Density-matrix evaluation of the enhancement to resonant Raman scattering and fluorescence of molecules confined in metallic nanoparticle dimers.

In the present work we study the surface-enhanced resonant Raman scattering (SERRS) and fluorescence (SEF) spectra of a general model molecule confined in metallic dimers consisting of Ag, Au and hybrid AuAg nanoparticles (NPs). The electromagnetic (EM) enhancement factors were simulated by the generalized Mie scatting method and the scattering cross section of the molecules were obtained by density-matrix calculations. The influence of the size of the NPs and the separation between the dimer on the Raman scattering and fluorescence were systematically studied and analyzed in detail. It was found that the SERRS mainly related to EM enhancement and the SEF depended on the competition between EM enhancement and quantum yield, both of which could be controlled by tuning the radius and separation of the metallic dimers. The optimal radius of the NPs for SERRS were found to be around 30 nm for AgNPs, 40 nm for AuNPs and 50 nm for hybrid AuAgNPs. The strongest Raman enhancement as predicted by the theoretical simulations were 6.2 × 10, 1.5 × 10 and 5.2 × 10 for the three types of structures, respectively. These results could offer valuable information for the design of metallic substrates for surface enhanced Raman and fluorescence measurements.