atomically thin semiconductors to make the world’s smallest lasers
transition-metal dichalcogenides are semiconducting materials that can be prepared in the shape of an atomically thin flake. these flakes can be brought into the confined electromagnetic field of microcavities to realize, amongst other things, very small lasers. while few of these nanolaser devices have been experimentally investigated, not much is yet known about the mechanism behind lasing and the coherence properties.
in two recent publications, we have addressed the material and quantum-optical properties of TMD-based nanolasers. in Lohof et al., Nano Lett. (2018) , the material gain has been calculated on a material-realistic footing and used in a rate-equation theory to predict cavity requirements and input-output characteristics for the common TMD materials. going further, in Lohof et al., Phys. Rev. Applied (2018)  we have used a simplified quantum-optical laser model to predict an unusual behavior of the coherence properties of such devices.
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Dr. Paul Gartner will be our guest for two months
we are very much looking forward to Dr. Paul Gartner again coming to Bremen to work with us for two months from may-july. he will be a guest at our institute and of the graduate school QM3.
how long does it take for stimulated emission to become coherent?
coherent emission of a laser is the result of stimulated emission, causing the excited gain medium to send out photons in phase. this is a fact that holds in many cases of conventional lasers. the truth is, however, that stimulated emission and coherence are two different things: the first results from an inversion of the gain medium and the presence of photons in the cavity. coherence, on the other hand, is the result of correlations that make the successive emission of two photons more than just the probabilites of their individual emission.
in collaboration with researchers at NIST we have identified operational regimes of nanolasers, in which stimulated emission and the formation of coherence do behave as two different things. in a photonic-crystal nanolaser that operates with short pulses, coherence forms only after the peak of the emission has ended, so that a large fraction of the stimulated emission is actually thermal. our results are published in Optica and shed new light on lasing in the presence of strong cavity-QED effects.
lasing from a GaN nanobeam
december 2017 / february 2018
our work on lasing from a two-dimensional GaN quantum well in a one-dimensional photonic crystal nanobeam cavity has been accepted for publication in nature communications right before christmas!
the publication is now online and can be downloaded here (open access).
Universität Bremen has released an official statement on the university homepage.
unveiling the physics of microcavity lasers
in a news and view article in light: science & applications, Wiliam Hayenga and Mercedeh Khajavikhan give their point of view on the importance of second-order coherence properties for future design and understanding of high-beta nanolasers. their comment is published in light of our article in nature LSA by Sören Kreinberg and co-workers. thank you!
strong coupling at the laser threshold
the cavity-QED regime of strong light-matter coupling is typically considered in the context of a weakly excited system. in a nanolaser that only operates with a handful of emitters, it is the natural regime because light-matter coupling needs to be strong to achieve sufficient gain to cross the threshold. at the same time, a laser is not a weakly excited system, but operates at stronger excitation. if you are interested to find out how the signatures and the presence of strong coupling become redefined in this „uncommon“ regime, please check out our article that has finally been published here.
textbook contribution on quantum dots for applications in the quantum-information technologies
our theory contribution to the new springer text book on the role and potential of semiconductor quantum dots for applications in the quantum information technologies is available as a free preview on the springer page.
Dr. Paul Gartner is staying as guest researcher at our institute
Paul Gartner has worked at the Institute for Theoretical Physics in Bremen for 15 years before moving back to his home town Bucharest. he is an expert on non-equilibrium greens functions, carrier kinetics, transport, quantum optics and the laser transition. we are pleased to welcome him in Bremen for two months to work on scaling behavior at the laser threshold and other problems with our group. we are grateful to the DFG to provide funding.
universality in a new class of nanolasers
in a detailed study on a variety of quantum-dot micropillar emitters, we have identified a universal dependence of the emission- and coherence properties on the intracavity photon number. superradiant coupling between emitters is identified as source of coherence that does not rely on the presence of photons in the cavity, thereby increasing the „coherence per photon“ of the device.
our combined theoretical and experimental work has been accepted in light: science & applications published by nature. a preview of the article can be found here.
graduate school quantum-mechanical materials modeling QM3 to begin in january 2017
bremen will be the main hub for the recently granted graduiertenschule involving the univeristies in bremen, north bremen (Jacobs), oldenburg, and the max planck institute in hamburg. the aim of the program is research and education in the field of a new class of materials whose macroscopic properties are defined by design and manipulation on a microscopic scale. a prominent example are atomically thin layers of transition-metal dichalcogenides, such as MoS2 or WSe2, which are direct semiconductors with exceptionally large coulomb effects. in the graduate school, i am one out of 12 subproject leaders. more information on the web page http://www.rtg-qm3.de/
NEWS: signatures of radiative coupling in a quantum-dot nanolaser published in nature communications
our theoretical work on superradiance (link to phys. rev. applied) in quantum-dot nanolasers is an extension to conventional laser models. together with researches from the universities magdeburg, dortmund, and würzburg, a combined theoretical and experimental work has recently been published in nature communications. you can find the press release here.
NEWS: DFG funding
together with researchers from the technical universities in berlin and würzburg a dfg research grant has been approved to study the the few-emitter limits of lasing. more information on the project is found here.
work on two-dimensional semiconductors
our recent interest in the optical properties of two-dimensional materials has led to a couple of successful collaborations and publications. this effort will be increased in the framework of the graduate school QM3 starting 2017.
review article on signatures of lasing in nanolasers
miniaturisation towards the ultimate limit of cavity-QED can fundamentally change the properties of a laser. in this regime, few solid state emitters or, in the extreme case even a single one, interact with the individual particles of light. how to identify and characterise the emission of such systems is the topic of our work in light: science&publications publised by nature.
FOPS conference gallery
photographs of the FOPS conference, taken place at lake junaluska in august this year, are available as a web gallery.
correlation dynamics of individual photons