The present semiconductor technology largely exploits the manipulation of the electronic charge through its entanglement with other degrees of freedom, such as spin, lattice and coupling with photons. Group-VI layered (2D) transition metal dichalcogenides (TMDs) like MoS2, MoSe2, WS2 and WSe2 are among the best materials for employing such scenario. Due to their layered hexagonal structure, when exfoliated to the atomic level, their electronic structure is dominated by two inequivalent valleys at the ±K points of the Brillouin zone, with opposite and complementary response in different valleys. This fact, together with the sizeable spin-orbit coupling and with the broken inversion symmetry of the monolayer compounds, constitutes the playground for making possible the control of spin and valley in these materials, reflected in the possibility of a selected spin/valley population controlled by the circular polarization of the impinging light beam. The possibility of exploiting such physics in materials with even number of layers, where the inversion symmetry is restored and no control of the valleys is feasible, hampers however straightforward applications to bulk materials.
We perform broadband time-resolved optical spectroscopy on bulk WSe2 and, by using a circularly polarized pump pulse, we show that a large transient anisotropic reflectivity signal, associated to the rotation of the reflected polarization, is obtained. This signal is bound in the spectral region of the A exciton at ≈1.6 eV. At room temperature, its dynamics, which is connected to the valley depolarization time, equals ≈300 fs and it is considerably faster than that of the valley exciton lifetime, showing a 10s of ps decay. We experimentally prove that, provided that the excitation is nearly-resonant to the A exciton, the valley degree of freedom can be exploited also in bulk materials when referring to the (transient) macroscopic optical properties. This fact is confirmed by a microscopic modelling of the dielectric properties of multi-layer non-centrosymmetric TMDCs, and paves the way to the exploitation of transition-metal dichalcogenide bulk materials for valleytronics-related applications beyond the stringent monolayer limit.