Tunable wave couplings induced by transverse properties of elastic valley topological edge states

Beyond well-known advantages such as robust and unidirectional wave transport, the valley topological edge states (TES) manifest distinctive transverse properties, i.e., the transversely symmetric (TS) and antisymmetric (TAS) properties. The TS and TAS properties provide an alternative choice for controlling wave behavior. In this work, the tunable wave coupling between an external bare plate and a valley topological phononic crystal (PnC) waveguide is investigated, together with the tunable TS and TAS properties. It is directly demonstrated in experiments that the TS and TAS properties of TES can efficiently and flexibly tune the wave coupling. Importantly, the TS and TAS properties encounter an inversion in a sliding valley topological PnC waveguide, where the PnC at one side of the interface slides along the interface. Finally, the TS and TAS properties are shown to support nonreciprocal wave transport, achieving a high asymmetric transport ratio, i.e., 26 times or 28.3 dB, between the forward and backward wave incidences. Our results may enrich the designs of functional phononic devices in the future.