The ultrafast dynamics of exciton-polaritons (EPs) in single plasmonic nanocavities are of fundamental interest due to extreme mode confinement ($V\sim {10}^{-6}{\lambda }^3$), which enables few-exciton strong coupling and strong nonlinearity of EPs. However, directly probing transient evolutions of EPs in a single nanocavity has remained a challenge. Here, we develop an EP-resonant enhanced ultrafast difference-frequency generation (DFG) microscopy for directly visualizing EP dynamics in a single ${\mathrm{WSe}}_2$-Au nanocavity. By scanning the pump photon energy across the polariton resonances and recording the DFG intensity as a function of pump-probe delay, we construct two-dimensional maps that directly visualize the transient evolution of the high and low energy branches of EPs. The maps reveal that the EPs collapse suddenly and then revive within 2 ps with a saturation threshold below 2 pJ, indicating strong nonlinearity of EPs in the few-exciton saturation regime. This Letter provides a new approach for visualizing the nonequilibrium dynamics of EPs in a single plasmonic nanocavity, offering important experimental evidence for ultrafast manipulation of EPs.

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• Exciton polariton
• Nanophotonics
• Nanoparticles
• Transition metal dichalcogenides
• Finite-element method
• Materials modeling
• Maxwell's equations
• Time-resolved light scattering spectroscopy