Influence of DC current direction in graphene on dispersion and amplification of plasmons in graphene

We investigate dispersion and amplification of plasmon eigen modes in graphene with a direct electric current (DC-current) directed arbitrarily relative to the direction of plasmon propagation. Graphene is described by tensor  conductivity obtained in the hydrodynamic approximation. We detected the possibility of amplification of plasmons in graphene in a certain range of DC current directions at terahertz frequencies. The most effective amplification is achieved when the drift of charge carriers and plasmons propagate co-directionally. This is due to the most effective interaction of DC current with the electric field of plasmons.