A Hybrid Intelligent Reflecting Surface with Graphene-based Control Elements for THz Communications

Terahertz (THz)-band (0.1-10 THz) communication is envisioned as a key wireless technology to fulfill the demand for increasing data rates and to accommodate denser networks. The THz-band, however, suffers from very high propagation losses further aggravated by the presence of obstacles in common scenarios that behave as opaque barriers at THz frequencies. Engineering non-line-of-sight (NLoS) communication links with reflecting intelligent surfaces (RIS), such as smart reflectarrays, is one possible method of overcoming the complex THz communication model. However, existing reflectarray designs at lower frequencies cannot be simply repurposed due to the operating failure of traditional semiconductor or electro-mechanical switch-based tunable control elements at THz frequencies. In this direction, the use of 2D nanomaterials, such as graphene, to design tuning elements and integrate these into THz-RIS is being explored. This paper presents a novel graphene-based tuning element for continuous phase control of the reflecting element, in situ. The fundamental building block of the reflectarray is designed to have high reflection efficiency and high tunability by leveraging the properties of metals and graphene, respectively. The radiating elements are integrated to form a hybrid graphene-metal reflectarray. First, the working principle and design of the proposed tuning element, comprised of a graphene-based plasmonic waveguide, is described and explained. Second, the trade-offs in the design of the hybrid tunable reflecting element, resulting from the integration of the tuning element with a metallic patch, are exhaustively studied. After discussing the integration of multiple reflecting elements in a reflectarray, the ability to perform complete continuous dynamic beamforming is presented. Extensive numerical results are provided to benchmark the performance of the reflectarray with other reflecting strategies and to demonstrate the ability to engineer NLoS paths for improved communication in the THz band.