Determination of the spectral variation origin in high-order harmonic generation in noble gases

One key parameter in the high-order harmonic generation (HHG) phenomenon is the exact frequency of the generated harmonic field. Its deviation from perfect harmonics of the laser frequency can be explained by considering (i) the single-atom laser-matter interaction and (ii) the spectral changes of the driving laser. In this work, we perform an experimental and theoretical study of the causes that generate spectral changes in the HHG radiation. We measured the driving-laser spectral shift after HHG in a long medium by using a correction factor to take into account the multiple possible HHG initiation distances along the laser path. We separate out the contribution of laser spectral shift from the resultant high-harmonic spectral shift in order to elucidate the microscopic effect of spectral shift in HHG. Therefore, in some cases we are able to identify the dominant electron trajectory from the experimental data. Our investigations lead to valuable conclusions about the atomic dipole phase contribution to a high-harmonic spectral shift. We demonstrate that the significant contribution of a long electron path leads to a high-harmonic shift, which differs from that expected from the driving laser. Moreover, we assess the origin of the high-order harmonics spectral broadening and provide an explanation for the narrowest high-harmonic spectral width in our experiment.