Electron energy loss spectroscopy on freestanding perforated gold films

We report on a combined far- and near-field study of surface plasmon polaritons on freestanding perforated gold films. The samples were fabricated by focused ion beam milling of a periodic hole array into a carbon membrane followed by thermal evaporation of gold and plasma ashing of the carbon film. Optical transmission spectra showed a series of characteristic features, which can be attributed to the excitation of surface plasmon modes via the periodic nanohole array. The corresponding near-field distributions were mapped by electron energy loss spectroscopy. In addition to the optically bright surface plasmon modes, we observed in the near-field a dark plasmon mode which is absent in the normal incidence far-field spectra. Our experimental results are in good agreement with numerical computations based on a discontinuous Galerkin time-domain method.