Doctor of Philosophy, The Ohio State University, 2020, Physics

Light-matter interactions are key in providing fundamental information about materials. The terahertz (THz) frequency range is a critically important region of the electromagnetic spectrum where the electronic properties of many quantum materials have resonant responses. It has also long been difficult to access; a property which has been termed the THz gap. In recent decades though, new techniques and methods in THz spectroscopy have come a long way to filling in that gap with more efficient emitters and detectors. We describe here our contributions to the field of THz spectroscopy. We detail the development and construction of devices and development of techniques to explore new categories of materials and generally expand the capabilities of THz spectroscopy. We also demonstrate the efficacy of these techniques in novel and interesting material systems.

Committee: Rolando Valdés Aguilar (Advisor); Jay Gupta (Committee Member); Yuan-Ming Lu (Committee Member); Andrew Heckler (Committee Member) Subjects: Condensed Matter Physics; Optics; Physics

Master of Science (MS), Wright State University, 2020, Physics

A recently constructed novel analytical tabletop terahertz (THz) chemical sensor capable of detecting a wide range of gases with high sensitivity and specificity was characterized to assess its performance over a range of operational parameters. The sensor was designed with an objective of quantifying composition of exhaled human breath, where target concentrations span part per trillion (ppt) to part per billion (ppb) level of dilutions. The sensor utilizes terahertz rotational spectroscopy of sampled gases for quantification of dilutions. The sensor occupies a volume of ~ 2 ft3 and incorporates a coiled absorption cell, thermal desorption tubes, and all necessary electronic components necessary for autonomous operation. Coiled absorption cell minimizes the sensor footprint while maintaining a large path length for sensitive spectral measurements. Preconcentration aides the detection of compounds by removing the background gases which would negatively affect the absorption signal if present during spectral analysis. Spectral parameters of the sensor were studied to optimize its sensitivity. Efficiencies of preconcentration over a range of gas sampling parameters were determined by comparing concentrations measured by the sensor to concentrations of a reference gas mixture. The sensor was characterized in its ability to detect acetaldehyde, acetone, ethanol, isoprene, and methanol – all known breath analytes. These gases were chosen for their range of volatility and absorption strength. Minimum detectable sample concentrations are well suited for breath sampling making this sensor a valuable new tool for environmental sensing and biosensing.

Committee: Ivan Medvedev Ph.D. (Advisor); Brent Foy Ph.D. (Committee Member); Jason Deibel Ph.D. (Committee Member) Subjects: Physics

Master of Science (MS), Wright State University, 2012, Physics

A highly sensitive and selective Terahertz gas sensor used to analyze a complex mixture of Volatile Organic Compounds (VOCs) has been developed. To best demonstrate analytical capabilities of a THz chemical sensor, we chose to perform analytical quantitative analysis of a certified gas mixture using a prototype gas phase chemical sensor that couples a commercial preconcentration system (Entech 7100A) to a custom high resolution THz spectrometer. A Method TO-14A certified mixture of thirty-nine VOCs was purchased. Twenty-six of the thirty-nine chemicals were identified as suitable for THz spectroscopic detection. The Entech 7100A system is designed and marketed as an inlet system for Gas Chromatography Mass Spectrometry (GC-MS) instruments with a specific focus on TO-14A sampling methods and has been incorporated into our spectrometer. Its preconcentration efficiency is high for the thirty-nine chemicals in the mixture used for this study and our preliminary results confirm this for many of the selected VOCs. Presented are the results of this study which will serve as a basis for our ongoing research in environmental sensing and exhaled human breath.

Committee: Ivan Medvedev PhD (Advisor); Doug Petkie PhD (Committee Member); Gary Farlow PhD (Committee Member) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 2019, Electrical and Computer Engineering

Utility of wireless connectivity has been steadily increasing as broadband internet becomes widely available and having low-cost technology leads to more devices built with Wi-Fi capabilities and sensors. As the traditional radio-frequency (RF) bands (sub 3 GHz) become congested, the mmW band offering vast amount of spectrum, is poised to be the backbone of 5G wireless networks. Particularly, thanks to much smaller wavelengths, antenna-integrated transceivers are viable solutions for the future 5G wireless networks. However, key challenges still remain for on-chip implementation of efficient radiators at such high frequencies. Namely, poor antenna bandwidths, severely low radiation efficiencies, as well as laborious and expensive antenna-transceiver integration (wire bonds, flip-chip, ball grid arrays, etc.) limit the utility of truly-integrated on-chip antennas. To overcome these prevailing obstacles we present an ultra-wideband (UWB), low-profile, high efficiency, tightly-coupled array topology which is adopted from RF-frequency realizations and modified as a multilayered structure suitable for standard micro-fabrication process. Through this work, we show that on-chip radiation efficiency is well above 60% over the entire impedance bandwidth. The proposed array exhibits wideband performance, covering 35-75 GHz, achieving an unprecedented coverage that spans most of the bands allocated for mobile communications. Utilization of low-loss materials in such designs can address the substrate coupling issues and improve the radiation efficiency. Moreover, the structural support and packaging materials that exhibit low loss are indispensable for cost-effective realization of integrated high frequency systems. To effectively address these requirements, polymers are a natural, low-cost choice for structural support and packaging of microchips due to their favorable chemical, thermal, and mechanical properties. However, many polymers have not been studied for mmW and TH (open full item for complete abstract)

