  Monolayer valley semiconductors, such as tungsten diselenide (WSe$_2$),
possess valley pseudospin degrees of freedom that are optically addressable but
degenerate in energy. Lifting the energy degeneracy by breaking time-reversal
symmetry is vital for valley manipulation. This has been realized by directly
applying magnetic fields or via pseudo-magnetic fields generated by intense
circularly polarized optical pulses. However, sweeping large magnetic fields is
impractical for devices, and the pseudo-magnetic fields are only effective in
the presence of ultrafast laser pulses. The recent rise of two-dimensional (2D)
magnets unlocks new approaches to control valley physics via van der Waals
heterostructure engineering. Here we demonstrate wide continuous tuning of the
valley polarization and valley Zeeman splitting with small changes in the laser
excitation power in heterostructures formed by monolayer WSe$_2$ and 2D
magnetic chromium triiodide (CrI$_3$). The valley manipulation is realized via
optical control of the CrI$_3$magnetization, which tunes the magnetic exchange
field over a range of 20 T. Our results reveal a convenient new path towards
optical control of valley pseudospins and van der Waals magnetic
heterostructures.
