Host: Prof. Dr. Sven Höfling, Technische Physik, Universität Würzburg
Fellow: Dr. Sebastian Klembt
The TOPOPOLIS project is a 24 month fellowship in the framework of Marie Skłodowska-Curie actions and got its name from TOPOlogical POLaritons In Semiconductor microstructures. It is aimed at the fundamental understanding and realization of topologically protected edge modes in a hybrid light-matter system. These hybrid states are so-called exciton-polaritons, hosted by technologically well-developed semiconductor structures. Topological insulators are a class of materials in which topological invariants lead to a robustness against perturbations. Probably the most striking feature is the emergence of topological edge states at the boundary areas with distinct topological invariants. The measurable physical effect is robust, unidirectional transport, unaffected by disorder or defects. Topological insulators have for the fist time been observed in the integer quantum Hall effect, meaning in electron transport physics. Since then, manifestations of topological effects have been demonstrated in a wide range of field, including cold atoms, acoustics, mechanics, microwaves and notably photonic systems. Topological insulators have been proposed theoretically for exciton-polaritons in lattice potential landscapes, such as e.g. the honeycomb lattice. The striking advantage of using a system that is part-light, part-matter is that the light part allows for propagation and the system to be measured by spectroscopy techniques, while the matter part allows for interactions, manipulation and is susceptible to magnetic fields.
The overall objectives of this fellowship, next to the mutual exchange of knowledge, is the epitaxial growth and processing of semiconductor mecrocavity landscapes, the subsequent spectroscopical investigation, all towards the final goal of a polariton topological insulator.
During the 24 month project period the MSCA Fellow Sebastian Klembt and the host institution, the TEP group lead by Prof. Höfling have intensely shared knowledge and ideas regarding sample preparation and device technology, experiment design and spectroscopic methods. We have made advance in the control of polariton lattice structures leading to publications demonstrating Flatband- and Dirac-cone dispersions, typical for honeycomb- and Lieb-lattice geometries. Furthermore we have studied and published on the propagation behavior and condensation dynamics of polaritons in confined structures, which are important pre-requisites for polariton topological insulators. Final experiments on polariton honeycomb lattices under magnetic field have shown first hints of polariton edge modes that might lead to a full demonstration of a polariton topological insulator. The collaboration has been highly beneficial for both sides and we firmly believe that the mutual support has been crucial for the success of the project.
In order to realize the proposals for polariton topological insulators, which use a combination of TE/TM-mode splitting and Zeeman-splitting to open a topological gap and to create a topologically protected edge mode in a honeycomb geometry, we have investigated various microcavity sample designs with a focus on half-etched and etch-and-overgrowth microcavities. Here, we find that both techniques allow for a good parameter control and a very similar mode hybridization. Subsequently, the cavity structures have been experimentally optimized (with respect to the theoretical proposals) for their optical quality, Zeeman- and TE/TM-splitting.
In a Lieb-lattice geometry, that shares some distinct features with the honeycomb lattice, we have observed condensation in S- and P-band flatband states. Within a careful investigation of the properties of micropillars - the building blocks for lattice potential structures - results on polariton propagation behavior and the temporal coherence of trapped polariton condensates have been published.
In a significant technical advance, we have been able to demonstrate that polaritons in lattices can, in addition to laser excitation, also be excited electrically by doping and contacting polariton lattices. Finally, experiments on polariton lattices under magnetic field have shown first hints of polariton edge modes that might lead to a full demonstration of a polariton topological insulator.
Results have been published in peer-reviewed journals and have been made available on open access platform, such as arXiv.org. In addition, results have been presented and discussed on international conferences.
The demonstration of a polariton topological insulator would certainly, not only expand the state-of-the-art of semiconductor physics but of a wide range of fields, dealing with the creation, control and understanding of topologically protected states. They can be envisaged to build the bases for new technologies for the transfer of information or for computational purposes, but so far a more fundamental understanding is in the focus of research. Our results, will and have certainly made some advance towards the realization of a polariton topological insulator.
K. Winkler, H. Flayac, S. Klembt, A. Schade, D. Nevinskiy, M. Kamp, C. Schneider, and S. Höfling, Exciton-polariton flows in cross-dimensional junctions, Phys. Rev. B 95, 201302(R) (2017).
S. Klembt, T. H. Harder, O. A. Egorov, K. Winkler, H. Suchomel, J. Beierlein, M. Emmerling, C. Schneider, and S. Höfling, Polariton condensation in S- and P-flatbands in a two-dimensional Lieb lattice, Appl. Phys. Lett. 111, 231102 (2017).
M. Klaas, H. Flayac, M. Amthor, I. G. Savenko, S. Brodbeck, T. Ala-Nissila, S. Klembt, C. Schneider, and S. Höfling, Evolution of Temporal Coherence in Confined Exciton-Polariton Condensates, Phys. Rev. Lett. 120, 017401 (2018).
H. Suchomel, S. Klembt, T. H. Harder, M. Klaas, O. A. Egorov, K. Winkler, M. Emmerling, S. Höfling, C. Schneider, An electrically pumped polaritonic lattice simulator, arXiv:1803.08306 (2018).