Abstract
We study the process of high-order harmonic generation using laser pulses with non-adiabatic turn-on and intensities well above saturation. As a main point, we report the existence of a valley structure in the efficiency of single-atom high-order harmonic generation with increasing laser intensities. Consequently, after an initial decrease, the high-frequency radiation yield is shown to increase for higher intensities, returning to a level similar to the case below saturation. Such behavior contradicts the general belief of a progressive degradation of the harmonic emission at ultrahigh intensities, based on the experience with pulses with smoother turn-on. We shall show that this behavior corresponds to the emergence of a new pathway for high-order harmonic generation, which takes place during the pulse turn-on. Our study combines trajectory analysis, wavelet techniques and the numerical integration of 3-Dimensional Time Dependent Schrödinger Equation. The increase in efficiency raises the possibility of employing ultrahigh intensities to generate high-frequency radiation beyond the water window.
Highlights
In strong field interactions, high order non-linear processes appear with efficiencies much higher than those expected by conventional perturbative approaches
The harmonic emission spectrum is characterized by the emergence of a plateau structure in which, for several orders, the intensity of the harmonics remains at similar levels [1]
The global behavior of the harmonic yield with the laser amplitude (Fig. 2(a)) follows the form of a valley: First, a decrease connected with the degradation of the efficiency of the SC trajectories, followed by an increase as the efficiency of the NAT trajectories becomes the relevant contribution to the dipole spectrum
Summary
High order non-linear processes appear with efficiencies much higher than those expected by conventional perturbative approaches. On the other hand, reaching shorter wavelengths by the increase of intensity is known to be limited by the saturation of ionization This threshold the harmonic yield is known to decrease [12, 13], and the generated plasma affects negatively the phase matching of the harmonic propagation. The robustness of the single-atom results against propagation for NAT pulses has been confirmed numerically [16] In light of these results, the main limitation for using ultrahigh intensities to generate shorter wavelengths would be the degradation of the harmonic yield with intensity in the single-atom response. We shall show in the single atom response that it is possible to generate high-frequency radiation using intensities about two orders of magnitude above the saturation threshold. We should note that the possibility of generating pulses with rapid rising edges has been recently reported in the context of filamentation in molecular aligned gases [21,22,23,24]
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