Light-emitting nanostructures for control of valley-selective exciton emission

The overarching goal of this thesis has been the photonic integration of monolayer transition metal dichalcogenides (TMDs) into light-emitting nanostructures, using resonant nanoantennas and their periodic arrangements into metasurfaces and arrays as the photonic platform. Resonant nanoantennas concentrate light from free-space radiation (farfield) into subwavelength volumes (nearfield), strongly enhancing the local field intensity. As a result, they provide a versatile platform for enhancing light–matter interaction of quantum emitters placed in their near field. For active nanophotonics, robust and scalable emitter platforms that also offer spectral tunability and polarisation control are crucial. Semiconducting monolayer TMDs have emerged as an attractive material system in this respect. Despite their atomic thickness, they can be prepared as stable, single-crystalline layers and exhibit bright direct-bandgap photoluminescence, strong nonlinear response, and electrical and optical tunability. Two aspects are particularly relevant for this thesis: (1) their optical response is dominated by tightly bound excitons, leading to strong resonances in the visible and near-infrared; and (2) their crystal symmetry and strong spin-orbit coupling give rise to a valley degree of freedom, in which excitons can be prepared in spin-selective valleys using circularly polarised light. Thus, monolayer TMDs provide a direct optical handle on valley-selective exciton populations and are a natural candidate for valleytronic light sources. The results presented in this thesis provide new insight into nanoscale light-valley interactions and delineate how resonant nanoantennas and metasurfaces can be used to engineer the excitation, emission, and directional properties of monolayer TMDs. This advances the understanding and design of light-emitting nanodevices that harness the valley pseudospin as an additional degree of freedom.