Browsing by Subject "Polarization"
Now showing 1 - 12 of 12
Results Per Page
Sort Options
Item Bio-inspired nanophotonics : manipulating light at the nanoscale with plasmonic metamaterials(2013-05) Zhao, Yang, active 21st century; Alù, AndreaMetals interact very differently with light than with radio waves and finite conductivities and losses often limit the way that RF concepts can be directly transferred to higher frequencies. Plasmonic materials are investigated here for various optical applications, since they can interact, confine and focus light at the nanoscale; however, regular plasmonic devices are severely limited by frequency dispersion and absorption, and confined signals cannot travel along plasmonic lines over few wavelengths. For these reasons, novel concepts and materials should be introduced to successfully manipulate and radiate light in the same flexible way we operate at lower frequencies. In line with these efforts, optical metamaterials exploit the resonant wave interaction of collections of plasmonic nanoparticles to produce anomalous light effects, beyond what naturally available in optical materials and in their basic constituents. Still, these concepts are currently limited by a variety of factors, such as: (a) technological challenges in realizing 3-D bulk composites with specific nano-structured patterns; (b) inherent sensitivity to disorder and losses in their realization; (c) not straightforward modeling of their interaction with nearby optical sources. In this study, we develop a novel paradigm to use single-element nanoantennas, and composite nanoantenna arrays forming two-dimensional metasurfaces and three-dimensional metamaterials, to control and manipulate light and its polarization at the nanoscale, which can possibly bypass the abovementioned limitations in terms of design procedure and experimental realization. The final design of some of the metamaterial concepts proposed in this work was inspired by biological species, whose complex structure can exhibit superior functionalities to detect, control and manipulate the polarization state of light for their orientation, signaling and defense. Inspired by these concepts, we theoretically investigate and design metasurfaces and metamaterial models with the help of fully vectorial numerical simulation tools, and we are able to outline the limitations and ultimate conditions under which the average optical surface impedance concept may accurately describe the complex wave interaction with planar plasmonic metasurfaces. We also experimentally explore various technological approaches compatible with these goals, such as the realization of lithographic single-element nanoantenna and nanoantenna arrays with complex circuit loads, periodic arrays of plasmonic nanoparticles or nanoapertures, and stacks of rotated plasmonic metasurfaces. At the conclusion of this effort, we have theoretically analyzed, designed and experimentally realized and characterized the feasibility of using discrete metasurfaces to realize phenomena and performance that are not available in natural materials, oftentimes inspired by the biological world.Item Depth-resolved measurements in multi-layer scattering media using linearly and elliptically polarized reflectance spectroscopy(2016-08) Bailey, Maria Jimenez; Sokolov, Konstantin V. (Associate professor); Dunn, Andrew; MacAulay, Calum; Markey, Mia; Rylander, H. GradyOptical reflectance spectroscopy is a non-invasive tool that can provide quantitative information on tissue morphology and biochemistry. However, interpreting the depth-dependent signals from multi-layer tissue can be challenging as optical signatures are from a range of depths. The focus of this dissertation is on the development of light-based systems for the depth-resolved characterization of multi-layered tissue. Specifically, this dissertation concentrates on the assessment of polarized reflectance spectroscopy (PRS) with linearly and elliptically polarized light. PRS is a form of elastic scattering spectroscopy that relies on polarization gating to selectively isolate polarization maintaining photons in order to probe superficial depths. Polarization gating relies on the observation that as light travels deeper into a turbid medium and undergoes scattering, the incident polarization will become depolarized. Polarization-sensitive techniques for imaging and spectroscopy have received attention for their relatively simple and low-cost instrumentation to favorably collect either superficial or deep penetration photons. In this dissertation, I will present a fiber optic probe that combines polarization gating with linearly polarized light and an oblique detection geometry via multiple beveled collection fibers to detect scattering within various depths in tissue. The performance of the oblique polarized reflectance spectroscopy (OPRS) probe was evaluated in a clinical trial of oral cavity cancer. I will also discuss in this dissertation a new system based on elliptical polarization. The probing depth of elliptical polarized reflectance spectroscopy (EPRS) was assessed in turbid media by tuning the ellipticity of polarized light.Item The geographical foundations of state legislative conflict, 1993-2012(2013-08) Myers, Adam Shalmone; Jones, Bryan D.; Trubowitz, PeterOver the past twenty years, the geographical bases of state legislative parties have shifted substantially. In statehouses across the country, legislators from densely-populated districts with large racial minority populations have become a larger presence inside Democratic caucuses while legislators from exurban and sparsely-populated districts have become a larger presence inside Republican caucuses. These changes have had important consequences for roll-call voting and policy outcomes inside legislatures, as new coalitional configurations formed by the intersection of party and geography have replaced older ones. In this