High-order harmonic generation in the soft X-ray to XUV by ultrashort laser pulses

High-order harmonic generation has been observed in a gaseous medium with laser pulses of intensities I=10¹³~10¹⁵ W/cm², of which radiation field becomes comparable to or in excess of the typical atomic Coulomb field of 10⁸~10⁹ V/cm. There has been a significant interest in this phenomenon at a strong field, because the harmonic generation properties observed so far have suggested a new domain of nonlinear optics associated with breakdown of the conventional radiative perturbation theory and provides a promising approach to a useful (tunable) coherent soft X-ray radiation source. This paper reports the high-order harmonic generation in He and Ne with a fsec Ti:sapphire laser at intensities above 10¹⁵ W/cm², where the highest order of harmonic has been observed up to the 103rd (7.6 nm) in He and the 95th (8.2 nm) in Ne, and a systematic study of competing processes at the high intensity has been made