Harmonic Generation in Presence of Ionization

Abstract

Most experiments and theoretical studies of High-Order Harmonics have been limited to situations of low levels of ionization. It is the purpose of this work to investigate experimentally and theoretically the situation of intensities around the saturation intensity since it is under such conditions that harmonic generation is most efficient. In case of strong ionization, both the spatial and temporal behavior of harmonics are expected to be affected. This study focuses on the temporal aspect, with special attention of the respective roles of the intensity-dependent atomic phase and the ionization. Since direct autocorrelation of the XUV pulses is not yet possible, a cross-correlation method is the only available technique to approach short pulses measurements in this wavelength range^1–4. Experimentally, the cross-correlation of the 19th harmonic pulses generated in argon by 800 nm pulses of various lengths from 120 fs to 1 ps and a 120 fs 800 nm probe pulse were measured at intensities above 10¹⁴W.cm ⁻². Theoretically, the corresponding profiles were calculated using numerical values of the atomic dipole and the ionization rate in a propagation code solving the coupled wave equations by a finite-difference method. The main result is that the harmonic pulse is strongly shortened by the combined effect of the intensity-dependent phase mismatch and the dipole reduction due to ionization.