Nano-plasmonic phenomena in graphene nanoribbons

Graphene plasmons, which are collective oscillations of Dirac fermions in graphene, are of broad interests in both fundamental research and technological applications. In this talk, we present the first nano-infrared (IR) imaging studies of graphene plasmons using the scattering-type scanning near-field optical microscopy-a unique technique allowing efficient excitation and high-resolution imaging of graphene plasmons. With this technique, we were able to show that common graphene/SiO ₂ /Si back-gated structures support propagating surface plasmons in the IR frequencies. The observed plasmons are highly confined surface modes with a wavelength around 200nm that are conveniently tunable by the back gate voltages [1]. In addition, we were able to map and characterize grain boundaries inside CVD graphene film by examining the distinct plasmonic interference patterns triggered by these line defects. Our modeling and analysis unveiled unique electronic properties associated with grain boundaries [2]. Furthermore, we investigate the plasmonic properties of Bernal-stacking bilayer graphene (BLG) and find that BLG supports gate-tunable IR plasmons with higher confinement compared to graphene and randomly stacked graphene layers. We also found that BLG plasmons can be turned off completely in a wide voltage range close to the charge neutrality point. Those unique plasmonic properties are attributed to both interlayer electron tunneling and bandgap opening in BLG [3]. Finally, we observed peculiar one-dimensional edge plasmons propagating strictly along the edges of patterned graphene nanostructures. Compared to commonly known two-dimensional surface plasmons, these one-dimensional edge modes have shown a slightly smaller plasmon wavelength [4].