Thesis (Ph.D.) - University of Glasgow, 1992. / Ph.D. thesis submitted to the Faculty of Engineering, University of Glasgow, 1992. Includes bibliographical references. Print version also available.

Thesis (Ph. D.)--University of Chicago, Dept. of Chemistry, March 2000. / Includes bibliographical references. Also available on the Internet.

Thesis (Ph. D.)--University of Washington, 2006. / Vita. Includes bibliographical references (leaves 164-174).

Made available in DSpace on 2014-06-11T19:22:32Z (GMT). No. of bitstreams: 0 Previous issue date: 2013-05-29Bitstream added on 2014-06-13T20:49:11Z : No. of bitstreams: 1 lima_ra_me_ilha.pdf: 2147608 bytes, checksum: ffed1e67ca6abfae1bf18b6925796fec (MD5) / Os transformadores de potencial baseados em tecnologia óptica têm sido desenvolvidos com as finalidades de melhorar o desempenho da proteção e medição dos sistemas elétricos de potência, para monitorar a tarifação do consumo ou a qualidade da energia desses sistemas. Apesar da tecnologia já consolidada dos transformadores para instrumentos convencionais, as versões ópticas possuem diversas vantagens, tais como: medições mais precisas, menor peso, reduzida necessidade de manutenção, facilidade na isolação física e galvânica, maiores faixa dinâmica e largura de banda, além dos enlaces de transmissão e recepção de sinais serem menos susceptíveis às interferências eletromagnéticas. Esses transformadores de potencial podem ser projetados em torno dos moduladores eletro-ópticos de amplitude que, por sua vez, podem ser baseados no efeito eletro-óptico em cristais que apresentam essa propriedade, em que a diferença de fase óptica induzida entre os modos ordinário e extraordinário pode ser relacionada à tensão elétrica aplicada. As medições foram realizadas para duas diferentes configurações: a primeira, dedicada a medir baixas tensões, apresenta campo elétrico externo aplicado na direção Z e propagação óptica na direção X dos eixos principais do cristal. Na segunda configuração, voltada para a medição de tensões mais elevadas (kV), a célula Pockels apresenta campo elétrico externo aplicado na direção Y e propagação óptica na direção Z (eixo óptico) do cristal. Para ambas as configurações foram usados cristais eletro-ópticos de Niobato de Lítio. Nesta dissertação de mestrado, relata-se a aplicação de diferentes formas de ondas periódicas à célula Pockels, a fim de realizar a medição óptica dessas funções comparando-se, posteriormente, os sinais de entrada e saída. Fazendo uso do... / Optical voltage transformers have been developed in order to enhance the performance of protection and measurement circuits in electric power systems, to monitor energy tax revenues and power quality of these systems. Although the technology for conventional instrument transformers has been consolidated for years, optical versions of these devices have several advantages, such as: more accurate measurements, lower weight, reduced maintenance requirements, easer insulation and isolation, higher dynamic range and bandwidth, and less susceptibility to electromagnetic interference when optical fiber links between transmitter and receiver are used. Such optical voltage transformers can be designed based on amplitude electro-optical modulator principle, and on Pockels cell devices. In turn, this last can be built according to the electro-optic effect in non-centro-symmetrical crystals, where the induced induced optical phase shift between ordinary and methods extraordinary modes can be directly related to applied voltage. Measurements were performed for two different sensor configurations: the first one is dedicated to measuring low voltages (few hundred of volts), in which the external electric field is applied in the Z direction (optical axis) and the optical propagation is in the X direction of principal axes of the crystal. In the second configuration, dedicated to measuring higher voltages (tens of kV), the external electric field is applied in the Y direction and optical propagation is in the Z direction of the crystal. For both configurations Lithium Niobate crystals were used. In this dissertation, the Pockels cell is driven by periodic waveform voltages, the photo detected signal is acquired and computationally processed, and the comparison between input and demodulated signals are compared. By using the Sampled Piece-Wise... (Complete abstract click electronic access below)

Thesis (Ph. D.)--University of Washington, 2006. / Vita. Includes bibliographical references (leaves 186-200).

Thesis (Ph. D.)--University of Texas at Austin, 2000. / Vita. Includes bibliographical references. Available also in a digital version from Dissertation Abstracts.

Thesis (Ph. D.)--University of Washington, 2005. / Vita. Includes bibliographical references (leaves 134-145).

Thesis (Ph. D.)--University of Washington, 2005. / Vita. Includes bibliographical references (leaves 126-130).

Thesis (M.S.)--University of Missouri-Columbia, 2006. / The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file (viewed on August 30, 2007) Includes bibliographical references.

