Experimental investigations of structurally-dictated surface waves supported by periodically textured metallic substrates with different symmetries, are the primary focus of the work presented in this thesis. The electromagnetic response of three near perfectly conducting substrates perforated with arrays of holes with different geometries,together with a low-profile high-impedance structure are characterised. Experimental measurement techniques are employed to record the transmission, and reflection from the structures under investigation, together with phase-resolved measurements to directly obtain the dispersion of the surface waves supported by these structures. From these measurements information about the nature of the surface modes supported by the structures under investigation can be observed. A study of diffractively coupled surface waves supported by a close-packed array of square cross-section, close-ended holes in the limit where the wavelength of incident radiation and periodicity of the hole array are comparable, is presented. An additional grating, which has a periodicity comparable to the hole array is used to control the strength of diffractive coupling to the mode. Using a free-space measurement technique,information about the dispersion of the modes supported by the structure is obtained by recording the azimuthal-dependent reflection from the structure. It is found that the relative positions of the hole array and `coupling-in' grating is significant, a key issue not addressed in the literature when investigating grating-coupling to surface modes. Good agreement with numerical predictions is demonstrated. Structurally-dictated surface waves on a metallic substrate pierced by a close-packed array of deep, rectangular holes is characterised. In this arrangement, the fundamental resonance in the holes in the orthogonal directions is different and the frequency therefore to which the dispersion of the surface waves supported by the structure is limited, varies with sample orientation. The anisotropic dispersion, resulting from an ellipsoid of limiting frequencies, is directly mapped using a phase-resolved measurement technique. Furthermore by exploiting the anisotropy of the unit cell, a family of higher order surface waves associated with the quantisation of the electromagnetic fields within the holes is explored in this chapter. Once again good agreement with numerical predictions is shown.The `enhanced transmission' recorded through a `zigzag' hole array, attributed to the excitation of diffractively coupled surface waves, is explored. Due to the specific symmetry of the unit cell of the zigzag hole array it is shown that coupling to these surface waves can be achieved with both transverse magnetic and transverse electric polarised incident radiation. Further, incident radiation can directly couple to the surface modes supported by the zigzag hole array, via scattering from its inherent in-plane periodicity. The observed polarisation-selective excitation of individual surface wave bands, agrees well with numerical predictions and is shown to be a direct consequence of the reduced symmetry of the system. Finally, the dispersion of the modes supported by an ultra-thin, high-impedance surface in the form of a Sievenpiper `mushroom' structure, with rectangular geometry is directly recorded. The behaviour of the Sievenpiper structure is rather complex and to aid understanding of the electromagnetic response of the structure, the results are compared with the modes supported by a simpler patch array structure. The anisotropy arising from the rectangular geometry is characterised and an in depth discussion of the origin of the modes presented.

Recent years have seen a growing interest in a new kind of periodic structures called ``metamaterials'. These new artificial materials exhibit many new appealing properties, not found in nature, and open many new possibilities in the domain of antenna design. This thesis describes efficient numerical tools and methods for the analysis of infinite and finite periodic structures. A numerical simulation code based on the Method of Moments has been developed for the study of both large phased arrays and periodic metamaterials made of metal and/or dielectrics. It is shown how fast infinite-array simulations can be used in a first instance to approximately describe the fields radiated by large antenna arrays or compute transmission and reflection properties of metamaterials. These infinite-array simulations rely on efficient computation schemes of the doubly periodic Green’s function and of its gradient. A technique based on eigenmode analysis is also described, that allows to efficiently compute the dispersion curves of periodic structures. Accounting for the finiteness of real structures is possible in good approximation thanks to a finite-by-infinite array approach. Moreover, the excitation of large finite periodic structures by a single (non periodic) source can be studied by using a combination of the Array Scanning Method with a windowing technique. All these techniques were validated numerically on several examples and it is finally shown how they can be combined to design high gain antennas, based on metamaterial superstrates excited by a slotted waveguide. The proposed design method relies on the separation of the whole structure in two different problems. An interior problem is used to optimize the input impedance of the antenna, while the radiation pattern can be optimized in the exterior problem.

