The pulse duration of the free electron laser is at tens femtosecond level, which is of great significance for the study of ultrafast science such as molecule recombination or molecule vibration. However, the timescales of electron motion in molecules are on several hundred attosecond level, showing strong demands on the attosecond pulse generation. High harmonic generation provides an important tool but the relatively low pulse energy limits the application. On the other way, several schemes have been proposed based on free electron laser, including the enhanced self-amplified spontaneous emission (ESASE) scheme or the pulse-compression scheme. In those schemes, a state-of-the-art ultrashort few-cycle laser is required to modulate the electron beam, which is relatively technical challenging. In addition, the satellite radiation pulse generated in the ESASE scheme will significantly degrade the free electron lasers (FEL) performance.

The research group led by Prof. Chao Feng and Zhen Wang from Shanghai Advanced Research Institute proposed a novel scheme using two infrared lasers with opposite chirps to modulate the electron beam with a beating frequency, aiming to generate a train of electron spikes with gradually varying spacing. Then the target isolated radiation will be amplified via adjusting the delay line inserted in the undulator segments. The proper delay line parameters are carefully optimized to amplify only one single spike and suppress the radiation pulses coming from other spikes because of the irregularly-spaced current peaks, as shown in Fig. 1. Comparing with the previous schemes, this novel scheme holds the advantage of using commercial infrared lasers to provide more stable frequency beating structure, thus the possibility of being implemented experimentally in the existing facilities.

Fig. 1 Schematic layout of the proposed scheme with optical frequency beating.

In the proposed scheme, two 1 ps long laser pulses with opposite frequency chirps are adopted to interact with the 3 GeV electron beam in the modulator to generate pulse train with peak current of about 5 kA. The simulation results demonstrate that an isolated fully coherent radiation pulse is achieved with a peak power of 330 GW and pulse duration of about 620 as at around 50 m undulator line, which is 1 order of magnitude higher than that of the normal SASE scheme. One can further improve the peak power of the radiation by optimizing the parameters of the frequency beating with more current spikes and undulator segments. In addition, one can further shorten the pulse duration of the radiation with shorter undulators, aiming to suppress the slippage effect.

This work presents the possibility of generating fully coherent attosecond radiation pulses based on Shanghai soft x-ray free electron laser facility (SXFEL). Future work will focus on the proving of principle experiments of the proposed scheme. The modulation undulator and the delay line inserted in the undulator segments have been installed at SXFEL. The frequency beating laser and the attosecond pulses detection diagnostics are under preparation.

The article is published in the High Power Laser Science and Engineering, Volume 11, Issue 3 (Zhen Wang, Chao Feng. Optical beat notes assisted attosecond soft X-ray pulse generation in high-gain free electron lasers[J]. High Power Laser Science and Engineering, 2023, 11(3): 03000e33).