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arXiv · 2601.16747

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

Doan-An Trieu·Duong D. Hoang-Trong·Cam-Tu Le·Sang Ha·Ngoc-Hung Phan·F. V. Potemkin·Van-Hoang Le·Ngoc-Loan Phan
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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.

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Doan-An Trieu, Duong D. Hoang-Trong, Cam-Tu Le, Sang Ha, Ngoc-Hung Phan, F. V. Potemkin, Van-Hoang Le, Ngoc-Loan Phan. 2026-01-23. Moderate-terahertz-induced plateau expansion of high-order harmonic generation to soft X-ray region. https://arxiv.org/abs/2601.16747

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