▎ 摘　　要

Plasmonic modes in a symmetrical structure comprised of two layers of doped or inverted graphene separated by a dielectric barrier layer and their dispersion properties at different parameters of the barrier layer were investigated. The emergence of additional modes that exist in a limited frequency range was detected; near its cut-off the phase velocity of plasmons substantially decreased. In the case of inverted graphene, the additional modes are amplifying modes. The regions of parameters at which the group velocities of main and additional modes have opposite signs have been revealed.