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 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 experiments which we carry out in the project are based on thin layers embedded in high quality photonic heterostructures providing optical confinement:

Firstly, we exploit the combination of ultra-large exciton binding energies, giant absorption and unique spin properties of such materials to study microcavity exciton polaritons. These composite bosons provide the unique possibility to study coherent quantum fluids up to room temperature. It is our believe that establishing bosonic condensation effects in atomic monolayers can lead to a paradigm shift in polaritonics.

Secondly, we study 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, hence we investigate possibilities of tuning the spectral properties of these localizations via external electric and strain-fields, to gain position control and make use of them as sources of single, indistinguishable photons.

In our project, we explore twisted bilayers of transition metal dichalcogenides in optical microcavities, together with our partners Dr. C. Gies (University of Bremen) and Prof. S. Reitzenstein (TU Berlin).

Together, we explore:

Material and device platforms for the integration of high-quality TMD hetero-bilayers in high-quality microcavities: We will realize high-quality monolayers and heterostructures by mechanical exfoliation and sophisticated transfer-methods, beyond simple dry-stamping methods.

Condensation and expansion of cavity polaritons based on Moiré excitons: We will particularly focus on the behaviour of Moiré excitons, which strongly couple to cavity modes in the limit of large particle densities controllable by the twist angle. We will explore the emergence of coherence in such systems via spatially imaging and interferometry- as well as quantum optical spectroscopy methods. We will explore the perspective to the evolution of chiral, topological polariton modes in the hetero-bilayer cavity coupled system. 

Quantum Correlations in arrays of Moiré-trapped excitons: This new class of excitons forms a solid-state analogue to an optical lattice of strongly correlated spinor bosons. Thus, it represents a completely new, solid-state platform to study collective quantum phenomena of non-linear bosons in lattices. We will test the collective behaviour of such emitter arrays, and explore their coherent coupling to a joint cavity resonance.