Second Harmonic Generation from a Single Plasmonic Nanorod Strongly Coupled to a WSe₂ Monolayer

Monolayer transition metal dichalcogenides, coupled to metal plasmonic nanocavities, have recently emerged as new platforms for strong light-matter interactions. These systems are expected to have nonlinear-optical properties that will enable them to be used in many classical and quantum photonic technologies. Here we report the first experimental investigation of the nonlinear properties of these strongly-coupled systems, by observing second harmonic generation from a WSe₂ monolayer strongly coupled to a single gold nanorod. The pump-frequency dependence of the second-harmonic signal displays a pronounced splitting that can be explained by a coupled-oscillator model with second-order nonlinearities. Rigorous numerical simulations utilizing a non-perturbative nonlinear hydrodynamic model of conduction electrons support this interpretation and reproduce experimental results. Our study thus lays the groundwork for understanding the nonlinear properties of strongly-coupled nanoscale systems.