Wavelength demultiplexing in topologically coupled valley sonic crystal waveguides

The control of topological sound transport has attracted increasing attention from researchers. Topological interface states, which have pseudospin-momentum-locked dispersions, render sound transport backscattering immune against defects. Here we report on the experimental observation of topological energy transfer through distinct passages, whose existence and elimination are attributed to the coupling and decoupling between the topological interface states. The phenomenon of topological coupling is elucidated well using interference theory and the coupling wavelength, including the propagation constants of the two supermodes and coupling length as key parameters. As a potential application, we numerically and experimentally demonstrate a topological wavelength demultiplexer, where the incident sound can be split into different channels, immune to the structural disorder along the passages, by employing different coupling wavelengths of interface states at different coupled frequencies. Our work provides the prototype for controlling acoustic topological transport, and may facilitate practical applications in multichannel sound signal processing devices for separating sound at different wavelengths.