We have investigated the effects of tuning the localized surface plasmon resonances (LSPRs) of silver nanoparticles on the fluorescence intensity, lifetime, and Raman signal from nearby fluorophores. The presence of a metallic structure can alter the optical properties of a molecule by increasing the excitation field, and by modifying radiative and non-radiative decay mechanisms. By careful choice of experimental parameters we have been able to decouple these effects. We observe a four-fold increase in fluorescence enhancement and an almost 30-fold increase in decay rate from arrays of Ag nanoparticles, when the LSPR is tuned to the emission wavelength of a locally situated fluorophore. This is consistent with a greatly increased efficiency for energy transfer from fluorescence to surface plasmons. Additionally, surface enhanced Raman scattering (SERS) measurements show a maximum enhancement occurs when both the incident laser light and the Raman signal are near resonance with the plasmon energy. Spatial mapping of the SERS signal from a nanoparticle array reveals highly localized differences in the excitation field resulting from small differences in the LSPR energy.
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