TIME-RESOLVED HARMONIC-GENERATION IN AN IONIZING GAS

We report on a time-resolved study of the fifth harmonic generated in xenon by 140 ps pulses from a Nd:YAG laser in the 10(13) W cm-2 intens ity regime. Absolute timing between the driving laser pulse and the ha rmonic pulse could be determined by means of reference harmonics gener ated in non-linear crystals and a single common streak camera. Above a certain laser intensity the centre of the observed harmonic pulse was shifted earlier in time relative to the laser pulse. We found the int ensity dependence of this shift to be approximately linear within the intensity range used which is consistent with the results of numerical simulations taking the ionization and dispersion of the xenon gas int o account.