Abnormal deflection of flexural waves by achromatic meta-slab

Various metasurfaces are proposed to abnormally modulate the elastic waves, but most of the elastic metasurfaces are wave-dispersive. In this paper, we theoretically design an achromatic meta-slab with subunits of different thickness, which can keep the refraction angle unchanged with the incident frequency. The designed meta-slab eliminates the wave dispersion without changing its configuration. Unlike the metasurface whose phase shift gradient is a constant, the designed meta-slab has a phase shift gradient that depends on the wavelength of the incident wave. Consequently, the meta-slab can compensate the wave path difference produced by the incident waves with different wavelengths. Through phase compensation, the meta-slab possesses an achromatic property. To show this property, we conduct corresponding finite element simulations for wave propagation and obtain good results consistent with the theoretical design. Our design broadens the working bandwidth of the meta-slab without changing its configuration, which makes it significant in the applications of vibration isolation, energy harvesting and health monitoring.