Survey of the Theoretical Background

High harmonic generation is a process in which an atom absorbs q photons of energy ω_L and emits one photon of energy qω _L . Harmonics are emitted by electrons which (i) tunnel through the atomic potential barrier, (ii) are accelerated by the laser field toward the nucleus and recombine. The theory of the whole process involves a microscopic level, relative to the nonlinear behavior of single atoms in strong fields, and a macroscopic level which deals with the coherent propagation of harmonic radiation in the excited medium. It is clear that the HHG study is an appealing topic for improving the understanding of strong radiation—matter interaction. Neverthless, the goal of this section is no more than a brief introduction to both microscopic and macroscopic aspects of HHG considered as a coherent source of XUV radiation. The reader can access original works (see for instance [4, 24–28]).