Abstract

Optical pumping in the transparent region of GaAs quantum well heterostructures produces modulation of the exciton absorption by two distinct physical mechanisms. First, photoexcitation below the band gap can produce carriers at the bandedge via phonon-assisted absorption. These carriers bleach the exciton through the effects of the exclusion principle. We have utilized this mechanism in studies of the phonon sidebands of the exciton in quantum well heterostructures. In contrast, the second mechanism is a direct interaction between the optical field and the exciton and involves no exciton or free carrier population. This is the optical Stark effect, which produces a shift of the exciton transition to higher energy. The exciton transition remains shifted only as long as the optical pump pulse is present. Hence the optical Stark effect is a new nonlinearity in quantum wells which occurs on an extremely fast time scale. In this paper we present experimental results representing both carrier-related modulation and Stark shifting o, the exciton resonance produced by picosecond photoexcitation. We describe studies of the Urbach tail and phonon sidebands and discuss the optical Stark shift in terms of a model developed for the ground state exciton transition.

© 1986 Optical Society of America