Metamaterials are artificial structures, which periodically arranged to exhibit fascinating electromagnetic properties, not existing in nature. A great deal of research in the field of metamaterial was conducted in a linear regime, where the electromagnetic responses are independent of the external electric or magnetic fields. Unfortunately, in linear regime the desired properties of metamaterials have only been achieved within a narrow bandwidth, around a fixed frequency. Therefore, nonlinearity is introduced into metamaterials by merging meta-atoms with well-known nonlinear materials. Nonlinear metamaterials are exploited in this dissertation to introduce and develop applications in microwave frequency with broadband responses. The nonlinearity was achieved via embedding varactor diode on to split ring resonator (SRR) design, which demonstrates tunability in resonance frequency and phase of the transmission signal. SRR exhibits power and frequency dependent broadband tunability and it is realized for external electro-magnetic signals. More importantly, the nonlinear SRR shows bi-stability with distinct transmission levels, where the transition between bi-states is controlled by the impulses of pump signal and it can be used as a switching device in microwave regime. In order to increase its functionality in other frequencies, a new design, double split ring resonator (DSRR) is introduced with two rings, which has two distinct resonance frequencies. The double split ring resonator also demonstrate similar behavior as the SRR but it is broadband. Furthermore, by designing the structure such that the inner ring has a frequency twice as outer ring resonance frequency; we observed the enhancement of harmonic generation. We exhibit enhancement in second harmonic generation and methods that can use to increase the harmonic signal power. Arranging the unit cells in an array and particular orientation further increases the harmonic power. In addition, we show that using a back plate to create a cavity will help to increase harmonic power. Furthermore, we have demonstrated that applying an external DC voltage can be used to tune resonance frequency as well as phase of the signal. Exploring these ideas in THz frequency regime is also important. So simulation results were obtained with advanced designs to achieve non-linearity in terahertz frequency regime to realize tunability, hysteresis and bi-stable states. A negative refractive index can be realized in metamaterials consisting of strong magnetic and electric resonators with responses at the same frequency band. However, high loss and narrow bandwidth resulting from strong resonances have impeded negative index optical components and devices from reaching expected functionalities (e.g. perfect lens). Here, we demonstrate experimentally and numerically that a 2D helical chiral metamaterial exhibits broadband negative refractive index with extremely low loss. With Drude-like dispersion, its permittivity leads to zero-index, and broadband chirality further brings the index to negative values for left-handed circularly polarized light in the entire range below the plasma frequency. Non-resonant architecture results in very low loss (<2% per layer) and an extremely high Figure-of-merit (>90). Tunable THz metamaterials has shown great potential to solve the material challenge due to the so-called “THz gap”. However, the tunable mechanism of current designs relies on using semiconductors insertions, which inevitably results in high Ohmic loss, and thereby significantly degrades the performance of metamaterials. In this work, we demonstrate a novel tunable mechanism based on polymeric microactuators. Our metamaterials are fabricated on the surface of patterned pillar array of flexible polymers embedded with magnetic nanoparticles. The transmission spectrum of the metamaterial can be tuned as the pillars are mechanically deformed though applied magnetic field. We observed and measured several type of deformation including bending, twisting and compressing when the applied magnetic field is polarized along different direction with respected to the axis of the magnetic particles. Compared to previous semiconductor based tunable mechanism, our structure has shown much lower loss. We demonstrate using simulations and experimentally that with an external magnetic field, we can achieve phase modulation using magnetic polymeric micro-actuators.

