Adiabatic Control of Surface Plasmon-polaritons in Graphene

Surface plasmon-polaritons (SPPs) on graphene can be treated analogously to light propagating along optical waveguides due to their guided transmission along the graphene surface. The characteristic length of a graphene device is much shorter than of an optical waveguide and thus SPPs on graphene hold the potential to be remarkably useful in integrated optics and nanophotonics.

Here we propose a novel adiabatic directional coupler of SPPs between three layers of graphene sheets. To realize a robust transfer of SPPs between the input and the output graphene sheets by we creating a counter-intuitive coupling in analogy to STIRAP in atomic systems. The input and output graphene sheets (the outer ones) are weakly curved, with opposite curvatures of radius, and are transversely displaced from each other by a fixed distance. According to coupled mode theory, the coupling of the SPPs is determined by the distance between them, and thus we obtain spatially-varying coupling that can be engineered to fulfill the adiabatic criteria. Fine control over the coupling strengths enables a complete and robust transfer of SPPs from the input to the output graphene sheet.

We perform numerical simulations of the propagation of the SPPs assuming a device length of less than 1 micrometer, which is shorter than the maximum propagation length of 4 micrometers. Our calculations show that SPPs are efficiently transferred from the input to the output graphene sheet without ever exciting the middle sheet. We analyze the sensitivity of our scheme to variations in the parameters and the wavelength of the excited SPPs, and find that it is robust due to its adiabatic design. We compare the proposed three-layer device to a previously reported two-layer one and find that the intensity of the SPPs ranges between 0.6 to 0.9.