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

Abstract

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 “optical diode”. 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.