A revisitation of the Forster energy transfer near a metallic spherical nanoparticle: (1) Efficiency enhancement or reduction? (2) The control of the Forster radius of the unbounded medium. (3) The impact of the local density of states

The central motivation of this theoretical revisitation comes from the fact that some experimental works about Forster energy transfer report improvement of the Forster efficiency when the donor-acceptor molecular pair is in the vicinity of a metallic particle, while others found efficiency deterioration. In the presence of a nanoscale metallic sphere, we calculate contour plots of the Forster energy transfer rate K-F and the Forster efficiency eta as a function of the acceptor position r(A) for a fixed donor position. These contour plots clearly highlight the influence of the sphere on K-F and eta as the donor position, the orientations of donor and acceptor dipoles, and the particle size are varied; also the impact on K-F(r(A)) and eta due to the excitation of surface plasmons is easily noticeable from these contour plots. Moreover, we obtain the enhancement factor K-F/K-F0 (K-F0 refers to the case without sphere) against the donor-surface separation for particular donor-acceptor spatial distributions, several particle sizes, and distinct molecular dipole orientations. Therefore, our calculations provide a systematic analysis of the Forster energy transfer in the presence of a metallic nanosphere. Based on these results, we formulate hypotheses for explaining the aforementioned contradictory experimental results about eta. To complement our study, we examine the impact of the local density of states rho on K-F. K-F is practically unperturbed by sphere when the intermolecular separation R is less than or similar to 3 nm, since the direct donor-acceptor electromagnetic interaction is dominant. On the contrary, when R greater than or similar to 3 nm, the nanosphere perturbs K-F and this perturbation is stronger if plasmonic resonances are excited. K-F/K-F0 can greatly be enhanced in certain regions, but these regions coincide with low-efficiency regions, compromising applications involving the Forster process. In the presence of the nanosphere, the high Forster efficiency region (eta >= 0.5) has the same shape as that for the case without sphere, but its extension (Forster radius R-o) is reduced; this effect is a consequence of the large increase of the donor direct decay rate and R-o depends strongly on donor position. Consequently, the sphere controls R-o that is associated with the efficiency pattern that corresponds to the unbounded medium; this effect can be exploited in the measuring technique of nanoscale displacements of proteins that is based on the fluorescence resonant energy transfer. The functional form of K-F(rho) is determined by the intermolecular separation R, the spatial configuration and the dipole orientations of the molecular pair, and the donor proximity to the nanoparticle. (C) 2013 AIP Publishing LLC.