Cross-correlation measurements of femtosecond extreme-ultraviolet high-order harmonics

He atoms have been ionized by a combination of two pulses: the fundamental (800 nm, 150 fs) from a Ti:sapphire laser, and its 21st harmonic (38 nm). The resulting above-threshold ionization spectra comprise peaks that scale as the cross-correlation function, which can be mapped out by variation of the time delay between the two pulses. Using this technique, we have carried out the first measurement, to our knowledge, of the duration of high-order harmonic pulses with subpicosecond resolution. The result indicates a duration of 40 fs. This duration is much shorter than the duration of the fundamental pulse, in agreement with existing model calculations. (C) 1996 Optical Society of America