Moderate-terahertz-induced plateau expansion of high-order harmonic generation to soft X-ray region

TL;DR

This paper demonstrates that weak terahertz fields can significantly extend high-order harmonic generation cutoff energies, enabling tabletop soft X-ray sources and advancing ultrafast electron dynamics studies.

Contribution

It provides a comprehensive analysis showing how weak THz assistance extends HHG cutoff via long electron excursions, supported by classical and Bohmian dynamics.

Findings

Weak THz fields extend HHG cutoff to soft X-ray region.

The cutoff extension forms a 'fish-fin' structure saturating near Ip + 8 Up.

The mechanism is robust across different atomic species and parameters.

Abstract

Extending the high-harmonic cutoff with experimentally accessible fields is essential for advancing tabletop coherent extreme ultraviolet (EUV) and soft X-ray sources. Although terahertz (THz) assistance offers a promising route, cutoff extension at weak, laboratory-accessible THz strengths remain poorly understood. In this report, we comprehensively investigate THz-assisted high-order harmonic generation (HHG) using time-dependent Schr\"odinger equation simulations supported by classical trajectory analysis and Bohmian-based quantum dynamics. By mapping the plateau evolution versus THz strength, we show that even weak THz fields can extend the cutoff, producing a pronounced ``fish-fin'' structure whose prominent rays saturate near $I_p + 8 U_p$. We trace this extension to long electron excursions spanning several optical cycles before recombination, and provide a fully consistent explanation using both classical analysis and Bohmian trajectories flow. Our findings reveal that this cutoff-extension mechanism is remarkably robust, persisting across different atomic species and remaining insensitive to variations in the driving parameters. These results demonstrate that cutoff control is achievable with laboratory-scale THz fields, offering practical guidelines for engineering coherent high-energy HHG, and providing a robust pathway for tracking ultrafast electron motion in real time.