Using light to reveal properties of 2D semiconductors
Electrons in a crystal do not simply move like tiny particles along fixed paths. Instead, their behaviour is determined by quantum mechanical states with geometric properties known as 'quantum geometry'.
Berry curvature is one key parameter that influences how electrons react to light or electric fields. But until now, it's mainly been calculated theoretically or investigated using complex experimental methods.
Now, researchers from the universities of Jena and Regensburg in Germany have developed a new approach that combines optical measurements with an analytical model. In this way they say they can quantitatively estimate the local Berry curvature of the material. Their work 'Nonlinear circular dichroism reveals the local Berry curvature' is published in Nature Photonics.
"We wanted to make a quantum property accessible that has so far been difficult to determine experimentally«, says lead author Nele Tornow from the Institute of Solid State Physics at the University of Jena. "What makes our approach special is that we combine an optical experiment with a theoretical model to obtain a specific value for the Berry curvature."
For the experiment, the researchers used WSe2, a 2D compound semiconductor whose layer under investigation is less than one nanometre thick. Such atomically thin materials react particularly strongly to light and are therefore well suited to non-linear optical experiments.
The team directed two ultrashort laser pulses at the sample. The first pulse alters the electronic states of the material for a tiny fraction of a second. The second serves to optically detect this change. This produces light at twice the frequency of the incident light. From its polarisation, researchers obtain information about the Berry curvature.
The effect takes place within a few hundred femtoseconds. If the team changes the direction of rotation of the first, circularly polarised laser pulse, the sign of the measured signal also reverses. From the measurement data, the researchers determined the Berry curvature, which was consistent with theoretical calculations.
"Using this optical method, we can investigate such properties on extremely short time scales", says Giancarlo Soavi from the Institute of Solid State Physics and the Abbe Centre of Photonics at the University of Jena. "This will enable us in future to track how the quantum geometry of a material changes during very rapid processes."
Although the experiment was demonstrated using WSe2, the team states that the fundamental approach is not limited to this class of materials. In the long term, the method could help to investigate the geometric properties of electronic states in greater detail and to better understand their behaviour in fast optical processes.
