An international team of scientists used the Linac Coherent Light Source (LCLS) 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.

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, which distinguishes wave-mixing from conventional x-ray diffraction.

Beyond the ability to directly probe atomic-scale details of how light initiates such changes as chemical reactions or phase transitions, sensitivity to valence charge creates new opportunities to track the evolution of chemical bonds or conduction electrons in a material – something traditional x-ray diffraction does poorly. Different components of the valence charge can be probed by tuning the so-called optical pulse; higher-frequency pulses of extreme ultraviolet light, for example, probe a larger portion of valence charge.

Because mixing x-ray and optical light waves creates a new beam, which shows up as a slightly higher-energy peak on a graph of x-ray diffraction, the process is called “sum frequency generation.” It was proposed almost half a century ago by Isaac Freund and Barry Levine of Bell Labs as a technique for probing the microscopic details of light’s interactions with matter, by separating information about the position of atoms from the response of valence charge exposed to light.

But sum frequency generation requires intense x-ray sources unavailable until recently. SLAC’s LCLS is just such a source. It’s a free-electron laser (FEL) that can produce ultrashort pulses of high-energy “hard” x-rays millions of times brighter than synchrotron light sources, a hundred times a second.

The 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.

Looking farther ahead, they can imagine experiments that observe the dynamic evolution of a complex system as it evolves from the moment of initial excitation by light. Photosynthesis is a prime example, in which the energy of sunlight is  transferred through a network of light-harvesting proteins into chemical reaction centers with almost no loss.

Such experiments will require high pulse-repetition rates that free electron lasers have not yet achieved. Synchrotron light sources like Berkeley Lab’s Advanced Light Source, although not as bright as FELs, have inherently high repetition rates and, says the researcher, “may play a role in helping us assess the technical adjustments needed for high repetition-rate experiments.”

Light sources with repetition rates up to a million pulses per second may someday be able to do the job. The researcher says, “FELs of the future will combine high-peak brightness with high repetition rate, and this combination will open new opportunities for examining the interactions of light and matter on the atomic scale.”

This story is reprinted from material from
Berkeley Lab, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.