This thesis is devoted to potential applications of metamaterials in antenna structures as well as metamaterials behaviour, characterisation, structure design, simulation and extraction of parameters. The focus of this work is on the practical application of metamaterial structures for antenna performance enhancement. This thesis comprises three key parts; In the first part, theory of metamaterials is investigated including fields, polarisation, effective and average parameters, parameters extraction and transmission line (TL) model. In part two, zero index metamaterials (ZIM) theory is studied. The use of ZIM to form a highly directive medium is illustrated. A comparative study between different ZIM structures is conducted with a special attention to their operational bandwidth. ANSYS HFSS is used to model ZIM structures where simulation results show a bandwidth between 7.4% and 14.0%. Then two novel ZIM structures with a bandwidth of up to 33% are proposed. The first proposed ZIM is used to form a highly directive shell. Four directive shells are designed and placed around the dipole antenna where a gain increase of up to 6.8 dBi is obtained along the desired direction. Further, proposed ZIM cells are integrated with a quasi-Yagi antenna in order to increase its gain. Simulation results demonstrate gain enhancement for frequencies over which the proposed structure expresses ZIM properties. In part three, a new technique is stablished to design a metamaterial lens. The new technique is based on wave interference phenomena where engineered wave interference results in a desired spatial energy distribution. It is shown theoretically that having 180° phase difference between interfering waves results in a focused emission. Both hypothetical and metamaterial realisation models of a 180° phase shifter for a patch antenna are designed and simulated where a gain enhancement of 8 dBi and 5.77 dBi are achieved, respectively. Further, the concept of intended phase shift between interfering waves is used to design a novel bi-reflectional ground plane which can focus the reflected emission and consequently, increase the antenna directivity. In the theoretical model, the Perfect-E and Perfect-H planes are combined to form a bi-reflectional plane, whereas the practical model is designed using the copper cladding for the Perfect-E plane and the mushroom structure for the perfect-H plane. Both square and hexagonal geometries are used to form the mushroom structure. Simulation results confirm a gain enhancement of 5.4 dBi for the design using the square mushroom structure and a gain enhancement of 3.3 dBi for the design using the hexagonal mushroom structure.

In recent years a new class of materials, the metamaterials, has emerged in the scientific community. The use of these materials makes possible to achieve unique electromagnetic properties, such as the negative refractive index. Today there exist several applications that take advantage of these special properties, such as sensors, antennas and invisibility cloaks, aiming at improving their intrinsic characteristics. Based on these considerations, this project aims at developing metamaterials structures to control the radiation properties of antennas in the microwave range, such as gain and directivity. More specifically, it was also chosen chiral metamaterials, mainly due to the phenomenon of electromagnetic field rotation which opens the possibility to control efficiently the radiation properties of antennas. In addition, chiral metamaterials, which have proved to be a more attractive alternative to obtain negative or zero refractive index, enable a greater degree of freedom in the design of different structures. This work encompasses all different phases of the structure design, namely: project, computational modeling, fabrication, and characterization of the proposed structures. We show improvements for the gain that in some cases reaches more than the double of the conventional monopole antenna gain and for the return loss parameter, which reaches minimum values. We also could maintain good efficiency and improve the input impedance matching. Finally, it is worth mentioning that this new technology also has the great potential to be applied in the telecommunication devices, particularly to improve communications based on antennas. / Nos últimos anos uma nova classe de materiais, os metamateriais, emergiu na comunidade científica. O uso desses materiais torna possível alcançar propriedades eletromagnéticas singulares, como o índice de refração negativo. Hoje existem vastas aplicações que usufruem destas propriedades especiais, como os sensores, mantas de invisibilidade e antenas, onde se procura o aperfeiçoamento de suas características intrínsecas. Com base nestas considerações, este projeto buscou desenvolver estruturas metamateriais para controle das propriedades de radiação de antenas na faixa de micro-ondas, tais como diretividade e ganho. Mais especificamente, foram utilizados os metamateriais quirais, principalmente devido ao fenômeno de rotação do campo eletromagnético que abre a possibilidade de controle mais eficiente das propriedades de radiação de antenas. Além disso, os metamateriais quirais, por se mostrarem uma alternativa mais atraente para se obter meios com índice de refração zero ou negativo, possibilitam um maior grau de liberdade no projeto de diferentes estruturas. Este trabalho contempla, ainda, todas as etapas de projeto de tais estruturas, quais sejam: projeto, modelagem computacional, fabricação, e caracterização das estruturas. Mostramos melhorias para o ganho que, em alguns casos, chega a mais do que o dobro do ganho da antena monopolo convencional e para o parâmetro de perda de retorno, que atinge valores mínimos. Nós também mantivemos uma boa eficiência e melhoramos o casamento de impedância de entrada. Finalmente, vale salientar que essa nova tecnologia também apresenta grande potencial de ser aplicada em dispositivos de telecomunicações, com o intuito de aprimorar a comunicação baseada em antenas.

