Broadband Tunable Acoustic Asymmetric Focusing Lens from Dual-Layer Metasurfaces

The acoustic asymmetric focusing effect attracts more and more attention, with its extensive applications in high-intensity focused ultrasound (HIFU) therapy and medical ultrasound imaging. Typical focusing devices have symmetric structures, though, which do not yield asymmetric transmission simultaneously. This work realizes a broadband acoustic lens using dual-layer metasurfaces for both subwavelength focusing and asymmetric transmission. Without changing the metasurface's structures, horizontal and vertical focusing are manipulated by mechanically changing the distance between metasurfaces, and the angle of incidence.