Now showing items 19396-19415 of 20274

    • The U.S. foreign policy in the Persian Gulf, 1968-1988: From regional surrogate to direct military involvement.

      Bahramzadeh, Mohammad Ali.; Whiting, Allen S.; Wahlke, John C.; Muller, Edward N. (The University of Arizona., 1993)
      This study examines the U.S. foreign policy toward the Persian Gulf from 1968 to 1988 with an attempt to explain why and how particular U.S. foreign policy decisions were made. It further attempts to determine whether each president, within this time frame, pursued a different foreign policy toward the region. The indicators used to longitudinally measure foreign policy change were trade, both imports and exports between the U.S. and the Persian Gulf countries, bilateral treaties between them, and U.S. military sales to them. By examining the effect of presidential succession on selected patterns of American foreign policy behavior toward the area it is apparent that the pattern of interaction exhibits a clear continuity and in fact different administrations have not drastically altered the fundamental thrust of U.S. foreign policy. Furthermore, from a broad historical perspective, this study challenges the conventional notion that U.S. foreign policy has been "short-sighted" and often erratic. By examining two case studies, namely the Iran-Iraq war and U.S. decision to reflag Kuwaiti oil tankers, one can readily see that U.S. foreign policy is far from being reactive in its approach. In general, the evident suggests that the U.S. foreign policy in the Persian Gulf, in a broad conceptual framework, can be explained as a part of the rational decision making process where the U.S. foreign policy makers select the alternatives best suited to maximize the strategic goals and objectives.
    • U.S. Transnational Higher Education in Cambodia: Soft Power and Orientalism in Context

      Lee, Jenny J.; Vance, Hillary; Rhoades, Gary; Koyama, Jill (The University of Arizona., 2021)
      In the expanding landscape of transnational higher education (TNHE), cases have begun to emerge that serve as possible bellwethers for future challenges. These challenges include geopolitical conflicts and concerns over foreign influence that threaten the viability of continued TNHE delivery. To explore this issue in depth, this study focused on a particular case, that of a U.S. TNHE program in Cambodia, at a time of increased tensions between home and host country.This study explored the perceptions, values, and attitudes of local students enrolled in a U.S. TNHE program in Cambodia and how their experiences and perceptions connected with the complex political relationship between the United States and Cambodia. 32 students, 4 staff members, and 5 faculty members involved in the program were interviewed to learn about their perceptions of the program and what values and attitudes they held related to both the program, its context, and their participation in it. Findings indicate that the soft power of the United States, in particular that of its higher education system, remains strong in spite of negative perceptions of U.S. politics and foreign policy, and despite the contentious nature of U.S./Cambodian relations. Additionally, the study found the presence of Orientalist tropes that were internalized by students when expressing their preference for U.S. higher education over Cambodian higher education, which have been reinforced by marketing and media, as well as by the opinions of family, friends, faculty, staff members, and acquaintances. By integrating these findings, this study illuminates how the soft power attraction of the United States helps TNHE programs exhibit resilience even in times of political crisis between sending and receiving countries. However, the long-term viability of such programs, and their reliance on U.S. soft power attraction, is questionable and subject to the changing nature of the political context. Additionally, with the increase in the number of TNHE providers and programs, practitioners should be cognizant of the messaging used to promote programs to students and their families. In this way, they can avoid reinforcement of negative perceptions of the host country to ensure that long term involvement in the local higher education landscape is rooted in respectful partnership and not by banking on the enduring attraction of U.S. soft power.
    • UA62784; a Putative Inhibitor of CENP-E Kinesin-like Protein and its Effects on Human Pancreatic Cancer Cells

      Dorr, Robert T.; Henderson, Meredith C.; Dorr, Robert T.; Bowden, G. Timothy; Gerner, Eugene W.; Cress, Anne E.; Martinez, Jesse D. (The University of Arizona., 2008)
      UA62784 is a novel fluorenone identified in a biologic screen of compounds that are selectively cytotoxic in DPC4 (deleted in pancreatic cancer)-deleted pancreatic cancer cells. We sought to determine the mechanism of action of UA62784, and discovered it to be a potent mitotic inhibitor. UA62784 affects the ATPase activity of the mitotic kinesin centromere protein E (CENP-E), but does not affect other known mitotic kinesins. This inhibition of ATPase activity is not caused by an inhibition of microtubule binding nor is it caused by a failure of the kinesin to translocate to the nucleus during mitosis. Despite the anti-cancer properties of this drug, UA62784 is relatively insoluble and is not suitable as a lead compound for further development.Once we determined the mechanism of action of UA62784, we sought to determine if analogs would demonstrate the same potent mitotic inhibition while also offering properties such as increased solubility. A small library of chemical analogs was generated wherein each compound was a slight variation of UA62784 (termed the DPC series). Several potential leads were identified which exhibited increased solubility and/or increased cytotoxic activity. When tested for CENP-E ATPase inhibition, some compounds were noted to inhibit other kinesins as well. We therefore created a screen where each of the DPC compounds was tested for activity in Eg5, CENP-E, MKLP-1, MCAK, and KIF3C kinesins. Within these data, there is a correlation between cellular IC50 and kinesin ATPase inhibition for CENP-E and MKLP-1. A few compounds emerged from these studies, including DPC046, which has a low cellular IC50 and inhibits all five kinesins to some degree. DPC046 was used in a mouse xenograft study to determine in vivo efficacy, but no significant tumor shrinkage was seen, likely due to solubility limitations affecting the amount of bioavailable compound.From these studies we conclude that the cytotoxic effects seen in UA62784 and its analogs are due, at least in part, to their inhibition of kinesin proteins. We demonstrate that compounds that inhibit CENP-E and other kinesin proteins hold promise in cytotoxically targeting pancreatic cancer cells. Further development is needed to optimize DPC046 compound solubility in order to increase in vivo efficacy.
    • Ultra narrow band fiber optic Bragg grating filters for atmospheric water vapor measurements

