Demonstrating Unprecedented Control of X-ray Pulses

The Science

X-ray free electron lasers (XFELs) are the brightest and fastest X-ray light sources. They can produce pulses shorter than a femtosecond. (A femtosecond is 10^-15 seconds.) At the core of an XFEL is a magnetic undulator, which makes electrons wiggle and emit X-rays. Normally, an XFEL produces short pulses at a single level of X-ray energy. This pattern is like a musical instrument playing a single note over and over. For the first time, scientists at the Linac Coherent Light Source (LCLS) - a Department of Energy Office of Science User Facility - carefully tuned the undulator's magnetic field to generate pulses across multiple energies. Instead of isolated X-ray "notes," they effectively played a full X-ray "melody." This technique opens new possibilities for advanced experiments.

The Impact

Quantum systems-atoms, molecules, and materials-react differently to varying X-ray "melodies." By shaping X-ray pulses, scientists can influence behavior at the atomic scale. This technique also allows experimental testing of theoretical models that scientists previously could not test. For example, this technique makes it possible to study how ultrafast electron motion connects with the slower movement of atomic nuclei. Insights from these experiments could improve our understanding of energy technologies, quantum devices, and photochemical processes. This information could provide an important bridge between fundamental science and practical applications. This approach also opens a new way to explore and manipulate the quantum world in unprecedented detail.

Summary

Researchers have developed a new way to shape X-ray pulses at the Linac Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory. The LCLS is the only XFEL in the United States. Scientists operated the XFEL at LCLS in two sections. The first section created an ultrafast burst of X-rays from a short, intense segment of the electron bunch. The second section adjusted this burst using another segment to enhance different "notes" or frequencies. This method allows scientists to produce a variety of pulse shapes. These shapes include positive chirps (higher-frequency notes following lower ones), negative chirps, or precisely synchronized sequences of attosecond (one billionth of a nanosecond) pulses.

The work marks the first publication following LCLS's recent upgrade. This upgrade dramatically increased LCLS's pulse rate, making this experiment possible. Previously, LCLS was limited to 120 pulses per second. The upgrade increased that rate to over 33,000 pulses per second. Ultimately, it is designed to go to 1 million pulses per second. Remarkably, these results were published in March 2025, just five months after data collection using the upgraded light source. By giving researchers unprecedented control over X-ray "melodies," this technique opens the door to new experiments that can probe matter at ultrafast timescales. These experiments will offer fresh insights into atomic and electronic dynamics, quantum behavior, and advanced materials.

Contact

Agostino Marinelli
SLAC National Accelerator Laboratory
[email protected]
(650) 926-7478 (office)

Funding

This research was funded by the Department of Energy Office of Science, Office Basic Energy Sciences, Scientific User Facility Division, and Stanford Graduate Fellowship and the Robert H. Siemann Fellowship. It used resources at LCLS, a DOE Office of Science User Facility.

Publications

Robles, River R., Larsen, Kirk A., Cryan, James, and Marinelli, Agostino, et al., "Spectrotemporal shaping of attosecond X-ray pulses with a fresh-slice free-electron laser", Physical Review Letters, 134, 115001, (2025), [10.1103/PhysRevLett.134.115001]

Related Links

New upgrade will supercharge atomic vision of the world's most powerful X-ray laser, SLAC News