Physics and Astronomy Theses

This thesis presents an engineering project measuring the effective wavelength of the CHARA Classic beam combiner on the CHARA Array. Knowing the actual effective wavelength of light observed is very important because that value is necessary for determining astrophysical parameters of stars. Currently, the value used for CHARA Classic data comes from a model of the system and is based on numbers published by the manufacturer of the filter; it is not derived from measurements done on the system directly. We use two data collection methods to observe standard stars of different spectral types and calculate the wavelength of light recorded by the instrument for each star. We find the best estimate of the effective wavelength for the CHARA Classic K′-band configuration to be 2.138±0.003μm, a 0.56% decrease from the previously adopted value of 2.150μm. Our result establishes the first estimate of the uncertainty in the effective wavelength.

Optical absorption spectroscopy has been applied to study properties such as the fundamental absorption edge and defect absorption centers of group III-nitride compound semiconductor epilayers. The investigation in this thesis focused on analyzing the band gap of indium-rich In1-xGaxN epilayers, which where grown by the high-pressure chemical vapor deposition (HPCVD) technique. Our results - together with literature data for gallium-rich In1-xGaxN alloys indicate that the shift of the fundamental band gap of In1-xGaxN with composition x can be described with a bowing parameter of b = 2.2eV. Temperature dependent transmission measurements show that the band gap variation with temperature follows a S-shape behavior for small gallium concentration and shifts towards a Varshni type behavior for a higher gallium concentrations. The S-shape behavior is attributed to nanoscale compositional fluctuations/clustering in the ternary alloy system. The thicknesses of the measured In1-xGaxN epilayers have been analyzed through multilayer stack model calculations of the transmission spectra. The free electron concentration in the In1-xGaxN epilayers has been obtained from simulations of infrared reflectance spectra.

This thesis analyzes the reflectance behavior of textiles in the short-wave infrared (SWIR) band (1 – 2 microns) in order to identify/design potential diagnostic tools that allow the remote detection of human presence in a scene. Analyzing the spectral response of fabrics in the SWIR band has gained significant interest in the remote sensing community since it provides a potential path to discriminate camouflaged clothing from backgrounds that appear similar to the object of interest in the visible band. Existing research, originating primarily from the textiles community, has thoroughly documented the behavior of clothing fabrics in the visible band. Other work has shown that the differences in spectral response in the SWIR band allows for discrimination of materials that otherwise have the same visible spectral response. This work expands on those efforts in order to quantify the reflectance behavior and to better understand the physical basis for that behavior.

Capacitance-voltage-frequency measurements on n+-GaN/AlxGa1−xN UV/IR dual-band detectors are reported. The presence of shallow Si-donor, deep Si-donor, and C-donor/N-vacancy defect states were found to significantly alter the electrical characteristics of the detectors. The barrier Al fraction was found to change the position of the interface defect states relative to the Fermi level. The sample with Al fraction of 0.1 shows a distinct capacitance-step and hysteresis, which is attributed to C-donor/N-vacancy electron trap states located above the Fermi level (200 meV) at the heterointerface; whereas, the sample with Al fraction of 0.026 shows negative capacitance and dispersion, indicating C-donor/N-vacancy and deep Si-donor defect states located below the Fermi level (88 meV). When an i-GaN buffer layer was added to the structure, an anomalous high-frequency capacitance peak was observed and attributed to resonance scattering due to hybridization of localized Si-donor states in the band gap with conduction band states at the i-GaN/n+-GaN interface.

Neuroprosthetics is at the intersection of neuroscience, biomedical engineering, and physics. A biocompatible neuroprosthesis contains artificial neurons exhibiting biophysically plausible dynamics. Hybrid systems analysis could be used to prototype such artificial neurons. Biohybrid systems are composed of artificial and living neurons coupled via real-time computing and dynamic clamp. Model neurons must be thoroughly tested before coupled with a living cell. We use bifurcation theory to identify hazardous regimes of activity that may compromise biocompatibility and to identify control strategies for regimes of activity desirable for functional behavior. We construct real-time artificial neurons for the analysis of hybrid systems and demonstrate a mechanism through which an artificial neuron could maintain duty cycle independent of variations in period.

