Broadband terahertz tunable metasurface linear polarization converter based on graphene
Zhang Hongtao, Cheng Yongzhi, Huang Mulin     
School of Information Science and Engineering, Wuhan University of Science and Technology, Wuhan, Hubei 430081, China

Overview:In recent years, terahertz science and related technologies have emerged as one of the rapidly evolving technologies, and they have shown good application prospects in the fields of communication, imaging, sensing and non-destructive testing. These applications require not only efficient terahertz sources, but also high-performance terahertz devices such as modulators, polarization converters, and more. At present, there are relatively few materials in nature that can effectively manipulate terahertz waves, and the corresponding devices are quite scarce. In order to promote the application of terahertz technology in the above related fields, the effective regulation of terahertz waves is particularly important. Since many applications of terahertz waves are related to their polarization states, terahertz polarization converters that control polarization states have become an important research direction. Conventional terahertz polarization converters are usually made of materials based on grating structure and dispersion, and generally have problems such as narrow frequency band and low efficiency, which greatly limits the practical application range. Therefore, it is very important to design and prepare high performance terahertz polarization control devices. Because graphene has very good optical transparency, adjustable electromagnetic properties and high electron mobility, it can be widely used in the design of optoelectronic devices. In addition, the addition of a bias voltage to the graphene can change its Fermi level and electron relaxation time, thereby achieving dynamic adjustment of its electromagnetic properties. In this paper, a terahertz broadband tunable reflective linear polarization converter based on oval-shape-hollowed graphene metasurface is proposed and verified by simulation and Fabry-Perot multiple interference theory. Our designed metasurface model is similar to a sandwiched structure, which is consisted of the top layer of anisotropic elliptical perforated graphene structure, an intermediate dielectric layer and a metal groundplane. The simulation results show that when the given graphene relaxation time and Fermi energy are τ=1 ps and μc=0.9 eV, respectively, the polarization conversion rate (PCR) of the designed metasurface structure is over 90% in the frequency range of 0.98 THz~1.34 THz, and the relative bandwidth is 36.7%. In addition, at resonance frequencies of 1.04 THz and 1.29 THz, PCR is up to 99.8% and 97.7%, respectively, indicating that the metasurface we designed can convert incident vertical (horizontal) linearly polarized waves into reflected horizontal (vertical) linearly polarized waves. We used the Fabry-Perot multi-interference theory to further verify the metasurface model. The theoretical predictions are in good agreement with the numerical simulation results. In addition, the designed metasurface reflective linear polarization conversion characteristics can be dynamically adjusted by changing the Fermi energy and electron relaxation time of graphene. Therefore, our designed graphene-based tunable metasurface polarization converter is expected to have potential application value in terahertz communication, sensing and terahertz spectroscopy.

Supported by the Science and Technology Research Project of Education Department of Hubei China (D20181107)