The term photonic crystals appears because of the analogy between electron waves in crystals and the light waves in artificial periodic dielectric structures. During the recent years the investigation of one-, two-and three-dimensional periodic structures has attracted a widespread attention of the world optics community because of great potentiality of such structures in advanced applied optical fields. The interest in periodic structures has been stimulated by the fast development of semiconductor technology that now allows the fabrication of artificial structures, whose period is comparable with the wavelength of light in the visible and infrared ranges.

http://ieeexplore.ieee.org/iel5/6354/16969/00781867.pdf

Photonic crystals

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Rfid Handbook Fundamentals And Applications In Contactless Smart Cards And Identification

Resonance conditions for a substrate-superstrate printed antenna geometry which allow for large antenna gain are presented. Asymptotic formulas for gain, beamwidth, and bandwidth are given, and the bandwidth limitation of the method is discussed. The method is extended to produce narrow patterns about the horizon, and directive patterns at two different angles.

Gain enhancement methods for printed circuit antennas

As stated in Chapter 1, one of the many advantages of microstrip patch technology over its competitors is its low profile and hence small volume. Another key advantage of this printed antenna is the relative ease in which it can be connected to the feed network, as was highlighted in Chapter 2. For these reasons antenna design engineers deduced that microstrip patch antennas could be utilized for applications requiring where there was very limited space to mount the antenna. One such global application is for wireless communication handset terminals.

Small microstrip patch antenna

In electrical impedance tomography an approximation for the internal resistivity distribution is computed based on the knowledge of the injected currents and measured voltages on the surface of the body. It is often assumed that the injected currents are confined to the two-dimensional (2-D) electrode plane and the reconstruction is based on 2-D assumptions. However, the currents spread out in three dimensions and, therefore, off-plane structures have significant effect on the reconstructed images. In this paper we propose a finite element-based method for the reconstruction of three-dimensional resistivity distributions. The proposed method is based on the so-called complete electrode model that takes into account the presence of the electrodes and the contact impedances. Both the forward and the inverse problems are discussed and results from static and dynamic (difference) reconstructions with real measurement data are given. It is shown that in phantom experiments with accurate finite element computations it is possible to obtain static images that are comparable with difference images that are reconstructed from the same object with the empty (saline filled) tank as a reference.

Three-dimensional electrical impedance tomography based on the complete electrode model

A mode matching methodology for the analysis of periodically loaded circular waveguides is proposed. Our present efiort is focused on inflnite periodic waveguides employing two difierent approaches. The flrst one is similar to the so called transfer matrix technique, where classical mode matching is employed to analyze the discontinuities involved within a unit cell. In turn periodic boundary conditions are enforced between the input and output ports of the unit cell. The resulting system of equations is formulated as an eigenproblem and solved for the Floquet wavenumbers. A more rigorous approach is also under development where the fleld within the unit cell is expressed directly as a Floquet expansion. While the methodology is general, it is herein applied particularly for the study of periodical irises in which complementary split ring resonators (CSRRs) are etched. 1. INTRODUCTION Periodic structures have been examined and investigated widely, due to their importance in design and implementation of a plethora of microwave devices, mainly microwave fllters. The early anal- ysis was based on approximate-analytical techniques and the resultant deviations were regulated with mechanical moving elements. The usage of complex geometries and anisotropic material load- ings, ofiers excellent potential. Additionally electromagnetic band gap (EBG) applications, where the periodic structure is designed to prevent the wave propagation either at certain frequencies (frequency band gap) or in certain directions (space gap or space selective propagation), have been acquired a special attention, particularly in recent years. In this paper, we will examine a circular waveguide loaded with inflnite or flnite periodic struc- tures. These are mainly comprised of irises etched on diaphragms or corrugations and we will try to investigate their propagation characteristics. Particular efiort is devoted to periodic structure loaded with periodically repeated split ring resonators (SRR), printed on dielectric diaphragms, which presents negative refraction phenomena and constitutes a popular physical realization of left handed (LH) medium. Explicitly, LH behavior is expected when circular waveguide is operated below cutofi and this phenomenon will be examined herein in order to be verifled for both inflnite and flnite periodic SRRs sections. Usually such structures are analyzed and designed by usage of advanced numerical methods. However, the increasing demand for accurate prediction of wave propagation behavior and the electrically large dimensions of such structures, render the numerical simulation, time consuming or almost impossible. Floquet theorem has been used to tackle this problem, but this is possible if only the periodic structure extends inflnitely. Then the analysis is restricted to the determination of only one spatial period-cell. The electromagnetic fleld in one cell is related with electromagnetic fleld of the next or the previous cell by an exponential function. Hence, periodic boundary conditions are enforced on the unit cell's periodic surfaces in order to introduce the periodicity into the formulation. Ini- tially the inflnitely extended circular waveguide is divided into unit cells. The unit cell internal structure is composed of discontinuities and uniform (geometrically) circular waveguide sections. Each discontinuity is analyzed with the aid of a Mode Matching Technique (MMT). Electromag- netic flelds at the terminal cross sections of each uniform unit cell section are correlated by known expressions (5). Finally the Floquet theorem, by mean of periodic boundary conditions, at the ports of the unit cell, is used in combination with previous expressions in order to derive a classical eigenvalue problem which is solved for the Floquet wavenumbers.

