Vasily V. Strelkov

Single attosecond soft-x-ray pulse generated with a limited laser beam

The problem of obtaining a single attosecond pulse with high-order harmonics generation is considered. A method for producing an exciting light pulse with a time-dependent degree of ellipticity from a one-frequency linearly polarized laser pulse by means of linear optics is suggested. The whole of the field generated with a limited beam of this light in a gas target is calculated in the far zone. For this calculation we use an original method based on the time approach of a theory of high-order harmonic generation. Contrasted and intense (focused) single attosecond soft-x-ray pulses are calculated by suppression of the low-frequency part of the field to as many as 11 harmonics of exciting frequency. The possibility of suppressing the low-frequency part of the field with pinholes is shown.

Off-axially phase-matched high-order harmonic generation in an extended medium

We investigate theoretically high-order harmonic generation in gases and show that, if the target thickness is high, off-axial phase matching is of importance. Results of a numerical study of harmonic generation that takes into account the self-action of the laser pulse are presented. We show that self-guiding of the laser pulse in a noble gas makes possible off-axially phase-matched high harmonic generation if some easily ionizable gas is added to the main generating gas. In calculations for such mixtures we obtained conversion efficiencies as great as approximately 10-3–10-2 for the 33rd harmonic of a Ti:sapphire laser and approximately 10-4 for the 121st harmonic.

Off-axial phase-matched high-order harmonic generation in extended medium under self-guiding of a laser beam

We present results of analytical and numerical calculations of the angular spectrum and the power of high-order harmonics generated with a limited laser beam in an extended medium. It is shown that the spectrum of harmonics generated in a thick target dramatically differs from that of the single-atom response. In particular, in this spectrum there are two plateaus due to off-axially phase-matched generation of some harmonics (phase-matching condition holds only for a part of the harmonic angular spectrum). The origin that makes the off-axially phase-matched generation effective is the amplitude modulation of the high-frequency single-atom response in the laser beam cross section that increases the divergence of the harmonic beam. In Gaussian beams the off- axially phase-matched generation in extended medium (i.e. under target thickness close to the confocal length) is effective under any value of geometrical dispersion (i.e. under arbitrary hard focusing). Our treatment explains features of the high harmonic spectra obtained experimentally using extended targets as well as high conversion efficiencies achieved in these experiments. It is shown that under self-guiding of a laser beam in a noble gas off-axially phase-matched high harmonic generation is possible if some easily ionizable gas is added to the main generating gas. In calculations of high-order harmonic generation in such mixtures conversion efficiency as high as approximately10-3 - 10-2 for 33-rd harmonic of Titanum-Sapphire laser and about 10-4 for 121-st one is obtained.