▎ 摘　　要

Highly tunable optical antennas in teraherz range based on graphene ring structures are proposed, which employ graphene plasmons instead of traditional metallic plasmons. The plasmon resonances of the perfect graphene ring (PGR) can be understood with the edge plasmons in graphene ribbons. While in the nonconcentric graphene ring, the multipolar plasmon modes appear and anti-symmetric mode splits due to symmetry breaking. Furthermore, the symmetric plasmon mode in a graphene ring can concentrate electromagnetic field with an enhancement factor as large as 10(3) in terahertz waveband, which is almost 20 times larger than a gold ring with the same size. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3702819]