Time-resolved high harmonic spectroscopy of dynamical symmetry breaking in bi-circular laser fields

TL;DR

This paper investigates the origins of forbidden high harmonic lines in bi-circular laser fields, revealing they encode sub-cycle electronic dynamics and symmetry breaking mechanisms beyond field ellipticity.

Contribution

It demonstrates that forbidden harmonic lines can arise from dynamical symmetry breaking due to Rydberg electron trapping, not just ellipticity in the driving fields.

Findings

Forbidden lines appear even with perfectly circular pulses.

Symmetry breaking occurs via Rydberg electron trapping (Freeman resonances).

Forbidden lines encode sub-cycle electronic dynamics.

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 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.