Amin Tayebi

Dynamic Beam Shaping Using a Dual-Band Electronically Tunable Reflectarray Antenna

An electronically reconfigurable dual-band reflectarray antenna is presented in this paper. The tunable unit cell, a ring loaded square patch with a single varactor diode connected across the gap between the ring and the patch, is modeled using both a full-wave solver and an equivalent circuit. The parameters of the equivalent circuit are calculated independently of the simulation and experiment using analysis techniques employed in frequency-selective surfaces (FSSs). The reflection phase of the proposed unit cell is shown to provide an excellent phase range of 335° in F-band and 340° in S -band. Results from the analysis are used to design and build a 10 × 10 element reflectarray antenna. The high tuning phase range of each element allows the fabricated reflectarray to demonstrate a very broad steering range of up to ± 60° in both frequency bands.

Quantum signal transmission through a single-qubit chain

A system of a two-level atom of an impurity (qubit) inserted into a periodic chain coupled to the continuum is studied with the use of the effective non-Hermitian Hamiltonian. Exact solutions are derived for the quasistationary eigenstates, their complex energies, and transport properties. Due to the presence of the qubit, two long-lived states corresponding to the ground and excited states of the qubit emerge outside the Bloch energy band. These states remain essentially localized at the qubit even in the limit of sufficiently strong coupling between the chain and the environment when the super-radiant states are formed. The transmission through the chain is studied as a function of the continuum coupling strength and the chain-qubit coupling; the perfect resonance transmission takes place through isolated resonances at weak and strong continuum coupling, while the transmission is lowered in the intermediate regime.

Design and development of an electrically-controlled beam steering mirror for microwave tomography

Microwave tomography has gained significant attention due to its reliability and unhazardous nature in the fields of NDE and medical industry. A new microwave tomography system is presented in this paper, which significantly reduces the design and operational complexities of traditional microwave imaging systems. The major component of the proposed system is a reconfigurable reflectarray antenna which is used for beam steering in order to generate projections from multiple angles. The design, modeling and fabrication of the building block of the antenna, a tunable unit cell, are discussed in this paper. The unit cell is capable of dynamically altering the phase of the reflected field which results in beam steering ability of the reflectarray antenna. A tomographically reconstructed image of a dielectric sample using this new microwave tomography system is presented in this work.

A dual-band tunable reflectarray

In this work, a dual-band reflectarray with reconfigurable beam angle is presented. The unit cell of the reflectarray is a square ring-patch structure which was optimized to perform within two distinct frequency bands. The full wave simulation software HFSS was used to analyze a unit cell inside a waveguide, approximating the infinite array scenario. The tuning of the unit cell was achieved by altering the capacitance of a varactor diode placed between the square ring and the patch. A 10 × 10 element array will be built and measured to demonstrate its capability of beam steering over a wide range of angles.

Improved backpropagation algorithms by exploiting data redundancy in limited-angle diffraction tomography

Filtered backpropagation (FBPP) is a well-known technique used in Diffraction Tomography (DT). For accurate reconstruction of a complex-valued image using FBPP, full 360 ◦ angular coverage is necessary. However, it has been shown that by exploiting inherent redundancies in the projection data, accurate reconstruction is possible with 270 ◦ coverage. This is called the minimal-scan angle range. This is done by applying weighting functions (or filters) on projection data of the object to eliminate the redundancies. There could be many general weight functions. These are all equivalent at 270 ◦ coverage but would perform differently at lower angular coverages and in presence of noise. This paper presents a generalized mathematical framework to generate weight functions for exploiting data redundancy. Further, a comparative analysis of different filters when angular coverage is lower than minimal-scan angle of 270 ◦ is presented. Simulation studies have been done to find optimum weight filters for sub-minimal angular coverage. The optimum weights generate images comparable to a full 360 ◦ coverage FBPP reconstruction. Performance of the filters in the presence of noise is also analyzed. These fast and deterministic algorithms are capable of correctly reconstructing complex valued images even at angular coverage of 200 ◦ while still under the FBPP regime.

Class of backpropagation techniques for limited-angle reconstruction in microwave tomography

Filtered backpropagation (FBPP) is a well-known technique used in Diffraction Tomography (DT). For accurate reconstruction using FBPP, full 360° angular coverage is necessary. However, it has been shown that using some inherent redundancies in the projection data in a tomographic setup, accurate reconstruction is still possible with 270° coverage which is called the minimal-scan angle range. This can be done by applying weighing functions (or filters) on projection data of the object to eliminate the redundancies and accurately reconstruct the image from 270° coverage. This paper demonstrates procedures to generate many general classes of these weighing filters. These are all equivalent at 270° coverage but vary in performance at lower angular coverages and in presence of noise. This paper does a comparative analysis of different filters when angular coverage is lower than minimal-scan angle of 270°. Simulation studies have been done to find optimum weight filters for sub-minimal angular coverage (<270°).

Reconstruction algorithm for limited-angle diffraction tomography for microwave NDE

Microwave tomography is becoming a popular imaging modality in nondestructive evaluation and medicine. A commonly encountered challenge in tomography in general is that in many practical situations a full 360° angular access is not possible and with limited access, the quality of reconstructed image is compromised. This paper presents an approach for reconstruction with limited angular access in diffraction tomography. The algorithm takes advantage of redundancies in image Fourier space data obtained from diffracted field measurements and couples it to an error minimization technique using a constrained total variation (CTV) minimization. Initial results from simulated data have been presented here to validate the approach.

A microwave tomography system using a tunable mirror for beam steering

Microwave tomography is a fast-growing technique in the fields of NDE and medical industry. This paper presents a new microwave tomography system which reduces the complexities of conventional microwave imaging systems by utilizing a reconfigurable mirror, a tunable reflectarray antenna. In order to build a tunable reflectarray with beam steering capabilities, the unit cell characteristics should dynamically alter. Modelling and experimental results of a single unit cell are presented in this work.

A Novel Time Reversal Based Microwave Imaging System

This paper presents an alternate microwave imaging system that greatly reduces design and operation complexities compared to traditional imaging systems. At the heart of this novel system lies an electronically reconfigurable beam-scanning reflectarray antenna. The high tuning capability of the reflectarray provides us a broad steering range of ±60 ◦ . The beam is steered across this range and the scattered field is recorded. The collected data are used for image reconstruction by means of the time reversal signal processing technique. Experimental results of the detection of various dielectric targets are presented.

Environment-protected solid-state-based distributed charge qubit

A novel solid state based charge qubit is presented. The system consists of a one-dimensional wire with a pair of qubits embedded at its center. It is shown that the system supports collective states localized in the left and right sides of the wire and therefore, as a whole, performs as a single qubit. The couplings between the ground and excited states of the two central qubits are inversely proportional making them fully asynchronized and allowing for coherent manipulation and gate operations. Initialization and measurement devices, such as leads and charge detectors, connected to the edges of the wire are modeled by a continuum of energy states. The coupling to the continuum is discussed using the effective non-Hermitian Hamiltonian. At weak continuum coupling, all internal states uniformly acquire small decay widths. This changes dramatically as the coupling strength increases: the width distribution undergoes a sharp restructuring and is no longer uniformly divided among the eigenstates. Two broad resonances localized at the ends of the wire are formed. These superradiant states (analogous to Dicke states in quantum optics), effectively protect the remaining internal states from decaying into the continuum and hence increase the lifetime of the qubit. Environmental noise is introduced by considering random Gaussian fluctuations of electronic energies. The interplay between decoherence and superradiance is studied by solving the stochastic Liouville equation. In addition to increasing the lifetime, the emergence of the superradiant states increases the qubit coherence.