This work is concerned with the modeling and analysis of lossy planar dielectric optical waveguides. Loss mechanisms which affect propagation characteristics are reviewed, and various representations of the propagation constant in the lossy case are defined. Waveguide structures which are susceptable to absorption and/or to leakage loss, in particular silicon-based structures, are discussed. The modeling and analysis of these waveguides by various computational techniques is considered. Two computational methods, the commonly used transfer matrix method and the recently developed impedance boundary method of moments (IBMOM), are reviewed and extended to the complex domain. A third computational method, which offers improved convergence of the IBMOM for structures with large stepwise changes in refractive index, is formulated. In this approach, the regions containing refractive index discontinuities are replaced by equivalent extended impedance boundary conditions, and expansion of the transverse field in the remaining region of continuous refractive index profile is carried out. A significant increase in the rate of convergence is demonstrated for various waveguide structures, including an anti-resonant reflecting optical waveguide (ARROW) structure. Two applications of the IBMOM with extended impedance boundary conditions are presented. In the first, the method is applied to the design of a chemical sensor. The sensor, a silicon-based ARROW structure, is designed to measure the refractive index of certain chemical substances with a high degree of accuracy. In a second application, graded index SiON waveguides fabricated at Oregon State University are characterized and compared to the theoretical model. Excellent agreement between the theoretical and measured coupling angles is shown. / Graduation date: 1996

Lately, there has been an increasing interest in studying the propagation of electromagnetic waves in periodic dielectric structures (photonic crystals). Like the electron propagation in semiconductors, these structures are represented by band diagrams in which gaps can be found where the electromagnetic propagation is forbidden. Much effort is dedicated to find structures that can prohibit the propagation of light in all directions. This effect could lead to light localization. / Singapore-MIT Alliance (SMA)

When polymer power cables were introduced into mediumvoltage applications in the late 1960s, the degradationphenomenon later referred to aswater treeingwas still unknown. However, this changedwithin a relatively short space of time. Just 5-10 years afterthese first polymer applications, faults began to appear thatcould be attributed to the new phenomenon-water treeing. Sincethen, cable manufacturers have been improving their designsgradually, and cables produced today are assumed to have muchlower susceptibility to water trees. However, water treeing isstill assumed to be the most important degradation process inthese applications. There are huge amounts of polymer cable installations thatare degraded by water trees. Since many of these cables areinstalled in the same grid nets, one cable fault oftengenerates another. If two or more cables fail at the same time,the consequences can be extensive. A non-destructive diagnosticmethod makes it possible to assess cables and take action priorto failures. This thesis presents the results of research work on thedevelopment of a measurement technique and diagnostic methodfor the assessment of medium voltage XLPE cables in the field.The method used is called high voltage dielectric spectroscopy,and involves the measurement of capacitance and loss as afunction of frequency at high voltages. The method is used in ashort frequency range of approximately one decade in thelow-frequency region (normally from 1 down to 0.1 Hz). Thevoltage levels used initially are all below the service-voltagelevel. If the cable is assessed to be free from water trees, aslightly increased voltage can be used. Based on dielectric-spectroscopy measurements, specificwater-tree analysis and breakdown tests, a set of measurementprocedures and criteria that take into account differences incable designs has been proposed for the assessment of mediumvoltage XLPE cables. The characteristic responses of XLPEcables (in both non and water-tree-deteriorated cases) make itpossible to separate the cable response from the influence ofaccessories. Field experience shows thatwater-tree-deteriorated cables with relatively low breakdownstrengths can remain in service for many years. <b>Keywords:</b>dielectric spectroscopy, high voltage,variable frequency, diagnostics, non-destructive diagnostics,cables, medium voltage, polymer, XLPE, water trees, watertreeing

MAX-phase materials are a new type of material class. These materials are potentiallyt echnologically important as they show unique physical properties due to the combination of metals and ceramics. In this project, spectroscopic ellipsometry in the spectral range of 0.06 eV –6.0 eV was used to probe the linear optical response of MAX-phases in terms of the complexd dielectric function ε(ω) = ε1(ω) + iε2(ω). Measured data were fit to theoretical models using the Lorentz and generalized oscillator models. Data from seven different samples of MAX-phase materials were obtained using two ellipsometers. Each sample dielectric function was determined, including their infrared spectrum.

