Cross-correlation measurement of femtosecond noncollinear high-order harmonics

The durations of extreme-ultraviolet femtosecond pulses (harmonics 21 and 27 of a Ti:sapphire femtosecond laser) have been measured by a cross-correlation technique. Photoelectron energy spectra resulting from ionization of helium atoms by one harmonic photon in the presence of the infrared laser field show sidebands that provide a cross-correlation signal from which the durations of the harmonic pulses can be obtained. The measurements yield a mean value of 130 fs for both harmonics. The accuracy of the method depends on the actual intensity dependence of the sidebands.