Open Collections

Abstract

Recently, parallel-stacked bilayer transition metal dichalcogenides (TMDs) with rhombohedral stacking (3R) have been the focus of interest due to their broken inversion and mirror symmetries. This gives rise to intriguing phenomena such as the polarization-induced spontaneous photovoltaic effect and sliding ferroelectricity. Additionally, the asymmetric interlayer coupling leads to an effective type-II band alignment at the K points. Despite the widely studied TMD heterostructures with similar band alignments, linear spectroscopy methods are unable to efficiently probe the ultrafast charge transfer in rhombohedral-stacked bilayer TMDs with a small energy splitting on the order of 10 meV. In this thesis, we study the exciton dynamics of rhombohedral bilayer MoSe₂ (2L R-MoSe₂) via multidimensional coherent spectroscopy (MDCS). This nonlinear optical method based on four-wave mixing (FWM) enables access to exciton-exciton interactions on the sub-100 fs timescale. We measure one-quantum rephasing (1QR) spectra, which show the splitting of intralayer exciton resonances as well as their couplings via cross peaks. The time evolution of the FWM in the 1QR measurements reveals that these peaks emerge from incoherent interactions, namely from energy and monodirectional charge transfers. We find that an ultrafast interlayer electron transfer occurs within 50±10 fs upon photoexcitation from the upper to the lower layer. In addition, an energy transfer in the opposite direction on a timescale of <50 fs is discovered, limited by the resolution of our experimental setup. This result was not reported before in homobilayer TMDs. Finally, we explore the mechanisms that lead to scatterings between optically bright and dark excitons, which eventually lead to the recombination of the momentum-indirect Γ-K intervalley excitons on the time scales ranging from a few to tens of picoseconds.