Abstract

Recently, phase-matched high harmonic generation (HHG) using 20fs laser pulses was demonstrated in the wavelength range around 20 nm.' This mechanism works only as long as the gas is weakly ionized thus, setting a limit to the shortest wavelengths for which phase-matching can be achieved. Our calculations reveal the existence of a different "self-phase-matching" (SPM) mechanism, which occurs only for sub-10fs pulses. SPM works best for high harmonic orders and for peak intensities larger as the saturation intensity for ionization. Our analysis predicts that due to SPM the generation of harmonics in He in the 1-2 keV range should be possible for the first time. Further, we find an enhancement of the harmonic signal in the water window generated in He by a factor 5-10.

© 1999 Optical Society of America