Far-field detection and imaging of surface plasmon polaritons by engineering sub-wavelength slit structures

We develop a method to detect surface plasmon polaritons (SPP) launched by a sub-wavelength slit structure using optical microscopy. The mechanism relies on an ultra-thin layer of polymer, whose thickness is varied with nano-scale precision to enable matching between the momentum of incident light and that of SPPs on the metal surface adjacent to the slit exit. At an optimal layer thickness, the SPP coupling efficiency is enhanced about six times relative to that without the layer. The enhanced efficiency results in distinctive and bright signatures visible under a far-field optical microscope. We show how this method can be used for parallel measurement of SPPs through a simple experiment in which the SPP propagation distance is extracted from a single microscope image. We also use optical microscopy to image SPPs focussed by a curved array of holes, obtaining results that are consistent with previous measurements using near-field optical microscopy.