Abstract:
The European XFEL (European XFEL) research team announced that they successfully generated an ultra-short hard X-ray flash lasting only about 300 attoseconds, and set a world record for such pulses in terms of intensity and energy density. This result is expected to help scientists observe the dynamic behavior of electrons inside atoms and molecules with unprecedented precision. The relevant research results have been published in "Optica".
Scientists have long hoped to understand the movement of electrons in atoms and molecules, so as to gain a deeper understanding of chemical reactions and even achieve active control of electron behavior in the future. X-rays are an important tool for studying the movement of electrons, but high-energy radiation itself can also change the sample being tested, so researchers must obtain enough information before noticeable changes occur in the sample.
In this experiment, the research team focused the X-ray pulses onto an extremely small area, which is about one hundred thousandth the cross-section of an ordinary human hair. Each pulse lasts about 300 attoseconds, which is only one billionth of a billionth of a second. The experiment was completed in April 2026 at the MID experimental station of the European XFEL facility, using hard X-rays with extremely short wavelengths.
Yan Jiawei, a scientist at the German Electron Synchrotron Center (DESY) who participated in the research, said that the team had demonstrated terawatt-level hard X-ray attosecond pulses a few years ago, but generating strong pulses is only the first step. What is really important is to be able to effectively deliver this energy to the experimental sample. With this breakthrough, scientists have been able to use attosecond-level X-rays to study the highly nonlinear interaction between X-rays and matter.
In order to achieve extreme focusing, the researchers used an advanced Kirkpatrick-Baez optical system (AKB) from Japan. Each X-ray pulse undergoes four reflections, with two ultra-high-precision mirrors responsible for vertical and horizontal focusing respectively. Ultimately, the X-rays are compressed into a tiny focal area of just 180 x 330 nanometers.
Based on pulse energy, duration and focus size calculations, the research team estimated that its peak intensity exceeded 10²⁰W per square centimeter. Ichiro Inoue, a researcher at the University of Tokyo, said that this result breaks the world record for hard X-ray attosecond pulses in terms of intensity and energy flux. Among them, energy flux is used to measure the energy received per unit area, while intensity further reflects how quickly this energy is released.
To verify this calculation, the researchers irradiated these ultra-intense pulses onto copper foil samples. X-rays knock out electrons from the deep inner shell of copper atoms, forming electron vacancies. When other electrons fill these vacancies, new X-ray radiation is released. At a high enough intensity, these radiations can induce other excited copper atoms to emit more X-rays in the same direction, creating a phenomenon known as amplified spontaneous emission (ASE), achieving signal amplification.
Anders Madsen, head of the MID Experiment Station, said that observing this phenomenon felt almost magical. Normal fluorescence emits in all directions, regardless of the direction of the incident beam; when the ASE threshold is exceeded, the researchers can clearly see the beam gradually shrinking and concentrating in the forward direction.
Experimental results show that as the pulse energy increases, the ASE signal initially increases rapidly and then tends to saturate at higher energies. The researchers pointed out that this saturation phenomenon only occurs at extremely high X-ray intensities, thus providing independent verification of the ultra-high intensities previously calculated.
European XFEL plans to receive and install its own AKB focusing system in early 2027. Researchers said that after the system is installed and debugged, scientific research teams from around the world will be able to use the MID experimental station to obtain the same short and powerful X-ray flash, opening up new possibilities for ultrafast physics, materials science, chemical reaction kinetics and nonlinear X-ray scientific research.
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