<p>This thesis presents a theoretical and experimental study of the temperature characteristics of 1.3μm strained layer multiple quantum well (MQW) lasers over a wide temperature range. A number of achievements have been made toward understanding the temperature sensitivity of the performance of the lasers.</p> <p>Under assumptions that the deterioration of optical gain with temperature dominates the temperature sensitivity of the laser performance and that the differential gain coefficient decreases linearly with temperature, two formulae, which include a maximum operating temperature, were derived to describe the threshold current, Ith, and the external quantum efficiency, ηd, as functions of temperature. The formulae produce a very good fit to the experimental data that were extracted from the shortpulse L-I characteristics of 1.3μm 0.7% compressive strained layer MQW lasers containing varying number of wells. The maximum operating temperatures obtained from fitting the formulae to Ith vs. T and ηd vs. T data are consistent with each other, which experimentally supports the theory and the underlying assumptions. Based on the same assumptions, the conventional method of determining the internal quantum efficiency and internal loss from a set of lasers with different length was scrutinized. It was concluded that the internal quantum efficiency is a function of temperature, even if the true internal quantum efficiency is independent of temperature, and that the internal loss is a sublinear function of temperature around room temperature, as available experimental results show. The experimental results from 1.3μm ten 0.7% compressively strained wells lasers with varying cavity length support the theoretical conclusions.</p> <p>The experimental observation of the far-field patterns for 1.3μm 1.2% tensile strained layer MQW lasers containing 3 wells with varying ridge width over a wide temperature range indicated that the injected carriers exert little effect on the refractive index, and that the change in the far field distribution with temperature is the result of spatial hole-burning.</p> <p>The final part of this thesis presents a technique to determine the temperature rise of the lasers during CW operation, which was then used to calculate the thermal impedances of different ridge width lasers. It was concluded that a wider ridge laser has a smaller thermal impedance and a lower available CW maximum operating temperature.</p> / Doctor of Philosophy (PhD)

<p>A positive feedback mechanism linking reactor power and loss of coolant exists in the current configuration of the CANDUˣ reactor. This mechanism creates a "coolant void effect" which is accommodated in both the safety system and control system designs: .although a political incentive exists to reduce the effect's magnitude. This dissertation includes a review of current methods and knowledge in the area of CANDU lattice cell analysis of the coolant void effect, presented in two parts: (1) an investigation of the methodology and modelling, and (2) an investigation of the contributing reactor physics. This study also explores several routes toward the goal of reducing the coolant void effect, drawing from both existing work and original contributions. A novel approach to CANDU fuel design is defined, combining the concepts of coolant displacement, concentritubular fuel, and differential uranium enrichment. Several variations of the new design are discussed, as well as practical difficulties that would be associated with the realization of this design. Finally, a corrective algorithm is developed and utilized which compensates for a modelling deficiency arising in the simulation of tubular fuel.</p> / Doctor of Philosophy (PhD)

