According to a recent study published in Nature Photonics, researchers from an international team led by Göttingen University have introduced a novel technique for ultrafast imaging of dark excitons.
Artistic representation showing how the newly developed technique – Ultrafast Dark-field Momentum Microscopy – allows both bright excitons (shown in red) and dark excitons (shown in blue) to be analyzed. Image Credit: Lukas Kroll
How can emerging technologies like solar cells be further improved? The research team explored this question with a groundbreaking technique. For the first time, they have precisely tracked the formation of elusive particles known as dark excitons, which are set to play a crucial role in the future of solar cells, LEDs, and detectors.
Dark excitons are tiny pairs consisting of an electron and the hole left behind when it becomes excited. They carry energy but do not emit light, giving them the name “dark.” An exciton can be visualized as a balloon (representing the electron) that drifts away, leaving an empty space (the hole) to which it remains bound by a force called Coulomb interaction.
Researchers describe these as “particle states” that are challenging to detect but are crucial in atomically thin, two-dimensional structures within specialized semiconductor compounds.
In a previous study, Professor Stefan Mathias's research group at the University of Göttingen's Faculty of Physics demonstrated how dark excitons form quickly and described their dynamics using quantum mechanical theory.
In their latest study, the team has developed a novel technique called “Ultrafast Dark-field Momentum Microscopy” and applied it for the first time. This breakthrough allowed them to observe the formation of dark excitons in a tungsten diselenide (WSe₂) and molybdenum disulfide (MoS₂) material, occurring in just 55 femtoseconds (0.000000000000055 seconds) with a precise resolution of 480 nm (0.00000048 m).
This method enabled us to measure the dynamics of charge carriers very precisely. The results provide a fundamental insight into how the properties of the sample influence the movement of the charge carriers. This means that this technique can be used in future to specifically improve the quality and therefore also the efficiency of solar cells, for example.
Dr. David Schmitt, Study First Author, Faculty of Physics, Göttingen University
Dr. Marcel Reutzel, Junior Research Group Leader in Mathias' research group, adds, “This means that this technique can be used not only for these specially designed systems but also for research into new types of materials.”
The study was funded by the DFG-funded Collaborative Research Centres: “Control of Energy Conversion on Atomic Scales” and “Mathematics of Experimentation” in Göttingen, as well as the “Structure and Dynamics of Inner Interfaces” in Marburg.
Journal Reference:
Schmitt, D., et al. (2025). Ultrafast nano-imaging of dark excitons. Nature Photonics. doi.org/10.1038/s41566-024-01568-y.