      Herman, Benjamin M.; Vann, Lelia Belle (The University of Arizona., 2003)
      Optical fibers have revolutionized telecommunications. Much of the success of optical fiber lies in its near-ideal properties: low transmission loss, high optical damage threshold, and low optical nonlinearity. The photosensitivity of an optical fiber was accidentally discovered by Hill, et al. in 1978. However, the technological advances made in the field of photosensitive optical fibers are relatively recent. This fascinating technology of photosensitive fiber is based on the principle of a simple in-line all-fiber optical filter. It has been shown that the transmission spectrum of a fiber Bragg grating can be tailored by incorporating multiple phase-shift regions during the fabrication process. Phase shifts open up ultra narrowband transmission windows inside the stop band of the Bragg grating. As a specific application, this research is focused on applying this technology in future space-based water vapor DIfferential Absorption LIDAR (DIAL) systems to improve the performance of space-based LIDAR systems by rejecting the reflected solar background. The primary goal of this research effort was to demonstrate the feasibility of using ultra narrow band fiber optic Bragg grating filters for atmospheric water vapor measurements. Fiber Bragg gratings were fabricated such that two transmission filter peaks occurred and were tunable, one peak at a 946 nm water vapor absorption line and another peak at a region of no absorption. Both transmission peaks were in the middle of a 2.66-nm stop band. Experimental demonstration of both pressure and temperature tuning was achieved and characterization of the performance of several custom-made optical fiber Bragg grating filters was made. To our knowledge these are the first optical fiber gratings made in this frequency range and for this application. The bandwidth and efficiency of these filters were measured and then these measurements were compared with theoretical calculations using a piecewise matrix form of the coupled-mode equation. Finally, an ultra narrow band water vapor DIAL filter was characterized having two pass bands less than 8 pm and peak transmissions greater than 80 percent. Such fiber optic filters are now ready for integrating into space-based water vapor LIDAR systems. More broadly, these filters have the characteristics that will revolutionized satellite remote-sensing.
    • Ultra-Precision Non-Contact Metrology for Optical Shop and Alignment Applications

      Liang, Rongguang; Khreishi, Manal A.; Schwiegerling, James; Kupinski, Matthew (The University of Arizona., 2019)
      New, unconventional designs for modern optical systems strive for superior performance at lower cost and volume. These compact optical instruments, with larger fields of view and faster f-numbers, call for more challenging prescriptions to meet their ambitious requirements. Freeform optical surfaces have the potential to enable these compact, high-performance systems, but are more challenging to characterize and hence to fabricate. This work enables the use of precision coordinate measuring machine (CMM) for a wide range of optical testing and alignment applications, including the testing and integration of freeform and aspheric optics. In this dissertation, the capabilities of this non-contact, precision metrology instrument have been stretched beyond its everyday applications. Advanced data collection and reduction techniques have been developed to determine the as-built prescription of optical surfaces, as well as the alignment with respect to the part and global coordinate systems. For measurement of low-order surface error and prescription parameters, when compared to interferometry and other optical shop techniques that rely on optical path difference or slope error, these techniques have greater dynamic range and, in some cases, precision that approaches the sensitivity of traditional methods. For example, the CMM, when equipped with a non-contact probe that utilizes a chromatic confocal principle, has a dynamic range measured in mm, a sensitivity measured in nm, yet does not require a null corrector or other custom-made components. In one case, the CMM was used to measure the surface error and as-built prescriptions of two freeform mirrors for a reflective telescope. The CMM was then utilized to align the telescope, as confirmed by an end-to-end interferometric test used to evaluate the system performance. In another example, the CMM was paired with a laser radar (LR) to align a different telescope that uses off-axis, polynomial aspheres in a three mirror anastigmat (TMA) configuration. The CMM enables fast determination of as-built prescription parameters and surface error without the types of systematic errors that are inherent in some traditional optical test setups, like the removal of power or other aberration from alignment of the test setup. The applicability of this work was further studied for large, convex aspheres using the spare secondary mirror for the Hubble Space Telescope (HST) and, via simulation, for large meter-class optics such as the Giant Magellan Telescope (GMT) secondary mirror segments. This measurement technique was further extended to other types of optical surfaces, like gratings and spatial filters. In the case of gratings, it is shown that one can derive as-built period, amplitude, and grating vector quickly and accurately from a few, fast scans.
    • Ultra-Sensitive and Selective Whispering Gallery Mode Microtoroid Chemical Sensor

