We have observed a coherent oscillation of zone-folded acoustic phonons in GaAs-AlAs superlattices for the first time by means of a femtosecond time-resolved pump-probe technique. The oscillatory component in the time-resolved reflection signal corresponds to the upper branch ($B_2$ symmetry) of the first-order doublet mode. Carriers were selectively excited in well layers and the $B_2$-symmetry phonon mode was selectively generated. The dephasing time of the $B_2$ mode was measured to be 70 ps from our experiments.

• Received 30 March 1994

DOI:https://doi.org/10.1103/PhysRevLett.73.740