Thesis (M. Phil.)--University of Hong Kong, 1998. / Includes bibliographical references (leaves 91-92).

Thesis (doctoral)--University of Oulu, 2008. / Includes bibliographical references. Also available on the World Wide Web.

Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2005. / Includes bibliographical references (leaves 137-141). Also available in electronic version.

Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2007. / Includes bibliographical references (leaves 110-116). Also available in electronic version.

Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2007. / Includes bibliographical references (leaves 129-147). Also available in electronic version.

Thesis (Ph. D.)--Lehigh University, 2006. / Vita. Includes bibliographical references and vita.

Ultraviolet (UV) light emitting diodes (LEDs) are promising light sources for purification, phototherapy, and resin curing applications. Currently, commercial UV LEDs are composed of AlGaN-based n-i-p junctions grown on sapphire substrates. These devices suffer from defects in the active region, inefficient p-type doping, and poor light extraction efficiency. This dissertation addresses the development of a novel UV LED device structure, grown on p-SiC substrates. In this device structure, the AlGaN-based intrinsic (i) and n-layers are grown directly on the p-type substrate, forming a p-i-n junction. The intrinsic layer (active region) is composed of an AlN buffer layer followed by three AlN/Al0.30Ga0.70N quantum wells. After the intrinsic layer, the n-layer is formed from n-type AlGaN. This device architecture addresses the deficiencies of UV LEDs on sapphire substrates while providing a vertical device geometry, reduced fabrication complexity, and improved thermal management. The device layers were grown by molecular beam epitaxy (MBE). The material properties were optimized by considering varying growth conditions and by considering the role of the layer within the device. AlN grown at 825 C and with a Ga surfactant yielded material with screw dislocation density of 1x10^7 cm^-2 based on X-ray diffraction (XRD) analysis. AlGaN alloys grown in this work contained compositional inhomogeneity, as verified by high-resolution XRD, photoluminescence, and absorption measurements. Based on Stokes shift measurements, the degree of compositional inhomogeneity was correlated with the amount of excess Ga employed during growth. Compositional inhomogeneity yields carrier localizing potential fluctuations, which are advantages in light emitting device layers. Therefore, excess Ga growth conditions were used to grow AlN/Al0.30Ga0.70N quantum wells (designed using a wurtzite k.p model) with 35% internal quantum efficiency. Potential fluctuations limit the mobility of carriers and introduce sub-bandgap absorption, making them undesirable in the n-AlGaN layers. n-Al0.60Ga0.40N grown under stoichiometric Ga flux and an In surfactant reduced the Stokes shift (compared to n-AlGaN grown without In) by 150 meV. However, even under these growth modes, some compositional inhomogeneity persisted which is speculatively attributed to the vicinal substrate. Device epitaxial layer stacks utilizing the optimum growth conditions were fabricated into prototype vertical UV LEDs which emit from 295-320 nm. In order to increase light extraction efficiency, UV distributed Bragg reflectors (DBRs) based on compositionally graded AlGaN alloys were designed using the transfer matrix method (TMM) and grown by MBE. DBRs were formed from repeated compositionally graded AlGaN alloys. This structure utilized the polarization doping and index of refraction variation of graded composition AlGaN. DBRs with square wave, sinusoidal, triangular, and sawtooth compositional profiles were realized, with reflectivity peaks over 50%, centered at 280 nm.

Theoretical and experimental studies of real-time interrogation of optical sensors based on wavelength-to-time (WTT) mapping are presented. The sensing information is encoded in the spectrum of an optical sensor, and transferred to the time domain by using WTT mapping. Utilizing digital electronics for post processing, the sensing information can be interrogated at an ultra-high speed and resolution. Two sensors based on WTT mapping are proposed and demonstrated. First, a random grating sensor for simultaneous measurement of the temperature and strain is investigated. An ultra-short pulse from a mode-lock laser is spectrum shaped by a high-birefringence random grating to generate two orthogonally polarized spectrums, which are then fed to an optical loop in which a linearly chirped fiber Bragg grating is incorporated. Linear WTT mapping is implemented, and two temporally separated optical pulses are generated, and then converted to two electrical waveforms at a photodetector. Pulse compression is then employed. By measuring the temporal intervals of the temporally compressed pulses, the strain and temperature information can be retrieved. Conventional fiber based sensors are not sensitive to the refractive index change of the environment. In the second sensor, a silicon photonic microdisk resonator (MDR) for temperature and liquid refractive index sensing is proposed and demonstrated. By using the notches in the spectrum of the MDR, a microwave photonic filter (MPF) is implemented. By feeding a linearly chirped microwave signal to the MPF, a filtered signal with its temporal location representing the spectrum is generated. By monitoring the time location of the filtered signal, the temperature or the refractive index information is revealed.

