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Researchers at Friedrich Schiller University Jena and the University of Regensburg have developed a method to measure a quantum property of electrons called Berry curvature using light. This quantity describes geometric properties of electron states that were previously difficult to access experimentally. The team measured it in tungsten diselenide, a semiconductor layer just one atomic layer thick, which is less than one nanometer. Their results are published in the journal Nature Photonics.

Electrons in crystals do not move like simple particles but exist in quantum mechanical states. Berry curvature influences how these electrons react to light or electric fields. Until now, this property was mainly calculated theoretically or studied with complex methods. The new approach combines optical measurements with an analytical model to quantitatively estimate the local Berry curvature of the material.

For the experiment, the team directed two ultrashort laser pulses at the sample. The first pulse alters electronic states for a tiny fraction of a second, while the second detects this change by producing light at twice the frequency of the incident light. From the polarization of this new light, researchers obtained information about the Berry curvature. The effect occurs within a few hundred femtoseconds, and changing the rotation direction of the first pulse reverses the signal sign.