Graphene layered systems as the terahertz source with tuned frequency

Propagation of an electron beam over a graphene/dielectric sandwich structure is considered assuming the distance between layers to be large enough to prevent interlayer tunnelling.  A dispersion equation for the surface electromagnetic modes propagating along
graphene sheets is derived and \v{C}erenkov synchronism between surface wave and non-relativistic electron beam is predicted at
achievable parameters of the system. The generation frequency tuning is proposed by varying the graphene doping, the number of graphene sheets, the distance between sheets, etc.