The gate-controllable complex conductivity of graphene offers unprecedented opportunities for reconfigurable plasmonics at THz and mid-IR frequencies. However, the requirement of a gating electrode close to graphene and the single `control knob that
this approach offers for graphene conductivity limits the practical implementation and performance of graphene-controllable plasmonic devices. Herein, we report on graphene stacks composed of two or more graphene monolayers separated by electrically thin dielectrics and present a simple and rigorous theoretical framework for their characterization. In a first implementation, two graphene layers gate each other, thereby behaving as a controllable single equivalent layer but without any additional gating structure. Second, we show that adding an additional gate --a third graphene layer or an external gate-- allows independent control of the complex conductivity of each layer within the stack and hence provides enhanced control on the stack equivalent complex conductivity. The proposed concepts are first theoretically studied and then demonstrated experimentally via a detailed procedure allowing extraction of the parameters of each layer independently and for arbitrary pre-doping. These results are believed to be instrumental to the development of THz and mid-IR plasmonic devices with enhanced performance and reconfiguration capabilities.
