▎ 摘　　要

Because of extreme three-dimensional field confinement and easy electrically tenability, plasmons in graphene nanostructures are promising candidates for many applications, such as biosensing, photodetectors and modulators. However, up to now, graphene plasmons have been explored mostly on substrates. Scatterers, corrugations and dopants induced by substrates not only add damping to plasmons but also obscure the intrinsic electronic properties of graphene. In this work, the near-field response of surface plasmons of suspended graphene circular resonators is studied with the scattering-type scanning near-field optical microscopy under different excitation wavelengths, lambda = 10.653 and 10.22 mu m, respectively. Fundamental and higher order breathing plasmon modes are revealed in real-space with the Fermi energy of graphene of only 0.132 eV. Moreover, the direct experimental evidence on near-field electric tuning in suspended graphene resonators is demonstrated by using back-gate tuning. Our work not only provides a foundation to truly understand the properties of electrons inside pure graphene, but shines light on the applications in optoelectronic devices with suspended two-dimensional materials.