Nonresonant Metasurface for Fast Decoding in Acoustic Communications

Acoustic communication is crucial in underwater exploration, where sound is the dominant information carrier, with significantly less loss and scattering than that of electromagnetic waves. However, the capacity of acoustic communication channels is limited due to the intrinsically low speed of sound relative to that of electromagnetic waves and because the attenuation of acoustic waves underwater increases with frequency. Recently, orbital angular momentum (OAM) has emerged as an alternative multiplexing degree of freedom to encode data onto vortex beams for increasing the capacity of acoustic communication. For information retrieval from the multiplexed acoustic vortices, an active scanning method and a passive resonant method are explored. Time-consuming scanning and complex postprocessing significantly restrict the data-transmission speed, while the large amount of resonant cascaded devices in the passive technique intrinsically results in a low efficiency and bulky volume of the system. Here, we propose and experimentally demonstrate a passive and nonresonant approach with the ability to separate different OAM states of multiplexed acoustic vortex beams in parallel using a parabolic-phased metasurface. The metasurface converts the spiral-phase patterns of vortex beams carrying various angular momenta into plane waves with different in-plane linear momenta. Our approach is compatible with multiplexing technologies, significantly enhancing the speed in acoustic communication.