Ultrafast quasiparticle dynamics and the role of screening in WS${}_2$ monolayers

The optical properties of transition metal dichalcogenides (TMDC) are dominated by excitons, due to quantum confinement and reduced screening characteristic of their 2D nature. Exactly the screening of the Coulomb interaction (CIA) has a fundamental role in determining the steady-state and dynamic properties of such material. Excitons are particularly sensitive to the dielectric screening of a substrate or, even to dynamic screening originating from the presence of quasiparticles which can be transiently present in the system upon photoexcitation for instance. Time resolved optical spectroscopies are a fundamental tool to investigate non-equilibrium physics of TMDC materials. Previous studies evidenced how different phenomena, such as dynamic screening of the CIA, phase-space filling, scattering, and thermal effects, contribute to the non-equilibrium response of TMDCs. Nevertheless, the comparison between previously reported studies, due to the different experimental conditions, is difficult if not impossible. Furthermore, the role of the screening in the non-equilibrium response of the TMDC is yet to be understood. I investigate monolayers WS${}_2$ placed on various substrates with time-resolved transmittance/reflectance contras. I report a simple formalism that allows the reliable comparison of the exciton static and dynamic response independently of sample, substrate and measurement technique. This is achieved by the retrieval of the exciton contribution to the dielectric function upon photoexcitation. Moreover, with this formalism I extracted the pump-photon energy and fluence-dependent exciton peak shift and broadening. These quantities are reproduced using a basic two/threelevel model which was found in good agreement with the data and supported by a good agreement of the extracted parameters with literature values. Through this model the competition of quasiparticle dynamic screening, scattering and thermal effects was unravelled. The excitons' or QFC's effective impact on the exciton dynamic response is unveiled: the broadening is governed by scattering, specifically QFC-exciton scattering when QFC are present in the system, and exciton-exciton scattering when excitons are present. Regarding the exciton shift, QFC induce a global red-shift of the exciton resonance, reproduced with an effective QFC dynamic screening-induced bandgap renormalization. Excitons, instead, induce a blueshift, which can be reproduced with an effective exciton dynamic screening induced binding energy reduction. It is concluded that the QFC induced red-shift is minor than the exciton-induced blue-shift, for the same excited quasiparticle density. Finally, the influence of the static screening on the non-equilibrium exciton response is addressed. It is found that the quasiparticle scattering and QFC dynamic screening are unaffected by a different dielectric environment. On the contrary, the exciton dynamic screening is more effective for higher dielectric permittivity of the substrate. This effect is tentatively attributed to a higher degree of delocalization of the exciton. Ultimately, this thesis contributes to a comprehensive picture of the non-equilibrium dynamics and the role of screening in TMDC and, possibly, other 2D materials.