Tunable soft-x-ray radiation by high-order harmonic generation

We have investigated the effect of free electrons on the spectral properties of high-order harmonics generated by 30-fs Ti:sapphire laser pulses in a neon gas jet. Our observations clearly indicate the possibility of continuously tuning the harmonic wavelength in the region below 20 nm, by taking advantage of the blue shift of the harmonic wavelength induced by the presence of free electrons in the gas. We have experimentally demonstrated that this allows one to cover the entire spectral region between two consecutive harmonics of the unshifted spectrum. Different amounts of blue shift are imparted:by simply changing the gas jet position relative to the laser beam waist, namely, by varying the effective laser intensity experienced by the gas jet when it is moved across the focal region. We have also interpreted the experimental results in terms of a simple model of the generation process based on the tunnel ionization of:an atom exposed to an ultraintense laser field.