  We present theoretical investigations of high-order harmonic generation (HHG)
resulting from the interaction of noble gases with localized surface plasmons.
These plasmonic fields are produced when a metal nanoparticle is subject to a
few-cycle laser pulse. The enhanced field, which largely depends on the
geometrical shape of the metallic structure, has a strong spatial dependency.
We demonstrate that the strong non-homogeneity of this laser field plays an
important role in the HHG process and leads to a significant increase of the
harmonic cut-off energy. In order to understand and characterize this new
feature, we include the functional form of the laser electric field obtained
from recent attosecond streaking experiments [F. S{\"u}{\ss}mann and M. F.
Kling, Proc. of SPIE, {\bf Vol. 8096}, 80961C (2011)] in the time dependent
Schr\"odinger equation (TDSE). By performing classical simulations of the HHG
process we show consistency between them and the quantum mechanical
predictions. These allow us to understand the origin of the extended harmonic
spectra as a selection of particular trajectory sets. The use of metal
nanoparticles shall pave a completely new way of generating coherent XUV light
with a laser field which characteristics can be synthesized locally.
