Ultrabroadband and Multifunctional Achromatic Mikaelian Lens on an Elastic Plate

The emergence and development of acoustic artificial materials stimulate a large number of applications for the manipulation of ultrasonic and elastic waves. Most of the reported metasurfaces and metamaterials only work effectively at a given frequency and target a fixed functionality. Reconfigurable designs may solve this problem, to a certain extent, but require the introduction of complex active control components. Here, by integrating multiple functionalities on a conformally mapped Mikaelian lens with a hyperbolic secant refractive-index distribution, a broadband passive lens is presented and accomplished on an elastic plate. Just by adjusting the location and type of source, different regulations of flexural waves can be switched from one sort to another. Through projecting the refractive-index profile onto the thickness of the plates, the proposed Mikaelian lens is designed and fabricated. Theoretical prediction and experimental results demonstrate beam scanning at an angle of up to 120° from 30 to 180 kHz and achromatic subwavelength focusing with a full width at half maximum of around 0.3λ from 30 to 120 kHz. Without using locally resonant materials or active control elements, this work provides a feasible way to construct multifunctional flexural-wave devices.