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

Doctor of Philosophy (Ph.D.), University of Dayton, 2014, Electro-Optics

The objective of this dissertation is to evaluate and develop novel sources for tunable narrowband IR generation, tunable narrowband THz generation, and ultra-wideband RF generation to be used in possible non-destructive evaluation systems. Initially a periodically poled Lithium Niobate (PPLN) based optical parametric amplifier (OPA) is designed using a double-pass configuration where a small part of the pump is used on the first pass to generate a signal, which is reflected and filtered by an off-axis etalon. The portion of the pump that is not phase matched on the first pass is retro-reflected back into the PPLN crystal and is co-aligned with the narrow bandwidth filtered signal and amplified. We demonstrate that the system is tunable in the 1.4 µm -1.6 µm signal range with a linewidth of 5.4 GHz. Next the outputs of seeded, dual periodically poled lithium niobate (PPLN) optical parametric amplifiers (OPA) are combined in the nonlinear crystal 4-dimthylamino-N-methyl-4-stilbazolium-tosylate (DAST) to produce a widely tunable narrowband THz source via difference frequency generation (DFG). We have demonstrated that this novel configuration enables the system to be seamlessly tuned, without mode-hops, from 1.2 THz to 26.3 THz with a minimum bandwidth of 3.1 GHz. The bandwidth of the source was measured by using the THz transmission spectrum of water vapor lines over a 3-meter path length. By selecting of the DFG pump wavelength to be at 1380 nm and the signal wavelength to tune over a range from 1380 nm to 1570 nm, we produced several maxima in the output THz spectrum that was dependent on the phase matching ability of the DAST crystal and the efficiency of our pyro-electric detector. Due to the effects of dispersive phase matching, filter absorption of the THz waves, and two-photon absorption multiple band gaps in the overall spectrum occur and are discussed. Employing the dual generator scheme, we have obtained THz images at several locations in the spect (open full item for complete abstract)

Committee: Joseph Haus (Committee Chair) Subjects: Optics

Doctor of Philosophy (PhD), Wright State University, 2014, Engineering PhD

Since the first demonstration of the generation of terahertz (THz) pulses from photoconductive (PC) antennas, research has pushed toward the development of smaller, cost efficient, and faster THz systems. This dissertation presents the work accomplished in order to realize these more practical terahertz (THz) photoconductive (PC) systems. First, this work will present a novel ErAs:GaAs photoconductive switch used to make a THz source excited by 1550 nm laser pulses. It will be shown that the excitation process taking place in the material relies on extrinsic (rather than intrinsic) photoconductivity. Then, several experiments will be presented that aim to improve the efficiency of the device and further the understanding of the underlying physical mechanisms. The erbium composition of the photoconductive layer will be varied and the effects of these variations on THz generation will be investigated. Then the wavelength of the drive laser used to excite the extrinsic photoconductive mechanism will be varied, while recording the photocurrent responsivity. This wavelength study will be used to find the optimal drive wavelength for maximum THz power. In conclusion, the results of these experiments will show that extrinsic PC THz generation is practical, cost effective, and capable of producing an average THz power of more than 100 μ W. Coinciding with this high power level, the bandwidth of this new source was found to be ~350 GHz, corresponding to a photocarrier recombination time of 450 fs. The work presented in this section will provide a path to develop superior THz PC sources that have a higher THz-power-to-cost ratio than the current state of the art. Photoconductive antennas are mostly used to conduct spectroscopy measurements, either in time domain systems (TDS) or in frequency domain systems (FDS). Currently, both techniques can reach high-frequencies (>1 THz) but struggle to do so while making fast, high-resolution measurements (<2 GHz). In ad (open full item for complete abstract)

Committee: Elliott Brown Ph.D. (Advisor); Jason Deibel Ph.D. (Committee Member); Doug Petkie Ph.D. (Committee Member); Daniel LeMaster Ph.D. (Committee Member); Julie Jackson Ph.D. (Committee Member) Subjects: Electrical Engineering; Electromagnetics; Engineering; Materials Science; Optics; Physics; Plasma Physics; Radiation

