Acoustic metaholograms for encrypted information transmission

We report on the theoretical, numerical, and experimental investigation of secure acoustic holography. This secure holography is achieved via an encoded hologram generated by a transducer array decoded by a transmissive acoustic metakey. An unreadable image is received if directly using the encoded hologram, while the desired image can be obtained only with the aid of a decoding metastructure, i.e., a correctly decoding metakey. The proposed metakey consists of a series of unit cells, each of them is composed of a straight channel decorated with several opening panels. By adjusting the opening size of unit cells within the metakey, its transmitted phase profile can be effectively controlled; hence, the desired decoding metakey can be successfully constructed. The patterns of a butterfly, sunglasses, and the letters NJUST are chosen as examples to demonstrate the performance of the secure acoustic holography. Vulnerability tests of the metakey further confirm the safety and reliability of secure acoustic holography. With the advantages of simple design, flexible functionality, and high quality for reconstructed images, our work opens an alternative avenue for acoustic holography and may find applications in secure acoustic communications based on holograms.