  We report on a quasi-nondegenerate pump-probe technique that is based on
spectral-filtration of femtosecond laser pulses by a pair of
mutually-spectrally-disjunctive interference filters. This cost- and
space-efficient approach can be used even in pump-probe microscopy where
collinear propagation of pump and probe pulses is dictated by utilization of a
microscopic objective. This technique solves the contradictory requirements on
an efficient removal of pump photons from the probe beam, to achieve a good
signal-to-noise ratio, simultaneously with a needed spectral proximity of the
excitation and probing, which is essential for magnetooptical study of many
material systems. Importantly, this spectral-filtration of 100 fs long laser
pulses does not affect considerably the resulting time-resolution, which
remains well below 500 fs. We demonstrate the practical applicability of this
technique with close but distinct wavelengths of pump and probe pulses in
spatially- and time-resolved spin-sensitive magnetooptical Kerr effect (MOKE)
experiment in GaAs/AlGaAs heterostructure, where a high-mobility spin system is
formed after optical injection of electrons at wavelengths close to MOKE
resonance. In particular, we studied the time- and spatial-evolutions of
charge-related (reflectivity) and spin-related (MOKE) signals. We revealed that
they evolve in a similar but not exactly the same way which we attributed to
interplay of several electron many-body effects in GaAs.
