Engineering acoustic reflections with programmable metasurfaces

Acoustic anomalous reflectors have recently drawn intense attention due to their ability to reflect impinging waves in non-specular directions. However, virtually all previously analyzed anomalous reflectors are passive structures with high spatial and/or temporal dispersion. Therefore, they are typically narrowband and unidirectional, namely they operate in the desired manner for only one direction of incidence. We discuss here a method to design quasi-omnidirectional and broadband anomalous metasurface reflectors composed of programmable non-resonant scatterers capable to manipulate the in-plane wave vector component in prescribed ways. We illustrate the method in 3D experiments in which the incident waves undergo negative reflections and show that the desired effect does not depend on the direction of incidence and has a bandwidth an order of magnitude larger than passive designs. Furthermore, we show experimentally that anomalous reflectors of various geometries can be realized by simply rearranging the active scatterers to fill-in the desired geometries as long as the periodicity of the scatterer lattice remains subwavelength. This is direct evidence that the presented metasurfaces behave like continuous materials rather than gratings or phonic crystal. The implications of this finding on the design of future sound manipulation devices will be discussed.