Tunable THz Edge-Coupled Graphene Directional Couplers: A General Analysis Resulting From Graphene Properties

A surface plasmon-polariton graphene waveguide directional couplers for THz region are discussed in this paper. The device has a very simple structure. The central part of it presents two side-coupled parallel graphene nanoribbons with a small gap between them. The graphene waveguides are deposited on a dielectric substrate. In one of the presented examples, the proposed 3 dB coupler has a 25% fractional bandwidth centered at 3.8 THz with isolation and return losses better than −17 dB. By changing the Fermi energy of graphene, the power division in a given frequency range can be dynamically controlled or, alternatively, a frequency band of the coupler with a fixed power division can be dislocated. The discussion is fulfilled in terms of the so-called frequency reference point corresponding to a 3-dB coupler. We determine also the limits of possible coupler parameters tuning in the frequency range from 2 to 8 THz. The suggested general analysis can be useful in the design of graphene couplers for high-density integrated THz circuit.