  Harnessing the optical properties of noble metals down to the nanometer-scale
is a key step towards fast and low-dissipative information processing. At the
10-nm length scale, metal crystallinity and patterning as well as probing of
surface plasmon (SP) properties must be controlled with a challenging high
level of precision. Here, we demonstrate that ultimate lateral confinement and
delocalization of SP modes are simultaneously achieved in extended
self-assembled networks comprising linear chains of partially fused gold
nanoparticles. The spectral and spatial distributions of the SP modes
associated with the colloidal superstructures are evidenced by performing
monochromated electron energy loss spectroscopy with a nanometer-sized electron
probe. We prepare the metallic bead strings by electron beam-induced
interparticle fusion of nanoparticle networks. The fused superstructures retain
the native morphology and crystallinity but develop very low energy SP modes
that are capable of supporting long range and spectrally tunable propagation in
nanoscale waveguides.
