High-order harmonic generation at high laser intensities beyond the tunnel regime

TL;DR

This study investigates high-order harmonic generation at intensities beyond the tunnel regime, revealing unexpected increases in harmonic yield due to unique electron trajectories during the laser pulse turn-on phase.

Contribution

It uncovers novel electron trajectories responsible for enhanced harmonic efficiency at ultrahigh intensities, challenging previous assumptions of degradation.

Findings

Harmonic yield initially drops then increases with laser intensity.

Unique electron trajectories emerge during the turn-on stage.

Quantum and classical analyses confirm the trajectory-based mechanism.

Abstract

We present studies of high-order harmonic generation (HHG) at laser intensities well above saturation. We use driving laser pulses which present a particular electron dynamics in the turn-on stage. Our results predict an increasing on the harmonic yield, after an initial dropping, when the laser intensity is increased. This fact contradicts the general belief of a progressive degradation of the harmonic emission at ultrahigh intensities. We have identified a particular set of trajectories which emerges in the turn-on stage of these singular laser pulses, responsible of the unexpected growth on the harmonic efficiency at this high intensity regime. Our study combines two complementary approaches: classical analysis and full quantum mechanical calculations resulting from the numerical integration of the 3-dimensional time-dependent Schr\"odinger equation complemented with the time-frequency analysis.