Σπιν-Valley πόλωση στα διχαλκογενίδια μετάλλων μετάπτωσης

After the discovery of graphene, new horizons opened up in the scientific community for two-dimensional materials. Their exceptional physical and optical properties, as they become thinner and thinner, have attracted an extensive scientific interest. As graphene is not so suitable for optoelectronic applications due to the zero bandgap, there is another category of materials more suitable for such applications. This category of materials is called Transition Metal Dichalcogenides (TMDS) which are semiconductors and therefore are presented as alternatives to graphene. Thin layers of these materials have a direct-gap at the K-point of the Brillouin zone with a range of bandgaps in the visible spectrum making them suitable for optoelectronic applications. A strong photoluminescence (PL) emission is created, as an indirect-todirect transition occurs, when they are thinned down to a monolayer level. In this thesis, temperature-dependent spin-valley polarization studies on monolayer WS2 samples with two different laser excitation sources, continuous wave (CW) and pulsed laser are presented. The spin-valley polarization of the monolayer WS2 with CW laser excitation is well-known and it has been practiced by many groups. The basic purpose was to compare the data of the same experiment but with different excitation sources. The spin-valley polarization with pulsed laser is a little more difficult to carry out, because it was necessary to have the same wavelength in both lasers. With various optical devices and filters, this was finally achieved. At the same time some temperature dependence photoluminescence (PL) experiments took place in order to characterize the behavior of the excitonic complexes involved in the emission process. Α series of two photon photoluminescence (2PPL) experiments were also performed simultaneously; in addition to the A-exciton emission of WS2, the second harmonic generation (SHG) signal was also recorded. During this study, the behavior of SHG signal was investigated while changing the repetition rate in pulsed laser settings. Finally, Transient Absorption Spectroscopy (TAS) experiments were performed to probe the exciton lifetime, in order to shed light in the behavior of the spin-valley polarization under different excitation laser sources