Attractive and repulsive exciton-exciton interaction in bilayer MoS₂

Transition metal dichalcogenide (TMD) bilayers provide a promising platform to study interaction-driven physics. In particular, optical spectroscopy of bilayer TMD structures unveils excitonic coupling effects similar to semiconducting coupled quantum wells (CQWs). Excitons in bilayer TMDs can be sorted into two categories: Intralayer and interlayer excitons. For intralayer excitons, the Coulomb-bound electron-hole pair resides in the same layer while for interlayer excitons the electron and hole reside in different layers. The interaction of intralayer and interlayer excitons is studied in a two-dimensional semiconductor, homobilayer MoS₂. The excitonic interaction is well-described by a model of two coupled optical dipoles with different oscillator strength driven by a light field. Applying the model to the excitonic absorption reveals the nature of the excitonic coupling, attractive or repulsive. While the interlayer excitons interact attractively with the A-excitons, they interact repulsively with the B-excitons. Our model also predicts constructive interference in one eigenmode ("bright"), destructive interference in the other eigenmode ("dark"), near the energetic crossing of the bare states. We argue that this is a general feature of coupled excitons.