We have observed a coherent oscillation of zone-folded acoustic phonon
s in GaAs-AlAs superlattices for the first time by means of a femtosec
ond time-resolved pump-probe technique. The oscillatory component in t
he time-resolved reflection signal corresponds to the upper branch (B2
symmetry) of the first-order doublet mode. Carriers were selectively
excited in well layers and the B2-symmetry phonon mode was selectively
generated. The dephasing time of the B2 mode was measured to be 70 ps
from our experiments.