Two dimensional semiconductors such as monolayer transition metal dichalcogenides (TMDs) and Lead iodide (PbI2) nanosheets exhibits remarkable optical properties due to their reduced dimensionality and crystal symmetry. The visible direct bandgap, robust valley polarization and strong tunability make them ideal for optoelectronic devices. The unique valley polarization can also be considered as a new degree of freedom, which enables new opportunities in the field of valleytronics. Manipulating the valley polarization by optical method is the key to realize valleytronic devices.

In this seminar, I will present how to spatially separate the photoluminescence from different valley of monolayer WSe2 by photonic nanostructures. I will also show the valley polarization at room temperature from bilayer WS2 by using optical excitation with vortex beams. Furthermore, I will show the nonlinear microscopy of PbI2 nanosheets. By using the nonlinear harmonic emission, we can precisely determine their thickness, strain and crystalline orientation with a non-invasive optical method.