Recently, the free electron laser team of the Shanghai Advanced Research Institute of the Chinese Academy of Sciences has made progress in the research of new mechanisms of high repetition rate fully coherent free electron laser (FEL). For the first time, the independently proposed new mechanism of "direct amplification driven harmonic generation" (DEHG-FEL) has been experimentally verified, and its light amplification and stable operation have been successfully achieved. This marks a key step on the road to achieving fully coherent extreme ultraviolet (EUV) and X-ray FEL light sources with a repetition rate of megahertz (MHz).
High repetition rate fully coherent EUV and soft X-ray FELs have important application prospects in cutting-edge fields such as advanced time-resolved spectroscopy, coherent diffraction imaging, and ultrafast dynamics research of nanostructures and nanodevices. However, traditional exogenous seeded FEL schemes usually require ultraviolet seed lasers with peak powers of up to hundreds of megawatts to drive them, which limits their repetition rates to the kilohertz level.
The free electron laser team of Shanghai Advanced Research Institute has previously verified the self-amplification mechanism of coherent energy modulation at the Shanghai Soft X-ray Free Electron Laser Facility (SXFEL). In this work, the research team used a long modulation segment to directly amplify the weak seed laser signal through a high-gain FEL process, thereby obtaining a high-power, stable modulated laser. In this process, the electron beam obtained sufficient energy modulation, and formed a coherent micro-bunch on the scale of the optical wavelength through the dispersion segment, thereby generating a fully coherent radiation output up to the 12th harmonic. The team further amplified the 7th harmonic to saturation, and obtained a saturated pulse energy of about 160 microjoules, with an energy stability of 5.5%. Through the harmonic cascade process, the team also successfully achieved 16th harmonic output, and the output spectral bandwidth was close to the Fourier transform limit, which shows that the DEHG-FEL has been well maintained in terms of longitudinal coherence.
DEHG technology can effectively amplify extremely weak seed laser signals with its simple device configuration. In principle, it can reduce the seed laser power by two to three orders of magnitude and has the ability to output high-order harmonics in a stable, controllable manner, which is particularly suitable for high-repetition-rate exo-seed free electron lasers. In addition, it has the potential to provide a solution for achieving high repetition rates for more complex two-stage seed schemes (such as echo-driven harmonic generation, EEHG). In the future, this scheme is expected to be combined with high-order harmonic generation technology (HHG) to provide a new means for ultrafast spectroscopy, imaging and materials science research in the soft X-ray band.
The relevant research results were published in
Physical Review Letters under the title "
First Lasing and Stable Operation of a Direct-Amplification Enabled Harmonic Generation Free-Electron Laser" and were selected as an "Editor's Recommendation" article .
This research was supported by the Shanghai Soft X-ray Free Electron Laser Facility, the National Key R&D Program, the National Natural Science Foundation, the Chinese Academy of Sciences' Stable Support Program for Young Teams in Basic Research, and Shanghai's Major Science and Technology Projects.