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.