Giant optomechanical spring effect in plasmonic nano- and picocavities probed by surface-enhanced Raman scattering
jats:titleAbstract</jats:title>jats:pMolecular 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.</jats:p>
Acknowledgements: We thank Mikolaj Schmidt and Adrián Juan Delgado for fruitful discussions, and Marlous Kamp for synthesizing SPARK constructs. We acknowledge EPSRC grants EP/N016920/1, EP/L027151/1, NanoDTC EP/L015978/1, NSFC grant 12004344, NSFC-DPG grant 21961132023, Basque Government grant IT1526-22, grant PID2019-107432GB-I00 funded by MCIN/AEI/10.13039/501100011033/, and EU THOR 829067, POSEIDON 861950 and PICOFORCE 883703. L.A.J. acknowledges support from the Cambridge Trust and EPSRC award 2275079. B.d.N acknowledges support from the Winton Programme for the Physics of Sustainability, and the Royal Society in the form of a University Research Fellowship URF \R1\211162. C.C. thanks NPL for PhD funding.
EC | EU Framework Programme for Research and Innovation H2020 | H2020 Priority Excellent Science | H2020 European Research Council (H2020 Excellent Science - European Research Council) (883703)
National Science Foundation of China | National Natural Science Foundation of China-Yunnan Joint Fund (NSFC-Yunnan Joint Fund) (12004344, 21961132023)
Ministry of Economy and Competitiveness | Agencia Estatal de Investigación (Spanish Agencia Estatal de Investigación) (PID2019-107432GB-I00)