Screening of coherent LO phonons by interwell plasmon oscillations along the growth direction in GaAs/AlGaAs superlattices

We study coherent phonon oscillations in GaAs/AlGaAs superlattices and mult iple quantum wells as a function of the barrier width. Strong, bulklike scr eened LO phonons accompany bare LO phonons for thin barrier samples, wherea s for thicker barrier samples, only the bare LO phonons exist. These result s strongly indicate that the coherent interwell plasmons oscillating across the barrier couple to LO phonons. The technique applied is a very sensitiv e way of probing the superlattice-multiple quantum well transition of colle ctive charge oscillations.