Exploring Hidden Acoustic Spin Underwater.

Investigating wave propagation in fluid enables a variety of important applications in underwater communications, object detections and unmanned robot control. Conventionally, momentum and spin reveal fundamental physical properties about propagating waves. Yet, vast previous research focused on the orbital angular momentum of acoustics without thinking about the existence possibility of spin due to the longitudinal wave nature. Here, we show that underwater acoustic wave processes the non-trivial spin angular momentum intrinsically, which is associated with its special spin-orbital coupling relation for longitudinal waves. Furthermore, we demonstrate that this intrinsic spin, although unobservable in plane wave form, can be detected by four approaches: wave interference, Gaussian exponential decay form, boundary evanescent wave, and waveguide mode. We further show that the strong spin-orbital coupling can be exploited to achieve unidirectional excitation and backscattering immune transport. We hope the present results can improve the geometric and topological understanding about underwater acoustic wave and pave the way on the spin-related underwater applications.