I report on suspected nearby stars with proper motions 1.0 arcsec > μ ≥ 0.5 arcsec/yr in the southern sky (DEC = −90° to 00°). This sample of slow-motion (SLOWMO) stars complements the work of Jao (2004), who reported on faster moving stars with μ ≥ 1.0 arcsec/yr in the entire sky for his doctoral dissertation, and the work of Finch (2007), who uncovered stars moving slower than 0.5 arcsec/yr between declinations −90° and −47°. Characterizations of SLOWMO systems include trigonometric parallaxes, optical and infrared photometry. For stars without trigonometric parallaxes, colors and apparent magnitudes are used to calculate photometric distance estimates and the statistics of this population of stars are analyzed. The SLOWMO sample is comprised of 1906 total stars − 560 estimated to be less than 25 parsecs away, and 245 stars without parallaxes estimated to be within 25 parsecs.

The co-existence of bursting activity and silence is a common property of various neuronal models. We describe a novel mechanism explaining the co-existence of and the transition between these two regimes. It is based on the specific homoclinic and Andronov-Hopf bifurcations of the hyper- and depolarized steady states that determine the co-existence domain in the parameter space of the leech heart interneuron models: canonical and simplified. We found that a sub-critical Andronov-Hopf bifurcation of the hyperpolarized steady state gives rise to small amplitude sub-threshold oscillations terminating through the secondary homoclinic bifurcation. Near the corresponding boundary the system can exhibit long transition from bursting oscillations into silence, as well as the bi-stability where the observed regime is determined by the initial state of the neuron. The mechanism found is shown to be generic for the simplified 4D and the original 14D leech heart interneuron models.

This research focuses on constructing a wavefront manipulation benchtop system. The primary goal is to replicate a hologram reconstruction method using a two-phase or two-pass modulation system with a spatial light modulator (SLM). The ability to precisely manipulate phase and amplitude is beneficial for simulating complex optical scenarios and enhancing astronomical observations. The chosen system allows independent control over spatial amplitude and phase distributions, improving wavefront manipulation capabilities. This precise control can be applicable across scientific, industrial, and defense sectors, enabling improved image quality and detailed analysis of low-intensity objects, a common challenge in astronomical observations and space surveillance. By constructing this benchtop setup, the research aims to create a scalable, hardware-based wavefront manipulation tool, capable of simulating various optical phenomena with high precision, offering significant potential for enhancing the observation of space objects and atmospheric conditions. The study addresses the technical challenges involved in constructing a wavefront manipulation benchtop tool for phase and amplitude modulation.

ABSTRACT

Fourier Transform Infrared Spectroscopy (FTIR) is a tool that allows for the characterization of molecular changes in biological systems. In this thesis, FTIR Microscopy is used to investigate the innate response of cells, via their biomolecular compositions, to nutrient stress and drug-induced changes. FTIR was used to study cyanobacteria (Synechocystis sp. PCC 6803 and Spirulina platensis) and algal (Neochloris minuta and Neochloris alveolaris) cells. A strain of yeast (Yarrowia lipolytica), and prostate cancer cells from the PC3 cell line were also studied. The chemometric analysis, in particular, principal component analysis (PCA), was used to differentiate different cell types, cells at various stages of growth, and metabolic modifications in cells in response to external stress.

Changes in the cell biomolecular composition indicate biomarkers that indicate how cells respond to changes in their environment. The cell is in effect a sentinel or biological sensor, which can provide information on drug efficacy, and even on pollutants and other metabolites in the environment in which cells grow, not to mention a variety of pathogenic diseased states of cells.

FTIR spectra of cells provide information on the composition of lipids, carbohydrates, and proteins within the cell. These biomolecules absorb IR radiation for the most part in distinct spectral regions. C-H, N-H, and O-H stretching vibrations occur in the 3300-2800 cm-1 region. Proteins absorb around 1650 and 1550 cm-1. While lipids, including phospholipids, nucleic acids, and polysaccharides display absorption in the 1400-900 cm^-1 region.

To test and verify the level of applicability of the FTIR technique we introduce the hypothesis that cyanobacterial cells in various stages of growth can be distinguished based on their FTIR spectrum. Different cyanobacterial strains at similar growth stages can also be characterized. IR spectra of prostate cancer cells in the presence of various drugs were also described based on their FTIR spectra.

Low surface brightness (LSB) disk dwarf galaxies are dark matter dominated down to their core, making their halo density distributions vital in testing cosmological model halo predictions. In this thesis, we characterize the photometric and kinematic properties of four nearby LSB dwarfs, determine if they are good candidates for dark matter halo studies, and provide the tools needed to isolate their dark matter component. We use optical broadband imaging and integral field spectroscopy to obtain photometric images and create 2D velocity maps of each galaxy. Using photometric and kinematic DiskFit models, we derived surface brightness profiles, rotation curves, and disk geometry parameters for each galaxy. We found our sample of dwarf galaxies to be free of bulges, bars, and non-circular motions, making them excellent candidates for future dark matter studies.

