We apply the quasiadiabatic approximation for the femtosecond pulse propagation in a collection of excitons in the case of weak interaction between the optical pulse and the semiconductor medium. Using the semiconductor Maxwell-Bloch equations beyond the slowly varying envelope approximation we show that the dynamics of femtosecond pulse propagation is described by the modified Korteweg-de Vries equation. Bright solitons superimposed on a continuous wave background are found and their stability against low amplitude perturbations is investigated. Possible experiments in semiconductor systems such as GaAs/AlGaAs are discussed.
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