Ionization-induced pulse shortening and retardation of high intensity femtosecond laser pulses

High-intensity, ultrashort laser pulses were focused into a gas jet to study the propagation at intensities up to 10 17 W/cm 2. The pulse intensity envelope and phase were measured using a modified version of the FROG (frequency-resolved optical gating) technique which allows an absolute time reference. Pulse shortening and retardation were observed and are explained by erosion of the leading part of the pulse due to ionization-induced refraction, and propagation of the remaining part of the pulse in the plasma. At sufficiently high pressures, multiple pulse formation followed by complete pulse break-up is observed. Numerical simulations are in qualitative agreement with experiment.