  While, for semiconductors photoexcited by a circularly polarized pump, the
polarization plane of a linearly polarized probe has been shown to rotate, we
here predict a spectacular change when the pump beam is linearly polarized,
from Faraday rotation to Faraday oscillation, the oscillation of the
polarization plane going along a change of the photon polarization from linear
to elliptical. This effect, which reduces to zero when the probe field is
parallel or perpendicular to the pump field, comes from coherence between the
real excitons created by the pump and the virtual exciton coupled to the
unabsorbed probe, as easy to see from the Shiva diagrams which represent the
many-body physics taking place in this coupled photon-composite-exciton system.
