Three-dimensional higher-order topological acoustic system with multidimensional topological states

Topologically protected gapless edge/surface states are phases of quantum matter which behave as massless Dirac fermions and are immune to disorders and continuous perturbations. Recently, a new class of topological insulators (TIs) with gapped edge states and in-gap corner states has been theoretically predicted in electronic systems and experimentally realized in two-dimensional (2D) mechanical and electromagnetic systems, electrical circuits, optical and sonic crystals, and elastic phononic plates. Here we elaborately design a three-dimensional topological acoustic system by arranging acoustic meta-atoms in a simple cubic lattice. Under the direct field measurements, besides the 2D surface propagations on all of the six surfaces, the one-dimensional hinge propagations behaving as acoustic fibers along the 12 hinges, and the zero-dimensional corner modes working as localized resonances at the eight corners are experimentally confirmed. As these multidimensional topological states are activated in different frequencies and independent spaces, our works may provide a new pathway for designing high-performance integrated acoustic devices that can manipulate the propagation of sound waves in multiple dimensions.