Role of Spin-Orbit Coupling in High-order Harmonic Generation Revealed by Super-Cycle Rydberg Trajectories

TL;DR

This paper provides experimental and theoretical evidence that long-lived Rydberg states significantly influence high-harmonic generation in atoms driven by complex laser fields, challenging previous assumptions about their negligible role.

Contribution

It reveals the active role of Rydberg trajectories in high-harmonic generation using spin-orbit dynamics and the Larmor clock, a novel insight into the process.

Findings

Long-lived Rydberg states contribute to harmonic emission.

Spin-orbit evolution tracks Rydberg dynamics during laser pulses.

Rydberg states can produce well collimated macroscopic harmonic signals.

Abstract

High-harmonic generation is typically thought of as a sub-laser-cycle process, with the electron's excursion in the continuum lasting a fraction of the optical cycle. However, it was recently suggested that long-lived Rydberg states can play a particularly important role in atoms driven by the combination of the counter-rotating circularly polarized fundamental light field and its second harmonic. Here we report direct experimental evidence of long and stable Rydberg trajectories contributing to high-harmonic generation. We confirm their effect on the harmonic emission via Time-Dependent Schr{\"o}dinger Equation simulations and track their dynamics inside the laser pulse using the spin-orbit evolution in the ionic core, utilizing the spin-orbit Larmor clock. Our observations contrast sharply with the general view that long-lived Rydberg orbits should generate negligible contribution to the macroscopic far-field high harmonic response of the medium. Indeed, we show how and why radiation from such states can lead to well collimated macroscopic signal in the far field.