  Graphene has emerged as a promising building block in the modern optics and
optoelectronics due to its novel optical and electrical properties. In the
mid-infrared and terahertz (THz) regime, graphene behaves like metals and
supports surface plasmon resonances (SPRs). Moreover, the continuously tunable
conductivity of graphene enables active SPRs and gives rise to a range of
active applications. However, the interaction between graphene and metal-based
resonant metamaterials has not been fully understood. In this work, a
simulation investigation on the interaction between the graphene layer and THz
resonances supported by the two-gap split ring metamaterials is systematically
conducted. The simulation results show that the graphene layer can
substantially reduce the Fano resonance and even switch it off, while leave the
dipole resonance nearly unaffected, which phenomenon is well explained with the
high conductivity of graphene. With the manipulation of graphene conductivity
via altering its Fermi energy or layer number, the amplitude of the Fano
resonance can be modulated. The tunable Fano resonance here together with the
underlying physical mechanism can be strategically important in designing
active metal-graphene hybrid metamaterials. In addition, the "sensitivity" to
the graphene layer of the Fano resonance is also highly appreciated in the
field of ultrasensitive sensing, where the novel physical mechanism can be
employed in sensing other graphene-like two-dimensional (2D) materials or
biomolecules with the high conductivity.
