Electrodynamic calculations of spontaneous emission coupled to metal nanostructures of arbitrary shape : nanoantenna-enhanced fluorescence

We present a theoretical study of the spontaneous emission of an optical emitter close to a metal nanostructure of arbitrary shape. The modification of the corresponding radiative and nonradiative decay rates and resulting quantum efficiencies, expressed on the basis of a semiclassical dipole model in terms of the local plasmonic mode density, is calculated by means of the rigorous formulation of the Green’s theorem surface integral equations. Metal losses and the intrinsic nonradiative decay rate of the molecules are properly considered, presenting relationships valid in general for arbitrary intrinsic quantum yields. Resonant enhancement of the radiative and nonradiative decay rates of a fluorescent molecule is observed when coupled to an optical dimer nanoantenna. Upon varying the dipole position, it is possible to obtain a predominant enhancement of radiative decay rates over the nonradiative counterpart, resulting in an increase of the internal quantum efficiency. For emitters positioned in the gap, quantum efficiency enhancements from an intrinsic value of 1% to ~75% are possible.