  We demonstrate the coupling of bright and dark Surface Lattice Resonances
(SLRs), which are collective Fano resonances in 2D plasmonic crystals. As a
result of this coupling, a frequency stop-gap in the dispersion relation of
SLRs is observed. The different field symmetries of the low and high frequency
SLR bands lead to pronounced differences in their coupling to free space
radiation. Standing waves of very narrow spectral width compared to localized
surface plasmon resonances are formed at the high frequency band edge, while
subradiant damping onsets at the low frequency band edge leading the resonance
into darkness. We introduce a coupled oscillator analog to the plasmonic
crystal, which serves to elucidate the physics of the coupled plasmonic
resonances and to estimate very high quality factors (Q>700) for SLRs, which
are the highest known for any 2D plasmonic crystal.
