Magnetic Response of Atomically Thin Mirrors Probed with Many-Body States

The thesis focuses on the interaction of light with matter and how light-matter coupling reveals many-body physics in transition metal dichalcogenides (TMDs) monolayers under an applied magnetic field. Stacking of TMDs, hexagonal boron nitride (h-BN) and graphene, was optimized in order to reduce the inhomogeneities of the produced heterostructures. Homogeneous h-BN encapsulated molybdenum diselenide (MoSe2) and tungsten diselenide (WSe2) monolayers with graphene gates were produced while optimizing stacking temperatures and speed. Exfoliation and stacking under inert argon atmosphere finally resulted in state of the art samples. Two dimensional photoluminescence maps showing homogeneous emission wavelength and intensity as well as homogeneous doping proved high quality of the samples. Atomic force microscope measurements confirmed the high homogeneity by showing very little variations in height on the heterostructures. Resonant white light reflection and transmission measurements were done on h-BN encapsulated MoSe2 monolayer samples at 4 Kelvin. The ratio of the radiative decay to the total exciton line broadening was extracted and demonstrated to be ∼1, implying a very small non radiative recombination and confirmed the high optical quality of the samples. In addition, this has the remarkable consequence that a monolayer TMD can be used as an atomically thin mirror. A reflectivity for light resonant with the exciton resonance of up to 72% and an extinction coefficient of the transmitted light reaching 97% were obtained. The reflectivity of such a mirror could be electrostatically turned on and off by applying a gate voltage. The valley degree of freedom of exciton-polarons was used to realize a chiral mirror, reflecting only right- or left-hand circularly polarized light. Magnetic response of MoSe2 monolayers was investigated with an optical probe, making use of exciton-polarons. To show that polaron formation is limited to the presence of itinerant charge carriers, optical measurements were combined with transport measurements. Exposing a monolayer MoSe2 to a magnetic field of Bz = 7 Tesla allowed to valley polarize itinerant charge carrier densities of ∼ 1 × 10^12 cm^−2 . This remarkable observation is indicative of a giant magnetic susceptibility of MoSe2 monolayers, presumably coming from exchange interactions. The differences between electron and hole doping in WSe2 monolayers were investigated without and with the presence of an out of plane magnetic field. In the latter case the amount of valley polarized charge carriers was extracted and the associated effective electronic g-factor was calculated. Surprisingly the presence of Landau quantization of charge carriers manifested itself by splitting in energy of the ”third” polaron resonance that appeared only at higher electron densities, whereas for hole doping no energy splitting of the attractive polaron resonance was observed