Graphene, a two-dimensional array of carbon atoms arranged in a hexagonal lattice, has attracted a great amount of attention due to its outstanding mechanical, thermal and electronic properties. Moreover, graphene shows an exceptionally strong tunable light-matter interaction that depends on the Fermi level and the electromagnetic resonance provided by intentionally engineered structures. In the optical regime, the nonlinearities of graphene originated from the Pauli blocking have already been exploited for mode-locking device applications in ultrafast laser technology, whereas nonlinearities in the terahertz regime, which arise from a reduction in conductivity due to carrier heating, have only recently been confirmed experimentally. 

In this talk, I will present 1) gate-voltage controlled optical chirality and circular dichroism of graphene chiral metamaterials, 2) a nonlinear transmission in randomly stacked graphene due to carrier heating, and 3) controlled nonlinearity of graphene metamaterials. First, based on a chiral metamaterial combined with a gated single layer graphene, the polarization state of terahertz waves can be electrically tuned. In particular,      transmission of a terahertz wave with one circular polarization can be electrically controlled without affecting that of the other circular polarization and the rotation angle of a linearly polarized terahertz wave can be controlled by the gate voltage. Second, in a randomly stacked graphene, the fluence-dependent nonlinear THz transmission can be enhanced compared to as-grown graphene. This enhancement of transmission will be explained using a thermo-modulational model. Finally, I will present how the nonlinear interactions of graphene with an intense terahertz field can be controlled using metamaterials. The induced transparencies of graphene can be controlled effectively by engineering meta-atoms and/or changing the number of charge carriers through electrical gating.