Researchers Unleash Record-Breaking X-Ray Flashes in Just 300 Attoseconds

Researchers used the European XFEL to produce ultrashort X-ray flashes that reached record intensity for this type of pulse.

To understand chemical reactions and perhaps one day control the electrons involved, scientists need a detailed picture of how electrons behave inside atoms and molecules. X-rays offer a way to probe that behavior, but the flashes can also change the sample they are meant to examine. Researchers therefore need enough light to gather useful information before those changes become substantial.

At the European XFEL, a team has now concentrated X-ray flashes onto an area roughly 100,000 times smaller than the cross-section of a typical human hair. The pulses lasted an estimated 300 attoseconds, with each attosecond equal to a billionth of a billionth of a second.

The experiment used hard X-rays, light with particularly short wavelengths, and took place in April 2026 at the facility’s MID instrument. The results are published in Optica.

Focusing ultrashort X-rays to extreme intensity

“A few years ago, we demonstrated terawatt-scale hard X-ray attosecond pulses at the European XFEL,” says DESY scientist Jiawei Yan, one of the corresponding authors of the study. “But generating powerful pulses is only the first step. For experiments, what really matters is what we can deliver to the sample. With this advance, we can now use attosecond X-rays to explore highly nonlinear interactions between X-rays and matter.”

To concentrate that energy, the researchers used an advanced Kirkpatrick–Baez, or AKB, optical system brought from Japan. Each flash underwent four reflections in total, with one of the system’s two extremely precise mirrors focusing the light vertically and the other horizontally. Together, they concentrated the X-rays into a spot measuring just 180 × 330 nanometers. A nanometer is a billionth of a meter.

From the pulse energy, estimated duration, and focal-spot size, the team calculated a peak intensity of more than 10²⁰ watts per square centimeter. “This is a world record in intensity and fluence for a hard-X-ray attosecond pulse,” says Ichiro Inoue from the University of Tokyo, first author of the publication. Fluence measures the energy delivered to a given area, while intensity also accounts for how quickly that energy arrives.

Copper atoms amplify the X-ray signal

To test that calculation, the researchers shone the pulses onto copper foil. The X-rays knocked electrons out of deep inner shells within the copper atoms, leaving gaps that other electrons could fill. As electrons moved into those gaps, they released X-ray light. At sufficiently high intensities, that light can stimulate other excited copper atoms to emit additional X-rays in the same direction, amplifying the signal through a process called amplified spontaneous emission, or ASE.

“It is almost magical to see this happening,” says Anders Madsen, group leader at the MID instrument. “Normal fluorescence goes in all directions, regardless of the direction of the incoming beam. But when the ASE threshold is crossed, you see the light cone starting to condense into the forward direction.”

The ASE signal initially rose rapidly as the pulse energy increased, then leveled off at higher energies. This saturation, expected only at extremely high X-ray intensities, provided independent confirmation of the team’s calculation.

European XFEL expects to receive its own AKB system in early 2027. Once installed and commissioned, the system is intended to give researchers from around the world using the MID instrument access to similarly brief, intense X-ray flashes.

Reference: “Generation of high-fluence and high-intensity hard X-ray attosecond pulses at the European XFEL” by 19 October 2026, Optica.
DOI: 10.1364/OPTICA.613284

The new system is funded through a collaboration with the University of Hamburg and support from the Federal Ministry of Research, Technology and Space (BMFTR)

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