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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.