  An applied field can modulate optical signals by resonance shifting via the
Stark effect. The optical Stark effect (OSE) uses ultrafast light in the
transparency region of a material to shift resonances with speeds limited by
the pulse duration or system coherence. In this Letter we investigate the OSE
in resonant optical harmonic generation (OHG) using the ground state exciton
transition of WS2 with a variety of morphologies. Multidimensional
pump-harmonic-probe measurements, in which the probe is second- or
third-harmonic emission, reveal not only large Stark shifts that are
commensurate with the large optical susceptibilities common to WS2 excitons,
but also behaviors more complex than simple OSE treatments predict. We show how
a new manifestation of the Stark Effect, brought forth by coherent photon
exchange between the pump and OHG fundamental fields, can strongly enhance or
suppress OHG.
