To unlock materials of the future, including better photocatalysts or light-switchable superconductors, researchers need to understand how the valence electrons within materials respond to light at the atomic scale. Materials are made of atoms, and an atom’s outer electrons, or valence electrons, are responsible for chemical bonding as well as a material’s thermal, magnetic and electronic properties.
But imaging valence electrons in bulk materials is extremely difficult because valence electrons are only a small subset of a typically large pool of electrons.
Now, researchers at the Department of Energy’s SLAC National Accelerator Laboratory have refined a way to track valence electrons using a unique method that shines both X-rays and lasers onto a material, then tracks the frequency generated by both sources. The method allows the researchers to understand more about extremely fast-moving valence electrons, including the symmetry of their local environment.
“Electrons are much lighter and move even faster than the atoms themselves,” said David Reis, professor of applied physics and photon science at SLAC and Stanford University and a faculty member of the Stanford PULSE Institute who led the research. “To find valence electrons and track them is a nice advance a long time in the making. Our hope is that this method becomes one of many tools to understand complex material properties.”
The results were published in the journal Physical Review X.
Mixing X-Rays With Optical Laser Light
Scientists have previously been able to track the ultrafast motion of valence electrons, but only on the surface of materials or by inferring their location during a chemical reaction with the help of theory. Because valence electrons are difficult to probe directly on a microscopic scale and are responsible for most of materials’ interesting properties, much of our understanding of materials remains theoretical.
The new method, called X-ray and optical wave mixing, was first demonstrated by Ernie Glover and his collaborators more than a decade ago but was optimized recently by Chance Ornelas-Skarin, a PhD student working with Reis at the time of the study.
The technique harnesses the short wavelength of X-rays of SLAC’s Linac Coherent Light Source to image electrons within the material while using the long wavelength of optical lasers to pick out the valence electrons. Each atom of silicon, the material used in the experiment, has 14 electrons, 4 of which are valence electrons.
“All the electrons will feel the hard X-rays, but only the valence electrons will feel the optical rays,” said Ornelas-Skarin, who is now a postdoctoral researcher at SLAC. “So, the sum of those frequencies shows us the valence electrons, which we wouldn’t be able to see in any other way without the need for theory to fill in the gaps.”
In particular, the research team focused on the atomic-scale motion of the electrons oscillating at twice the frequency of the laser. Even more, the research team rotated the optical laser field to follow the details of how the optically induced motion of the electrons changes. That gives researchers much more information about how valence electrons are distributed in the material.
“With that, we can understand the structure of bonds, which allows us to learn more about the structure and dynamics of materials, giving us much more information about how complicated materials work,” Ornelas-Skarin said.
This proof-of-concept experiment tracked the valence electrons of silicon, but Reis is confident the technique could be used on more exotic materials, helping to pave the way toward better photocatalysts and materials that change with emergent properties under light illumination.
Next, the team plans to refine the technique and use other X-ray wavelengths to understand even more about electrons and the structure of materials.
“We’re just getting a glimpse of what’s possible,” Reis said.
In addition to researchers from SLAC and Stanford, the team included collaborators from the University of Hamburg, the Karlsruhe Institute of Technology, and Brandenburg University of Technology Cottbus-Senftenberg, all in Germany, as well as Bar-Ilan University in Israel. This research was funded in part by the DOE’s Office of Science and was performed at SLAC’s Linac Coherent Light Source, an Office of Science user facility. Preliminary experiments were conducted at SACLA in Japan and SwissFEL in Switzerland.