New technologies improve temporal coherence of XFEL pulses

X-ray free-electron lasers produce pulses of light that are exceptionally bright, making them powerful tools for studying ultrafast chemical reactions, biological processes, or probing the structure of materials at atomic scales.

However, these pulses are noisy in time and frequency due to the way the light is generated through a process known as self-amplified spontaneous emission, or SASE for short: i.e., the pulses are not temporally coherent.

This spectral randomness can be a limitation for experiments requiring ultra-high spectral control to follow electron and structural dynamics.

In true Swiss style, researchers at SwissFEL have now found a way to make the light neat and orderly. Their developments are published in the journal, Physical Review Letters, where it was selected as an “Editors’ Suggestion.”

At the Athos beamline of SwissFEL, by inserting magnetic chicanes to control the timing of the electron beam between undulator modules, the researchers achieved two major breakthroughs:

1. Tuneable frequency combs: They created frequency combs, where the number of spectral lines and their separation can be varied.
2. High brightness, narrow bandwidth pulses: They collapsed the frequency combs to a single, ultra-bright “tooth”—i.e., a single spectral line with a bandwidth about a third of that of a normal XFEL pulse.

Both these advances will open new scientific opportunities with XFELS in areas that need very precise spectral control across a wide range of fields from fundamental physics to applied sciences.