Softening molecular bonds through the giant optomechanical spring effect in plasmonic nanocavities

Molecular vibrations couple to visible light only weakly, have small mutual interactions, and hence are often ignored for nonlinear optics. Here we show the extreme confinement provided by plasmonic nanocavities can sufficiently enhance optomechanical coupling so that intense laser illumination drastically softens the molecular bonds. This optomechanical pumping regime produces strong distortions of the Raman vibrational spectrum that are related to giant vibrational frequency shifts from an 'optical spring effect' which is hundred-fold larger than in traditional cavities, as predicted by an optomechanical theory fully accounting for the multimodal nanocavity response as well as for near-field-induced collective phonon interactions that hybridize the vibrations of hundreds of identical molecules. The theoretical results are consistent with the strongly non-linear behavior exhibited in the Raman spectra of molecular monolayers placed in more than a thousand nanoparticle-on-a-mirror constructs illuminated by microwatt ultrafast laser pulses. Driving this collective phonon in the nanocavity paves the way to control reversible bond softening, as well as irreversible chemistry. Optomechanical dressing thus provides a new tool for molecular dynamics distinct from coherent control, vibrational strong coupling, or vibrational heating.