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

We present novel realizations of computational terahertz (THz) imaging techniques based on compressive sensing and phase-retrieval algorithms and a single-pixel THz sensor. Imaging in the THz band covering 300 GHz-10 THz is being considered for key applications in biomedical imaging, security screening and non-destructive evaluation. State-of-the-art in THz imaging is based on mechanical raster scanning using a single, high-performance sensor. Such raster-scanning imagers are rather bulky and suffer from very low frame-rates, as well as mechanical noise due to the moving parts in the hardware. Alternatively, multi-detector imagers such as THz focal plane arrays (FPAs) can speed-up image acquisition time, potentially reaching real-time video rates. However, such devices require complex and expensive fabrication and they typically exhibit limited sensitivity due to additional noise introduced by the read-out circuit. In this dissertation, we demonstrate novel THz imaging techniques based on a single THz sensor that concurrently circumvent the slow acquisition time and mechanical noise of raster scan imaging. This is achieved by using an optically reconfigurable spatial wave modulation scheme to "serialize'' the scene measurements. Subsequently, compressive sensing (CS) and reconstruction algorithms are employed to computationally generate 2D images of the scene from a set of serial measurements, each corresponding to a different spatial modulation. Similar to well-developed optical CS methods, compressive THz imaging allows far fewer measurements than the conventional Nyquist rate to accurately reconstruct sparse scenes. In addition, compressive THz reconstruction exhibits better signal-to-noise (SNR) performance compared to the FPA cameras. To enable the study and experimental demonstration of various computational imaging algorithms, we realize a generalized compressive THz imaging setup using conventional quasi-optical components and a semiconductor-based ph (open full item for complete abstract)

Committee: Kubilay Sertel (Advisor); Niru K. Nahar (Committee Member); Fernando Teixeira (Committee Member); Robert Burkholder (Committee Member) Subjects: Electrical Engineering; Electromagnetics; Optics

Master of Science in Electrical Engineering (MSEE), Wright State University, 2024, Electrical Engineering

The uncertainty of the standard calibration procedure for radar cross-section (RCS) measurement is studied for different targets measured in the near-field from 550 to 700 GHz. Using common calibration spheres and squat cylinders mounted on a styrofoam pedestal at waterline (zero-degrees elevation), the calibration difference measure is determined for each target. Similarly, the difference metric is determined for square trihedral and tophat targets placed on a ground plane and measured at different elevation angles. The mean calibration measure is calculated using the dual calibration target method and repeated measurements in an anechoic chamber. The specific THz system is described and the results show how the near field scattering behaviors degrade the accuracy of the scattering measurement. Additional analysis shows the measurement uncertainty to be within a few decibels for frequencies within 580 to 650 GHz.

Committee: Michael A. Saville Ph.D., P.E. (Advisor); Josh Ash Ph.D. (Committee Member); Cheryl B. Schrader Ph.D. (Committee Member) Subjects: Electrical Engineering

Doctor of Philosophy, The Ohio State University, 2024, Electrical and Computer Engineering

The central theme of this dissertation is to explore material interaction with electromagnetic waves in millimeter-wave (mmWave) and terahertz (THz) frequency bands spanning the range from 30 GHz to 3 THz. The implications of these interactions in the context of on-vehicle integration of mmWave automotive radars is discussed. Furthermore, specific mechanisms are exploited for broadband material characterization, and biomedical imaging. First, this research outlines the traditional broadband methods to characterize the electromagnetic properties of isotropic non-magnetic dielectric materials. In mmWave and THz regime, this data is not readily available in many cases. Utilizing established free-space techniques such as terahertz time-domain spectroscopy (THz-TDS) and quasi-optical transmission measurements, this research extracts this data for a diverse range of materials. In particular, we discuss the challenges related to the reliability of the permittivity extraction process in situations where the measurement may not have a high SNR across the bandwidth of interest. We circumvent this problem by cross-validating the data across multiple modalities to ensure consistency. Additionally, for thin dielectric films for which conventional methods fail, this research proposes a novel permittivity extraction technique from calibrated two-port S-parameter measurements of a coplanar waveguide. Interaction of mmWave radar signal with the near zone radome and bumper layers can impair the radar performance through reduction of signal-to-noise ratio and distortion of the pattern. Therefore, towards the goal of a `transparent' radome, the dissertation proposes a novel textured radome design aimed at optimizing transmission efficiency for mmWave automotive radar. Through a strategic optimization based on first-principles, this design exhibits an enhanced signal transmission throughout the entire automotive radar band of 76 – 81 GHz. The optimized design demonstrates an avera (open full item for complete abstract)

