Tri-Band, Polarization-Independent Reflectarray at Terahertz Frequencies: Design, Fabrication, and Measurement

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.

Multi-band reflectarray antennas in Ku and THz frequency bands

Hamed Hasani

Printed reflectarrays are low-cost, low-profile high gain antennas demonstrating distinctive advantages over conventional parabolic reflectors and phased-arrays. The flat, low weight reflecting surface of a reflectarray makes it an attractive alternative with respect to bulky parabolic reflectors specially for space and satellite systems. As compared to high-cost phased-array antennas, with incorporation of solid state devices, reflectarrays are able to demonstrate electronic beam scanning in a very low-cost way. A distinctive advantage of a reflectarray antenna lies in its potential to be readily designed as a multi-band antenna which demonstrates independent performance at several frequencies. A characteristic that is difficult to achieve using conventional parabolic reflectors. The aim of this thesis is to present low-cost, simple, multi-band printed reflectarray antennas in Ku and THz frequency bands. In Ku band we present a dual-band reflectarray performing at 12 and 14 GHz and a quad-band reflectarray antenna performing at 12, 13, 14 and 15.5 GHz. The presented prototypes benefit from the advantage of having a single-layer structure which reduces the design complexity as well as the fabrication cost. In addition, multi-band reflectarrays are able to perform at any polarization due to the dual-linear polarized design of their unit-cells. Furthermore, the design of the unit-cell is such that, at each frequency, the phase response depends on only one parameter of the cell. This advantage eliminates the need for time consuming optimizations. Based on proposed unit-cells dual-band and quad-band reflectarrays with arbitrary beam direction versus frequency have been simulated, fabricated and measured. Simulation and measurement results as well demonstrate the satisfactory independent performance of the prototypes at each intended frequency. In THz region, for the first time we present a tri-band unit-cell based on which reflectarray prototypes performing at the three frequencies 0.7, 1.0 and 1.5 THz, are designed. The presented reflectarrays possess all the advantages of those designed for Ku band with the additional advantage of having high resistivity silicon as the substrate thanks to a sophisticated fabrication process. The use of silicon as substrate is a big advantage since it facilitates the integration of solid state devices for reconfigurability. Based on the proposed unit-cell reflectarray samples with arbitrary independent performance at each frequency are designed, simulated, fabricated and measured. Measurement results obtained using a THz-TDS (Terahertz Time-Domain Spectroscopy) measurement system, demonstrate the satisfactory independent performance of the reflectarray samples at each frequency. This thesis also presents a dual-band, dual-polarized reconfigurable unit-cell for beam-scanning reflectarray operating at 12 and 14 GHz. The cell however suffers from high-cross-polarization level. A chessboard cell arrangement is proposed to mitigate the high cross-polarization level at the reflectarray far-field region. Simulation results show the effectiveness of the chessboard arrangement in eliminating the cross-polarization allowing the design of a low-cross polarization reconfigurable reflectarray antenna out of a unit-cell with high cross-polarization level. Finally, the thesis presents the concept of a versatile flat prism which is a reflectarray with a pre-designed frequency-scanning behaviour. The limitations and challenges as well as solutions for implementation of such a device are presented and discussed.

EPFL2015

Theory, design and measurement of near-optimal graphene reconfigurable and non-reciprocal devices at terahertz frequencies

Michele Tamagnone

This thesis explores the applications of graphene for terahertz and far infrared optical components and antennas, with particular emphasis on tunable and non-reciprocal devices. Both terahertz technologies and graphene are emerging fields which hold many promises for a number of future applications, including ultra-broadband communications, sensing and security. A very important amount of research has been devoted to explore the potential applications of graphene and its advantages over existing technologies. Conversely, there is a clear set of applications that could benefit from the development of terahertz technologies, but there are several technical challenges in terms of very limited availability of materials and components to generate, manipulate and detect terahertz waves. The main idea of this work is to bring these two topics together to demonstrate that terahertz and far infrared technologies can greatly benefit from the unique optical properties of graphene. The first original contribution of this thesis is an important theoretical upper bound for the performance of non-reciprocal and tunable devices, demonstrating that both these components can achieve a target performances at the expense of an unavoidable optical loss, which depends uniquely on the properties of graphene. If graphene with higher mobility is used, this unavoidable loss can be reduced; however, independently of the design geometry (waveguide devices, free space planar devices, ...), the loss will always appear. This theoretical limit is an important guideline for the design of graphene optical devices, as it can predict the best possible performances prior to any design effort or numerical simulation. It is also demonstrated that devices able to reach the upper bound are actually possible, and hence these devices (modulators, isolators among others) are optimal. The thesis explores then a number of designs of graphene antennas for terahertz and mid infrared frequencies, where it is shown that gated graphene can be used to achieve frequency reconfiguration in resonant plasmonic antennas and beam steering in graphene based reflectarrays. Circuit models are provided as a simple way to understand the behavior of the device in a simple way. Furthermore, an experimental technique able to measure the complex conductivity of graphene at infrared frequency is demonstrates, providing a very useful evaluation of graphene quality at those frequencies. The potential of graphene for non-reciprocal applications is then demonstrated experimentally, with the design, fabrication and measurement of the first terahertz isolator (operation frequency between 1 THz and 10 THz). The isolator is a device which allows the unilateral propagation of light, and for that reason is often called âoptical diodeâ. The isolator uses graphene immersed in a magnetostatic field, and exhibits approximately 7 dB of loss in one direction and more than 25 dB in the other. The device is shown to be quasi-optimum according to the theoretical bound and greatly improved performances are predicted for devices with next generation CVD graphene. Finally, the first tunable graphene reflectarray is presented, which is a metasurface able to steer in a desired direction an incoming beam of terahertz radiation. The device acts as a mirror, but, upon graphene gating, the direction of the reflected beam can be controlled and the beam itself can be modulated with complex modulation schemes.

EPFL2016

Modulated scattering technique in the terahertz domain enabled by current actuated vanadium dioxide switches

Santiago Capdevila Cascante, Mihai Adrian Ionescu, Juan Ramon Mosig, Anja Skrivervik Favre, Michele Tamagnone, Wolfgang Amadeus Vitale

The modulated scattering technique is based on the use of reconfigurable electromagnetic scatterers, structures able to scatter and modulate an impinging electromagnetic field in function of a control signal. The modulated scattering technique is used in a wide range of frequencies up to millimeter waves for various applications, such as field mapping of circuits or antennas, radio-frequency identification devices and imaging applications. However, its implementation in the terahertz domain remains challenging. Here, we describe the design and experimental demonstration of the modulated scattering technique at terahertz frequencies. We characterize a modulated scatterer consisting in a bowtie antenna loaded with a vanadium dioxide switch, actuated using a continuous current. The modulated scatterer behavior is demonstrated using a time domain terahertz spectroscopy setup and shows significant signal strength well above 0.5 THz, which makes this device a promising candidate for the development of fast and energy-efficient THz communication devices and imaging systems. Moreover, our experiments allowed us to verify the operation of a single micro-meter sized VO2 switch at terahertz frequencies, thanks to the coupling provided by the antenna.