Effect of multiphoton ionization on high-order harmonic generation and propagation in rare gases with subpicosecond laser pulses

An experimental study was made on the effect of multiphoton ionization on high-order harmonic generation and propagation in rare gases with subpicosecond dye-laser (λ∼616 nm) pulses at the intensity up to ∼2×${10}^{14}$ W/${\mathrm{cm}}^2$. Both high-order harmonic intensity and the ion signal were measured as a function of the position of the laser focus. The harmonic signal in Ar, Kr, and Xe shows a remarkable decrease near the peak position of the ion signal. We have observed the asymmetry in the shape of the harmonic signal and the change of the asymmetry with harmonic order. Furthermore, the dependence of harmonic signals on fundamental intensity were found to be quite different at the specific positions of the laser focus. These experimental results are consistently explained by the medium ionization and the resulting self-defocusing of the laser beam. The results obtained with the XeCl laser (λ∼308 nm) pulses back up our interpretations for those obtained with the dye-laser pulses.