Giant optomechanical spring effect in plasmonic nano- and picocavities probed by surface-enhanced Raman scattering

TL;DR

This paper demonstrates that plasmonic nano- and pico-cavities can significantly enhance optomechanical interactions, leading to large vibrational frequency shifts and bond softening in molecules, with potential applications in controlling chemical reactions.

Contribution

It reveals the giant optomechanical spring effect in plasmonic cavities and provides a combined theoretical and experimental analysis of non-linear Raman spectra under ultrafast laser illumination.

Findings

Giant vibrational frequency shifts observed in Raman spectra.

Theoretical simulations match experimental non-linear behavior.

Indications of accessing the optical spring effect in single molecules.

Abstract

Molecular vibrations couple to visible light only weakly, have small mutual interactions, and hence are often ignored for non-linear optics. Here we show the extreme confinement provided by plasmonic nano- and pico-cavities 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 related to giant vibrational frequency shifts from an optical spring effect which is hundred-fold larger than in traditional cavities. The theoretical simulations accounting for the multimodal nanocavity response and near-field-induced collective phonon interactions are consistent with the experimentally-observed non-linear behavior exhibited in the Raman spectra of nanoparticle-on-mirror constructs illuminated by ultrafast laser pulses. Further, we show indications that plasmonic picocavities allow us to access the optical spring effect in single molecules with continuous illumination. Driving the collective phonon in the nanocavity paves the way to control reversible bond softening, as well as irreversible chemistry.