A new benchmark for ultrafast X-ray science
Researchers at the Linac Coherent Light Source (LCLS), based at the U.S. Department of Energy’s SLAC National Accelerator Laboratory, have measured the shortest high-energy X-ray pulses yet recorded at an X-ray free-electron laser. The reported pulses carried photon energies of about 9 kiloelectronvolts and lasted from 100 to 400 attoseconds.
An attosecond is a billionth of a billionth of a second. At that timescale, researchers aim to study extremely rapid processes in matter, including electronic motion. The findings were published in Nature Photonics.
The result is significant not simply because of the pulse duration, but because the team was able to measure it. According to SLAC researcher and study author Agostino Marinelli, producing high-energy attosecond X-ray pulses had been learned over several years; demonstrating that the pulses were only a few hundred attoseconds long was the harder task.
Hard X-rays combine speed and penetration
The experiment focused on hard, or high-energy, X-rays. The approximately 9-keV pulses are energetic enough to probe deep into atomic systems while also being brief enough to capture electronic motion on ultrafast timescales. That combination makes them relevant to experiments that seek to examine matter at its smallest and fastest scales.
For an X-ray free-electron laser, short pulses alone are not enough to establish a usable scientific capability. Researchers need a way to characterize the pulse’s energy, spectrum, shape and duration. Without such a diagnostic, an ultrashort pulse cannot be confirmed with the confidence needed for experiments that depend on precise timing.
Using fluorescence as a pulse diagnostic
The study’s lead author, Ichiro Inoue of the University of Tokyo, proposed an indirect measurement approach based on amplified spontaneous emission, or ASE. ASE occurs when an intensely focused X-ray pulse excites a material and causes it to fluoresce in a forward direction. Information about the original X-ray pulse is embedded in that emitted fluorescence.
In the LCLS experiment, the team generated a beam believed to contain attosecond X-rays. Part of the beam was sent to a detector that measured its energy and spectral shape. The remaining beam was focused to a spot only 150 nanometers wide on a copper foil. That interaction generated ASE, which was captured by a second detector.
By using the fluorescence signal alongside measurements of the original beam, the researchers could infer properties of the X-ray pulse, including its duration. The diagnostic provided evidence that the hard X-ray pulses were in the 100-to-400-attosecond range.
Why measurement matters
Ultrashort-pulse research depends on matching the timing tool to the event under study. Electronic motion can unfold much faster than chemical changes visible on longer timescales. A pulse lasting a few hundred attoseconds can, in principle, provide a much sharper temporal window than longer X-ray flashes.
The newly demonstrated diagnostic therefore adds an important practical capability to the generation of hard attosecond X-rays. Rather than relying on an assumption about pulse duration, researchers can evaluate whether the beam has the characteristics required for an ultrafast experiment.
The work also illustrates the role of experimental methods in advancing scientific instruments. The reported record comes from a combination of beam generation, nanometer-scale focusing, a copper target, and two complementary measurements. Each element was needed to turn an ultrafast beam into a measurable result.
What comes next
The source article describes the pulses as valuable for work at the smallest and fastest scales. That framing points toward studies where high-energy X-rays can probe atomic systems and attosecond timing can resolve rapid electronic behavior. The reported result does not itself describe a particular material or process observed with the new pulses; it establishes and verifies the pulse capability.
For LCLS and the wider X-ray free-electron-laser field, the advance is a new measurement benchmark: hard X-ray pulses with photon energies near 9 keV and durations down to 100 attoseconds, confirmed through an ASE-based diagnostic. Future experiments can build on that foundation when they require both the penetrating reach of hard X-rays and the temporal resolution of attosecond science.
Key points
- LCLS measured hard X-ray pulses lasting 100 to 400 attoseconds.
- The pulses had photon energies of about 9 keV.
- The team used amplified spontaneous emission from a copper foil as an indirect diagnostic.
- The study was published in Nature Photonics.
This article is based on reporting by Phys.org. Read the original article.
Originally published on phys.org
