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.