Phase-matched high-order harmonic generation in the nonadiabatic limit

Our calculations show that nonadiabatic effects dominate phase-matched high-order. harmonic generation with few cycle laser pulses. The nonadiabatic behavior is a direct consequence of the large ionization rate realized with few-cycle laser pulses, which introduces a nonlinear growth of the harmonic phase with propagation distance. The deviation from linear growth has two striking consequences. On the one hand the nonadiabatic effect drastically reduces the efficiency of conventional phase-matching mechanisms, such as quasi-phase-matching, and on the other hand, it is responsible for a novel phase-matching mechanism, termed nonadiabatic self-phase-matching that is predicted to make high-order harmonic generation in the x-ray regime possible.