  Graphene plasmonics is a rapidly growing field with multiple potential
applications. One of the standard ways to study plasmons in graphene is by
fabricating an array of graphene nanoribbons where nanoribbon edges provide the
efficient photon-plasmon coupling. We systematically analyze the problem of
optical plasmonic response in such systems and demonstrate the purely geometric
nature of the size quantization condition for graphene plasmons. Accurate
numerical calculations allowed us to tabulate the universal geometric
parameters of plasmon size quantization, which is expected to become useful in
analysis of experimental data on plasmonic response of graphene nanoribbons. A
simple analytical theory has also been developed which accurately reproduces
all the qualitative features of optical plasmonic response of graphene
nanoribbons.
