  Electrostatic gating and optical pumping schemes enable efficient time
modulation of graphene's free carrier density, or Drude weight. We develop a
theory for plasmon propagation in graphene under temporal modulation. When the
modulation is on the timescale of the plasmonic period, we show that it is
possible to create a backwards-propagating or standing plasmon wave and to
amplify plasmons. The theoretical models show very good agreement with direct
Maxwell simulations.