In recent years a new class of materials, the metamaterials, has emerged in the scientific community. The use of these materials makes possible to achieve unique electromagnetic properties, such as the negative refractive index. Today there exist several applications that take advantage of these special properties, such as sensors, antennas and invisibility cloaks, aiming at improving their intrinsic characteristics. Based on these considerations, this project aims at developing metamaterials structures to control the radiation properties of antennas in the microwave range, such as gain and directivity. More specifically, it was also chosen chiral metamaterials, mainly due to the phenomenon of electromagnetic field rotation which opens the possibility to control efficiently the radiation properties of antennas. In addition, chiral metamaterials, which have proved to be a more attractive alternative to obtain negative or zero refractive index, enable a greater degree of freedom in the design of different structures. This work encompasses all different phases of the structure design, namely: project, computational modeling, fabrication, and characterization of the proposed structures. We show improvements for the gain that in some cases reaches more than the double of the conventional monopole antenna gain and for the return loss parameter, which reaches minimum values. We also could maintain good efficiency and improve the input impedance matching. Finally, it is worth mentioning that this new technology also has the great potential to be applied in the telecommunication devices, particularly to improve communications based on antennas. / Nos últimos anos uma nova classe de materiais, os metamateriais, emergiu na comunidade científica. O uso desses materiais torna possível alcançar propriedades eletromagnéticas singulares, como o índice de refração negativo. Hoje existem vastas aplicações que usufruem destas propriedades especiais, como os sensores, mantas de invisibilidade e antenas, onde se procura o aperfeiçoamento de suas características intrínsecas. Com base nestas considerações, este projeto buscou desenvolver estruturas metamateriais para controle das propriedades de radiação de antenas na faixa de micro-ondas, tais como diretividade e ganho. Mais especificamente, foram utilizados os metamateriais quirais, principalmente devido ao fenômeno de rotação do campo eletromagnético que abre a possibilidade de controle mais eficiente das propriedades de radiação de antenas. Além disso, os metamateriais quirais, por se mostrarem uma alternativa mais atraente para se obter meios com índice de refração zero ou negativo, possibilitam um maior grau de liberdade no projeto de diferentes estruturas. Este trabalho contempla, ainda, todas as etapas de projeto de tais estruturas, quais sejam: projeto, modelagem computacional, fabricação, e caracterização das estruturas. Mostramos melhorias para o ganho que, em alguns casos, chega a mais do que o dobro do ganho da antena monopolo convencional e para o parâmetro de perda de retorno, que atinge valores mínimos. Nós também mantivemos uma boa eficiência e melhoramos o casamento de impedância de entrada. Finalmente, vale salientar que essa nova tecnologia também apresenta grande potencial de ser aplicada em dispositivos de telecomunicações, com o intuito de aprimorar a comunicação baseada em antenas.

<p>One of the longstanding goals of photonics research has been to obtain strong optical nonlinearities. A promising method to achieve this goal is to operate in the so-called epsilon near zero (ENZ) spectral regime, where the real part of the dielectric permittivity changes sign. If accompanied by low losses, this region enables a platform to achieve extraordinarily high nonlinear response, along with many other interesting optical phenomena. In this work, some of the common all-optical switching structures employing homogeneous ENZ materials are investigated under varying conditions of frequency, incidence angle, and polarization. The optimum switching conditions have been highlighted to pave the way forward to the best experimental configurations in future studies. Moreover, the properties of some of the emerging novel plasmonic materials such as aluminum-doped zinc oxide (AZO) and titanium nitride (TiN) are investigated, specifically for ENZ applications. Their thickness-dependent crystalline structure and carrier densities are employed as a method to control their optical properties. A near-perfect absorption scheme is demonstrated utilizing the Ferrell-Berreman mode occurring at the ENZ region of ultrathin AZO and TiN film. The ENZ frequency and the associated absorption peak of AZO are engineered through thickness-dependence to cover most of the telecom range. This work covers the theoretical background for ENZ nonlinearities and looks into the materials aspect for better control of nonlinearities in experimental realizations.</p>

Since its inception, the field of nonlinear optics has only increased in importance as a result of a growing number of applications. The efficiency of all parametric nonlinear optical processes is limited by challenges associated with phase-matching requirements. To address this constraint, a variety of approaches, such as quasi-phase-matching, birefringent phase matching, and higher-order-mode phase matching have historically been used to phase-match interactions. However, the methods demonstrated to date suffer from the inconvenience of only being phase-matched for one specific arrangement of beams, typically co-propagating along the same axis. This stringency of the phase-matching requirement results in cumbersome optical configurations and large footprints for integrated devices. In this thesis, we show that phase-matching requirements in parametric nonlinear optical processes may be satisfied for all orientations of input and output beams when using zero-index media: a condition of omnidirectional phase matching. To validate this theory, we perform experimental demonstrations of phase matching for five separate FWM beam configurations to confirm this phenomenon. Our measurements constitute the first experimental observation of the simultaneous generation of a forward- and backward-propagating signal with respect to the pump beams in a medium longer than a free-space optical wavelength, allowing us to determine the coherence length of our four-wave-mixing process. Our demonstration includes nonlinear signal generation from spectrally distinct counter-propagating pump and probe beams, as well as the excitation of a parametric process with the probe beam's wave vector orthogonal to the wave vector of the pump beam. By sampling all of these beam configurations, our results explicitly demonstrate that the unique properties of zero-index media relax traditional phase-matching constraints, and provide strong experimental evidence for the existence of omnidirectional phase matching in zero-index media. This property can be exploited to facilitate nonlinear interactions and miniaturize nonlinear devices, and adds to the established exceptional properties of low-index materials.