A Mode Matching Methodology for the Analysis of Circular Waveguides Loaded with Inflnite and Finite Periodic Structures

The establishment of finite element based eigenanalysis as a numerical tool for the indepth understanding and revealing the characteristics of Terahertz and photonic structures, constitute the scope of this work A tree-cotree splitting formulation method is used for the removal of imaginary and dc spurious modes The Ritz vectors are appropriately restricted during the linearization technique of the polynomial eigenproblem The capabilities of the method are demonstrated presenting certain microdisk and microrings whispering gallery modes eigenvalues and eigenvectors

Eigenanalysis of photonic structures based on finite element with efficient suppression of spurious modes

A Hybrid technique for the analysis of pyramidal horn antennas is proposed in this paper. The transition from a relatively small feeding waveguide to a larger radiating aperture is analyzed using the mode matching technique, while the discontinuity between the horn and free space is analyzed using the method of auxiliary sources. The resulting procedure is very stable and accurate while the computational time is quite small since all the coupling integrals involved in the mode matching are evaluated analytically. I. Introduction The proposed hybrid method aims at the accurate prediction of the return loss and the radiation pattern of horn antennas. Approximate methods used in the past for the analysis of this type of antennas failed to accurately calculate the VSWR as well as the far-out sidelobes and backlobe radiation patterns. A hybrid method was proposed in [1] for the analysis of conical horn antennas, where the Mode Matching Technique (MMT) was used for the evaluation of the scattering parameters of the antenna and the method of moments for the analysis of the transition between the horn and the free space. This method, restricted to circularly symmetric structures, proved to be very accurate. Liu et al [2], proposed a similar technique for the analysis of rectangular horn antennas. In the proposed paper the horn antenna is divided into a series of waveguide sections and step junctions, which are analyzed using a closed-form MMT developed in our previous work [3]. In this manner the field within the horn as well as on its aperture can be described by the MMT. In [3] we assumed that the aperture was a perfect termination for all the incident waveguide modes. This approximation causes inaccuracies, especially in the antenna input impedance and when the aperture electrical dimensions are relatively small (in terms of wavelengths). In order to overcome this shortcome this paper aims at the characterization of the aperture discontinuity as an imperfect junction between the horn and the free space. The resulting hybrid technique can accurately and efficiently evaluate the return loss and the radiation characteristics of the antenna under investigation. This is due to the closed form representation of the coupling integrals involved in the MMT, while the key feature of the MAS is the displacement of the location of the auxiliary sources with respect to the actual boundary. This last feature provides a non-vanishing distance between sources and observation points, leading to a stable numerical code. Moreover, this procedure is applicable to any waveguide or horn geometries, but it could also be applied to other structure like the leaky waveguides.

https://www.piers.org/piers2k4Pisa/session_02a/02a_12.pdf

Horn Antennas Analysis Using a Hybrid Mode Matching-Auxiliary Sources Technique

This work aims at the establishment of a rigorous full-wave eigenmode analysis technique based on a finite element scheme for the study of terahertz (THz) or photonic/optical unbounded structures. This numerical tool follows the last decades' trend to migrate the technological knowledge from microwave to THz and photonic regimes. The performed eigenanalysis offers an insightful view of the studied structures, revealing their characteristics. For the truncation of the infinite solution domain, the first kind absorbing boundary conditions are employed, while the involved spurious modes are eliminated with the incorporation of a tree-cotree splitting formulation. The study focuses on tunable microring resonators supporting leaky-radiating wave propagation and bounded-resonating whispering gallery modes. Tunability is achieved by integrating an electro-optical layer and in turn through the enforcement of an external applied DC electric field. An innovative approach constitutes the numerical determination of the altered dielectric permittivity as a piecewise constant distribution rather than a constant mean value.

/pdf/an-eigenanalysis-study-of-tunable-thz-and-photonic-unbounded-1sdwcqwuml.pdf

An Eigenanalysis Study of Tunable THz and Photonic Unbounded Structures Employing Finite Element Method

A wideband switched beam beamforming network based on an 8£8 Butler matrix is designed. This structure is implemented in a multilayer microstrip slot technology that employs two dielectric layers with common ground plane and coupling slots. Elliptically shaped microstrip hybrids and phase shifters are adopted to realize the required wide bandwidth. The flnal design of the Butler matrix provides a compact and without any crossovers beamforming network. The Butler matrix outputs feed a tapered slot Vivaldi antenna array that provides operating bandwidth covering the whole desired frequency range of 3.1 to 10.6GHz.

George A. Kyriacou

Papers

A Mode Matching Methodology for the Analysis of Circular Waveguides Loaded with Inflnite and Finite Periodic Structures

Eigenanalysis of photonic structures based on finite element with efficient suppression of spurious modes

Horn Antennas Analysis Using a Hybrid Mode Matching-Auxiliary Sources Technique

An Eigenanalysis Study of Tunable THz and Photonic Unbounded Structures Employing Finite Element Method

Design of a Wideband RF Front End Based on Multilayer Technology