Fig. 1. Schematic of free-space acoustic communication with high spatial information density by multi-path twisting of acoustic wave.

a The designed communication system consists of N pairs of transmitting and receiving units with each transmitting unit containing M coaxial vortex beams with different orders, resulting in a total number of NM independent data channels. At the receiving terminal, real-time and passive decoding of the transmitted information is realized by using N metamaterial-based demultiplexers with planar profile and ultrathin thickness. Ch: channel. b The schematic of simultaneous demultiplexing of synthesized vortex beams and beam focusing into prescribed locations with single metamaterial-based demultiplexer. Inset: partially enlarged view of the metamaterial layer. c Simulated phase shift and transmission provided by the metamaterial unit cells with different structural parameter h. Inset: section view of an individual element.