This thesis focuses on the analytical modeling of periodic structures which contain bands with multiple modes of propagation. The work is motivated by several structures which exhibit dual-mode propagation bands. Initially, transmission line models are focused on. Transmission line models of periodic structures have been used extensively in a wide variety of applications due to their simplicity and the ease with which one can physically interpret the resulting wave propagation effects. These models, however, are fundamentally limited, as they are only capable of capturing a single mode of propagation. In this work multiconductor transmission line theory, which is the multi-mode generalization of transmission line theory, is shown to be an effective and accurate technique for the analytical modeling of periodically loaded structures which support multiple modes of propagation. Many results from standard periodic transmission line analysis are extended and generalized in the multiconductor line analysis, providing a familiar intuitive model of the propagation phenomena. The shielded Sievenpiper structure, a periodic multilayered geometry, is analyzed in depth, and provides a canonical example of the developed analytical method. The shielded Sievenpiper structure exhibits several interesting properties which the multiconductor transmission line analysis accurately captures. It is shown that under a continuous change of geometrical parameters, the dispersion curves for the shielded structure are transformed from dual-mode to single-mode. The structure supports a stop-band characterized by complex modes, which appear as pairs of frequency varying complex conjugate propagation constants. These modes are shown to arise even though the structure is modeled as lossless. In addition to the periodic analysis, the scattering properties of finite cascades of such structures are analyzed and related to the dispersion curves generated from the periodic analysis. Excellent correspondence with full wave finite element method simulations is demonstrated. In conclusion, a physical application is presented: a compact unidirectional ring-slot antenna utilizing the shielded Sievenpiper structure is constructed and tested.

This thesis focuses on the analytical modeling of periodic structures which contain bands with multiple modes of propagation. The work is motivated by several structures which exhibit dual-mode propagation bands. Initially, transmission line models are focused on. Transmission line models of periodic structures have been used extensively in a wide variety of applications due to their simplicity and the ease with which one can physically interpret the resulting wave propagation effects. These models, however, are fundamentally limited, as they are only capable of capturing a single mode of propagation. In this work multiconductor transmission line theory, which is the multi-mode generalization of transmission line theory, is shown to be an effective and accurate technique for the analytical modeling of periodically loaded structures which support multiple modes of propagation. Many results from standard periodic transmission line analysis are extended and generalized in the multiconductor line analysis, providing a familiar intuitive model of the propagation phenomena. The shielded Sievenpiper structure, a periodic multilayered geometry, is analyzed in depth, and provides a canonical example of the developed analytical method. The shielded Sievenpiper structure exhibits several interesting properties which the multiconductor transmission line analysis accurately captures. It is shown that under a continuous change of geometrical parameters, the dispersion curves for the shielded structure are transformed from dual-mode to single-mode. The structure supports a stop-band characterized by complex modes, which appear as pairs of frequency varying complex conjugate propagation constants. These modes are shown to arise even though the structure is modeled as lossless. In addition to the periodic analysis, the scattering properties of finite cascades of such structures are analyzed and related to the dispersion curves generated from the periodic analysis. Excellent correspondence with full wave finite element method simulations is demonstrated. In conclusion, a physical application is presented: a compact unidirectional ring-slot antenna utilizing the shielded Sievenpiper structure is constructed and tested.

