Lasing of a cavity-based X-ray source.
basic_science · Level V
Where this comes from
- Record sourced from PubMed, PMID 41606320.
- Also identified by DOI 10.1038/s41586-025-10025-x and PMC identifier 12872447.
- Licence recorded as CC BY.
- The licence permits redistribution, so the abstract is shown in full and the full text is available from the publisher.
Abstract
The invention of the laser transformed optics by providing intense, coherent light in the visible region, but extending this concept to X-rays has been hindered by a lack of suitable gain media and mirrors. Current hard X-ray free-electron laser (XFEL) facilities<sup>1-5</sup> overcome this by amplifying shot noise from a high-peak-current electron bunch via self-amplified spontaneous emission<sup>6</sup> in a single pass through long undulators, delivering very high brightness but with a noisy, multi-spiked temporal and spectral profile. Cavity-based XFELs (CBXFELs)<sup>7-9</sup> were proposed to close this gap by recirculating spectrally filtered X-ray pulses in a Bragg-reflecting cavity synchronized to a high-repetition-rate electron beam. Here we show lasing with multi-pass gain at 6.952 keV in a 132.8-m round-trip diamond-based Bragg cavity<sup>10</sup> at the European XFEL, matched to the 2.23-MHz bunch spacing of the superconducting accelerator<sup>5</sup>. Under stringent length and angular stability requirements, a ring-up in the cavity across successive bunches was observed, producing spectrally pure, microjoule-level pulses. This establishes the feasibility of CBXFELs in an accelerator environment and validates diamond Bragg optics for X-ray resonators. The demonstrated spectral purity opens a path to next-generation X-ray science, which demands highly coherent, stable sources.