The free-electron laser team at the Shanghai Advanced Institute of Research (SARI) of the Chinese Academy of Sciences (CAS) has made progress in the diagnostic study of ultrafast free-electron laser pulses. The team proposed and validated a new method for single-shot diagnosis of ultrafast free-electron laser pulses based on self-referenced interference spectroscopy, which provides a brand new idea for cracking the problem of high-precision real-time diagnosis of attosecond free-electron lasers. The related research results, titled Self-Referenced Spectral Interferometry for Single-Shot Characterization of Ultrashort Free-Electron Laser Pulses, were published in Physical Review Letters ( Physical Review Letters.
Advanced light sources with attosecond (10-18 s) time-resolving capabilities are urgently needed to explore the fundamental processes of matter transformations in the microscopic world, such as photoelectric emission delays, the motion of valence electrons, and the transfer of electric charge. The attosecond light source can be used to observe and manipulate the electron motion inside atoms and molecules, which helps scientists to explore chemical reactions, electronic structure and molecular dynamics in greater depth, and is of great significance to materials science and chemistry research. In recent years, important breakthroughs in X-ray free-electron laser physics and technology have enabled the generation of attosecond X-ray pulses with extremely high peak brightness, which is expected to provide a revolutionary tool for attosecond scientific research. In addition to the generation of attosecond pulses, the complete time-frequency domain information diagnosis of attosecond X-ray free-electron laser is equally important for ultrafast scientific experiments, and how to perform high-precision real-time diagnosis of this information has become a bottleneck limiting the application of attosecond X-ray free-electron laser. To address this issue, the team has carried out systematic research work based on China's free-electron laser large-scale scientific device.
Scheme layout and time-frequency domain reconstruction method for attosecond pulsed X-ray free-electron laser pulses
In recent years, spectral phase interferometry with direct electric field reconstruction (SPIDER) has become one of the rapidly developing pulse reconstruction methods in the field of ultrafast lasers. The key to this method is to generate a pair of replica pulses with appropriate spectral shear. This process generally requires the use of nonlinear crystalline materials, making the expansion of the method to short wavelengths rather difficult. In this study, it is innovatively proposed to utilize the frequency traction effect of the free-electron laser to generate the spectral shear quantity, and both the ultrafast radiation pulse and the reference pulse are generated by the same electron beam, which cleverly realizes the self-reference spectral interference of the radiation pulse; by applying the wavelet transform algorithm in order to improve the SPIDER, the signal-to-noise ratio and the efficiency of the reconstruction can be further improved, and at the same time, by using the parameters of the Shanghai soft-X-ray free-electron laser device, it is demonstrated that the complete time-frequency domain information of an attosecond X-ray pulse can be reconstructed accurately (the reconstruction error is less than 6%) using this method. Compared with the diagnostic method of ultrafast pulses in traditional free-electron laser devices, this method has the advantages of simple equipment, high diagnostic efficiency (real-time, single-shot), obtaining complete time-frequency domain information at the same time, and higher diagnostic accuracy with shorter radiation pulses, which provides a brand-new diagnostic means for the optimization of the debugging of ultrafast X-ray free-electron lasers as well as for future attosecond science experiments based on X-ray free-electron lasers. The results are summarized in the following table.
