Strong light-matter coupling and room temperature exciton polaritons in atomically-thin WS2 crystals

Abstract

Monolayer transition metal dichalcogenide crystals (TMDCs) hold great promise for semiconductor optoelectronics because their bound electron-hole pairs (excitons) are stable at room temperature and interact strongly with light. When TMDCs are embedded in an optical microcavity, their excitons can hybridise with cavity photons to form exciton polaritons (polaritons herein), which inherit useful properties from their constituents. For example, the low effective mass inherited from the photonic component enables polaritons in principle to macroscopically occupy their ground state at room temperature, i.e., to form a polariton condensate. Such a polariton condensate can behave like a superfluid and hence, polariton condensation in TMDCs might be a route towards dissipationless transport of information carriers on a microchip. However, because of the low effective interactions between the excitons in TMDCs, spontaneous formation of a polariton condensate in a single monolayer is challenging and clear evidence was not demonstrated yet. In other material systems it was shown that maximising the polariton lifetimes and increasing their density through strong confinement can help to reach this regime. A prevailing approach to generate polaritons with large lifetimes in conventional semiconductors are all-dielectric planar microcavities, in which polaritons can be spatially confined by patterning the cavity spacer. However, integrating monolayer TMDCs in these high-quality structures remains a challenge since they are notoriously fragile and their excitonic properties are extremely sensitive to many nanofabrication techniques. This thesis presents experimental work performed with the aim to integrate TMDCs in high-quality all-dielectric microcavities and create optimal conditions for driving the system to the regime of bosonic condensation at room temperature. In particular, we focus on monolayer WS2, which has a high exciton quantum yield and displays strong light-matter interaction at room temperature. First, the challenge presented of integrating monolayer WS2 into high-Q microcavities without causing damage to the monolayer is overcome by mechanical assembly. We show that WS2 polaritons in such a microcavity can propagate ballistically over tens of micrometres in the thermal regime, before the onset of condensation, and possess enhanced macroscopic coherence due to strong motional narrowing and weak inter-particle interactions. However, the mechanical assembly process of the microcavities is intrinsically non-scalable. To enable the integration of TMDC monolayers into functional devices on larger scales, we developed a new passivation and protection technology utilising liquid-metal printed, ultrathin Ga2O3 glass. We show that the Ga2O3 film strongly suppresses exciton-exciton annihilation in monolayer WS2, which prohibits large exciton densities in blank TMDC monolayers, and that it provides excellent protection against dielectric material deposition. The latter allows us to integrate the monolayer into all-dielectric environments with conventional deposition techniques while maintaining its high optical performance. Finally, we engineer an effective trapping potential for WS2 polaritons at room temperature by placing a WS2/Ga2O3/WS2 structure with a small top layer inside a microcavity. This design allows us to compare the properties of trapped and free polaritons. Remarkably, the ground state emission and the macroscopic coherence is strongly enhanced when trapped due to efficient energy relaxation, long photon lifetimes and strong motional narrowing. Overall, this thesis provides significant insights into the properties and dynamics of free and trapped WS2 polaritons in the thermal regime at room temperature, which can guide future work towards demonstrating unambiguous signatures of polariton condensation in this novel material class.