  Plasmonic crystals are well known to have band structure including a bandgap,
enabling the control of surface plasmon propagation and confinement. The band
dispersion relation of bulk crystals has been generally measured by
momentum-resolved spectroscopy using far field optical techniques while the
defects introduced in the crystals have separately been investigated by near
field imaging techniques so far. Particularly, defect related energy levels
introduced in the plasmonic band gap have not been observed experimentally. In
order to investigate such a localized mode, we performed electron energy-loss
spectroscopy (EELS), on a point defect introduced in a plasmonic crystal made
up of flat cylinders protruding out of a metal film and arranged on a
triangular lattice. The energy level of the defect mode was observed to lie
within the full band-gap energy range. This was confirmed by a
momentum-resolved EELS measurement of the band gap performed on the same
plasmonic crystal. Furthermore, we experimentally and theoretically
investigated the emergence of the defect states by starting with a corral of
flat cylinders protrusions and adding sequentially additional shells of those
in order to eventually forming a plasmonic band-gap crystal encompassing a
single point defect. It is demonstrated that a defect-like state already forms
with a crystal made up of only two shells.
