Voltage-adjustable terahertz hyperbolic metamaterial based on graphene and doped silicon

We propose a special structure of the hyperbolic metamaterial (HMM) in terahertz frequency range that is composed of graphene-dielectric multilayer fishnet and silicon-dielectric nanowire. The results based on the full-wave numerical simulation show that the proposed metamaterial can exhibit Type I or Type II hyperbolic behavior at some special frequency ranges and can also realize the optical topological transition by tuning the frequency or the bias voltage. The dynamical tunability of the permittivity with the applied voltage is analyzed at the specific frequencies of 2.8 THz and 4.5 THz. The geometry of the dispersion relation of the designed metamaterial can be mutually converted between the hyperbolic and elliptical geometry when the bias voltage changes. Moreover, the difference between the vacuum background and the proposed effective medium at different frequencies is also analyzed with the comparison of the corresponding isofrequency surfaces. The proposed metamaterial may be expected to provide many tunable facilities in the optical topological transitions and the field of THz technologies.