FIG. 11B shows a radiation source 1101, which uses two graphene layers in a sandwich configuration. The radiation source includes a dielectric layer 1115 sandwiched by two graphene layers 1111a and 1111b. Alternatively, the dielectric layer 1115 can be replaced by air or vacuum. The advantage of this sandwich structure includes that the effective index n of the SPPs will then grow by a factor of almost 2, due to the high index of the dielectric layer 1115. In practice, the radiation source 1101 can be grown on a layer-by-layer basis. In addition, a multi-layered structure can also be constructed. The multi-layered structure can include alternating layers of graphene and dielectric material, i.e. dielectric-graphene-dielectric-graphene-dielectric.
FIG. 11C shows a radiation source 1102 in which a graphene layer 1112 is disposed on a dielectric substrate 1142. An electron beam 1135 is delivered by an electron source 1132 into the graphene layer 1112 so as to interact with any SPP field within the graphene layer 1112. An electromagnetic wave (EM) source 1122 is configured to couple an EM wave 1125 into the graphene layer 1112 to excite the SPP field. This approach can be helpful in constructing on-chip devices, at least because the electrons are moving inside the graphene layer 1112 and electron beam control can be simpler (e.g., without vacuum chamber).
