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Scientists at TU Dortmund University, Paderborn University and the University of Würzburg have for the very first time succeeded in using laser pulses to precisely control ‘photon echoes’, which can occur when light waves superimpose on each other. The findings of the research have now been published in the scientific journal “Communications Physics”, published by the Nature Publishing Group.
‘Wie man in den Wald hineinruft, so schallt es heraus’ (‘shout into the forest and a similar echo will return’, or ‘What goes around, comes around’), is not only a well-known German proverb, but is also literally true. When a sound wave is reflected, the echo is heard. When exactly it comes back, however, depends on the ‘forest’ – but first and foremost on the distance between the caller and the place of reflection,” explains Professor Torsten Meier from Paderborn University. “Just imagine that you could tailor when you wanted the echo to come back to you,” continues the physicist. A team of scientists has now achieved just this. Not for acoustic signals, however, but for optical signals: The scientists succeeded in controlling photon echoes emitted by semiconductor quantum dots with sub-second precision.
Meier explains: “Optical echoes are somewhat different to conventional acoustic echoes, because they are not generated by the reflection of waves, but rather in a non-linear optical process. Two short laser pulses are sent to a sample: The first represents the signal and the second the forest. This provides for the reflection. When the lag time of these pulses is doubled, a new light pulse, the photon echo, is emitted by the system exposed to the light.” Using a further control pulse, the researchers were able to control this photon echo within the picoseconds range (i.e. 10-12 of a second), and thereby delay it to a desired point in time. Such control is particularly pertinent for nanophotonic circuits in which multiple optical systems need to be precisely synchronised with each other.
The phenomenon is set to be further developed in the future
The theoretical prediction of the effect was developed in Professor Torsten Meier’s research group. A big challenge was the experimental implementation, which was carried out in the research group led by Professor Ilya Akimov (TU Dortmund University): “The temporal control of optical echoes is a highly dynamic effect, whereby the control pulse virtually pauses the system,” says Hendrik Rose, a PhD student in Paderborn. Alexander Kosarev, a PhD student at TU Dortmund University, adds: “This effect was recently theoretically predicted, was successfully experimentally implemented by us and offers a wealth of possibilities for manipulating light emissions from semiconductor systems.” The samples used were produced in Professor Sven Höfling’s (University of Würzburg) research group.
The research findings are the result of a collaboration funded by the German Research Foundation (DFG) as part of the Transregio 142 “Tailored Nonlinear Photonics” project. Based on this first demo, the scientists now want to optimise the effect, by increasing the time lags, for example. The phenomenon is set to be further developed in the future, in particular for novel applications in the field of photonic quantum technologies, which are the subject of intensive research at the Institute for Photonic Quantum Systems (PhoQS) at Paderborn University.
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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.