Ultrafast science has made great advances in recent years. Attosecond pulses with photon energies lying in the soft x-ray range corresponding to the fundamental absorption edges of matter, permit the studiesy of electron dynamics in live biological samples and next-generation semiconductor materials, such as diamond and graphene. The urgent need of intense attosecond pulses at x-ray wavelengths, especially in the water-window range, has promoted the development of attosecond x-ray free-electron lasers (FELs). A common method for producing ultrafast pulses is the enhanced self-amplified spontaneous emission (ESASE) technique, and there are many improvements based on the ESASE to further enhance the peak power or shorten the pulse duration. Whereas, it is still very challenge to generate stable and isolated x-ray pulses with durations of several tens of attoseconds since SASE starts from electron beam shot noise and the shortest pulse duration is eventually limited by the slippage length. To overcome these problems, several methods based on the echo-enabled harmonic generation (EEHG) have been proposed. However, in these methods, few-cycle laser pulses are generally required, leading to additional challenges for the laser generation and transmission.
The authors propose a simple and feasible method based on EEHG to generate intense isolated x-ray pulses covering the water-window range with a duration of tens of attoseconds. The schematic of the proposed scheme is similar to the conventional EEHG setup as shown in Fig. 1. The difference is that the second seed laser is replaced by a wavefront rotation (WFR) laser, i.e., the seed laser is sent through a dispersion element, e.g., double gratings, to induce spatiotemporally coupling and control the wavefront of the beam. The function of WFR laser is to tailor the longitudinal profile of the radiation pulse. Due to the sensitivity of seeded FEL to external lasers, this method can effectively inhibit the bunching on both sides while preserving an isolated bunching in the middle (see Fig. 2). The generated isolate attosecond pulses are natural synchronization to external lasers, making them capable of driving high resolution pump-probe experiments and providing a new avenue for attosecond sciences. Comparing with previous methods with few-cycle lasers, the proposed method only requires a 100 fs conventional laser, which greatly relaxes the requirements on for the seed laser and makes it reliable based on currently existing FEL facilities.
This kind of coherent x-ray light sources can make it possible to study the electronic dynamic of the valence electrons of which the time scale is about 100 attoseconds and may open up a new frontier of ultrafast science.
