  A new degree of freedom, the valley pseudospin, emerges in atomically thin
two-dimensional transition metal dichalcogenides (MX2) and has attracted great
scientific interest. The capability to manipulate the valley pseudospin, in
analogy to the control of spin in spintronics, can open up exciting
opportunities in valleytronics. Here we demonstrate that an ultrafast and
ultrahigh valley pseudomagnetic field can be generated using circularly
polarized femtosecond pulses to selectively control the valley degree of
freedom in monolayer MX2. Employing ultrafast pump-probe spectroscopy, we
observed a pure and valley-selective optical Stark effect in WSe2 monolayers
from the non-resonant pump, which instantaneously lift the degeneracy of valley
exciton transitions without any dissipation. The strength of the optical Stark
effect scales linearly with both the pump intensity and the inverse of pump
detuning. An energy splitting more than 10 meV between the K and K_prime valley
transitions can be achieved, which corresponds to an effective pseudomagnetic
field over 170 Tesla. Our study demonstrates efficient and ultrafast control of
the valley excitons with optical light, and opens up the possibility to
coherent manipulate the valley polarization for quantum information
applications.
