31 August 2012
A long-awaited technique for watching how light interacts with matter on the atomic scale was demonstrated by mixing x-ray and optical lightwaves. 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.
Simulated valence-charge density from x-ray and optical wave-mixing shows the nuclei of carbon atoms as dark spots revealed by diffracted x-rays and the peaks of some of the bonds between them as white and blue spots induced by the polarized optical pulse. In diamond, the optical pulse primarily wiggles the charge that makes up chemical bonds. The technique measures how light changes matter on the atomic scale.
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. 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.
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.
Pulses of 8000-eV x-rays from the LCLS are synchronized with 1.55-eV pulses from an optical laser, so that both strike the diamond sample at the same time and mix to form upconverted pulses of 8,001.55 eV. The detector first sees the diffracted x-ray pulse and then, after the sample is gently “rocked,” the slightly more energetic mixed pulse. The optical pulse exerts localized force on the chemical bonds among the carbon atoms.
“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, which 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.”
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 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.
Mixing x-ray and optical lightwaves creates a new beam which 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 (FELs) 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.
“The breadth of the science impact of LCLS is still before us,” said Jerome Hastings, a professor of photon science at the LCLS and an author of an article on the research. “What is clear is that it has the potential to extend nonlinear optics into the x-ray range as a useful tool. Wave mixing is an obvious choice, and this first experiment opens the door.”
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 says. “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.”
Looking farther ahead, Glover imagines 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.
The team reports its work in the Aug. 30 issue of Nature.