Reprogrammable phononic metasurfaces

Mechanical metamaterials are materials with tailored, architected geometry, designed to retain properties that do not exist or rare in nature. Most of these mechanical properties are inscribed in the material’s frequency dispersion spectrum, ranging form its stiffness at zero frequency to its wave attenuation capacity at finite frequencies. These materials usually feature a structural pattern that repeats spatially (i.e., unit cell). A special class of these metamaterials can manipulate elastic waves (i.e., phonons). Most of the existing design frameworks for phononic metamaterials capitalize only one of there mechanisms; scattering, resonance or inertia amplification. In addition, once these designs are realized, their operational frequencies cannot be altered, limiting their potential for practical applications. Here, we present a reporgrammable metamterial platform for manipulating phonons utilizing all the aforementioned wave manipulation mechanisms [Foehr and Bilal et al.,PRL, 2018]. We program our nonlinear metamaterial to redirect stress waves, in real-time, in an element-wise fashion [Bilal et al., Adv. Mater. 2017]. Moreover, we use it to realize the first purely acoustic transistor (switching and cascading sound with sound) [Bilal et al., PNAS 2017].