High-order harmonic generation spectroscopy by recolliding electron caustics

Abstract

Spectral focusing of the recolliding electron in high-order harmonic generation driven by twocolor fields is shown to be a powerful tool for isolating and enhancing hidden spectral features of the target under study. In previous works we used this technique for probing multi-electron effects in xenon and we compared our experimental results with time-dependent configurationinteraction singles calculations. We demonstrate here that this technique can be exploited for reconstructing the enhancement factor of the xenon giant dipole resonance and we discuss the sensitivity of this method to macroscopic effects. We then extend the technique to argon in order to test the applicability of this procedure to other targets.

Correlation enhancement of high-order harmonic generation in Xe

“…The top frame of Fig. 7 exhibits the experiment from [5]. We find solid qualitative agreement between our results and that of the experiment, and as desired, applying the enhancement ratio appropriately magnifies the caustic in the cut-off region.…”

“…This process leads to enhancement of the 5p PICS which can exceed an order of magnitude. Not surprisingly, a similar correlation enhances the HHG spectrum of Xe in the same photon energy range [3][4][5]. Indeed, the photorecombination, the last stage of the three-step picture of the HHG process [6], is the inverse to photoionization and their respective crosssections are related through the principle of detailed balance.…”

“…Agreement with the TDCIS calculation was achieved only when the intershell correlation between the 5p, 5s and 4d shells was taken into account. However, there was a noticeable disagreement around 63 eV photon energy discussed both in [4] and [5] and attributed to interference between the various photoelectron trajectories recombining with the parent ion from the opposite sides. It was concluded that this interference was blurred in the experiment due to a likely spatial and temporal averaging effects.…”

“…In the more recent measurements [4,5], the correlation enhanced HHG spectrum of Xe was recorded using the two-color ω/2ω IR laser field. This spectrum was systematically studied as a function of the relative ω/2ω phase which allowed for a tunable enhancement of a narrow spectral range to be overlapped with the giant resonance of Xe.…”

Spectral caustic in two-color high-order harmonic generation: Role of Coulomb effects

“…The intensity of the narrow group of the harmonics enhanced due to this phenomenon can be an order of magnitude [12] (or even more [33]) higher than the average intensity outside this spectral region. This effect has been used recently in the HHG spectroscopy [34,35].…”