Experimental Demonstration of Dual-Band Nano-Electromechanical Valley-Hall Topological Metamaterials.

Suppression of undesired backscattering of very-high-frequency elastic signals has been considered as a grand challenge in integrated phononic circuits. Originating from condensed-matter physics, valley-Hall topological insulators provide an intriguing strategy to overcome this challenge. To date, phononic valley-Hall topological insulators have been demonstrated only in bulk acoustic and mechanical systems operating at relatively low frequencies. Here, an integrated nano-electromechanical valley-Hall topological insulator operating in the very-high-frequency regime is experimentally realized. Valley kink states that are backscattering-immune against sharp bends and exhibit the "valley-momentum locking" effect simultaneously in the fundamental (≈60 MHz) and second-order (≈120 MHz) frequency bands are demonstrated. It is further shown that the propagation directions of these dual-band valley kink states are always locked to their valley pseudospins. The results not only enable various applications in very-high-frequency integrated phononic circuits with enhanced robustness and capacity, but also open the door to experimental exploration of mechanical nonlinearities, particularly those involving the fundamental and second-order frequencies, in topologically nontrivial nanostructures.