Artificial metamaterials composed of specifically designed subwavelength unit cells can support an exotic material response and present a promising future for various microwave, terahertz and optical applications. Metamaterials essentially provide the concept to microscopically manipulate light through their subwavelength inclusions, and the overall structure can be macroscopically treated as homogeneous bulk material characterized by a simple set of constitutive parameters, such as permittivity and permeability. In this dissertation, we present a complete homogenization theory applicable to one-, two- and three-dimensional metamaterials composed of nonconnected subwavelength elements. The homogenization theory provides not only deep insights to electromagnetic wave propagation among metamaterials, but also allows developing a useful and efficient analysis method for engineering metamaterials. We begin the work by proposing a general retrieval procedure to characterize arbitrary subwavelength elements in terms of a polarizability tensor. Based on this system, we may start the macroscopic analysis of metamaterials by analyzing the scattering properties of their microscopic building blocks. For one-dimensional linear arrays, we present the dispersion relations for single and parallel linear chains and study their potential use as sub-diffractive waveguides and leaky-wave antennas. For two-dimensional arrays, we interpret the metasurfaces as homogeneous surfaces and characterize their properties by a complete six-by-six tensorial effective surface susceptibility. This model also offers the possibility to derive analytical transmission and reflection coefficients for metasurfaces composed of arbitrary nonconnected inclusions with TE and TM mutual coupling. For three-dimensional metamaterials, we present a generalized theory to homogenize arrays by effective tensorial permittivity, permeability and magneto-electric coupling coefficients. This model captures comprehensive anisotropic and bianisotropic properties of metamaterials. Based on this theory, we also modify the conventional retrieval method to extract physically meaningful effective parameters of given metamaterials and fundamentally explain the common non-causality issues associated with parameter retrieval. Finally, we conceptually propose an inverse design procedure for three-dimensional metamaterials that can efficiently determine the geometry of the inclusions required to achieve the anomalous properties, such as double-negative response, in the desired frequency regime. / text

A set of tools are proposed for the efficient modeling of several classes of problems related to periodic structures in microwave and optical regimes with Finite-Difference Time-Domain method. The first category of problems under study is the interaction of non-periodic sources and printed elements with infinitely periodic structures. Such problems would typically require a time-consuming simulation of a finite number of unit cells of the periodic structures, chosen to be large enough to achieve convergence. To alleviate computational cost, the sine-cosine method for the Finite-Difference Time-Domain based dispersion analysis of periodic structures is extended to incorporate the presence of non-periodic, wideband sources, enabling the fast modeling of driven periodic structures via a small number of low cost simulations. The proposed method is then modified for the accelerated simulation of microwave circuit geometries printed on periodic substrates. The scheme employs periodic boundary conditions applied at the substrate, to dramatically reduce the computational domain and hence, the cost of such simulations. Emphasis is also given on radiation pattern calculation, and the consequences of the truncated computational domain of the proposed method on the computation of the electric and magnetic surface currents invoked in the near-to-far-field transformation. It has been further demonstrated that from the mesh truncation point of view, the scheme, which has a unified form regardless dispersion and conductivity, serves as a much simpler but equally effective alternative to the Perfectly Matched Layer provided that the simulated domain is periodic in the direction of termination. The second category of problems focuses on the efficient characterization of nonlinear periodic structures. In Finite-Difference Time-Domain, the simulation of these problems is typically hindered by the fine spatial and time gridding. Originally proposed for linear structures, the Alternating-Direction Implicit Finite-Difference Time-Domain method, as well as a novel spatial filtering method, are extended to incorporate nonlinear media. Both methods are able to use time-step sizes beyond the conventional stability limit, offering significant savings in simulation time.

