Generation of Nonadiabatic Blueshift of High Harmonics in an Intense Femtosecond Laser Field

Abstract

By applying intense 30-fs Ti:sapphire laser pulses to Ar gas well above the saturation intensity for optical-field ionization, a strong blueshift of high harmonics 2 times as large as the laser frequency was generated. A semiclassical calculation showed that the observed large blueshift resulted from a rapidly increasing electric field, existing much earlier in time than the peak of the laser pulse, i.e., a nonadiabatic effect.