SLAC Research Team Creates Shortest High-Energy X-Ray Pulses Ever, Lasting Less Than 400 Femtoseconds

Researchers at SLAC's Linac Coherent Light Source (LCLS) have generated and measured the shortest high-energy ("hard") X-ray pulses recorded in an X-ray free electron laser (XFEL). The study confirmed that at high photon energies (around 9 keV), the pulse duration ranges from 100 to 400 attoseconds. These "hard" X-ray flashes fundamentally change the way we observe and control quantum matter. Scientists are already using these ultra-short pulses to capture the real-time motion of electrons within atoms and in dense plasmas.

These ultra-short pulses enable scientists to observe pristine structures, surpassing the radiation damage caused by the X-rays themselves. "This is an important milestone for researchers hoping to use attosecond hard X-rays at LCLS," said Ichiro Inoue, a professor at the University of Tokyo.

Production and Measurement of Attosecond Pulses

Producing attosecond pulses is extremely challenging, and measuring a pulse is even more difficult. "Over the years, we learned how to create these high-energy attosecond pulses, but measuring such short pulses is really very difficult. This is the first diagnostic tool that proves the pulse is indeed only a few hundred attoseconds long," explained Agostino Marinelli, a professor at SLAC and one of the authors of the study. The solution came from lead author Inoue, who proposed an indirect measurement technique using amplified spontaneous emission (ASE). ASE essentially splits the X-ray beam to directly measure its energy spectrum while focusing the remaining light onto a copper foil to generate forward fluorescence.

The researchers tested SLAC's Linac Coherent Light Source by firing a high-energy beam and splitting it into two paths. One part directly reached a detector recording the energy spectrum, while the remaining light was focused onto a 150-nanometer spot on the copper foil to produce fluorescence radiation. Combining measurements from the two detectors allowed the team to calculate the precise, instantaneous duration of the pulse. This technique extends attosecond science to a photon energy range of about 9 keV, representing a significant leap beyond earlier soft X-ray experiments that operated below 5 keV.

Why are these ultra-short hard X-rays so important? Depth and speed. Low-energy soft X-rays are suitable for observing chemical bonds in atomic shells. High-energy hard X-rays (above 5 keV) can penetrate dense materials, allowing scientists to determine electron positions with atomic precision. "Plasma physics is very chaotic. Typically, we excite a sample, and then the system relaxes and reorganizes before we can extract measurements. We are often forced to rely on modeling and make assumptions about what is actually happening," said Sam Vinko, a physicist at the University of Oxford.

Applications of Attosecond Hard X-Rays

Scientists can now target extreme systems, such as those in inertial confinement fusion energy experiments, and capture frozen core excited states before they become blurred or evolve. X-rays typically destroy the fine samples they scan, but attosecond hard X-rays prevent this damage due to their extremely rapid speed. Since the flash ends before damage or decay occurs, researchers can take snapshots of the system in an undamaged state. Tais Gorkhover from the University of Hamburg believes this capability will not only improve imaging at the nanoscale but may also help fully control or even reverse interactions between light and matter.

SLAC's latest milestone opens a new era for quantum dynamics, providing researchers with a camera shutter faster than atomic decay.

Impact of Attosecond Hard X-Rays on Quantum Research

The development of attosecond hard X-rays marks a significant advancement in quantum dynamics research, as they can capture instantaneous phenomena without damaging samples. This enables scientists to observe and control quantum systems more precisely, especially in cases involving high energy and dense materials. As technology advances, future research will be able to delve deeper into the dynamic behaviors at the atomic and molecular levels, which holds significant implications for fields such as materials science and energy research.