      Su, Judith; Li, Cheng; McLeod, Euan; Kolesik, Miro (The University of Arizona., 2022)
      Optical Whispering gallery mode (WGM) microresonators, which benefit from an ultra-high quality (Q) factor and small mode volume to significantly enhance light-matter interaction, stand out from other sensors, and are utilized in a variety of biochemical sensing or physical parameter detection applications. Physical or chemical reactions occurring in the evanescent field of the polymer-treated microtoroid equatorial plane will be translated into variations of the WGM spectra, which will, in turn, be recorded and analyzed through techniques such as frequency locking, balanced detection, and post data processing. The overall platform is known as the Frequency-locked optical whispering evanescent resonator (FLOWER) system. The performance and characteristics of ultra-sensitive and selective WGM gas sensors are evaluated and demonstrated in this dissertation. Two approaches to further improve the system are proposed, one based on plasmonic near-field enhancement to improve the sensitivity and the other on a fiber metrology method using Rayleigh backscattering to eliminate the thermal noise of the sensing system. Finally, another sensing application using the dual-FLOWER system for particle shape analysis is introduced.
    • Ultrafast carrier and gain dynamics in strongly confined semiconductor quantum dots.

      Giessen, Harald Willi.; Peyghambarian, Nasser; Kippelen, Bernard; Binder, Rudolf (The University of Arizona., 1995)
      This thesis investigates the carrier and gain dynamics of semiconductor quantum dots in the strong quantum confinement regime (i.e. the dot radius is smaller than the bulk excitonic Bohr radius). The materials under investigation are InP and CdSe. We can summarize our findings as follows: For the first time, the quantum confined ground state in InP quantum dots has been observed at room temperature by femtosecond spectral holeburning. This is the first confirmation of the observation of a strongly confined quantum dot made of III-V semiconductor materials. In CdSe quantum dots with a radius of half the bulk exciton Bohr radius, the carrier and gain dynamics have been investigated. The predicted phonon bottleneck, which should slow down carrier relaxation up to nanoseconds, has not been found. The carrier relaxation rates are rather on the order of 1 eV/ps. Gain has been found for the first time in strongly confined quantum dots. The existence of gain was proven by spectral holeburning in the gain region. The gain buildup and decay dynamics have been studied on a femtosecond and picosecond timescale. A multi-level model including biexcitons accounts for the gain formation. The model has been confirmed by three-beam spectral holeburning experiments and femtosecond pump-probe experiments with circularly polarized light. Some quantum dots did not show gain under high optical excitation but instead exhibited photodarkening. The carrier separation and localization dynamics of this photodarkening process has been studied on a femtosecond timescale. For the first time, the shift of the bleaching towards lower energy during the localization process could be observed on a femtosecond timescale. Finally, pulse propagation in bulk CdSe at multiple Pi-pulses has been studied. For the first time, strong evidence for the observation of self induced transparency in semiconductors has been found. Also, optical precursors, probably of nonlinear nature, have been found.
    • Ultrafast carrier dynamics and enhanced electroabsorption in (gallium,indium)arsenide/(aluminum,indium)arsenide asymmetric double quantum well structures.

      Krol, Mark Francis.; Peyghambarian, Nasser; Binder, Rudolf; Kippelen, Bernard (The University of Arizona., 1995)
      An experimental study, utilizing a novel nondegenerate transmission pump/probe technique, of ultrafast electron and hole tunneling in (Ga,In)As/Al,In)As asymmetric double quantum wells (ADQWs) is presented. A single time constant is observed at low carrier densities indicating the holes tunnel from the narrow well (NW) to the wide well (WW) at least as fast as electrons. At high carrier densities a two component decay is observed, consistent with phase-space filling and space-charge effects blocking tunneling carriers. The fast transfer of electrons was confirmed to be a LO-phonon assisted process. A detailed theoretical study of ultrafast hole tunneling at low carrier densities indicates that in ternary materials alloy disorder is responsible for fast hole transfer between the wells. Enhanced electroabsorption in selectively doped (Ga,In)As/(Al,In)As ADQWs by the use of real space electron transfer is demonstrated. The electron concentration in both the WW and NW is investigated by field-dependent absorption and photoluminescence spectroscopy. The results are compared to absorption changes in an undoped ADQW structure which utilizes the quantum confined Stark effect. The doped modulator exhibits a significantly larger red-shift with applied field than the undoped structure.
    • Ultrafast Dynamics of Two Dimensional Materials

