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 (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.
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Yamamoto et al. (1994) studied this question.
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