It has been found that non-magnetic organic semiconductors can show some magnetic responses in low magnetic field (<100 >mT). When applying magnetic field, the electroluminescence, electrical current, photocurrent, and photoluminescence could change with magnetic field, which are called magnetic field effects. Magnetic field effects are generated through spin-dependent process affected by the internal magnetic interaction. In nonmagnetic materials, hyperfine interaction has been supposed to dominantly affect the spin-dependent process recently. But the conclusion was made in weak spin-orbital coupling organic semiconductor. The hyperfine interaction might not be the main reason responsible for magnetic field effects in strong spin-orbital coupling materials. Therefore, the study of magnetic field effects in strong spin-orbital coupling organic semiconductor is important to get a whole view of the origin of the magnetic field effects in nonmagnetic organic semiconductors. This dissertation will clarify the generation mechanism of magnetic field effect in nonmagnetic organic semiconductors and further explore how the strong spin-orbital coupling affecting the magnetic field effect. It has been found the intermolecular excited states are important inter-median for magnetic field effects. The change of intersystem crossing at intermolecular excited states will change the singlet/triplet ratio and further generate magnetic field effects through different recombination and dissociation properties of singlet and triplet intermolecular excited states. Both the energy transfer effect coupled spin orbital coupling and energy transfer effect free spin orbital-coupling are discussed in the dissertation. The tuning of the magnetic field effect by adjusting the spin-orbital coupling is also established through distance effect and interface effect. It has been found that changing inter-molecular spin-orbital coupling is a critical factor to generate magnetic field effects in organic semiconductors. And the sensitivity of different magnetic field effects to strong spin-orbital coupling strength is depending on the final product. The internal magnetic interaction can be hyperfine interaction, spin orbital coupling and spin-spin interaction between electrons. The hyperfine interaction and spin orbital coupling are important in nonmagnetic organic semiconductors. But the electron spin-spin interaction is important in magnetic organic semiconductors. The magnetocurrent for magnetic and nonmagnetic organic semiconductors at different temperature has been compared.

This work focuses on discovery and development of novel binary halide scintillation materials for radiation detection applications. A complete laboratory for raw materials handling, ampoule preparation, material rapid synthesis screening, single crystal growth, sample cutting, polishing and packaging of hygroscopic halide scintillation materials has been established. Ce3+ and Eu2+ activated scintillators in three binary systems: Alkali Halide – Rare Earth Halide (AX–REX3), Alkali Halide – Alkaline Earth Halide (AX–AEX2) and Alkalin Earth Halide – Rare Earth Halide (AEX2–REX3) were systematically studied. Candidates for new scintillation materials in the three systems were selected based on a set of selection rules. A total of 42 Ce3+ or Eu2+ activated binary halide scintillation material candidates were synthesized and characterized. Among all synthesized candidates, 10 - 15 candidate materials were found to be highly promising in terms of high scintillation light output, fast scintillation decay or desirable emission wavelength. Three most promising candidates, Cs3EuI5, CsGd2Cl7:Ce3+ and CsSrI3:Eu2+ were selected for single crystal growth and further evaluation. Technologies for single crystal growth of hygroscopic halide scintillation materials were developed. Detailed design of experimental apparatuses was discussed. Single crystals were successfully grown via Bridgman or Vertical Gradient Freeze techniques. Study on optical and scintillation properties was performed. Possibility of using CsGd2Cl7:Ce3+ as a neutron detector was confirmed. CsSrI3:Eu2+ shows extraordinary scintillation light output (73,000 ph/MeV), excellent energy resolution (3.9%) and ease for crystal growth. A scaled-up crystal growth was carried out. A bulk crystal of 1” diameter CsSrI3:Eu2+ was successfully grown. Energy level structure and charge carrier traps in CsSrI3:Eu2+ were investigated. Potential of CsSrI3:Eu2+ in various radiation detection applications were evaluated.