      Sandhu, Arvinder S.; Golla, Dheeraj; Sandhu, Arvinder S.; LeRoy, Brian J.; Schaibley, John R.; Wang, Weigang (The University of Arizona., 2017)
      Two dimensional (2D) materials are poised to revolutionize the future of optics and electronics. The past decade saw intense research centered around graphene. More recently, the tide has shifted to a bigger class of two-dimensional materials including graphene but more expansive in their capabilities. The so called ‘2D material zoo’ includes metals, semi-metals, semiconductors, superconductors and insulators. The possibility of mixing and matching 2D materials to fabricate heterostructures with desirable properties is very exciting. To make devices with superior electronic, optical and thermal properties, we need to understand how the electrons, phonons and other quasi particles interact with each other and exchange energy in the femtosecond and nanosecond timescales. To measure the timescales of energy distribution and dissipation, I used ultrafast pump-probe spectroscopy to perform time-domain measurements of optical absorption. This approach allows us to understand the impact of manybody interactions on the bandstructure and carrier dynamics of 2D materials. After a brief introduction to femtosecond laser spectroscopy, I will explore the transient absorption dynamics of three classes of 2D materials: intrinsic graphene, graphene-hBN heterostructures and Transition Metal Dichalcogenides (TMDs). We will see that using pumpprobe measurements around the high energy M-point of intrinsicgraphene, we can extract the value of the acoustic deformation potential which is vital in characterizing the electron-acoustic phonon interactions. In the next part of the thesis, I will delineate the role of the substrate in the cooling dynamics in graphene devices. We will see that excited carriers in graphene on hBN substrates cool much faster that on SiO2 substrates due to faster decay of the optical phonons in graphenehBN heterostructures. These results show that graphene-hBN heterostructures can solve the hot phonon bottleneck that plagues graphene devices at high power densities. In the last part, I will demonstrate the role of phonon induced bandgap renormalization in the carrier dynamics of TMD materials and measure the timescale of phonon decay through the generation of low-energy phonons and transfer to the substrate. This study will help us understand carrier recombination in TMD devices under high-bias conditions which show great potential in opto-electronic applications such as photovoltaics, LEDs etc.
    • Ultrafast optical nonlinearities in aluminum phthalocyanine organic thin films and a picosecond all-optical organic etalon switch.

      Peyghambarian, Nasser; Williams, Valorie Sharron, 1960-; Sarid, Dror; Armstrong, Neal R.; Mazumdar, Sumit; Lindberg, Markus (The University of Arizona., 1991)
      The history of femtosecond laser pulse generation is summarized and a current state-of-the-art femtosecond laser system described. The femtosecond pulses are used to observe coherent coupling effects in a fluoro-aluminum phthalocyanine thin film. The polarization dependency of the coherent coupling indicates that orthogonal polarization states in the phthalocyanine ring are effectively uncoupled. The coherent coupling effect evolves into a nonequilibrium exciton population spectrally coincident with the pump pulse. This population rapidly decays to the bottom of the π- π* absorption band. These singlet excitons exhibit rapid bimolecular decay characteristics. In addition, some singlet excitons relax into the triplet manifold by intersystem crossing. Excited-state triplet-triplet absorption is then observed. The triplet excitons relax to the ground state, apparently via nonradiative decay mechanisms. Femtosecond techniques are also employed to demonstrate a picosecond all-optical organic NOR gate. A dye-doped polymer is used as the nonlinear material inside a Fabry-Perot etalon.
    • Ultrafast phenomena in gallium arsenide/aluminum gallium arsenide multiple quantum well waveguide structures using a near infrared femtosecond laser system.

      Harten, Paul Alexander.; Peyghambarian, Nasser; Koch, Stephan W.; Burke, James J. (The University of Arizona., 1992)
      A near infrared hybridly mode-locked dye laser system consisting of a femtosecond oscillator and a high repetition rate dye amplifier was designed and built. This system was then applied to the study of room temperature below-bandgap femtosecond switching and coherent pulse propagation in GaAs/GaAlAs multiple quantum well waveguides. The noise properties of the oscillator output were studied using radio frequency spectrum analysis techniques. Two distinct modes of operation were identified: The first is characterized by the shortest pulse duration and its real-time autocorrelation signal appears more strongly modulated. The second mode of operation, which exhibits a slightly longer pulse duration and a smoother real-time autocorrelation signal, is obtained for a relative cavity length detuning of ΔL = -0.7 μm. Unexpectedly, the second mode features larger pulse duration fluctuations than the first mode and self-pulsing, while the pulse repetition timing and pulse energy fluctuations were found to be similar in both cases, making the first mode preferable for use in time-resolved experiments. Femtosecond all-optical switching under off-resonance room temperature excitation was demonstrated in a passive GaAs/AlGaAs multiple quantum well directional coupler for the first time. The required phase mismatch originates from an ultrafast refractive index change caused by the optical Stark effect. The main obstacle regarding practical device applications is its low transmission (less than 10%). The use of electrically pumped semiconductor waveguides that provide gain promises to remove this disadvantage. Below-resonance, coherent pulse breakup in a room temperature semiconductor waveguide was observed for the first time. Numerical simulations of the coupled semiconductor Maxwell-Bloch equations show that the light-matter interaction can induce enough chirp through self-phase modulation during propagation in order to violate the initial adiabatic following regime and cause pulse breakup. This coherent effect is distinctly different from self-induced transparency, because it does not involve Rabi-oscillations at the start of propagation, from temporal solitons, because it does not require group velocity dispersion, and from self-steepening. However, it should be ubiquitous under off-resonance pulse propagation with a pulse duration less than the polarization dephasing time.
    • Ultrafast Photocarrier Relaxation Mechanisms in Sputter-Deposited CdTe Quantum Dot Thin Films

