Nonlinear excitation power dependence of surface enhanced fluorescence from a nanostructured Ag film

We investigate the excitation power dependence of fluorescent emission from Cy3-tagged molecules separated from an Ag film prepatterned with arrays of nanostructures by a thin spacer. While the fluorescent intensities from both the patterned area and the flat Ag surfaces increase monotonically with the power of excitation light, the fluorescent contrast between them decreases with excitation power in a nonlinear fashion. We propose a simple theoretical model which includes basic properties of molecular fluorescence, the effect of near field enhancement from surface plasmon excited on the patterned structure, and the effect of enhancement of fluorescent emission rate and non-radiative decay rate. Our results agree qualitatively with the prediction of a model for which there is a larger enhancement of the excitation rate than that of the total decay rate of the excited molecule.