Spin-Orbit Angular Momentum Conversion in Coupled Nonparaxial Bessel Acoustic Vortices

Acoustic waves as longitudinal waves are recently demonstrated to have both spin and orbital angular momentums (AMs). The separation of orbital and spin components is proposed as the most current idea. However, it remains extremely difficult to develop studies sensitive to both types of AM using direct wave field measurements. Here, theoretical explanation and experimental proof of the acoustic spin-orbit AM conversion are reported. A formulaic description of spin-orbit AM conversion in coupled fields of two nonparaxial Bessel acoustic vortices is given by a thorough theoretical derivation. Experimental results of wave field measurements provide compelling evidence for spin-orbit AM conversion via passive artificial structures. Based on this conversion, contactless high-resolution manipulation of micro-objects is further achieved, as demonstrated by the controlled rotation of Rayleigh particles and their translation along a circular trajectory. This work provides deeper insights into the AM physics of classical waves and has significant implications for acoustic sensing, acoustic imaging, and particle manipulation based on acoustic radiation force or torque.