Record X-Ray Flashes Last 300 Attoseconds in a Spot Far Smaller Than a Hair

Physicists have set a world record for the intensity of ultrashort hard X-ray pulses, according to a release from European XFEL published by ScienceDaily. Each flash lasted an estimated 300 attoseconds and was focused into a spot roughly 100,000 times smaller than the cross-section of a typical human hair.

The team says the result could eventually let scientists watch, and perhaps influence, electrons as they move during chemical reactions. The work was done at the European XFEL X-ray research facility and published in the journal Optica.

The key numbers behind the record

• 300 attoseconds: the estimated length of each X-ray pulse. An attosecond is a billionth of a billionth of a second.
• 180 × 330 nanometers: the size of the focal spot. A nanometer is a billionth of a meter.
• 100,000 times smaller: how the focused area compares with the cross-section of a typical human hair.
• Above 10²⁰ watts per square centimeter: the peak intensity that the copper test indicated the pulses reached.
• Four reflections: the number of times each pulse bounced off mirrors on its way to the focus.
• April 2026: when the experiment took place.

How the team squeezed the beam

The researchers used a setup called an advanced Kirkpatrick-Baez, or AKB, system. It has two extremely precise mirrors, one focusing the beam vertically and the other horizontally. Each pulse reflected four times in total before reaching the target.

The release describes two ways of measuring the power of such a beam. Fluence is the amount of energy delivered to a given area. Intensity also takes into account how quickly that energy arrives. Ichiro Inoue of the University of Tokyo, the study's first author, said the work sets a record in both for a hard X-ray attosecond pulse.

A test with copper

To check the intensity independently, the team aimed the pulses at a thin copper sheet. The X-rays knocked electrons out of the innermost shells of the copper atoms, leaving empty positions known as holes. Electrons from elsewhere in the atoms then dropped into those gaps and released X-ray light.

Normally that light spreads in all directions. At sufficiently high intensity, the emitted light can instead trigger other excited copper atoms to release more X-rays in the same direction, producing an amplified beam. The effect is called amplified spontaneous emission, or ASE. Anders Madsen, group leader at the MID instrument where the study was conducted, said it was "almost magical to see this happening," describing how the light cone condenses into the forward direction once the ASE threshold is crossed.

The researchers also saw saturation: as pulse energy rose, the amplified signal first grew strongly, then the increase slowed and leveled off. According to the release, this pattern is expected only under exceptionally intense X-ray illumination, and it supported the intensity calculated from the pulse energy, duration and focal size.

Why it matters for studying electrons

Electrons largely determine how atoms bond, how molecules interact and how chemical reactions unfold. Their motion is so fast that it is hard to observe directly. Pulses this short and this intense could capture information about electrons before the radiation has time to substantially change the material being studied, according to the release.

DESY scientist Jiawei Yan, a corresponding author, said that producing powerful pulses was only a first step, and that what counts for experiments is what reaches the sample. He said the advance allows researchers to use attosecond X-rays to explore highly nonlinear interactions between X-rays and matter. In those interactions, very intense X-rays make matter respond in ways it would not under weaker light.

For now, the result is a demonstration of capability rather than a new chemical measurement. The release says the technique could provide a clearer picture of electron behavior inside atoms and molecules, and, in the longer term, potentially lead to ways of influencing electron motion during chemical reactions. It does not give a timeline for that.

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Around Prague, based on reporting by ScienceDaily