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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - unLiMIt-2D (Unique Light-Matter Interactions with Two-Dimensional Materials)

Teaser

Summary

Controlling light- and matter excitations down to the microscopic scale is one major challenge in modern optics. Applications arising from this field, such as novel coherent- and quantum light sources have the potential to affect our daily life. One particularly appealing material platform in quantum physics consists of monolayer crystals. The most prominent species, graphene, however remains rather unappealing for photonic applications due to the lack of an electronic bandgap in its pristine form. Monolayers of transition metal dichalcogenides and group III-VI compounds comprise such a direct bandgap, and additionally feature intriguing spinor properties, making them almost ideal candidates to study optics and excitonic effects in two-dimensional systems.
unLiMIt-2D aims to establish these materials as a new platform in solid-state cavity quantum electrodynamics. The targeted experiments will be based on thin layers embedded in high quality photonic heterostructures providing optical confinement.
unLiMIt-2D will exploit the combination of ultra-large exciton binding energies, giant absorption and unique spin properties of such materials to form microcavity exciton polaritons. These composite bosons provide the unique possibility to study coherent quantum fluids up to room temperature. Due to the possibility of fabricating such structures by relatively simple means, establishing bosonic condensation effects in atomic monolayers can lead to a paradigm shift in polaritonics.
Secondly, the project will Focus on exciton localization in layered materials, with the perspective to establish a new generation of microcavity-based quantum light sources. Light-matter coupling effects will greatly improve the performance of such sources.

Work performed

Within the reporting period, the PI and his Team have concluded a variety of breakthrough results in the field light-matter coupling with atomically thin materials. The Team could show, that it is possible to reach the strong coupling Regime of exciton and Photons in a microcavity with a single embedded monolayer (N. Lundt et al. 10.1038/ncomms13328), and they demonstrated the valley selective strong coupling at cryogenic, (N. Lundt et al. 2D Materials 10.1088/2053-1583/aa6ef2), as well as at room temperature (N. Lundt et al. Pysical Review B 10.1103/physrevb.96.241403). The Team furthermore demonstrated the Formation of so-called hybrid polaritons composed of III-V and TMDC excitons (M. Wurdack et al. Nature Communications 10.1038/s41467-017-00155-w) and subsequently the bosonic condensation of exciton-polaritons in a hybrid cavity at cryogenic temperature (M. Waldherr et al. Nature Communications 10.1038/s41467-018-05532-7).

The ERC Team identified techniques to Isolate single excitons in WSe2 monolayers via strain Engineering (O. Iff et al. Optics Express 10.1364/oe.26.025944) and subsequently, the coupling of a strain-localized single exciton (single Photon sources) to a plasmonic resonance (L. Tripathi et al. ACS Photonics 10.1021/acsphotonics.7b01053). Further, the Team demonstrated the formation of Biexciton states via Quantum corelations (He et al. Nature Communications 10.1038/ncomms13409 28)

Final results

The results indicated in the section above indeed all go well beyond the state of the art, yielding original journal publications. Altogether, within the first 2.5 years of the Project, 11 peer reviewed papers were published by the team.
The Formation of a bosonic condensate in a microcavity with a monolayer Crystal (M. Waldherr et al. Nature Communications 10.1038/s41467-018-05532-7) is considered as a Landmark Experiment for the field. It is anticipated, that the temperature of bosonic condensation in monolayer-based cavities will be increased up to room temperature until the end of the Project, and that the interplay between spin-valley effects, magnetic fields and bosonic condensates will reveal new physics associated with Transition metal dichalcogenides.

On the other Hand, while the ERC Team has now repeatedly demonstrated the implementation of single photon sources based on WSe2 monolayers, the accomplished first cavity experiments outline following steps in the Project. This includes the imrovement of the extraction efficiencies from the selected structures by improving the light-matter coupling, as well as the coherence of the emitted Photons by exploiting electrstatic gating in the high-Purcell regime.