High-order harmonic generation in rare gases with subpicosecond XeCl laser pulses

A systematic study of the high-order harmonic generation in He, Ne, Ar, Kr, and Xe has been made using subpicosecond XeCl laser pulses at a maximum laser intensity of 3.5×${10}^{15}$ W/${\mathrm{cm}}^2$. The highest orders of harmonics observed are the 15th in He, the 13th in Ne, and the 9th in Ar, Kr, and Xe, and no broad plateau is formed in the harmonic distributions. The ultraviolet laser pulse, which has a large multiphoton ionization rate, was observed to induce a strong dielectric breakdown of the nonlinear medium at the high intensity. Multiphoton ionization and the subsequent dielectric breakdown of the medium are shown to have a significant influence on the harmonic-generation processes, as well as on the harmonic distribution. The laser-wavelength dependence of the high-order harmonic generation is discussed by comparing the results with those obtained by the subpicosecond visible dye-laser pulses.