Effects of edge on graphene plasmons as revealed by infrared nanoimaging

We used scattering-type scanning near-field optical microscopy (s-SNOM) to investigate the plasmonic properties of edges in well-defined graphene nanostructures, including sharp tapers, nanoribbons and nanogaps, which were all fabricated via the growth-etching chemical vapor deposition (GECVD) method. The obtained near-field images revealed the localized plasmon modes along the graphene nanoribbon; these modes strongly depended on the size of the graphene pattern, the angle of the tapered graphene and the infrared excitation wavelength. These interesting plasmon modes were verified by numerical simulations and explained by the reflection, and interference of electromagnetic waves at the graphene–SiO2 edge. The constructive interference at the graphene nanogap caused by charge accumulation was demonstrated for the first time. Using the infrared nanoimaging technique, greater plasmon broadening was observed in the zigzag edge than in the armchair edge. Our study suggests that graphene edges should be separated by an effective working distance to avoid the overlapping of localized plasmon modes, which is very important for the design of graphene-based plasmonic circuits and devices. Scanning near-field optical microscopy has been used to explore plasmon modes on nanoribbons with well-defined edges. Since plasmons involve both electrons and light, plasmonics can combine the speed of optics with the tiny size of electronics. Qiaoliang Bao of Soochow University, China, and co-workers investigated the interaction of infrared light with various graphene nanostructures and found that the resulting plasmons strongly depend on the dimensions of the pattern and the wavelength of the infrared light. Based on their results, the researchers recommend separating graphene edges in real devices to prevent plasmons from overlapping. They anticipate that these findings may pave the way for the development of active sub-wavelength optics and a wide range of nano-optoelectronic devices, including graphene-based waveguides, quantum information devices and ultra-sensitive sensors.