1
Dept. of Communications and Electronics, School of Electrical and Computer Engineering, Shiraz University, Shiraz, Iran
2
Zand Ave., Shiraz, Iran
Abstract
Ultrathin tunable broadband terahertz transmission mode linear-to-circular polarizers are proposed and numerically validated. Each structure consists one or two stack of a dielectric substrate with slotted graphene. Both polarizers yield tunable broadband conversion with large transmission coefficients in terahertz frequency range. Compared to the previous polarizers, the proposed structure with one stack exhibits a 29% larger fractional bandwidth and 49% larger transmittance. The polarizer with two stack features a 77% larger fractional bandwidth and 13% larger transmittance, compared to the previous polarizers, with excellent axial ratio. The tuning frequency range for the proposed one- and two-stack structures is (2.9 to 5.1) and (2.2 to 5.3) THz, respectively. The tuning is achieved by varying the graphene Fermi energy (chemical potential) from 0.2 to 1 eV, by the application of a DC bias voltage. In addition, in the previous structures, the graphene layer is discrete, so in order to control the structures by applying an external voltage, it is necessary to connect all the graphene patches with thin metal wires, which in practice can be very complicated and difficult. While in the presented structures, the graphene layer is continuous and we will not have these problems. Moreover, the polarization conversion performance is well maintained under oblique incidence condition, over a wide range of incident angles up to 60°. The size of the square-shaped unit cell and the thickness are less than λ_0⁄10, much smaller than the free-space wavelength of the incident waves, λ_0. With the above features, the present work offers a further step in developing controllable polarization converters with potential applications in imaging, sensing, and communications.
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