Higher-order surface plasmon contributions to passive and active plasmonic interferometry.

We compare the intensity modulation of passive light transmission and active fluorescence emission in planar plasmonic interferometers consisting of a nano-scale hole flanked by circular grooves etched in a silver film. Discrete fast Fourier transform applied to plasmonic interferograms - i.e., optical interferograms obtained by varying the propagation phase of surface plasmon polaritons (SPPs) - reveals higher-order interference effects that can be enhanced by optimizing in-plane SPP scattering and reflection. The experimental SPP dispersion relations agree with finite-difference frequency-domain calculations. Finally, we show that odd-order SPP contributions can be suppressed by reducing the spatial coherence of the incident beam.