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of the plasmonic cavity. This insightallows the coherent nonlinear interaction between quantized plasmons and molecular vibrations in SERS to be described using a typical optomechanical Hamiltonian that couples the population of the cavity with the vibrational degree of freedom. The strength of this coupling is determined by the Raman activity of the molecule andthe eective mode volume into which the plasmonic cavity can squeeze light. As a result, a SERS system can be considered as an optomechanical set-up with very high mechanical frequencies of the vibrations (tens of terahertz) and signicant coupling parameters, in spite of the bad quality factors of plasmonic cavities.
This model suggests that the vibrations of a molecule can be amplied in SERSby illuminating a plasmonic cavity with a blue-detuned laser, just like the heating of mirror vibrations in an optomechanical cavity can be induced by a similarly detuned laser. Under these conditions, a signicant increase of the Raman signal intensity is expected. In fact, Kippenberg and colleagues point out that such an eect
may have been observed in recent Raman experiments in which blue detuning ofthe incident laser light was required for maximum enhancement2, and an increase in power intensity led to a narrowing of the spectral signal3. While these results would not be predicted by standard SERS theory, they can be foreseen and explained by this new model. This theoretical framework also has implications for future SERS experiments. In particular, the relative position of a vibrational band with respect to the energy of the plasmonic cavitiesand the incident laser should be taken into consideration.
Furthermore, special attention should be paid to the analysis of nonlinear signals involving stimulated emissionof vibrations5, as well as the correlations between the Stokes (which occurs whena molecule gains energy) and anti-Stokes (which occurs when a molecule loses energy) photon scattering6. The low thermal populations of vibrations and large coupling parameters could enable the control of entanglement between the states of the optical cavity and vibrations,
as well as the study of fundamental optomechanical eects related to quantum correlations and metrology. Molecular spectroscopy suddenly opens the doorto explore the interactions of light and vibrational states of...