  Photoreflectance is used for the characterisation of semiconductor samples,
usually by sweeping the monochromatized probe beam within the energy range
comprised between the highest value set by the pump beam and the lowest
absorption threshold of the sample. There is, however, no fundamental upper
limit for the probe beam other than the limited spectral content of the source
and the responsivity of the detector. As long as the modulation mechanism
behind photoreflectance does affect the complete electronic structure of the
material under study, sweeping the probe beam upstream towards higher energies
from that of the pump source is equally effective in order to probe high energy
critical points. This fact, up to now largely overseen, is shown experimentally
in this work. E1 and E0+{\Delta}0 critical points of bulk GaAs are
unambiguously resolved using pump light of lower energy. Upstream modulation
may widen further applications of the technique.
