Graphene is a two-dimensional (2D) carbon-based material, whose unique electronic and optical properties have attracted a great deal of research interest [1]. Despite the fact that graphene is an atomically thin layer the optical absorption of a single layer can be as high as 2.3% (defined by the fine structure constant). Nevertheless, for light-matter interactions this number is imposing challenges and restrictions for graphene-based optoelectronic devices. One promising way to enhance optical absorption is to excite graphene-plasmon polaritons (GPPs) supported by graphene.