CALCULATIONS OF HIGH-ORDER-HARMONIC GENERATION IN THE STRONGLY IONIZING REGIME

Harmonic spectra have been calculated by numerically integrating the o ne-dimensional Schrodinger equation for a model potential and a laser field sufficiently intense to cause significant or complete ionization . The use of a one-dimensional model allows realistic pulse lengths (1 ps) and laser wavelengths (0.5 and 1 mum) to be considered. The depen dence of the plateau cutoff on laser intensity has been studied in det ail, from far below to far above the saturation intensity. We find tha t below saturation the cutoff scales approximately linearly with laser intensity, and that the cutoff continues to increase beyond saturatio n, although at a slower rate. At high intensities, where ionization oc curs rapidly during the leading edge of the laser pulse, the edge of t he plateau becomes less clearly defined. In this regime, the harmonic cutoff will be determined by a combination of the atomic response and the phase mismatch due to the presence of free electrons.