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W. H. Knox, D. S. Chemla, and
D. A. B. Miller, "Femtosecond ac Stark Effect in Semiconductor Quantum
Wells: Extreme Low- and High-Intensity Limits," Phys. Rev. Lett., 62,
1189-1192, (1989). The authors show that in quasi-2D GaAs quantum wells,
below-resonance low-intensity excitation produces a pure shift of the excitons, contrary
to AC Stark shifts in atomic systems. At high pump intensities two-photon 3D-real-carrier
generation competes with the virtual 2D-exciton effects. They also show that DC fields
applied perpendicular to the layers reduce the excitonic AC Stark effect
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