Shanghai Institute of advanced research has made breakthroughs in the research o

The free electron laser team of Shanghai Institute of higher research, Chinese Academy of Sciences has made an important breakthrough in the research of fully coherent free electron laser. Based on the Shanghai soft X-ray free electron laser device, the new mechanism of echo harmonic cascade free electron laser independently proposed by China has been successfully verified, and soft X-ray coherent radiation with excellent performance has been obtained. Recently, relevant research results were published on optica, a top optical journal, under the title "coherent and ultra short soft X-ray pulses from echo enabled harmonic cascade free electron lasers".

X-ray free electron laser is one of the most advanced light source large scientific devices in the world. At present, most X-ray free electron lasers in the world are based on self amplified spontaneous emission mechanism (SASE). SASE has excellent performance such as extremely high peak brightness and femtosecond ultrashort pulse width, but SASE is vibrated by noise, and its radiation pulse coherence and stability are not high, so it is not a "laser" in the X-ray band.

One of the most important development directions in the international free electron laser field is to produce fully coherent x-ray radiation with conventional laser quality. The important way is to use the external seed free electron laser operation mechanism. The radiation of external seed free electron laser inherits the characteristics of seed laser, and has excellent characteristics such as full coherence, phase control and precise synchronization with external pump laser. However, due to the limitation of the wavelength and pulse width of the seed laser, the short wavelength coverage and pulse length adjustment range of the outer seed free electron laser are limited.

In order to further expand the short wavelength coverage of external seed free electron lasers, new free electron laser operation modes such as echo harmonic generation are being vigorously developed in the world in recent years.

Echo harmonic cascade FEL has excellent spectral performance: the left figure shows the conventional cascade mode, and the right figure shows the echo harmonic cascade mode（图图图）

Echo harmonic cascade can realize X-ray pulse length adjustment and ultrafast pulse generation（图图图）

The external seed free electron laser is one of the main technical routes for the development of high gain free electron laser in China. At present, all four high gain free electron laser devices in China have adopted the external seed operation mode. Based on Shanghai deep ultraviolet free electron laser device and Shanghai soft X-ray free electron laser device, we have successively achieved the world's first echo type free electron laser output amplification and the first extreme ultraviolet band echo type free electron laser saturation amplification. In order to further promote the outer seed free electron laser to short wavelength, the free electron laser team of our hospital independently proposed a new mechanism of fully coherent free electron laser with echo harmonic cascade. Subsequently, this mechanism was adopted as the basic scheme by Shanghai soft X-ray free electron laser device, and completed the whole process from principle verification to light output amplification in soft X-ray band.

The results show that this new mechanism has very excellent spectral characteristics compared with the traditional external seed mechanism. By using our self-developed ultrafast X-ray pulse diagnosis technology (doi:https: / / doi.org / 10.1016 / j.fmre.2022.01.027), we also verify the superior performance of this new mechanism in pulse length control and ultrafast pulse generation. These research results provide a practical technical route for the generation of fully coherent free electron lasers in the sub nanometer band, and will provide an ideal research tool for the fields of X-ray nonlinear optics and ultrafast physical chemistry.

At present, both Fermi FEL device in Italy and swissfel device in Switzerland have proposed plans to further improve its radiation performance by adopting this new mechanism.
This work has been supported by the national key R & D program, the national fund for outstanding young people, the general program of the National Natural Science Foundation and the Shanghai talent program.

Source: Shanghai Institute of higher studies, Chinese Academy of Sciences