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Publication in Physical Review Letters

On the Trail of the Mysterious Dark Exciton Matter

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Four men standing in a laboratory © Oliver Schaper​/​TU Dort­mund University
Assoc. Prof. Dmitri Yakovlev, Prof. Dietmar Fröhlich, An­dre­as Farenbruch and Prof. Manfred Bayer have gained new knowledge about excitons.

LEDs form the basis for energy-efficient generation of light: inside them, electricity creates particles called excitons, which are converted into light. These “light” excitons have “dark twins”. A research team at TU Dort­mund University has now characterized these in detail for the first time – and in doing so has made astonishing observations. In this way, the team was able to provide evidence for quantum mechanical phenomena that can improve understanding of parasitics in LEDs.

Today LEDs are built into smartphones, televisions, and lamps, and everyday life is unimaginable without them. Their widespread use was only made possible by the development of the blue light-emitting diode, for which the 2014 Nobel Prize in Physics was awarded. Before that, there already were red and green light-emitting diodes. With the advent of the blue light-emitting diode, it now became possible to generate white light too.

To generate light, ne­ga­ti­ve and positive electrical charges are injected into a crystal. When two meet, they transform into light and disintegrate. Before that, they enter into a bound state. This state corresponds to a new particle called an exciton. Excitons can only have certain energies that are specified by quantum mechanics. Each light-emitting crystal exhibits a specific series of exciton energy states, the values ​​of which depend on the material. If you want to optimize this, you need to draw conclusions about the excitons and their characteristic energies. Excitons were first detected in the material cuprous oxide (Cu2O).

Bright and Dark Excitons

Besides the bright, light-emitting excitons, there are also dark excitons that cannot decay into light. Through a quantum mechanical interaction, the so-called exchange interaction, their energies differ from those of the bright excitons. Up to now, in all known materials, in­clu­ding cuprous oxide, only the lowest ground state of this dark exciton matter could be observed. In keeping with their name, these states had previously remained dark and hidden.

Graphic of a laser pulse and excitons © TU Dort­mund University
A laser pulse stimulates the dark and light excitons in the cuprous oxide crystal.

Now, for the first time, the Dort­mund physicists were able to gain a deeper insight into the world of the dark excitons. They used strong magnetic fields to mix together dark and light excitons. In addition, a special experimental technique came into play in which two photons, each with half the exciton energy, are used to stimulate the dark exciton. When this decays again, a photon is created, which can be observed. It is only with this trick that the extremely weak signals can be measured at all.

The research team at TU Dort­mund University succeeded in observing the six energetically lowest dark excitons and systematically measuring the exchange energy. On the basis of quantum mechanics, clear differences were revealed with regard to atomic physics and its predictions. The energies of dark excitons were supposed to lie systematically below those of the light excitons. But the Dortmunders found an exception, specifically the state with the second-lowest energy. Here the order is reversed; the light exciton has a lower energy than the dark one. They were also able to clarify the reason for this: The light exciton is strongly coupled with another exciton of higher energy, and whenever such a coupling is present in quantum mechanics, the two levels involved repel each other. This lowers the energy of the light exciton, while that of the dark exciton hardly changes at all. As a consequence, their order is reversed.

Publication in Renowned Journal

With this knowledge, the influence of the dark excitons and the possibility of manipulating them can now be better understood. Dark excitons can massively interfere with the brightness of a light-emitting diode, for example when excitons accumulate in the energetically lowest dark state. Conversely, dark excitons could also be used to store in­for­mation, since they do not decay. This opens up new vistas for using them constructively.

The results were published in the current issue of the re­nowned journal Physical Review Letters.

Original publication:
An­dre­as Farenbruch, Dietmar Fröhlich, Dmitri R. Yakovlev und Manfred Bayer: Rydberg Series of Dark Excitons in Cu2O. Physical Review Letters 125, 207402 (2020).

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