XFEL attosecond X-ray pulses set a 118.8 GW intensity record

By Priya Raman

A hard X-ray attosecond pulse at the European X-ray Free Electron Laser (XFEL) in Hamburg, Germany, concentrated 118.8 GW of power onto a 180 × 330 nm focal spot, equivalent to 2 × 10²⁰ W/cm², the highest intensity reported for this pulse class. This result measures intensity at the sample, not merely energy available at the source.

The researchers directed the pulses to XFEL’s Materials Imaging and Dynamics (MID) instrument and focused them with a total-reflection Advanced Kirkpatrick–Baez (AKB) mirror. Jumpei Yamada of the University of Osaka and Ichiro Inoue of the University of Tokyo developed the optics.

Co-leader Jiawei Yan said: “For real-world experiments, what really matters is the intensity we can deliver to the sample.” He added: “We undertook this new study to see whether we could tightly focus these pulses to increase their intensity.”

What does the intensity record change?

In the semi-classical Bohr model, an electron in hydrogen takes roughly 150 attoseconds, or 150 × 10⁻¹⁸ seconds, to orbit the nucleus. A sample’s electrons can distort a passing attosecond pulse; by monitoring that distortion, researchers can construct three-dimensional maps and even movies of electron motion.

Attosecond pulses usually come from high-harmonic generation (HHG), a method developed in the late 1980s. HHG works well in the extreme-ultraviolet and soft X-ray regions, but Yan said extending the approach to shorter, hard X-ray wavelengths had not been easy.

A few years ago, Yan and colleagues at XFEL and the neighboring DESY facility developed hard X-ray attosecond pulses at a terawatt scale. XFEL is a linear electron accelerator that generates coherent light through self-amplified spontaneous emission, producing photon energies from hundreds to many thousands of electron volts.

The new work used a self-chirping scheme. Researchers compressed a bunch of electrons into a short spike of current; the bunch’s own electromagnetic fields then imprinted an energy variation along it, and a magnetic arc used that variation to compress the bunch further.

The ultrashort, high-current section emitted intense hard X-ray attosecond pulses in the accelerator’s undulator. Their energy reached up to a few hundred microjoules, more than 10 times the energy of pulses produced by other hard X-ray attosecond methods, Yan said.

Focusing that output produced the 118.8 GW result. For R&D managers selecting beam time or comparing techniques, the practical specification is intensity at the specimen, together with evidence of focus, stability and timing control, rather than a source-energy figure alone. A higher source energy offers little practical value if transport and focusing erase the gain.

Can the pulses image materials before radiation damage?

Yan said the pulses could collect diffraction signals before substantial electronic damage develops, potentially improving structural measurements of radiation-sensitive samples. That is a proposed application; the reported experiment established source intensity, not a material-imaging benchmark.

The account also frames the new attosecond X-ray spectroscopy technique as capable of “freezing” atomic nuclei in place, a proposed basis for resolving ultrafast structural changes.

The experiment concerned pulse generation and focusing rather than a specimen series, so the account provides no sample count. It also gives no uncertainty range for the 118.8 GW figure, no independent replication at another beamline and no funding attribution; the number remains a single-instrument report.

What comes next?

The Optica report says the team is now developing stabilized attosecond pulses and multiple pulses with controlled timing. Yan also wants experiments on nonlinear X-ray interactions and ultrafast electronic dynamics, which remain research goals rather than demonstrated applications.

Yan told Physics World: “We now have a source that can reach an extreme intensity regime, and I think the next important step is to find out what new physics we can do with it.” The near-term test is whether stabilized, synchronized pulses can turn the intensity milestone into repeatable structural and electronic measurements.