Acoustic Pancharatnam-Berry geometric phase for structured sound manipulation

Phase control is fundamental to acoustic wave manipulation. The propagation and resonant phases have been widely used to modulate acoustic waves. However, the real-space geometric phase, also known as the Pancharatnam-Berry (PB) phase, has remained elusive in acoustics, owing to the fact that airborne sounds are curl-free longitudinal waves lacking intrinsic polarization degrees of freedom. Here, we theoretically and experimentally demonstrate that the PB phase can emerge in inhomogeneous sound waves with polarization evolution of velocity field. Using surface sound waves as an example, we uncover the intriguing Janus property of the PB phase arising from spin-momentum locking, and experimentally demonstrate acoustic PB metasurfaces for versatile wavefront manipulation. We further extend the mechanism to free-space structured sound and realize acoustic q-plates for converting vortex topological charge through spin-orbit interaction. Our work uncovers a type of acoustic phase and provides a simple yet effective mechanism for structured sound manipulation, with promising applications in acoustic communications, imaging, and on-chip devices.