Committee: Niru Nahar (Advisor); Kubilay Sertel (Committee Member); Asimina Kiourti (Committee Member); Alebel Arage (Committee Member) Subjects: Electrical Engineering; Electromagnetics

Doctor of Philosophy, The Ohio State University, 2023, Electrical and Computer Engineering

Automotive radar systems are poised to become key enablers for improving the overall functionality and safety of commercial automobiles. In this proposal, three separate investigations into improving the electromagnetic modeling, design, and testing, of current automotive radar systems are presented. Due to their extremely low profile and ease of manufacturing, patch antenna arrays are the common choice of these radar systems. A particular problem with patch arrays is the inter-element coupling though substrate modes. In particular, such mutual coupling introduces radiation pattern ripple, or distortion, from observation angles between -30 and +30 degrees in the elevation plane. To mitigate mutual coupling, we propose a double-slot array with an integrated alumina lens to maintain low mutual coupling between radiating elements while simultaneously reducing radiation pattern ripple from 5dB in the microstrip patch array to 0.8dB in the double-slot array. In addition, an improvement of 4.5dB in maximum gain was achieved for the individual element patterns with the inclusion of the alumina lens. The proposed double-slot array is also presented with a “common-differential” mode functionality that can aid in beam scanning the intended area of the radar system with finer resolution than traditional constructive phase beam forming. As the demand for bandwidth across the electromagnetic (EM) spectrum continues to grow, the need for technology in vacant frequency bands of the spectrum also increases. Reconfigurable filters are one way to increase the efficacy of devices in all parts of the EM spectrum allowing applications to coexist in nearby frequency bands. Our vanadium dioxide (VO2) reconfigurable filters were designed on a thin, transparent, flexible, polyimide substrate intended to operate in the Far-IR. We present novel fabrication recipes for VO2 on a polyimide substrate, and verify that it exhibits a prodigious metal to insulator transition at 48⁰C that is (open full item for complete abstract)

Committee: Niru Nahar (Advisor); Robert Burkholder (Committee Member); Kubilay Sertel (Committee Member) Subjects: Electrical Engineering; Electromagnetics

Master of Science (MS), Wright State University, 2021, Physics

In order to support ongoing terahertz time-domain spectroscopic experiments involving plasma characterization, it is beneficial to simulate the interaction of THz pulses with varying plasma configurations. In this approach, a 1-D Finite Difference Time Domain (FDTD) model was constructed to simulate the interaction of terahertz radiation with a plasma medium. In order to incorporate the plasma properties into the simulation, a Z-transformation was applied. This model is capable of simulating the following properties of plasmas including electron density, collision frequency, and the interaction length of the plasma medium. The simulated model was characterized using terahertz time-domain spectroscopy. The effects of electron density, collision frequency, and the interaction length of the plasma medium on the amplitude and phase of the terahertz pulse were studied and calculated values of electron density were compared with the values used in simulation.

Committee: Jason A. Deibel Ph.D. (Advisor); Amit R. Sharma Ph.D. (Committee Member); Sarah F. Tebbens Ph.D. (Committee Member) Subjects: Physics

Doctor of Philosophy (PhD), Wright State University, 2018, Engineering PhD

This dissertation entails the investigation of ultrafast photoconductive (PC) THz sources driven by fiber and semiconductor lasers around λ= 1550-nm to utilize commercial fiber-optic telecom technology. The preferred approach is to use GaAs with a high concentration of erbium, which has performed well when driven with laser sources at both 800 nm wavelength, through intrinsic photoconductivity, and 1550 nm, through extrinsic photoconductivity. Studies in the early 1990s showed that the Er doping level has a solubility limit of ~ 7 × 1017 cm−3 at 580 °C, above which erbium is incorporated into GaAs as ErAs nanoparticles which promote resonant absorption around λ= 1550-nm. This research is focused on improving the GaAs:Er extrinsic-photoconductive device performances by engineering the material and improving the design of THz antennas. Antennas with different dimensions have been fabricated and tested, and substrates with different doping levels and epi-layer thicknesses have been studied and characterized to improve absorption of the 1550 nm radiation, increase the dark resistivity and get more THz radiation. The antennas were then fabricated with a planar-processing technique, packaged, and tested as 1550-nm driven PC THz sources. These are the first THz devices fabricated in the history of Wright State University and have already set a record in terms of power generated by THz photoconductive devices driven at 1550-nm.

