Ultrafast Dynamics of 2D Materials and their Heterostructures

The extreme confinement and reduced screening in monolayer Transition Metal Dichalcogenides (TMDs) leads to the appearance of tightly bound excitons which can also couple to free charges, forming trions, owing to strong Coulomb interactions. Low temperatures and encapsulation in hexagonal Boron Nitride (hBN) can narrow the excitonic linewidth, approaching the regime of homogeneous broadening, mostly dominated by the radiative decay. Ultrafast spectroscopy is a powerful tool to study exciton formation and relaxation dynamics in TMD monolayers. However, high-quality hBN-encapsulated structures have usually lateral sizes of the order of a few microns, calling for the combination of high spatial and temporal resolution in pump-probe experiments. Here we use a custom broadband pump-probe optical microscope to measure the ultrafast dynamics of excitons and their complexes in high-quality hBN-encapsulated monolayer and bilayer of MoS2 at 8K. In monolayers, neutral excitons with a narrow linewidth of 7.5 meV, approaching the homogeneous limit, show very fast recombination time of ∼ 130 fs measured in pump-probe. Moreover, we observe markedly different dynamics of the trions and biexcitons over the neutral ones. Finally, we investigate the static and dynamic nonlinear behavior of intra and interlayer excitons in bilayers, showing very peculiar and unprecedented inter-species scattering effects, linked to the charge tunneling between the layers.