Self-phase-matched high harmonic generation

Our theoretical analysis reveals that tunnel ionization significantly modif ies the electric field of few-cycle laser pulses within a single oscillatio n period. This subcycle self-modulation is predicted to result in phase mat ching, making high harmonic generation in the x-ray regime possible for the first time. Such a radiation source opens novel possibilities in the inves tigation of matter with x-ray techniques, such as time resolved x-ray diffr action and absorption.