      Simmons, Joseph H.; Juncker, Christophe Rene Henri; Simmons, Joseph H.; Simmons, Joseph H.; Potter, Barrett G.; Lucas, Pierre; Peyghambarian, Nasser; Armstrong, Neal R. (The University of Arizona., 2007)
      Photocarrier relaxation mechanisms in CdTe quantum dots in the strong confinement regime were investigated using femtosecond pump-probe measurements. The quantum dots were formed in films deposited on silica substrates using a sequential RF magnetron sputtering process with heat treatment to grow crystallites of various sizes. Size selection was achieved by tuning the laser to various wavelengths across the first excitation transition. The recombination mechanism showed a biexponential decay, which was fitted to a three-level model. It was shown that recombination occurs increasingly through the intermediate energy level as the size of the dots decreases. The nature of the intermediate level and the role of Auger recombination is discussed.
    • Ultrafast XUV Spectroscopy: Unveiling the Nature of Electronic Couplings in Molecular Dynamics

      Sandhu, Arvinder; Timmers, Henry Robert; Sandhu, Arvinder; Cronin, Alex; Huxter, Vanessa; Wang, Weigang (The University of Arizona., 2014)
      Molecules are traditionally treated quantum mechanically using the Born-Oppenheimer formalism. In this formalism, different electronic states of the molecule are treated independently. However, most photo-initiated phenomena occurring in nature are driven by the couplings between different electronic states in both isolated molecules and molecular aggregates, and therefore occur beyond the Born-Oppenheimer formalism. These couplings are relevant in reactions relating to the perception of vision in the human eye, the oxidative damage and repair of DNA, the harvesting of light in photosynthesis, and the transfer of charge across large chains of molecules. While these reaction dynamics have traditionally been studied with visible and ultraviolet spectroscopy, attosecond XUV pulses formed through the process of high harmonic generation form a perfect tool for probing coupled electronic dynamics in molecules. In this thesis, I will present our work in using ultrafast, XUV spectroscopy to study these dynamics in molecules of increasing complexity. We begin by probing the relaxation dynamics of superexcited states in diatomic O₂. These states can relax via two types of electronic couplings, either through autoionization or neutral dissociation. We find that our pump-probe scheme can disentangle the two relaxation mechanisms and independently measure their contributing lifetimes. Next, we present our work in observing a coherent electron hole wavepacket initiated by the ionization of polyatomic CO₂ near a conical intersection. The electron-nuclear couplings near the conical intersection drive the electron hole between different orbital configurations. We find that we can not only measure the lifetime of quantum coherence in the electron hole wavepacket, but also control its evolution with a strong, infrared probing field. Finally, we propose an experiment to observe the migration of an electron hole across iodobenzene on the few-femtosecond timescale. We present experimental modifications made to the high harmonic generation set-up in order to probe this ultrafast and elusive charge migration. These results demonstrate the potential of ultrafast, XUV spectroscopy in probing the inner-workings of electronic couplings occurring in nature.
    • Ultrahigh-Resolution Endoscopic Optical Coherence Tomography for In Vivo Mouse Colonoscopy

