Attosecond X-ray pulses: revealing the hidden world of quantum processes

Background: Attosecond light sources serve as advanced optical tools that are crucial for investigating electron dynamics at the subfemtosecond scale in quantum systems. However, generating high-intensity isolated X-ray pulses remains a significant scientific challenge. X-ray free-electron lasers (FELs) are advanced light sources based on linear accelerators, capable of generating ultrashort and ultra-intense laser pulses. Enhanced Self-Amplified Spontaneous Emission (ESASE) serves as an effective method for producing ultrashort pulses in FELs by locally enhancing the peak current of electron beams. However, obtaining isolated current spikes has remained a critical challenge for ESASE, which directly determines the signal-to-noise ratio of ultrafast FEL pulses.

Our team proposed a novel scheme for generating high-intensity, isolated attosecond soft X-ray FELs using a mid-infrared (MIR) sub-cycle modulation laser from gas-filled hollow capillary fibers (HCFs). The multi-cycle MIR pulses are first compressed to sub-cycle using a krypton-filled HCF with decreasing pressure gradient due to soliton self-compression effect. By utilizing such sub-cycle MIR laser pulse to modulate the electron beam, we can obtain a quasi-isolated current peak, which can then produce an isolated FEL pulse with high signal-to-noise ratio, naturally synchronizing with the sub-cycle MIR laser pulse.

Research Overview: One remarkable advantage of gas-filled HCF system is the tunable nonlinearity and dispersion landscape through adjusting gas type and pressure. Due to the soliton-effect self-compression in gas-filled HCFs, the laser pulses can be significantly compressed with the combined effect of waveguide-induced anomalous dispersion and self-phase modulation (SPM). Compared with the traditional post compression schemes, soliton self-compression holds the capability of directly generating sub-cycle pulses without additional dispersion compensation. In the simulation, we used 40 fs (FWHM), 4 μm, 640 μJ Gaussian-shape MIR pulses as input, and achieved ~5.7 fs, ~40 GW sub-cycle MIR pulses from the gas-filled HCFs, as shown in Figure 2.

After transmitted into the modulator (see Figure 1), the sub-cycle MIR pulses generated at the HCF output port will interact with the electron beams, and this interaction can lead to a large energy chirp in the electron beams. Then, the energy chirp will be compressed after the electron beam passes through the chicane, resulting in the generation of isolated current spike, as illustrated in Figure 3.

After the electron beam is transmitted to the radiator, the soft X-ray FEL pulses with duration of hundreds of attoseconds, peak power of tens of GW can be obtained, as shown in Figure 4. Most importantly, the generated isolated FEL pulse has an extremely high signal-to-noise ratio. Such high-intensity ultrashort pulses can be used for numerous cutting-edge scientific applications, such as probing valence electron motion, photoemission delay, tunneling delay time and so on. Remarkably, by adopting the ultra-intense sub-cycle pulse synchronized with the attosecond FEL pulse as the pump laser, in pump-probe experiments, our proposed method will have broad application prospects.