Dirac electronic states in graphene systems: optical spectroscopy studies

Electronic properties of two-dimensional allotropes of carbon, such as graphene and its bilayer, multi-layer epitaxial graphene, few-layer Bernal-stacked graphene, as well as of three-dimensional bulk graphite are reviewed from the viewpoint of recent optical spectroscopy studies. Attention is focused on relativistic-like character of quasi-particles in these systems, which are referred to as massless or massive Dirac fermions.