Chiral symmetry and its manifestation in optical responses in graphene: interaction and multi-layers

Chiral symmetry, fundamental in the physics of graphene, guarantees the existence of topologically stable doubled Dirac cones and anomalous behaviors of the zero-energy Landau level in magnetic fields. The crucial role is inherited in the optical responses and many-body physics in graphene, which are explained in this paper. We also give an overview of multilayer graphene from the viewpoint of the optical properties and their relation with the chiral symmetry.