Acoustic Pancharatnam-Berry Geometric Phase Induced by Transverse Spin

Common wisdom believes that the Pancharatnam-Berry (PB) geometric phase is absent in acoustics due to the spin-0 nature of sound waves. We theoretically and experimentally demonstrate that the PB phase can emerge in surface sound waves (SSWs) carrying transverse spin. The phase differs for the SSWs propagating in opposite directions due to spin-momentum locking. We further realize acoustic PB metasurfaces for nearly arbitrary wavefront manipulation. Our work provides the missing piece of acoustic geometric phases, offering new insights into the fundamental properties of sound waves and opening a new avenue for sound manipulation based on the PB phase.