Spectral properties of a fluorescing molecule within a spherical metallic nanocavity

The electrodynamics of a fluorescing molecule ( an electric dipole emitter) within a spherical metallic nanocavity is studied. It is shown that, for a favorable cavity size, the fluorescence properties such as fluorescence emission rate or photostability are strongly enhanced. This is caused by two effects: the enhancement of the exciting electromagnetic field within the cavity, and the dramatic reduction of the fluorescence lifetime due to the strong near-field interaction between dye and cavity. Both effects can largely outweigh the fluorescence losses caused by energy absorption within the cavity metal. Special emphasis is given to calculating the wavelength dependence of the fluorescence properties, allowing the modeling of real dyes with broad absorption and emission spectra. As an example of practical interest, the cavity-enhanced fluorescence properties of the widely used dye Rhodamine 6G are extensively studied. It is shown that the presence of the cavity also has a strong impact on the absorption and emission spectra of the dye. Last but not least, it is demonstrated that the interaction between dye and nanocavity can dramatically boost its fluorescence quantum yield.