Thermally driven hybrid metastructure for multi-functional surface acoustic wave engineering

Acoustic wave engineering through solid mediums are essential for improving or optimizing the performances of various platforms, such as biomedical and industrial applications. Metamaterials or metasurfaces have been recently developed to artificially manipulate the wave propagation passing through mediums. However, they only conduct one fixed function passively, determined by the structure design for the particular wavelength, and their fabrication involves high-cost and complex procedures. Here, we report a thermally driven hybrid metastructure (HMS) for actively controlling surface acoustic waves. The HMS is designed as the layered structure of a thermoresponsive polymer (TP), thermal metamaterials (TMs) and an intermediate layer (IL) made of silica aerogel. For providing multi-functions, TPs conduct the considerable elastic modulus changes and corresponding wave manipulation at a certain glass transition temperature while TMs enable the programmed heating configuration, and the IL mitigates thermal and acoustic wave interference. By introducing the IL, the entire energy of the propagating wave is captured at the interface and interference between the wave and thermal energy is prevented, which directly reflects the temperature control through the TMs into the TP heating configuration. Versatile functions of programmable wave engineering including omnidirectional wave focusingcollimation through modified Luneberg lenses, dual-functioning focusing-bifurcation, and double refraction through 3-phase wave steering are demonstrated using the proposed HMS implemented by the temperature gradient design of TMs. This work will inspire tunable and scalable platforms for active controls of wave propagation enabled by thermal energy as a driving force.