  Valley-selective optical selection rules and a spin-valley locking in
transition-metal dichalcogenide (TMDC) monolayers are at the heart of
"valleytronic physics", which exploits the valley degree of freedom and has
been a major research topic in recent years. In contrast, valleytronic
properties of TMDC bilayers have not been in the focus so much by now. Here, we
report on the valleytronic properties and optical characterization of bilayers
of WS2 as a representative TMDC material. In particular, we study the influence
of the relative layer alignment in TMDC homo-bilayer samples on their
polarization-dependent optical properties. Therefore, CVD-grown WS2 bilayer
samples have been prepared that favor either the inversion symmetric AA'
stacking or AB stacking without inversion symmetry during synthesis.
Subsequently, a detailed analysis of reflection contrast and photoluminescence
spectra under different polarization conditions has been performed. We observe
circular and linear dichroism of the photoluminescence that is more pronounced
for the AB stacking configuration. Our experimental findings are supported by
theoretical calculations showing that the observed dichroism can be linked to
optical selection rules, that maintain the spin-valley locking in the
AB-stacked WS2 bilayer, whereas a spin-layer-locking is present the inversion
symmetric AA' bilayer instead. Furthermore, our theoretical calculations
predict a small relative shift of the excitonic resonances in both stacking
configurations, which is also experimentally observed.
