Coherent Second Harmonic Generation Enhanced by Coherent Plasmon–Exciton Coupling in Plasmonic Nanocavities

Efficient nonlinear optical process at the nanoscale is in high demand for the trend of integrating photonic devices. However, limited by the short interaction length, the nonlinear conversion efficiency in nanostructures is generally weak. Inheriting the strong confinement of light and the large nonlinear response of the matter, plasmon–exciton polaritons (plexcitons) provide a promising platform for nonlinear nanophotonics. Here, we engineered the effective second-order susceptibility of a hybrid nanostructure by coherent plasmon–exciton coupling. More than 3000-fold enhancement in the second harmonic generation (SHG) is achieved at the plexciton states in a plasmonic nanocavity embedded with multilayer WSe2 compared with that in the bare exciton material on film. The polarimetric and layer-dependent measurements reveal that the SHG originates from both plasmon and exciton components and their nonlinear interaction. Our work provides an additional degree of freedom for enhancing the nonlinear conversion efficiency and gives an insight into the nonlinear plasmon–exciton interaction and their coherent control.