Observation of coherence-related phenomena in experiments with surface plasmon polaritons excited by fluorescence

We describe experiments using a plasmon-coupled leakage radiation microscope where Surface Plasmon Polaritons (SPP) were excited by spontaneous emission of dye molecules. We report the seemingly contradictory observation of various coherence-related phenomena in these experiments and present a detailed explanation of our findings. We show that the apparent discrepancy between the incoherent character of the fluorescence and the coherent nature of the observed phenomena can be simply explained by recognizing that the short coherence length of the fluorescence is comparable to the propagation length of the excited SPPs. Our analysis then demonstrates that SPPs excited by fluorescence can be considered as a coherent illumination source in SPP microscopy.