Abstract
Abstract Localised surface plasmons can couple strongly with the electronic transitions of a molecule, inducing new hybridised states of light and matter, the plasmon–exciton polaritons. Furthermore, molecules support vibrational degrees of freedom that interact with the electronic levels, giving rise to inelastic resonant Raman scattering under coherent laser illumination. Here we show the influence of strong plasmon–exciton coupling on resonant Raman processes that populate the vibrational states of the molecule and that lead to the characteristic surface-enhanced Raman scattering spectra. We develop analytical expressions that give insight into these processes for the case of moderate illumination intensity, weak electron–vibration coupling and no dephasing. These expressions help us to elucidate the twofold role of plasmon–exciton polaritons to pump the system efficiently and to enhance the Raman emission. Our results show a close analogy with the optomechanical process described for off-resonant Raman scattering but with a difference in the resonant reservoir. We also use full numerical calculations to study the effects reaching beyond these approximations and discuss the interplay between the fluorescence background and the Raman lines. Our results allow for better understanding and exploitation of the strong coupling regime in vibrational pumping and in the surface-enhanced resonant Raman scattering signal.
Highlights
Metallic nanoparticles support plasmonic modes at optical frequencies than can confine the electromagnetic energy to effective volumes [1,2,3,4,5,6] even below ≈10–1000 nm3 and give rise to efficient coupling of the particle plasmons with excitations in nearby molecules
It has been shown that in off-resonance surface-enhanced Raman scattering (SERS), where the electronic transitions of the molecules are strongly detuned with respect to the exciting laser, the underlying physics can be understood as a molecular optomechanical process [5, 14, 16,17,18,19,20,21], in which the vibration of the molecules and the plasmonic resonance replace the macroscopic mechanical vibrations and the optical mode in conventional optomechanics [22], respectively
We present a theoretical study of vibrational pumping and Raman scattering in resonant SERS systems [50] in the framework of cavity Quantum Electrodynamics (QED), focusing on the regime of strong coupling between the single molecule and a plasmonic mode
Summary
Metallic nanoparticles support plasmonic modes at optical frequencies than can confine the electromagnetic energy to effective volumes [1,2,3,4,5,6] even below ≈10–1000 nm and give rise to efficient coupling of the particle plasmons with excitations in nearby molecules (e.g. molecular excitons). It has been shown that in off-resonance SERS, where the electronic transitions of the molecules are strongly detuned with respect to the exciting laser, the underlying physics can be understood as a molecular optomechanical process [5, 14, 16,17,18,19,20,21], in which the vibration of the molecules and the plasmonic resonance replace the macroscopic mechanical vibrations and the optical mode in conventional optomechanics [22], respectively This QED-based theory has described regimes of nonlinear SERS with the use of a single incident laser and has naturally addressed the effect of optomechanical vibrational pumping [23,24,25,26,27], a process where enhancement of the Stokes–Raman emission leads to population of the vibrational mode of the molecule. In surface-enhanced resonant Raman scattering (SERRS), an electronic transition close to the energy of the exciting laser considerably affects the Raman process and gives rise to complex nonlinear phenomena [28] In this case, it is possible to describe the light–matter interaction by a similar Hamiltonian as the. In the second part of the article, we numerically solve the system’s dynamics without simplifications to show successively how large electronic– vibrational coupling, pure dephasing, and intense laser illumination, effects that might be important in real experiments, affect the response of the system
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