Abstract
The bi-circular scheme for high harmonic generation, which combines two counter-rotating circular fields with frequency ratio 2:1, has recently permitted to generate high harmonics with essentially circular polarization, opening the way for ultrafast chiral studies. This scheme produces harmonic lines at 3N + 1 and 3N + 2 multiples of the fundamental driving frequency, while the 3N lines are forbidden owing to the three-fold symmetry of the field. It is generally established that the routinely observed signals at these forbidden harmonic lines come from a slight ellipticity in the driving fields, which breaks the three-fold symmetry. We find that this is neither the only nor it is the dominant mechanism responsible. The forbidden lines can be observed even for perfectly circular, long driving pulses. We show that they encode rich information on the sub-cycle electronic dynamics that occur during the generation process. By varying the time delay and relative intensity between the two drivers, we demonstrate that when the second harmonic either precedes or is more intense than the fundamental field, the weak effects of dynamical symmetry breaking caused by finite pulse duration are amplified by electrons trapped in Rydberg orbits (i.e., Freeman resonances), and that the forbidden harmonic lines are a witness of this.
Highlights
High harmonic generation serves as an indispensable source of bright, coherent XUV and soft X-ray light
By varying the time delay and relative intensity between the two drivers, we demonstrate that when the second harmonic either precedes or is more intense than the fundamental field, the dynamical symmetry of the system is broken by electrons trapped in Rydberg orbits (i.e., Freeman resonances), and that the forbidden harmonic lines are a witness of this
While the experimentally observed spectra are likely affected by the macroscopic propagation effects, we focus on the features that must originate in the single-atom response: the dependence of the forbidden harmonic lines on the delay and the relative intensities of the two driving fields
Summary
High harmonic generation serves as an indispensable source of bright, coherent XUV and soft X-ray light. It allows one to generate XUV radiation and trains of attosecond pulses with controlled polarization properties [29, 30, 40] and carries the potential to generate isolated circularly polarized attosecond pulses [41, 42] These pulses would open the route to studying chiral-sensitive light-matter interactions with unprecedented temporal resolution in gas and condensed phase, e.g., the study of ultrafast chiral-specific dynamics in molecules, ultrafast chiral recognition via photoelectron circular dichroism [19, 43], ultrafast magnetization and spin dynamics [44,45,46], etc. This situation stands in stark contrast to the two-dimensional high harmonic spectroscopy which uses the combination of linearly polarized fundamental and its second harmonic [11, 13,14,15,16,17,18, 47,48,49,50,51,52,53], allowing one to track electronic and vibronic [13] dynamics with temporal resolution from tens of femtoseconds down to tens of attoseconds
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