Committee: Elliott Brown Ph.D. (Advisor); Marian Kazimierczuk Ph.D. (Committee Member); Pradeep Misra Ph.D. (Committee Member); Jason Deibel Ph.D. (Committee Member); John Middendorf Ph.D. (Other); Weidong Zhang Ph.D. (Other) Subjects: Electrical Engineering; Solid State Physics

Doctor of Philosophy (PhD), Wright State University, 2018, Interdisciplinary Applied Science and Mathematics PhD

The use of terahertz time-domain spectroscopy provides one of the most versatile and promising techniques for the robust determination of optical parameters, which is needed to enable identification of materials for quality control, materials science advancement, tamper prevention, drug enforcement, and hidden explosives detection. Previously, the state-of-the-art relied on legacy error measures for minimization of simulation error and the standard practice was to use a single unique measurement for each unknown material in a sample. Successful optical parameter extraction for uniformly varying optical property materials is correlated with low variation in extracted optical properties. This work advances the state-of-the-art in optimization-based physical parameter extraction using terahertz time-domain spectroscopy. This is achieved by standardizing the signal processing methodology, clearly defining the best optimization formulation to yield low simulation error and optical property variation, and leveraging multiple measurements to reduce the impact of system-dependent artifacts on extracted optical properties. A thorough analysis of alternative error measures across numerous objective function formulations demonstrates that a 28% reduction in the Fabry-Perot etalon effect in the optical property of materials is achievable, compared with legacy approaches. The research conclusively demonstrates that time-domain objective function formulations yields simulation error that is 83% less than frequency-domain objective function formulations. Furthermore, the research shows that multi-measurement optimizations reduce oscillations in optical properties caused by the Fabry-Perot etalon effect by as much as 92%, compared with single-measurement optimizations. The research validates the numerical solutions to less than 6% error compared with analytical solutions, for uniform and non-uniform optical property materials. Importantly, the research extends the state-of-the-art (open full item for complete abstract)

Committee: Jason Deibel Ph.D. (Advisor); Elliott Brown Ph.D. (Committee Member); Sara Pollock Ph.D. (Committee Member); Michael Saville Ph.D., P.E. (Committee Member) Subjects: Engineering; Mathematics; 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

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

Over the last couple of decades, the field of millimeter-wave (mmW) and terahertz (THz) radiation has seen an accelerated growth thanks to developments in high quality mmW/THz sensors. These developments have contributed to many useful applications in the fields of medicine, security, construction, etc. Non-destructive evaluations such as the detection of defects in materials is currently one of the major applications of mmW/THz radiation. These evaluations are performed using mmW/THz continuous-wave radar systems. In this study, we discuss how Frequency Modulated Continuous Wave (FMCW) techniques can enhance sensing applications through the use of a Michelson interferometer. This allows multiple objects or surfaces to be range resolved. Range resolution is dependent upon the bandwidth of the frequency sweep of the FMCW system and we will discuss several experiments performed that lead to achieving a range resolution of around 1mm.

Committee: Douglas Petkie Ph.D. (Advisor); Jerry Clark Ph.D. (Committee Member); Jason Deibel Ph.D. (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 (PhD), Wright State University, 2015, Engineering PhD