      Barton, Jennifer K; Tumlinson, Alexandre Rex; Barton, Jennifer K; Drexler, Wolfgang; Gerner, Eugene; Utzinger, Urs (The University of Arizona., 2007)
      In vivo monitoring of mouse models of colon cancer promises to reduce the cost of research by improving sacrifice timing and allowing serial studies that observe the progression of disease and drug efficacy in a relatively small set of animals. Optical coherence tomography (OCT) is an optical analog of ultrasound imaging, capable of minimally-invasive mapping of light scatter intensity up to 2 mm deep in tissue. In this work, factors limiting resolution in OCT were examined and devices were created and applied to mouse colon imaging that extended the state-of-the-art in endoscopic ultrahigh-resolution OCT. First, axial chromatic aberration of the objective optics acts as a spectral filter in the sample arm limiting the effective bandwidth of the system. An achromatized endoscope design was demonstrated that achieved axial resolution of 2.3 mum in tissue and 4.4 mum lateral spot diameter with 101 dB sensitivity when interfaced with a time domain OCT system utilizing a 10-femtosecond laser (bandwidth=150 nm FWHM, center wavelength=800 nm). Second, dispersion matching between the sample and reference arms presents the practical resolution limit to endoscopic implementations including a separate, fiber-based reference arm. A second endoscope incorporated the reference arm into the tip of the endoscope using a novel custom beamsplitter prism and achieved 2.4 mum axial resolution in tissue without adjustments for pathlength or dispersion matching when interfaced with a spectrometer-based frequency domain OCT system and a similar laser. Third, non-linear dispersion of the sample media with respect to wavelength causes distortion and broadening of the axial point spread function when data are sampled uniformly in optical frequency. An experiment was performed on high dispersion glass to demonstrate that dispersion artifact free imaging can be achieved without post process corrections if the samples are acquired at equal intervals of media index of refraction divided by vacuum wavelength. Finally, other microscopic modalities that depend on tissue scatter intensity are used to find the origins of scatter in the mouse colonic mucosa. These observations are used to explain unexpected features found in ultrahigh-resolution tomograms collected with the two endoscopes presented.
    • Ultrasensitive spectroelectrochemistry of monolayer and submonolayer thin films using an electroactive integrated optical waveguide

      Armstrong, Neal R.; Dunphy, Darren Robert (The University of Arizona., 1999)
      To increase the applicability of spectroelectrochemistry to ultrathin films at a transparent semiconductor electrode, a single-mode, step-index electroactive integrated optical waveguide (the EA-IOW) incorporating an indium tin oxide top layer as an electrode was developed. The EA-IOW is much more sensitive to absorbance by molecular adlayer species than previous electroactive waveguide designs; a sensitivity increase of ca. 4000 relative to a single-pass transmission experiment has been measured by monitoring the reduction of a surface-adsorbed dye molecule. An important characteristic of the present three-layer EA-IOW structure is that its design is close to being optimized in terms of maximizing sensitivity while maintaining acceptable optical losses, as determined by theoretical modeling. Before the EA-IOW can be applied to measure absorbance changes arising from electron transfer in ultrathin films, the background optical changes that occur as a function of potential must be understood. There is a linear decrease in outcoupled intensity as the EA-IOW is scanned negative which is a result of an increase in the number of free carriers inside the ITO, a highly reproducible effect. There is also a poorly reproducible non-linear component to the optical background, accompanied by a hysteresis between the forward and reverse potential scans, that disappears after conditioning the EA-IOW in electrolyte solution for a period of several days. It is hypothesized that his effect is due to hydroxylation of the ITO network. To test the EA-IOW experimentally, the reduction of surface-adsorption methylene blue was monitored, along with the formation of Prussian blue during the electrochemistry of ferricyanide. Two experimental applications of the EA-IOW will be reviewed; first, the EA-IOW was used to measure the spectroelectrochemistry of submonolayer films of phthalocyanine polymeric assemblies to compare the electrochemistry at submonolayer and multilayer coverages. Finally, the use of the EA-IOW in protein electrochemistry will be discussed. The dichroic ratio of cytochrome c adsorbed to indium tin oxide was measured as a function of potential, and found to be consistent with an orientation of the heme ligand that is almost parallel to the electrode surface. Also, a change in heme orientation was detected during reduction of the protein.
    • Ultrasonic Field Modeling in Non-Planar and Inhomogeneous Structures Using Distributed Point Source Method

      Kundu, Tribikram; Das, Samik; Frantziskonis, George; Kemeny, John; Missoum, Samy; Frantziskonis, George; Kemeny, John; Missoum, Samy (The University of Arizona., 2008)
      Ultrasonic wave field is modeled inside non-planar and inhomogeneous structures using a newly developed mesh-free semi-analytical technique called Distributed Point Source Method (DPSM). Wave field inside a corrugated plate which is a non-planar structure is modeled using DPSM when the structure is excited by a bounded acoustic beam generated by a finite-size transducer. The ultrasonic field is computed both inside the plate and in the surrounding fluid medium. It is observed that the reflected beam strength is weaker for the corrugated plate in comparison to that of the flat plate, as expected. Whereas the backward scattering is found to be stronger for the corrugated plate. DPSM generated results in the surrounding fluid medium are compared with the experimental results.Ultrasonic wave field is also modeled inside inhomogeneous structures. Two types of inhomogeneity are considered - a circular hole and a damaged layered half-space. Elastic wave scattering inside a half-space containing a circular hole is first modeled using DPSM when the structure is excited with a bounded acoustic beam. Then the ultrasonic wave field is computed in presence and absence of a defect in a layered half-space. For the layered problem geometry it is shown how the layer material influences the amount of energy that propagates through the layer and that penetrates into the solid half-space when the solid structure is struck by a bounded acoustic beam. It is also shown how the presence of a crack and the material properties of the layer material affect the ultrasonic fields inside the solid and fluid media.After solving the above problems in the frequency domain the DPSM technique is extended to produce the time domain results by the Fast Fourier Transform technique. Time histories are obtained for a bounded beam striking an elastic half-space. Numerical results are generated for normal and inclined incidences, for defect-free and cracked half-spaces. A number of useful information that is hidden in the steady state response can be obtained from the transient results.
    • Ultrasonic Non-Destructive Evaluation: Impact Point Prediction and Simulation of Ultrasonic Fields

