Acoustic vortex beams interacting with a metasurface and the acoustic Hall effect

Acoustic metasurfaces are capable of modulating the wavefronts of sound beams, governed by generalized laws of reflection and refraction. However, these laws fall short in describing the angular momenta carried by acoustic vortex beams. Such beams, characterized by twisted wavefronts arising from a rotating phase in their cross sections, exhibit unique phenomena with promising applications. Our numerical simulations reveal that an acoustic vortex beam refracted by a metasurface exhibits an asymmetry in its pressure field relative to the incident plane [Fan and Zhang, Phys. Rev. Res. 3(1), 013251 (2021)]. This asymmetry is analogous to that seen in the refraction of vortex beams propagating through stratified media [Fan et al., Phys. Rev. Res. 1(3), 032014 (2019)]. The asymmetry depends on the twist direction of the beam's rotating phase, a phenomenon referred to as the acoustic Hall effect. The asymmetry can also be observed via a geometric projection of the vortex beam, which interprets angular momentum carried by the beam when obliquely reflected from a flat interface [Zou et al., Phys. Rev. Lett. 125(7), 074301 (2020)]. Here, we experimentally observed the asymmetry of a vortex beam interacting with a metasurface, along with the associated conservation of angular momentum.