Multiband acoustic helical interface states in inverse-designed sonic crystals with glide symmetry

Acoustic topological insulators (ATIs) with topological states that are insensitive to defects and impurities offer a robust way to steer acoustic waves. However, current ATIs in square lattice only host topological interface states within one bulk bandgap, restricting their multiband applications. Here, we design the ATI, made of glide-symmetric sonic crystals (SCs), hosting multiband topological interface states within multiple bulk bandgaps. First, SCs restricted with glide and mirror symmetries are inversely designed to host multiple bulk bandgaps. Then, the ATI with multiband helical interface states is constructed by selecting two kinds of unit cells (UCs) from the inverse-designed SC and arranging them to form an interface. Both dual-band and triple-band ATIs are designed and experimentally validated. The total size of interface states hosted by the triple-band ATI is about 8.5 times of the record. Besides, by exploiting the mismatch of frequency windows of interface states at the horizontal and vertical interfaces, we realize acoustic demultiplexers for routing interface states. Our work suggests a route to engineering multiband ATIs, having promising applications in designing novel acoustic devices for multiband information processing and communication.