dissertation, I examine the causes and consequences of these changes in a new way, one that more closely approximates a legislator's relationship to her "geographical constituency" (to use Richard Fenno's famous term). Unlike traditional studies of the social origins of legislative conflict, which have focused on how the constituency bases of legislative parties can be distinguished by reference to a small set of district-level demographic variables examined independently of each other, my approach views district demographic variables as the empirical manifestations of a wide variety of distinct, if latent, geographical contexts. My efforts to model the geographical constituency are centered upon a technique called Latent Profile Analysis (LPA), which estimates a latent categorical variable (in this case, legislative district categories indicative of distinct socioeconomic contexts) that captures covariation among a set of observed continuous variables (in this case, district-level demographic and geographical variables). The LPA analysis, which incorporates over 3,500 districts from seventeen chambers in the 1990s and 2000s, yields a nine-fold district categorization scheme that serves as the basis for subsequent inquiries of the dissertation. These inquiries examine how demographic and electoral change have interacted to influence trends in partisan representation of the district categories, how party and district category come together to explain patterns of roll-call ideology among state legislators, and how social cleavages over public policy within state electorates are translated into particular voting alignments involving the district categories. The dissertation speaks to a large literature in political science on the constituency-legislator relationship, as well to current debates about geographical sorting, legislative polarization, and the role of policy content in shaping voting coalitions.Item The impact of selective exposure on political polarization and participation : an exploration of mediating and moderating mechanisms(2013-05) Kim, Yonghwan; Gil de Zúñiga, Homero; McCombs, Maxwell E.; Stroud, Natalie JominiThis dissertation seeks to improve our understanding of the process by which citizens' selective exposure contributes to attitudinal polarization and engagement in political activities. In this dissertation, I test two models that explicate the relationship between selective exposure and political polarization and participation. The knowledge model suggests that the effects of selective exposure on individuals' attitudinal polarization and political engagement are mediated by knowledge of candidate issue stances. The stereotype model proposes that selective exposure indirectly influences polarized attitudes and political participation via stereotypical perceptions of candidates (i.e., McCain's age and the prospect of a Black presidency). By posing issue knowledge and stereotypical perceptions as potential mediators, this study extends current literature to analyze why and how selective exposure leads to polarization and political participation. The results provide evidence that selective exposure influences individuals' stereotypical perceptions of the candidates' age and race, and these stereotypic perceptions influence attitudinal polarization and participation in campaign activities. There was no support for the knowledge model; selective exposure did not have a significant relationship with citizens' issue knowledge nor did it play a mediating role in the relationship between selective exposure and political polarization and participation. This dissertation thus challenges the argument that selective exposure is normatively desirable due to its contribution to citizens' greater levels of political participation. The findings of this study call into question such a contention because the results show that individuals who engage in selective exposure are motivated to participate in political activities by forming stereotypic perceptions of candidates rather than by gaining factual issue knowledge, which is in contrast to democratic theories' assumptions of informed citizenship. Turning to the role of exposure to dissonant media outlets, two contrasting roles were found. On one hand, results offer some evidence that dissonant media use contributes to gaining issue knowledge and inspiring citizen participation. On the other hand, some findings suggest that it reinforces, rather than attenuates, citizens' attitudinal polarization and stereotypical perceptions of candidates. Thus the findings from this study offer mixed support for encouraging citizen exposure to dissimilar viewpoints.Item Optical coherence tomography for retinal diagnostics(2013-08) Yin, Biwei; Milner, Thomas E.; Rylander, H. Grady (Henry Grady), 1948-Optical Coherence Tomography (OCT) is a non-invasive three-dimensional imaging technique. OCT synthesizes a cross-sectional image from a series of lateral adjacent depth scans, and with a two-dimensional scanning scheme, three-dimensional intensity image of sample can be constructed. Due to its non-invasive capability, OCT has been widely applied in ophthalmology, cardiology and dermatology; and in addition to three-dimensional intensity image construction, various functional OCT imaging techniques have been developed for clinical application. My research is focused on developing functional OCT systems for application in ophthalmology, including polarization-sensitive optical coherence tomography (PS-OCT) for retinal nerve fiber layer (RNFL) birefringence measurement and dual-wavelength photothermal optical coherence tomography (DWP-OCT) for microvasculature blood oxygen saturation (SO2) measurement. In the study, a single-mode-fiber based polarization-sensitive swept-source OCT (PS-SS-OCT) with polarization modulator, polarization-sensitive bulk-optics balanced detection module is constructed and polarization processing methods