Valley polarization control in WSe2 monolayer by a single-cycle laser pulse

The valley degree of freedom in two-dimensional materials provides an opportunity to extend the functionalities of valleytronic devices. Very short valley lifetimes demand the ultrafast control of valley pseudospin. Here we theoretically demonstrate the control of valley pseudospin in WSe2 monolayer by a single-cycle linearly polarized laser pulse. We use the asymmetric electric field controlled by the carrier-envelope phase (CEP) to make the valley polarization between K and K' point in the Brillouin zone (BZ). Time-dependent density functional theory with spin-orbit interaction reveals that no valley asymmetry and its CEP dependence is observed within the linear-optical limit. In the nonlinear-optical regime, a linearly polarized pulse induces a high degree of valley polarization and this polarization is robust against the field strength. Valley polarization strongly depends and oscillates as a function of CEP. The carrier density distribution forms nodes as the laser intensity increases, our results indicate that the position of the carrier density in the BZ can be controlled by the laser intensity. From the analysis by the massive Dirac Hamiltonian model, the nodes of the carrier density can be attributed to the Landau-Zener-Stfickelberg interference of wave packets of the electron wave function.