Programme

I.7 -- Invited, VCTP-50

Date: Tuesday, 5 August 2025

Time: 14:00 - 14:40

Terahertz-Assisted High-Order Harmonic Generation for Waveform Sampling and Coherent XUV Emission

Ngoc-Loan Phan1, Doan-An Trieu2, Van-Hoang Le1

1 Ho Chi Minh City University of Education, Ho Chi Minh City, Vietnam 2 Duy Tan Universtiy, Da Nang, Vietnam

Terahertz (THz) fields—particularly THz pulses with durations ranging from picoseconds to nanoseconds—play a vital role in fundamental science, material control, and practical applications such as high-speed wireless communication technologies [1, 2]. Beyond generating THz pulses, accurately characterizing them is a critical step toward their full utilization. Among existing techniques, THz time-domain spectroscopy (THz-TDS) is widely used as it provides both amplitude and phase information of the THz field. However, THz-TDS remains technically challenging due to its strong dependence on the properties of the detection material, which limits its accuracy and measurable bandwidth [1, 2]. This motivates the development of alternative, material-independent methods for THz waveform sampling. In this report, we demonstrate that high-order harmonic generation (HHG)—a nonlinear optical process emitted when atoms or molecules interact with an intense ultrashort laser field—can serve as a powerful and general tool for THz field characterization. When the laser interaction is assisted by a THz field, the resulting HHG spectrum encodes detailed information about the THz waveform through symmetry-breaking signatures. By establishing analytical relationships between the THz electric field and the odd-even harmonic intensity ratio or frequency shift in the HHG spectrum, we propose two universal THz-TDS methods [3-5]. The first measures the even-to-odd harmonic intensity ratio in atoms or symmetric molecules irradiated by a multicycle mid-infrared laser pulse [3]. The second extracts the frequency shift of harmonic peaks in the cutoff region using few-cycle laser pulses [4]. More specifically, we show that the two proposed methods are general and robust, being independent of the atomic or molecular species and laser parameters. This universality paves the way for a compact, all-optical scheme for THz waveform sampling using standard tabletop HHG setups. Additionally, we explore the potential of THz-assisted HHG for generating high-coherence extreme ultraviolet (XUV) and soft X-ray photons. We show that moderate THz fields can significantly extend the HHG cutoff, making it possible to achieve coherent high-energy radiation [6]. Such sources hold great promise for applications such as diagnosing microscopic defects in microelectronic components. References: [1]. X. C. Zhang, A. Shkurinov, and Y. Zhang, Nat. Photon. 11, 16 (2017). [2]. Y. Zhang, K. Li, and H. Zhao, Front. Optoelectron. 14, 4 (2021). [3]. D.-A. Trieu, N.-L. Phan, Q.-H. Truong, H. T. Nguyen, C.-T. Le, D. Vu, and V.-H. Le, Phys. Rev. A 108, 023109 (2023). [4]. D.-A. Trieu, T.-T. D. Nguyen, T.-D. D. Nguyen, T. Tran, V.-H. Le, and N.-L. Phan, Phys. Rev. A 110, L021101 (2024). [5]. D.-A. Trieu, V.-H. Le, and N.-L. Phan, Phys. Rev. A 110, L041101 (2024). [6]. D.-A. Trieu, D. D. Hoang-Trong, C.-T. Le, S. Ha, V.-H. Le, and N.-L. Phan, in preparation.

Presenter: Phan Ngoc-Loan