European XFEL sets hard X-ray attosecond pulse intensity record
Researchers at European XFEL have generated record-breaking hard X-ray attosecond pulses, focusing them into a spot 100,000 times smaller than a human hair to achieve unprecedented intensities exceeding 10²⁰ watts per square centimeter.
Physicists at European XFEL have produced a hard X-ray attosecond pulse with world-record intensity and fluence. The experiment, conducted in April 2026, created X-ray flashes lasting just 300 attoseconds and focused them to a microscopic spot, achieving peak intensities above 10²⁰ watts per square centimeter.
"This is a world record in intensity and fluence for a hard-X-ray attosecond pulse," said Ichiro Inoue of the University of Tokyo, the first author of the paper published in Optica. The team's work confirms a leap in the ability to probe ultrafast electron dynamics. Each pulse carried an average of 45 microjoules at the focus, with some reaching 100 microjoules. The energy spread implies a duration of about 300 attoseconds.
Focus and delivery to the sample
Generating powerful pulses is only the first step. What matters is what can be delivered to the sample. To concentrate the X-rays, the team used an advanced Kirkpatrick-Baez mirror system transported from Japan. Each pulse underwent four reflections, with one mirror focusing vertically and the other horizontally.
The result was a focal spot measuring just 180 by 330 nanometers. This area is roughly 100,000 times smaller than the cross-section of a typical human hair. By directing energy into this microscopic area, researchers achieved the record-breaking intensity. "For experiments, what really matters is what we can deliver to the sample," added corresponding author Jiawei Yan, a scientist at DESY.
Amplified spontaneous emission and saturation
The team confirmed the record intensity by observing a specific behavior in copper atoms. When the intense X-ray pulses struck a thin copper sheet, they knocked electrons from atoms' innermost shells. Electrons from outer shells then filled these vacancies, releasing X-ray light.
Under ordinary fluorescence, this light travels in all directions. But at high intensity, the incoming X-rays stimulate neighboring excited atoms to emit in the same direction, a process called amplified spontaneous emission (ASE). When the ASE threshold is crossed, you see the light cone starting to condense into the forward direction, the researchers noted.
As pulse energy increased, the amplified signal initially grew much stronger. At higher energies, the increase slowed and leveled off, a saturation behavior expected only under exceptionally intense X-ray illumination. This pattern provided independent confirmation of the record intensity calculated from pulse parameters.
Implications for electron dynamics
The breakthrough enables researchers to capture information about electron behavior at a level of detail previously difficult to achieve. Electrons determine how atoms bond and how chemical reactions unfold. Their movements happen on extraordinarily short timescales.
By combining ultrashort X-ray flashes with extreme intensity, scientists hope to capture information about electrons before the incoming radiation substantially alters the material. With this advance, we can now use attosecond X-rays to explore highly nonlinear interactions between X-rays and matter, said Yan.
The key performance figures for the record-breaking pulse are summarized below:
| Parameter | Value |
|---|---|
| Pulse Duration | ~300 attoseconds |
| Focal Spot Size | 180 × 330 nanometers |
| Peak Intensity | >10²⁰ W/cm² (210 quintillion W/cm²) |
| Fluence | 67,000 J/cm² |
| Pulse Energy at Focus (avg.) | 45 microjoules |
Facility upgrades and future access
European XFEL is preparing to receive its own advanced Kirkpatrick-Baez mirror system, with delivery expected in early 2027. The new equipment is funded through a collaboration with the University of Hamburg and the Federal Ministry of Research, Technology and Space.
Once installed and commissioned at the MID instrument, where the record-setting experiment was conducted, scientists are expected to gain regular access to these exceptionally brief and intense X-ray flashes. It is almost magical to see this happening, said Anders Madsen, group leader at the MID instrument. The upgraded capabilities will allow researchers to investigate electron behavior in new detail. Scientists using the MID instrument are expected to gain access to exceptionally brief and intense X-ray flashes once the new equipment is installed and commissioned in early 2027.
