Nonplanar acoustic metasurface for focusing

Acoustic metasurfaces have attracted considerable attention in recent years because of their unprecedented manipulation of acoustic waves within subwavelength planar structures. However, planar rigid structures are not compatible with human skin, which possesses dynamically varying and highly unconventional geometries. This limits the applicability of acoustic metasurfaces in a promising industry. In this paper, we describe a nonplanar acoustic metasurface for focusing based on the ability of metasurfaces to impart nontrivial phase shifts. We propose a common mapping method for the pattern design that enables focusing behind the free surface. By cutting hollowed-out patterns on a nonplanar metasurface membrane, forming a specialized transmitted phase distribution, we obtain the approximate constructive interference on the focal spot, thus implementing high-efficiency focusing behind an arbitrary nonplanar metasurface. Specifically, we demonstrate focusing on spherical, conical, and chaotic surfaces and discuss the scope of application of our design rule. Moreover, by applying the pattern to conical surfaces, we design an “umbrella structure” to implement high-efficiency focusing with a dynamic, tunable focal length. The proposed nonplanar metasurface not only expands the application range of metamaterials but also demonstrates the real-world applicability of cutting-edge metamaterial research.