Committee: Kubilay Sertel (Advisor); Niru Nahar (Committee Member); Fernando Teixeira (Committee Member) Subjects: Electrical Engineering; Electromagnetics; Electromagnetism

Doctor of Philosophy, The Ohio State University, 2019, Physics

In crystalline materials, the symmetry of the crystal lattice imposes strict conditions on the observable properties of the material. These symmetry restricted conditions can be, in turn, probed by light via the electromagnetic interaction. Studying the electromagnetic excitations in solids can reveal many fundamental properties of these systems. A quick introduction and guide to symmetry in solids will be given, with an emphasis on how it can be used to interpret spectroscopic measurements. The measurement techniques used will also be described. Time domain Terahertz spectroscopy (TDTS) is the main technique used in this dissertation. Important experimental considerations pertaining to the construction of the THz spectrometer will be given. In the multiferroic Sr_2 FeSi_2O_7, we found multiple excitations in the few meV energy scale (THz), in the material's paramagnetic phase. Measurements with varying temperature and magnetic field revealed that these excitations are both electric and magnetic dipole active. By considering the ground state of the Fe 2+ magnetic ion in Sr 2 FeSi 2 O 7 , we concluded that our observation is coming from the spin-orbital coupled states of the ion. This realization demonstrated that spin-orbit coupling plays a crucial role in these exotic materials. Interestingly, these spin-orbital THz excitations persist into the magnetically ordered phase. The single-ion picture of the paramagnetic phase needs to be expanded theoretically to explain our observations. CaFe_2O_4 orders antiferromagnetically below ~ 200 K. Two co-existing magnetic structures (A and B phase) have been measured previously by neutron diffraction. The anti-phase boundaries between these two phases have been proposed to be the cause of the quantized magnetic excitations (magnons) measured by an inelastic neutron scattering study. We measured two antiferromagnetic resonances (magnons) with TDTS. Our observation can be explained by the orthorhombic crystal anisotropy of CaF (open full item for complete abstract)

Committee: Rolando Valdes Aguilar (Advisor); P. Chris Hammel (Committee Member); Nandini Trivedi (Committee Member); Douglass Schumacher (Committee Member) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 2017, Physics

A time-domain terahertz spectrometer was built for the purposes of studying condensed matter systems.

Committee: Rolando Valdes Aguilar (Advisor); P. Chris Hammel (Committee Member); Christopher Hill (Committee Member); Mohit Randeria (Committee Member) Subjects: Physics

Master of Science (MS), Wright State University, 2017, Physics

Terahertz spectroscopy has found use as an analytical tool in determining chemical composition of exhaled human breath. This thesis demonstrates a novel application of this technology - analytical sensing of gaseous metabolic products of several types of human cell cultures. An innovative experimental system was developed for probing cellular metabolism using terahertz [THz] rotational spectroscopy. Gaseous emissions of cell cultures were analyzed and compared between several cell types. Cancerous and healthy lung cells as well as cancerous liver cells were studied. This technique carries a lot of promise as a noninvasive method of distinguishing between cell types and identifying cell pathologies. In this set of experiments, prominent variance in the rates of acetaldehyde metabolism was identified, which can potentially be used as a diagnostic method of cellular identification. Possible applications of this novel technique might extend to the medical field, where it will be used as a non-invasive detection and diagnostic tool.

