Researchers at SLAC’s Linac Coherent Light Source (LCLS) have generated and measured the shortest high-energy (“hard”) X-ray pulses ever recorded at an X-ray free-electron laser (XFEL).
The study confirms pulse durations between 100 and 400 attoseconds at high photon energies (~9 keV). These “hard” X-ray flashes are fundamentally changing how we observe and control quantum matter.
Scientists are already using these ultrashort flashes to capture the real-time motion of electrons deep within atoms and dense plasmas. These ultrashort flashes let scientists observe pristine structures, outrunning the radiation damage caused by the X-rays themselves.
“This is an important milestone for researchers who want to use attosecond hard X-rays at LCLS,” said Ichiro Inoue, a professor at the University of Tokyo.
Outrunning time
Making an attosecond pulse is immensely hard. Measuring one is even harder.
“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. This was the first diagnostic that proves the pulses are indeed only a few hundred attoseconds long,” explained Agostino Marinelli, a study author and professor at SLAC
The solution came from lead author Inoue, who proposed an indirect measurement technique using amplified spontaneous emission (ASE).
ASE is basically splitting an X-ray beam to measure its energy spectrum directly while focusing the remainder onto copper foil to produce forward fluorescence.
Researchers tested SLAC’s Linac Coherent Light Source by firing a high-energy beam and splitting it into two paths. One portion went directly to a detector that recorded its energy spectrum, while the remaining light was focused onto a 150-nanometer spot on copper foil to produce fluorescent radiation. Combining measurements from both detectors enabled the team to calculate the precise, fleeting duration of the pulse.
This technique extends attosecond science into photon-energy ranges around 9 kiloelectronvolts, an order-of-magnitude leap beyond earlier soft X-ray experiments operating below 5 kiloelectronvolts.
Freezing plasma in motion
The question might be: why do these ultrashort hard X-rays matter? Depth and speed.
Lower-energy soft X-rays work well for observing chemical bonds in an atom’s outer shell. High-energy hard X-rays above 5 kiloelectronvolts penetrate deep into dense matter, letting scientists pinpoint electrons with atomic precision.
“Plasma physics is very messy. Normally, we excite our sample, then the system relaxes and recombines before we can extract a measurement. We’re often forced to rely on modeling and make assumptions about what exactly is happening,” said physicist Sam Vinko at the University of Oxford.
Scientists can now target extreme systems, such as those in inertial fusion energy experiments, and capture frozen core-excited states before they blur or evolve.
X-rays often destroy the delicate samples they scan, but attosecond hard X-rays prevent this destruction because they are so fast. Since the flash finishes before damage or decay can occur, researchers can take snapshots of a system in its untouched state.
Tais Gorkhover of the University of Hamburg believes this ability won’t just improve nanoscale imaging, but it might also help control or even reverse light-matter interactions entirely.
SLAC’s latest milestone ushers in a new era in quantum dynamics it giving researchers a camera shutter faster than atomic decay.