Abstract
We show the generation of intense isolated pulses from a single harmonic (photon energy 27.9 eV) by using sub-lO-femtosecond blue laser pulses. Nonadiabatic evolution of the blue laser pulse produces a large dipole moment at the relatively low (ninth) harmonic order and the ionization of neutral gas interacting with the laser electric field shortens the harmonic pulse duration, resulting in the high peak intensity at a focus of the high harmonic light field. By virtue of the high peak intensity of the generated harmonic pulse, nonlinear optical pprocess in the extreme ultraviolet (XUV), i.e. two-photon above threshold ionization (ATI), was successfully observed. The XUV pulses with temporal durations of 950 attoseconds and 1.3 femtoseconds were characterized by an autocorrelation technique, based on two-photon ATI of helium atoms. Because of the small cross-section for above-threshold ionization, such an autocorrelation measurement of xUv pulses with photon energy larger than the ionization energy of helium has not hitherto been demonstrated. The technique can be extended to the characterization of higher harmonics in shorter wavelengths.
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