Broadband integrative acoustic asymmetric focusing lens based on mode-conversion meta-atoms

Acoustic asymmetric focusing (AAF) has a great potential application in medical ultrasound imaging, focused ultrasound therapy, and nondestructive testing. The advancement of acoustic metamaterials and metasurfaces has provided various mechanisms to design AAF lenses. However, due to the existence of a certain difficulty in realizing sound focusing and asymmetric transmission simultaneously, the realization of the AAF lens with a broad bandwidth and an integrative planar structure still remains a challenge. Here, we theoretically propose and experimentally demonstrate an integrative planar AAF lens based on a type of mode-conversion meta-atom composed of two types of phased unit cells with a phase difference of pi and a step waveguide. The meta-atom can realize the conversion between the zero-order and first-order waves and asymmetric sound transmission, which arises from the phase modulation and the cutoff frequency of the first-order wave, respectively. Based on the proposed mode-conversion meta-atoms, we design a broadband AAF lens with high performance, in which its fractional bandwidth can reach about 0.19 and the focus size is only about 0.38 lambda. The proposed mode-conversion meta-atom and its associated AAF lens provide diverse routes for designing asymmetric multi-functional sound devices with promising applications.