The research team completed the first electron beam pulse train-driven superradiant terahertz free-electron laser experiment at the Dalian Coherent Light Source, achieving tunable narrowband terahertz radiation output in the 1-20 THz frequency range with energy levels of hundreds of microjoules, and obtaining a single-pulse energy of nearly one millijoule at 10 THz.
Recently, Professor Yan Lixin's research group from the Department of Engineering Physics at Tsinghua University, in collaboration with the Dalian Institute of Chemical Physics of the Chinese Academy of Sciences and the Shenzhen Advanced Light Source Research Institute, reported significant progress in the field of accelerator-based terahertz light sources. The research team completed the first electron beam pulse train-driven superradiant terahertz free-electron laser experiment at the Dalian Coherent Light Source, achieving tunable narrowband terahertz radiation output in the 1-20 THz frequency range with energy levels of hundreds of microjoules, and obtaining a single-pulse energy of nearly one millijoule at 10 THz. This achievement sets a new international record for radiation energy in this frequency range and provides an effective solution to the "terahertz gap" problem in light sources.
Terahertz radiation (THz) usually refers to electromagnetic waves with frequencies between 0.1 and 10 THz, corresponding to wavelengths from 3 mm to 30 μm, which is the electromagnetic spectrum between microwaves and infrared light. Due to its unique frequency characteristics, terahertz radiation can effectively excite various collective modes in materials, such as phonons, magnons, and excitons, and therefore has important applications in cutting-edge research such as superconductivity and quantum materials. However, because it is located in the transition region of the spectrum covered by optical and electronic technologies, the development of high-power, narrow-bandwidth terahertz radiation sources has long faced technological bottlenecks. In particular, the 1-10 THz frequency range, which is the most urgently needed, still lacks effective methods for generating high-power, narrowband radiation across the entire frequency range. This dilemma is known as the "terahertz gap" problem.
Based on the coherent radiation mechanism of ultrashort electron beams, Professor Yan Lixin's research group has conducted continuous exploration and research in this area for nearly two decades. Although free-electron lasers (FELs) have achieved great success in short-wavelength bands such as extreme ultraviolet and X-rays, and various high-gain schemes such as SASE, HGHG, and EEHG have been developed, in the terahertz band, due to the severe diffraction effects of its long wavelength, the interaction between the electron beam and the radiation is significantly weakened, making traditional high-gain schemes difficult to implement. The research group proposed a solution using the superradiant free-electron laser mechanism. The core of this mechanism is to use a pre-formed terahertz electron beam pulse train to drive the free-electron laser, bypassing the microbunching stage in traditional FELs and directly utilizing the pulse train to achieve coherent superposition of the radiation field, thereby generating high-power coherent radiation whose power is proportional to the square of the number of electrons. During the transmission of the microbunches in the undulator, they continuously interact with the generated radiation field at the maximum deceleration phase, rapidly extracting electron beam energy and significantly improving radiation efficiency, ultimately overcoming the limitations of diffraction effects and achieving higher terahertz radiation energy.
Based on the above idea, building upon the research group's 2023 work on generating 1-10 THz broadband tunable ultrashort electron beam pulse trains using a space-charge force modulation scheme, the research team further introduced an X-band accelerating cavity to compensate for the nonlinearity in the radio frequency (RF) chirp, extending the frequency tuning range of the electron beam pulse train to 15 THz. This was then used to drive a superradiant free-electron laser, successfully achieving continuously tunable high-power narrowband terahertz radiation across the entire 1-20 THz frequency range.
Figure 1. Schematic diagram of the superradiant terahertz free-electron laser principle (a) Experimental setup (b) Electron beam longitudinal distribution (c) Radiation spectral characteristics (d) Relationship between radiation pulse energy and the square of the charge quantity
The experimental setup is shown in Figure 1a. A quasi-flat-top ultraviolet laser pulse train generated by pulse stacking illuminates the photocathode, producing an electron beam pulse train with initial density modulation. This is then transformed into periodic energy modulation through a space-charge oscillation process, which deepens during subsequent transmission. Through subsequent accelerating tubes and X-band harmonic cavities, the electron beam's energy and energy chirp can be adjusted. A downstream magnetic compressor converts the electron beam's energy chirp into density modulation at the target frequency, which is then fed into the undulator to drive the superradiant free-electron laser. Figures 1b to 1d show the electron beam's longitudinal density modulation measured by a deflection cavity, the radiation spectral characteristics, and the relationship between the radiation pulse energy and the square of the charge quantity, respectively. These results not only verify the coherence of the radiation but also indicate that the radiation energy can be further increased by increasing the charge quantity.
Figure 2. Main characteristics of the radiation and simulation results of energy growth (a) Radiation spectral characteristics (b) Single-pulse energy measurement results (c) Simulation results of radiation energy growth
As shown in Figure 2, the experiment achieved a pulse energy output of hundreds of microjoules across the entire 1-20 THz frequency range using a planar undulator configuration. At 10 THz, the single-pulse energy reached 390 μJ. Further use of a tapered undulator increased the single-pulse energy to a maximum of 900 μJ, which is the highest radiation energy record achieved internationally in narrowband terahertz sources in this frequency range to date.
The research results, titled "Superradiant terahertz free-electron laser driven by electron microbunch trains," were published in *Light: Science & Applications* on January 8th, Beijing time.
Dr. Liang Yifan from the Shenzhen Advanced Light Source Research Institute and Li Tong, a 2022 doctoral student from the Department of Engineering Physics at Tsinghua University, are the co-first authors of the paper; Professor Yan Lixin and Professor Tang Chuanxiang from the Department of Engineering Physics at Tsinghua University, and researchers Zhang Weiqing and Wu Guorong from the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, are the co-corresponding authors. The research was supported by the Beijing Municipal Excellent Young Scientists Program, the Chinese Academy of Sciences Research Instrument and Equipment Development Project, the National Natural Science Foundation of China, and the Tsinghua University "Dushi" Special Project.
