Resolving Exciton Dynamics and Couplings in Atomically Thin Semiconductors with Multidimensional Spectroscopy

Monolayer transition metal dichalcogenides (TMDs) such as MoS2 feature strong light absorption, pronounced exciton physics at room-temperature, and an emergent "valley" degrees of freedom. These and other atomically thin materials have emerged as a growing library for constructing and tuning novel device architecture and optical properties up from the atomic limit. Realizing next-generation optoelectronics built around these materials requires an understanding and control over the excited state dynamics. In this dissertation, I employ coherent multidimensional spectroscopy to map the exciton dynamics and couplings in atomically thin semiconductors in an effort to disentangle the many-body interactions dictating the optical response on the femtosecond timescale. Leveraging broadband two-dimensional spectroscopy (2DES), I provide evidence that bandgap renormalization, a collective many-body effect on the exciton optical resonance, dominates on the sub-100 fs timescale over bound biexciton formation in monolayer MoS2 as a dynamic screening process. By following the exciton dynamics with simultaneous femtosecond and valley resolution using broadband helicity-resolved 2DES, I show that intervalley coupling occurs between all exciton states on the timescale of excitation (<10 fs). This coupling is largely insensitive to temperature, excitation fluence, and material grain size, pointing to a persistent and intrinsic picture of intervalley coupling distinct from dynamic scattering mechanisms and which poses large challenges for TMD-based "valleytronic" applications. To further probe excited state dynamics in two-dimensions, I investigate colloidal CdSe quantum well superlattices which feature high quantum yields, narrow emission linewidths, and atomic control over the layered well structure. Carrier relaxation in these materials occurs in two distinct steps, an initial femtosecond thermalization or delocalization followed by sub-picosecond carrier cooling before eventual bandedge emission. The work presented in this dissertation highlights the role of many-body effects and the initial ultrafast carrier dynamics on the optical properties of new materials, providing a set of design parameters for engineering excited state behavior to realize new functionalities for next-generation optoelectronic applications.