Resonance energy transfer from a fluorescent dye to a metal nanoparticle

A quantum mechanical theory of the rate of excitation energy transfer from a fluorescent dye molecule to the surface plasmonic modes of a spherical metal nanoparticle is presented. The theory predicts the distance dependence of the transfer rate to vary as 1/d(sigma), with sigma=3-4 at intermediate distances, in partial agreement with the recent experimental results. Förster's 1/d(6) dependence is recovered at large separations. The predicted rate exhibits nontrivial nanoparticle size dependence, ultimately going over to an asymptotic, a(3) size dependence. Unlike in conventional fluorescence resonance energy transfer, the orientational factor is found to vary between 1 and 4.