Since Ha experimental evidence in semiconductors in 1986, the optical Stark effect has attracted much attention both from a fundamental point of view and for possible use in ultrafast twitching devices without dissipating energy in the material. Up to now this effect has been observed as a blue shift of the ground state exciton line in the presence of a strong pump beam with frequency in the semiconductor spectral transparency range. Numerous theoretical studies have been reported, because in principle the whole physical process can be described exactly since it involves only virtual excitations. In particular, it has been predicted1,2 that the biexciton should play an important role. Its influence, however, can only be evidenced in specific semiconductors for which the biexcitonic molecule is stable and its binding energy large enough so that the related optical transition is well separated from the exciton absorption line. In these conditions, one expects to observe a change in the manifestation of the optical Stark effect from the usual blue shift toward a red shift. The exact transition should occur for a pump photon energy corresponding to the creation of a biexciton starting from the excitonic state.

© 1990 Optical Society of America

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