To guide research into formation scenarios for giant planets, I present a catalogue of 199,391 sources with candidate protoplanetary disks. These candidates are identified through an iterative color selection process in a 5,460-dimensional color-color space. As part of this process, photometry and SIMBAD classifications are aggregated from multiple large surveys, collectively covering 14 optical and infrared bands from 0.35 – 22 microns. An estimated false positive rate of 36.1% makes this catalogue a suitably sturdy foundation for a wide range of subsequent studies. After a discussion of the context surrounding this research, the forthcoming journal article describing this catalogue is reprinted in its entirety, and commentary is offered on the results.

I report 1684 new proper motion systems in the southern sky (declinations -90 degrees to -47 degrees) with 0.50 arcsec/yr > mu >= 0.18 arcsec/yr. This effort is a continuation of the SuperCOSMOS-RECONS (SCR) proper motion search to lower proper motions than reported in Hambly et al. (2004); Henry et al. (2004); Subasavage et al. (2005a,b). Distance estimates are presented for the new systems, assuming that all stars are on the main sequence. I find that 34 systems are within 25 pc, including three systems --- SCR 0838-5855, SCR 1826-6542, and SCR 0630-7643AB --- anticipated to be within 10 pc. These mini-motion (MINIMO) discoveries constitute a more than ten-fold increase in new systems found in the same region of sky searched for systems with mu >= 0.50 arcsec/yr, suggesting a happy hunting ground for new nearby slower proper motion systems in the region just north (declinations -47 degrees to 0 degrees), much of which has not been rigorously searched during previous efforts.

Bi-directional photocurrent cancellation in 30 period GaAs/Al_xGa_1-xAs split-off band photodetectors will be discussed. This cancellation results in a distinctive zero-response “notch” in spectral responsivity curves that can be controlled over the entire response range of the detector by using applied bias voltage. This phenomenon occurs at low negative bias, indicating a built-in potential offset in the Al_xGa_1-xAs barriers, with higher potential occurring at GaAs-on-AlGaAs interfaces. This asymmetry is also shown in threshold wavelength difference between negative and positive applied bias, and shows increasing potential offset with increasing aluminum fraction of the Al_xGa_1-xAs barriers. This barrier asymmetry is a major contributor to the photovoltaic operation in otherwise symmetric device structures, and a thorough understanding of this characteristic could lead to better operating and design parameters used for multi-junction photodetectors.

The surface morphology of thermally annealed copper foils utilized for graphene growth by chemical vapor deposition (CVD) has been studied by Optical microscopy, Scanning Electron Microscopy (SEM), and Scanning Tunneling Microscopy (STM) as a function of heat treatment. This study reports on the surface roughness and relative grain size before and after thermal annealing. The main results are that (a) the graphene covered foil exhibits reduced surface roughness, and (b) the graphene film preserves an imprint of the Cu grain structure. In the second part of the work, the transfer of CVD graphene is experimentally investigated using Poly(methyl methacrylate) (PMMA), Polycarbonate (PC), and Polystyrene (PS). Noticeable improvement to surface cleanness as well as electrical properties of graphene is observed after the ethanol treatment. Finally, Raman characterization showed apparent blue-shifts of the G and 2D bands suggesting that the graphene is heavily doped after the ethanol treatment.

The Engage to Excel (PCAST) report, the National Research Council's Framework for K-12 Science Education, and the Next Generation Science Standards all call for transforming the physics classroom into an environment that teaches students real scientific practices. This work describes the early stages of one such attempt to transform a high school physics classroom. Specifically, a series of model-building and computational modeling exercises were piloted in a ninth grade Physics First classroom. Student use of computation was assessed using a proctored programming assignment, where the students produced and discussed a computational model of a baseball in motion via a high-level programming environment (VPython). Student views on computation and its link to mechanics was assessed with a written essay and a series of think-aloud interviews. This pilot study shows computation's ability for connecting scientific practice to the high school science classroom.

Transient absorption spectroscopy was used to study light induced electron transfer processes in Type 1 photosynthetic reaction centers. Flash induced absorption changes were probed at 800, 703 and 487 nm, and on multiple timescales from nanoseconds to tens of milliseconds. Both wild type and menB mutant photosystem I reaction centers from the cyanobacterium Synechocystis sp. PCC 6803 were studied. Photosystem I reaction centers from the green algae Chlamydomonas reinhardtii, and the newly discovered chlorophyll-d containing organism Acaryochloris marina, were also studied.

