Spin-orbit interactions of transverse sound

TL;DR

This paper demonstrates that airborne sound can exhibit spin-orbit interactions through an acoustic metamaterial, enabling phenomena like negative refraction and spin-dependent vortices, thus opening new avenues in sound manipulation.

Contribution

It introduces the concept of artificial transversality in airborne sound using acoustic metamaterials, enabling spin-orbit interactions in longitudinal waves.

Findings

Demonstration of negative refraction of transverse sound.

Generation of spin-dependent acoustic vortices.

Experimental validation of spin-orbit interactions in sound.

Abstract

Spin-orbit interactions (SOIs) endow light with intriguing properties and applications such as photonic spin-Hall effects and spin-dependent vortex generations. However, it is counterintuitive that SOIs can exist for sound, which is a longitudinal wave that carries no intrinsic spin. Here, we theoretically and experimentally demonstrate that airborne sound can possess artificial transversality in an acoustic micropolar metamaterial and thus carry both spin and orbital angular momentum. This enables the realization of acoustic SOIs with rich phenomena beyond those in conventional acoustic systems. We demonstrate that acoustic activity of the metamaterial can induce coupling between the spin and linear crystal momentum k, which leads to negative refraction of the transverse sound. In addition, we show that the scattering of the transverse sound by a dipole particle can generate spin-dependent acoustic vortices via the geometric phase effect. The acoustic SOIs can provide new perspectives and functionalities for sound manipulations beyond the conventional scalar degree of freedom and may open an avenue to the development of spin-orbit acoustics.