Femtosecond excitonic bleaching recovery in the optical Stark effect of GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As multiple quantum wells and directional couplers

Femtosecond pump-probe spectroscopy was used to study the ultrafast-excitonic-bleaching-recovery feature in the optical Stark effect of several GaAs/${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As multiple-quantum-well samples at room and low temperatures. We also have seen this effect in a nonlinear directional coupler as a total transmission increase of the waveguides. The semiconductor Bloch equations for the system were solved numerically and gave good qualitative agreement with the experiments. The transient absorption saturation and recovery observed in the experiments is explained as an ultrafast adiabatic following of the semiconductor excitation.