Optical Stark effect in semiconductors

The optical Stark effect on the exciton is seen as a blue shift of the line when a direct-gap semiconductor is excited in the transparency region. The magnitude of the Stark blue shift depends on the strength of the electromagnetic dipolar coupling between the different levels. In the case of degenerate levels, splitting can occur. Such a splitting has been predicted and observed, in GaAs, using adequately polarized laser beams to enhance the detection of this phenomenon in the small-signal limit. The dynamics of the optical Stark effect is expected to be ultrafast, since it should follow the pump pulse. However, femtosecond pump and probe experiments may show slower behavior due to the medium coherence. Large oscillatory structures then distort the differential spectra, leading to a signal maximum clearly before the pump pulse maximum. Integration analysis, which allows recovery of the true dynamics, has been performed. By Fourier transforming any of the differential spectrum at a time delay that shows these oscillations, the complete temporal behavior is deduced.