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Summary form only given. The occurrence of Rabi flopping in excitonic transitions of semiconductors demonstrate that an intense light field can induce a non-perturbative ac Stark effect despite the very fast excitation-induced dephasing. However the homogeneous broadening induced by the strong excitation prevented the observation in quantum wells of the spectral counterpart of Rabi oscillations in time, the ac Stark splitting. We report the evolution of the absorption spectrum of a specially designed single InGaAs quantum well as a function of the growing excitation density. The excitation is provided by a pulsed Ti:sapphire laser (100 fs of pulse duration) and the measurements are taken in a pump-probe geometry.