  Inter-slit interactions across one-dimensional arrays of sub-micro meter
rectangular holes in gold films are explored. Using electron energy loss
spectroscopy combined with scanning transmission electron microscopy, a series
of cavity standing waves is resolved, indicating particularly high interslit
interactions, about an order of magnitude larger than the intra-slit edge to
edge coupling. Pronounced signal enhancements are thus induced, dominated by
short-range interactions and high mode-localization, while yet, relatively
long-range coherence is retained. The sub-nm electron beam, in spite of
principal differences from broad-area probes, yields results similar to
extraordinary optical transmission (EOT). Implications to EOT mechanisms,
including its sub-wavelength, off-resonance limit, are pointed out.
