Innovative technique makes subtle dynamic processes visible

17 February 2026

Photo: C. Ornelas-Skarin et al.

The new technique enables high-sensitivity measurements that reveal the asymmetric shifts of electrons along atomic bonds.

Valence electrons determine a crystal’s material and chemical properties, shaping how solids conduct electricity, interact with light, and form chemical bonds. Yet directly measuring how these electrons rearrange and move on atomic length scales remains a major experimental challenge. In Physical Review X, an international research team has now demonstrated an innovative technology that responds to subtle dynamic processes that were previously undetectable. The results could help to develop quantum materials whose functional properties change under the influence of light.

The new technique builds on a method known as X-ray–optical mixing (XOM). In XOM experiments, a polarized optical laser drives the motion of valence electrons, while hard x rays probe the resulting time-dependent electron density with angstrom-scale spatial resolution. Because the optical field couples primarily to valence electrons, their motion leaves subtle fingerprints in the x-ray scattering signal. These fingerprints appear as weak “sidebands” near a much stronger Bragg diffraction peak originating from the core-electron density.

The researchers developed a significantly more sensitive experimental setup now, that cleanly separates the weak, wavelength-shifted sidebands from the dominant elastic signal. This setup made it possible to resolve a second-order sideband in silicon, a signal that is not expected to appear in conventional optical measurements.

In bulk optical experiments, the symmetry of silicon causes second-order nonlinear effects to cancel out. At the atomic level, however, the experiment becomes sensitive to tiny asymmetries in how valence electrons shift along individual atomic bonds. The researchers interpreted the detected second-order sideband as a direct signature of bond-direction dependent electron motion, invisible to standard optical probes.

The measured signal is in good agreement with first-principles theoretical calculations, confirming that the technique isolates the optically driven valence-electron density without requiring phase information. Beyond silicon, the approach provides a powerful new route for probing subtle electronic ordering phenomena and light-induced states in more complex quantum materials, both near equilibrium and in strongly driven, nonequilibrium regimes.

By combining ultrafast optical excitation with atomic-resolution X-ray probes, XOM exemplifies how advanced imaging techniques can reveal hidden degrees of freedom that govern material functionality.

The study is the result of a broad international collaboration involving researchers at the SLAC National Accelerator Laboratory and the Linac Coherent Light Source, the University of Hamburg, the Hamburg Centre for Ultrafast Imaging (CUI), Karlsruhe Institute of Technology, Bar-Ilan University, and Brandenburg University of Technology Cottbus–Senftenberg (BTU).

Among the contributors is Daria Gorelova, who, prior to her professorship at BTU, was a junior professor at the University of Hamburg and a young investigator research group leader in the Cluster of Excellence “CUI: Advanced Imaging of Matter.” Her research combines first-principles theory with ultrafast dynamics, thus supporting the close integration of theory and experiment.

Such collaborations are part of the research culture at the Cluster of Excellence, which actively promotes interdisciplinary exchange and takes diversity in science seriously. This also includes providing targeted support for young researchers and creating visible role models who can encourage young female scientists in particular.