      Kundu, Tribikram; Hajzargarbashi, Talieh; Frantziskonis, George; Zhang, Lianyang; Kundu, Tribikram (The University of Arizona., 2011)
      This work has two parts. The first part of the work (in Chapters II, III, IV and V) presents a method for locating the point of impact using acoustic emission techniques.The second part of the work is modeling the ultrasonic fields generated by one and two spherical cavities placed in front of a point focused acoustic lens using the semi-analytical distributed point source method (DPSM).Acoustic emission (AE) refers to the generation of transient elastic waves during the rapid release of energy from localized sources within a material.In this work the acoustic emission has been used for locating the point of impact on anisotropic and homogeneous or non-homogenous flat plates and cylindrical structures. In these cases the wave speed is a function of the angle of propagation. An optimization function is introduced and minimized to get the location of the impact point.This method has been used on a flat (fiber reinforced polymer) plate. The proposed new objective function reduces the amount of time needed for solving the problem and improves the accuracy of prediction. The method is extended to cylindrical structures for which the objective function is written in cylindrical coordinates and the method is tested on a FRP shell.In Chapter IV an alternative method is introduced called the near-field acoustic emission (AE) beamforming method. It has been used to estimate the source locations by using a small array of sensors closely placed in a local region. To validate the effectiveness of the AE beamforming method a series of experiments on a FRP shell are conducted. The experimental results demonstrate that the proposed method can correctly predict the point of impact.The semi-analytical mesh-free technique DPSM is then used to model the ultrasonic field in front of a point focused acoustic lens; anomalies such as cavities are introduced in the medium in front of the acoustic lens and the effect of those cavities are studied. Solution of this problem is necessary to get an idea about when two cavities placed in close proximity can be distinguished by an acoustic lens and when it is not possible.

      KUNDU, TRIBKRAM; Lee, Joon Pyo; KUNDU, TRIBKRAM; Haldar, Achintya; Cho, Myung Kyu (The University of Arizona., 2005)
      The determination of material properties for very small specimens such as biological cells or semiconductor microchips is extremely difficult and has been a challenging issue for several decades. One important constraint during these measurements is not to harm the specimens during the test process because the specimens, biological cells in particular, are vulnerable to the test itself even during a short period of testing time.Nondestructive evaluation (NDE) is the only suitable precess for such applications. It is fast, causes no disturbance and can give a real time response while being cost effective. Many NDE methods are available today, such as, laser based techniques, Radiography, Magnetic techniques, High resolution photography and other optical techniques, MRI, acoustic and ultrasonic techniques to name a few. Ultrasound is the most popular tool for NDE. As specimens become smaller, the need for shorter wave length ultrasound increases dramatically.The use of acoustic waves in microscopy technology provides many more benefits than its conventional optical microscope counterpart. One such benefit is its ability to inspect a specimen in dark. Another is the capability to see inside an optically opaque specimen. Today, very high frequency, higher than 1 Giga Hertz (109 Hz), ultrasound is being used. This technology has improved at the same pace as the development of electronics and computer science. In acoustic microscopy experiments wave speed and wave attenuation in the specimen are measured by the V(f) technique. A specimen's density, Poisson's ratio and Young's modulus are directly related to the wave speed. V(f) method, as discussed in this dissertation, has some advantages over the more commonly used V(z) method. In order to correctly estimate the wave speed and attenuation in the specimen, the transducer modeling should be completed first. The Distributed Point Source Method (DPSM) is used in this dissertation to model a 1 GHz acoustic microscope lens. Then the model-predicted pressure field is used in a FORTRAN program to calculate the thickness profile and properties of biological cell specimens from experimental data.Transducer modeling at 1 GHz has rarely been attempted earlier because it requires an immense amount of computer time and memory. In this dissertation 1 GHz transducer modeling is conducted by taking advantage of the axisymmetric geometry of the acoustic microscope lens. This exploitation of symmetry in the modeling process has not been attempted prior to this dissertation.
    • Ultrasonic Wave Propagation on an Inclined Solid Half-Space Partially Immersed in a Liquid

