Surface-enhanced Raman scattering and fluorescence near metal nanoparticles

We present a general model study of surface-enhanced resonant Raman scattering and fluorescence, focusing on the interplay between electromagnetic (EM) effects and the molecular dynamics as treated by a density matrix calculation. The model molecule has two electronic levels, is affected by radiative and non-radiative damping mechanisms, and a Franck-Condon mechanism yields electron-vibration coupling. The coupling between the molecule and the electromagnetic field is enhanced by placing it between two Ag nanoparticles. The results show that the Raman scattering cross section can, for realistic parameter values, increase by some 10 orders of magnitude (to $\sim10^{-14}$ cm$^2$) compared with the free-space case. Also the fluorescence cross section grows with increasing EM enhancement, however, at a slower rate, and this increase eventually stalls when non-radiative decay processes become important. Finally, we find that anti-Stokes Raman scattering is possible with strong incident laser intensities, $\sim 1$ mW/$\mu$m.