The development and application of a combinatorial sputtering thin film technique to screen potential scintillation material systems was investigated. The technique was first benchmarked by exploring the binary lutetium oxide-silicon oxide material system, which successfully identified the luminescence phases of the system, Lu2SiO5 (LSO) and Lu2Si2O7 (LPS). The second application was to optimize the activator concentration in cerium doped LSO. The successfully optimized cerium concentration in the thin film LSO of 0.34 atomic percent was much greater than the standard cerium concentration in single crystal LSO. This lead to an intensive study based on temperature dependent steady-state and lifetime photoluminescence spectroscopy to understand the different concentration quenching mechanisms involved in the bulk single crystal versus the thin film LSO. The results were used to develop configuration coordinate models which were employed to explain the observed concentration dependent behavior. The nature of single crystal LSO:Ce concentration quenching was determined to be due to radiative energy transfer, and ultimately self-absorption. For the thin films it was found self-absorption was not a dominant factor due to the thin dimension of the film and also its nano-crystalline nature. Instead, the photoluminescence excitation and emission spectra as a function of concentration demonstrated the concentration quenching behavior was due to an increase in defect-mediated non-radiative transitions with increasing cerium. The final application of the thin film screening technique was the exploration of the ternary Lu2O3-SiO2-Al2O3 material system doped with cerium. It was found that the presence of aluminum and silicon hindered LSO and Al5Lu3O12 (LuAG) emission, respectively. However, the presence of aluminum was found to increase LPS emission intensity. The percent of aluminum in the LPS phase was estimated at 2.5 atomic percent.

Photonic crystals (PCs), consisting of a periodic pattern of variations in the material properties, are one of the platforms proposed as synthetic optical materials to meet the need for optical materials with desired properties. Recently, applications based on dispersive properties of the PCs have been proposed in which PCs are envisioned as optical materials with controllable dispersive properties. Unlike the conventional use of PCs to achieve localization, in these new applications propagation inside the photonic crystal is studied, and their dispersive properties are utilized. Among these applications, the possibility of demultiplexing light using the superprism effect is of particular interest. Possibility of integration and compactness are two main advantages of PC-based wavelength demultiplexers compared to other demultiplexing techniques, for applications including compact spectrometers (for sensing applications), demultiplexers (for communications), and spectral analysis (for information processing systems). I develop the necessary simulation tools to study the dispersive properties of photonic crystals. In particular, I will focus on superprism-based demultiplexing in PCs, and define a phenomenological model to describe different effects in these structures and to study important parameters and trends. A systematic method for the optimization and design of these structures is presented. Implementation of these structures is experimentally demonstrated using the devices fabricated in a planar SOI platform based on designed parameters. In the next step, different approaches to improve the performance of these devices (for better resolution and lower insertion loss) are studied, and extension of the concepts to other material platforms is discussed.

Advanced Laser Interferometric Gravitational Wave Detectors, based on current infrastructure (in particular, the Advanced LIGO detectors), are being planned to significantly increase the sensitivity to gravitational wave strain in the near future. To upgrade the existing detectors requests implementing very high optical power, as well as very high circulating power in the arm cavities; these measures will increase the sensitivity at the shot noise floor by one order of magnitude. However, such extremely high power circulation in the cavities will cause optical distortions in the test masses. Thermal distortions arise from the optical power absorption by defects or inhomogeneities in test masses, resulting in wavefront deformations, which have important consequences for the power buildup of the Radio-Frequency (RF) sidebands in the recycling cavities, thus degrading the performance of the detectors. The degree of this sensitivity degradation in the shot noise floor, due to optical distortions induced by defects or inhomogeneities (i.e. imperfections) in test masses in Advanced Laser Interferometric Gravitational-wave Detectors, is dependent on the test mass optical quality; while the sensitivity degradation in the thermal noise floor is dependent on the test mass mechanical properties. For this reason, it is compulsory to use high optical and mechanical quality test mass materials in the advanced interferometer detectors. Fused silica has been used for test masses in detectors, while sapphire has been planned to be used for test mass substrates in the proposed Large-scale Cryogenic Gravitational-wave Telescope (LCGT) project. Other materials, such as calcium fluoride (CaF2), are also attractive, especially for cryogenic detectors. However, for the state-of-theAbstract II art facilities, it is difficult to manufacture very uniform, defect-free, inhomogeneity-free, high-quality, and large-size samples. Thus, the qualities of sapphire and calcium fluoride single crystal samples were investigated and evaluated, to ensure that they have suitable properties for use in interferometer detectors, i.e. with an adequately low level of imperfections, but also with high mechanical quality factor (Q-factor). This thesis describes research done in the endeavour to investigate bulk defects or inhomogeneities in test masses, as well as their induced thermal distortions, which appear at a high optical power in Laser Interferometric Gravitational-wave Detectors. An Automatic Rayleigh Scattering Mapping System (ARSMS) to examine the optical property of large-size test masses is described. This ARSMS enables quantitative high-resolution 3D mapping of defects or inhomogeneities in optical materials. The measured 3D defect distribution mapping of optical materials can assist in the design of suitable configurations of test masses in high optical power interferometers. In addition, a very sensitive Hartmann wavefront sensor was used to actively monitor the thermal distortions due to bulk and coating absorption in test masses. A very strong thermal distortion in these test masses was observed in the Gingin facility, demonstrating that thermal distortions could be a critical issue in advanced interferometer detectors. A negative thermo-optical coefficient material, to be used in a thermal distortion compensation method, was investigated for the compensation of very localised distortions due to imperfections. This thesis also includes experimental and theoretical studies of the scattering, absorption, and birefringence mechanisms, thermal distortion effects, and optimal compensation methods for test masses.

