6. Conclusion

of the graphene sheets in each region, a small height difference Δh is introduced to ensure the

To obtain a desirable reflection phase in each region, a metamaterial unit cell composed of a three-layer sandwich structure has been designed, as shown in Figure 17. Each sandwich structure comprises graphene/Al2O3/SiO2 materials from the top to the bottom. An insulating layer of Al2O3 material is inserted between two adjacent sandwich structures. A ground consisting of an Au material is placed at the bottom of the unit cell. The designed unit cell periodicity is P = 20 μm, and the thicknesses of SiO2, Al2O3, and Au materials are 12 μm, 10 nm, and 5 μm, respectively. In each sandwich structure, an external DC voltage is applied between the graphene layer and the SiO2 layer to control the conductivity of graphene. For convenience, the chemical potentials of the graphene layers from the top to the bottom are denoted as "μc1," "μc2," and "μc3," respectively. With the equivalent circuit model given in Figure 17(b), the maximum reflection phase range of the proposed unit cell can be obtained. As shown in Figure 18, the reflection phase range of the proposed unit cell can cover 360o in a wide frequency band from 1.8 to 2.8 THz, when three chemical potentials independently vary

A wideband horn antenna as the excitation is used to generate a wave incident on the reflectarray. Figure 19 shows the OAM vortex waves with different modes generated by the reflectarray at 2.3 THz. We can clearly observe the spiral phase distributions of the OAM

Figure 20. Simulated OAM beams with l = 1 mode at different frequencies. Reprinted from Shi et al. [43], with permission

insulation between two adjacent regions, as shown in Figure 16(b).

from 1 to 1 eV.

186 Metamaterials and Metasurfaces

from the IEEE.

In sum, several characteristics of graphene including the conductivity model and equivalent circuit model have been presented. Two graphene-based devices, i.e., metamaterial absorber and metamaterial reflectarray, have been designed. By varying graphene's chemical potential, the wideband tunable absorption for the designed absorber and the broadband tunable OAM modes for the developed reflectarray have been demonstrated, respectively. Graphene provides more degrees of freedom for the design of the tunable metamaterial systems.
