SLAC researchers take important step toward developing cavity-based X-ray laser technology

Researchers have announced an important step in the development of a next-gen technology for making X-ray free-electron laser pulses brighter and more stable: They used precisely aligned mirrors made of high-quality synthetic diamond to steer X-ray laser pulses around a rectangular racetrack inside a vacuum chamber.

Setups like these are at the heart of cavity-based X-ray free-electron lasers, or CBXFELs, which scientists are designing to make X-ray laser pulses brighter and cleaner – more like regular laser beams are today.

“The successful delivery of a cavity-based X-ray free-electron laser will mark the start of a new generation of X-ray science by providing a huge leap in beam performance,” said Mike Dunne, director of the Linac Coherent Light Source (LCLS) X-ray laser at the Department of Energy’s SLAC National Accelerator Laboratory, where the work was carried out.

“There are still many challenges to overcome before we get there,” he said. “But demonstration of this first integrated step is very encouraging, showing that we have the approach and tools needed to sustain high cavity performance.”

The SLAC research team described their work in a paper published in Nature Photonics. Early results were so encouraging, they said, that the lab is already working with DOE’s Argonne National Laboratory, its longtime collaborator on the subject, to design and install the next, bigger version of the experimental cavity system in the LCLS undulator tunnel.

Despite their name, X-ray laser pulses are not yet fully laser-like. They’re created by making accelerated electrons wiggle through sets of magnets called undulators. This forces them to give off X-rays, which are shaped into powerful pulses for probing matter at the atomic scale. At LCLS, pulses arrive 120 times a second, a rate that will soon increase to a million times per second. 

But because of the way X-ray laser pulses are generated, they vary in intensity and contain an unpredictable mix of wavelengths. This creates what scientists call “noise,” which muddles their view of samples they are probing. 

The introduction of a cavity has been proposed to overcome this problem, adopting the approach used by conventional optical lasers. Cavities increase the coherence of lasers ­by preferentially selecting light of a single wavelength whose peaks and troughs line up with each other. ­But the mirrors that bounce light around in regular laser cavities won’t work for X-ray laser pulses – all you would get would be a smoking hole in your mirror where the X-rays drilled through. 

The idea of using crystals – and, more recently, synthetic diamond crystals – as mirrors to smooth and help amplify X-ray pulses inside a cavity has been around for a long time, said Diling Zhu, who led the experimental team with fellow SLAC scientist Gabriel Marcus.

Read more on SLAC website

Image: A top-down view of one of the cavity vacuum chambers. Two diamond mirrors can be seen in the upper and lower left corners, each one mounted on four motors that control its angle and position. At upper right, the precision diamond grating that brings X-ray pulses into the chamber is mounted on a screen holder. 

Credit: Diling Zhu/SLAC National Accelerator Laboratory

Ultrafast surface processes observed


In a world first, an international team of scientists led by European XFEL and the University of Siegen has demonstrated that the intense pulses produced by an X-ray laser can be used to investigate ultrafast processes occurring on and just below material surfaces with unprecedented depth and time resolution. This allows researchers to capture processes that are more than a billion times faster than what could previously be observed. The results, which the team has just published in Physical Review Research, pave the way for versatile applications that rely on our understanding of ultrafast surface dynamics. Examples are the laser processing of material surfaces to create tailor-made nanoscale structures or the realization of compact laser-based particle or radiation sources.

Using intense laser pulses, nanoscale surface structures can be created with optimized optical, mechanical, and chemical properties. Such tailored structures play a decisive role in many fields with significant societal and economic impact. They can be used to fashion antimicrobial coatings, to improve the bonding of dental implant screws with bone, and to build advanced optical components with high damage thresholds. To be able to better create these structures and comprehend their effects, scientists first need to observe and understand the ultrafast processes that happen when the intense femtosecond laser pulses used in the surface processing hit the material and react with it.

Read more on the European XFEL website

Image: Grazing-incidence small-angle X-ray scattering image obtained from a multilayer sample, measured using single X-ray pulses of the SACLA X-ray laser in Japan. The central black circle is the beamstop used to block the main mirror-like reflection peak, which is much more intense than the scattering pattern. The pattern contains information on the depth-resolved density profile (horizontal axis) and the surface structure (vertical axis).