based on Stokes vectors are applied to determine birefringence. PS-OCT is able to provide human subject's RNFL thickness, phase retardation, and birefringence information. Degradation in the degree of polarization (DOP) along depth is investigated and its difference between four quadrants of RNFL (superior, temporal, inferior and nasal) indicates the structural property difference. DWP-OCT is a novel functional OCT system consisting of a phase-sensitive optical coherence tomography system (PhS-OCT) and two photothermal excitation lasers. PhS-OCT is based on a swept-source laser operating in the 1060 nm wavelength range; the two photothermal excitation lasers with wavelength 770 nm and 800 nm are intensity modulated at different frequencies. PhS-OCT probe beam and two photothermal excitation beams are combined and incident on the sample, optical pathlength (op) change on the sample introduced by two photothermal excitation beams are measured and used for blood SO2 estimation. A polarization microscope is proposed for future study. The polarization microscope is an imaging technique providing molecular structure and orientation based on probe light's polarization state information. The polarization microscope uses a wavelength tunable light source, and can achieve any incident polarization state by a retarder-rotator combination. Specimen's birefringence can be determined based on the changing of detected light amplitude.Item Optical effects in photonic crystals and metamaterials(2011-05) McIlhargey, James Garland; Shvets, G.; Li, XiaoquinIn this thesis, I will describe the polarization properties of two separate but similar optical systems. I will begin by showing anisotropy in a dielectric photonic crystal slab patterned with a periodic circular hole array. This anisotropy can be utilized in manipulating the gain properties of surface emitting photonic crystal lasers. I will then describe a metallic, planar metamaterial patterned similarly with a 2d periodic array of holes. The enhanced optical transmission of this system is demonstrated computationally and experimentally, with a good agreement between the two. I will also demonstrate polarization rotation in this array. The effect is shown to minimize the background contribution to the transmission resulting in the narrowing of the line width and improvement between on and off resonance contrast. I then provide a theory behind the polarization rotation in transmission through a metamaterial based upon a Jones matrix formulation, which is dependent only upon the existence of separate s and p resonances in a photonic system.Item Polarization and charge transport in polymer and 2-D material based field-effect transistors(2016-12) Kim, Seohee; Dodabalapur, Ananth, 1963-; Lee, Jack; Bank, Seth R.; Akinwande, Deji; Tao, HuThis work investigates new semiconductors that are currently being considered for use in flexible electronics. It is critical to understand charge transport physics in field-effect transistors (FETs) to get better device performance and integrate them into commercial products. The charge transport studies include understanding of effect of wide-range of applying voltage (sub-threshold and above threshold operation), dominant charge transport mechanisms under specific conditions (band, band-like and hopping transport), and of polarization effect (polaronic and non-polaronic transport). Charge transport in high-mobility diketopyrrolopyrrole (DPP) co-polymers FETs is investigated. Sub-threshold regime conduction, including diffusion- and drift-limited regimes, are accurately modeled with above threshold regime. Based on modeling results, the realistic density of states (DOS) curve of polymer FETs in a wide range of gate voltages is calculated. From modeled sub- and above- threshold regime mobility data, dominant charge transport mechanisms under specific conditions are investigated. Shallow states charge transport is well-described by multiple trap and release (MTR) transport, while hopping transport models such as variable range hopping (VRH) or Gaussian disorder based model (GDM) can describe deep states charge transport. The transition between the conduction regimes is a function of temperature and carrier density. In addition, a polarization effect from polar molecules in both atmosphere and dielectric on the electronic properties of polymer FETs is demonstrated. High-k and low-k surface dielectric devices are measured in polar and non-polar atmospheres. Dipoles in both conditions affect conduction in polymer FETs, but have different aspects, such as uniformly shifted DOS or only shallow states shifted DOS. The improved electrical characteristics of graphene monolayer sheet FETs with fluoropolymer capping is explained via measurements under polar vapor flow, which is reversible and non-destructive. It is found that the higher dipole moment of polar molecules corresponds to better improvement of electrical properties of graphene FETs, including the Dirac voltage shift, mobility and residual carrier concentration. In addition, a similar experiment is applied to graphene nano-ribbon (GNR) FETs. GNR FETs, which have high on/off ratio but some degraded electrical characteristics due to a considerable number of edge defects, also show a highly improved performance under polar vapor.Item Polarization rotation upon reflection of direct shear waves in purely isotropic media(2006-12) Lyons, Eric, 1980-; Tatham, R.H. (Robert H.), 1943-The reflection process alters the polarization of a direct shear wave in a purely isotropic medium. I show that where the alteration occurs is governed by the shear reflection amplitude versus incidence angle for the reflecting interface. Specifically, I address the zero crossing in the SV--SV component's reflectivity (where the amplitude changes sign), and how the zero crossing governs the alteration's location. The severity of the deviation of the polarization of the