This thesis presents both theoretical and experimental investigations of the performance and capabilities of frequency selective surfaces (FSS) applied at THz frequencies. The aim is to explore and extend the use of FSS, traditionally limited to microwave frequencies, towards the THz regime of the spectrum, where interesting applications such as imaging, sensing and communications exist. The contribution of this work lies in three main areas within the scope of THz FSS, namely, performance, prototyping and applications. Unlike microwave FSS where extensive research has been performed to evaluate the performance of different FSS designs, particular problems arise at THz frequencies, significantly, the ohmic losses. While a few notable studies can be found on the issue of ohmic losses, part of this thesis investigates, for the first time, the power dissipation due to the presence of both ohmic and dielectric losses, in relation to the power stored in the vicinity of the FSS, the currents induced in the elements of the array and the array’s terminal impedance. By doing so, a better understanding of the performance of THz FSS has been given in terms of their quality factor, allowing for design guidelines previously unavailable. In order to demonstrate multiband operation experimentally, a novel fabrication process has been designed and developed to manufacture capacitive or dipole-based THz FSS on a dielectric layer. Dry deep-reactive ion etching has been employed in order to avoid the use of wet etching to provide better control of etch characteristics. Various FSS operating around 15THz have been demonstrated experimentally. In addition, THz FSS have been investigated theoretically in the realm of three different applications, namely, multiband operation, sensing capability and reconfigurability. Multiband characteristics using single-screen FSS have been achieved by perturbed dipole FSS exhibiting up to four resonances due to the excitation of even and odd current modes. After studying the near-fields in perturbed FSS, it has been found that this type of FSS represent a very attractive candidate for sensing applications due to the revealed near-field enhancement phenomena related to the excitation of the odd mode, where currents flow in opposite directions. Finally, a novel tunability approach to reach frequency reconfigurability by varying the near-field coupling between two closely spaced layers in a dual-layer configuration has been proposed. A MEMS movable four-arm membrane has been suggested to vary the distance between the two layers mechanically, leading to the frequency tuning effect. This approach has been shown to be particularly suitable for THz frequencies, and has been applied to demonstrate theoretically tunable FSS and other periodic structures, such as artificial magnetic conductors and dielectric gratings.

A set of tools are proposed for the efficient modeling of several classes of problems related to periodic structures in microwave and optical regimes with Finite-Difference Time-Domain method. The first category of problems under study is the interaction of non-periodic sources and printed elements with infinitely periodic structures. Such problems would typically require a time-consuming simulation of a finite number of unit cells of the periodic structures, chosen to be large enough to achieve convergence. To alleviate computational cost, the sine-cosine method for the Finite-Difference Time-Domain based dispersion analysis of periodic structures is extended to incorporate the presence of non-periodic, wideband sources, enabling the fast modeling of driven periodic structures via a small number of low cost simulations. The proposed method is then modified for the accelerated simulation of microwave circuit geometries printed on periodic substrates. The scheme employs periodic boundary conditions applied at the substrate, to dramatically reduce the computational domain and hence, the cost of such simulations. Emphasis is also given on radiation pattern calculation, and the consequences of the truncated computational domain of the proposed method on the computation of the electric and magnetic surface currents invoked in the near-to-far-field transformation. It has been further demonstrated that from the mesh truncation point of view, the scheme, which has a unified form regardless dispersion and conductivity, serves as a much simpler but equally effective alternative to the Perfectly Matched Layer provided that the simulated domain is periodic in the direction of termination. The second category of problems focuses on the efficient characterization of nonlinear periodic structures. In Finite-Difference Time-Domain, the simulation of these problems is typically hindered by the fine spatial and time gridding. Originally proposed for linear structures, the Alternating-Direction Implicit Finite-Difference Time-Domain method, as well as a novel spatial filtering method, are extended to incorporate nonlinear media. Both methods are able to use time-step sizes beyond the conventional stability limit, offering significant savings in simulation time.

