Förster Energy Transfer in the Vicinity of Two Metallic Nanospheres (Dimer)

We present a detailed theoretical analysis of the Förster energy transfer process when a pair of molecules (donor and acceptor) is located nearby a cluster of two metallic nanospheres (dimer). We consider the case in which plasmonic resonances are within the overlap between the donor emission and acceptor absorption spectra, as well as the case that excludes such resonances from the aforementioned spectral overlap. Moreover, we explore the dependence of the Förster energy transfer rate on different dimer configurations (size and separation of nanospheres) and several dipole orientations of molecules. The dimer perturbs strongly the Förster energy transfer rate when plasmons are excited, donor dipole is oriented along the longitudinal axis of the dimer, and the radii of nanospheres and the sphere-gap distance are on the order of a few nanometers. In case of plasmonic excitation, the Förster energy transfer rate is degraded as the sphere-gap distance and size of the nanoparticles increase due to the dephasing of electronic motion arising from ohmic losses of metal. Also, we study the Förster efficiency influenced by the dimer, finding that the high efficiency region (delimited by the Förster radius curve) is reduced as a consequence of significant enhancement of the direct donor decay rate. Our study could impact applications that involve Förster energy transfer.