  Plasmons in low-dimensional systems respresent an important tool for coupling
energy into nanostructures and the localization of energy on the scale of only
a few nanometers. Contrary to ordinary surface plasmons of metallic bulk
materials, their dispersion goes to zero in the long wavelength limit, thus
covering a broad range of energies from terahertz to near infrared, and from
mesoscopic wavelengths down to just a few nanometers. Using specific and most
characteristic examples, we review first the properties of plasmons in
two-dimensional (2D) metallic layers from an experimental point of view. As
demonstrated, tuning of their dispersion is possible by changes of charge
carrier concentration in the partially filled 2D conduction bands, but for the
relativistic electron gas like in graphene only in the long wavelength limit.
For short wavelengths, on the other hand, the dispersion turns out to be
independent of the position of the Fermi level with respect to the Dirac point.
A linear dispersion, seen under the latter conditions in graphene, can also be
obtained in non-relativistic electron gases by coupling between 2D and 3D
electronic systems. As a well investigated example, the acoustic surface
plasmons in Shockley surface states, coupled with the bulk electronic system,
are discussed. Also the introduction of anisotropy, e.g. by regular arrays of
steps, seems to result in linearization (and to partial localization of the
plasmons normal to the steps, depending on wavelengths). In quasi-one
dimensional (1D) systems, such as arrays of gold chains on regularly stepped Si
surfaces, only the dispersion is 1D, whereas shape and slope of the dispersion
curves depend on the 2D distribution of charge within each terrace and on
coupling between wires on different terraces.
