Second harmonic generation via double topological disclination states

The topological disclination state (TDS) in topological insulators (TIs) has strong localization, and its impact on nonlinear effects has garnered significant attention. Second harmonic generations (SHGs) have been proven to be generated individually in topological corner states and topological edge states. However, the SHGs in TDSs have not been discussed so far. Here, a scheme for satisfying the SHG condition through the coupling of double TDS has been proposed, and the high conversion efficiency and strong robustness with disorder have been proven due to the doubly resonant TDS. Our method provides a new perspective for exploring the nonlinear effects of topological states caused by topological defects.