Graphene layered systems as the terahertz source with tuned frequency

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