Optical Stark effect of the exciton

When a semiconductor is irradiated by below gap photons, the exciton line blue-shifts. We show that this so-called exciton Optical Stark effect is due to a coupling between the exciton and all (bound and unbound) biexcitonic states. More precisely this effect mesure, the interactions between the two electrons and the two holes forming the biexcitonic states. At large detuning, the Fermi (statistical) interaction dominates and the exciton shift is nothing but the dressed-atom shift. At small detuning, the Coulomb interaction dominates and, in materials having a well- bound biexcitonic molecule, transforms the large detuning blue-shift into a red shift, just below the exciton-biexciton resonance.