New Spectral Control Techniques Enhance XFEL Capabilities

X-ray free-electron lasers are powerful tools for studying matter at the atomic level, but their light pulses are often noisy and lacking in precision. Researchers at SwissFEL have now developed a method to organize and refine this light, ensuring it is more coherent and orderly. The results are detailed in Physical Review Letters.

X-ray free-electron lasers generate extremely bright pulses of light, making them invaluable for examining ultrafast chemical reactions, biological processes, and the atomic-scale structure of materials.

However, these pulses exhibit noise in both time and frequency due to the self-amplified spontaneous emission (SASE) process used in their generation. As a result, the pulses lack temporal coherence.

This spectral randomness poses challenges for experiments that demand ultra-high spectral precision to track electron and structural dynamics effectively.

At the Athos beamline of SwissFEL, researchers introduced magnetic chicanes to regulate the timing of the electron beam between undulator modules, achieving two significant advancements:

1. Tuneable Frequency Combs: They generated frequency combs with adjustable spectral line spacing and numbers.
2. High-Brightness, Narrow-Bandwidth Pulses: They collapsed the frequency combs into a single, ultra-bright spectral line, or “tooth,” with a bandwidth approximately one-third that of a standard XFEL pulse.

These innovations pave the way for new scientific applications of XFELs in fields requiring precise spectral control, spanning fundamental physics to applied sciences.

Journal Reference:

Prat, E., et al. (2024) Experimental Demonstration of Mode-Coupled and High-Brightness Self-Amplified Spontaneous Emission in an X-Ray Free-Electron Laser. Physical Review Letters. doi.org/10.1103/physrevlett.133.205001.