We have studied the optical properties of WSe2 monolayers (ML) by means of photolu-minescence (PL), PL excitation (PLE) and Raman scattering spectroscopy at room temperature and as a function of hydrostatic pressure up to ca. 12 GPa. For comparison, the study comprises two cases: A single WSe2 ML directly transferred onto one of the diamonds of the diamond anvil cell and a WSe2 ML encapsulated into hexagonal boron nitride (hBN) layers. The pressure dependence of the A and B exciton, as determined by PL and PLE, respectively, is very different for the case of the bare WSe2 ML and the hBN/WSe2-ML/hBN heterostructure. Whereas for the latter the A and B exciton energy increases linearly with increasing pressure at a rate of 3.5 to 3.8 meV/GPa, for the bare WSe2 ML the A and B exciton energy decreases with a coefficient of -3.1 and -1.3 meV/GPa, respectively. We interpret that this behavior is due to a different stress situation. For a single ML the stress tensor is essentially uniaxial with the compressive stress component in the direction perpendicular to the plane of the ML. In contrast, for the substantially thicker hBN/WSe2-ML/hBN heterostructure, the compression is hydrostatic. The results from an analysis of the pressure dependence of the frequency of Raman active modes comply with the interpretation of having a different stress situation in each case.

We have investigated the electron and hole spin dynamics in p-doped semiconductor InAs/GaAs quantum dots by time resolved photoluminescence. We observe a decay of the average electron spin polarisation down to 1/3 of its initial value with a characteristic time of T<FONT FACE=Symbol>D »</FONT> 500ps. We attribute this decay to the hyperfine interaction of the electron spin with randomly orientated nuclear spins. Magnetic field dependent studies reveal that this efficient spin relaxation mechanism can be suppressed by a field of the order of 100mT. In pump-probe like experiments we demonstrate that the resident hole spin, "written" with a first pulse, remains stable long enough to be "read" 15ns later with a second pulse.