Coherent dynamics of molecular vibrations in single plasmonic nanogaps

Coupling with a resonant optical cavity is well known to modify the coherence of molecular vibrations. However, in the case of molecules coupled to a plasmonic nanocavity mode, the local mechanisms of vibrational coherence decay remain unclear. Here, the dynamics of a few hundred molecules of nitrothiophenol (NTP) within a single plasmonic nanocavity are studied by sum-frequency generation. Time-resolved experiments reveal a coherence lifetime sensitive to intermolecular dipole coupling strength, and is found to compete with cavity-induced vibrational dephasing above a threshold ∼10 cm^{-1}. Tuning of the system dipole strength presents a quantitative means to explore the energy landscape of coherent interactions within nanoscale vibrational platforms, relevant for studies from vibrational chemistry to cavity quantum optomechanics.