Orientador: Carlos Kenichi Suzuki / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica / Made available in DSpace on 2018-08-14T05:46:53Z (GMT). No. of bitstreams: 1 Fujiwara_Eric_M.pdf: 1778608 bytes, checksum: 8a3e06a4a074e3e5e58ea236d126397a (MD5) Previous issue date: 2009 / Resumo: Fibras ópticas mantedoras de polarização (PMF) permitem preservar o estado de polarização da luz através de uma elevada birrefringência, induzida, por exemplo, com a nãocircularidade do núcleo, e são utilizadas em sistemas ópticos coerentes e na fabricação de sensores, lasers e giroscópios ópticos. Entretanto, os métodos de produção de preformas para PMF demandam etapas adicionais de processamento e elevado custo. Assim, foi desenvolvido um processo automatizado, baseado na plataforma LabVIEW, para produção de preformas nãocirculares pelo método VAD, através da variação da velocidade de rotação da preforma entre valores máximo e mínimo, durante a etapa de deposição. O método foi também simulado através de um modelo matemático, possibilitando o estudo do efeito das variáveis de processo, como a velocidade máxima de rotação e tempo de "delay", na geometria final da preforma. Foram produzidas preformas soot de diferentes secções transversais, como geometrias elípticas com razão de diâmetros de até 1,59, bem como preformas triangulares e quadrangulares, de sorte que a não-circularidade se manteve mesmo após a consolidação. Dessa forma, a metodologia apresentada permite produzir preformas não-circulares com custos acessíveis e sem agregar etapas adicionais aos processos convencionais de fabricação, abrindo fronteiras para o desenvolvimento de novos produtos para a área de tecnologia de fibras ópticas especiais, e sensores fotônicos no país. / Abstract: Polarization-maintaining fibers (PMF) preserve the state of light polarization due to a high birefringence index, induced by the non-circularity of fiber core. These fibers are necessary on the implementation of coherent optical systems, sensors, lasers and fiber gyroscopes. However, the PMF preform fabrication methods demand additional stages of processing, increasing the manufacturing costs. Concerning these problems, an automated methodology for non-circular preform fabrication based on LabVIEW platform was developed, based on controlling the rotation velocity between maximum and minimum values during the deposition stage of VAD method. Moreover, focusing the study of the process variables effect, like the maximum velocity and delay time, on the preform geometry, the methodology was also simulated by using a mathematical model. Soot preforms were produced according to different cross sections, resulting in elliptical geometries with diameters proportion of 1,59, as well triangular and square shaped preforms, and the geometries were preserved even after the consolidation. Results show that presented methodology allows producing non-circularly shaped preforms with low costs, and without demanding the accomplishment of additional stages on conventional process, assisting the development of new products for the optical fiber technology and photonic devices scenario. / Mestrado / Materiais e Processos de Fabricação / Mestre em Engenharia Mecânica

