  As 2D materials with subwavelength structures, elastic metasurfaces show
remarkable abilities to manipulate elastic waves at will through artificial
boundary conditions. However, the application prospects of current metasurfaces
may be restricted by their phase-only modulating boundaries. Herein, we present
the next generation of elastic metasurfaces by additionally incorporating
amplitude-shift modulation. A general theory for 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 the incidence by 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.
