Low Temperature Steady State Nonlinear Spectroscopy of Excitons in Monolayer MoSe2

Transition metal dichalcogenides (TMDs), such as MoSe2, can be thinned to a single layer molecular monolayer and combined with numerous atomically flat van der Waals materials to form novel heterostructures as a potential basis for quantum materials1–3 and photoelectrodes for solar cells4,5. Monolayer TMDs exhibit tightly bounded excitons in both neutral and charged species, spin-valley physics, and strong non-linear properties when probed at the neutral and charged exciton (trion) resonances. Previous non-linear optical measurements have consisted of transient response from ultrafast pump lasers, however a systematic study of the steady state non-linear response is lacking.In this thesis, I will discuss the fundamentals of two-dimensional van der Waals materials in Chapter 1. In Chapter 2, I discuss the fundamentals of non-linear optics as applied to 2-, 3-, and 4-level systems, as a method to model the non-linear optical response of MoSe2. In Chapter 3, I discuss fabrication methods. In Chapter 4, I discuss our optical setups used to measure both linear and non-linear response of a TMD heterostructure. In Chapter 5, I discuss the steady-state non-linear spectroscopy using a modulated two-color pump-probe system described in Chapter 4. We measure the non-linear response of hBN-encapsulated MoSe2 We observe differential transmission (DT) signals associated with both the neutral exciton and the intrinsically doped trion response, showing a strong dependence on the intensity and polarization of the pump and probe beam. A non-linear signal was observed at the trion resonance that has not appeared in previous work. The results are first compared to a theoretical model based on a multi-level system, where the signal was compared to a biexciton resonance. We further explore these results by analyzing the results of a T-matrix formulation based on the exciton-exciton, exciton-trion, and trion-trion correlations. The parameters in the T-matrix formalism can be tuned to reproduce the experimental DT results. The theory remains consistent with the assumption of exciton-trion correlations, the absence of a bound biexciton, and strong spin-mixing through incoherent spin-valley relaxation