An increasing need for effective thermal sensors, together with dwindling energy resources, have created renewed interests in thermoelectric (TE), or solid-state, energy conversion and refrigeration using semiconductor-based nanostructures. Effective control of electron and phonon transport due to confinement, interface, and quantum effects has made nanostructures a good way to achieve more efficient thermoelectric energy conversion. This thesis studies the two well-known approaches: confinement and energy filtering, and implements improvements to achieve higher thermoelectric performance. The effect of confinement is evaluated using a 2D material with a gate and utilizing the features in the density of states. In addition to that, a novel controlled scattering approach is taken to enhance the device thermoelectric properties. The shift in the onset of scattering due to controlled scattering with respect to sharp features in the density of states creates a window shape for transport integral. Along with the controlled scattering, an effective utilization of Fermi window can provide a considerable enhancement in thermoelectric performance. The conclusion from the results helps in selection of materials to achieve such enhanced thermoelectric performance. In addition to that, the electron filtering approach is studied using the Wigner approach for treating the carrier-potential interactions, coupled with Boltzmann transport equation which is solved using Rode's iterative method, especially in periodic potential structures. This study shows the effect of rapid potential variations in materials as seen in superlattices and the parameters that have significant contribution towards the thermoelectric performance. Parameters such as period length, height and smoothness of such periodic potentials are studied and their effect on thermoelectric performance is discussed. A combination of the above two methods can help in understanding the effect of confinement and key requirements in designing a nanostructured thermoelectric device that has a enhanced performance.

Sneak path current is a significant remaining obstacle to the utilization of large crossbar arrays for non-volatile memories and other applications of memristors. A two-terminal selector device with an extremely large current-voltage nonlinearity and low leakage current could solve this problem. We present here a Ag/oxide-based threshold switching (TS) device with attractive features such as high current-voltage nonlinearity (~1010), steep turn-on slope (less than 1 mV/dec), low OFF-state leakage current (~10-14 A), fast turn ON/OFF speeds (<75/250 ns), and good endurance (>108 cycles). The feasibility of using this selector with a typical memristor has been demonstrated by physically integrating them into a multilayered 1S1R cell. Structural analysis of the nanoscale crosspoint device suggests that elongation of a Ag nanoparticle under voltage bias followed by spontaneous reformation of a more spherical shape after power off is responsible for the observed threshold switching of the device. Such mechanism has been quantitatively verified by the Ag nanoparticle dynamics simulation based on thermal diffusion assisted by bipolar electrode effect and interfacial energy minimization.

Gallium Nitride (GaN) high electron mobility transistors (HEMT) have superior performance characteristics compared to Silicon (Si) and Gallium Arsenide (GaAs) based transistors. GaN is a wide bandgap semiconductor which allows it to operate at higher breakdown voltages and power. Unlike traditional semiconductor devices, the GaN HEMT channel region is undoped and relies on the piezoelectric effect created at the GaN and Aluminum Gallium Nitride (AlGaN) heterojunction to create a conduction channel in the form of a quantum well known as the two dimensional electron gas (2DEG). Because the GaN HEMTs are undoped, these devices have higher electron mobility crucial for high frequency operation. However, over time and use these devices degrade in a manner that is not well understood. This research utilizes low frequency noise (LFN) as a method for analyzing changes and degradation mechanisms in GaN-on-Si devices due to gate stress. LFN is a useful tool for probing different regions of the device that cannot be measured through direct means. LFN generation in GaN HEMTs is based on the carrier fluctuation theory of 1/f noise generation which states fluctuations in the number of charge carriers results in conductance fluctuations that produce a Lorentzian noise spectrum. The summing Lorentzian noise spectra from multiple traps leads to 1/f and random telegraph signal (RTS) noise. The primary cause of carrier fluctuations are electron traps near the 2DEG and in the AlGaN bulk. These traps occur naturally due to dislocations and impurities in the manufacturing process, but new traps can be generated by the inverse-piezoelectric effect during gate stress. This thesis introduces noise and presents a circuit to bias the devices and measure gate and drain LFN simultaneously. Three measurements are performed before and after gate DC stress at three different temperatures: DC characterization, capacitance-voltage (C-V) measurements, and LFN measurements. The DC characteristics show an increase in gate leakage after stress caused by an increase in traps after degradation consistent with trap assisted tunneling. However, the leakage current on the drain and source side differ before and after stress leading to the conclusion that the source side of the gate is more sensitive to gate stress. Gate leakage current on the drain side is also sensitive to temperature due to thermionic trap assisted tunneling. Hooge parameter calculations agree with previous research. The LFN results show an increase in gate and drain noise power, SIg(f) and SId(f), in accordance with increased gate leakage current under cutoff bias. RTS noise is also observed to increase in frequency with increased temperature. Activation energies for RTS noise are extracted and qualitatively linked to trap depth based on the McWhorter trap model.

