Engineering Excitonic Resonances and Light-Matter Interactions in 2D Transition Metal Dichalcogenides

Abstract

This thesis investigates the optical properties of transition metal dichalcogenide (TMD) monolayers. These ultrathin semiconductors are highly suitable for on-chip optical devices due to their ease of integration on existing photonic platforms, broadband applicability, strong excitonic effects at room temperature, and large second-order nonlinear susceptibility. Their peculiar symmetry properties further lead to helicity-dependent selection rules in the ±K valleys of the Brillouin zone, opening the field of valleytronics. Despite the plethora of interesting characteristics, there is a lack of promising tuning knobs to modulate them on ultrafast timescales. This work advances the state of the art by investigating all-optical band gap modulations, caused by the optical Stark and Bloch-Siegert shifts, and probing their impact using nonlinear optical processes. Depending on the polarization state of the excitation laser these effects lead to different observations. Linearly polarized light causes a symmetric band gap opening in ±K, thus inducing a shift of the excitonic resonance. This is quantified by evaluating intensity-dependent second-harmonic generation measurements in combination with an analytical model and numerical simulations, which further allow to extract fundamental materials parameters. In contrast to the linear excitation, circularly polarized light introduces an asymmetric shift, causing the breaking of the underlying time-reversal symmetry, which is quantified by both polarization-resolved second-harmonic generation and by comparing the intensity ratio of circularly to linearly polarized second-harmonic emission, also in agreement with the analytical model. Furthermore, this thesis provides new insights into how light-matter interactions in TMDs can be further tailored in hybrid structures. First, the work investigates the recombination dynamics of excitons in TMD samples with different doping levels, leveraging potassium doping and charge transfer to few-layer graphene. A simple rate equation model is used to explain the influence of trions and exciton-exciton annihilation in photoluminescence measurements in different excitation intensity regimes. Second, it suggests a TMD/metasurface system to simplify the investigation of spin-forbidden dark excitons. Simulations of a metasurface with particular geometry show that such a structure can be used to excite these quasi-particles under normal incidence and further redirect their photoluminescence emission. All together, this thesis highlights the role of TMD monolayers as a promising basis for ultrafast photonic devices and also a versatile playground for fundamental light-matter interactions. Part of the results presented within this thesis have been originally published in Refs. [1–5].