Efficient Excitation and Active Control of Propagating Graphene Plasmons with a Spatially Engineered Graphene Nanoantenna

Graphene plasmons (GPs) are of great importance in photonics and optoelectronics due to ultrahigh near-field confinement and enhancement. However, the large momentum mismatch between GPs and incident light hinders the efficient excitation of GPs. Conventional excitation schemes, such as prism coupling, grating coupling, and resonant metal antennae, go against the tunability and multifunction of the GP device. Here, we numerically demonstrate the efficient excitation and active control of propagating GPs in a resonant graphene nanoantenna (GNA)-based GP launcher. The resonant GNA provides high-momentum near-field components to match the wavevector of GPs, and the excitation efficiency is significantly enhanced by the quarter-wavelength condition in a reflective configuration. Furthermore, the propagating behavior of GPs is gate-tunable with a GNA. Using spatially engineered GNAs, a tunable directional GP launcher with an extinction ratio of larger than 1000 is achieved. Moreover, we design a vertically crossed GNA-based propagating GP launcher that can serve as the incident polarization information recording. Finally, some graphene plasmonic circuits at the nanoscale, such as a GP waveguide, splitter, and prism, are realized using spatial conductivity patterns in graphene. The efficient excitation and flexible control of propagating GPs with engineered GNAs associated with the spatial conductivity patterns in graphene provide a gate-tunable and multifunctional platform for nanoscale graphene plasmonic devices

Anomalous Frictional Behaviors of Ir and Au Tips Sliding on Graphene/Ni(111) Substrate: Density Functional Theory Calculations

The atomic force microscope (AFM) provides a facilitating tool to investigate the atomic-scale friction properties of surfaces through the sliding of the scanning tip; therefore, the interaction between the tip and the surface should play an important role to determine the frictional behaviors. In this study, density functional theory (DFT) calculations have been carried out to perform the pushing down processes of a tip (10 atom Ir or Au tip) on the top, hollow, and bridge sites of the graphene/Ni(111) substrate. The calculation results indicate that the interactions between the tips and the graphene/Ni(111) substrate influence the adsorption energy remarkably, leading to the sequence of bridge < top < hollow for Ir and Au tips, which is totally different from the adsorption energy of an inert Ar atom, following the sequence of hollow < bridge < top. The strong interactions between the (Ir or Au) tip and the graphene/Ni(111) substrate will introduce novel frictional properties into the system, and an anomalous negative friction coefficient could be obtained. Further investigations show that these interactions arise from the hybridizations between the 2p_<i>z</i> orbitals of C atoms and the 5d_<i>z</i>² orbitals of the tip apex atom