Electro-optic phase modulators play a vital role in various large-scale photonic systems including Light Detection and Ranging (LIDAR), quantum circuits, optical neural networks and optical communication links. The key requirement of these modulators include strong phase change with low modulation induced optical loss, low electrical power consumption, small device footprint and low fabrication complexity. Conventional silicon phase modulators have either high power consumption (thermo-optic effect) or high optical loss (plasma-dispersion effect). On the other hand, low-loss phase modulation can be achieved using electro-optic 𝑋² effect such as LiNbO₃ which has a large device footprint (in mm's) and requires complex fabrication. The need of the hour is a material or a device that is strongly tunable with low optical loss and capable of picosecond switching speed. Transition metal dichalcogenides (TMDs) have been widely studied for optoelectronic applications due to their strong and tunable excitonic response. In fact, TMDs have been shown to experience massive changes of upto 20 % in their refractive index with doping, but this modulation is accompanied with large absorption change (60 %), which greatly limits their utility in photonic applications. In contrast, very litte is known about the effect of doping on the electro-optic response of TMDs at energies far below the exciton resonances, where the material is transparent and therefore could be used for photonic circuits. In this work, we first probe the electro-optic properties of TMDs in the near-infrared using a dielectric SiN microring resonator platform. We measure a strong doping induced change in the refractive index (Δn) of 0.52 in WS2 with minimal induced absorption (Δk) of 0.004. The |Δn/Δk| of 125, is an order of magnitude higher than the measured |Δn/Δk| for 2D materials including graphene and TMD monolayer at excitonic resonances, and for bulk electro-refractive materials commonly employed in silicon photonics. We next utilize this strong electro-refractive response to demonstrate low power, lossless optical phase modulation based on a composite SiN-TMD platform. The WS₂ based photonic modulator achieves a modulation efficiency (V_π⋅L) of 0.8 V ⋅ cm with a RC limited bandwidth of 0.3 GHz and DC electrical power consumption of 0.64 nW. The measured index change in monolayer TMDs (∼ 15%) in TMDs is unprecedented, considering the change in index of bulk (LiNbO₃) - the 'gold standard' for photonics - is typically 0.04 %. Despite the observed strong electro-refractive effect in TMDs and the enhanced light-matter interaction, the change in effective index of the propagating mode is 6.5 × 10⁻⁴ RIU, thereby requiring WS₂ phase modulators that are 1.3 mm long. This is due to the low optical mode overlap of 0.03 % with the monolayer that necessitates long phase shifter length. There is an urgent need for a compact, low-loss and high-speed optical phase shifter. Conventional phase modulators with low optical loss require long lengths to achieve strong phase change. On the contrary, traditional intensity modulators leverage compact high-finesse ring resonators to modulate output intensity. However, such cavities with conventional electro-refractive materials such as silicon where Δn/Δk = -20 cannot be used for phase modulation, owing to the high insertion loss associated with the phase change. Here, we show that we can leverage high-finesse ring resonators to achieve strong phase change with low optical loss. We achieve this by simultaneously modulating both the real and imaginary part of the effective index in the cavity to the same extent i.e. Δn/Δk ≈1. We design a hybrid SiN-2D platform that modulates the complex effective index of the propagating mode, by tuning the loss and index in monolayer graphene (Gr) and WSe₂ embedded on a SiN waveguide, respectively. We engineer the Gr-WSe₂ capacitor design to achieve a linear phase change of (0.50 ± 0.05)π radians with a low transmission modulation of 1.73 ± 0.20 dB and insertion loss of 2.96 ± 0.34 dB. We measure a 3 dB electro-optic bandwidth of 14.9 ± 0.1 GHz in the SiN-2D hybrid platform. We measure a phase modulation efficiency (V_(π/2)⋅L) of 0.045 V ⋅ cm with an insertion loss of 4.7 dB for a phase change of π/2 radians in the 25 μm SiN-2D platform. We show that the V_(π/2)⋅L for our SiN-2D hybrid platform is significantly lower than V_(π/2)⋅L of electro-refractive phase modulators based on silicon PN, PIN and MOS capacitors with comparable insertion loss. The TMD or TMD-graphene capacitor is incorporated as a post-fabrication process, transforming any passive substrate into an active photonic platform. The demonstrated enhanced light-matter interaction in monolayer TMDs could open up routes to a range of novel applications with these 2D materials and enable highly reconfigurable photonic circuits with low optical loss and power dissipation. We estimate that the efficiency of our TMD platform can be improved by optimizing the optical mode overlap with the monolayer through photonic mode optimization or reducing the dielectric thickness. For large-scale photonic systems, wafer-scale integration of TMD materials with silicon photonics can be done either as a direct TMD growth process on silicon wafers or a post-processing step where large wafer-scale TMD films are transferred onto a silicon photonics platform fabricated in a standard foundry.