  The transmission of ultrashort (7 fs) broadband laser pulses through periodic
gold nano-structures is studied. The distribution of the transmitted light
intensity over wavelength and angle shows an efficient coupling of the incident
p-polarized light to two counter-propagating surface plasmon (SP) modes. As a
result of the mode interaction, the avoided crossing patterns exhibit energy
and momentum gaps, which depend on the configuration of the nano-structure and
the wavelength. Variations of the widths of the SP resonances and an abrupt
change of the mode interaction in the vicinity of the avoided crossing region
are observed. These features are explained by the model of two coupled modes
and a coupling change due to switching from the high frequency dark mode to the
low frequency bright mode for increasing wavelength of the excitation light.
PACS numbers: 73.20.Mf, 42.70.Qs, 42.25.-p,