<p>The fabrication and characterization of the double heterojunction opto-electronic switch (DOES) and the heterojunction field effect transistor (HFET) in the Si/Si_(1-x)Ge_(x) materials system is described. The significant achievements of this work are summarized below. The theory of operation of the DOES and HFET are reviewed. A computer-based, one-dimensional, analytical model is used to calculate the current-voltage characteristics and operational parameters of the DOES. From this, the design of the inversion-channel Si/Si_(1-x)Ge_(x) heterostructure is shown to contain a manifold parameter space. For example, the switching voltage of the DOES is shown to be extremely sensitive to the magnitude of doping in the charge sheet. In the HFET, the threshold voltage is calculated to have a non-linear dependence on the magnitude of the doping in the charge sheet. The measured switching voltages and currents compare well with the calculations of the model. Separately, the effects of illumination and temperature on the dc electrical characteristics of the device are investigated. Illumination is seen to reduce the switching voltage and holding current. With decreasing temperature, the switching voltage and holding current are observed to increase. The effects of illumination and temperature are explained as phenomena related to carrier injection (or the lack thereof). An account of the fabrication of these devices is given in detail. A self-aligned technology for the Si/Si_(1-x)Ge_(x)-based HFET was developed using the aluminum gate as a mask for reactive ion etching and ion implantation procedures. Oscillatory electrical behavior of the DOES is examined. Self-induced oscillations are shown to be correlated to the regime of negative differential resistance. The dc current-voltage characteristics of the DOES are demonstrated to be affected by this oscillatory behavior. In addition, an enhancement in the optical emission over a narrow range of drive currents in the DOES is shown to be a result of device oscillations. Changes in the electrical characteristics of the DOES in response to third terminal current injection are measured for both active layer contact and inversion layer contact. Contact to the inversion layer is shown to be more effective than a third terminal contact to the active layer. These experimental observations are supported by the computer-based model. Concurrent usage of optical and electrical injection to affect the I-V characteristic of the device is demonstrated. Electrical extraction via the third terminal is shown to negate the effect of optical injection. The highest reported transconductance in a Si/SiGe-based metal-semiconductor FET is found in the HFETs reported in this thesis. Peak transconductance of 8mS/mm is measured for a depletion mode device. Subthreshold slope is measured as 720mV/decade and a high frequency 3-dB point for voltage gain is seen to be 1.8GHz. The suitability of this technology for integration is examined by demonstrating a Reset-Set Flip-Flop which is comprised of two HFETs and a three-terminal DOES. The range of operational voltages for the circuit are shown to be determined primarily by the operational characteristics of the DOES. Gate leakage from the HFET is seen to hinder circuit performance.</p> / Doctor of Philosophy (PhD)

<p>InGaAsP and AlGaAs near-IR laser diodes with a short external cavity (SXC) were characterized and used as single-mode radiation sources for molecular spectroscopy. A module for extended temperature-tuning of the lasers was tested to evaluate their single-mode frequency tuning range. AlGaAs SXC lasers operating at 0.76 μm tuned up to 400 cm⁻¹ in a single mode, a few wavenumbers at a time, with no gaps in frequency coverage. InGaAsP SXC lasers operating at 1.3-1.55 μm tuned up to 200 cm⁻¹ in a similar fashion. A technique was developed to control the behaviour of the longitudinal modes and enhance laser operation. The technique employed interference fringes in the laser output intensity distribution to produce a discriminating signal used in an electrical feedback loop to maintain the external cavity at the optimum length for single-mode operation. Control theory was used to optimize the operation of this feedback loop. The SXC lasers were used in a high-sensitivity absorption spectrometer. The noise sources of the spectrometer were assessed, and the sensitivity obtained with SXC lasers was compared with that obtained with distributed feedback lasers. Both lasers yielded sensitivities of ≈3x10⁻⁶ in units of equivalent absorbance in an equivalent noise bandwidth of 1.25 Hz. The sensitivity was limited by etalon fringes produced by optical components in the detection path. The SXC laser spectrometer was employed to monitor the relative abundance of H₂O/HDO and ¹²CO₂/¹³CO₂. Changes in the relative abundance as small as 1% could be observed. The crystal-field and impurity splitting of the W₁₋₀(0) transition of solid hydrogen were observed with the spectrometer. Finally, frequency measurements were made of the 3v₂← 0 ro-vibrational band of the H₃⁺ molecular ion.</p> / Doctor of Philosophy (PhD)

<p>An investigation into the effects of material strain on the temperature sensitivity and operating characteristics of InGaAsP/InP strained-layer multiple-quantum-well (MQW) ridge-waveguide lasers is reported. This thesis contains the first comparison between the performance of strained and unstrained all quaternary MQW lasers having, not only similar emission wavelength, but also identical quantum well thicknesses. Results show that significant improvements in device performance can be obtained through the application of strain. These improvements include: a substantial reduction in the temperature sensitivity of laser threshold current, Ith; a reduction in Ith; an increase in maximum output power; an increase in the maximum operating temperature; and, an increase in the internal device efficiency, η. From investigations into the effects of strain, and QW width on the temperature performance of InGaAsP/InP lasers, the root of the temperature sensitivity of Ith is found to be much more complicated than can be explained by a single dominant physical process, as proposed by other authors. In addition the exponential relationship proposed by Pankove in 1968, which is commonly used to describe the Ith-T sensitivity in terms of the parameter, To, is found to be inappropriate. A new, relationship to describe the variation of Ith with temperature for InGaAsP/InP based lasers is proposed, and the parameter To is replaced by the parameter Tmax which represents the lasers' maximum operating temperature. In addition an expression relating Tmax to adjustable device parameters, such as: cavity length; facet reflectivity; optical confinement factor; internal efficiency; and transparency current density, is derived mathematically from an empirically determined relationship between the temperature rate of change of the threshold current density, ∇TJth, and the threshold current density, Jth. Achieved are the highest reported maximum operating temperature (140℃) and largest characteristic temperature (To = 88 K) for InGaAsP based lasers.</p> / Doctor of Philosophy (PhD)

