  High-frequency response of a system with drifting electrons in a highly doped
graphene and surface polar optical phonons of a polar substrate is considered.
For this interacting system, we obtained a dielectric function, frequencies and
decrement/increment of cooperative plasmon-optical phonon oscillations. We
found, that the response is significantly depends on the degree of
nonequilibrium of the electrons. In particular, interaction between drifting
plasmons and surface polar optical phonons leads to an instability of the
electron subsystem due to Vavilov-Cherenkov effect. We suggest that the hybrid
system - graphene on a polar substrate - is capable to be used in amplifiers or
generators of THz electromagnetic radiation.
