Fluorescence near metal tips: The roles of energy transfer and surface plasmon polaritons.

We simulate the remarkable changes that occur to the decay rates of a fluorescent molecule as a conical metal tip approaches. A new and simple model is developed to reveal and quantify which decay channels are responsible. Our analysis, which is independent of the method of molecular excitation, shows some universal characteristics. As the tip-apex enters the molecule's near-field, the creation of surface plasmon polaritons can become extraordinarily efficient, leading to an increase in the nonradiative rate and, by proportional radiative-damping, in the radiative rate. Ehancements reaching 3 orders of magnitude have been found, which can improve the apparent brightness of a molecule. At distances less than ~5nm, short-ranged energy transfer to the nano-scale apex quickly becomes dominant and is entirely nonradiative.