Experimental demonstration of valley-protected backscattering suppression and interlayer topological transport for elastic wave in three-dimensional phononic crystals

Abstract Topological insulator (TI) that possesses topologically protected characteristic of guiding the wave against disorders and structural perturbations without backscattering has attracted significant research interest in various fields including electromagnetic, acoustic and elastic system. However, for the mechanical system, realizing an elastic analogue of three-dimensional (3D) TI supporting the topologically protected wave propagation in two-dimensional (2D) plane is still a challenge due to the complicated mode polarization of elastic wave in 3D. This paper theoretically and experimentally investigates the robust and layer-selective transports of elastic wave in 3D monolayer- and bilayer-stacked plate-like metamaterial structures. Firstly, considering 3D monolayer-stacked structure, the 2D valley surface states are achieved numerically along the 2D projected plane based on the mechanism of quantum valley Hall effect. The simulation and experimental measurement are performed to confirm the robust transport of 3D elastic wave and backscattering immunity against the straight channel and sharp bends. Then, by stacking the monolayer into bilayer with a twisted angle of 60°, non-zero interlayer coupling of elastic valley layer is introduced and the layer-related topological phase is revealed in the 3D bilayer-stacked structure, giving rise to the 2D topological layer-dependent surface states. Finally, the 3D robust layer-selective transports of elastic wave are tested by experiment. This research provides exciting application perspectives for ultrasonic devices with robustness, lower-power consumption and high-dimensional manipulation.