Multiband acoustic topological metamaterials with valley-switchable higher-order corner states

Topological insulators have become a hot research area in the field of acoustics in recent years, particularly the recently-appearing higher-order topological insulators (HOTIs) that support low-dimensional corner states. However, most HOTIs reported are implemented in tetragonal lattices, and only support single-frequency operation, which enables the higher-order states to be fixed at specific corners and at specific frequencies, lacking the multiband response and flexibility of switching. Herein, the experimental realization of flexibly valley-switchable topological corner states in multiple frequency bands with acoustic metamaterials in the triangular lattice are reported in a 2D system. The designed topological metamaterials, whose second-order topology is characterized by nontrivial bulk polarization, exhibit diverse topological valley-switchability in different bandgaps, as verified by simulation and experiments. Furthermore, an arbitrary structure containing complex patterns with various crossing angles is designed and experimentally implemented, where selected corners can be switched on or off by valley switching, and the switching modes are distinct in different bandgaps. The reported valley topological metamaterials enrich the fundamental understanding of the interaction between multiband higher-order topology and valley degrees of freedom, which inspire potential applications for multiband communication devices and integrated acoustics and photonics.