The Effect of Linear Gradation of Thickness on Tunable Graphene Photonic Crystals in THz Region

By using the transfer matrix method, the optical properties of two one-dimensional tunable photonic crystal structures are investigated and compared with each other. The structure of the first photonic crystal consists of alternating layers of two dielectrics SiO2 and Si, with graphene layers between the dielectric layers. By adding polystyrene polymer as the defect layer to the photonic crystal structure, the defect mode appears within the photonic bandgap. This defect mode is tunable with the chemical potential of graphene and the wave incidence angle. In the structure of the second photonic crystal, one of the dielectric layers of the first crystal structure has a linear gradation. The defect mode is plotted for three structures with different linear gradation thicknesses and compared with the non-graded. The location of Defect modes in the crystal structure with linear graded in comparison with non-graded, shifts to lower frequencies. In both structures, the effect of the chemical potential of graphene and the incident angle of the transverse electric and transverse magnetic waves on the tuning of the location of the defect mode and photonic bandgap and graphene photonic bandgap are investigated. By increasing the chemical potential of graphene and the wave incidence angle, the location of the defect mode shifts to higher frequencies. These two structures can be used in designing the tunable terahertz filters