▎ 摘　　要

The radiation efficiency improvement of graphene plasmonic antennas via synthesized substrates with metamaterial resonators is introduced in this letter. Graphene, represented as an ultrathin layer, can support highly confined surface waves of significantly decreased wavelength compared to the vacuum one. Although this concept yields more compact devices, its applicability to graphene antennas is limited, due to the degradation of radiation efficiency. Thus, the effect of a substrate material on the surface wave wavelength is thoroughly examined, while radiators of larger dimensions and enhanced efficiency are designed for an epsilon-negative medium. Also, a novel realistic metamaterial substrate is developed for graphene terahertz (THz) antennas, successfully verifying all theoretical estimations. Numerical results are extracted via an accurate finite-difference time-domain algorithm that treats graphene as an efficient surface boundary condition.