Manipulation of light-beams with subwavelenth metallic devices has motivated intensive studies, following the discovery of extraordinary transmission of electromagnetic waves through sub-wavelength apertures in thin noble-metal films. The propagation of light in these holes can be investigated at greately improved spatial resolution by means of focused electron-beams. Here we demonstrate direct e-beam excitation of radiative cavity modes well below the surface plasmon (SP) frequency, of isolated rectangular holes in gold films, illuminating the hotly debated phenomenon of the extraordinary optical transmission through subwavelength holes. The exceptionally long range e-beam interaction with the metal through the vacuum, involving electromagnetic excitations within the light cone, is allowed by momentum conservation breakdown along the e-beam axis. Two types of lowlying excited modes are revealed: radiative cavity modes which are nearly unaffected by SPs, and SP polariton modes with waveguide character in the near field region of the slit walls, which in spite of the strong hybridization preserve the waveguide cutoff frequencies and symmetry characteristics.
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