A research team led by Professor Wu Xiaojun from the Hangzhou International Innovation Institute of Beihang University has achieved a major breakthrough in high-average-power, high-repetition-rate strong-field THz source. The team successfully generated terahertz radiation with an average power of 104 mW at room temperature, setting a new world record for such configurations. The findings have been published in Chinese Physics Letters under the title "100-mW high-average power strong-field terahertz source."

Postdoctoral researcher Xu Aojie is the first author of the paper, with Professor Wu Xiaojun serving as the corresponding author. The Hangzhou International Innovation Institute of Beihang University is the primary affiliation for the study.

High-average-power strong-field terahertz (THz) pulses, generated via optical rectification of hundred-watt average-power ytterbium lasers in a nonlinear crystal, have been used to study extreme physical phenomena and enable various applications. However, this THz generation method suffers from the trade-off where high repetition rates lead to low optical-to-THz energy conversion efficiency, as well as the risk of damaging the crystals under high average pumping power.

Strong-field THz sources possess a single-pulse energy > 1μJ, a peak electric field > 100 kV/cm, and a peak magnetic field > 1 mT. These sources support research areas such as ultrafast spin control and THz high-harmonic generation, and are also applied in X-ray Free-Electron Laser (FEL) facilities. Meanwhile, next-generation X-ray sources (e.g., the European XFEL) are developing toward higher repetition rates to enable more complex experiments, which in turn requires THz sources to have a repetition rate≥100 kHz.More recently, advanced precision measurement techniques and commercial applications, such as THz-coupled angle-resolved photoemission spectroscopy and scanning near-field optical microscopy, require these sources to possess high average power.

This high average power enables the sources to generate much stronger signals than weak-field THz spectrometers, and further enhances the quality of THz imaging and computed tomography. To meet these demands, the choice of nonlinear and pumping method is critical. Lithium niobate crystals, with their high nonlinear coefficient and damage threshold, are the priority candidate for high-intensity laser pumping. Furthermore, the tilted-pulse front pumping technique in lithium niobate enables high conversion efficiencies (in the percent range) by optimizing phase matching during THz generation.

Fig. 1. Overview graph of the THz pulse energy generated by optical rectification in lithium niobate for Ti:Sapphire laser and ytterbium laser pumping: room temperature (RT) cooled displayed in black square, RT in red dot, and cryogenic temperature (CT) in blue triangle.

In this work, the authors demonstrate a high-average-power, high-repetition-rate strong-field THz source in lithium niobate driven by a 1030 nm, 1 ps, 2 mJ, 100 kHz ytterbium femtosecond laser with a tilted-pulse front pumping configuration. By characterizing two key experimental parameters, the pump spot size and the pulse duration, they achieve the highest THz average power of 104 mW at 100 kHz, with a conversion efficiency of 0.1% without any cooling operation at room temperature. In addition, a strong electric field of 421 kV/cm is achieved at 1 kHz.

Fig. 2. The experimental setup for generation of the high-repetition rates and average power THz source.

Fig. 3. THz energy results for three different pump beam spots at the laser parameters of 800 fs and 100 Hzmeasured by (Gentec-EO: Beamage-4M).

This research successfully demonstrates a high-repetition-rate, high-average-power strong-field THz source based on an industrial-grade ytterbium-doped femtosecond laser. Through systematic optimization of pump spot size and pulse width, the team achieved a record 104 mW average power output at room temperature without cooling—a milestone that enables straightforward beam visualization with a liquid crystal sheet. The work significantly advances strong-field THz technology from complex laboratory setups toward practical, tool-like systems. By overcoming the power bottleneck, high-power terahertz sources hold great promise for aerospace, non-destructive evaluation, biomedical sensing, and fundamental research, providing a powerful new tool for materials science, biomedical studies, and industrial inspection.

Fig. 4. The methods of characterizing the maximum THz signal at 1-kHz.

Fig. 5. Characterization of the THz source via pump power, energies, and fluence with varying repetition rates.

Link to the article: https://iopscience.iop.org/article/10.1088/0256-307X/42/12/120406?sessionid=

Editor: Lyu Xingyun