Nonlocal optical effects on the fluorescence and decay rates for admolecules at a metallic nanoparticle.

A phenomenological model is implemented to study the decay rates of fluorescing molecules in the vicinity of a metallic nanoparticle, wherein the nonlocal optical response of the particle is accounted for via the hydrodynamic model for the description of the free electrons in the metal. These nonlocal effects are examined for each of the radiative rate and the nonradiative rate of the admolecule, respectively. In addition, the overall fluorescence rate which includes the enhancement ratio for the driving field intensity is also studied. It is found that for particles of very small sizes (<10 nm), the nonlocal effects, in general, lead to significantly greater fluorescence rates and smaller nonradiative decay rates for the admolecules, with the effects on radiative rates depending crucially on the orientation of the molecules. Furthermore, the effects are mostly noticeable for molecules close to the metal particle and in processes where higher multipolar interactions are significant such as those in nonradiative decay processes. Above all, these nonlocal effects can still be observable in the presence of large surface damping imposed on the metallic electrons due to the ultrasmall sizes of these nanoparticles. The relevance of these effects to some of the latest experiments is discussed.