out-of-phase, the particle velocity vector rotates, resulting in acoustic spin as shown in Fig. 1. This acoustic spin was applied to generate a torque that controls the rotation of a particle remotely. In evanescent sound waves, the acoustic spin interlocks with the wave vector such that sound waves with clockwise spin propagate only towards the right, while counterclockwise spin sound waves propagate only towards the left. This acoustic spin-momentum locking property will be critical for acoustic communication. This ground-breaking discovery of acoustic spin will lay the foundation of the development of acoustic spin-related physics that will be significant for emerging topics in fundamental physics and acoustics. The work is supported by the Office of Naval Research (ONR) MURI Program, the Georgia Tech Faculty Startup Funding, and the National Natural Science Foundation of China and the National Key Research Program of China.

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

Observation of acoustic spin

Natl Sci Rev (May 2019) doi: 10.1093/nsr/nwz059

https://doi.org/10.1093/nsr/nwz059