Near-Field-Mediated Enhancement of Two-Photon-Induced Fluorescence on Plasmonic Nanostructures

We investigate two-photon-induced fluorescence for organic chromophores next to elongated gold disks which support plasmonic resonances. By polyelectrolyte coating, the dye is placed in a defined concentration at a defined distance from the surface. A fluorescence enhancement for two-photon excitation is found in spite of a quenched emission. The measured fluorescence is always accompanied by an autoluminescence from the gold, which is of similar intensity. The fluorescence signal is strongly reduced when no plasmonic resonance is excited, while the gold autoluminescence remains of considerable strength, pointing to different enhancement mechanisms for the two processes. When the dimension of the nanostructure is within the resolution limit of the microscope, we observe double-spot patterns at the edges of the structure. We suggest this is a direct visualization of the electrical near-field.