Measurements of chirp-induced frequency shift in high-order harmonic generation in xenon

A precise metrology experiment on the ninth harmonic generated in the nonperturbative regime in xenon is presented. Harmonic generation is performed with a 300-ps 4-GW narrowband tunable laser, and well-calibrated atomic resonances are used as a frequency markers in the extreme ultraviolet, enabling an absolute frequency calibration at the 10-6-10-7 level. A harmonic frequency is observed that deviates from the expected integer multiple of the fundamental frequency. This frequency shift is found to increase with gas density starting from negative values, i.e., redshift, at a low gas density. Possible sources of the observed frequency shift are critically discussed. The increase of the frequency shift toward high gas densities is consistent with self-phase modulation of the fundamental beam due to the presence of free electrons in the interaction region that are generated via multiphoton ionization, while the observed redshift toward lower gas densities may be attributable to plasma-dynamics effects.