Steering flexural waves by amplitude-shift elastic metasurfaces

As 2D materials with subwavelength thicknesses, elastic metasurfaces show remarkable abilities to manipulate elastic waves at will through artificial boundary conditions. However, current elastic metasurfaces are still far away from arbitrary wave manipulations since they just play a role of phase compensator. Herein, we present the next generation of elastic metasurfaces by incorporating amplitude discontinuities as an additional degree of freedom. A general theory predicting target wave fields steered by metasurfaces is proposed by modifying the Huygens-Fresnel principle. As examples, two amplitude-shift metasurfaces concerning flexural waves in thin plates are carried out: one is to transform a cylindrical wave into a Gaussian beam by elaborating both amplitude and phase shifts, and the other one is to focus incident waves by metasurfaces of amplitude modulations only. These examples coincide well over theoretical calculations, numerical simulations, and experimental tests. This work may underlie the design of metasurfaces with complete control over guided elastic waves and may extend to more sophisticated applications, such as analog signal processing and holographic imaging.