Control of the photoelectron dynamics for the effective conversion of short-pulse, frequency-modulated optical radiation into X-ray radiation

We report a theoretical investigation of high-order harmonic generation (HHG) in the ionisation of atoms by nonlinear frequency-modulated laser pulses of short duration. It is shown that the reduction in the instantaneous value of the laser pulse frequency can lead to a significant increase in the width of the plateau in the HHG spectrum. We have found optimal parameters of frequency modulation at which photons with energies of 1 keV are efficiently generated at a relatively low laser-pulse intensity. High HHG efficiency at optimal parameters is explained by the peculiarities of atomic ionisation dynamics and acceleration of photoelectrons by a frequency-modulated laser field.