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For the first time, researchers have amplified X-ray light multiple times in a resonator cavity, in a way highly similar to traditional lasers. With great success: the new technique delivers extremely energetic X-ray pulses for high-precision experiments. This development opens up entirely new possibilities for research in physics, chemistry, or biology. The system is called “XFELO”. Researchers from European XFEL, DESY and Hamburg University have published their findings in the latest edition of the journal Nature.
The team of engineers and scientists have shown for the first time that a hard-X-ray cavity can provide net X-ray gain, with X-ray pulses being circulated between crystal mirrors and amplified in the process, much like happens with an optical laser. The result of the proof-of-concept at European XFEL is a particularly coherent, laser-like light of a quality that is unprecedented in the hard X-ray spectrum. Lasing inside a cavity had been challenging to achieve with short-wavelength X-rays for a variety of reasons, including – on a basic level – that the nature of the light makes it difficult to reflect the beam at large angles. The “XFELO” (short for: X-Ray Free-Electron Laser Oscillator) technique opens new perspectives for scientific investigations, from ultrafast chemical reactions to detailed analyses of the smallest biological structures.
From XFEL to XFELO
Today, free-electron lasers generate X-ray pulses with the help of linear electron accelerators. Electric fields accelerate bunches made from around 100 billion electrons to near-light speed. Next, the negatively charged elementary particles pass through special magnetic structures called undulators. They force the electrons onto a high-speed slalom course. The constant change of direction forces the electrons to send out strongly bundled X-ray light in the forward direction. At the European XFEL, up to 27,000 electron bunches per second speed through the undulators and generate X-ray flashes at the same rate.
Although these X-ray flashes are already of an exceptionally good quality, they still retain a certain blurriness in terms of energy. The new XFELO system helps to dramatically reduce this blurriness and generate X-ray light of an exactly defined energy. For precision experiments, this is particularly important.
In XFELO, the X-ray light bounces through a circuit called a resonator cavity several times. The cavity comprises two sets of diamond mirrors and a row of undulators between them. With every pass, the X-ray light interacts with a “fresh” bunch of electrons from the electron accelerator. “That strengthens and concentrates it every time,” explains Harald Sinn, X-ray optics expert and leader of the Instrumentation Department at European XFEL.
A razor-sharp spike
“With every round trip, the noise in the X-ray pulse gets less and the concentrated light more defined,” says DESY accelerator scientist Patrick Rauer, whose doctoral thesis laid the groundwork for the resonator cavity and who is now the DESY lead of the implementation. “It gets more stable and you start to see this single, clear frequency – this spike.” That spike represents the singular pulse of X-ray light with a razor-sharp definition.
The concept of using the resonator cavity at the European XFEL was first proposed by Jörg Rossbach, who was a professor of physics at the University of Hamburg at the time. In the decades since that proposal, the idea was further studied and modeled for years, before Rauer and other scientists from DESY’s accelerator division and scientists and engineers from Sinn’s instrumentation teams at European XFEL could design a specific resonator cavity setup. Coincidentally, at a beamtime at the European XFEL examining the results of the resonator, it was Jörg Rossbach, now a professor emeritus, who first noticed the spike among the data.
Unique level of precision
The resonator cavity structure at European XFEL is about 66 metres long. High-quality diamond crystals reflect the light in such a way that it repeatedly moves through the resonator cavity. Optical mirrors ensure additional focusing and stability. Particular challenges included the exact positioning of the crystals and the synchronization of the motion of the X-ray light pulses with the electron bunches. The stability of the 1.7-kilometer-long accelerator in terms of energy, timing (down to femtoseconds) and position (down to micrometers) over multiple days was crucial for the success of the project. “It took years to bring the accelerator to that state, which is now unique in the world of high-repetition-rate accelerators”, explains Rauer.
“The successful demonstration shows that the resonator principle is practical to implement,” Sinn says. “In comparison with methods used up to now, it delivers X-ray pulses with a very narrow wavelength as well as a much higher stability and coherence.” This in turn enables completely new opportunities for precise experiments in physics, materials science, chemistry, or biology. “With this system, researchers can investigate structures and processes that were until now barely measurable,” says Thomas Feurer, managing director at European XFEL.
In the coming years, the team will work on further strengthening the X-ray light, securing the stability over longer operating times, and preparing the technique for a large research user community. DESY Accelerator Division Director Wim Leemans says: “This joint team has made real a long-considered possibility of enhancing the laser characteristics of coherent hard X-ray pulses at the European XFEL, and the users will profit considerably from their efforts.” The goal is a new generation of X-ray sources that can achieve exceptional precision and brilliance and enable unprecedented glimpses into the smallest and fastest processes.
Dr. Harald Sinn harald.sinn@xfel.eu
Phone +49 40 8998-1744
Dr. Patrick Rauer patrick.rauer@desy.de
Phone +49 40 8998-2579
https://www.nature.com/articles/s41586-025-10025-x
https://doi.org/110.1038/s41586-025-10025-x
Animation briefly explaining XFELO
Video interview with Patrick Rauer and Harald Sinn
Illustration of the XFELO system
Copyright: European XFEL
Emergence of a sharp energy peak in the resulting X-ray beam, as electron bunches pass through the X ...
Copyright: DESY
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Illustration of the XFELO system
Copyright: European XFEL
Emergence of a sharp energy peak in the resulting X-ray beam, as electron bunches pass through the X ...
Copyright: DESY
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