Ahmed Kord, PhD

Postdoctoral Research Scientist, Columbia University

Chip-scale Floquet topological Insulators, 2019

A topological insulator is a 2D or a 3D material which behaves as an insulator in the bulk yet as a conductor on the surface. What is unique about TIs is that their edge states are unidirectional and immune against reflections from any surface imperfections or local disorders. The reflection-free property is particularly of interest in quantum systems for it can be used to mitigate the decoherence problem of current qubit technologies. On the other hand, the unidirectionality of TIs is also very useful in classical electronic and photonic systems, as has already been established in recent years in the context of non-reciprocity. I have been working recently on synthesizing a chip-scale Floquet TI based on a reconfigurable 4x4 array of magnet-free circulators using GF 45nm RF-SOI process, which would be the first-ever experimental validation of such artificial materials with potential applications in wireless communications as a programmable antenna interface for full-duplex MIMO and phased array systems.

Magnetless Circulators Based on Linear Time-Varying Circuits, 2015-2019

In a crowded electromagnetic spectrum with an ever‐increasing demand for higher data rates to enable multimedia‐rich applications and services, an efficient use of the available wireless resources becomes crucial. For this reason, full‐duplex communication, which doubles the transmission rate over a certain bandwidth compared to currently deployed half-duplex radios by operating the uplink and the downlink simultaneously on the same frequency, has been brought back into the spotlight after decades of being presumed impractical. This long‐held assumption has been particularly due to the lack of high performance low-cost and small-size circulators that could mitigate the strong self-interference at the RF frontend interface of full-duplex transceivers while, at the same time, permitting low-loss bi-directional communication using a single antenna. Traditionally, such non-reciprocal components were almost exclusively based on magnetic biasing of rare-earth ferrite materials, which results in bulky and expensive devices that are not suitable for the vast majority of commercial systems. Despite significant research efforts over the past few decades, none of the previous works managed to eliminate the magnet while satisfying all the challenging requirements dictated by the standards of real systems. In this work, I introduced several new topologies and architectures of magnetless circulators based on linear time-varying circuits that overcome for the first time the limitations of all previous approaches. I analyzed the presented circuits rigorously and validated them through simulations and measurements, showing unprecedented performance in all relevant metrics, thus holding the promise to enable full-duplex radios in the near future.

Active Microwave Cloaking Based on Parity-Time Symmetric Satellites, 2018

The possibility of minimizing the scattering from an object and making it undetectable to external observers has intrigued the imagination of mankind for centuries. This process, known in the scientific literature as cloaking, is not only of fundamental significance from a basic research standpoint but it also has many applications at different portions of the electromagnetic spectrum. For example, at RF and microwave frequencies, cloaking can be used in radar systems to enhance the stealth technology by suppressing the shadow of hidden objects. It can also be used in wireless communications to mitigate the self-interference in MIMO radios and in wireless energy transfer to improve the charging efficiency by eliminating the loss due to scattering. Research towards these goals has seen different breakthroughs since the advent of metamaterials and numerous approaches have been proposed, yet very little advancement has been made to realize broadband cloaks that can hide arbitrarily large objects which, in turn, casted a lot of doubt on whether this is even possible. Nevertheless, I find it astonishing that nearly all cloaking devices presented to-date are based on passive approaches which already suffer from high intrinsic losses and fundamentally narrow bandwidth. In principle, these problems can be overcome by using an active metamaterial which incorporates a balanced distribution of gain and loss. For this reason, I started exploring in this project the idea of covering a scattering object with an active metasurface that absorbs the incident signal on one side and re-emits it on the other side. Rigorous theoretical analysis and simulation results show that such an approach can indeed overcome the limits of passive cloaks. An experimental demonstration will be presented in the near future.

Composite Right-/Left-Handed Metamaterials, 2013-2014

Composite right-/left-handed (CRLH) metamaterials are structures that exhibit simultaneous negative permittivity and permeability over a finite BW resulting in left-handed transmission within this band, in addition to the natural double-positive and right-handed characteristics far from it. These structures can be built using low-loss and high-power waveguide structures operating near their dominant mode cutoff and periodically loaded with stubs having a lower cutoff frequency. In order to optimize the design of the stubs, I developed a general form of asymptotic corrugation boundary conditions (ACBCs) that can be applied to any interface between two materials with arbitrary cross-sections. This, in turn, permitted a systematic design of several CRLH components with enhanced performance such as backward-to-forward leaky wave antennas with a wide beam-scanning range.

Computational Electromagnetics, 2013-2014

In this project, I developed several GUI tools to analyze various RF circuits and EM scattering problems based on FDTD, FEM, and MOM methods. All programs were written in Matlab and were validated by comparing the results mutually and to analytical solutions of standard problems. Further improvements in terms of speed, memory efficiency, and post-processing capabilities were implemented later during my PhD.

Low-power Implantable Pressure Sensor for Biomedical Applications, 2010-2011

This was my senior-year project and I worked on it with five other students. We designed the entire chip from system-level to layout using a standard 130 nm CMOS process. I was responsible for the energy harvesting unit which included a 1.8V LDO regulator, a 1.2V bandgap reference, an RF rectifier, an off-chip antenna, and a backscattering communication module. I also engaged in regular discussions with the group members on the design of all other analog blocks. This project was sponsored by Si-Ware Systems, a fabless semiconductor company in Egypt.