Two-Dimensional Semiconductors for Chiral Nanophotonics

The interplay of light and matter at the nanoscale has spurred recent advancements across a broad swath of diverse disciplines, from biosensing and energy harvesting to quantum information science and optical engineering. Confined to these small length scales – somewhere between the extent of a few atoms and the wavelength of visible light – materials and photons begin to look strikingly different than in their "bulk" counterparts. This dissertation builds on and bridges the promising research on two such systems: atomically thin semiconductors and chiral nanophotonics. Two-dimensional transition metal dichalcogenides – van der Waals semiconductors only three atoms thick – host tightly bound, optically bright excitons and strongly interacting electrons with spin and valley degrees of freedom addressable via circularly polarized light. Chiral nanophotonics exhibit guided modes with circularly polarized evanescent waves, wherein the electric field ellipticity is strictlylocked to the propagation direction. Leveraging these properties, an integrated TMDC-nanophotonic interface is demonstrated for on-chip, active routing of excitonic emission into a waveguide. Additionally, long-range spin polarization of itinerant electrons, which arises from interaction-driven magnetic ordering, is generated by microwatt optical pumping. Together, these results establish a novel route for low-power, integrated non-reciprocal photonics and provide new tools for optoelectronic, valleytronic, and spintronic technologies.