Valley-dependent Bloch-Siegert shift in monolayer WSe2: transition to the strong-field regime

Recently, two-dimensional transition metal dichalcogenides (2D TMDs) have attracted a lot of interest due to their potential applications in spin-/valleytronics. The strong spin-obit interaction leads to a large spin-orbit splitting of the states in the extrema of the conduction and valence bands and to the coupling between the spin and valley degrees of freedom (valley pseudospin). Additionally, the exciton binding energy is exceptionally high thanks to a weak screening of Coulomb interaction in 2D layers. Strong optical selection rules in the K and K´ points of the Brillouin zone allow to control the valley polarization of carriers via excitation by circularly polarized light. Apart from this direct access, the degeneracy between the two groups of valleys in K and K´ points can be lifted by an interaction with nonresonant circularly-polarized light fields via optical Stark effect [1] , [2] . The principle is based on the fact that the shift of the energy level of the excitonic resonance in the presence of circularly-polarized light fields with small detuning of the photon energy from the resonance occurs only when the selection rules are fulfilled. It is thus possible to remove the degeneracy between the two pseudospin levels via a virtual process, which does not excite real populations.