  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.
