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.

Figure 1

(a) Radiative transitions to different ground state sublevels are enhanced in dimer resonators of different lengths. (b) Plasmonic nanoresonators with embedded fluorescent molecules. (c) Experimental setup.

Figure 2

Experimental (a),(b) and theoretical (c),(d) scattering and fluorescence spectra of nanoparticle dimer resonators with incorporated Cy3 dye molecules. The nanoresonators shape the fluorescence spectra by favoring specific resonant transitions. (a) Experimental scattering spectra and (b) experimental fluorescence spectra of single dimers; spectra with identical colors are measured on the same dimer. (c) Theoretical plane wave scattering cross sections of nanoparticle dimers whose interparticle distance is chosen such that the resonance wavelengths are those of (a). (d) Enhanced fluorescence spectra for Cy3 molecules located in the center of the same dimers calculated according to Eq. (5); the molecular dipole is oriented along the dimer axis [10]. (e) Experimental (▪) and theoretical (+) correlation between the wavelengths of maximum fluorescence enhancement and the wavelengths of maximum scattering, which indicate the resonance wavelengths of the nanoresonators.