Interlayer excitons and polaritons in atomically thin van der Waals heterostructures

In two dimensional materials, arbitrary combinations of stacks are possible where each layer constitutes an atomically thin building block to engineer unprecedented properties. In monolayer form, 2D semiconductors show outstanding light-matter interactions, stemming from the confined nature of Coulomb-bound complexes known as excitons. When two Transition Metal Dichalcogenide (TMD) layers are brought together, new excitonic species of interlayer excitons (IXs) can be formed. Their properties can, hence, be engineered with material choice and stacking order. In Chapter 4, we stack pure monolayers of MoSe2 to single-layer MoxW1−xSe2 alloys, which allows a continuous tuning of the IX energy by more than 50 meV introducing a chemical degree of freedom. Profound modifications of the electronic band structure occur when the alloy composition become increasingly similar to the adjacent pure TMD. In addition, we are able to observe optical signatures of interlayer exciton hybridisation and moiré superlattice potentials. We therefore expand the design possibilities for atomically thin devices. In the next chapters, we employ encapsulated homobilayers of MoS2 to observe hybridised interlayer excitons (hIX) that uniquely combine high oscillator strength and promise for increased nonlinear interactions. Compared to excitons in MoS2 monolayers, hIX exhibit ≈ 8 times higher nonlinearity, which is further strongly enhanced when hIX and intralayer excitons, sharing the same valence band, are excited simultaneously. This provides access to an unprecedented highly nonlinear regime which, through collaborative work, we describe theoretically by introducing the concept of hole crowding. In Chapter 6, the aforementioned system is embedded in energy-tunable optical microcavities, in which the TMD excitons strongly couple to photonic modes, forming composite exciton-polaritons. This allows the realisation of dipolar polaritons (dipolaritons).We find dipolaritons to be an order of magnitude more nonlinear than monolayer polaritons, showing a rapid quenching of the Rabi splitting with increased density. The presented insight into many-body interactions can provide new avenues for accessing sought-after few-polariton quantum correlations.