SLAC researchers confirm hard X-ray pulses lasting 100 to 400 attoseconds

Researchers at SLAC National Accelerator Laboratory's Linac Coherent Light Source in the United States have confirmed hard X-ray pulses lasting 100 to 400 attoseconds. The result, published in Nature Photonics on 20 October, gives experimental evidence for exceptionally short, high-energy pulses whose duration had been difficult to measure.

An attosecond is one billionth of a billionth of a second. SLAC described the pulses, with photon energies of about 9 kiloelectronvolts, as the shortest high-energy X-ray pulses measured at an X-ray free-electron laser. The paper reports that their duration varied from shot to shot across the 100-to-400-attosecond range.

How researchers measured the X-ray pulses

Lead author Ichiro Inoue, a University of Tokyo professor, proposed an indirect method based on amplified spontaneous emission. Rather than timing each X-ray flash with a conventional clock, the experiment analysed collective X-ray fluorescence induced by the pulses. The paper says that signal allowed the team to identify attosecond pulses and variations in duration between shots.

Interesting Engineering described the setup as splitting the X-ray beam: researchers measured the energy spectrum of one part and directed the rest onto copper foil to produce fluorescence. Those observations provided a way to infer how long the incoming pulses lasted. The paper identifies the measured phenomenon as amplified spontaneous emission from a transition metal.

What the Nature Photonics result establishes

The paper calls the finding an experimental confirmation of attosecond pulse generation in the hard X-ray range at a free-electron laser. For pulses inferred to last 100 attoseconds, it reports a time–bandwidth product of approximately 2 femtosecond-electronvolts. The authors say that suggests a fraction of the pulses were nearly transform-limited, a measure of how closely pulse duration and spectral width approach their physical relationship.

The result therefore concerns both the production of very short hard X-ray flashes and scientists' ability to verify their length. Its measured 100-to-400-attosecond spread also matters when interpreting individual shots from the laser: a single duration would not describe every pulse in the experiment.