In this age of digital electronics the quest for faster computational devices and high speed communications have driven a need for new materials that are capable of fulfilling these goals. In both areas the need for a thinner channel in transistors, faster carrier transport characteristics, and better magnetic materials dominate the direction of research. Recently 2D materials have been realized. These single layer atomic thick materials show potential in having extremely high carrier transport velocities at room temperature and, due to their natural 2D structure, are the thinnest material possible in nature. On the other hand spin-spray ferrites have showed potential in producing high permeability, low loss materials with a low processing temperature compatible with current CMOS technology. One of the largest hindrances in the implementation of these materials are the lack of measurement capabilities. Both 2D materials and spin-spray ferrites have nm sized features that significantly change how the material behave. To further investigate these materials scanning microwave microscopy (SMM) is being developed as a possible characterization tool. SMM has the unique ability to collect the complex reflection coefficient simultaneously with the topography at nm horizontal spatial resolutions. The complex reflection coefficient is able to supply valuable information about materials such as conductivity and permittivity. This dissertation provides an in depth look at the potential applications for SMM and supplies a rigorous characterization, both experimentally and numerical simulations, of the SMM system. In detail we re- port first time SMM measurements of graphene's conductivity and permittivity along with characterization of graphene defects induced by oxygen plasma etching and graphene wrinkles. We have also experimentally show conductive grain boundaries in spin-spray ferrites leading to larger than expected losses. Lastly we show Fourier transform inferred spectr (open full item for complete abstract)

Committee: Yan Zhuang Ph.D. (Advisor); Marian Kazimierczuk Ph.D. (Committee Member); Douglas Petkie Ph.D. (Committee Member); LaVern Starman Ph.D. (Committee Member); Shin Mou Ph.D. (Committee Member) Subjects: Atoms and Subatomic Particles; Electrical Engineering; Electromagnetics; Engineering; Materials Science; Nanoscience; Nanotechnology; Optics

Doctor of Philosophy, Case Western Reserve University, 2015, Physics

Van der Waals (vdW) materials are layered structures bonded by the weak vdW force. As such, stable single atomic layers can be isolated either by mechanical exfoliation or chemical methods as chemical vapor deposition. Atomically thin vdW materials have emerged as new types of two-dimensional (2D) systems with unique electronic and optical properties that are distinct from that of their bulk counterparts. Studies of this new class of material are not only interesting fundamentally; they can potentially also lead to applications in next-generation electronics and optoelectronics devices. In this thesis, we investigate two prototypes of 2D vdW materials, graphene (a semimetal) and semiconducting transition metal dichalcogenides (TMD) based on optical spectroscopy. Electro-magnetic radiation ranging from the far-infrared (or terahertz (THz)) to the visible has been utilized to investigate two questions: (1) the excitonic effects in Mo/W dichalcogenides; and, (2) the free carrier response in graphene. For the first topic, exciton series in monolayer WSe2 and the effect of electric field on the excitons is studied. A exciton series of WSe2 is observed by a complimentary measurement of linear absorption and two-photon photoluminescense excitation (2PPLE). Strong exciton binding energy ($\sim$ 0.4 eV) and non-Rydberg series are observed arising from 2D screening of Coulomb interactions. Using field-effect transistor structures we apply electrostatic doping and/or perpendicular electric field to WSe2 monolayer through the gates. Trion peak is observed under doping, which further splits under high electric fields. This phenomenon can be explained by Rashba spin-orbit interaction induced spin sub-bands hybridization. For the second topic, the free carrier response in monolayer graphene is investigated using the Fourier transform infrared (FTIR) spectroscopy in steady state conditions and the optical pump-THz probe spectroscopy under non-equilibrium conditions. We ob (open full item for complete abstract)

Committee: Jie Shan (Committee Chair); Kenneth Singer (Committee Member); Jesse Berezovsky (Committee Member); Philip Feng (Committee Member) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 2014, Physics

THz systems are useful in a variety of fields such as astrophysics, analytical chemistry, security and communication. In this dissertation I present a computer simulation which allows evaluation of system performance, as well as the first submillimeter spectra of two molecular species: NdO and UO. The submillimeter spectra were recorded with a new high temperature spectroscopic system capable of temperatures of 2500 K while withstanding chemical and metallurgical attack. The simulation code addresses problems related to noise and nonlinear effects while maintaining a reasonable memory footprint and computational speed. It serves as a tool for system design and evaluation and bridges the information gap between technology developers and spectroscopists. The code is validated against experimental results from a well known ``standard'' spectrometer, as well as against fundamental theoretical results. It is then used to simulate a CMOS based system in development. The submillimeter spectra of NdO is presented and analyzed, building on the previous work in the optical region. The analysis results in improved spectroscopic constants, as well as constants for some never before measured states. In total, 1336 lines were recorded, of these, 643 were identified as belonging to series and 568 have vibronic assignments. The spectra also contain examples of resolved hyperfine structure for isotopes with nuclear spin, as well as examples of Ω doubling including one which resolved within the range of J recorded. Unlike NdO, UO has only one detectable isotope. As a result many fewer lines were recorded, a total of 274. However the analysis provided spectroscopic constants for four electronic states, including some vibrationally excited states which were previously unmeasured. In addition several series were identified that do not currently have assignments. In total 85 of the 274 lines were identified as belonging to series and 45 have vibronic assignments.

