Enhancing 5G propagation into vehicles and buildings using optically transparent and polarisation insensitive metasurfaces over wide-incidence angles.

This article introduces two transmissive metasurfaces applied to normal windows, aiming to improve the 5G outdoor-to-indoor (O2I) coverage. These windows can be utilized in various settings, such as vehicles or buildings. The proposed unit cells, designed to be wide-incident angle and polarization insensitive, are implemented in both single-glazing and double-glazing glasses, arranged in a periodic structure to form the transmission surfaces. Both metasurfaces maintain optical transparency by incorporating Indium Tin Oxide (ITO) as the conductive element in each unit cell. These engineered transmission surfaces enhance the 5G signal indoor coverage at the 3.5 GHz band across a broad range of incident angles. While multi-layer structures typically exhibit heightened sensitivity to the angle of incidence, the proposed two-layered transmissive surfaces demonstrate substantial angular stability, reaching up to 65 and 75 degrees for double- and single-glazed glass, respectively. To achieve this wide and stable angular response, evolutionary optimization techniques were employed to fine-tune the proposed unit cells. Both designs exhibit a high transmission coefficient across the operating frequency for a variety of incident angles, surpassing those reported in the existing literature. Experimental evaluations of the fabricated prototypes indicate that both metasurfaces hold significant potential for enhancing signal propagation into buildings and vehicles.

Improvement of Terahertz Photoconductive Antenna using Optical Antenna Array of ZnO Nanorods.

An efficient terahertz (THz) photoconductive antenna (PCA), as a major constituent for the generation or detection of THz waves, plays an essential role in bridging microwave-to-photonic gaps. Here, we propose an impressive approach comprising the use of arrayed zinc oxide nanorods (ZnO NRs) as an optical nanoantenna over an anti-reflective layer (silicon nitride) in the antenna gap to boost the photocurrent and consequently the THz signal. The numerical approach applied in investigating the optical behavior of the structure, demonstrates a significant field enhancement within the LT-GaAs layer due to the optical antenna performing simultaneously as a concentrator and an antireflector which behaves as a graded-refractive index layer. ZnO NRs have been fabricated on the PCA gap using the hydrothermal method as a simple, low cost and production compatible fabrication method compared to other complex methods used for the optical nanoantennas. Compared to the conventional PCA with a traditional antireflection coating, the measured THz power by time domain spectroscopy (TDS) is increased more than 4 times on average over the 0.1-1.2 THz range.

Reprogrammable Graphene-based Metasurface Mirror with Adaptive Focal Point for THz Imaging.

Recent emergence of metasurfaces has enabled the development of ultra-thin flat optical components through different wavefront shaping techniques at various wavelengths. However, due to the non-adaptive nature of conventional metasurfaces, the focal point of the resulting optics needs to be fixed at the design stage, thus severely limiting its reconfigurability and applicability. In this paper, we aim to overcome such constraint by presenting a flat reflective component that can be reprogrammed to focus terahertz waves at a desired point in the near-field region. To this end, we first propose a graphene-based unit cell with phase reconfigurability, and then employ the coding metasurface approach to draw the phase profile required to set the focus on the target point. Our results show that the proposed component can operate close to the diffraction limit with high focusing range and low focusing error. We also demonstrate that, through appropriate automation, the reprogrammability of the metamirror could be leveraged to develop compact terahertz scanning and imaging systems, as well as novel reconfigurable components for terahertz wireless communications.

Reconfigurable THz Plasmonic Antenna Based on Few-Layer Graphene with High Radiation Efficiency.

Graphene plasmonic antennas possess two significant features that render them appealing for short-range wireless communications, notably, inherent tunability and miniaturization due to the unique frequency dispersion of graphene and its support for surface plasmon waves in the terahertz band. In this letter, dipole-like antennas using few-layer graphene are proposed to achieve a better trade-off between miniaturization and radiation efficiency than current monolayer graphene antennas. The characteristics of few-layer graphene antennas are evaluated and then compared with those of antennas based on monolayer graphene and graphene stacks, which could also provide such improvements. To this end, first, the propagation properties of one-dimensional and two-dimensional plasmonic waveguides based on the aforementioned graphene structures are obtained by transfer matrix theory and finite-element simulation, respectively. Second, the antennas are investigated as three-dimensional structures using a full-wave solver. Results show that the highest radiation efficiency among the compared designs is achieved with the few-layer graphene, while the highest miniaturization is obtained with the even mode of the graphene stack antenna.