  We have studied the Faraday rotation and ellipticity signals in ensembles of
singly-charged (In,Ga)As/GaAs quantum dots by pump-probe spectroscopy. For
degenerate pump and probe we observe that the Faraday rotation signal amplitude
first grows with increasing the time separation between pump and probe before a
decay is observed for large temporal separations. The temporal behavior of the
ellipticity signal, on the other hand, is regular: its amplitude decays with
the separation. By contrast, for detuned pump and probe the Faraday rotation
and ellipticty signals both exhibit similar and conventional behavior. The
experimental results are well described in the frame of a recently developed
microscopic theory [Phys. Rev. B 80, 104436 (2009)]. The comparison between
calculations and experimental data allows us to provide insight into the
spectral dependence of the electron spin precession frequencies and extract the
electron g-factor dependence on energy.
