Excitonic optical nonlinearities induced by virtual absorption

Below-gap excitation of semiconductors induces a blue shift of the absorption spectrum, known as the optical Stark effect. Using femtosecond pump and probe spectroscopy, we monitor the temporal behavior of the optical Stark shift and observe an apparent non-instantaneous response. However we show that the actual response of the material is ultrafast, only its observation is perturbed by the exciton coherence. We also investigate the dependence of the Stark shift upon the band-structure, leading to a light-induced exciton splitting.