Shaping emission spectra of fluorescent molecules with single plasmonic nanoresonators

We show that plasmonic nanoresonators composed of two gold nanoparticles change not only the intensity but also the spectral shape of the emission of fluorescent molecules. The plasmonic resonance frequency can be tuned by varying the distance between the nanoparticles, which allows us to selectively favor transitions of a fluorescent molecule to a specific vibrational ground state. Experimental data from correlated scattering and fluorescence microscopy agree well with calculations in the framework of generalized Mie theory. Our results show that the widely used description of a dye molecule near a metal surface as a mere two-level system is inadequate.