Spectral dependence of single molecule fluorescence enhancement

The fluorescence from a single molecule can be strongly enhanced near a metal nanoparticle acting as an optical antenna. We demonstrate the spectral tunability of this antenna effect and show that maximum enhancement is achieved when the emission frequency is red-shifted from the surface plasmon resonance of the particle. Our experimental results, using individual gold and silver particles excited at different laser-frequencies, are in good agreement with an analytical theory which predicts a different spectral dependence of the radiative and non-radiative decay rates.