▎ 摘　　要

Collective excitations of coupled electron-phonon systems are calculated for both monolayer and bilayer graphenes, taking into account the nonperturbative Coulomb coupling between electronic excitations in graphene and the substrate longitudinal-optical phonon modes. We find that the plasmon-phonon coupling in monolayer graphene is strong at all densities but in bilayer graphene the coupling is significant only at high densities satisfying the resonant condition omega(pl) approximate to omega(ph). The difference arises from the peculiar screening properties associated with chirality of graphene. Plasmon-phonon coupling explains the measured quasilinear plasmon dispersion in the long-wavelength limit, thus resolving a puzzle in the experimental observations.