We performed differential transmission spectroscopy and time resolved pump probe measurements on biased InGaAs/InGaAsP multiple quantum well structures. At low carrier excitation levels we observe a shift in the energy of the excitonic absorption resonance caused by the photogenerated carriers screening the applied electric field within the quantum wells. Increasing the pump power the exciton transmission energy blue shifts until it reaches the zero internal field energy position. For our

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... robe measurements field screening dominates at pump powers lower than 50 W /~r n .~ while at higher powers exciton saturation becomes dominant. We deduce a saturation intensity of 250 W/cm2.