<p>Physical mechanisms responsible for the above-threshold spectral output of 1.3 μm InGaAsP semiconductor diode lasers are presented and discussed. Measurements<br />of the facet emission of a large number of devices indicate modulations in the below-threshold<br />RmGm product which can be used to predict and explain the shape of the above-threshold mode profile for output power levels of less than approximately 5 mW. Above-threshold<br />measurements using devices incorporated into a shon-external-cavity<br />configuration show that a symmetric, nonlinear gain mechanism is required to explain the<br />spectral properties for output power levels in a single mode which are greater that 5 mW. Thus it is concluded that both the effects of scattering centres and nonlinear gain are required to model accurately the spectral output of 1.3 μm InGaAsP semiconductor diode lasers.</p> / Doctor of Philosophy (PhD)

<p>Thermally tunable planar Bragg gratings in silicon-on-insulator (SOl) rib waveguides with external and integrated heaters were simulated, fabricated, and measured. Planar Bragg gratings were fabricated using selective silicon self-implantation at a dose of 2 x 10^15 ions/cm^2 to amorphize and increase the refractive index by 0.3 due to the induced damage. External heating resulted in a positive shift of 0.080 nm/degree in a good agreement with the 0.082 nm/degree predicted by theory. This demonstrated the tunability of planar Bragg gratings in SOl rib waveguides and motivated the fabrication of integrated heaters on waveguides. The planar surface of implanted Bragg gratings makes it desirable for subsequent processing such as surface bonding and its tunability makes it applicable as an active device.</p> <p>Simulations performed with the heat transfer module of COMSOL in conjunction with beam propagation method (BPM) simulations predicted that application of 200 mA current to 100 nm thick aluminum strips is sufficient to generate a 10 degree temperature increase in a waveguide and that a minimum thickness of 300 nm of oxide layer is required to prevent mode loss in a waveguide due to the presence of the metal layer on top. Due to temperature restrictions on implanted damage and high temperatures involved in growth of oxide, non-cured spin-on-glass was used as the insulating layer between the rib waveguide and metal strips. Although, the combination of spin-on-glass (SOG) and aluminum worked on the test samples, it was unsuccessful on the device samples, such that the integrated heaters failed and the deposited aluminum affected the propagation optical mode.</p> <p>The use of Bragg gratings to couple a broad spectrum of surface illuminating light to a multimode SOl slab waveguide was also demonstrated. This has potential applications in enhancing the long wavelength performance of solar cells. The addition of Bragg gratings on top of slab waveguides promotes light coupling, trapping reflected light and also increasing the optical path length of the incident beam, which increases the efficiency of the solar cells. Bragg gratings with a pitch of 500 nm were fabricated on part of the surface of a multimode planar SOl waveguide and the coupling of light into waveguides with and without Bragg gratings were measured. The measurements and the simulations were in good agreement, confirming that broadband Bragg grating couplers are a good candidate for coupling in more light with higher efficiencies.</p> / Master of Applied Science (MASc)

Phase separated domains in multicomponent vesicles form spherical buds to reduce interfacial energy. We study the response of a multicomponent budded vesicle to an imposed shear flow with dissipative particle dynamics. We find that shear can either act to stretch the bud open or separate the bud from the vesicle, depending on bud orientation. We examine the interplay of interfacial tension, bending energy, and shear in determining the behavior of the vesicle, and provide criteria for the design of vesicles for controlled bud release. The neck connecting the budded domain with the bulk vesicle assumes a catenoid shape to minimize bending energy. We model the mechanism for pinch-off of catenoid necks with continuum elastic theory and dissipative particle dynamics. We examine pore nucleation and growth driven by Gaussian energy, by the adhesion energy of an encapsulated particle, and by the line energy of an interface between two amphiphile species, aiming to provide principles for the design of vesicles for biomimetic phagocytosis.