The subject of this thesis are channel waveguide lasers in epitaxial garnet films grown by liquid phase epitaxy. Results of this thesis include: The feasibility of a waveguide laser with erbium-doped YIG as the active material is discussed. Nonlinear rate equations that describe the behavior of this laser are formulated and solved numerically. The material parameters that are needed as the input are taken from literature. The simulations reveal that the performance of the laser depends critically on the magnitude of the upconversion parameter W22. A thorough analysis shows, however, that the values of W22 that are found in literature are most likely too small. The other parameter of importance is the background loss of the waveguide. Our results indicate that, unless the absorption losses of the LPE grown iron garnet films can be significantly reduced, the chances to realize such a device are only minimal. The mechanisms behind the absorption losses in LPE grown YIG films are investigated. During the LPE growth process, lead, platinum and other non-trivalent impurities are incorporated into the films in high concentrations. While platinum is tetravalent only, lead can be both di- and tetravalent. Depending on the ratio of the lead and the platinum concentrations, the near infrared absorption can be caused by three entirely different mechanisms. In case the tetravalent impurities are the majority, charge compensation is achieved by the formation of Fe2+ and the absorption is caused by crystal field transitions between the 5E and the 5T2 states of the Fe2+-ion. If the divalent impurities are in the majority, the absorption is due to crystal field transitions between the 5T2 and the 5E states of the Fe4-ion. In LPE-grown iron garnets, however, the impurity with the highest concentration is usually lead. We prove that lead performs self-compensation as it is incorporated in Pb2+/4+ pairs on neighboring lattice sites, and that the optical absorption is caused entirely by a charge transfer transition between the two neighboring lead ions. The electrical conductivity is measured in a temperature range from 300 to 800 K. It is demonstrated that in this temperature range the so-called impurity conduction is the predominant conduction mechanism in all types of films. Around room temperature, the conductivity shows an abnormal temperature behavior as the measured Bohr radius of the impurity ions almost doubles between 295 and 370 K. This can be explained by the fact that film and substrate have different temperature expansion coefficients. As the temperature increases, the stress in the film changes from tensile to compressive. When that happens, the {Pb2+} and the [Pb4+] ions counteract the strain in that they interchange their lattice sites. Also, Fe4+, if present, switches from the tetrahedral to the octahedral site which reduces the lattice constant of the film. This interpretation finds support in the results on the optical absorption, in the lattice mismatch, and in MCD spectra that were previously measured by Milani and Paroli. Based on the above findings, an easy way to eliminate the growth induced absorption in LPE iron garnet films is presented. By co-doping the films with other di- and tetravalent ions like Mg and Ge in high concentrations, the concentration of absorbing Pb2+/Pb4+ centers is greatly reduced due to the formation of non-absorbing Pb2+/Ge4+ and Ca2+/Pb4+ centers. As a result, absorption losses as low as 0.05 cm-1 at 1.3 µm wavelength have been achieved. In the second part of this thesis the fabrication of a channel waveguide laser in epitaxial Nd:GGG films is described. The films are grown by LPE on [111] oriented GGG substrates from a PbO/B2O3 flux. To increase the index contrast between film and substrate, Bi is added to the melt. Initially, the films showed strong brown discoloration and a high defect density. Luminescence lifetime measurements revealed severe quenching problems. The chemical analysis of the films by EPMA shows that platinum from the crucible enters the films but no lead from the flux. It is found that the platinum in GGG, similar to the lead in YIG, forms Pt2+/Pt4+ centers in order to achieve charge compensation. The quenching is caused by the charge transfer transition between the two platinum ions. To eliminate the quenching, MgO is added to the melt. As a result, the brown discoloration, the quenching and the growth defects disappear. From the planar film rib waveguides are fabricated using ion beam etching. The unclad waveguides are cut and the endfaces are polished to couple light in and out. Both endfaces are perpendicular to the waveguide within 0.1°. The losses are derived from the Fabry-Perot interferences. Losses as low as 0.2 dB/cm are achieved for both TE and TM modes. Laser oscillation starts at a launched pump power of 5 mW. The resonator is formed by the polished endfaces of the waveguide, no dielectric mirrors have been applied. Thus, the facet reflectivity is only about 10%. The launched pump power is measured precisely using a novel technique at which the luminescence intensity emitted through the surface is spatially resolved with a beam profile analyzer. Coupling efficiencies of over 90% are measured. The slope efficiency is found to be 48%.