01.03.2013

Best stopwatch for X-ray Lasers

Timing tool allows new insights into ultrafast molecular dynamics

With X-ray lasers like DESY's FLASH or the LCLS at the US National Accelerator Laboratory SLAC, timing is everything. Their pulses are designed to explore atomic-scale processes that are measured in femtoseconds, or quadrillionths of a second. Determining the instant in time at which the laser strikes a sample, either by itself or in concert with another laser pulse, can be vital to the success of an experiment.

In pump-probe experiments an optical laser (depicted red) pumps a sample into a desired state, then probed by an X-ray pulse (depicted blue). Credit: Greg Stewart / SLAC National Accelerator Laboratory

In the scientific journal "Nature Photonics", an international team of researchers detail a new set of tools that better pinpoints the arrival time of X-ray and other laser pulses to within a few femtoseconds of accuracy.

"The development of such a timing tool as well as the demonstration of a few-femtosecond time resolution is opening a large field of applications in trying to resolve ultrafast dynamics in physics, chemistry and biology," said DESY scientist Marion Harmand, the paper’s lead author.

Many experiments at X-ray lasers rely on conventional laser systems, known as optical lasers, that excite and prepare samples in the instant before they are struck by the ultrabright, ultrafast X-ray laser pulses. These experiments are often referred to as "pump-probe." The optical laser pulse "pumps" the sample to a desired state, and the X-ray laser pulses serve as a high-resolution "probe" of the sample's properties at the molecular scale.

The precision of pump-probe experiments is usually limited by how well laser and X-ray laser pulse can be synchronised. "In electronically synchronised systems, there can be a "jitter" in the arrival times of several hundred femtoseconds," said DESY scientist Sven Toleikis from the team. "New research, pioneered at DESY among other centres, synchronises the lasers with an optical fibre and can achieve a precision of well below 100 femtoseconds." To overcome the jitter problem, the team attempted to measure the exact delay between optical and X-ray pulses at each individual shot with a kind of ultra precise stopwatch. This way, the individual recordings can be time sorted afterwards.

In the LCLS experiment, the researchers installed two sets of timing tools to detect changes in samples using X-ray and optical laser pulses. The pulses were ultimately directed to a sample of bismuth metal, triggering atomic vibrations that provided a final test of the timing tools. A fraction of each individual laser pulse is used for the timing tools. They each consist of a thin, optically transparent membrane, that changes it's optical properties when hit by an X-ray laser pulse.

"Each of these 'stopwatches' are essentially like conducting a second pump-probe experiment," explains Toleikis. "When the X-ray photons pass through the membrane, they create free charge carriers in the material via photoionisation, which changes the optical properties of the material." Theoretical calculations tell the researchers how these changes occur on the timescale of femtoseconds.

The membrane is tilted, so that the X-ray pulse strikes one side before the other. To measure the optical properties, a part of the conventional laser pulse is sent down a "delay line" to let it arrive after the X-ray pulse at the membrane. The exact location of the modified optical transmission of the membrane tells the researchers, how many femtoseconds ago the X-ray pulse struck. Together with the known temporal length of the delay line, this reveals the interval between the two pulses at the experiment.

Aspects of the experiment were demonstrated earlier in lower-energy or "soft" X-ray experiments at the LCLS and at FLASH. "For the first time we proved these timing tools can work in the hard X-ray regime, and can dramatically improve the accuracy of measurements," Harmand said.

The researchers sampled more than 15,000 sets of laser pulses at the LCLS, and the correlation in measurements from the separate timing tools provided the high degree of accuracy. Some of the tools used in the experiment have been adopted at several LCLS experimental stations. New experiments are planned to explore other materials and operating conditions at LCLS that could benefit from the timing tools, the researchers noted.

The research team included scientists from SLAC, the CFEL (Center for Free-Electron Laser Science in Germany), the Polish Academy of Sciences, and the Institute of Physics and University of Rennes in France.

Reference:
"Achieving few-femtosecond time-sorting at hard X-ray free-electron lasers"; Marion Harmand et al.; "Nature Photonics, 2013; DOI: 10.1038/nphoton.2013.11