Ultrafast refractive index control of THz graphene metamaterials

We present an ultrafast dynamics of THz graphene-metamaterial hybrid devices, where the refractive index and the conductivity are largely modulated by electrical and optical methods. Unprecedentedly large modulation of refractive index and the pump-induced effective negative conductivity are investigated by an ultrafast time-resolved optical-pump THz-probe spectroscopy with varying gate voltage.