Shape Approaches for Enhancing Plasmon Propagation in Graphene

Abstract

Graphene plasmonics is a promising alternative for on-chip high speed communication that integrates optics and electronics, where the strong confinement of the electromagnetic energy at subwavelength scale and the tunability of the plasmon frequency via an external gate voltage are key advantages. The main drawback of graphene plasmons is their rather short decay and propagation length, which is due to intrinsic losses and substrate-related defects. Toward plasmonic devices, noble metal antennas represent a viable approach for plasmon launching in graphene waveguides, with the challenge of efficient coupling and plasmon propagation that are feasible for on chip communication. Here we discuss and analyze, using numerical simulations, different designs of metal antennas and their coupling to graphene plasmons (GP), as well as graphene based nanopatterned waveguides that can lead to a more efficient GP propagation. A Yagi-Uda antenna leads to stronger coupling to GPs and allows for directive propagation as compared to a simple dipole antenna. This is especially advantageous to launch plasmons in graphene nanowire waveguides, where propagation up to 3 μm and frequency and phase control can be achieved. In tapered graphene waveguides, the constructive interference of the plasmon reflection at the edges can lead to strong plasmon signals up to 8 μm distant from the launching dipole antenna. Nanostructuring of rectangular waveguides into asymmetric chains of truncated triangles greatly enhances directionality of GP propagation and conserves phase information. A comparison of the propagation length and electric near-field strength of these different approaches is presented, and confronted with the efficiency of GP launching by light scattering on scanning near field optical microscopy (SNOM) tips.