Autonomous heavy vehicles will enable the promise of decreased energy consumption through the ability to platoon in closer formation than is currently safe or legal. It is therefore increasingly important to understand the complex and dynamic wake interactions between vehicles operating in close proximity for aerodynamic gains. In recent years, a growing body of research has documented a bi-stable, shifting wake generated behind the Ahmed reference bluff body. At the same time, studies of platooning Ahmed bodies have focused on changes to the body forces and moments at different following distances or lateral offsets, typically based around time-averaged measurements or steady-state CFD. The present study attempts to understand the implications of bi-stability in the wake of two square-back, platooning Ahmed bodies, given the potential for transient instabilities. Temporally-correlated static pressures were measured on two identical wind tunnel models at various following distances to uncover the time-dependent interactions between platooning vehicles. Bi-stability is highly dependent on symmetry and the uniformity of oncoming flow, and it is shown that a shifting bi-stable wake behind the lead vehicle leads to correlated, bi-stable flow patterns on the following vehicle, even in the absence of a lateral offset. At a following distance of 0.25L, pressure data indicated there may be a point where this bi-stable behavior reaches a critical point between suppression and amplification, significantly affecting the aerodynamic loads on the lead vehicle. This leads to the conclusion that bi-stable wake interactions between vehicles may be useful to consider in the context of real-time organization of vehicle platoons.

Made available in DSpace on 2015-05-14T12:14:08Z (GMT). No. of bitstreams: 1 arquivototal.pdf: 2614177 bytes, checksum: 64cd8eee6f69230497da81e691be96a2 (MD5) Previous issue date: 2005-11-24 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / Interactions with surfaces modify internal and external liberty degrees of atoms next to these surfaces. The dominant long range interaction (which extends itselfs to about an atomic transition wavelength is the van der Waals interaction, usually attractive. Firstly, this thesis treats C3 coefficient, which is characteristic of this interaction and depends on the type of surface and temperature. Other theme is determination of this coefficient using a spectroscopic technique (Selective Reflection) in the atomic cesium system (6S1=2 - 8P3=2 transition) on a dielectric surface. This interaction allows the long range excitation transfer (Förster effect) from atom to the dispersive dielectric surface. Considering small distances (atomic dimensions), interaction becomes repulsive because of electronic orbital overlap between the atom and surface components. The sum of these two kinds of interaction (far and close range) results in a potential well, with discrete energy levels. Next, are presented results of simulations on optic transfer from free atoms to atom-surface bounded states. The radiation sources used on the experiments to evidence atom-surface effects described in the first part of this thesis are resonant diode lasers, with spectral characteristics that must be modified on the laboratory to became useful tools to perform high resolution experiments. The Part II treats diode lasers, beginning from stabilization techniques description and going to a new technique developed on the laboratory during doctoral work, where the laser frequency is controlled by an coupled optical orthogonal feedback with atomic transition to diode. Other effects related to this stabilization technique, as the bi-stability phenomena, are described and interpreted on the last chapter. / Interações dos átomos com superfícies modificam os graus de liberdade internos e externos desses átomos quando próximos a elas. A interação dominante de longo alcance (até a ordem do comprimento de onda das transições atômicas) é a interação de van derWaals, geralmente atrativa. Nesta tese é abordado o coeficiente C3, característico dessa interação e dependente da superfície e temperatura, determinado através do uso de uma técnica espectroscópica (Reflexão Seletiva). Trata-se também da transferência de excitação de longo alcance (efeito Förster) do átomo para a superfície. A curtas distâncias (dimensões atômicas), a interação torna-se repulsiva, devido à sobreposição dos orbitais eletrônicos do átomo incidente e dos constituintes da superfície. A soma das contribuições de curto e de longo alcance resulta em um poço de potencial com níveis discretos de energia que são simulados em uma transferência ótica para esses estados ligados. As fontes de radiação utilizadas nas experiências da primeira parte desta tese são lasers de diodo ressonantes, cujas características espectrais precisam ser modificadas no laboratório para eles se tornarem ferramentas adequadas para a realização de tais experiências de espectroscopia de alta resolução. A Parte II da tese trata de diodos lasers, iniciando com a descrição de técnicas de estabilização e chegando a uma nova técnica desenvolvida no laboratório durante este trabalho de doutorado, onde a freqüência do laser é controlada através de um retorno ótico com polarização ortogonal no diodo acoplado à transição atômica. Outros efeitos relacionados a essa técnica de estabilização, como o fenômeno de bi-estabilidade, são descritos e interpretados no último capítulo desta tese.