Highly efficient tunable broadband terahertz polarizers based on a graphene metasurface

Abstract

Ultrathin tunable broadband terahertz transmission mode linear-to-circular polarizers are proposed and numerically validated. Each structure consists of a dielectric substrate with slotted graphene patches on one or both sides of the substrate. Both polarizers yield tunable broadband conversion with large transmission coefficients in terahertz frequency range. Compared to the previous polarizers, the proposed structure with a single-layer of graphene patches exhibits a 120% larger fractional bandwidth and 60% larger transmittance. The polarizer with two layers of graphene patches features a 138% larger fractional bandwidth and 25% larger transmittance, compared to the previous polarizers, with excellent axial ratio. The tuning frequency range for the proposed single- and double-layer structures is (2.4 to 4.9) and (1.8 to 5.8) 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. Moreover, the polarization conversion performance is well maintained under oblique incidence condition, over a wide range of incident angles up to 70°. The size of the square-shaped unit cell is λ0/10, much smaller than the free-space wavelength of the incident waves, λ 0 , while the thickness is λ0/22. With the above features, the present work offers a further step in developing controllable polarization converters with potential applications in imaging, sensing, and communications.