CROSS-CORRELATION MEASUREMENTS OF FEMTOSECOND EXTREME-ULTRAVIOLET HIGH-ORDER HARMONICS

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