Cellulose whiskers (CWs) obtained from naturally occuring cellulose are nano-inclusions which show a lot of promise as mechanical reinforcements in polymers. Typically, a relatively high content is added to realize improvement in effective mechanical behavior. This enhancement in modulus is usually followed by a modest increase in strength but generally the ductility and toughness decrease. Our approach is to use small concentrations of CWs so as not to detrimentally affect processability, toughness and ductility. By aligning the small concentrations, we target the same kind of improvement in modulus and strength as reported in the literature, but at much smaller volume contents. In this work, we investigate the effect of AC electric field on the alignment of dispersed nanoscale CW in a polymer. Polyvinyl acetate (PVAc) is used as the model polymer because of the good interaction between CWs and PVAc. A low concentration of 0.4wt% was used for the study. Two dispersion methods, namely basic and modified, were developed. The basic method led to micron scale dispersion. Using the modified method, CWs were individually dispersed in PVAc with average lengths and diameters of 260 nm and 8 nm respectively yielding an aspect ratio of approximately 30. The behavior of CWs (alignment and chain formation) under an applied electric field was found to be a function of applied electric field magnitude, frequency and duration. Following alignment, the CW/PVAc nanocomposites are thermally dried in the presence of electric field to maintain the aligned microstructure. Improvements in dielectric constant and mechanical properties were observed for the aligned cases as compared to random case and pure PVAc. The optimal electric field magnitude, frequency and duration for the alignment and chain formation were found to be 200Vpp/mm, 50 KHz for duration of 20 minutes for the microcomposite and 250Vpp/mm, 10KHz for a duration of 1hr for the nanocomposite. At 0.4wt% concentration, 21% increase in dielectric constant for the optimal nanocomposite case. Above Tg, a 680% improvement in elastic modulus at 0.4wt% concentration for the optimal nanocomposite case. The reason for the significant reinforcement is attributed to alignment (rotation and chain formation) and chain-chain interaction (3D network formation and hydrogen bonding).

Im Rahmen der vorliegenden Arbeit wurden Zinkoxid (ZnO)-basierte Metal-Isolator-Halbleiter Feldeffekttransistoren (MISFETs) mit Wolframtrioxid als transparentes Dielektrikum untersucht. Im ersten Teil werden die morphologischen, optischen, elektrischen und chemischen Eigenschaften der mittels gepulster Laserabscheidung (PLD) gewachsenen Wolframoxiddünnfilme, in Abhängigkeit vom Züchtungsdruck, diskutiert. Mit Hilfe dieser Ergebnisse konnte schließlich das hochisolierende Wolframtrioxid erfolgreich mit einer transparenten Gateelektrode, bestehend aus dem entarteten Halbleiter Zink-Galliumoxid (ZGO) bzw. Zink-Aluminiumoxid (AZO), kombiniert und somit MISFETs auf kristallinen und amorphen Substraten realisiert werden. Zur Optimierung der Transistoreigenschaften wurde die Dicke des Dielektrikums variiert und der Einfluss auf die Transfereigenschaften diskutiert. Des Weiteren wurde zur Verschiebung der Einschaltspannung eine Variation der Kanaldicke und des Elektrodenmaterials des Gates untersucht, wodurch die Möglichkeit der Herstellung von Verarmungs- bzw. Anreicherungstyptransistoren gegeben wurde. Um einen Vergleich der Transfereigenschaften des MISFETs gegenüber einem Metall-Halbleiter Feldeffekttransistor mit einem Schottky-Gatekontakt, bestehend aus oxidiertem Platin bzw. einem Sperrschicht-Feldeffekttransistor mit Zinkkobaltit als p-dotierten Bereich zu ermöglichen, wurden alle drei Transistorarten auf einem Substrat hergestellt und umfassend verglichen. Schließlich wird die Stabilität der Transistoren untersucht. Dabei wird der Einfluss einer permanenten Spannungsbelastung auf die Transfereigenschaften unter verschiedenen einflussnehmenden Bedingungen diskutiert. Abschließend werden aufgrund einer sich ausbildenden Hysterese der Transistoreigenschaften mögliche Ursachen derselben und Wege zur Passivierung der Bauelemente untersucht.

