We employ single-crystal silicon dielectric resonator antennas for a beam-shaping terahertz reflectarray. The reflected wavefront is shaped by local phase discontinuities introduced by the resonators. Unit-cell simulations show potential for highefficiency operation, and the performance of the reflectarray is experimentally verified.

This paper describes the implementation and performance analysis of the first fully operational beam-space multiple-input multiple-output (MIMO) antenna for the spatial multiplexing of two QPSK streams. The antenna is composed of a planar three-port radiator with two varactor diodes terminating the passive ports. Pattern reconfiguration is used to encode the MIMO information onto orthogonal virtual basis patterns in the far field. A measurement campaign was conducted to compare the performance of the beam-space MIMO system with a conventional 2 × 2 MIMO system under realistic propagation conditions. Propagation measurements were conducted for both systems and the mutual information and symbol error rates were estimated from Monte-Carlo simulations over the measured channel matrices. The results show the beam-space MIMO system and the conventional MIMO system exhibit similar finite-constellation capacity and error performance in nonline-of-sight scenarios when there is sufficient scattering in the channel. In comparison, in line-of-sight channels, the capacity performance is observed to depend on the relative polarization of the receiving antennas.

Isolators, or optical diodes, are devices enabling unidirectional light propagation by using non-reciprocal optical materials, namely materials able to break Lorentz reciprocity. The realization of isolators at terahertz frequencies is a very important open challenge made difficult by the intrinsically lossy propagation of terahertz radiation in current non-reciprocal materials. Here we report the design, fabrication and measurement of a terahertz non-reciprocal isolator for circularly polarized waves based on magnetostatically biased monolayer graphene, operating in reflection. The device exploits the non-reciprocal optical conductivity of graphene and, in spite of its simple design, it exhibits almost 20 dB of isolation and only 7.5 dB of insertion loss at 2.9 THz. Operation with linearly polarized light can be achieved using quarter-wave plates as polarization converters. These results demonstrate the superiority of graphene with respect to currently used terahertz non-reciprocal materials and pave the way to a novel class of optimal non-reciprocal devices.

Advances in terahertz technology rely on the combination of novel materials and designs. As new devices are demonstrated to address the terahertz gap, the ability to perform high-efficiency beam control will be integral to making terahertz radiation a practical technology. Here, we use a metasurface composed of nonuniform dielectric resonator antennas on a ground plane to achieve efficient beam focusing at 1 THz. The dielectric resonators are made of high-resistivity silicon, which is a low-loss, nondispersive material for terahertz waves. The resonators operate around the resonance of the displacement current in the silicon, which is crucial to attaining high efficiency. The reflectarray's capacity to focus terahertz radiation is experimentally verified, and hence by the principle of antenna reciprocity, it can also be employed as a terahertz collimator. The demonstrated device can therefore be deployed for high-gain terahertz antennas. Further measurements show that the loss of the reflectarray is negligible, which confirms the high efficiency of the dielectric resonators. This finding will enable the design of efficient flat-profile terahertz reflectarrays and metasurfaces to serve arbitrary beam control requirements in the near and far fields.

In this paper, two THz reflectarray surfaces have been designed and fabricated in order to deflect a plane wave with any polarization and with a specific incident angle to three different specific directions each at distinct three frequencies of 0.7, 1.0 and 1.5 THz. The surface is composed of an array of 100 100 cells, each comprised of gold crosses and parasitic dipoles printed on thin grounded high resistivity silicon. Finite-element method (FEM) simulations are in line with the measurement results obtained using THz time-domain spectroscopy (THz TDS) showing the intended deflections for the two fabricated samples each with an arbitrary frequency-vs-deflection angle relationship. In addition, the use of silicon as the substrate paves the way for the integration of reconfigurable technologies which enhances the reflectarray versatility.

We present the experimental demonstration of the first terahertz isolator (working frequency >1 THz), obtained using the non- reciprocal electromagnetic properties of graphene under magnetostatic bias. The presentation will cover in detail the design, fabrication and measurement of the device. The isolator operated in reflection for circularly polarized waves, showing two bands at 3 and 7.5 terahertz respectively. In spite of the simplicity of design and fabrication, the proposed device geometry achieves near-optimal performances. We believe these results to be groundbreaking and of great importance for the terahertz community.

Nonlocal electromagnetic effects of graphene arise from its naturally dispersive dielectric response. We present semianalytical solutions of nonlocal Maxwell's equations for graphene nanoribbon arrays with features around 100 nm, where we found prominent departures from its local response. Interestingly, the nonlocal corrections are stronger for light polarization parallel to the ribbons, which manifests as an additional broadening of the Drude peak. For the perpendicular polarization case, nonlocal effects lead to blue-shifts of the plasmon peaks. These manifestations provide a physical measure of nonlocal effects, and we quantify their dependence on the ribbon width, doping, and wavelength.

