  We review studies of superintense laser interaction with solid targets where
the generation of propagating surface plasmons (or surface waves) plays a key
role. These studies include the onset of plasma instabilities at the irradiated
surface, the enhancement of secondary emissions (protons, electrons, and
photons as high harmonics in the XUV range) in femtosecond interactions with
grating targets, and the generation of unipolar current pulses with picosecond
duration. The experimental results give evidence of the existence of surface
plasmons in the nonlinear regime of relativistic electron dynamics. These
findings open up a route to the improvement of ultrashort laser-driven sources
of energetic radiation and, more in general, to the extension of plasmonics in
a high field regime.
