Probe of Multielectron Dynamics in Xenon by Caustics in High-Order Harmonic Generation

We investigated the giant resonance in xenon by high-order harmonic generation spectroscopy driven by a two-color field. The addition of a nonperturbative second harmonic component parallel to the driving field breaks the symmetry between neighboring subcycles resulting in the appearance of spectral caustics at two distinct cutoff energies. By controlling the phase delay between the two color components it is possible to tailor the harmonic emission in order to amplify and isolate the spectral feature of interest. In this Letter we demonstrate how this control scheme can be used to investigate the role of electron correlations that give birth to the giant resonance in xenon. The collective excitations of the giant dipole resonance in xenon combined with the spectral manipulation associated with the two-color driving field allow us to see features that are normally not accessible and to obtain a good agreement between the experimental results and the theoretical predictions.

Figure 1

Photon energies (a), ionization rates (b), and excursion times (c) associated to the cutoff trajectories as a function of the phase difference between the two fields for different values of $\alpha$ ranging from 0 to 0.5 with step $\mathrm{\Delta }\alpha =0.05$. The dashed line represents the lower branch and the solid line represents the upper branch. The two cutoff energies are calculated with the semiclassical three-step model; the ionization rates of the two cutoff trajectories are calculated with the Ammosov-Delone-Krainov model [15]; the excursion times of the two cutoff trajectories are calculated with the semiclassical three-step model and they are normalized to the period of the fundamental field. The intensity of the fundamental field is equal to $I=7\times 10^{13}\mathrm{W}/{\mathrm{cm}}^2$.

Figure 2

HHG spectra as a function of the phase delay between the two colors—linear scale color map. (a) Experiment (b) TDCIS simulations with only the $5p_z$ orbital being active. (c) TDCIS simulations with all $5p$, $4d$, and $5s$ orbitals being active. The solid and the dashed curves represent the two cutoffs for the upper and lower branches, respectively, associated with the semiclassical calculations.

Figure 3

Maximum value of HHG intensity (linear scale) with respect to the phase delay as a function of the photon energy. (Solid line) Experiment. (Dotted line) TDCIS simulations considering only $5p$ orbitals. (Dashed line) TDCIS simulations including all $5p$, $4d$, and $5s$ orbitals.

Figure 4

(a) Gabor (time-frequency) analysis of the HHG spectra calculated with the TDCIS approach including all $5p$, $4d$, and $5s$ orbitals (Gaussian time gate $\sigma =86$ as). (b) Temporal structure of the HHG field calculated with the TDCIS approach including all $5p$, $4d$, and $5s$ orbitals (solid line) and considering only $5p$ orbitals (lighter area) in the energy region corresponding to $75\pm 5\mathrm{eV}$. The driving electric field is shown as the dash-dotted curve (value divided by $10^3$). (c) Temporal structure of the HHG field calculated with the TDCIS approach including all $5p$, $4d$, and $5s$ orbitals (solid line) and considering only $5p$ orbitals (lighter area) in the energy region corresponding to $75\pm 15\mathrm{eV}$. The driving electric field is shown as a dash-dotted curve (value divided by $10^3$. All calculations refer to one period of the driving field and $\phi =0.5\pi \mathrm{rad}$.