Engineering excitonic resonances and light-matter interactions in 2D transition metal dichalcogenides

This thesis investigates the optical properties of transition metal dichalcogenide (TMD) monolayers. These ultrathin semiconductors are promising for on-chip optical devices due to their broadband applicability, strong excitonic effects at room temperature, and large second-order nonlinear susceptibility. Their symmetry properties further lead to helicity-dependent selection rules in the ±K valleys of the Brillouin zone. Despite these appealing properties, efficient tuning knobs for ultrafast modulation remain scarce. This work investigates all-optical band gap modulations caused by optical Stark and Bloch-Siegert shifts and their impact on nonlinear optical processes. Excitation with linearly polarized light causes a symmetric band gap opening in ±K, shifting the excitonic resonance. This is quantified by intensity-dependent second-harmonic generation (SHG) measurements combined with an analytical model and numerical simulations. In contrast, circularly polarized light introduces an asymmetric shift, breaking the underlying time-reversal symmetry. This is quantified by polarization-resolved SHG and by comparing the intensity ratio of circularly to linearly polarized SH emission. Further, this thesis provides insights into tailoring light-matter interactions in hybrid TMD structures. First, the recombination dynamics of excitons in TMD samples with different doping levels are studied. A rate equation model explains the influence of trions and exciton-exciton annihilation in photoluminescence measurements in different excitation regimes. Second, a TMD/metasurface system is proposed to simplify the investigation of spin-forbidden dark excitons. Simulations show that a metasurface with suitable geometry enables excitation of these quasi-particles under normal incidence and redirects their emission. Overall, this thesis highlights TMD monolayers as a promising platform for ultrafast photonic devices and a versatile playground for fundamental light-matter interactions.