摘要： Graphene has unique plasmonic properties because of its unusual electromagnetic properties and its 2D geometric structure. Graphene supports plasma oscillations when it has a nonzero chemical potential. Different from a metal, which has a fixed carrier density, thus a fixed plasma frequency, graphene is similar to a semiconductor in that its carrier density, thus its plasma frequency, can be varied by varying its chemical potential through, for example, impurity doping, electrical modulation, or optical illumination. Nevertheless, graphene is very different from an ordinary semiconductor because of its unique band structure and its 2D geometry. As a 2D material, graphene naturally supports true surface plasmons and, in a certain spectral region depending on its chemical potential, surface plasmon polaritons. As discussed in preceding chapters, the electromagnetic properties of graphene approach those of a conductor at low frequencies and those of a dielectric at optical frequencies. Between the two limits, particularly in the terahertz frequency region, graphene has sophisticated electromagnetic properties that can be tuned through varying its chemical potential. For this reason, graphene has very interesting plasmonic properties that lead to many useful applications in the terahertz region.