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The optical Stark shift of the exciton is due to a coupling between the exciton and "semivirtual" biexcitonic states (bound and unbound). We show that at large detuning the excitonic shift is the same as the "dressed atom" blue shift. We predict that at small detuning the exciton-exciton interaction plays an important role and leads to an excitonic red shift at the two-photon absorption threshold when the molecular biexciton is stable.
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Combescot et al. (1988) studied this question.
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