Strongly Confined Gap Plasmon Modes in Graphene Sandwiches and Graphene-on-Silicon

Abstract

Graphene has emerged as a radically new platform in nanotechnology and its tunable optical and electrical properties make it a material of choice for future nanocircuitry (Vakil and Engheta, Science 332(6035):1291–1294, 2011). A key element towards actual devices is the exploration of graphene nanostructures. For instance, graphene nanoribbons, readily fabricated by electron beam lithography, have been shown to be attractive waveguides for plasmons. These bound surface waves arising from the coupling between light and collective oscillations of the charge carriers exhibit indeed unusually strong confinement in graphene (Nikitin et al., Phys Rev B 84:161407, 2011; Christensen et al., ACS Nano 6(1):431–440, 2012). Our work focuses on the physics and the classification of plasmon waveguide modes in structures consisting of two infinitely long graphene ribbons vertically offset by a gap, a “sandwich” geometry. We find strongly hybridized plasmonic modes, some of which are tightly confined within the gap region, and therefore hold promise for nanodevices (Francescato et al., New J Phys 15(6):063020, 2013). In order to aid the understanding of the fundamental physics of the different classes of waveguide modes encountered, we introduce a convention for plotting the mode spectrum which allows to group the modes by shared characteristics. This representation is particularly useful when coupling occurs, because the mode density increases considerably. In this manner, and varying the critical parameters of width, gap and operation wavelength, different regimes, coupling mechanisms and mode families can be recognized. We confirm our findings by considering experimentally realizable systems with tunable graphene doping in a geometry where a single ribbon is placed on top of a highly doped silicon substrate via a dielectric spacer layer. Remarkably, we show that the new gap modes still survive in the latter case. More, we report on an unprecedented level of confinement of a terahertz wave of nearly 5 orders of magnitude. Because of their remarkable field distributions and extreme confinement, the families of modes presented here could serve as the building blocks for both graphene-based integrated optics and ultrasensitive sensing modalities (Francescato et al., New J Phys 15(6):063020, 2013) (Fig. 40.1).