Orbital angular momentum- and frequency-dependent high-capacity encrypted hologram through multi-dimensional multiplexing acoustic metasurface.

Sound holography has shown great capability in reconstructing the arbitrary complex sound fields, with potential applications in various scenarios. To overcome the limitations of existing technologies in terms of capacity and security, here we propose and experimentally demonstrate a mechanism of orbital angular momentum- (OAM-) and frequency-dependent high-capacity encrypted hologram through multi-dimensional multiplexing acoustic metasurface. By customizing the acoustic response of the metasurface based on a theoretically-derived transfer function, we bestow the metasurface with mode and spectrum selectivity simultaneously such that multiple predesigned images can be stored in parallel and restored with specific combinations of vortex mode and wavelength, thereby realizing the double-key-secured high-capacity holography. A distinct example of image reconstruction of multiple patterns is experimentally demonstrated, achieving as many independent hologram channels as the working frequency number multiplied by the OAM mode number and high-quality reconstruction of sound field information with strong confidentiality. Our methodology offers an avenue for building ultrahigh-capacity holographic information systems in acoustics, with far-reaching impact on encrypted sound communication and beyond.