Asymmetric coding metasurfaces for the controllable projection of acoustic images

Abstract

The projection of acoustic images is of relevance for a wide range of applications, including acoustic communication and medical imaging. Various acoustic projection devices that use holograms have been proposed; however, they inevitably involve multiple basic elements and lack reconfigurability. In this paper, we demonstrate that an acoustic coding metasurface that consists of a single type of asymmetric acoustic unit (AAU) can be used to project the desired acoustic images. The AAU is constructed using a perforated panel and coating unit cell, which can be tunable in terms of the transmission coefficient and reflection phase asymmetry. Patterned in regular and inverse forms, the AAU can function as the coding elements ``0'' and ``1,'' respectively. We demonstrate, both numerically and experimentally, the efficient projection of complex acoustic images such as the letters ``N,'' ``J,'' and ``U.'' More importantly, we develop a simple composite metasurface system to achieve the controllable encryption and decryption of acoustic images. Our proposed approach provides a design methodology for the coding metasurface and may have applications in acoustic security, information storage, and wave manipulation.