      Kundu, Tribikram; Dao, Cac Minh; Kundu, Tribikram; Contractor, Dinshaw; Frantziskonis, George; Desai, Chandrakant; Haldar, Achintya (The University of Arizona., 2007)
      The interaction between a bounded ultrasonic beam and a liquid wedge over a solid half-space is studied theoretically as well as experimentally. A semi-analytical technique called Distributed Point Source Method (DPSM) is adopted for modeling the ultrasonic field in a wedge-shaped fluid structure on a solid half-space. This study is important for analyzing and understanding the propagation of ultrasonic waves used for underwater communications and inspections. A better understanding of the elastic wave propagation in water and in submerged marine strata near the seashore requires extensive investigations of such problem geometries. The semi-analytical technique used in this dissertation considers a bounded acoustic beam striking a fluid-solid interface between a fluid wedge and a solid half-space. Solution of this problem is beyond the scope of the currently available analytical methods when the beam is bounded. However, it is important to model the bounded beams because, in all underwater communications and inspections, bounded beams are used. Currently, only numerical method [Boundary Element Method (BEM) or Finite Element Method (FEM)] based packages (e.g., PZFlex) are in principle capable of modeling ultrasonic fields in such structures. However, these packages are not very accurate and are very CPU-intensive for high-frequency ultrasonic problems. At high frequencies, FEM- and BEM-based packages require huge amount of computation memory and time for their executions that the DPSM technique can avoid. The effect of the angle variation between the fluid-solid interface and the fluid wedge on the wave propagation characteristics is studied and presented.
    • Ultrasound Current Source Density Imaging in Live Rabbit Hearts Using Clinical Intracardiac Catheter

      Witte, Russell S.; Li, Qian; Witte, Russell S.; Xin, Hao; Furenlid, Lars (The University of Arizona., 2015)
      Ultrasound Current Source Density Imaging (UCSDI) is a noninvasive modality for mapping electrical activities in the body (brain and heart) in 4-dimensions (space + time). Conventional cardiac mapping technologies for guiding the radiofrequency ablation procedure for treatment of cardiac arrhythmias have certain limitations. UCSDI can potentially overcome these limitations and enhance the electrophysiology mapping of the heart. UCSDI exploits the acoustoelectric (AE) effect, an interaction between ultrasound pressure and electrical resistivity. When an ultrasound beam intersects a current path in a material, the local resistivity of the material is modulated by the ultrasonic pressure, and a change in voltage signal can be detected based on Ohm's Law. The degree of modulation is determined by the AE interaction constant K. K is a fundamental property of any type of material, and directly affects the amplitude of the AE signal detected in UCSDI. UCSDI requires detecting a small AE signal associated with electrocardiogram. So sensitivity becomes a major challenge for transferring UCSDI to the clinic. This dissertation will determine the limits of sensitivity and resolution for UCSDI, balancing the tradeoff between them by finding the optimal parameters for electrical cardiac mapping, and finally test the optimized system in a realistic setting. This work begins by describing a technique for measuring K, the AE interaction constant, in ionic solution and biological tissue, and reporting the value of K in excised rabbit cardiac tissue for the first time. K was found to be strongly dependent on concentration for the divalent salt CuSO₄, but not for the monovalent salt NaCl, consistent with their different chemical properties. In the rabbit heart tissue, K was determined to be 0.041 ± 0.012 %/MPa, similar to the measurement of K in physiologic saline: 0.034 ± 0.003 %/MPa. Next, this dissertation investigates the sensitivity limit of UCSDI by quantifying the relation between the recording electrode distance and the measured AE signal amplitude in gel phantoms and excised porcine heart tissue using a clinical intracardiac catheter. Sensitivity of UCSDI with catheter was 4.7 μV/mA (R² = 0.999) in cylindrical gel (0.9% NaCl), and 3.2 μV/mA (R² = 0.92) in porcine heart tissue. The AE signal was detectable more than 25 mm away from the source in cylindrical gel (0.9% NaCl). Effect of transducer properties on UCSDI sensitivity is also investigated using simulation. The optimal ultrasound transducer parameters chosen for cardiac imaging are center frequency = 0.5 MHz and f/number = 1.4. Last but not least, this dissertation shows the result of implementing the optimized ultrasound parameters in live rabbit heart preparation, the comparison of different recording electrode configuration and multichannel UCSDI recording and reconstruction. The AE signal detected using the 0.5 MHz transducer was much stronger (2.99 μV/MPa) than the 1.0 MHz transducer (0.42 μV/MPa). The clinical lasso catheter placed on the epicardium exhibited excellent sensitivity without being too invasive. 3-dimensional cardiac activation maps of the live rabbit heart using only one pair of recording electrodes were also demonstrated for the first time. Cardiac conduction velocity for atrial (1.31 m/s) and apical (0.67 m/s) pacing were calculated based on the activation maps. The future outlook of this dissertation includes integrating UCSDI with 2-dimensional ultrasound transducer array for fast imaging, and developing a multi-modality catheter with 4-dimensional UCSDI, multi-electrode recording and echocardiography capacity.