By Amanda Solliday

Researchers can now use pairs of X-ray pulses to trigger and observe changes in matter with exceptional timing. The new system expands flexibility for multi-beam free-electron laser experiments.

The science and engineering team stands next to the split-delay system for the X-ray Correlation Spectroscopy (XCS) instrument, located at the end of the X-ray transport tunnel between the Near Experimental Hall and Far Experimental Hall. From left: Diling Zhu, Sanghoon Song, Tyler Johnson, Takahiro Sato, Lin Zhang, Matt Hayes, Matt Seaberg, Matthieu Chollet, Abdullah Rashed Amed, Aymeric Robert, Paul Fuoss, Yanwen Sun, Ernesto Paiser, Justin James, Hongliang Shi, Drew Barada. (Diling Zhu/SLAC National Accelerator Laboratory)

A team of engineers and scientists at the Linac Coherent Light Source (LCLS) designed and built a new system of X-ray optics that allows researchers to perform unique, precisely timed experiments where two X-ray pulses travel together and capture a double exposure image of the system. The sharpness or blurriness of the image indicates whether the system under investigation remained static or moved during the small interval between the two X-ray pulses.

This technique is called X-ray speckle visibility spectroscopy. Researchers can also use the double pulses for X-ray pump-probe experiments, where the first X-ray pulse energizes the sample, while the second pulse captures another image that reveals any changes triggered by the first one. 

Yanwen Sun, a Stanford physics doctoral student, next to the split-delay system at the Linac Coherent Light Source (LCLS). (Andy Freeberg/SLAC National Accelerator Laboratory)

This new optical system generates two X-ray pulses from a single pulse from LCLS. The time separation between the pulses can be controlled from a few femtoseconds (quadrillionths of a second) to a few hundred picoseconds (trillionths of a second).

Experience with a previous system that produced paired X-ray pulses, as well as collaboration with the SPring-8 Angstrom Compact free electron Laser (SACLA), gave us more confidence in designing and building this new installation that now covers a wider range of wavelengths and time separations, says Diling Zhu, LCLS staff scientist.

Diling Zhu, SLAC Staff Scientist and Yanwen Sun, a Stanford physics doctoral student, stand in front of the new split-delay system developed for the X-ray Correlation Spectroscopy (XCS) instrument. (Andy Freeberg/SLAC National Accelerator Laboratory)

“With this experimental set-up, research teams can now have two X-ray free-electron laser beams at their disposal,” Zhu says. “And we can precisely control the time separation and wavelength of the two beams.”

Yanwen Sun, a Stanford physics doctoral student, works on the split-delay system at the Linac Coherent Light Source (LCLS). The new system allows scientists to do unique, precisely timed double-probe and pump-probe experiments that investigate natural changes or X-ray excited atomic motion in various materials. (Andy Freeberg/SLAC National Accelerator Laboratory)

The new optics—dubbed the Hard X-ray Split-Delay system—at the X-ray Correlation Spectroscopy (XCS) instrument started commissioning in September 2017, and first became available to scientists at the Department of Energy’s SLAC National Accelerator Laboratory in December the same year. The first experiment using the new system looked at how semiconductor materials respond to femtosecond X-ray pulses differently compared to optical light.

In the “split-delay” process, a first crystal splits a single X-ray pulse in two at the upstream end of the system. One of those pulses is sent on a longer mirrored path than the other, guided by more crystal diffractions. At the end of the system, the pulses converge back onto the same path, but will arrive on the same spot on the sample at a slightly different time. Scientists measure changes in the sample by looking at the difference in the detected signal when the timing of the two pulses is varied.

One of the silicon crystals (top right) splits the X-ray beam as it enters the split-delay system. Multiple bounces of the beams by silicon crystals along two separate paths eventually reunite the beams at the output end of the system. (Andy Freeberg/SLAC National Accelerator Laboratory)

Because these experiments require two distinct micron-sized X-ray beams to strike the sample at the same spot and separated by just several trillionth of a second, any unwanted vibrations could muddy the data when the timing is this exact.

SLAC mechanical engineers Hongliang Shi and Drew Barada inspect the motion mechanics that split the X-ray beam and controls the time delay. The underlying granite base provides precision and stability for the system. (Andy Freeberg/SLAC National Accelerator Laboratory)

“Lack of precision and repeatability caused issues in similar, previously attempted designs,” says Drew Barada, who led the mechanical engineering effort at LCLS. “This was one of our primary concerns while creating the system at XCS, and it drove our decision to use an air-bearing system.”

The split-delay equipment sits on a granite table polished to a flat surface within a few microns, which provides exceptional stability and precision.

“We also use a novel air bearing system to float the key sections of the equipment on a few microns-thick layer of air above the table and maintain position, while a unique mechanism bends to prevent unwanted motions,” says Hongliang Shi, LCLS mechanical engineer.

“It’s a great example of our scientists and engineers working together to translate a high impact science requirement into reality, and provide really interesting opportunities for innovation across the board,” says Mike Dunne, director of LCLS.

The engineering team next to the split-delay system at the X-ray Correlation Spectroscopy (XCS) instrument. From left: Drew Barada, Ted Osier, Randy Whitney, Don Schafer, Justin James, and Hongliang Shi. (Andy Freeberg/SLAC National Accelerator Laboratory)

LCLS is a DOE Office of Science user facility.

For questions or comments, contact the SLAC Office of Communications at communications@slac.stanford.edu.

SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, Calif., SLAC is operated by Stanford University for the U.S. Department of Energy's Office of Science.

SLAC National Accelerator Laboratory is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.