High-intensity field interaction for the generation of soft X-ray harmonics

In recent years much interest has been focused on the high-order harmonic generation (HHG) using high-intensity femtosecond laser pulses, because the HHG provides a promising approach to develop a compact, high-brightness coherent soft X-ray source. The HHG is a typical nonperturbative phenomenon induced by a strong laser field, and the observed harmonics have shown some characteristic behaviors, e.g., representing a broad plateau formation with a sharp cutoff. The detailed interaction process at the high intensity has been studied extensively, and the results have revealed a lot of new physical processes for the HHG. Here we review some fundamental aspects of the HHG, based on our studies concerning the single-atomic response to the strong field, the laser wavelength and intensity dependence of the HHG, competing processes in the interaction, the influence of self-phase modulation, the effect of laser-pulse width on the HHG, and so forth. The laser field used for the HHG is strong enough to induce multiphoton ionization (MPI) or tunneling ionization of atoms and molecules.