Customizable multiband second-order sonic topological insulators via inverse design

The second-order sonic topological insulators (SSTIs) with topologically protected corner states offer promising opportunities for developing novel acoustic devices. However, most of the current SSTIs are designed via trial-and-error and are only able to host the second-order topological phases within a single bandgap, leaving the topic of second-order topological phases within multiple bandgaps rarely studied. Here, we exploit a topology optimization method to customize and optimize multiband SSTIs. To begin with, we create multiple dual-band SSTIs with customizable dual bandgaps for hosting dual-band corner states. On that basis, a three-band SSTI with three bandgaps is constructed for hosting three-band corner states. Experimental validation is performed to prove the existence of the three-band corner states. This study ushers in a route for customizing high-performance multiband SSTIs, and the designed multiband SSTIs have potential for designing robust multiband acoustic devices.