La photonique intégrée offre la possibilité d’exploiter un vaste bouquet de phénomènes optique nonlinéaires pour la génération et le traitement de signaux optiques sur des puces très compactes et à des débits potentiels extrêmement rapides. De nouvelles solutions et technologies de composants pourraient être ainsi réalisées, avec un impact considérable pour les applications télécom et datacom. L’utilisation de phénomènes optiques nonlinéaires (e.g. effet Kerr optique, effet Raman) permet même d’envisager la réalisation de composants actifs (e.g. amplificateurs, modulateurs, lasers, régénérateurs de signaux et convertisseurs en longueur d’onde).Pendant cette dernière décennie, les efforts ont principalement porté sur la plateforme Silicium sur isolant (SOI), profitant du fort confinement optique dans ce matériau, qui permet la miniaturisation et intégration de composants optiques clés (e.g. filtres passifs, jonctions coupleurs et multiplexeurs). Cependant, la présence de fortes pertes nonlinéaires dans ce matériau aux longueurs d’onde d’intérêt (i.e. autour de 1.55 µm dans les télécommunications) limite certaines applications pour lesquelles une forte réponse nonlinéaire est nécessaire et motive la recherche de nouvelles plates-formes, mieux adaptées. L’objectif premier de cette thèse était ainsi l’étude de matériaux alternatifs au Si cristallin, par exemple le silicium amorphe hydrogéné, alliant de très faibles pertes nonlinéaires et une compatibilité CMOS, pour la réalisation de dispositifs photoniques intégrés qui exploitent les phénomènes nonlinéaires. Alternativement, l’utilisation de longueurs d’onde plus élevées (dans le moyen-IR) permet de relaxer la contrainte sur le choix de la filière matériau, en bénéficiant de pertes nonlinéaires réduites, par exemple dans la filière SiGe, également explorée dans cette thèse. Ce travail est organisé de la façon suivante. Le premier chapitre donne un iii panorama des phénomènes nonlinéaires qui permettent de réaliser du traitement tout-optique de l’information, en mettant en évidence les paramètres clés à maitriser (confinement optique, ingénierie de dispersion) pour les composants d’optique intégrée, et en présentant le cadre de modélisation de ces phénomènes utilisé dans le travail de thèse. Il inclut également une revue des démonstrations marquantes publiées sur Silicium cristallin, donnant ainsi des points de référence pour la suite du travail. Le chapitre 2 introduit les cristaux photoniques comme structures d’optique intégrée permettant d’exalter les phénomènes nonlinéaires. On s’intéresse ici aux cavités, avec une démonstration de génération de deuxième et troisième harmoniques qui exploite un design original. Ce chapitre décrit également les enjeux associés à l’utilisation de guides à cristaux photoniques en régime de lumière lente, qui serviront de fondements pour le chapitre 4. Le chapitre 3 présente les résultats de caractérisation de la réponse nonlinéaire associée à des guides réalisés dans deux matériaux alternatifs au silicium cristallin : le silicium amorphe hydrogéné testé dans le proche infrarouge et le silicium germanium testé dans le moyen infrarouge. Le modèle présenté au chapitre 1 est exploité pour déduire la réponse de ces deux matériaux, et il est même étendu pour rendre compte d’effets nonlinéaires d’ordre plus élevé dans le cas du silicium germanium à haute longueur d’onde. Ce chapitre inclut également une discussion sur la comparaison des propriétés nonlinéaires de ces deux matériaux avec le SOI standard. Le chapitre 4 combine l’utilisation d’une plate-forme plus prometteuse que le SOI, avec des structures photoniques plus avancées que les simples guides réfractifs utilisés au chapitre 3 : il décrit l’ingénierie de modes (lents) dans des guides à cristaux photoniques en silicium amorphe hydrogéné et enterrés dans la silice. [...] / Integrated photonics offers a vast choice of nonlinear optical phenomena that could potentially be used for realizing chip-based and cost-effective all-optical signal processing devices that can handle, in principle, optical data signals at very high bit rates. The new components and technological solutions arising from this approach could have a considerable impact for telecom and datacom applications. Nonlinear optical effects (such as the optical Kerr effect or the Raman effect) can be potentially used for realizing active devices (e.g. optical amplifiers, modulators, lasers, signal regenerators and wavelength converters). During the last decade, the silicon on insulator (SOI) platform has known a significant development by exploiting the strong optical confinement, offered by this material platform, which is key for the miniaturization and realization of integrated optical devices (such as passive filters, splitters, junctions and multiplexers). However, the presence of strong nonlinear losses in the standard telecom band (around 1.55 µm) prevents some applications where a strong nonlinear optical response is needed and has motivated the research of more suitable material platforms. The primary goal of this thesis was the study of material alternatives to crystalline silicon (for instance hydrogenated amorphous silicon) with very low nonlinear losses and compatible with the CMOS process in order to realize integrated photonics devices based on nonlinear optical phenomena. Alternatively, the use of longer wavelengths (in the mid-IR) relaxes the constraints on the choice of the material platform, through taking advantage of lower nonlinear losses, for instance on the SiGe platform, which is also explored in this thesis. This work is organized as follows. In the first chapter we provide an overview of the nonlinear optical effects used to realize all optical signal processing functions, focusing on the key parameters that are essential (optical confinement and dispersion engineering) for integrated optical components, and presenting the main models used in this thesis. This chapter also includes a review of the main demonstrations reported on crystalline silicon, to give some benchmarks. Chapter 2 introduces the use of photonic crystals as integrated optical structures that can significantly enhance nonlinear optical phenomena. First we present photonic crystal cavities, with a demonstration of second and third harmonic generation that makes use of an original design. In the second part of the chapter, we describe the main features and challenges associated with photonic crystal waveguides in the slow light regime, which will be used later in chapter 4. In chapter 3, we report the experimental results related to the characterization of the optical nonlinear response of integrated waveguides made of two materials that are alternative to crystalline silicon : the hydrogenated amorphous silicon, probed in the near infrared, and the silicon germanium, probed in the mid-infrared. The model presented in chapter 1 is extensively used here for extracting the nonlinear parameters of these materials and it is also extended to account for higher order nonlinearities in the case of silicon germanium tested at longer wavelengths. This chapter also includes a comparison of the nonlinear properties of these two material platforms with respect to the standard SOI. In chapter 4, we combine the use of a material platform that is better suited than SOI for nonlinear applications with integrated photonics structures that are more advanced that those used in chapter 3. Here we describe the design of (slow) modes in photonic crystal waveguides made in hydrogenated amorphous silicon fully embedded in silica. [...]

