Electron energy loss spectroscopy on freestanding perforated gold films

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