Abstract
High-power, continuously tunable narrowband terahertz (THz) sources are essential for advancing nonlinear optics, THz-driven material dynamics and ultrafast spectroscopy. Conventional techniques typically impose a trade-off between pulse energy and frequency tunability. Here we demonstrate a novel free-electron laser approach that overcomes these limitations by premodulating a relativistic electron beam with a frequency-beating laser pulse and leveraging bunch compression along with collective effects to enhance microbunching. Experimental results demonstrate that this technique generates narrowband THz emission with continuous frequency tunability from 7.8 to 30.8 THz, achieving pulse energies up to 385 \(\upmu {\rm{J}}\) and maintaining spectral bandwidths between 7.7% and 14.7%. Moreover, the method exhibits exceptional robustness and scalability, highlighting its unique ability to bridge the long-standing THz gap and offering a promising solution for diverse cutting-edge scientific applications.
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The data that support the findings of this study are presented in the article and the Supplementary Information. Other relevant data and findings of this study are available from the corresponding authors upon reasonable request.
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Acknowledgements
We thank M. Chen and H. Zhu for helpful discussions. This work was supported by the National Natural Science Foundation of China under grant numbers 12275340 (K.Z.), 12105347 (K.Z.) and 12435011 (C.F.), CAS Project for Young Scientists in Basic Research under grant number YSBR-115 (C.F.), Shanghai Municipal Science and Technology Major Project (C.F.) and Innovation Program of Shanghai Advanced Research Institute, CAS, under grant number 2024CP001 (K.Z.).
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C.F., K.Z. and Y.K. conceived and designed the experiments. K.Z. and Y.K. conducted the experiments with help from Z.W., T. Li, Y. Wang, Z.G., H.L., Xiaofan Wang, T. Liu and Z.Q. and with software and hardware support by C.Y., W.Y., H.X., J.C., T. Li, X.L., J.W., H.Z., F.G., L.S., Y.Z., Y. Wen, C.X., Xingtao Wang, J.X. and B. Li. The THz diagnostic system is designed and constructed by Y. Wang, T. Li, L.Y., Q.T., T. Lan and Y.K. The simulations on beam dynamics were performed by Y.K. and Z.W. The paper was written by C.F., K.Z. and Y.K. with contributions from Z.W. Management and oversight of the project was provided by C.F., B. Liu and Z.Z.
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Kang, Y., Li, T., Wang, Z. et al. Continuous terahertz band coverage through precise electron-beam tailoring in free-electron lasers. Nat. Photon. 20, 96–101 (2026). https://doi.org/10.1038/s41566-025-01775-1
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DOI: https://doi.org/10.1038/s41566-025-01775-1
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