Master of Science / Department of Physics / Brian Washburn / This research sought to characterize the length dependent loss of hollow core photonic crystal fibers (HC-PCF) in the mid-infrared. These fibers are used in gas-filled fiber lasers that operate in the mid-infrared range. A black body source which provided a broad mid-infrared spectrum was coupled into a HC-PCF and a fiber cut-back method was implemented to make the length dependent loss measurement. A monochromator was used to observe narrow bands of the broad spectrum provided by the black body source and the loss as a function of wavelength was constructed. The loss for four unique HC-PCF fibers was characterized across the wavelength range [lambda] =1754 nm to [lambda] =3220 nm. The best fibers demonstrated a loss of less than 2 dB/m across this range, with some fibers even exhibiting loss below 1 dB/m.

Master of Science / Department of Physics / Itzhak Ben-Itzhak / Imaging the interaction of molecular ion beams with ultrafast intense laser fields is a very powerful method to understand the fragmentation dynamics of molecules. Femtosecond laser pulses with different wavelengths and intensities are applied to dissociate and ionize molecular ions, and each resulting fragmentation channel can be studied separately by implementing a coincidence three-dimensional (3D) momentum imaging method. The work presented in this master’s report can be separated into two parts. First, the interaction between molecular ion beams and femtosecond laser pulses, in particular, the dissociation of CO[superscript]+ into C[superscript]++O, is studied. For that purpose, measurements are conducted at different laser intensities and wavelengths to investigate the possible pathways of dissociation into C[superscript]++O. The study reveals that CO[superscript]+ starts to dissociate from the quartet electronic state at low laser intensities. Higher laser intensity measurements, in which a larger number of photons can be absorbed by the molecule, show that the doublet electronic states with deeper potential wells, e.g. A [superscript]2Π, contribute to the dissociation of the molecule. In addition, the three-body fragmentation of CO[subscript]2[superscript]+ into C[superscript]++O[superscript]++O[superscript]+ is studied, and two breakup scenarios are separated using the angle between the sum and difference of the momentum vectors of two O[superscript]+ fragments. In the second part, improvements in experimental techniques are discussed. Development of a reflective telescope setup intended to increase the conversion efficiency of ultraviolet (UV) laser pulse generation is described, and the setup is used in the studies of CO[superscript]+ dissociation described in this report. The other technical study presented here is the measurement of the position dependence of timing signals picked off of a microchannel plate (MCP) surface. The experimental method is presented and significant time spread over the surface of the MCP detector is reported [1].

Master of Science / Department of Physics / Brett D. Esry / In the last 20 years, advancements in laser technology have allowed for the production of intense laser pulses with durations in the femtosecond (10⁻¹⁵ second) regime, giving scientists the ability to probe nuclear dynamics on their natural time scale. Study of the dissociated fragments created by these intense fields can be used to learn about the molecular structure and dynamics. The work presented in this thesis focuses on controlling this light–molecule interaction in such a way that we can preferentially dissociate the molecule to a desired final product. The hydrogen molecular ion, HD⁺, as well as LiF serve as simple systems that can be studied theoretically for a broad range of laser parameters. Our goal in using these relatively simple systems is to capture the essential physics of the light–molecule interaction and develop general methods to describe these interactions in more complex systems.

Master of Science / Department of Mechanical and Nuclear Engineering / Youqi Wang / Electrospinning is one of the most versatile methods used to fabricate nanofibers. Sub micron and nano level fibers can be continuously produced with the help of an external electric field induced on the polymer melt. These nanofibers can be used in a large variety of applications such as biosensors, three dimensional tissue scaffolds, composites, electronic devices, etc. A unique feature of electrospinning is its ability to work with different fiber assemblies. This helps in making application specific changes and also increases the quality and performance of the fibers. PEO (polyethylene oxide) and electrostrictive graft elastomer (an electroactive polymer developed by NASA) were used in our experiments which focus on controlling the shape and alignment of the fibers. Electroactive polymers (EAP’s) are seen as the basis for future artificial muscles because of their ability to deform when external voltage is applied and quickly recover to their original form when the polarity of the applied voltage is reversed. Hence, aligned fibers of the electrostrictive graft elastomer were produced to mimic the alignment in human muscle fibers. Alignment of fibers is the main objective of this research and was facilitated using vacuum technology. The research was basically divided into three phases, starting with checking of the repeatability of the previously developed techniques using polyethylene oxide. Next, the electrostrictive graft elastomer was spun using the electrospinning techniques and was checked for alignment using the Coaxial Electrode method and PLC controlled secondary electric field method. Finally, a vacuum chamber was designed and built with new components and the elastomer was tested for improved alignment in vacuum using the PLC controlled secondary electric field method.

