Observation of topological valley transport of sound in sonic crystals

Valleytronics — exploiting a system’s pseudospin degree of freedom — is being increasingly explored in sonic crystals. Now, valley transport of sound is reported for a macroscopic triangular-lattice array of rod-like scatterers in a 2D air waveguide. The concept of valley pseudospin, labelling quantum states of energy extrema in momentum space, is attracting attention1,2,3,4,5,6,7,8,9,10,11,12,13 because of its potential as a new type of information carrier. Compared with the non-topological bulk valley transport, realized soon after predictions1,2,3,4,5, topological valley transport in domain walls6,7,8,9,10,11,12,13 is extremely challenging owing to the inter-valley scattering inevitably induced by atomic-scale imperfections—but an electronic signature was recently observed in bilayer graphene12,13. Here, we report the experimental observation of topological valley transport of sound in sonic crystals. The macroscopic nature of sonic crystals permits a flexible and accurate design of domain walls. In addition to a direct visualization of the valley-selective edge modes through spatial scanning of the sound field, reflection immunity is observed in sharply curved interfaces. The topologically protected interface transport of sound, strikingly different from that in traditional sound waveguides14,15, may serve as the basis for designing devices with unconventional functions.