Gas medium ionization and harmonic wavelength tunability in high-order harmonic generation with ultra-short laser pulses

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 x 10(14) W/cm(2). 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.