Valley Dynamics and Tailored Light-matter Interaction in Two-dimensional Transition Metal Dichalcogenides

Two-dimensional transition metal dichalcogenides (TMDCs), with direct bandgaps in the visible to near-infrared wavelength, offer a tantalizing platform for making optoelectronic devices with enhanced and novel functionalities. In this dissertation, we explore the valley dynamics and various excitonic states in monolayer TMDCs, as well as demonstrate tunable and enhanced exciton emission using plasmonic nanocavities. First, we probe the origin of the excitonic and localized states in monolayer WSe2 using polarization-resolved PL spectroscopy at temperatures from 10 K to room temperature. Next, Kerr rotation experiments are used to investigate the temporal and spatial valley dynamics of monolayer TMDCs using femtosecond pump and probe pulses.

Despite these remarkable optical properties, atomically thin TMDC monolayer suffer from intrinsically weak light absorption (~3 %) and low photoluminescence (PL) quantum yield (~0.4 %). Furthermore, among the complex excitonic states of monolayer TMDCs, the B exciton emission is inherently weak compared to the dominant A exciton emission. Thus, we demonstrate a tunable plasmonic nanocavity where emitters are sandwiched in a sub-10-nm dielectric gap between a metallic film and colloidally synthesized silver nanocubes. When emitters with an intrinsic long lifetime are embedded in the gap region, the spontaneous emission rate enhancements can be exceeding 1,000 times while the structure maintains a high quantum efficiency (>50 %) and directional emission. Incorporating semiconductor quantum dots into the plasmonic cavity enable ultrafast spontaneous emission with emission rates exceeding 90 GHz. Finally, when MoS2 monolayers are integrated into this plasmonic nanocavity with tunable plasmon resonances, we observe a 1,200-fold enhancement for the A exciton emission and a 6,100-fold enhancement for the B exciton emission. Moreover, we show a strong modification of the PL emission peaks, which exhibits a strong correlation between the emission wavelengths and the nanocavity resonance. Manipulating the optical properties of these 2D materials using tunable plasmon resonances is promising for the design of novel optical devices with precisely tailored responses, which is critical for optimizing the performance of future optoelectronic and nanophotonic devices.