Electrons move so quickly inside atoms and molecules that studying their behavior requires flashes of light lasting just a fraction of a billionth of a billionth of a second.
Now, scientists have found a way to concentrate these ultrashort X-ray flashes into a tiny spot, making them more powerful at the point where they hit a sample.
In an experiment at the European X-ray Free-Electron Laser (European XFEL), researchers achieved record intensity and fluence for hard-X-ray attosecond pulses. Their peak intensity exceeded 1020 watts per square centimeter.
“This is a world record in intensity and fluence for a hard-X-ray attosecond pulse,” Ichiro Inoue, lead researcher and a scientist at the University of Tokyo, said.
This advancement could help scientists investigate how electrons respond to intense light and, eventually, explore ways to control them during chemical reactions.
How precision mirrors pack X-ray energy
The experiment, conducted in April 2026, used X-ray flashes lasting an estimated 300 attoseconds. An attosecond is a billionth of a billionth of a second, making these pulses useful for investigating some of the fastest processes involving electrons.
The researchers directed the pulses through an advanced Kirkpatrick–Baez (AKB) optical system consisting of two precision mirrors. Each flash reflected off the mirrors four times in total. One mirror focused the X-rays vertically, while the other focused them horizontally, concentrating the beam into a spot measuring just 180 × 330 nanometers.
In short, the researchers somehow focused the X-rays into an area roughly 100,000 times smaller than the cross-section of a typical human hair.
The achievement builds on earlier work at the European XFEL, where researchers demonstrated hard-X-ray attosecond pulses at terawatt-scale power. However, generating powerful pulses is only part of the problem.
The energy must also reach the sample in a sufficiently concentrated form to produce the desired effects, a challenge also faced by other X-ray laser technologies used to study matter at tiny scales.
“A few years ago, we demonstrated terawatt-scale hard X-ray attosecond pulses at the European XFEL. 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,” Jiawei Yan, one of the researchers, said.
Copper atoms help verify the record intensity
To check the intensity independently, the team directed the X-ray pulses onto a copper foil and examined the light emitted by its atoms.
The incoming X-rays knocked electrons out of deep inner shells of copper atoms, leaving vacancies, or holes. Electrons from outer shells then fell into those vacancies, releasing energy as X-rays.
Under sufficiently intense illumination, this emission can stimulate other excited copper atoms to release X-rays traveling in the same direction. The resulting amplification produces a stronger, more concentrated signal. This process is called amplified spontaneous emission (ASE), a phenomenon related to the emission processes that make X-ray lasers possible.
“It is almost magical to see this happening. 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,” said Anders Madsen, who wasn’t directly involved in the study but is the project leader of the facility where the experiment was conducted.
Initially, the ASE signal grew rapidly as the researchers increased the pulse energy. At higher energies, however, the signal began to level off, a behavior known as saturation.
This response was consistent with the extremely high intensities estimated from the pulse energy, duration and focused spot size, providing an independent check on the calculations.
The next step in capturing electrons in action
The achievement could help researchers study how electrons behave in atoms and molecules, including how matter responds to intense X-ray illumination. In the longer term, these studies could contribute to efforts to control electrons during chemical reactions.
Researchers are already exploring ways to capture such rapid changes, including experiments imaging electrons and atoms during chemical reactions. However, the experiment demonstrated the production and verification of intense X-ray pulses, not direct control of electrons during a reaction.
For now, the researchers used an AKB mirror system brought from Japan. European XFEL expects to receive its own system in early 2027. Once installed and tested, it is intended to make these short, intense X-ray flashes available to researchers worldwide using the facility’s MID instrument.
The study is published in the journal Optica.