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The method of interferometric cross-correlation is used to obtain precise information about the phase and amplitude of the laser pulse induced interband polarization in semiconductors. With this technique it is possible to study the linear as well as nonlinear response of a sample. The light transmitted through a thin GaAs crystal after excitation with femtosecond pulses near the band gap is measured. The phase shift between the driving laser pulses and the induced polarization is measured in the time domain. The influence of additional pump pulses on the decay of the polarization is studied. The instantaneous frequency of the polarization depends on the laser center frequency, on the pump pulse delay time, and on the relative polarization of the pump pulse with respect to the test pulse.

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