EXCITON MANIPULATION IN 2D SEMICONDUCTOR HETEROSTRUCTURES
The discovery of graphene marked the start of research in 2D electronic materials which was expanded in new directions with MoS2 and other layered semiconducting materials. They have a wide range of interesting fundamental properties and potential applications. New opportunities are enabled by the band structure of transition metal dichalcogenides (TMDCs) in which we could harness the valley degree of freedom for valleytronics and next-generation photonics. Long-lived interlayer excitons in van der Waals heterostructures based on TMDCs have recently emerged as a promising platform for this, allowing control over exciton diffusion length, energy and polarisation. I will show here how by using MoS2/WSe2 van der Waals heterostructures, we can realize excitonic transistors with switching action, confinement and control over diffusion length at room temperature in a reconfigurable potential landscape. On the other hand, the weak interlayer interaction and small lattice mismatch in MoSe2/WSe2 heterostructures results in brightening of forbidden optical transitions, allowing us to resolve two separate interlayer transitions with opposite helicities and meV-scale linewidths. These have opposite helicities under circularly polarized excitation, either preserving or reversing the polarization of incoming light. By using externally applied electrical fields, we can control their relative intensities and polarisation by different regions in the moiré pattern, characterized by different local symmetries and optical selection rules.
 A. Ciarrocchi, D. Unuchek, A. Avsar, K. Watanabe, T. Taniguchi, A. Kis. Nature Photonics 13, 131–136 (2019).
 D. Unuchek, A. Ciarrocchi, A. Avsar, K. Watanabe, T. Taniguchi, A. Kis. Room-Temperature Electrical Control of Exciton Flux in a van Der Waals Heterostructure. Nature 560, 340–344 (2018).