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Optical characterisation of metamaterials

This master project has, as main target, the realisation of an experimental set-up capable to optically characterise metamaterials. Besides that, the project includes the design and the analysis of different fishnet structures fabricated in DTU Danchip facility and the characterisation of woodpile structures, realised by Laser Zentrum Hannover e.V. Metamaterials are artificial man-made materials exhibiting particular properties such as negative refraction [5] which can lead to several applications like the realisation of superlenses [6] or invisibility cloaking [57], [58], [59]. In order to reach a negative refractive index, simultaneously negative electric permittivity and magnetic permeability need to be reached as pointed out by Veselago in 1968 [5]. A negative permittivity is a relatively easy task to obtain, since all metals below their plasma frequency exhibit it. On the other hand, a negative permeability is more difficult to achieve; the lack of magnetic charges and the weak interaction between matter and the magnetic part of light, make scientists capable to attain it only in certain frequency regions. Combining thin metallic wires and rods in a three layers’ Metal/Dielectric/Metal structure, constitutes the so called fishnet structure which has been proved to exhibit peculiar characteristics both in metamaterials [34] and plasmonics fields [40]. Recently, an increasing interest is given to three-dimensional metamaterials: the practical realisation of the aforementioned applications needs 3D, possibly isotropic, structures. An intuitive extension to the 3D case of the fishnet structures, are the so called woodpile structures whose study is important both for metamaterials and electromagnetic bandgap materials [54]. A complete theory about metamaterials doesn’t exist yet, thus, the realisation of an experimental set-up able to optically characterise them, is the first fundamental step for the analysis and for the study of their properties. This thesis is organised as follows: in the first chapter an introduction about metamaterials field of study is presented. In the second chapter a theoretical background is given in order to comprehend metamaterials’ main properties and how they can be realised. In the third chapter the requirements to characterise metamaterials are pointed out and the set-up, which has been built during the project, is presented. In chapter four, our results about the fabrication and characterisation of fishnet structures are shown and discussed. The focus is then shifted to 3D metamaterials, in chapter five, where polymer woodpiles are studied and their unexpected behaviour, when covered with silver [53], is highlighted.
At the end, in the conclusive chapter, impressions about the whole project are given together with considerations about future improvements for the experimental set-up.

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5 1. Introduction The word metamaterial can be split into the word meta-, that takes its origin from the greek term µετά which means beyond , and the word material. These kinds of materials can be, theoretically, synthesized embedding some composites with specific shapes into a host medium forming a particular pattern. This means we are going to talk about something beyond our usual concept of material that takes its optical properties from the atoms it is naturally made of. We will rather consider a kind of medium whose primitive characteristics (as light-matter interaction) strongly depend on man made sub-unities (containing many atoms) which constitute its structure. The features of the structure involved in the fabrication, by a physical point of view, are so crucial that their combination endows the whole structure of new macroscopic optical effects which don t exist in nature. The growing interest in metamaterials lies in the possibility to tailor a medium optical response in order to obtain a variety of applications. Artificial materials with peculiar electromagnetic properties are not a new challenge. In 1898 Sir Jagadish Chandra Bose (1858 1937) first experimented microwave responses from twisted structures geometries today known as chiral elements1 [1]. In 1914 Lindman (1874 1952) created artificial chiral elements embedding randomly oriented small helices coiled from copper wires in a host medium [2]. Later in 1948 Kock actually was able to tailor the refractive index of a microwave lens, periodically arranging conducting elements [3]. The capability in manipulating the interaction between matter and electromagnetic fields can be pushed further leading to extraordinary consequences such as negative refraction that, in the middle of the last century, was considered no more than an abstract speculation. In 1944 Mandelshtam [4] noticed that, considering Snell s law2 between two media, for a given incidence angle θ1 at the interface, mathematically two solutions are possible for the direction of the refracted ray: not only the 1 Chiral materials are a subclass of bi-anisotropic media showing an intrinsic asymmetry with respect to the left and right. The effect of chirality on electromagnetic field propagation is a rotation of the plane of linearly polarized waves. 2 Snell Law rules light rays directions propagating through an interface between two dielectric media. Let n1, n2 be respectively medium 1 and medium 2 refractive index. If θ1 is the angle of incidence of the ray in medium 1, the angle of refraction θ2 in medium 2 can be obtained from the relation: n1sinθ1=n2sinθ2 .

Laurea liv.II (specialistica)

Facoltà: Ingegneria

Autore: Alessandro Alabastri Contatta »

Composta da 111 pagine.


Questa tesi ha raggiunto 167 click dal 24/11/2010.

Disponibile in PDF, la consultazione è esclusivamente in formato digitale.