Graphene plasmonic nanostructures enable subwavelength confinement of electromagnetic energy from the mid-infrared down to the terahertz frequencies. By exploiting the spectrally varying light scattering phase at the vicinity of the resonant frequency of the plasmonic nanostructure, it is possible to control the angle of reflection of an incoming light beam. We demonstrate, through full-wave electromagnetic simulations based on Maxwell equations, the electrical control of the angle of reflection of a mid-infrared light beam by using an aperiodic array of graphene nanoribbons, whose widths are engineered to produce a spatially varying reflection phase profile that allows for the construction of a far-field collimated beam towards a predefined direction.

|We propose different concepts of mechanically recon¯gurable true-time-delay phaseshifters using micro-actuators at millimeter-waves (MMW). The mechanical reconfiguration allows to achieve phase shift reconfiguration with very low losses. One of the proposed concepts has been fully implemented and tested, demonstrating state-of-the-art performance in terms of phase-shift/loss ratio.

A novel single layer, dual linear polarized unit cell is introduced for a quad-band reflectarray antenna in the Ku-band centered around 12, 13, 14, and 15.5 GHz. The unit-cell design is such that the resultant reflectarray antenna can be considered as four different reflectarray antennas, each with a specific beam shape, sharing one aperture. As proof-of-concept, two quad-band reflectarray antenna prototypes having a plate size of 20 × 20 cm 2 have been designed and fabricated each with a certain beam direction versus frequency. Measurement and simulation results show satisfactory performance of both reflectarray antennas at each frequency.

We describe the current status of our research on reflectarray antennas operating at terahertz frequencies. We present preliminary measurements of a fixed beam reflectarray operating at 1 THz fabricated on silicon and characterized with a THz time domain system. Secondly, we study theoretically the applicability of novel reconfigurable materials, namely graphene and vanadium dioxide to attain phase modulation in terahertz reflectarray cells, providing theoretical upper bounds on the performance in terms of loss and phase range.

Recently the orbital angular momentum (OAM) multiplexing scheme has been proposed to increase without limit the channel capacity between a transmitter and a receiver node communicating in line of sight. A controversy arose about the far-field effectiveness of this method and about it being or not a subset of MIMO communication schemes. In this contribution we first illustrate that OAM multiplexing can be completely understood as line of sight MIMO, and hence that the technique cannot bring additional advantages in far field communications. Secondly, we show that it is possible to build similar line of sight near field MIMO systems based on lenses with similar or better performance than their OAM counterparts.

X-band reconfigurable reflectarray (RRA) cells having two, three, and four discrete reflection phase states are presented in this paper. The cell topology enables independent dynamic reflection phase control of each linear component of a dual-polarized wave. The reflection phase step deviation is smaller than the quantization error in 12.5% bandwidth for the two and three states designs and in 6.5% bandwidth for the four states design. Owing to particular design considerations and implementation in monolithic micro-electromechanical systems (MEMS) fabrication process, the average reflection loss is kept very low, from 0.3 dB for the two states design to 0.8 dB for the four states design. Measurement results of the two states (1-bit) prototype are in good agreement with the simulation results, thereby confirming the viability of the concept.

We present a X-band 1-bit reconfigurable reflectarray (RRA) cell capable of independent reflection phase control of each linear component of a dual-polarized wave. It presents the lowest loss ever reported in an operational RRA cell at microwave frequencies, thanks to particular design considerations and implementation in a monolithic micro-electromechanical systems (MEMS) process. The average reflection loss, including oblique incidence up to theta = 40 degrees, is below 0.6 dB at the operating frequency. Low phase error and loss are preserved in a 12.5% bandwidth. Measurements demonstrate the viability of the concept as well as very good agreement between simulated and measured results.

Recently, a technique called beam-space MIMO has been demonstrated as an effective approach for transmitting multiple signals while using a single RF-chain.In this work, we present novel design considerations and a compact antenna solution to stimulate the deployment of beam-space MIMO in future wireless applications.Targeting integration in small wireless devices, the novel antenna is made of a single integrated radiator rather than an array of physically-separated dipoles.It also drastically simplifies the implementation of variable loads and DC bias circuits for BPSK modulated signals, and does not require any external reconfigurable matching circuit.Finally, we show that this antenna system could be reconfigured by dynamic adjustment of terminating loads to preserve its beam-space multiplexing capabilities over a 1:2 tuning range, thereby promoting the convergence of MIMO and dynamic spectrum allocation via reduced-complexity hardware.A prototype achieving single-RF-chain multiplexing at a fixed frequency is designed and measured, showing excellent agreement between simulations and measurements.