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.