Espectroscopia de femtossegundos e propriedades ópticas não lineares em nanomateriais bidimensionais

Abstract

Two-dimensional materials are crystalline structures with just a few atoms thick. The graphene, as well as molybdenum disulde (MoS2) are two well-known materials by this feature. In this thesis, we present how these nanostructures were investigated using spectroscopy and optical microscopy techniques to clarify about the energy dissipation mechanisms of electrons in graphene and the nonlinear optical interaction with few-layered MoS2 thin lms. Excited electrons in graphene lose energy due to various scattering processes. However, little is known about the efects of increasing disorder in the description of the electron dynamics in graphene. We investigated the dynamics of photo-excited electrons in graphene with diferent densities of defects using ultrafast optical spectroscopy in femtoseconds. Thus we obtain the energy relaxation time for these charge carriers and its dependence on the defect density. We observed a linear relationship between the scattering rate and the defect density. In addition, through a nonlinear optical microscopy, we measured the hot photoluminescence from graphene analyzing the electron temperature dependence on the defect density. These results can be explained using a theoretical model of supercollision. We also investigated the second-harmonic generation (SHG) in samples of MoS2 by nonlinear optical microscopy. We obtained SHG images for few layers MoS2 exfoliated samples and we observed SHG for regions with odd number of layers. By Analyzing the intensity of the SHG with the polarization and energy of the laser allowed to study the crystallography of this two-dimensional material.