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

We have investigated the effect of free electrons on the spectral propertie s of high-order harmonics generated in a neon gas jet by a 30 fs Titanium:S apphire 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 continuous ly tuning the harmonic wavelength in the spectral region 20-7 nm, by taking advantage of the blue shift of harmonic wavelengths induced by the presenc e of free electrons to cover the entire spectral region between two consecu tive 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 als o interpreted our experimental results with a simple model for the generati on process based on the "barrier suppression" ionization of an atom exposed to an ultraintense laser field.