X-ray, optical wave mix probes light at atomic scale

Mixing x-ray and optical lightwaves enables observations of how light interacts with matter on the atomic scale. The capability can reveal certain properties of matter, and may enable the observation of changes during chemical reactions, such as the making and breaking of chemical bonds.

Vision, photosynthesis and solar cells are a few examples of the ways light changes matter, but how light makes those changes hasn’t been measured on the atomic scale until now. Mixing x-rays and optical waves was first proposed as an atomic-scale probe of optical interactions nearly 50 years ago, but has been difficult to achieve due to a lack of sufficiently strong x-ray sources.

An international team led by scientists at Lawrence Berkeley National Laboratory used the Linac Coherent Light Source (LCLS) at SLAC National Accelerator Laboratory to mix a pulse of superbright x-rays with a pulse of lower-frequency “optical” light from an ordinary laser. By aiming the combined pulses at a diamond sample, the team was able to measure the optical manipulation of chemical bonds in the crystal directly, on the scale of individual atoms.

X-ray and optical wave mixing is an x-ray diffraction technique similar to that long used in solving the structures of proteins and other biological molecules in crystalline form. But in contrast to conventional diffraction, wave mixing selectively probes how light reshapes the distribution of charge in a material. It does this by imposing a distinction between x-rays scattered from optically perturbed charge and x-rays scattered from unperturbed charge.

“You can think of the electrons orbiting atoms in a material as belonging to one of two groups,” said lead scientist Thornton Glover of Berkeley Lab. “The ‘active’ electrons are the outer, loosely bound valence electrons that participate in chemical reactions and form chemical bonds. The ‘spectator’ electrons are the ones tightly wrapped around the nucleus at the atom’s core.”

In a typical scattering experiment, all electrons scatter with comparable strength, making the core electrons indistinguishable from the valence electrons, he said.

“So x-rays can tell you where atoms are, but they usually can’t reveal how the chemically important valence charge is distributed,” Glover said. “However, when light is also present with the x-rays, it wiggles some portion of the chemically relevant valence charge. X-rays scatter from this optically driven charge, and in doing so, the x-ray photon energy is changed.”

The modified x-rays have a frequency (or energy) equal to the sum of the frequencies of both the original x-ray pulse and the overlapping optical pulse. The change to a slightly higher energy provides a distinct signature that distinguishes wave mixing from conventional x-ray diffraction.

“Conventional diffraction does not provide direct information on how the valence electrons respond to light, nor on the electric fields that arise in a material because of this response,” Glover said. “But with x-ray and optical wave mixing, the energy-modified x-rays selectively probe a material’s optically responsive valence charge.”

Mixing x-ray and optical lightwaves creates a new beam that shows up as a slightly higher-energy peak on a graph of x-ray diffraction, a process called “sum frequency generation.” That process requires an intense x-ray source that was unavailable until free-electron lasers such as the LCLS came online. The LCLS produces ultrashort pulses of high-energy “hard” x-rays millions of times brighter than synchrotron light sources.

Glover’s team chose diamond to demonstrate x-ray and optical wave mixing because diamond’s structure and electronic properties are already well-known. With this test bed, wave mixing has proved its ability to study light-matter interactions on the atomic scale and has opened new opportunities for research.

“The easiest kinds of diffraction experiments are with crystals, and there’s lots to learn,” Glover said. “For example, light can be used to alter the magnetic order in advanced materials, yet it’s often unclear just what the light does, on the microscopic scale, to initiate these changes.”

The work was published in Nature (doi: 10.1038/nature11340).