Nonlocal Metasurface for Acoustic Focusing

Under the passive and lossless requirement, acoustic metasurfaces are mostly designed based on the general Snell's law (GSL), under which the power flow-conservation condition is locally satisfied along the whole surface. Recent investigations on metagrating show, however, that such kinds of structures have an intrinsic problem in their efficiency. To overcome this problem, the restriction of the local power flow-conservation condition has to be loosened and replaced by a nonlocal one. Here, we show that a similar problem also exists in the GSL-based acoustic focusing metasurface and the nonlocal idea needs to be introduced into the design. Based on this understanding, a two-dimensional nonlocal metasurface for acoustic focusing is designed by first customizing a desired reflective field and then searching for a power flow-conformal surface profile. Compared with a focusing metasurface based on the GSL theory, our design has an increased focusing effect, especially when the numerical aperture of the metasurface becomes large. The suggested method provides a general technique for designs with a complicated target field.