<p> More than half of all cancer patients in the United States undergo fractionated, daily radiation therapy as a component of treatment. While radiotherapy is widely regarded as life-saving, errors during delivery can be severe or even fatal, despite their rarity. These incidents may not be noticed immediately, because the treatment beam is not observed directly, and radiation-induced injuries take time to manifest by nature. It is in this context that the goal of the research presented here is to develop new Cherenkov imaging techniques to be used during radiotherapy for quality assurance testing, as well as patient treatment verification. </p><p> Cherenkov imaging allows for direct optical visualization of radiation dose deposition as ionizing beams pass through dielectric material via the Cherenkov Effect, by which fast-moving charged particles excite optical emission from media as they relax after rapid polarization. The number of Cherenkov photons produced is proportional to radiation dose delivered for a monoenergetic beam. However, the number of photons that can be detected is governed by efficiency in reaching the detector, which is influenced by variation in tissue optical properties in living subjects. This thesis first focuses on clinical patient imaging requirements, and techniques to improve the correlation between detected Cherenkov light intensity and metrics relating to delivered dose. Patients undergoing whole-breast radiotherapy (WBI), volumetric modulated arc therapy (VMAT), and total skin electron therapy (TSET) are discussed. </p><p> Quality assurance imaging is more straightforward, since the optical properties of the irradiated subject can be controlled. The remainder of this thesis employs water tanks and solid plastics to measure relative dose optically. These methods are adapted to accommodate atypical large field setups for TSET, as well as cutting edge clinical systems combining concurrent magnetic resonance imaging with radiotherapy delivery systems (cobalt-60 and linac based).</p><p>

<p>A technique to measure void fraction in a two-phase flow system using the multiple scattering of neutrons from portable neutron sources was investigated. There exists evidence from a previous investigation with a neutron beam extracted from a nuclear research reactor, that the basic technique is attractive. The counting rate of multiply-scattered, thermalized neutrons was found to be nearly linear with void fraction and independent of flow regime. However, early investigation with a portable neutron source revealed that these advantages were not realized in all situations. In the present work, the objective, then was to determine the conditions under which the advantages of linearity and flow pattern independence could be achieved with portable neutron sources. Experiments were carried out to study the effect of various parameters like symmetry of neutron sources, test-section to detector distances, and detection energy bn did not affect the technique significantly. More significant parameters were flow patterns, test-section inside diameter, test-section wall thickness, presence of neutron reflector material around the test section, and incident neutron spectrum. From the trends observed in the parametric study, it was found that near linearity with void fraction and flow pattern independence could be achieved by tailoring the energy spectrum of the incident neutron beam, e.g. by moderating neutrons emitted by a given source. The amount of moderation required decreased with test-section inside diameter, test section wall thickness, and he addition of neutron reflector material surrounding the test-section. This finding was verified with experiments conducted for four different test sections. The reasons for these experimental observations were clarified by studying the effect of monoenergetic incident neutron groups with Monte Carlo simulation, the result of which was further interpreted with thermalization probability considerations. The Monte Carlo study revealed that each monoenergetic neutron group results in a different response in terms of thermalized neutron counting rate, which, in general, is neither linear with void fraction nor flow regime independent. However, linearity and flow regime independence could be achieved by superimposing the effect of different energy groups, i.e. by tailoring the neutron spectrum. In practice, this is achieved by appropriately moderating the neutron beam from a given source. Based on the understanding achieved, a conceptual design of a portable void fraction meter, and a design procedure are suggested.</p> / Doctor of Philosophy (PhD)