The modulation of the electric field within the intrinsic layer of surface-intrinsic-doped structures by means of a chopped pump beam and in the presence of a constant probe beam is calculated by considering the modulation of the surface voltage by the photoactivated majority carrier flow. The analysis is focused as to determine the dependency on the probe and pump photocurrents and on the chopping frequency of the observed rise and fall transient time constants of the time resolved photoreflectance signal. The non linear features of the carrier dynamics are worked out in detail. A rescaling of the time constants by the power of the probe beam is predicted. ©1997 American Institute of Physics.