The high-order harmonic generation process was studied using laser pump pulses of different duration, down to 7 fs. The time profile of this kind of pulse corresponds to a few optical cycles at the fundamental frequency, and therefore a cycle-to-cycle variation of the maximum amplitude occurs. These facts violate the adiabatic approximation of the generation process, giving rise to new aspects of the physical process. The comparison of results with pulses in the two regimes are reported and discussed. The modeling of the complete generation and propagation process in three dimensions is also presented.

We have investigated the effect of free electrons on the spectral properties of high-order harmonics generated in a neon gas jet by a 30 fs Titanium:Sapphire pumping laser with intensities in the range 5–10 × 1014 W/cm2. The main feature of our observations concerns the possibility of continuously tuning the harmonic wavelength in the spectral region 20–7 nm, by taking advantage of the blue shift of harmonic wavelengths induced by the presence of free electrons to cover the entire spectral region between two consecutive harmonics of the unshifted spectrum. Different amounts of blue shift, which can be as large as 0.3–0.4 nm, are imparted to the given harmonic by simply changing the gas-jet-laser-beam-waist relative position. We have also interpreted our experimental results with a simple model for the generation process based on the “barrier suppression” ionization of an atom exposed to an ultraintense laser field.