Ultra wideband THz graphene four-port circulators

In the presence of a DC magnetic field, the conductivity of graphene acquires tensor character with antisymmetric nondiagonal elements. This property can be used for creation of nonreciprocal elements in THz region. In this work, we propose and investigate two 4-port circulators with a planar structure. They are composed of a graphene layer placed on a dielectric substrate. The orientation of the external DC magnetic field is normal to the graphene layer. The graphene geometry of the first circulator presents four strips with surface plasmon-polariton waves. The strips are connected symmetrically to the magnetized central region such that the whole structure possesses 4-fold rotational symmetry. The operating region of the circulator at low frequency end is limited by the cyclotron frequency of magnetized graphene and at high frequencies by the appearance of the higher mode in the graphene waveguidings structures. Using numerical calculus, we give an example of the circulator project with the following scattering parameters: S-11 is better than -10 dB, the isolation level is less than -15 dB, insertion loss is better than -3 dB in the (1.6 divided by 2.65) THz region with the bandwidth of 44%. The second four-port circulator has a linear geometry with lower symmetry where the number of ports can be easily increased. It has the bandwidth also of about 44%. In both cases, the DC magnetic field is 1.5 T and the Fermi energy of graphene mu(c) = 0.15 eV.