In this thesis the concept of inhomogeneous dielectrics is demonstrated for various optical coating applications. Compositionally-varying silicon oxynitride (SiON) dielectric layers, with the refractive index varying as a function of position, are grown by computer-controlled plasma-enhanced chemical vapor deposition (PECVD) using silane, nitrogen, and nitrous oxide reactant gases. Compositionally graded and superlattice-like SiON layers are grown and their compositional profiles are confirmed by Auger electron spectroscopy sputter profiling. Inhomogeneous antireflection coatings and rugate filters, with sinusoidally varying refractive index profiles, are designed and fabricated and their measured spectral responses are found to be in excellent agreement with simulated results. Alternating-current thin film electroluminescent (ACTFEL) devices with multiple layer dielectrics also are designed, fabricated, and the insulating layers are shown to increase the optical outcoupling efficiency of an ACTFEL devices by approximately 14 % compared to that of a conventional ACTFEL structure. / Graduation date: 1993

This thesis provides the study of graphene’s electrostatic interaction with the substrate surrounding it. Mathematical models based on current experimental configurations of graphene field-effect transistors (FET) are developed and analyzed. The conductivity and mobility of charge carriers in graphene are examined in the presence of impurities trapped in the substrate near graphene. The impurities encompass a wide range of possible structures and parameters, including different types of impurities, their distance from graphene, and the spatial correlation between them. Furthermore, we extend our models to analyze the influence of impurities on the fluctuations of the electrostatic potential and the charge carrier density in the plane of graphene. The results of our mathematical models are compared with current experimental results in the literature.

Several novel optical devices, which were designed to manipulate terahertz waves for broadband near-field imaging, wave-guiding (invisible space), and sensing (resonator), are presented in this thesis. We developed the original working concepts of each device, and demonstrated the prototype experimentally in our lab. The working concepts of physics were investigated in experiment, in simulation and in theoretical analysis. We exploited a tapered parallel-plate waveguide (PPWG) as a novel probe for broadband near-field imaging. This imaging probe consists of two metal plates with the plate spacing gradually tapered from one end to the other. We proved that the space tapering enables this probe to propagate the broadband THz waves efficiently (with low-loss, no cut-off and nearly no dispersion) from the input end of large spacing into the narrow end of sub-wavelength spacing. Working in a reflection mode, this imaging probe is proved to be able to differentiate the dielectric features as well as topographic information on the sample. Combined with the methodology of filtered back projection, we reconstructed a two-dimensional image of a gold pattern on a GaAs chip by using this tapered PPWG probe. The smallest feature of ~100 µm is resolved by using the waves with average wavelength of 1.5 mm. We studied the phenomenon of surface plasmon-polariton in THz range on the platform of a parallel-plate waveguide (PPWG). In this thesis, we show the characterization of the waveguide mode of a finite-width parallel plate waveguide by using an improved scattering-probe technique. An abrupt waveguide mode transition was observed at a very narrow frequency range. We demonstrated that this transition frequency is determined by the material properties of the waveguide, the frequencies of the electromagnetic waves as well as the geometry of the waveguide. This result provides a good guidance for the waveguide design for THz transmission. We also exploited the capability of using the spoof surface plasmon to enhance the reflectivity of an interface between free space and a PPWG. We demonstrated that the reflection coefficient of this interface can be enhanced up to ~100 % at a designed frequency, by cutting a designed pattern of periodic rectangular groove on the output facet of the PPWG. A lateral shift and a phase shift of the reflected beam is observed in the experiment, which is a strong reminiscent of Goos-Hanchen shift. We carried out the experimental, simulation and theoretical characterizations of the lateral and phase shift. As an application, we designed and demonstrated a prototype of a band-pass THz resonator. We introduced the concept of a waveguide-based two-dimensional inhomogeneous artificial dielectric into THz range. This artificial dielectric is the space between the two metal plates of a PPWG working in TE1 mode. We designed a THz mirage device (or an invisible space device) by using ray-tracing and full-wave simulations, which contributed to the first experimental demonstration of such a device. A metal coin of size several times larger than the working wavelength can be hidden in the device without casting any shadow. This work is in collaboration with Dr. Rajind Mendis and the author of this thesis contributed to the design and characterization of the device in simulations.

This thesis studies two important carbon structures, graphene and carbon nanotubes, with the purpose of understanding how their three-dimensional electron density distribution affects the way fast ions interact with them. <br /><br /> A brief introduction to research in pure carbon structures is made. We then use different models to calculate the equilibrium electron density distribution in graphene and carbon nanotubes. <br /><br /> In the second part of the thesis we investigate fast ions moving parallel to a graphene sheet and experiencing forces due to the dynamic polarization of carbon valence electrons. Using the three-dimensional electron density distribution of graphene, we calculate the force directly opposing the ion's motion (stopping force), as well as the force which bends the ion's trajectory towards the sheet (image force). It is our purpose to compare these results with those based on a two-dimensional hydrodynamic model of graphene, which approximates the electron distribution of graphene by a charged fluid confined to the two-dimensional plane of the sheet. <br /><br /> The results obtained for interactions of ions with a single graphene sheet should be useful for a further analysis of ion channeling through carbon nanostructures.