Scientists Measure X-ray Laser's Heartbeat

Ultrafast stopwatch determines duration of femtosecond FLASH pulses

Many fundamental processes in nature, including certain chemical and biological reactions, occur on very small timescales. Researchers have been dreaming of the opportunity to film these fast processes in real time and to understand their underlying principles. X-ray free-electron lasers (FEL), with their intense light bundled into femtosecond pulses, hold promise for enabling breakthroughs in the ultrafast sciences. One femtosecond is one-thousandth of a trillionth of a second, or 0.000 000 000 000 001 seconds. Using DESY's pioneering FEL FLASH, a German-Polish research team has developed a novel pulse duration monitor, which allows the measurement of FEL pulses while simultaneously performing ultrafast experiments. The scientists reported their method in the journal "Nature Communications".

Spatial representation of a full FEL pulse (top). The X-rays turn the membrane opaque for optical laser light, which is reflected. In this image, bright areas correspond to low transparency. As the FEL pulse slants through the membrane, different locations of the membrane are hit at different times. This enables the scientists to follow the temporal development of the FEL pulse (bottom), measured as a reflection curve. This curve gives the duration of the FEL pulse. Credit: Robert Riedel, University of Hamburg/Helmholtz-Institute Jena

“Previous methods for the determination of X-ray pulse durations are very difficult to realize experimentally and are subject to certain limitations,” says Hamburg University scientist and first author Robert Riedel from Franz Tavella’s group at Helmholtz-Institut Jena. In particular, pulse length determinations had not been achieved without interrupting the actual experiment. “In contrast, our method can determine pulse lengths concurrently with experiments, is easier to implement, and can be used for a range of FEL wavelengths and pulse durations.”

With this new method, the researchers measured femtosecond pulses from FLASH, which produces light in the extreme ultraviolet (XUV) and soft X-ray energy range. However, the timing tool will also be of interest for FELs that operate at higher energies such as the U.S. Linac Coherent Light Source (SLAC National Accelerator Laboratory, Menlo Park, California) and the future European XFEL currently under construction in Hamburg.

Researchers study ultrafast processes in what is known as pump-probe experiments. A first laser (pump) pulse sets off a chemical reaction or other process in a sample. It is followed by a second laser (probe) pulse, which examines the pump pulse’s effect on the sample. The researchers record the time course of the process by varying the time delay between the two pulses. Exact timing is crucial in such experiments. Depending on the type of experiment, the FEL pulse can either act as a pump or probe pulse. The other pulse is typically provided by an optical laser.

Irregular "heartbeat"

A major complication for the synchronization of the two pulses is the irregular “heartbeat” of FELs. The length of individual pulses emerging from the FEL and the time between them vary from shot to shot. Therefore, researchers require a fast timing tool that has the ability to tell them precisely when each single pulse arrives at the sample and how long it lasts. While pulse characterization methods for optical pulses of visible or infrared light are quite advanced, they cannot be easily applied to XUV radiation and X-rays. “At present, there is no general method for these FEL pulses,” says DESY scientist Sven Toleikis who coordinated the team’s research efforts. “Although various methods have been tested before, they are basically experiments on their own and interrupt the actual pump-probe experiment. Furthermore, many methods average over several individual pulses or only work in a narrow wavelength range.”

Together with other members of the research team, Toleikis had recently been involved in the development of the world’s most precise stopwatch for hard X-ray lasers, which determines laser pulse arrival times at LCLS with an unsurpassed precision of ten femtoseconds. As the researchers demonstrate in the current study, the same technique can be applied to measure the FEL’s “heartbeat” by determining both the arrival times and the equally important lengths of individual FEL pulses at the same time.

The method is based on reflectivity changes in a target caused by the FEL radiation. Using an experimental setup conceived by DESY scientist Nikola Stojanovic and Franz Tavella, the researchers shot infrared laser pulses onto the target made from either glass or silicon nitride. A camera behind the target, which is initially transparent for the infrared light, detects the transmitted intensity. “When we simultaneously send an X-ray pulse through the sample, we notice an attenuation of the transmitted light,” says Riedel. “The absorption [of the X-ray pulse] in the material creates an electron plasma, which acts like a mirror for the optical laser light.” Thus, under the influence of the FEL pulse, the target becomes less transparent for infrared radiation.

Measuring each individual pulse

Because the X-ray pulse slants through the target, it reaches different positions on the target sample at different times and, consequently, different locations on the target reflect the optical laser light differently as well. In other words, the time structure of the X-ray pulse translates into a spatial variation of the target’s reflectivity. From the transition between dark and light areas on the camera, the researchers can derive the duration of the X-ray pulse.

The key to this analysis is that every photon of the absorbed X-ray pulse is directly converted into a constant amount of mirroring electrons – a fact which the researchers confirmed with theoretical calculations at the Center for Free-Electron Laser Science (CFEL), a scientific collaboration of DESY, Hamburg University, and the Max Planck Society. “Only with this finding were we able to exclude other excitation processes in the material and ascribe the reflectivity change exclusively to the X-ray pulse,” Riedel points out.

The researchers used the pulse duration monitor to characterize FLASH pulses of 21 and 184 femtoseconds at two different laser wavelengths. “With our method, users can directly measure the FEL pulse length in their experiment, and they can do so for every individual pulse,” says Toleikis. Moreover, silicon nitride targets can be manufactured into very thin membranes. The researchers used a membrane that was a mere 20 millionth of a millimetre (20 nanometres) thick. “The thin material absorbs only fifty to eighty percent of the FEL radiation, letting enough intensity through for other experiments,” Riedel says. Thus, experimentalists will now be able to simultaneously perform ultrafast pump-probe experiments and crucial pulse duration measurements.   

The research team included scientists from Hamburg University, Helmholtz-Institut Jena, DESY, Helmholtz-Zentrum Dresden-Rossendorf, Center for Free-Electron Laser Science, University of Duisburg-Essen (all in Germany), and the Polish Academy of Sciences (Kraków, Poland).

"Single-shot pulse duration monitor for extreme ultraviolet and X-ray free-electron lasers"; Robert Riedel et al.; Nature Communications (2013); DOI: 10.1038/ncomms2754