Doctor of Philosophy / Department of Physics / Chii-Dong Lin / In this thesis, we establish the theoretical tools to investigate high-order harmonic generation (HHG) by intense infrared lasers in a gaseous medium. The macroscopic propagation of both the fundamental and the harmonic fields is taken into account by solving Maxwell’s wave equations, while the single-atom (or single-molecule) response is obtained by quantitative rescattering theory. The initial spatial mode of the fundamental laser is assumed either a Gaussian or a truncated Bessel beam. On the examples of Ar, N[subscript]2 and CO[subscript]2, we demonstrate that the available experimental HHG spectra with isotropic and aligned target media can be accurately reproduced theoretically even though the HHG spectra are sensitive to the experimental conditions. We show that the macroscopic HHG can be expressed as a product of a macroscopic wave packet and a photorecombination cross section, where the former depends on laser and experimental conditions while the latter is the property of the target only. The factorization makes it possible to retrieve the single-atom or single-molecule structure information from experimental HHG spectra. As for the multiple molecular orbital contribution in HHG, it causes the disappearance of the minimum in the HHG spectrum of aligned N[subscript]2 with the increase of laser intensity, and the position of minimum in HHG spectrum of aligned CO[subscript]2 depending on many factors is also attributed to it, which could explain why the minima observed in different laboratories may differ. For an important application of HHG as ultrashort light source, we show that measured continuous harmonic spectrum of Xe due to the reshaping of the fundamental laser field can be used to produce an isolated attosecond pulse by spectral and spatial filtering in the far field. For on-going application of using HHG to ionize aligned molecules, we present the photoelectron angular distribution from aligned N[subscript]2 and CO[subscript]2 in the laboratory frame, which can be compared directly with future experiments.

Doctor of Philosophy / Department of Physics / Bharat Ratra / The expansion rate of the Universe changes with time, initially slowing (decelerating) when the universe was matter dominated, because of the mutual gravitational attraction of all the matter in it, and more recently speeding up (accelerating). A number of cosmological observations now strongly support the idea that the Universe is spatially flat (provided the dark energy density is at least approximately time independent) and is currently undergoing an accelerated cosmological expansion. A majority of cosmologists consider ``dark energy" to be the cause of this observed accelerated cosmological expansion. The ``standard" model of cosmology is the spatially-flat $\Lambda$CDM model. Although most predictions of the $\Lambda$CDM model are reasonably consistent with measurements, the $\Lambda$CDM model has some curious features. To overcome these difficulties, different Dark Energy models have been proposed. Two of these models, the XCDM parametrization and the slow rolling scalar field model $\phi$CDM, along with ``standard" $\Lambda$CDM, with the generalization of XCDM and $\phi$CDM in non-flat spatial geometries are considered here and observational data are used to constrain their parameter sets. In this thesis, we start with a overview of the general theory of relativity, Friedmann's equations, and distance measures in cosmology. In the following chapters we explain how we can constrain the three above mentioned cosmological models using three data sets: measurements of the Hubble parameter $H(z)$, Supernova (SN) apparent magnitudes, and the baryonic acoustic oscillations (BAO) peak length scale, as functions of redshift $z$. We then discuss constraints on the deceleration-acceleration transition redshift $z_{\rm da}$ using unbinned and binned $H(z)$ data. Finally, we incorporate the spatial curvature in the XCDM and $\phi$CDM model and determine observational constraints on the parameters of these expanded models.

Gas hydrates are a solid, ice-like mixture of water and low molecular weight hydrocarbons. They are found under the permafrost and to a far greater extent under the ocean, usually at water depths greater than 300m. Hydrates are a potential energy resource, a possible factor in climate change, and a geohazard. For these reasons, it is critical that gas hydrate deposits are quantitatively assessed so that their concentrations, locations and distributions may be established. Due to their ice-like nature, hydrates are electrically insulating. Consequently, a method which remotely detects changes in seafloor electrical conductivity, such as marine controlled source electromagnetics (CSEM), is a useful geophysical tool for marine gas hydrate exploration. Hydrates are geometrically complex structures. Advanced electromagnetic modelling and imaging techniques are crucial for proper survey design and data interpretation. I develop a method to model thin resistive structures in conductive host media which may be useful in building approximate geological models of gas hydrate deposits using arrangements of multiple, bent sheets. I also investigate the possibility of interpreting diffusive electromagnetic data using seismic imaging techniques. To be processed in this way, such data must first be transformed into its non-diffusive, seismic-like counterpart. I examine such a transform from both an analytical and a numerical point of view, focusing on methods to overcome inherent numerical instabilities. This is the first step to applying seismic processing techniques to CSEM data to rapidly and efficiently image resistive gas hydrate structures. The University of Toronto marine electromagnetics group has deployed a permanent marine CSEM array offshore Vancouver Island, in the framework of the NEPTUNE Canada cabled observatory, for the purposes of monitoring gas hydrate deposits. In this thesis I also propose and examine a new CSEM survey technique for gas hydrate which would make use of the stationary seafloor transmitter already on the seafloor, along with a cabled receiver array, towed from a ship. I furthermore develop a modelling algorithm to examine the electromagnetic effects of conductive borehole casings which have been proposed to be placed in the vicinity of this permanent marine CSEM array, and make preliminary recommendations about their locations.

