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Fluorescence enhancement and quenching in tip-enhanced fluorescence spectroscopy

We study fluorescence enhancement for a dipole emitter placed between a tip and different dielectric substrates. Resonant behavior is observed as a function of tip-substrate distance for a silver tip placed above a glass substrate and displays a strong dependence on the radius of the curvature of the tip. By choosing appropriate sizes for the tip radius and the tip-substrate gap to optimize the competition between field enhancement and fluorescence quenching, we have found fluorescence enhancement exceeding three orders of magnitude can be achieved. The enhancement can be further improved by matching the silver tip with an appropriate dielectric substrate to resonantly excite gap plasmons. This is verified by comparing the matched silver tip-TiO2 substrate pairing with non-matched tip-dielectric substrate pairings. Compared with the large fluorescence enhancement observed in the case of a silver tip above a glass substrate, modest enhancement is obtained for a silicon tip and can be further improved by using a high dielectric material as the substrate. The optimal tip-enhanced fluorescence obtained in a silver tip-TiO2 substrate pairing may be useful in obtaining efficient fluorescence signal in the same setup used for tip-enhanced Raman spectroscopy.