  Among their amazing properties, graphene and related low-dimensional
materials show quantized charge-density fluctuations--known as plasmons--when
exposed to photons or electrons of suitable energies. Graphene nanoribbons
offer an enhanced tunability of these resonant modes, due to their
geometrically controllable band gaps. The formidable effort made over recent
years in developing graphene-based technologies is however weakened by a lack
of predictive modeling approaches that draw upon available {\it ab initio}
methods. An example of such a framework is presented here, focusing on
narrow-width graphene nanoribbons organized in periodic planar arrays.
Time-dependent density-functional calculations reveal unprecedented plasmon
modes of different nature at visible to infrared energies. Specifically,
semimetallic~(zigzag) nanoribbons display an intraband plasmon following the
energy-momentum dispersion of a two-dimensional electron gas.
Semiconducting~(armchair) nanoribbons are instead characterized by two distinct
intraband and interband plasmons, whose fascinating interplay is extremely
responsive to either injection of charge carriers or increase in electronic
temperature. These oscillations share some common trends with recent
nanoinfrared imaging of confined edge and surface plasmon modes detected in
graphene nanoribbons of $100$-$500$~nm width.