Orientadores: Marcelo Knobel, Lucila Helena Deliesposte Cescato / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin. / Made available in DSpace on 2018-08-12T12:21:48Z (GMT). No. of bitstreams: 1 Rosa_WagnerdeOliveirada_M.pdf: 19528973 bytes, checksum: 7a1e2008a2b3dee6576a7022583102ab (MD5) Previous issue date: 2005 / Resumo: Nesta dissertação utilizamos a técnica de litografia holográfica para reproduzir padrões periódicos em fotorresinas, depositadas em substratos de quartzo e vidro, que posteriormente foram utilizados como molde para a deposição de materiais magnéticos, que, portanto reproduzirem a periodicidade da estrutura gravada na fotorresina. Uma vantagem desta técnica é a dimensão pequena das estruturas que podem ser produzidas com uma boa razão de aspecto em áreas da ordem de 1000 cm 2 , assim como o controle do seu espaçamento. A união desta técnica com a deposição de materiais por sputtering foi muito vantajosa, pois usando este tipo de deposição podemos, em princípio, selecionar qualquer tipo de material e reproduzir com grande eficiência os padrões determinados pelas máscaras holográficas. Os sistemas aqui estudados são nanofitas de três tipos de materiais magnéticos: Co, Ni e Ni80Fe20(permalloy). Isto nos dá a liberdade de estudar como varia a interação magnética entre as nanofitas em função da sua separação e também da sua espessura. Comparamos os resultados obtidos das amostras estruturadas com as sem estruturas e foi possível observar que a interação dipolar exerce uma função importante na mudança dos campos coercivos destas. Entretanto, observamos que o campo de troca é praticamente inalterado para as diversas amostras de mesma composição estudadas que possuem a mesma espessura, assinalando que este mecanismo independe da anisotropia de forma e da interação dipolar. Os resultados de ressonância ferromagnética (RFM) mostram que os campos de anisotropia das amostras são menores do que os previstos pelas curvas de magnetização destas, indicando que o campo medido pode ser na verdade um campo efetivo no qual poderíamos ter a contribuição do campo dipolar. / Abstract: In this thesis we employ the holographic lithography (HL) technique to reproduce periodic patterns that were subsequently used as masks for the deposition of magnetic materials by sputtering. The structures present the same periodicity of the previously recorded mask. One advantage of this technique is the very good aspect ratio, even for areas of the order of 1000 cm2, and the low dimensions achieved. Moreover, one has a complete control of the structures¿ periodicity. The combination of the HL with sputtering is very oportune because this kind of deposition allows one to work with any type of material. Then, one can reproduce with great efficiency the patterns introduced by the holographic masks. The studied magnetic systems were formed by three different types of magnetic materials: Co, Ni and Ni80Fe20(permalloy). We studied the variation of the magnetic interactions among the nanostripes both as a function of the separation among them and their thickness. By comparing the results from the structured samples with the continous one it was possible observe the important role of the dipolar interaction on the coercivity. Nevertheless, we noted that the exchange field does not change for different studied samples with the same composition and thickness, indicating that this behavior is independent of the shape anisotropy and the dipolar interactions. The results of ferromagnetic resonance (FMR) showed that the anisotropy fields of the samples are smaller than the ones predicted by the magnetic measurements, indicating that we measured an effective field with the contribution of the dipolar field. / Mestrado / Física da Matéria Condensada / Mestre em Física

A theoretical investigation of one-dimensional planar photonic crystal waveguides is carried out. These waveguides consist of a dielectric layer sandwiched between two semiinfinite periodic dielectric structures. Using a novel approach, exact analytical solutions for guided modes in such waveguides are presented. The se rigorous solutions allow one to distinguish clearly between the index-guiding regime and guidance due to the photonic crystal effect. In the first part of this investigation, a rigorous analysis of the reflection of uniform plane waves from a semi- infinite periodic dielectric structure is undertaken. Both parallel and perpendicular polarizations for the incident plane wave are considered. Exact expressions for the reflection coefficients corresponding to two polarization cases are obtained using an impedance approach. The results for the reflection coefficient are then used to study propagation properties of guided modes in one-dimensional photonic crystal waveguides with semiinfinite periodic cladding regions. Characteristic equations, from which propagation constants of guided modes can be obtained, and solutions for electromagnetic fields of these modes are derived. Solutions for both TE (transverse electric) and TM (transverse magnetic) modes are presented. Numerical results for the propagation constant and field distributions of several lower-order modes are presented. The solutions unique to photonic crystal waveguides are emphasized. / Master of Science