Committee: Ivan Medvedev Ph.D. (Advisor); Jason Deibel Ph.D. (Committee Member); Jerry Clark Ph.D. (Committee Member) Subjects: Biology; Biomedical Engineering; Biomedical Research; Biophysics; Cellular Biology; Physics

Doctor of Philosophy, The Ohio State University, 2017, Physics

Plasmas used for manufacturing processes of semiconductor devices are complex and challenging to characterize. The development and improvement of plasma processes and models rely on feedback from experimental measurements. Current diagnostic methods are not capable of measuring absolute densities of plasma species with high resolution without altering the plasma, or without input from other measurements. At pressures below 100 mTorr, spectroscopic measurements of rotational transitions in the submillimeter/terahertz (SMM) spectral region are narrow enough in relation to the sparsity of spectral lines that absolute specificity of measurement is possible. The frequency resolution of SMM sources is such that spectral absorption features can be fully resolved. Processing plasmas are a similar pressure and temperature to the environment used to study astrophysical species in the SMM spectral region. Many of the molecular neutrals, radicals, and ions present in processing plasmas have been studied in the laboratory and their absorption spectra have been cataloged or are in the literature for the purpose of astrophysical study. Recent developments in SMM devices have made its technology commercially available for applications outside of specialized laboratories. The methods developed over several decades in the SMM spectral region for these laboratory studies are directly applicable for diagnostic measurements in the semiconductor manufacturing industry. In this work, a continuous wave, intensity calibrated SMM absorption spectrometer was developed as a remote sensor of gas and plasma species. A major advantage of intensity calibrated rotational absorption spectroscopy is its ability to determine absolute concentrations and temperatures of plasma species from first principles without altering the plasma environment. An important part of this work was the design of the optical components which couple 500 - 750 GHz radiation through a commercial inductively couple (open full item for complete abstract)

Committee: Frank De Lucia (Advisor); Louis DiMauro (Committee Member); Richard Furnstahl (Committee Member); Thomas Lemberger (Committee Member) Subjects: Physics

Master of Science (MS), Wright State University, 2016, Physics

The ability to quantify trace chemicals in human breath enables the possibility of identifying breath biomarkers to aid in diagnosis. The vast majority of the studies in the analytical breath analysis rely on GC-MS techniques for quantification of the human breath composition1,2,3,4. THz spectroscopy of breath is rapid, sensitive, and highly specific molecular identification in complex mixtures containing 10-100 analytes with near `absolute' specificity. THz spectroscopic breath analyzers require chemical preconcentration. A newly developed custom preconcentrator was constructed and compared in its performance to a commercial system. Unlike the commercial counterpart, the new system does not require cryogenic liquids, is compact, and offers significant advantages in terms of ease of operation and facilitates further development of THz breath sensors. Its preconcentration efficiency was assessed. The THz spectrometer coupled with the custom preconcentrator demonstrated first THz detection of breath isoprene, a chemical not detected with the commercial device.

Committee: Ivan Medvedev Ph.D. (Advisor); Doug Petkie Ph.D. (Committee Member); Brent Foy Ph.D. (Committee Member) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 2015, Electrical and Computer Engineering

THz-frequency spectroscopic imaging has recently drawn increasing attention as a novel modality for bio-medical analysis of diseases and conditions of living tissues. More importantly, detection of cancerous tumors as well as necrotic tissue regions is being studied using THz waves with the aim of translating research studies into clinical practice. THz radiation provides unique sensing capabilities applicable to a variety of areas including non-destructive inspection, security screening, as well as bio-medical imaging. THz waves are safe (non-ionizing), and they can provide high-resolution with better specificity compared to X-rays. In addition, THz waves enable the spectroscopic analysis of organic molecules, since many of their rotational and vibrational resonances fall within the THz band. Perhaps more importantly, THz waves are extremely sensitive to the degree of sample hydration and this property has been utilized to differentiate cancerous tissue regions. However, previous studies on human tissue groups have been largely disconnected, with publications focusing on only limited tissue groups at a time. In addition, assessment of cancer margins to differentiate in-situ extent of disease has rarely been a major focus. As such, a more general in-depth study of the THz response of extended human tissue groups is much desired to demonstrate the potential of THz sensing as a clinical tool. In this work, we initially focus on a comprehensive experimental study of the THz response of major human tissue malignancies to investigate the efficacy of THz sensing as a clinical bio-medical tool. In particular, using the THz-band spectroscopic reflectivity and transmission properties of bulk and thin tissue samples, we characterize optical properties associated with the corresponding tissue characteristics. To do so, we develop calibration techniques to take into account experimental fixture effects. In addition, the specificity and sensitivity of the commercial time-doma (open full item for complete abstract)

Committee: Kubilay Sertel (Advisor); Fernando Teixeira (Committee Member); Umit Catalyurek (Committee Member); Niru Nahar (Committee Member) Subjects: Biomedical Engineering; Electrical Engineering

Master of Science (MS), Wright State University, 2012, Physics

An optical material parameter predictive model that accounts for sample to air interfaces was developed. The model predicts how a terahertz time-domain spectroscopy time domain pulse will be affected as it passes through a given thickness of a material. The model assumes a homogenous, linear, isotropic dielectric or semiconductor. The inputs to the model are the real and imaginary refractive indices across the desired frequency band. Different dielectric material's optical parameters were taken from the literature and the predicted time domain pulses were shown. It was also shown that the refractive index and absorption coefficient for samples that were optically thick and low-loss could be determined from measurements analytically. It was also shown that for non-dispersive media with a flat absorption coefficient, the predictive model could be used to determine an average value for both the index of refraction and the absorption coefficient across the frequency band, (0.1-4 terahertz).

