  Abstract The valley degree of freedom in two-dimensional materials provides
an opportunity to extend the functionalities of valleytronics 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 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, 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-St\"uckelberg interference of wave packets of the electron
wave function.
