Valley polarization in stacked MoS₂ induced by circularly polarized light

Manipulation of valley pseudospins is crucial for future valleytronics. The emerging transition metal dichalcogenides (TMDs) provide new possibilities for exploring the interplay among the quantum degrees of freedom, including real spin, valley pseudospin, and layer pseudospin. For example, spin–valley coupling results in valley-dependent circular dichroism in which electrons with particular spin (up or down) can be selectively excited by chiral optical pumping in monolayer TMDs, whereas in few-layer TMDs, the interlayer hopping further affects the spin–valley coupling. In addition to valley and layer pseudospins, here we propose a new degree of freedom—stacking pseudospin—and demonstrate new phenomena correlated to this new stacking freedom that otherwise require the application of external electrical or magnetic field. We investigated all possible stacking configurations of chemical-vapor-deposition-grown trilayer MoS₂ (AAA, ABB, AAB, ABA, and 3R). Although the AAA, ABA, 3R stackings possess a sole peak with lower degree of valley polarization than that in monolayer samples, the AAB (ABB) stackings exhibit two distinct peaks, one similar to that observed in monolayer MoS₂ and an additional unpolarized peak at lower energy. Our findings provide a more complete understanding of valley quantum control for future valleytronics. [Figure not available: see fulltext.]