  We theoretically investigate high-order harmonic generation (HHG) in Rydberg
atoms driven by spatially inhomogeneous laser fields, induced, for instance, by
plasmonic enhancement. It is well known that the laser intensity should to
exceed certain threshold in order to generate HHG, when noble gas atoms in
their ground state are used as an active medium. One way to enhance the
coherent light coming from a conventional laser oscillator is to take advantage
of the amplification obtained by the so-called surface plasmon polaritons,
created when a low intensity laser field is focused onto a metallic
nanostructure. The main limitation of this scheme is the low damage threshold
of the materials employed in the nanostructures engineering. In this work we
propose to use Rydberg atoms, driven by spatially inhomogeneous,
plasmonic-enhanced laser fields, for HHG. We exhaustively discuss the behaviour
and efficiency of these systems in the generation of coherent harmonic
emission. To this aim we numerically solve the time-dependent Schr\"{o}dinger
equation for an atom with an electron initially in a highly excited $n$-th
Rydberg state, located in the vicinity of a metallic nanostructure, where the
electric field changes spatially on the scales relevant for the dynamics of the
laser-ionized electron. We first use a one-dimensional model to investigate the
phenomena systematically. We then employ a more realistic situation, when the
interaction of a plasmonic-enhanced laser field with a three-dimensional
Hydrogen atom is modelled. We discuss the scaling of the relevant input
parameters with the principal quantum number $n$ of the Rydberg state in
question, and demonstrate that harmonic emission could be achieved from Rydberg
atoms well below the damage threshold, thus without deteriorating the geometry
and properties of the metallic nanostructure.
