Inversely Designed Second-Order Photonic Topological Insulator With Multiband Corner States

Second-order photonic topological insulators (SPTIs) with topologically protected corner states possess extraordinary abilities of robust light steering in lower dimensions. However, previous SPTIs are difficult for multiband on-chip applications. To overcome this challenge, we design, via the inverse design method, a SPTI supporting four highly localized corner states within four sizeable band gaps that are robust to bulk impurities. Importantly, the designed SPTI is made of fully connected dielectric materials, which can be readily fabricated in nanoscale via electron-beam lithography and integrated into on-chip circuits. Our work offers potential applications in designing multiband on-chip photonic integrated devices with high efficiency, high capacity, and high robustness for both linear and nonlinear optical processing.