Three-dimensional acoustic asymmetric focusing by mode-conversion structure

Acoustic asymmetric focusing (AAF) has attracted widespread attention owing to its great potential in medical ultrasound. The recent development of acoustic metamaterials and metasurfaces has provided various concepts to achieve AAF systems. Generally, the previously demonstrated AAF lenses were limited to one-dimensional structures, leading to difficulties in practical applications. The realization of AAF in three-dimensional (3D) space based on a two-dimensional (2D) device still remains a challenge. Here, we experimentally and numerically study a 2D AAF lens based on mode converters composed of two pairs of phased unit cells I and II and a step waveguide. Based on the phase profile of sound focusing and the step waveguide of mode converters, we experimentally design and demonstrate a type of 2D AAF lens and observe acoustic focusing and asymmetric transmission simultaneously in 3D space under the excitation of the zero-order wave. Both the phenomena arise from the phase modulation of the lens and the cutoff frequency of the first-order wave for the step waveguide, respectively. The fractional bandwidth (the ratio of the bandwidth to the center frequency) of the lens can reach about 0.14. The proposed 2D AAF lens, as well as its associated mode converter, paves the way for the asymmetric manipulation of sound in 3D space that has potential applications in practical sound devices.