The flash induced absorption changes obtained for menB mutant photosystem I reaction centers are distinguishable from wild type at 800 nm. MenB mutant photosystem I reaction centers displays a large amplitude decay phase with lifetime of ~50 ns which is absent in wild type photosystem I reaction centers. It is hypothesized that this ~50 ns phase is due to the formation of the triplet state of primary electron donor.

A two-scintillator paddle muon telescope with variable angular acceptance at the earth's surface was used to study correlations between flux distribution and barometric pressure. The detector was placed in 2 different locations around Georgia State University with varying paddle separations of 0, 7, and 14 inches. Correlation and anti-correlation analyses were conducted by using the muon count from the detector along with the barometric pressure, surface temperature, stratospheric temperature and solar activity. It was observed that there was a short and long-term variation relationship between cosmic ray counts and barometric pressure and also cosmic ray counts and temperature. No significant relationship was found between cosmic ray flux and solar activity. A new two-scintillator paddle telescope with larger detecting area was constructed in order to observe a stronger correlation between cosmic ray flux and pressure.

This document presents a study of astronomical observing conditions of Hard Labor Creek Observatory. Analysis of factors such as sky brightness, astonomical seeing, and patterns in the level of cloudiness at the site are presented. Characteristics of the observatory's Apogee Alta U230 camera are also measured and calculated. These characteristics include loss of linearity in the CCD's response to light, read noise, gain, dark current, and stability in the camera's bias levels. The camera is also used in conjunction with the 20-inch RC Optics telescope to determine the system's pixel scale and a set of limiting magnitudes for the Johnson-Cousins photometric filters that are used with the camera. Observations of a transit of known transiting exoplanet Qatar-2 b as well as observations of the open cluster Messier 29 are also performed to demonstrate the ability of the equipment to perform precise photometric observations.

Regional tissue temperature dynamics in the brain are determined by the balance of the metabolic heat production rate and heat exchange with blood flowing through capillaries embedded in the brain tissue, the surrounding tissues and the environment. Local changes in blood flow and metabolism during functional activity can upset this balance and induce transient temperature changes. Invasive experimental studies in animal models have estab- lished that the brain temperature changes during functional activity are observable and a definitive relationship exists between temperature and brain activity. We present a theoreti- cal framework that links tissue temperature dynamics with hemodynamic activity allowing us to non-invasively estimate brain temperature changes from experimentally measured blood- oxygen level dependent (BOLD) signals. With this unified approach, we are able to pinpoint the mechanisms for hemodynamic activity-related temperature increases and decreases. In addition to these results, the potential uses and limitations of optical measurements are dis- cussed.

This study uses a combination of quantitative and qualitative enquiry to focus on determining the most salient factors that affect international students’ learning of introductory physics in Georgia State University. For purposes of the study, “international students” were defined as those who attended high school in a country other than the US. These students comprise a significant portion of the physics courses at Georgia State, and this study was motivated by the desire to support their success. The study involved a collaboration with the newly emerging Physics and Astronomy Education Research Group who has recently begun the routine collection of student learning data in all of its introductory physics courses. The factors considered in the research design were informed by the literature on student learning for all students while including the possibility of new factors emerging in interviews with international students. Factors probed included students’ previous study of mathematics, previous study of physics, language issues, pedagogical differences (i.e., style of teaching, classroom culture & environment) between GSU and the student’s country of origin. For international students who are proficient in English, classroom environment and culture (pedagogy) emerged as the most important factor. For International students who are not very proficient in English, language remains the most important factor. The effect of language issues on international students’ learning of physics turned out to be more complex than originally considered. Some students understood instructors differently depending on what country the students come from and on what country the instructor comes from. Instructor office hours and general accessibility for addressing questions emerged as especially important options for international students who felt uncomfortable asking questions in front of the whole class. An unanticipated outcome of the study was to discover how the vast differences in the structure of high school mathematics education in non-US countries has serious implications for the way we advise and query international students in physics vis-à-vis their academic background before entering Georgia State. Moreover, the study revealed that students who had taken a high school physics course generally scored no better than those who had not taken a high school course on a pre-test of conceptual knowledge in physics. However, students who had taken a physics class in high school had dramatically higher learning gains when given a post-test near the end of the Georgia State physics course. This phenomenon suggests that more consideration should be given to prior course-work in combination with a diagnostic pre-test to advise students about which math and physics courses to take when they arrive at Georgia State.