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Physikgebäude (Neubau Otto-Hahn-Str. 4a) an der TU Dortmund © Nikolas Golsch​/​TU Dortmund
PUBLICATION IN NATURE COMMUNICATIONS

Research Group led by Dr. Marc Aßmann finds exotic Interactions in Semiconductors

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A detail shot of a laser in the lab in dark green colors. © Roland Baege​/​TU Dortmund
The team led by Dr Marc Aßmann has tailored two laser beams to precisely investigate the interactions of Rydberg excitons.
A research team led by the group of Dr Marc Aßmann from the Faculty of Physics at TU Dortmund University has investigated the extraordinarily strong interactions of Rydberg excitons in copper oxide in a transnational cooperation with partners from the universities of Rostock, Aarhus and Harvard. In the process, the group discovered a blockade effect between excitons that, with a size of several micrometres, appear like giants in the quantum mechanical system. The controllability of such effects is highly relevant for optical circuits and quantum information processing. The results were recently published in the well-known journal Nature Communications.

Excitons are hydrogen-like bound states of negatively charged electrons and positively charged electron vacancies - so-called holes - in a semiconductor. They play an important role in such diverse areas as organic solar cells, photosynthesis or semiconductor lasers. Analogous to hydrogen, excitons also have excited states. Excitons in highly excited states, the Rydberge excitons, exhibit astonishing properties that are all the stronger the higher the quantum number of the excited state: the volume of the twentieth excited state of an exciton is already 64 million times larger than in the ground state, while the polarisability, i.e. the sensitivity to external electric fields, is even 1.2 billion times greater. These properties make Rydberge excitons very interesting for precision sensor technology.

Investigations with customised laser beams

As part of his doctoral thesis, which was awarded the Wilhelm and Else Heraeus Dissertation Prize by the Dortmund Physics Faculty, Dr Julian Heckötter investigated the interactions between several such Rydberge excitons in different states. To do this, he tailored two laser beams so that each beam produced a precisely defined Rydberge exciton state and was thus able to precisely measure the interactions between the two states. In doing so, he was able to demonstrate a complex blocking effect. "We found that a sphere forms around each exciton in which no further excitons can be generated," says Dr Marc Aßmann. "The excitons have to maintain a certain minimum distance from each other, which can be several micrometres in size."

This also revealed a systematic asymmetry that depends on whether the effects on a larger or a smaller exciton are studied. Together with the theoreticians Dr. Valentin Walther from Harvard, Prof. Thomas Pohl from Aarhus and Prof. Stefan Scheel from Rostock, this phenomenon could be elucidated. Detailed computer simulations showed that the cause lies in Van der Waals interactions. These are the same forces that are mainly responsible for geckos being able to walk along walls and ceilings.

The results of the interdisciplinary research team were recently published in the renowned journal Nature Communications. The project was funded, among other things, within the framework of the joint German-Russian Collaborative Research Centre TRR 160, in which research institutions in Dortmund and St. Petersburg are involved.

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