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Molecular cavity optomechanics as a theory of plasmon-enhanced Raman scattering

Abstract

The exceptional enhancement of Raman scattering by localized plasmonic resonances in the near field of metallic nanoparticles, surfaces or tips (SERS, TERS) has enabled spectroscopic fingerprinting down to the single molecule level. The conventional explanation attributes the enhancement to the subwavelength confinement of the electromagnetic field near nanoantennas. Here, we introduce a new model that also accounts for the dynamical nature of the plasmon–molecule interaction. We thereby reveal an enhancement mechanism not considered before: dynamical backaction amplification of molecular vibrations. We first map the system onto the canonical Hamiltonian of cavity optomechanics, in which the molecular vibration and the plasmon are parametrically coupled. We express the vacuum optomechanical coupling rate for individual molecules in plasmonic ‘hot-spots’ in terms of the vibrational mode's Raman activity and find it to be orders of magnitude larger than for microfabricated optomechanical systems. Remarkably, the frequency of commonly studied molecular vibrations can be comparable to or larger than the plasmon's decay rate. Together, these considerations predict that an excitation laser blue-detuned from the plasmon resonance can parametrically amplify the molecular vibration, leading to a nonlinear enhancement of Raman emission that is not predicted by the conventional theory. Our optomechanical approach recovers known results, provides a quantitative framework for the calculation of cross-sections, and enables the design of novel systems that leverage dynamical backaction to achieve additional, mode-selective enhancements. It also provides a quantum mechanical framework to analyse plasmon–vibrational interactions in terms of molecular quantum optomechanics.

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Figure 1: Cavity-optomechanical model of the interaction between plasmon and molecular vibration.
Figure 2: Feedback diagram of dynamical backaction in the SERS process.
Figure 3: Sharpening of the Raman excitation spectral linewidth
Figure 4: Anomalous anti-Stokes/Stokes ratio under dynamical backaction amplification.

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Acknowledgements

The authors thank P. Fischer, H.-H. Jeong, V. Sudhir, D. Wilson and E. Verhagen for discussions and C. Corminboeuf and E. Bremond for help with running the chemical simulations. This work was partially supported by an ERC Advanced Grant (QREM), the NCCR of Quantum Engineering (QSIT) as well as the Swiss National Science Foundation. P.R. acknowledges the support of the Max Planck-EPFL Center for Molecular Nanoscience and Technology. C.G. acknowledges the support of the Swiss National Science Foundation through an Ambizione Fellowship. N.P. acknowledges the support of a Marie-Curie Fellowship.

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P.R., C.G. and T.J.K. conceived the study. P.R. and C.G. developed the model and performed the calculations. P.R., C.G. and T.J.K. co-wrote the paper. All authors discussed and analysed the results.

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Correspondence to Christophe Galland or Tobias J. Kippenberg.

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Roelli, P., Galland, C., Piro, N. et al. Molecular cavity optomechanics as a theory of plasmon-enhanced Raman scattering. Nature Nanotech 11, 164–169 (2016). https://doi.org/10.1038/nnano.2015.264

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