  We theoretically demonstrate that a system formed by two coupled graphene
sheets enables a negative damping regime wherein graphene plasmons are pumped
by a DC current. This effect is triggered by electrons drifting through one of
the graphene sheets and leads to the spontaneous light emission (spasing) and
wave instabilities in the mid-infrared range. It is shown that there is a deep
link between the drift-induced instabilities and wave instabilities in moving
media, as both result from the hybridization of oscillators with oppositely
signed frequencies. With a thickness of few nanometers and wide spectral
tunability, the proposed structure may find interesting applications in
nanophotonic circuitry as an on-chip light source.
