Thesis (Ph.D.) - University of Queensland, 2004. / Includes bibliography.

Thesis (Ph. D.)--University of Hong Kong, 2006. / Title proper from title frame. Also available in printed format.

Thesis (Ph. D.)--University of Washington, 2000. / Vita. Includes bibliographical references (leaves 120-131).

nÃo hà / Geradores quÃnticos de nÃmeros aleatÃrios (GQNA) tÃm importantes aplicaÃÃes em protocolos criptogrÃficos, jogos e loterias, entre outros. Em contraste com geradores de nÃmeros pseudoaleatÃrios baseados em software, a sequÃncia de nÃmeros gerada à verdadeiramente aleatÃria. A maioria dos GQNA encontrados na literatura à baseado em dispositivos optoeletrÃnicos, como fontes de fÃtons Ãnicos e detectores de fÃtons. Nesta direÃÃo, a presente dissertaÃÃo trata da teoria e experimento de GQNAs baseados em sistemas fotÃnicos, considerando geradores com variÃveis discretas e contÃnuas. Em particular, trÃs problemas foram considerados: 1) um novo modelo de GQNA de variÃvel contÃnua utilizando a polarizaÃÃo da luz foi proposto. 2) a anÃlise de desempenho de um GQNA usando apenas um detector de fÃtons, levando em consideraÃÃo o afterpulsing e o tipo de estado quÃntico da luz utilizado, coerente ou tÃrmico, foi realizada. 3) um GQNA com distribuiÃÃo binomial foi construÃdo. / Quantum random number generators (QRNG) have important applications in cryptographic protocols, gaming and lotteries, among others. In contrast to pseudo-random number generators based on software, the sequence of random numbers generated is truly random. Most QRNG found in the literature are based on optoelectronic devices like single-photon sources and single-photon detectors. In this direction, the present dissertation deals with the theory and experiment QRNG based photonic systems, taking into account QRNG using discrete and continuous variables. In particular, three issues were considered: 1) a new model of continuous variable QRNG based on light polarization was proposed. 2) The performance of a QRNG employing only one single-photon detector, taking into account the afterpulsing and the quantum light state used, coherent or thermal, was realized. 3) A QRNG with binomial distribution was built.

No description available.

Group- III nitride based semiconductors have emerged as the leading material for short wavelength optoelectronic devices. The InGaN alloy system forms a continuous and direct bandgap semiconductor spanning ultraviolet (UV) to blue/green wavelengths. An ideal and highly efficient light-emitting device can be designed by enhancing the spontaneous emission rate. This thesis deals with the design and fabrication of a visible light-emitting device using GaN/InGaN single quantum well (SQW) system with enhanced spontaneous emission. To increase the emission efficiency, layers of different metals, usually noble metals like silver, gold and aluminum are deposited on GaN/InGaN SQWs using metal evaporator. Surface characterization of metal-coated GaN/InGaN SQW samples was carried out using atomic force microscopy (AFM) and scanning electron microscopy (SEM). Photoluminescence is used as a tool for optical characterization to study the enhancement in the light emitting structures. This thesis also compares characteristics of different metals on GaN/InGaN SQW system thus allowing selection of the most appropriate material for a particular application. It was found out that photons from the light emitter couple more to the surface plasmons if the bandgap of former is close to the surface plasmon resonant energy of particular metal. Absorption of light due to gold reduces the effective mean path of light emitted from the light emitter and hence quenches the quantum well emission peak compared to the uncoated sample.

This thesis focuses on the synthesis, properties and applications of two types of contorted organic molecules: contorted molecular ribbons and conjugated corrals. We utilized the power of reaction chemistry to writing information into conjugated molecules with contorted structures and studied “structure-property” relationships. The unique properties of the molecules were expressed in electronic and optoelectronic devices such as field-effect transistors, solar cells, photodetectors, etc. In Chapter 2, I describe the design and synthesis of a new graphene ribbon architecture that consists of perylenediimide (PDI) subunits fused together by ethylene bridges. We created a prototype series of oligomers consisting of the dimer, trimer, and tetramer. The steric congestion at the fusion point between the PDI units creates helical junctions, and longer oligomers form helical ribbons. Thin films of these oligomers form the active layer in n-type field effect transistors. UV−vis spectroscopy reveals the emergence of an intense long-wavelength transition in the tetramer. From DFT calculations, we find that the HOMO−2 to LUMO transition is isoenergetic with the HOMO to LUMO transition in the tetramer. We probe these transitions directly using femtosecond transient absorption spectroscopy. The HOMO−2 to LUMO transition electronically connects the PDI subunits with the ethylene bridges, and its energy depends on the length of the oligomer. In Chapter 3, I describe an efficiency of 6.1% for a solution processed non-fullerene solar cell using a helical PDI dimer as the electron acceptor. Femtosecond transient absorption spectroscopy revealed both electron and hole transfer processes at the donor−acceptor interfaces, indicating that charge carriers are created from photogenerated excitons in both the electron donor and acceptor phases. Light-intensity-dependent current−voltage measurements suggested different recombination rates under short-circuit and open-circuit conditions. In Chapter 4, I discuss helical molecular semiconductors as electron acceptors that are on par with fullerene derivatives in efficient solar cells. We achieved an 8.3% power conversion efficiency in a solar cell, which is a record high for non-fullerene bulk heterojunctions. Femtosecond transient absorption spectroscopy revealed both electron and hole transfer processes at the donor-acceptor interfaces. Atomic force microscopy reveals a mesh-like network of acceptors with pores that are tens of nanometers in diameter for efficient exciton separation and charge transport. This study describes a new motif for designing highly efficient acceptors for organic solar cells. In Chapter 5, I compare analogous cyclic and acyclic π-conjugated molecules as n-type electronic materials and find that the cyclic molecules have numerous benefits in organic photovoltaics. We designed two conjugated cycles for this study. Each comprises four subunits; one combines four electron-accepting, redox-active, diphenyl-perylenediimide subunits, and the other alternates two electron-donating bithiophene units with two diphenyl-perylenediimide units. We compare the macrocycles to acyclic versions of these molecules and find that, relative to the acyclic analogs, the conjugated macrocycles have bathochromically shifted UV-vis absorbances and are more easily reduced. In blended films, macrocycle-based devices show higher electron mobility and good morphology. All of these factors contribute to the more than doubling of the power conversion efficiency observed in organic photovoltaic devices with these macrocycles as the n-type, electron transporting material. This study highlights the importance of geometric design in creating new molecular semiconductors. In Chapter 6, I describe a new molecular design that enables high performance organic photodetectors. We use a rigid, conjugated macrocycle as the electron acceptor in devices to obtain high photocurrent and low dark current. We directly compare the macrocyclic acceptor devices to an acyclic control device; we find that the superior performance of the macrocycle originates from its rigid, conjugated, and cyclic structure. The macrocycle’s rigid structure reduces the number of charged defects originating from deformed sp2 carbons and covalent defects from photo/thermo-activation. With this molecular design we are able to suppress dark current density while retaining high responsivity in an ultra-sensitive non-fullerene organic photodetector. Importantly, we achieve a detectivity of ~10^14 Jones at near zero bias voltage. This is without the need for extra carrier blocking layers commonly employed in fullerene-based devices. Our devices are comparable to the best fullerene-based photodetectors, and the sensitivity at low working voltages (< 0.1 V) is a record for non-fullerene OPDs.

