Tunable fluid-like metasurface for elastic mode decoupling

Mode coupling between longitudinal and shear waves in elastic media is a well-known phenomenon, leading to the simultaneous reflection of both wave types when they strike a surface at an oblique angle. However, this coupling often results in unwanted reflected waves, introducing noise in practical applications. In this work, we present a novel elastic metasurface—termed a fluid-like elastic metasurface—that effectively eliminates mode coupling. The key concept is to implement a mixed boundary condition, a principle derived from classical elastic wave theory, by appropriately designing a metasurface unit. Furthermore, we have developed a boundary configuration that not only supports in-phase reflection of incident longitudinal and shear waves but also enables their reflection with an opposite phase. Moreover, we have designed a tunable fluid-like elastic metasurface capable of switching between in-phase and out-of-phase reflection states.