Nondivergent Behavior of Spin-Orbit Features in Acoustic Vortex Beams

We consider effects due to interplay of the orbital angular momentum and spin in the propagating acoustic vortex beams. It is shown that the relation beween longitudinal and transverse wave motion is strictly controlled by the continuity equation, resulting in observable features that do not diverge during wave propagation in a paraxial limit, while the overall wave's intensity profile expands due to diffraction. Similar non-divergent features reveal themselves in the sound generation from acoustic multipoles. We show that point-like acoustic dipoles, quadrupoles, etc. generate sound waves with inherent cylindrically-symmetric properties with embedded chiral topologies. Direct analogies are drawn between acoustic waves and effects previously identified in electromagnetic radiation [1,2].

References

1. Afanasev, Andrei, Jack J. Kingsley-Smith, Francisco J. Rodríguez-Fortuño, and Anatoly V. Zayats. "Nondiffractive three-dimensional polarization features of optical vortex beams." Advanced Photonics Nexus 2, no. 2 (2023): 026001-026001.

2. Vernon, Alex J., Andrew Kille, Francisco J. Rodríguez-Fortuño, and Andrei Afanasev. "Non-Diffracting Polarisation Features around Far-Field Zeros of Electromagnetic Radiation." arXiv preprint arXiv:2306.03278 (2023).