Materials processing using microwave radiation is emerging as a novel and innovative technology that has proven useful in a number of applications. It has various advantages over conventional processing, such as; time and energy saving, very rapid heating rates, considerably reduced processing time and temperature, fine microstructures and improved mechanical properties, better product performance, etc. Microwave irradiation has shown great potential for the processing of different semiconductor materials and inorganic solids for various advanced electronic and optoelectronic devices such as solar cells, batteries, supercapacitors, fuel cells etc. This work intends to investigate the effect of microwave radiation on various semiconductor materials and inorganic solids, in particular the changes in their chemical, physical and photoelectrochemical properties after microwave treatment. Microwaves have been used as an alternative method to conventional thermal annealing for post annealing of widely used semiconductors (TiO2, ZnO nanorods), battery materials (lithium aluminium titanium phosphates), and synthesis of materials (ZnO, Ti0.97Pd0.03O1.97). It is found that, in contrast to conventional thermal annealing, microwave treatment of such materials improves the crystallinity without any structural changes by preserving their nanostructure due to the difference in the heating mechanism (volumetric heating). The results demonstrate that microwave processing is a promising alternative method to the traditional conventional sintering for materials processing for advanced electronic and optoelectronic devices. Also the microwave annealing method offers energy savings of up to ~75%, which would make it highly desirable for industrial scale up.

Photonic Crystal structures have been heralded as a disruptive technology for the miniaturization of opto-electronic devices, offering as they do the possibility of guiding and manipulating light in sub-micron scale waveguides. Applications of photonic crystal guiding - the ability to send light around sharp bends or compactly split signals into two or more channels have attracted a great deal of attention. Other effects of this waveguiding mechanism have become apparent, and attracted much interest - the novel dispersion surfaces of photonic crystal structures allow the possibility of “slow light” in a dielectric medium, which as well as the possibility of compact optical delay lines may allow enhanced light-matter interaction, and hence miniaturisation of active optical devices. I also consider a third, more traditional type of photonic crystal, in the form of a grating for surface coupling. In this thesis, I address many of the aspects of passive photonic crystals, from the underlying theory through applied device modelling, fabrication concerns and experimental results and analysis. Further, for the devices studied, I consider both the relative merits of the photonic crystal approach and of my work compared to that of others in the field. Thus, the complete spectrum of photonic crystal devices is covered. With regard to specific results, the highlights of the work contained in this thesis are as follows: Realisation of surface grating couplers in a novel material system demonstrating some of the highest reported fibre coupling efficiencies. Development of a short “injecting” taper for coupling into photonic crystal devices. Optimisation and experimental validation of photonic crystal routing elements (Y-splitter and bend). Exploration of interfaces and coupling for “slow light” photonic crystals.

This thesis describes the process of developing a robust phase measurement technique with which to analyse semiconductor based devices intended for use in optoelectronic/all optical networks. The devices measured are prospective dispersion compensators, based either on planar photonic crystal waveguides or coupled microcavities connected by ridge waveguide. The technique was validated by measuring the phase transfer function of a Fabry Perot etalon. This demonstrated that even when detecting low optical powers (sub μW), accurate measurement of phase could quickly be carried out over a significant spectral range (~10nm). Comparison of experimental data taken from the prospective dispersion compensators with theory showed excellent agreement, which provided qualitative (cavity spacing and reflectivity) and quantitative (loss) measures of device performance. The phase measurement technique has been designed to be capable of measuring other classes of device also, including active devices such as semiconductor optical amplifiers. This suggests the phase measurement technique may be valuable in analysing the variation of dispersion as a function of applied bias, peak power or temperature for a variety of devices.