Researchers Discover and Experimentally Observe Existence of Acoustic Spin in Airborne Sound Waves

Spin, which is a basic physical quantity, describes a particular type of angular momentum. For instance, consider Earth’s motion, Earth’s spin characterizes the angular momentum related to the rotation about its axis.

This classification differentiates the self-rotation of Earth from the angular momentum resulting from orbiting around the Sun, called orbital angular momentum. In the field of particle physics, the spin of the fundamental particles assists in classifying them as Bosons and Fermions. Electrons that have spin ½ are Fermions that obey the Pauli Exclusion Principle.

By contrast, photons that have spin 1 and obey Boson-Einstein distribution are Bosons. The rotation of the polarization of the photons (optical waves) characterizes their spins. The polarization rotation typically occurs in circular polarized light. In the case of acoustic waves that propagate in fluids, it is usually regarded as spinless due to the longitudinal wave nature.

The lack of acoustic spin will render various interesting spin-related phenomena like quantum spin Hall effect and spin-momentum locking challenging to be achieved for sound waves that usually require tremendous design effort to develop 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 found out and experimentally observed the presence of acoustic spin in airborne sound waves. This advancement has recently been reported in National Science Review (NSR).

The acoustic spin that was discovered was noticed in the interference of two perpendicular propagating acoustic beams and evanescent sound waves that propagated along a groove waveguide. In the case of two perpendicular acoustic beams, each of them contributes to a component of the local particle velocity field characterizing the polarization of the sound wave.

The particle velocity vector rotates when the two local particle velocity components are 90° out of phase, leading to an acoustic spin. This acoustic spin was used to produce a torque that remotely governs the rotation of a particle. In the case of evanescent sound waves, the acoustic spin gets interlocked with the wave vector such that sound waves that have a clockwise spin propagate only toward the right, while sound waves with a counterclockwise spin propagate only toward the left. This acoustic spin-momentum locking property will be crucial for acoustic communication.

This pioneering discovery of acoustic spin will be the groundwork for the advancement of acoustic spin-related physics that will be important for emerging topics in fundamental physics and acoustics. The study has been supported by the Office of Naval Research (ONR) MURI Program, the Georgia Tech Faculty Startup Funding, the National Natural Science Foundation of China, and the National Key Research Program of China.