  A novel type of spaser with the net amplification of surface plasmons (SPs)
in doped graphene nanoribbon is proposed. The plasmons in THz region can be
generated in a dopped graphene nanoribbon due to nonradiative excitation by
emitters like two level quantum dots located along a graphene nanoribbon. The
minimal population inversion per unit area, needed for the net amplification of
SPs in a doped graphene nanoribbon is obtained. The dependence of the minimal
population inversion on the surface plasmon wavevector, graphene nanoribbon
width, doping and damping parameters necessary for the amplification of surface
plasmons in the armchair graphene nanoribbon is studied.