Committee: Jason Deibel PhD (Advisor); Douglas Petkie PhD (Committee Member); Gregory Kozlowski PhD (Committee Member) Subjects: Physics

Master of Science (MS), Wright State University, 2006, Physics

With sub-millimeter wave or terahertz devices becoming more readily available, there is interest in developing sensors in this region of the spectra. To support this interest, we have developed a Fourier Transform Far InfraRed (FTFIR) spectrometer to characterize broadband transmission and reflectance coefficients of materials. The spectrometer utilizes a broadband blackbody source, a Michelson interferometer, and silicon bolometer. The path difference in the Michelson is obtained using a linear stage and data acquisition and stage control were both implemented in a Labview programming environment. The details of the experimental setup and experimental results are presented in this thesis. The instrument demonstrated capability to measure the broadband transmission spectra of cloth and cardboard samples and we found that these spectra, which showed transmission < ~0.5 THz and increased in attenuation at higher frequencies, agreed with accepted general trends.

Committee: Doug Petkie (Advisor) Subjects: Physics, Optics

Doctor of Philosophy, Miami University, 2012, Chemistry and Biochemistry

This dissertation contains six chapters demonstrating the use of Terahertz Time-Domain spectroscopy and imaging in a variety of applications, from the principle analysis of observed absorption features to the quantitation of threat agents. Chapter 1 focuses on the background of Terahertz, starting with its roots in Microwave and Infrared Spectroscopies and continuing on to modern time-domain techniques that dominate the field at present. Terahertz's interaction with different types of matter, various instrumentation setups, and several types of common time-domain measurements are also discussed. Chapter 2 discusses two separate studies attempting to further the understanding of collective mode absorption peaks observed in the THz spectral region. Absorption peaks found in the THz region of crystalline solids are typically described generically as collective modes or computationally analyzed with no supporting experimental data. These two studies demonstrate an experimental method that can be used concurrently with computational techniques to elucidate a more complete understanding of observed collective modes. Chapter 3 probes the feasibility of detecting a possible threat agent, dipicolinic acid, which is a major component in bacterial spores, such as Anthrax. It focuses on qualitative discovery and the ability to quantify its presence with Terahertz Spectroscopy and imaging. Chapter 4 presents a library of quality cryogenic and room temperature spectra for the 20 standard amino acids to be used as a reference for future research. In addition, trends observed by the groups of amino acids were assessed. Chapter 5 examines the spectral properties of a large biomolecule, heparin, in the terahertz spectral region. Several sample configurations are investigated, from heparin as-is to crystallized nitric acid digestion remnants. A novel trace metal analysis method of heparin utilizing Inductively Coupled Plasma Optical Emission Spectroscopy is also presented. Chapter 6 d (open full item for complete abstract)

Committee: Gilbert E. Pacey PhD (Advisor); Shouzhong Zou PhD (Committee Chair); Richard T. Taylor PhD (Advisor); C. Scott Hartley PhD (Committee Member); James R. Gord PhD (Committee Member) Subjects: Analytical Chemistry

Doctor of Philosophy, Miami University, 2009, Chemistry and Biochemistry

This dissertation consists of four chapters detailing terahertz time-domain spectroscopy (THz-TDS) and the application thereof to the investigation of biological samples and related analytes. Chapter 1 provides a background of the evolution of THz-TDS from a historical and physics/engineering perspective followed by an extensive section from a chemistry perspective dedicated to the interaction of THz radiation with matter. A variety of measurement techniques are discussed, culminating in a literature review. Chapter 2 focuses on the investigation of the 20 essential amino acids as crystalline solids using THz spectroscopy, in which absorbance of THz frequencies by crystal lattice phonon propagation is documented. A study dedicated to qualitative analysis of L-alanine introduces a novel approach for the spectral assignment of the observed THz absorbance frequencies to the crystal structure of the analyte. Chapter 3 presents the study of five types of excised human breast tissue – normal, benign, lobular carcinoma, ductal carcinoma in situ and invasive ductal carcinoma – using THz pulsed imaging, which consists of successive THz-TDS measurements over a two-dimensional space. The presentation and evaluation of the tissue images are supplemented with the examination of frequency-domain reflectance spectra of selected tissue areas. The results of the studies presented herein are followed by detailed guidelines for the maximization of the efficiency and benefit of continued pursuits in this field. Chapter 4 investigates the potential of reflectance-mode THz-TDS as a method for the non-destructive determination of the gel-point of sol-gel monoliths. Results are used to compare the gel point properties of sol-gels with and without a stabilizing dendrimer. The conclusions are found to support a particular theory concerning the stabilization mechanism of the dendrimer in the sol-gel process.

