Scientists Innovate Concentrated Ultra-Short X-Ray Pulses, Achieving Unprecedented Intensity Record

Electrons move extremely quickly within atoms and molecules, requiring light flashes lasting only a billionth of a billionth of a second to study their behavior. Now, scientists have found a way to concentrate these ultra-short X-ray flashes onto a tiny point, increasing their energy when they strike a sample. In experiments at the European X-Ray Free Electron Laser (European XFEL), researchers achieved a new record for the intensity and flux of hard X-ray attosecond pulses, with peak intensities exceeding 10²⁰ watts per square centimeter.

Ichiro Inoue, the lead researcher from the University of Tokyo, stated, "This is a world record for hard X-ray attosecond pulses in terms of intensity and flux." This advancement could help scientists study how electrons respond to intense light, ultimately exploring ways to control them during chemical reactions.

Focusing X-Rays Using Advanced Optical Systems

The experiment, conducted in April 2026, utilized X-ray flashes lasting approximately 300 attoseconds. An attosecond is a billionth of a billionth of a second, making these pulses suitable for studying some of the fastest processes involving electrons. Researchers directed the pulses through an Advanced Kirkpatrick–Baez (AKB) optical system, consisting of two precision mirrors. Each flash is reflected four times off the mirrors. One mirror focuses the X-rays vertically, while the other focuses them horizontally, concentrating the beam into an area of just 180 × 330 nanometers.

In short, researchers focused the X-rays to an area approximately 100,000 times smaller than the cross-section of a typical human hair. This achievement builds on earlier research at the European XFEL, where researchers demonstrated kilowatt-level hard X-ray attosecond pulses. However, generating powerful pulses is only part of the challenge. The energy must reach the sample in a sufficiently concentrated form to produce the desired effects, which is also a challenge faced by other X-ray laser technologies used to study materials at small scales.

One of the researchers, Jiawei Yan, remarked, "A few years ago, we demonstrated kilowatt-level hard X-ray attosecond pulses at the European XFEL. But generating powerful pulses is just the first step. What truly matters for the experiment is the results we can deliver to the sample. With this advancement, we can now explore the highly nonlinear interactions between X-rays and matter using attosecond X-rays."

To independently verify the intensity, the research team directed the X-ray pulses at a piece of copper foil and examined the emitted light from its atoms. The incoming X-rays knock electrons out of the deep inner shells of copper atoms, leaving vacancies or holes. Electrons from the outer shells then fill these vacancies, releasing energy in the form of emitted X-rays. Under sufficiently strong illumination, this emission can stimulate other excited copper atoms to release X-rays traveling in the same direction. The resulting amplification effect produces a stronger, more concentrated signal. This process is known as Amplified Spontaneous Emission (ASE), a phenomenon associated with the emission processes feasible with X-ray lasers.

Anders Madsen, the project leader for the experiment, stated, "Seeing this process is almost magical. Normal fluorescence disperses in all directions, regardless of the direction of the incident beam. But when the ASE threshold is surpassed, you see the beam start to concentrate forward."

Initially, as researchers increased the pulse energy, the ASE signal grew rapidly. However, at higher energies, the signal began to plateau, a behavior known as saturation. This response is consistent with the extremely high intensities estimated based on pulse energy, duration, and focal spot size, providing an independent check for the calculations.

Potential Applications of Strong X-Ray Pulses

This achievement could help researchers study the behavior of electrons in atoms and molecules, including how materials respond to intense X-ray illumination. In the longer term, these studies may assist in controlling electrons during chemical reaction processes. Researchers are already exploring methods to capture these rapid changes, including experiments imaging electrons and atoms during chemical reactions. However, the experiment demonstrated the generation and verification of strong X-ray pulses, rather than direct control of electrons during reaction processes. Currently, researchers are using an AKB mirror system brought from Japan.

The European XFEL is expected to acquire its own system by early 2027. After installation and testing, this system will make these short and powerful X-ray flashes available to researchers using the facility's MID instruments worldwide.

The Impact of This Technology on Electron Research

The breakthrough of this technology lies in the ability to concentrate extremely short X-ray pulses, which is crucial for studying the behavior of electrons in atoms and molecules. With the increase in intensity and flux, scientists will be able to delve deeper into how electrons react under intense light, potentially achieving control over electrons during chemical reaction processes in the future. This could not only advance fundamental science but also have revolutionary implications for materials science and chemical reactions.