When bound to metals, molecular vibrations play a key role in sensing, catalysis, molecular electronics and beyond, but investigating their coherence and dynamics is difficult as pulsed experiments prove very challenging. In this thesis, I study vibrations of 1-1000 molecules in a plasmonic nanocavity when driven by picosecond pulsed lasers out of the linear regime. This unravels new non-linear effects such as room-temperature vibrational pumping, giant optomechanical spring shifts, collective molecular vibrations, accelerated decay of vibrational coherence, and the generation of correlated photon pairs.

In plasmonic nanocavities, optical fields are enhanced 100-fold and focused to a nanometre-thin gap. Vibrations of molecules placed in the cavity interact strongly with the optical resonances, forming a coupled optomechanical system. Using pulsed laser illumination, I find that surface-enhanced Raman scattering can significantly increase the phonon population above the thermal equilibrium. This vibrational pumping leads to non-linear anti-Stokes scattering observable at room temperature. Further, the optomechanical coupling induces a red-shift of the vibrational energy by >100 cm⁻¹ and broadening of the Raman line at high peak laser powers (optomechanical spring shift). These non-linear effects are strongly enhanced by the excitation of collective molecular phonon modes. Further experiments show that Stokes-induced anti-Stokes scattering exhibits strong cross-frequency photon bunching. These correlated Stokes – anti-Stokes photon pairs show non-classical behaviour and could be used for applications in quantum computing and communication.

To study the dynamics of molecular vibrations, I use time-resolved incoherent and coherent anti-Stokes Raman scattering. Developing a new single-photon lock-in detection technique, it is possible to simultaneously record the decay of the vibrational population and vibrational dephasing for each nanocavity. The vibrational dephasing is found to strongly accelerate depending on the exciting laser intensity. Understanding these modified vibrational dynamics on plasmonically-active substrates is crucial for improving surface-enhanced catalysis of chemical reactions and heat transfer in molecular electronics.