Electron quantum path control in high harmonic generation via chirp variation of strong laser pulses

Abstract

The quantum phases of the electron paths driven by an ultrafast laser in high harmonic generation in an atomic gas depends linearly on the instantaneous cycle-averaged laser intensity. Using high laser intensities, a complete single ionisation of the atomic gas may occur before the laser pulse peak. Therefore, high harmonic generation could be localised only in a temporal window at the leading edge of laser pulse envelope. Varying the laser frequency chirp of an intense ultrafast laser pulse, the centre, and the width of the temporal window, that the high harmonic generation phenomenon occurs, could be controlled with high accuracy. This way, both the duration and the phase of the electron trajectories, that generate efficiently high harmonics, is fully controlled. A method of spectral control and selection of the high harmonic extreme ultraviolet light from distinct quantum paths is experimentally demonstrated. Furthermore, a phenomenological numerical model enlightens the physical processes that take place. This novel approach of the electron quantum path selection via laser chirp is a simple and versatile way of controlling the time-spectral characteristics of the coherent extreme ultraviolet light with applications in the fields of attosecond pulses and soft x-ray nano-imaging.

Spectral and Divergence Characteristics of Plateau High-Order Harmonics Generated by Femtosecond Chirped Laser Pulses in a Semi-Infinite Gas Cell

“…Both sets of measurements were performed maintaining a constant laser pulse energy of 1.0 mJ, as well as the same focusing conditions, i.e., f -number = 4. The laser peak intensity for the FTL pulse of 26 fs was experimentally estimated to be 2.3 ± 0.5 × 10 15 W/cm 2 , which is far above the ionisation saturation intensity for Ar [15]. The pulse durations, indicated in the spectral images in Figures 2 and 3 with plus and minus signs, correspond to the positive (instantaneous frequency increasing with time) and negative (instantaneous frequency decreasing with time) chirp values, respectively.…”

“…A closer inspection of the spectral area of each harmonic in Figure 2 shows that it consists of two components that are attributed to the long and the short electron trajectory contributions, respectively. Their identification is determined by their different divergence, with the long trajectory presenting, in general, a higher divergence than the short trajectory [15]. Moreover, their spectral splitting is related to their inherent spectral blue shift, with the long trajectory suffering a stronger shift and thus appearing at shorter wavelengths [12].…”

“…In more detail, for negatively chirped pulses, HHG is taking place mainly at the leading edge of the pulse, since at higher intensities the competing process of ionisation prevails, as evident from the intense plasma formation images shown in Figures 2 and 3. Therefore, HHG conditions are largely affected by the slope of the temporal intensity and frequency of the laser pulse at this temporal window [15]. For positively chirped pulses, plasma formation is seen to be marginal, and thus HHG is expected to be realised over a wider temporal window of the laser pulse, which, for high positive chirp values, includes the envelope peak area.…”

“…Even though similar investigations exist in the literature [16,17], these report only HHG spectra, unlike our work where we record both the HHG spectra and the HH divergence simultaneously, the latter being a fine tool for investigating the delicate dynamics behind the HHG and the role of the short and long trajectories in it. Phenomenological model calculations [15], which take into account the aforementioned physical processes, accompany our experimental findings on the spectral characteristics of the generated harmonics.…”

“…Thus, developing methods for controlling the relevant contribution of long and short trajectories of the plateau harmonics, and the role of the SPM effects, is of fundamental significance. Recently, we reported on such a proof-of-principle method for the coherent control of the two quantum paths in plateau harmonics generated in a noble gas, based on the accurate control of the laser pulse chirp variation [15]. The latter allowed for the accurate control of the time-dependent slope of the cycle-averaged laser intensity, the temporal evolution of the ionisation degree of the medium, and the SPM of the laser pulse inside the partially ionised medium.…”