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 cro ss-correlation technique. Photoelectron energy spectra resulting from ionization of helium atoms by one harmonic photon in the presence of t he infrared laser field show sidebands that provide a cross-correlatio n signal from which the durations of the harmonic pulses can be obtain ed. 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. (C) 1997 Optical Society of America.