Graphene layered systems as a terahertz source with tuned frequency

The propagation of an electron beam over a graphene/dielectric sandwich structure is considered assuming the distance between the graphene layers in sandwich is large enough to prevent interlayer tunneling. A dispersion equation for the surface electromagnetic modes propagating along graphene sheets is derived and \ifmmode \check{C}\else \v{C}\fi{}erenkov synchronism between a surface wave and a nonrelativistic electron beam is predicted at achievable parameters of the system. Generation frequency tuning is proposed by varying the graphene doping, the number of graphene sheets, the distance between sheets, etc.