Enhancing the phase shift of tunable phase shifters based on graphene nanoplatelets

Graphene exhibits tunable conductivity in a wide frequency band ranging from DC to microwaves [1]. The tunable conductivity of graphene opens a new paradigm of innovative microwave components that can be dynamically tuned by a DC voltage. The tunable conductive behavior of graphene exists not only in monolayer graphene but also in few layer graphene nanoplatelets [2]. Few layer graphene nanoplatelets are easier to fabricate and deposit as compared to monolayer graphene. The availability of commercial graphene nanoplatelets paves the way for mass scale usage. Graphene nanoplatelets were used in designing tunable attenuators [3] and phase shifters. By applying a DC bias voltage across graphene nanoplatelets, their sheet resistance drastically reduces. This variation of resistance can be converted to a variation of reactance by the help of stubs and tapered transmission line sections. The introduction of a variable reactance in the middle of a two-port transmission line causes a shift in the phase of the signal passing through the line. The lengths of the tapered lines are optimized to maximize the phase shift variation and minimize the additional insertion loss caused by the variation of the resistance of graphene. By adopting such structure, a phase shift of 40 degrees was demonstrated with an additional insertion loss of 3 dB [4].