In Ultrafast Science, a group of researchers presented a unique plan for producing isolated, high-intensity, attosecond soft X-ray FELs from gas-filled hollow capillary fibers (HCFs) utilizing a mid-infrared (MIR) sub-cycle modulation laser.
Layout of attosecond soft X-ray generation driven through MIR sub-cycle pulses from gas-filled HCF. Image Credit: Ultrafast Science
Attosecond light sources are sophisticated optical instruments essential for studying electron dynamics at the subfemtosecond scale in quantum systems. However, producing high-intensity isolated X-ray pulses remains a major scientific difficulty. X-ray free-electron lasers (FELs) are sophisticated light sources based on linear accelerators that can produce ultrashort and powerful laser pulses.
Enhanced Self-Amplified Spontaneous Emission (ESASE) is an efficient approach for creating ultrashort pulses in FELs by locally increasing the peak current of electron beams. However, producing isolated current spikes has remained a significant issue for ESASE, which dictates the signal-to-noise ratio of ultrafast FEL pulses.
The multi-cycle MIR pulses are first compressed to sub-cycle using a krypton-filled HCF with a diminishing pressure gradient caused by the soliton self-compression effect. Using a sub-cycle MIR laser pulse to modulate the electron beam results in a quasi-isolated current peak, which may then be used to generate an isolated FEL pulse with a high signal-to-noise ratio that naturally synchronizes with the sub-cycle MIR laser pulse.
Research Overview: One notable feature of a gas-filled HCF system is the ability to tune the nonlinearity and dispersion landscape by modifying the gas type and pressure. Due to the soliton-effect self-compression in gas-filled HCFs, laser pulses may be greatly compressed, which is paired with waveguide-induced anomalous dispersion and self-phase modulation (SPM).
Soliton self-compression systems may generate sub-cycle pulses without further dispersion adjustment than standard post-compression systems. The simulation employed 40 fs (FWHM), 4 μm, 640 μJ Gaussian-shaped MIR pulses as input, resulting in ~5.7 fs, ~40 GW sub-cycle MIR pulses from gas-filled HCFs.
After being transmitted into the modulator, the sub-cycle MIR pulses generated at the HCF output port will interact with the electron beams, resulting in a significant energy chirp in the electron beams. The energy chirp will be compressed when the electron beam passes through the chicane, resulting in an isolated current spike.
After the electron beam is delivered to the radiator, soft X-ray FEL pulses with durations of hundreds of attoseconds may be generated, yielding peak power of tens of GW, as illustrated in Figure 4. Most notably, the resulting isolated FEL pulse has an incredibly high signal-to-noise ratio.
These high-intensity ultrashort pulses may be employed in various cutting-edge scientific applications, including investigating valence electron mobility, photoemission delay, tunneling delay time, and so on. Surprisingly, the suggested approach will have extensive application possibilities by using an ultra-intense sub-cycle pulse synced with an attosecond FEL pulse as the pump laser in pump-probe experiments.
Journal Reference:
Xiao, Y., et al. (2025) Isolated attosecond free-electron laser based on a sub-cycle driver from hollow capillary fibers. Ultrafast Science. doi.org/10.34133/ultrafastscience.0099.