Defected elastic metasurfaces for structured focusing with the extension of Babinet principle

Abstract

Defects offer a new geometric freedom in metamaterials or phononic crystals to functionally modulate waves, but remain unexplored in a low-dimensional version of artificial structures. We here introduce the concept of a defected metasurface that enables structured focusing by breaking the traditional design notion of perfect metasurfaces for single focus. We theoretically and experimentally demonstrate that the distance between focal points is smaller than the wavelength, which is a challenging task previously. Moreover, the number and the energy distribution of foci can be tailored via integrating defects with the metasurface, which can be well described based on the Babinet principle. We further realize the Talbot effect to generate periodically focusing and digital coding. This defected prototype offers a promising strategy to shape structured elastic waves for nondestructive testing, and may be extended to other fields on the design of efficient acoustic or optical tweezer arrays.