Microcavity-enhanced light-matter interaction in van der Waals heterostructures

The recently emerging layered two-dimensional materials provide a new material class for novel opto-electronic devices. These materials have a unique crystal structure with strong intra-layer bonding and weak van der Waals inter-layer bonding. This allows to thin down the crystal to single atomic layer thickness using an adhesive tape. With the discovery of this method to produce stable monolayer sheets of graphene and the observation of its
remarkable properties, a new research area started to develop. Besides graphene there is a whole class of two-dimensional materials with similar crystal structure. One of the most prominent are transition-metal dichalcogenides, molybdenum and tungsten selenide and
sulphide. They are semiconducting materials that experience an indirect-to-direct bandgap transition when the material is thinned down to monolayer thickness. This change of the bandstructure leads to a remarkable increase in the emission efficiency of those materials in
monolayer form. Strong spin-orbit coupling, inversion symmetry breaking, large exciton binding energy and large oscillator strength means that this class of materials are very promising for future room temperature opto-electronic devices. In this work monolayer sheets of transition-metal dichalcogenides, as well as vertically stacked heterostructure of two-dimensional materials, are coupled to microcavity structures in order to study lightmatter interaction of these materials. A tunable open-acces microcavity structure has been developed to have full control of the light-matter interaction. In this system monolayer sheets of molybdenum disulphide have been studied, where the weak coupling regime
with a Purcell enhancement of a factor of 10 has been observed. Monolayer sheets of molybdenum diselenide have been investigated where the first conclusive demonstration of strong exciton-photon coupling is demonstrated. Finally, a light emitting diode, produced
by a heterostructure consisting of graphene, boron nitride and tungsten diselenide has been embedded in a microcavity structure where a significant change in the emission pattern of photo- and electroluminescence has been demonstrated.