SLAC Free-Electron Laser Fires Record-Short High-Energy Attosecond Pulses

Inside the Linac Coherent Light Source at the SLAC National Accelerator Laboratory in California, a tightly packed bunch of electrons has generated the shortest high-energy X-ray pulses ever recorded. Clocking in at just 100 to 400 attoseconds, the flashes were successfully measured by a research team led by physicist Agostino Marinelli.[1][5]

The achievement, published this week in Nature Photonics, establishes a new benchmark for X-ray free-electron lasers. It provides researchers with a diagnostic tool fast enough to freeze the motion of electrons in real time, opening a new window into the fundamental dynamics of matter.[5]

An attosecond is one billionth of a billionth of a second. To understand the brevity of this window, an attosecond is to a single second what a second is to the entire age of the universe. This is the natural timescale on which electrons orbit and chemical bonds break.

The pursuit of attosecond pulses was recognized with the 2023 Nobel Prize in Physics, awarded for generating these flashes using tabletop optical lasers. However, those methods primarily produce lower-energy extreme ultraviolet light. Scaling the technology to the hard X-ray regime required the massive infrastructure of billion-dollar particle accelerators.

The mechanics of hard X-rays

Capturing electron motion requires a camera shutter of equivalent speed. X-ray free-electron lasers overcome the limitations of standard optics by accelerating electron bunches to near light speed through a linear accelerator. At the European XFEL, superconducting cavities push these electrons to energies of 17.5 gigaelectronvolts.[4]

The accelerated electrons are then forced through an undulator, a precisely aligned series of alternating permanent magnets. As the electrons wiggle through the magnetic field, they emit coherent bursts of X-ray light through a process called self-amplified spontaneous emission.[1]

The SLAC team focused specifically on "hard" X-rays, which possess photon energies of about 9 kiloelectronvolts. Unlike soft X-rays, hard X-rays have wavelengths comparable to the distance between atoms. This allows them to penetrate deep into dense plasmas and the inner orbitals of heavy elements.[1][2]

Generating the pulses was only half the battle; the primary hurdle was proving their duration. Because no physical stopwatch operates on an attosecond scale, the pulses must effectively measure themselves.[1]

"Over the course of several years, we've learned how to create these high-energy attosecond pulses, but measuring pulses this short is really, really difficult," Marinelli explained.[1]

The SLAC researchers developed a novel diagnostic method to indirectly measure the pulse length. By analyzing the energy distribution of electrons ejected from a target gas, the team reconstructed the temporal profile, confirming the record-breaking duration.[1][5]

A parallel milestone in intensity

The SLAC achievement coincides with a parallel breakthrough at the European XFEL, a 3.4-kilometer underground facility near Hamburg, Germany. While the California team focused on pulse duration, an international consortium tackled the challenge of pulse intensity, publishing their results simultaneously in Optica.[4]

To concentrate the beam, the European team utilized an advanced Kirkpatrick-Baez optical system developed in Japan. Because X-rays tend to pass straight through conventional lenses, they must be reflected at extremely shallow, grazing angles using highly polished mirrors.[4]

The setup consists of two mirrors that reflect the X-ray flash four times in total. One mirror compresses the pulse vertically, while the other squeezes it horizontally. This dual-axis focusing concentrated the 300-attosecond pulses into an area of just 180 by 330 nanometers.[4]

That extreme focusing produced a world-record peak intensity exceeding 10²⁰ watts per square centimeter. Fluence measures the total energy delivered to a specific area, while intensity accounts for how rapidly that energy arrives.[4]

"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 lead author of the European study.[4]

Delivering such immense power in a fraction of a femtosecond opens the door to highly nonlinear light-matter interactions, allowing researchers to push the boundaries of experimental physics.[4]

"Generating powerful pulses is only the first step," noted Jiawei Yan, a scientist at Deutsches Elektronen-Synchrotron. "For experiments, what really matters is what we can deliver to the sample."[4]

Diffract before destroy

With these tools now operational, researchers can begin probing the fundamental mechanisms of chemical reactions. By firing a "pump" pulse to initiate a reaction and a subsequent "probe" pulse to capture the state of the system, scientists can compile stop-motion movies of charge migration.[3]

In the realm of condensed matter physics, attosecond hard X-rays offer a new window into quantum phenomena. Researchers plan to use the pulses to investigate how high-temperature superconductors transfer energy without resistance, and how magnetic domains in data storage devices flip their orientation.[2]

However, the extreme intensity of these pulses introduces a significant experimental challenge. When a biological or chemical sample is struck by 10²⁰ watts per square centimeter, it is rapidly ionized and destroyed, instantly turning into a superheated plasma.[3]

The attosecond duration is crucial here, as it allows the X-ray flash to scatter off the atoms and record their precise positions before the sample physically blows apart. This technique, known as "diffract before destroy," ensures the radiation damage does not blur the final image.[4]

The dual milestones at SLAC and the European XFEL mark the transition of attosecond hard X-ray science from theoretical physics to an applied experimental technique. As facilities refine their optical systems, the focus will shift toward increasing the repetition rate of the pulses for faster data collection.[1][4]