Surface-Enhanced Raman Scattering (SERS) is a fundamental spectroscopic technique that allows to access the rich vibrational structure of molecules. A typical SERS configuration with a molecule located in a plasmonic cavity acting as an optical nanoantenna enhances the vibrational (Stokes or anti-Stokes) signal of the molecule. A number of recent implementations of Raman experiments in plasmonic nanocavities appear to provide results which escape the standard description of the Raman process based on the classical treatment of the electromagnetic fields enhancement inside the cavity.
We establish a novel analogy between non-resonant SERS in molecular spectroscopy and typical optomechanical processes. By adopting an optomechanical hamiltonian which describes the interaction between cavity plasmons and molecular vibrations, we are able to trace the quantum dynamics of both plasmons and vibrations in a SERS process. The solution of the master equation of this optomechanical hamiltonian allows to identify novel quantum effects such as the existence of different regimes of molecular vibrational build-up: a thermal vibrational regime, a vibrational pumping regime, and a strongly nonlinear vibrational regime, which emerge as a consequence of the quantum dynamics induced by the optomechanical interaction. Correlations between the Stokes and anti-Stokes Raman signals can also be traced for different temperatures and pumping powers.
The presence of strong optomechanical effects in Raman has been recently addressed experimentally in special "picocavities" formed by a few metallic atoms in a plasmonic cavity. The strong optomechanical coupling achieved in this situation is found to activate the pumping regime in the Raman signal, thus corroborating the validity of this description.
Mikolaj Schmidt, Alejandro Gonzalez-Tudela, Geza Giedke, Tomas Neuman, Yao Zhang, Rubén Esteban, and Javier Aizpurua, "Molecular optomechanics in plasmonic cavities (Conference Presentation)," Proc. SPIE 10359, Quantum Nanophotonics, 103590J (Presented at SPIE Nanoscience + Engineering: August 08, 2017; Published: 21 September 2017); https://doi.org/10.1117/12.2272962.5583466066001.
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