Tunable soft-x-ray radiation hy high-order harmonic generation - art. no. 021801

We have investigated the effect of free electrons on the spectral propertie s of high-order harmonics generated by 30-fs Ti:sapphire laser pulses in a neon gas jet. Our observations clearly indicate the possibility of continuo usly tuning the harmonic wavelength in the region below 20 nm, by taking ad vantage of the blue shift of the harmonic wavelength induced by the presenc e of free electrons in the gas. We have experimentally demonstrated that th is allows one to cover the entire spectral region between two consecutive h armonics of the unshifted spectrum. Different amounts of blue shift are imp arted:by simply changing the gas jet position relative to the laser beam wa ist, namely, by varying the effective laser intensity experienced by the ga s jet when it is moved across the focal region. We have also interpreted th e 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.