Ultrawideband graphene three-port circulator for THz region.

The suggested circulator is formed by a concave pattern graphene junction and three waveguides symmetrically connected to it. The graphene is supported by SiO/Si layers. The circulation behavior is based on the nonsymmetry of the graphene conductivity tensor which appears due to magnetization by a DC magnetic field applied normally to the graphene plane. The symmetrical mode propagating in the nonmagnetized graphene waveguide, is transformed in magnetized region to an edge-guided one providing the propagation from one port to another port and isolating the third port. The device characteristics depend on the physical parameters of the graphene junction, its dimensions and parameters of the substrate. We discuss a choice of these parameters to maximize the frequency band and isolation level and to minimize the losses and the applied DC magnetic field. The theoretical arguments are confirmed by full-wave computations. In an example, we demonstrate that the circulator can have the frequency band of 42% (from 2.75 THz to 4.2 THz), with the isolation higher than 17 dB and the insertion losses better that 2 dB, provided by the biasing DC magnetic field 1.5 T and the chemical potential of graphene 0.15 eV.