Acoustic Pancharatnam-Berry Geometric Phase

Abstract

Geometric phases provide a unified framework for understanding diverse phenomena in quantum and classical physics. The Pancharatnam-Berry (PB) geometric phase, arising from variation of optical transverse polarization, has transformed light manipulation. However, this phase has never been observed in sound waves due to their curl-free longitudinal nature. Here, we theoretically and experimentally demonstrate that the PB phase can emerge in general 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 realize acoustic PB metasurfaces for versatile wavefront manipulation. We further extend the mechanism to free-space structured sound and realize acoustic $q$-plate for generating acoustic vortices through spin-orbit interaction. Our work provides new insights into sound wave properties and enables the manipulation of inhomogeneous acoustic fields via the PB phase, with potential applications in acoustic communications and imaging.