Nature 488, 603–609 (2012)

Credit: © NPG 2012

The interactions between light and matter are still poorly understood at the microscopic level, despite being of paramount importance in a wide range of research fields. Proposed over 40 years ago, the mixing of X-rays and optical waves can serve as a probe of light–matter interactions on the atomic scale. Thornton Glover and researchers from the United States and France have now demonstrated X-ray and optical sum-frequency generation in a diamond sample. The process may be interpreted as optically modulated X-ray diffraction. A 2-ps-duration optical pulse is fired at the diamond, redistributing the polarizable charge density in a time-varying manner. The 80-fs X-ray pulses are fired simultaneously and mix with the optical wave in the diamond. Some X-rays interact with the temporally oscillating part of the charge density and are inelastically scattered; the frequency of the optical waves is added to that of these X-rays. Most X-rays, however, are elastically scattered, such that their frequency is unchanged. As in standard X-ray crystallography, this enables a reconstruction of the crystal's charge density. Although this demonstration was performed for one crystal orientation of the diamond only, an extension to different orientations would reveal a three-dimensional charge-density variation. The researchers report that such light-induced measurements of atomic-scale charges and fields will contribute to a better understanding of materials.