Non-plasmonic vs. plasmonic graphene-based THz leaky-wave antennas

The propagation of surface plasmon polaritons (SPPs) along a graphene sheet is a topic of increasing interest for both the engineering and the physics communities, since it would allow for opening very interesting perspectives in the context of reconfigurable THz nano-antennas. As is known [1], the ambipolar electric-field effect allows for tuning the graphene surface conductivity by the simple application of an electrostatic field. This scheme has recently been proposed for the design of reconfigurable THz nano-antennas based on the propagation of surface plasmons. These structures (see Fig. 1(a)) mainly consist of a grounded dielectric substrate covered with a monolayer graphene sheet biased through the application of a DC voltage between the sheet itself and an extremely thin conductive polymer (e.g., PEDOT) placed just beneath it [2]. The proposed devices exhibit high degree of reconfigurability, but rather limited efficiency [2, 3]. This is mainly due to the non-negligible ohmic losses exhibited by graphene in the THz range. As a matter of fact, the high confinement of the electric field of an SPP in proximity of the graphene sheet has a two-fold effect. On one hand, the surface conductivity strongly interacts with the modal fields, thus the radiation features of the antenna (i.e., directivity, pointing angle, beamwidth, etc.) can considerably be reconfigured with a small variation of the bias. On the other hand, graphene ohmic losses impact more and consequently the radiation efficiency of the antenna is reduced. In this work, based on previous theoretical work by the Authors [4, 5], we analyze the potential advantages of using ‘ordinary’ (i.e., non-plasmonic) leaky modes in place of SPPs. In particular, two different graphene-based multilayered structures are proposed: a grounded-dielectric substrate antenna (GSA) covered with a graphene sheet [4], and a grounded substrate-superstrate antenna (GSSA), in which the graphene sheet is placed at a suitable position within the substrate [5]. Note that the fundamental leaky-mode TE-TM pair can simply be excited by placing a horizontal magnetic dipole (this can model, e.g., a slot etched on the ground plane and backilluminated by a coherent non-directive THz source). In particular, it is shown (see Fig.1(b)) that the leaky-wave modal configuration of a GSA has a maximum on the middle plane of the antenna cavity, being no longer confined in proximity of the graphene sheet, as instead occurs for a SPP. As a consequence, the proposed device has a reduced reconfigurability, but at the same time a significantly improved efficiency. We finally note that the graphene position in a GSSA can be properly adjusted (see Fig. 1(c)) in order to obtain the desired performance in terms of efficiency, reconfigurability, and directivity as well. Numerical results revealed us that a trade-off must be established amongst these figures of merit.