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

Abstract

A systematic study of the high-order harmonic generation in He, Ne, Ar, Kr, and Xe has been made using subpicosecond XeC1 laser pulses at a maximum laser intensity of 3.5 X 10' %/cm . 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 inhuence 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.

High-order harmonic generation in the tunneling regime

“…We found that in an identical extreme ultraviolet spectral range, the harmonic signal was much smaller than those observed in our previous experiments using visible dye and XeC1 lasers [3,11].The results obtained with three different kinds of femtosecond lasers have shown that the harmonic yield decreases with increasing the laser wavelength, while the harmonic order to be detected increases with an increase in the wavelength. In the present experiment, the highestorder harmonic (shortest wavelength) detected was the 103rd (7.6 nm) for He and the 95th (8.2 nm) for Ne.…”

“…From the above discussion, it is more reasonable in the present experiment to compare F. "with the highest order observed. Furthermore, as in our previous experiment, where we had strong multiphoton ionization of atoms with the femtosecond uv laser pulses [11], I* in the medium must be used to estimate F. , ".Using I* -(7.3 -8.0) X 10' W/cm obtained at Io) -2 X 10' W/cm for the 150-Torr He, we have F, =157 -169 eV.…”

“…The experimental arrangement for the harmonic generation was the same as that in our previous studies [3,11,13].…”

“…The harmonic signal detected was processed by a boxcar averager and recorded on a chart recorder. The wavelength dependence of detection sensitivity for the whole system has been calibrated in our previous studies [3,11].…”

“…Because of the resonant enhancement, the harmonic generation through the new channel is expected to be efficient. When the incident high-intensity field shifts the excited states to a higher energy by the ac Stark effect [36],one can expect additional resonance enhancement [3,11,37] through the 2p 4d and other excited states of Ne atoms.…”