MgxZn1-xO has emerged as a material of great technological importance. Having a direct energy band gap that is tunable throughout much of the ultraviolet (UV) region of the spectrum from the near-UV (~370 nm) to the deep-UV (~176 nm), this compound is of interest for a variety of optoelectronic devices operating in this part of the electromagnetic spectrum. MgxZn1-xO offers advantages over the more mature compound semiconductor AlGaN which stem mainly from the unusually high exciton binding energy (60 meV in ZnO). In this study the growth of ZnO and MgxZn1-xO thin films using metal organic chemical vapour deposition (MOCVD) is systematically investigated. The films are mainly grown on c-Al2O3 and Si (100) and characterized using various techniques, such as photoluminescence (PL), x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and auger electron spectroscopy (AES). The optical and the structural properties are essentially inspected in order to improve their quality. In this thesis the optimisation of ZnO grown using oxygen gas as a new oxidant in our reactor is investigated. The growth temperature and VI/II ratio are varied in order to find optimum parameters giving high quality layers. The effects of Si (100), Si (111), c- and r-sapphire, glass, GaAs and ZnO substrates on the optical, structural and morphological properties of ZnO thin films grown with tert-butanol (TBOH) is examined. Similar morphologies are observed for all substrates, with the films comprising hexagonal columns having cone shaped ends. The photoluminescence spectra are similar, but the various transitions have different relative intensities. It is clear that the different substrates influence neither the orientation of the films, nor the surface morphology, significantly. The photoluminescence hints at larger stacking fault densities in films grown on silicon and glass, however, as evidenced by stronger basal plane stacking fault-related luminescence at ~3.319 eV in the relevant low temperature photoluminescence spectra. The morphology changes with Mg incorporation, from hexagonal columnar structures to cubic faceted columns. From PL, the full with at half maximum is found to gradually increase with Mg content due to alloy broadening. The deep level emission (DLE) is observed to shift with Mg content. By changing the Mg content, the band gap of MgxZn1-xO film is tuned by ~450 meV, which provides an excellent opportunity for band gap engineering for optoelectronic applications. The c-lattice constant of ZnO (5.205 Å) decreases by only 0.6% when the Mg content reaches x=0.39. The introduction of Mg into ZnO is shown to increase the relative PL intensity of stacking fault-related transitions (at 3.314 eV for ZnO). This becomes the dominant near band edge emission. Using TEM a thin Mg rich layer is observed at the interface between the film and the Si or Al2O3. Temperature dependent PL measurements on layers with low Mg concentration (x=0.05 and 0.1) show that the main bound exciton peak exhibits an “s-shaped” temperature dependence, characteristic of localization in a disordered alloy. The origin of the PL line broadening of MgxZn1-xO (x≤0.04) is also analyzed with respect to alloy broadening, taking into account a random cation distribution and alloy clustering. The influence of various MOCVD growth parameters such as growth temperature and VI/II ratio is studied. Varying the temperature from 280 ˚C to 580 ˚C reveals strong morphological changes and optical degradation of the films. Low (<280 ˚C) and high (>580 ˚C) growth temperatures reduce the Mg incorporation. High VI/II ratios also decrease the Mg incorporation, as evidenced by the red-shift of the donor bound exciton (D°X) line. This is ascribed to a stronger premature reaction between (MeCp)2Mg and the oxidant or a preferential heterogeneous interaction between the Mg and oxygen species on the growth front. For both oxidizing agents (O2 and TBOH), the growth at 420 ˚C and a VI-II ratio of 60 on c-Al2O3 gave optimal quality layers in terms of their optical and structural quality. A comparison of films grown using TBOH and O2 gas as oxidizing agent shows no major difference in terms of Mg incorporation. The effect of annealing, the inclusion of a buffer layer and the influence of growth rate on the properties MgxZn1-xO thin films are also reported.