Calculations of high-order-harmonic generation in the strongly ionizing regime

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Abstract
Harmonic spectra have been calculated by numerically integrating the one-dimensional Schrödinger 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 μm) to be considered. The dependence of the plateau cutoff on laser intensity has been studied in detail, from far below to far above the saturation intensity. We find that below saturation the cutoff scales approximately linearly with laser intensity, and that the cutoff continues to increase beyond saturation, although at a slower rate. At high intensities, where ionization occurs rapidly during the leading edge of the laser pulse, the edge of the 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.