  Coherent optical dressing of quantum materials offers technological
advantages to control their electronic properties, such as the electronic
valley degree of freedom in monolayer transition metal dichalcogenides (TMDs).
Here, we observe a new type of optical Stark effect in monolayer WS2, one that
is mediated by intervalley biexcitons under the blue-detuned driving with
circularly polarized light. We found that such helical optical driving not only
induces an exciton energy downshift at the excitation valley, but also causes
an anomalous energy upshift at the opposite valley, which is normally forbidden
by the exciton selection rules but now made accessible through the intervalley
biexcitons. These findings reveal the critical, but hitherto neglected, role of
biexcitons to couple the two seemingly independent valleys, and to enhance the
optical control in valleytronics.
