THz graphene circulator with quadrupole mode resonator

We present a compact and efficient three-port graphene-based circulator operating in the terahertz (THz) frequency range. Conventional designs are predicated on dipole resonances. In contrast, the present approach exploits the quadrupole mode of a circular graphene resonator, magnetized by a perpendicular direct current magnetic field. The structure is composed of a single-layer graphene resonator that is coupled to three graphene waveguides. These waveguides are supported by silica and silicon substrates. Through the optimization of resonator geometry and the tuning of graphene chemical potential, a substantial reduction in operational requirements was achieved, enabling functionality with a magnetic field of 0.2 T and a Fermi energy of 0.1 eV. Full-wave simulations performed in COMSOL Multiphysics demonstrate excellent nonreciprocal performance, with isolation better than –21 dB, insertion loss around –2.6 dB, and reflection of –18 dB at 5.38 THz. The frequency response is in good agreement with the predictions of temporal coupled-mode theory (TCMT), which confirms a fractional bandwidth of approximately 6.3% around the central frequency of 5.58 THz under the applied magnetic bias. A comparison of the proposed circulator with existing designs reveals a substantial reduction in both its physical dimensions and its weight. Furthermore, the circulator functions under conditions that demand less voltage and magnetic field strength than existing designs. In conclusion, the practical feasibility of device fabrication is discussed, with a focus on the compatibility of the proposed structure with current graphene-based photonic manufacturing technologies.