Highly efficient nonlocal metasurface for transmission-type acoustic focusing

The design of a highly efficient planar focal lens with a near-unity numerical aperture remains a challenge, despite efforts made on this topic. In this paper, we propose a two-dimensional nonlocal transmission-type metasurface composed of subwavelength-sized channels and groove arrays, aiming to attain highly efficient acoustic focusing. We show that such a nonlocal structure can be efficiently designed by an inverse method based on the multistructural model, even when the number of constituent units becomes considerably large. As examples, metasurfaces both for the plane-wave source and for the point source have been designed. Numerical results show that our proposed nonlocal metasurface exhibits a much higher focusing efficiency than the corresponding structures based on the principle of the generalized Snell law. The structure designed for the point source has also been verified experimentally, showing good agreement between the numerical and experimental results. Our design contributes an alternative idea for wave control, which should be useful in acoustic sensors, energy harvesting, and so on.