The central motivation of this theoretical revisitation comes from the fact that some experimental works about Förster energy transfer report improvement of the Förster 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 Förster energy transfer rate KF and the Förster efficiency η as a function of the acceptor position rA for a fixed donor position. These contour plots clearly highlight the influence of the sphere on KF and η as the donor position, the orientations of donor and acceptor dipoles, and the particle size are varied; also the impact on KF(rA) and η due to the excitation of surface plasmons is easily noticeable from these contour plots. Moreover, we obtain the enhancement factor KF/KF0 (KF0 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 Förster energy transfer in the presence of a metallic nanosphere. Based on these results, we formulate hypotheses for explaining the aforementioned contradictory experimental results about η. To complement our study, we examine the impact of the local density of states ρ on KF. KF is practically unperturbed by sphere when the intermolecular separation R is ≲ 3 nm, since the direct donor-acceptor electromagnetic interaction is dominant. On the contrary, when R ≳ 3 nm, the nanosphere perturbs KF and this perturbation is stronger if plasmonic resonances are excited. KF/KF0 can greatly be enhanced in certain regions, but these regions coincide with low-efficiency regions, compromising applications involving the Förster process. In the presence of the nanosphere, the high Förster efficiency region (η ≥ 0.5) has the same shape as that for the case without sphere, but its extension (Förster radius Ro) is reduced; this effect is a consequence of the large increase of the donor direct decay rate and Ro depends strongly on donor position. Consequently, the sphere controls Ro 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 KF(ρ) 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.

A close look at the Förster energy transfer in the vicinity of a metallic nanoparticle Una examinación de la transferencia de energía Förster en la cercanía de una nano-partícula metálica Keywords: Intermolecular interaction; surface plasmons. Palabras clave:Interacción intermolecular; plasmones de superficie. ABSTRACTThe energy of an excited molecule (donor) can be transferred to a nearby molecule (acceptor) in ground state (Förster energy transfer). This mechanism, due to intermolecular electromagnetic interaction, depends on the environment in which the donor-acceptor pair is embedded. We closely examine the influence of a gold nanosphere on the Förster energy transfer rate K F including the impact of excitation of surface plasmons. When the intermolecular distance R is 3 nm, the influence by the metallic nanosphere on K F is weak. However, when the donor-surface separation is a few nanometers from the surface and R is  3 nm, K F is modified by the presence of the nanoparticle. The excitation of surface plasmons causes a stronger perturbation of K F . In the aforementioned region (R  3nm), K F can be enhanced with respect to K F0 (Förster energy transfer rate without nano-sphere). RESUMENLa energía de una molécula excitada (donador) puede ser transferida a una molécula cercana (aceptor) en estado base (transferencia de energía Förster). Este mecanismo, debido a la interacción electromagnética intermolecular, depende del entorno en el cual el par donadoraceptor es embebido. Se examina detalladamente la influencia de una nano-esfera de oro sobre la rapidez de transferencia de energía Förster K F , incluyendo el impacto de la excitación de plasmones de superficie. Cuando la distancia intermolecular es R es 3 nm, la influencia de la nano-esfera metálica sobre K F es débil. Por otro lado, cuando la separación donadorsuperficie es de unos cuantos nanómetros y la distancia intermolecular R es  3 nm, K F es modificada por la presencia de la nano-partícula; la excitación de plasmones de superficie causa una perturbación más fuerte de K F . En la mencionada región (R  3nm), K F puede aumentar con respecto a K F0 (rapidez de transferencia de energía Förster sin nano-esfera). (Förster, 1946). Typically, this process occurs when the intermolecular separation R is close to, or less than 10 nm. This energy transfer mechanism is due to donor-acceptor electromagnetic interaction. Commonly, Förster energy transfer is associated with plant photosynthesis; because of the Förster process, the energy absorbed by chlorophyll molecules is transported long distances until it reaches the cell reaction centers. However, Förster energy transfer also has a very important practical use in measuring nanometric displacements of the conformational folding of proteins. Consequently, Förster energy transfer can be applied as a tool for studying biological processes. For Cómo citar:Gonzaga Galeana, J. A. & Zurita Sánchez, J. R (2014). A close look at the Förster energy transfer in the vicinity of a metallic nanoparticle. Acta Universitaria...

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We study theoretically the absorbed power by a dielectric sphere when it is illuminated with partially coherent light coming from two pinholes. We present a general theory of Mie scattering of partially coherent light (based on the angular spectrum method); this theory is applied to the aforementioned particular scattering problem which is solved analytically. We found that, if the diameter of the sphere is smaller than the skin depth, the absorbed power by the sphere depends complicatedly on the degree of coherence of light between the pinholes. The absorbed power for coherent illumination can be smaller or greater than that for incoherent light between pinholes, depending on the geometrical configuration. Furthermore, there are particular setups in which the absorbed power is independent of the degree of coherence, despite that the intensity distribution of the electric field inside the sphere depends significantly on the spatial coherence. Hence, by tuning the coherence length between the pinholes, the absorbed power by the sphere can be controlled; if a whispering gallery mode is excited, the absorbed power can be varied over a wide range. Our study might have implications in the understanding of light absorption in photovoltaic nano-devices.