Seminarium Fizyki Ciała Stałego

Understanding the fundamental properties of exciton physics in the presence of 2D carrier gas in Transition Metal Dichalcogenides (TMDs) has been the subject of interest for many works in recent years. The development of electrostatically-gated heterostructures of these materials facilitated precise control of free carrier density, which opened the way for the studies of strong exciton-carrier interactions present in TMDs, while also leading to such milestone achievements in the field of correlated electron systems like the observation of the Wigner crystal phase or the significant enhancement of the magnetic susceptibility. Despite, however, all of this progress, there are still limited investigations into the particular influence of the Fermi sea of carriers on the optical response and dynamics of the exciton states, especially in the ultrafast limit. In this work, I will present the results of resonant pump-probe spectroscopy measurements of MoSe2 monolayer heterostructures with tunable electron density. This experimental approach will allow me to showcase various optical effects stemming from exciton-carrier interactions in two dimensions. Special focus will be put on the influence of free electrons on different relaxation mechanisms, and how their presence leads to increased decay rates of excitons and intervalley scattering of carriers. Furthermore, by presenting comparative measurements of heterodyne Four-Wave-Mixing spectral interferometry, I will also discuss the dependence of coherent exciton properties on the increasing density of the Fermi sea. Analysis of the observed changes in the homogeneous and inhomogeneous broadenings of exciton transitions shows that free carriers lead to increased exciton decoherence rates and screening of the spatial dielectric disorder. This valuable finding may also explain the observed strengthening of the coherent coupling between charged and neutral exciton states.