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Scientists are working to control materials precisely and down to the atomic level and to combine them with each other. The objective is to produce new hybrid structures that are multifunctional and suitable for use in both optoelectronics and magnetism, for example. An international team of researchers from Sheffield, Clermont-Ferrand, Tokyo and Dortmund has now succeeded in creating such a system, which opens up completely new possibilities, above all for optically controlled magnetism. Their findings were published recently in the renowned journal Nature Photonics.
“In collaboration with our international partners, we have found a new regime of interaction between light and magnetism that promises many exciting results for the future,” says Professor Ilya Akimov from the Department of Physics at TU Dortmund University, who led key studies on magnetization dynamics on the Dortmund side.
To achieve their objective, the scientists first had to systematically “assemble” the new material structure. The starting point for the new structure was a monatomic layer of molybdenum diselenide (MoSe2), which is known for its very strong absorption and emission of light. The aim was to imprint magnetic properties onto this MoSe2 layer: For this purpose, it was attached to an ultrathin europium sulfide (EuS) crystal. The desired magnetic properties were achieved by injecting electrons from the europium sulfide into MoSe2 so that a two-dimensional electron gas formed inside it. The two-dimensional electron gas then became a magnet itself, as it was “spin-polarized” when an external magnetic field was applied.
Strong coupling between light and matter
To make not only the magnetic but also an optical control of the material as efficient as possible, the team had to manipulate the interaction of light with MoSe2 in such a way that all incident light was captured in the material. Although MoSe2 absorbs light very efficiently, it does not absorb it completely. This property was enabled by embedding the material between two mirrors at a precisely controllable distance of less than a millionth of a meter. With this trick, it was possible to achieve what it known as the “regime of strong coupling between light and matter”.
If the system is now illuminated with a laser, each photon of the laser light is captured by shifting one electron per photon into a state of higher energy. This state produced by means of light is called an exciton. The exciton now grabs an electron from the two-dimensional electron gas and forms a new particle known as a trion, which starts to interact with the electron gas magnet. This trion is also magnetic via its spin: The spin can be parallel or antiparallel to the magnetization of the two-dimensional electron gas.
Accordingly, this interaction produces different outcomes: The oscillator strength of the trion, that is, its light-matter coupling, depends on the spin polarization of the electron gas. If the electron gas is fully polarized with spin parallel to the magnetic field, a trion can no longer form because for this it would need an electron with spin antiparallel to the magnetic field: The oscillator strength, that is, the strength of its coupling to light, has dropped to zero for such a trion as a result of the spin polarization.
The conclusions that the team has drawn from this are as follows: The trion shows strong light-matter interaction for parallel orientation, in the anti-parallel case it is cancelled. As a consequence, the reaction to an external magnetic field is five times stronger than for a trion in a simple MoSe2 layer. The researchers were thus able to show that the optical properties of the MoSe2 layer can be precisely controlled by the external magnetic field. Conversely, optical excitation determines the magnetic response: The more trions the laser generates, the more the strength of the light-matter interaction is reduced.
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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 South Campus), and from B 1 / A 40 “Dortmund-Dorstfeld” (closer to North Campus). 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 North Campus to South Campus by car, there is the connection via Vogelpothsweg/Baroper Straße. We recommend you leave your car on one of the parking lots at North Campus 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 15 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 North Campus. 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 South Campus) Eichlinghofen. The other station is located at the dining hall at North Campus and offers a direct connection to South Campus every five minutes.
The facilities of TU Dortmund University are spread over two campuses, the larger Campus North and the smaller Campus South. Additionally, some areas of the university are located in the adjacent “Technologiepark”.