The free-electron laser team of the Shanghai Institute of Advanced Studies of the Chinese Academy of Sciences has made progress in the research of fully coherent free-electron lasers. Based on the Shanghai soft X-ray free-electron laser device, the new mechanism of echo-harmonic cascade free-electron laser proposed by China has been successfully verified and the soft X-ray coherent radiation with excellent performance was obtained.The relevant research results were recently published in Optica under the title Coherent and ultra-short soft X-ray pulses from echo-enabled harmonic cascade free-electron lasers.
The X-ray free electron laser is one of the most advanced light source big science installations in the world. The vast majority of international X-ray free electron lasers are based on the self-amplifying spontaneous radiation mechanism (SASE), SASE has a very high peak brightness and femtosecond ultra-short pulse width and other excellent performance, but SASE by the noise vibration, its radiation pulse coherence and stability is not high, not yet the X-ray band "laser". One of the most important international developments in the field of free electron lasers is the generation of fully coherent X-ray radiation with conventional laser quality, and an important way to do this is to use an external seed type free electron laser operating mechanism. The radiation of the external seeded free-electron laser inherits the characteristics of the seed laser, with excellent features such as full coherence, phase control and precise synchronisation with the externally pumped laser. However, the short wavelength coverage and pulse length adjustment range of the outer seeded free-electron laser is limited by the wavelength and pulse width of the seed laser. In order to further extend the short wavelength coverage of the outer seed type free-electron laser, new free-electron laser operation modes such as echo-harmonic generation are being vigorously developed internationally in recent years.
The outer seed type free electron laser is one of the main technical routes for the development of high gain free electron lasers in China, and at present all four high gain free electron laser devices in China have adopted the outer seed operation mode. Based on the Shanghai Deep Ultraviolet Free Electron Laser Unit and the Shanghai Soft X-ray Free Electron Laser Unit, scientists have achieved the world's first echo-type free electron laser emission amplification and the first echo-type free electron laser saturation amplification in the extreme ultraviolet band. To further advance the outer seed type free electron laser to shorter wavelengths, the research team independently proposed a new mechanism of fully coherent free electron laser with echo harmonic cascade, which was adopted as the basic scheme by the Shanghai soft X-ray free electron laser device, and completed the whole process from the proof of principle to the soft X-ray band outgoing light amplification. The results show that this new mechanism has very superior spectral properties compared to the conventional exogenous seed-type operating mechanism, which is further validated in terms of pulse length control and ultrafast pulse generation by using the researchers' self-developed ultrafast X-ray pulse diagnostic technique (https://doi.org/10.1016/j.fmre.2022.01.027). The superior performance of this new mechanism in terms of pulse length control and ultrafast pulse generation has been further demonstrated by the use of an ultrafast X-ray pulse diagnostic technique developed in-house. The research results provide a practical route for generating fully coherent free-electron lasers at sub-nanometer wavelengths and will provide ideal research tools for the fields of X-ray nonlinear optics and ultrafast physicochemistry.
Both the Italian FERMI-FEL installation and the SwissFEL installation in Switzerland are currently proposing plans to further enhance their radiation performance using this new mechanism. The research work was supported by the National Key Research and Development Program of China, the National Excellent Youth Fund Program, the National Natural Science Foundation of China and the Shanghai Talent Program.
Echo-harmonic cascade free-electron laser with excellent spectral performance: conventional cascade mode on the left, echo-harmonic cascade mode on the right.
Echo harmonic cascade for X-ray pulse length adjustment and ultrafast pulse generation.