Committee: Gilbert Pacey PhD (Advisor); James Cox PhD (Committee Chair); Michael Crowder PhD (Committee Member); James Gord PhD (Committee Member); Richard Taylor PhD (Committee Member) Subjects: Analytical Chemistry; Biomedical Research; Chemistry; Experiments; Molecules; Radiation; Scientific Imaging; Statistics

Doctor of Philosophy, Miami University, 2008, Chemistry and Biochemistry

This dissertation details results of using terahertz (THz) radiation to investigate material properties. Progress in instrumentation has allowed researchers to work in this frequency region, yet most of the effort has been in hardware design and not directly in application. Here, select studies of foam drainage, alcohol absorption and dielectric properties, and amino acid spectral features are presented.Foam drainage has been investigated with THz spectroscopy to characterize various aqueous-based surfactant solutions. Anionic, cationic, nonionic, mixed ionic, and protein surfactant systems were studied. Nitrogen gas was bubbled into the respective aqueous systems to create a wet foam, which then drained characteristically based on interacting forces between the surfactant molecules with both water and each other. THz radiation, readily absorbed by water, was then used to identify the water content throughout the foam at various positions and times, yielding static and kinetic absorption data. In addition, basic imaging was attempted using an x-y translation stage to raster across a longer-lasting foam. Absorption and dielectric data of alcohols were collected by passing THz radiation through a high-density polyethylene cell containing the liquids. Octanol down to ethanol were investigated, and material properties were calculated from the time-domain waveforms collected. Pure alcohols were studied along with dilutions in hexane, as hexane has negligible absorption in the THz region. These data extend the available resources from the microwave region regarding dielectric properties of polar molecules. Amino acids, important in body function, have also been investigated using THz spectroscopy. Pure powders were "diluted" in polyethylene and pressed into pellets that were probed in transmission mode. Mixed amino acid pellets were also created to use as a basic polypeptide study. Attenuated total reflectance modules, included with the instrument purchased from TeraView, (open full item for complete abstract)

Committee: Gilbert Pacey PhD (Advisor); James Cox PhD (Committee Chair); Michael Crowder PhD (Committee Member); James Gord PhD (Committee Member); Shouzhong Zou PhD (Committee Member) Subjects: Analytical Chemistry

Doctor of Philosophy, Case Western Reserve University, 2008, Physics

Spectroscopy in the far-infrared part of the electromagnetic spectrum based on the time-domain measurements of transient terahertz pulses has become a standard experimental technique. In the first part of this thesis we present results regarding applications of this technique to the problem of near-field, sub-wavelength imaging and the effect of finite-size beams in optical pump/terahertz probe experiments. The second part presents time-resolved far-infrared measurements performed on semiconductor quantum dots. Amongst many applications, terahertz time-domain spectroscopy (THz-TDS) has been successfully used for imaging. We present a method based on highly-localized THz generation through a nonlinear process that achieves sub-wavelength resolution and a favorable power throughput, essential for the sensitivity of the measurement. With respect to standard optical pump/THz probe measurements the finite size of the beams intersecting at the sample introduces non-trivial effects. We modeled the problem as THz-induced radiation from the optically-generated polarization to obtain useful requirements for the relative dimensions of the pump and probe beams that allow readily interpretable measurements. The ability to directly measure the electric field of the THz pulse opens the possibility to perform spectroscopic measurements with picosecond time-resolution. We used this feature of the THz-TDS to explore the response of short-lived, optically-induced excitons to external THz electric fields in colloidal semiconductor quantum dots (QDs). In the limit of single exciton per QD we performed a comparative study between two systems of QDs, CdSe and PbSe, possessing different electronic structure. For both samples the response was found to be atom-like, and was successfully simulated with the help of an effective-mass model. The presence of multiple excitons within a QD is accompanied by strong many-body interactions manifested by the extremely fast Auger recombination. We inves (open full item for complete abstract)

Committee: Jie Shan (Advisor) Subjects: Physics, Condensed Matter