Near-field coupling of a single fluorescent molecule and a spherical gold nanoparticle.

Near-field coupling of a single gold nanoparticle (GNP) to a single fluorescent molecule is investigated here for varying separation d between the two. While the emission quantum efficiency of the coupled system generally decreases for d!0, a pronounced near-field enhancement is observed under certain conditions, partly outweighing the efficiency loss at small distances. We report on optimizing these conditions by varying the excitation field direction and the three-dimensional relative configuration between the GNP and the fluorophore. Furthermore, we examine how the sphere diameter, the surrounding medium, as well as the absorption and emission wavelengths of the molecular dipole influence the fluorescence yield. Our results are of high practical relevance for all GNP-mediated application fields such as fluorescence microscopy, scattering near-field optical microscopy, bioanalytics, and medical applications.