Flow-Permeable and Tunable Metalens for Subdiffraction Waterborne-Sound Focusing

Metalenses with high-efficiency focusing functionality and a water-flow-permeable structure are desired in various acoustic applications, such as medical imaging and underwater navigation. Here, we propose a design paradigm for a metalens for waterborne sound with a compact and simple configuration. The metalens is composed of an open central region reserved as a steady water-flow channel, and a metagrating-based peripheral region consisting of a grating of meta-atoms. Each meta-atom containing two elliptical iron cylinders is smartly designed according to the grating diffraction theory and intelligent optimization algorithm, so that it can deflect a normally incident wave along the desired direction toward the focal spot. In this way, subdiffraction focusing with a high energy concentration ratio is achieved, which breaks the conventional Rayleigh-Abbe diffraction limit in the focal plane. Here the subdiffraction focusing is due to the coherent interference in the far field of the \ifmmode\pm\else\textpm\fi{}first-order diffracted waves from each meta-atom, and is attributed to the superoscillation phenomenon. Interestingly, the focal depth of the metalens can be conveniently tuned by applying a background water flow with different velocities and directions, and the superresolution focusing effect is sustained regardless of whether there is a water flow or not. Since the magnitude and direction of the water-flow velocity can be electrically controlled, the compact and open configuration of the metalens not only provides a flexible and practical solution for sharp and controllable sound focusing, but also has potential applications in metagrating-based planar acoustic devices.