Committee: Frank De Lucia (Advisor); Douglass Schumacher (Committee Member); Thomas Lemberger (Committee Member); Ciriyam Jayaprakash (Committee Member) Subjects: Physics

Master of Science in Engineering (MSEgr), Wright State University, 2014, Electrical Engineering

The most common technology for electrical characterization of THz devices is DC-coupled contact probes. In this Masters thesis, a non-contact, antenna-free probe is analyzed for characterizing THz devices and integrated circuits. The probe consists of on-chip receiving or transmitting THz photomixers in a co-planar waveguide environment. Our probes are coupled to a CPW-embedded DUT by polarization current rather than conduction current and then down converted in frequency to baseband by an optically pumped photomixer. We investigated probe performance through numerical simulations using High Frequency Structure Simulator (HFSS) carried up to 1 THz and yielded a broadband design with DUT to photomixer promising coupling efficiency above -20 dB with an operational frequency range of 700 GHz between 0.3 and 1 THz, and the average increase in the coupling is ~ 8dB compared to the previous design. Several integrated-circuit techniques are necessary to achieve this performance, such as symmetric side-coupled CPW (SSC CPW), ¼ wave backshort impedance-matching. These will be addressed along with design trade offs.

Committee: Elliott R. Brown Ph.D. (Advisor); Mike Saville Ph.D. (Committee Member); Yan Zhuang Ph.D. (Committee Member) Subjects: Electrical Engineering; Electromagnetism

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

Measurements that use terahertz frequency radiation to characterize materials are beneficial for scientists trying to determine the physical parameters that govern the interaction of electromagnetic waves and matter at those frequencies. Results will be presented of time domain terahertz spectroscopy measurements taken in forward and backward scattering directions from vertically aligned arrays of multi‐walled carbon nanotubes and thin films of perforated copper. The intent of this research is to both corroborate results from independent research groups conducting similar experiments and to further increase understanding in the scientific community with respect to carbon nanotube reflection phenomena at terahertz frequencies.

Committee: Jason Deibel PhD (Advisor); Jason Deibel PhD (Committee Chair); Benjamin Maruyama PhD (Committee Member); Gregory Kozlowski PhD (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

Master of Science (MS), Wright State University, 2010, Physics

This thesis is a study of the propagation of millimeter wavelength (MMW) and submillimeter wavelength (sub-MMW) electromagnetic radiation (a.k.a. THz radiation) through the Earth's atmosphere. THz radiation is electromagnetic radiation that exists between the microwave and far infrared regions of the electromagnetic spectrum. It is nonionizing radiation but can penetrate through materials that are opaque to visible light so therefore has many new and useful applications. Unfortunately, THz radiation is heavily attenuated by the Earth's atmosphere as it propagates through it. This therefore represents a challenge to communications and sensing applications at millimeter and sub-millimeter wavelengths. In this work, the general theory of how the atmosphere attenuates propagating THz radiation by absorption and scattering is discussed. From this discussion, we find that water vapor is the constituent of the Earth's atmosphere most responsible for the absorption of THz radiation. The absorption of THz radiation by water vapor was measured at 325 and 620 GHz using steady state or frequency domain absorption spectroscopy. The absorption lines of these frequencies lie in the far wings of a very strong water absorption line at 557 GHz. Spectral line shapes were recorded across a range of pressures and fitted to Voigt profiles. The resulting relationship between the line width and pressure was shown to be linear and very close to published values. Finally, transient signals associated with population and polarization relaxation times were measured at 325 GHz using transient or time domain spectroscopy techniques. Experimental results associated with the steady state and transient measurements will be presented and discussed.

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