Influence of the density of a medium on high-order harmonic generation by atoms in strong laser fields

An investigation is reported of the maximum possible increase in the density of a medium needed to increase the efficiency of high-order harmonic generation by atoms ionised with high-intensity radiation. This investigation is based on a classical description of the motion of a quasi-free photoelectron in a continuum and of its perturbation by the surrounding ions. A strong dependence of the optimal concentration of atoms on the pump radiation wave- length is revealed. The spectral characteristics of high-order harmonic generation resulting from an increase in the density of the medium are predicted.