- Top News
- Press Releases
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.
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.
Andreas Farenbruch, Dietmar Fröhlich, Dmitri R. Yakovlev und Manfred Bayer: Rydberg Series of Dark Excitons in Cu2O. Physical Review Letters 125, 207402 (2020).
Contact for further information:
Search & People Search
Location & approach
The campus of TU Dortmund University is located close to interstate junction Dortmund West, where the Sauerlandlinie A 45 (Frankfurt-Dortmund) crosses the Ruhrschnellweg B 1 / A 40. The best interstate exit to take from A 45 is "Dortmund-Eichlinghofen" (closer to Campus Süd), and from B 1 / A 40 "Dortmund-Dorstfeld" (closer to Campus Nord). Signs for the university are located at both exits. Also, there is a new exit before you pass over the B 1-bridge leading into Dortmund.
To get from Campus Nord to Campus Süd by car, there is the connection via Vogelpothsweg/Baroper Straße. We recommend you leave your car on one of the parking lots at Campus Nord and use the H-Bahn (suspended monorail system), which conveniently connects the two campuses.
TU Dortmund University has its own train station ("Dortmund Universität"). From there, suburban trains (S-Bahn) leave for Dortmund main station ("Dortmund Hauptbahnhof") and Düsseldorf main station via the "Düsseldorf Airport Train Station" (take S-Bahn number 1, which leaves every 20 or 30 minutes). The university is easily reached from Bochum, Essen, Mülheim an der Ruhr and Duisburg.
You can also take the bus or subway train from Dortmund city to the university: From Dortmund main station, you can take any train bound for the Station "Stadtgarten", usually lines U41, U45, U 47 and U49. At "Stadtgarten" you switch trains and get on line U42 towards "Hombruch". Look out for the Station "An der Palmweide". From the bus stop just across the road, busses bound for TU Dortmund University leave every ten minutes (445, 447 and 462). Another option is to take the subway routes U41, U45, U47 and U49 from Dortmund main station to the stop "Dortmund Kampstraße". From there, take U43 or U44 to the stop "Dortmund Wittener Straße". Switch to bus line 447 and get off at "Dortmund Universität S".
The AirportExpress is a fast and convenient means of transport from Dortmund Airport (DTM) to Dortmund Central Station, taking you there in little more than 20 minutes. From Dortmund Central Station, you can continue to the university campus by interurban railway (S-Bahn). A larger range of international flight connections is offered at Düsseldorf Airport (DUS), which is about 60 kilometres away and can be directly reached by S-Bahn from the university station.
The H-Bahn is one of the hallmarks of TU Dortmund University. There are two stations on Campus Nord. One ("Dortmund Universität S") is directly located at the suburban train stop, which connects the university directly with the city of Dortmund and the rest of the Ruhr Area. Also from this station, there are connections to the "Technologiepark" and (via Campus Süd) Eichlinghofen. The other station is located at the dining hall at Campus Nord and offers a direct connection to Campus Süd every five minutes.