Broadband Transformation of the High-Order Harmonic X-Ray Spectrum in a Gas, Mediated by a Synthesized Laser Field of Intense Femtosecond Laser Pulses in the Near- and Mid-Infrared Range

The possibility of controlling the spectrum of high harmonics in the range of 50–200 eV under the excitation of a gas jet by a pair of intense femtosecond near- (1.24 μm) and mid-infrared (4.5 μm) laser pulses has been experimentally demonstrated. It has been established that an increase in the intensity of long-wavelength radiation from ~10 9 to ~10 12 W/cm 2 under the peak intensity 10 14 W/cm 2 of the short-wavelength pulse leads to a transition from a discrete spectrum containing sum and difference frequencies to a quasi-continuous broadband spectrum. This transition has been explained by a change in the regime of influence of the mid-infrared field on the dynamics of the generating electron. The obtained results demonstrate the prospects for generating radiation with the quasi-continuous broadband spectrum up to the soft X-ray region when generating high harmonics by a synthesized laser field.