Meta-device for acoustic spin–orbital angular momentum conversion

Abstract

Spin and orbital angular momentum, along with their intricate spin–orbit interactions (SOI), are fundamental to understanding wave phenomena across various physical systems. In acoustics, while orbital angular momentum has been a traditional focus due to sound's longitudinal nature, the existence of transverse spin has recently been identified in inhomogeneous sound fields. However, the practical realization of acoustic SOI for efficient angular momentum conversion and advanced wave manipulation remains a significant challenge. Here, we propose an acoustic meta-device that combines a comb-like waveguide with an anisotropic annulus to specifically engineer configurable spin–orbit conversion in acoustics. Numerical simulations reveal transverse spin and spin-momentum locking of spoof surface acoustic waves excited by the waveguide, alongside quadrupole resonances in the anisotropic annulus. Within this coupled system, an incident spin-carrying wave is effectively converted into a high-order acoustic vortex characterized by its distinct orbital angular momentum. Comparative experimental results confirm the efficacy of our design. This work advances acoustic SOI from theoretical observation to a practical, designable device implementation. The resulting spin-induced acoustic vortex generated by this angular momentum conversion mechanism holds promise for non-contact manipulations.