Optomechanical Pumping of Collective Molecular Vibrations in Plasmonic Nanocavities

In surface-enhanced Raman scattering (SERS), vibrations of molecules couple with optical modes of a plasmonic nanocavity via a molecular optomechanical interaction. This molecule-plasmon coupling gives rise to optomechanical effects such as vibrational pumping-the excitation of molecular vibrations due to Stokes scattering. Here, we investigate the influence of vibrational pumping and collective effects on biphenyl-4-thiol (BPT) molecules in nanoparticle-on-mirror nanocavities, both experimentally by pulsed SERS spectroscopy and theoretically with optomechanical modeling. From the anti-Stokes to Stokes ratio of hundreds of individual nanostructures, we provide clear experimental evidence of vibrational pumping in high-wavenumber vibrational modes at room temperature and investigate the emergence of collective vibrational effects experimentally by varying the spacing and number of BPT molecules in the nanocavity. This is achieved by preparing mixed monolayers of different molecular species with distinct vibrational spectra. We show a 3-fold reduction of the vibrational pumping rate in experiments by tuning the collective coupling through the intermolecular spacing. Including the full plasmonic multimode response as well as collective molecular vibrations in the optomechanical theory leads to good agreement with experiments. The optomechanical control of molecular vibrations may thus enable bond-selective plasmonic chemistry, collective parametric instabilities, and phonon lasing.