  We study the plasmonic properties of coupled noble-metal nanochains in the
case of different number of coupled chains and doping by different
transition-metal (TM) atoms within the time-dependent density-functional theory
(TDDFT) approach. We find that as the number of chains in the array increases
the plasmon peak shifts from the sub-eV towards the visible range. As doping
with TM atoms increases, the visible absorption band broadens, owing to
formation of additional plasmon peaks. The optical response is very sensitive
to the type of doped atoms, their number and position; in particular, the
additional peaks are most pronounced in the case of weak doping when they
correspond to local plasmon oscillations around the impurity atom. These
effects have a potential to be used in various modern technologies, from
sensors to solar cells. Most of the studies of nano-plasmon effects have been
focused on alkali- and noble-metal systems with extended s-electron states,
while it was believed that doping with TM atoms with their more localized
charge as a rule leads to an attenuation of the plasmon modes. We demonstrate
that TM atoms can play a constructive role in plasmon generation in small chain
systems, and that plasmonic modes can emerge even in some pure TM nanochains.
