Dynamically controllable graphene terahertz splitters with nonreciprocal properties

Two novel graphene-based nonreciprocal four-port splitters for the terahertz region are proposed. The input power is divided between two output ports, whereas the input port is isolated from the output ports due to the presence of the fourth port. The splitters consist of a circular graphene resonator and four graphene waveguides coupled to the resonator. These elements are placed on the two-layer dielectric substrate. The central part of the splitter is under a biasing DC magnetic field normal to the graphene layer. The surface plasmon-polariton wave in the input port excites the dipole resonance in the resonator. The splitters have the following parameters: the input power is divided between two output ports almost equally with -4.4  dB. The input port is isolated from two output ports by -15  dB. The bandwidth is 4.0% with a central frequency of 7.4 THz. The biasing DC magnetic field is 0.8 T, and the Fermi energy of graphene ϵ_F=0.15  eV. Changing the Fermi energy by electrostatic gating allows one to dynamically control the central frequency of the splitters.