Gas hydrates are a solid, ice-like mixture of water and low molecular weight hydrocarbons. They are found under the permafrost and to a far greater extent under the ocean, usually at water depths greater than 300m. Hydrates are a potential energy resource, a possible factor in climate change, and a geohazard. For these reasons, it is critical that gas hydrate deposits are quantitatively assessed so that their concentrations, locations and distributions may be established. Due to their ice-like nature, hydrates are electrically insulating. Consequently, a method which remotely detects changes in seafloor electrical conductivity, such as marine controlled source electromagnetics (CSEM), is a useful geophysical tool for marine gas hydrate exploration. Hydrates are geometrically complex structures. Advanced electromagnetic modelling and imaging techniques are crucial for proper survey design and data interpretation. I develop a method to model thin resistive structures in conductive host media which may be useful in building approximate geological models of gas hydrate deposits using arrangements of multiple, bent sheets. I also investigate the possibility of interpreting diffusive electromagnetic data using seismic imaging techniques. To be processed in this way, such data must first be transformed into its non-diffusive, seismic-like counterpart. I examine such a transform from both an analytical and a numerical point of view, focusing on methods to overcome inherent numerical instabilities. This is the first step to applying seismic processing techniques to CSEM data to rapidly and efficiently image resistive gas hydrate structures. The University of Toronto marine electromagnetics group has deployed a permanent marine CSEM array offshore Vancouver Island, in the framework of the NEPTUNE Canada cabled observatory, for the purposes of monitoring gas hydrate deposits. In this thesis I also propose and examine a new CSEM survey technique for gas hydrate which would make use of the stationary seafloor transmitter already on the seafloor, along with a cabled receiver array, towed from a ship. I furthermore develop a modelling algorithm to examine the electromagnetic effects of conductive borehole casings which have been proposed to be placed in the vicinity of this permanent marine CSEM array, and make preliminary recommendations about their locations.

Doctor of Philosophy / Physics / Nobel S. Rebello / This dissertation investigates the effects of visual cueing and outcome feedback on students' performance, confidence, and visual attention as they solve conceptual physics problems that contain diagrams. The research investigation had two parts. In the first part of the study, participants solved four sets of conceptual physics problems that contain diagrams; each set contained an initial problem, four isomorphic training problems, a near transfer problem (with a slightly different surface feature as the training problems), and a far transfer problem (with considerably different surface feature as the training problems). Participants in the cued conditions saw visual cues overlaid on the training problem diagrams, while those in the feedback conditions were told if their responses were correct or incorrect. In the second part of the study, the same students solved the near and far transfer problems from the first study two weeks later. We found that the combination of visual cueing and outcome feedback improved performance on the near transfer and delayed near transfer problems compared to the initial problem, with no significant difference between them. Thus, the combination of visual cueing and outcome feedback can promote immediate learning and retention. For students who demonstrated immediate learning and retention on the near and far transfer problems, visual cues improved the automaticity of extracting relevant information from the transfer and delayed transfer problem diagrams, while outcome feedback helped automatize the extraction of problem-relevant information on the delayed far transfer problem diagram only. We also showed that students' reported confidence in solving a problem is positively related to their correctness on the problem, and their visual attention to the relevant information on the problem diagram. The most interesting thing was how changes in confidence occurred due to outcome feedback, which were also related to changes in accuracy and visual attention. The changes in confidence included both reductions in confidence and increases in confidence due to feedback when the student was wrong (first) and right (later). This seems to have led to learning (change in accuracy), and also changes in attentional allocation (more attention to the thematically relevant area).

Doctor of Philosophy / Department of Physics / Nobel S. Rebello / Many introductory calculus-based physics students have difficulties when solving physics problems involving calculus. This study investigates students’ retention and transfer from calculus to physics. While retention is the ability to recall your knowledge at a later point in time, transfer of learning is defined as the ability to apply what one has learned in one situation to a different situation. In this dissertation we propose a theoretical framework to assess students’ transfer of learning in the context of problem solving. We define two kinds of transfer – horizontal transfer and vertical transfer. Horizontal transfer involves applying previously learned ideas in a problem. Vertical transfer involves constructing new ideas to solve the problem. Students need to employ both horizontal and vertical transfer when they solve any problem. This framework evolves through this research and provides a lens that enables us to examine horizontal and vertical transfer. Additionally, this proposed framework offers researchers a vocabulary to describe and assess transfer of learning in any problem solving context. We use a combination of qualitative and quantitative methods to examine transfer in the context of problem solving. The participants in this study were students enrolled in a second-semester physics course taken by future engineers and physicists, calculus instructors and physics instructors. A total of 416 students’ exam sheets were collected and reviewed. Statistical methods were used to analyze the quantitative data. A total of 28 students and nine instructors were interviewed. The video and audio recordings were transcribed and analyzed in light of the aforementioned theoretical framework. A major finding from this study is that a majority of students possess the requisite calculus skills, yet have several difficulties in applying them in the context of physics. These difficulties included: deciding the appropriate variable and limits of integration; not being clear about the criteria to determine whether calculus is applicable in a given physics problem, and others. This study also provides a detailed understanding of students’ difficulties in terms of our theoretical framework. Instructional strategies are suggested at the end to facilitate the transfer from calculus to physics.