  Graphene is a unique two-dimensional (2D) material that has been extensively
investigated owing to its extraordinary photonic, electronic, thermal, and
mechanical properties. Excited plasmons along its surface and other unique
features are expected to play an important role in many emerging photonic
technologies with drastically improved and tunable functionalities. This review
is focused on presenting several recently introduced photonic phenomena based
on graphene, beyond its usual linear response, such as nonlinear, active,
topological, and nonreciprocal effects. The physical mechanisms and various
envisioned photonic applications corresponding to these novel intriguing
functionalities are also reported. The presented graphene-based technologies
promise to revolutionize the field of photonics at the relatively unexplored
terahertz (THz) frequency range. They are envisioned to lead to the design of
compact harmonic generators, low-power wave mixers, linear and nonlinear
sensors, magnet-free isolators and circulators, photonic topological
insulators, modulators, compact coherent optical radiation sources, and
subwavelength imaging devices.
