Broadband tunable focusing lenses by acoustic coding metasurfaces

Acoustic tunable focusing lens has important applications in acoustic fast imaging and three-dimensional displays. Previously developed metasurface lenses suffer from narrow bandwidth and fixed geometric configuration, thereby limiting their further practical applications. Here, we design an acoustic coding metasurface (ACM) lens to achieve a broadband tunable focusing in air. This simple ACM lens only includes two coding bits: a space-folding structure is designed to act as the bit '1' and an air unit with the same size is used as the bit '0'. By adjusting the coding sequence of the ACM, the focus location of the ACM lens can be modified efficiently and be steered in the whole working plane. Moreover, the focusing behaviors of the ACM lens are well performed in a large frequency range of similar to 3.1 to similar to 4.6 kHz. Our ACM lens may be beneficial to applications that traditionally need different and complicated structures for tunable acoustic focusing.