Contribution to characterization techniques for practical metamaterials and microwave applications

Metamaterials (MTMs) are broadly defined as artificial composite materials specifically engineered to produce desired unusual electromagnetic properties not readily available in nature. The most interesting unusual property achievable with MTMs is probably negative refraction, which is achieved when both the permittivity and the permeability of a medium are negative. Such structures are also referred to as left-handed media (LHM). From the first evidences in the early 2000's showing that materials with a negative refractive index were indeed physically realizable, numerous entirely new devices or improvements of existing devices have been reported in the microwave and antenna fields. In this context, the objective of this thesis is to contribute to the development of new characterization techniques for practical implementations of MTMs, aiming at determining a set of relevant equivalent medium parameters describing the structure from a macroscopic point of view. For this purpose, analysis techniques were developed based on the theory of wave propagation in periodic structures, and tested on selected existing or entirely new MTM structures of the two main reported categories: arrays of resonant particles and loaded transmission lines. In the first part of the work, an improved retrieval procedure which allows the determination of equivalent dyadic permittivity and permeability of MTMs from reflection and transmission coefficients obtained for several incidences was developed and tested, thereby extending current techniques which only dealt with normal incidence. The main achievement obtained with this technique is the ability to evaluate to which extent a given MTM slab can be considered as an equivalent homogeneous medium obeying some specific constitutive relations. This technique was tested on various structures, including a novel highly isotropic artificial magnetic material which was shown to exhibit a negative permeability in the three dimensions. In a second step, MTMs based on the transmission line approach have been investigated. In this context, the theory of the so-called composite right/left-handed transmission line (CRLH TL) has been revisited, and several planar implementations of this structure in various technologies were designed and realized. Subsequently, a volumetric LHM obtained by layering several planar artificial TLs of the CRLH type was proposed and fully characterized. This volumetric structure was shown to support left-handed propagation over a quite large bandwidth, compared to other resonant LHM made of split-ring resonators and wires. We provided an extensive experimental assessment of potential applications of this structure as an exotic substrate for microstrip patch antennas. An important contribution here consisted in the assessment of the ability of such a volumetric structure based on the TL approach to behave as a material filling in this type of configurations. The next part presents an enhanced analysis technique for periodic structures which allows accurately characterizing MTMs exhibiting higher order coupling phenomena between successive cells. This technique also allows an accurate and complete description of more elaborated structures such as periodically loaded multiconductor TLs. The main idea of this technique is to model the periodic structure with an equivalent multiconductor TL, a model which provides all the parameters needed to describe the phase response (dispersion) and terminations (excitation and matching) of finite size periodic structures. In the last part, we introduced and analyzed a novel unit cell topology for the CRLH TL which employs a lattice network in place of the conventional ladder-type topology. This new CRLH TL was shown to exhibit a more wideband behaviour than its conventional counterpart, both in terms of impedance and phase. These performances were numerically and experimentally demonstrated on several practical implementations. The possibilities of using this unit cell to reduce the beam squinting in leaky-wave antennas and in series-fed arrays were highlighted. It is foreseen that this new CRLH TL can be potentially used to improve the performances of many of the well-known CRLH TL applications

Métamatériau Mécano-Acoustique basé sur le Concept de Ligne de Transmission Duale

Les métamatériaux acoustiques sont des structures artificielles conçues pour réaliser des propriétés macroscopiques inédites dans la nature, telles qu’un indice de réfraction négatif. Parmi les différentes configurations permettant d’obtenir de telles propriétés, les structures basées sur le concept de ligne de transmission duale sont connues en électromagnétisme pour présenter les meilleures performances en termes de largeur de bande. De plus, ces structures se prêtent particulièrement bien à une représentation par le formalisme de Kirchhoff, utilisé habituellement pour le dimensionnement des transducteurs électroacoustiques. Dans ce contexte, un métamatériau unidimensionnel présentant un indice de réfraction négatif et basé sur l’approche de ligne duale est présenté. La structure proposée consiste en un guide d’onde acoustique chargé périodiquement par des membranes réalisant la fonction de « compliances acoustiques séries », ainsi que par des conduits transverses ouverts réalisant la fonction de « masses acoustiques parallèles ». Cette structure présente un indice de réfraction négatif sur environ une octave (de 600 Hz à 1000 Hz), suivi d’une seconde bande à indice de réfraction positif, sans transition discontinue entre les deux domaines. Cet exemple présente avantageusement les méthodes de circuits acoustiques à constantes localisées comme une solution pour simplifier le dimensionnement de structures autrement complexes à modéliser. Une validation du principe, d’une part avec un modèle en circuits équivalents, et d’autre part avec une méthode numérique multiphysique basée sur la résolution formelle des équations constitutives, soutient l’approche envisagée, et aboutit à des premières perspectives en termes de design et de performances

Experimental verification of broadband cloaking using a volumetric cloak composed of periodically stacked cylindrical transmission-line networks

Cloaking using a volumetric structure composed of stacked two-dimensional transmission-line networks is verified with measurements. The measurements are done in a waveguide, in which an array of metallic cylinders is inserted causing a short-circuit in the waveguide. The metal cylinders are cloaked using a previously designed and simulated cloak that hides the cylinders and thus enables wave propagation inside the waveguide.Comment: 8 pages, 3 figure

Transmission Line Based Metamaterials for Acoustic Waves

We present our recent work on a one-dimensional acoustic negative refractive index metamaterial based on the concept of dual transmission line extensively investigated in microwave engineering. The proposed structure consists of an acoustic waveguide periodically loaded with membranes realizing the function of series “capacitances” and transversally connected open channels realizing shunt “inductances”. It exhibits a negative refractive index band over almost one octave, from 0.6 to 1 kHz. Using formal analogies, we describe how simple acoustic circuit models can be used for efficient design of metamaterials both in terms of dispersion and impedance

Design of Acoustic Metamaterials based on the Concept of Dual Transmission Line

Within the last years, an increasing number of studies have been carried out in the field of acoustic metamaterials. These artificial composite materials aim at achieving new macroscopic properties, like negative refraction, that are not readily present in nature. In analogy to electromagnetics, where such concepts are more mature, a novel concept of artificial acoustic transmission line has recently been reported, which presents such artificial behavior. In this article, the design of the proposed transmission line is presented and a validation is made with the help of a finite element model. Moreover, these results are compared to a usual circuit description of the problem. One cell and 10-cell long structures are implemented in Comsol Multiphysics® and confirm the good performances of the different models, in terms of dispersion diagram, Bloch impedance, as well as reflection and transmission coefficients