source from the observed polarization of the reflection varies with the difference in azimuth between source position and the receiver location. I develop a correction based on the observed shear amplitude versus incidence angle, which corrects to the equivalent of nearangle polarization for mid and large incidence angle (large offset) data. When the exact parameters from the model data are used to create the correction, the reflected shear polarization matches the source polarization out to moderate offsets. A universal, model-independent correction does not perform as well as the exact correction, but it may be suitable for many applications where knowledge of regional geology is not complete. I also show preliminary results from applying the correction to anisotropic model data. The correction is most effective for small percentages of anisotropy (<5%), after which the anisotropy signature of the observed polarization dominates over the isotropic reflection distortion.Item The role of emotion in selective exposure, information processing, and attitudinal polarization(2010-05) Kim, Soohee, 1980-; Stroud, Natalie Jomini; Jarvis, Sharon E.This thesis reviews the role of emotions in one’s choice of information, information processing, and political attitudes. Theoretical and empirical endeavors to date have focused primarily on how emotions influence attitudes and information processing, leaving the actual processes guiding these outcomes in the margins. Specifically, it has been largely unexplored how emotions influence individuals’ information search behavior and then attitudes and information processing. Noting that the purposeful selection of likeminded information, often referred to as selective exposure, is commonly enacted when an individual first initiates information processing, and is also likely influenced by emotions, this study explores how emotions may affect people’s tendency to seek out congruent information. In addition, this study examines how the relationship between emotions and selective exposure in turn may affect aspects of information processing and attitudes. By designing an online experiment, I first tested how certain negative emotions (anger/fear) affected one’s pursuit of certain types of information (consistent/inconsistent) and second, I investigated how these emotions and information selections influenced subsequent information processing and attitudes. Results showed that while anger motivated more likeminded exposure for Republicans than fear, fear promoted more likeminded exposure for Democrats than anger. Further, anger prompted people to process messages more closely and to develop more polarized attitudes compared to fear. In addition, pro-attitudinal exposure produced more message-relevant thoughts for Republicans than counter-attitudinal message exposure, while it was counter-attitudinal exposure that yielded more message-relevant thoughts for Democrats. No such effect, however, was shown for attitudinal polarization.Item The roles of Shroom family proteins during Xenopus development(2009-05) Lee, Chan-jae; Wallingford, John B.The Shroom family of proteins is currently comprised of four members, Shroom1, 2, 3 and 4. Since Shroom3 was shown to be a critical protein for neural tube closure, the other three proteins are also expected to play an important role for proper development. However, their functions during development were not clear. To address this, my study started with Shroom3 function in the neural plate. Shroom3 had been previously known to induce apical constriction by controlling actin filaments in neuroepithelial cells. My studies show that Shroom3 induces apico-basal cell heightening by controlling parallel microtubule assembly. Shroom3 is able to change the distribution of γ-tubulin, suggesting that Shroom3 controls apical constriction and apico-basal cell elongation via both actin filaments and microtubules. The ability to control γ-tubulin distribution is possessed not only by Shroom3, but also by all other Shroom proteins, although they can not induce apical constriction. In addition, they are expressed in tissues which contain apico-basally elongated cells. Data from functional assays with Shroom2 show that it induces cell elongation and is required for proper cell shape in deep layer neuroepithelial cells in Xenopus. These data suggest that Shroom family proteins control cell architecture during morphogenetic development. I have discovered another role for Shroom2. By comparative analysis with Xenopus and Physalaemus, which have different pigment patterns in eggs, I show that a high level of maternal Shroom2 mRNA is important for pigment polarity in Xenopus. Furthermore, Shroom2 controls the distribution of spectrin which plays a role in pigment granule movement. Thus, Shroom2 is suggested to be a key molecule to control the pigment polarity in amphibian eggs. Together all these data suggest that Shroom family proteins play a role in cell morphogenesis and polarization via controlling the cytoskeleton during Xenopus development.Item The Implementation of Photon Polarization into the Mercury Transport Code(2014-06-04) Windsor, EthanPolarization effects have been ignored in most photon transport codes to date, but new technology has created a need for portable, massively parallel, versatile transport codes that include the effects of polarization. In this project, the effects of both linear and elliptic polarization on the angular distribution of coherently and incoherently scattered photons are incorporated into the Monte Carlo transport code Mercury. In Mercury, photons are given a polarization fraction, polarization direction, and a polarization ellipticity. These new variables are tracked throughout each particle?s history. They impact and are impacted by interactions with the medium. The determination of how these variables affect the photon?s interactions with the medium uses Stokes vectors and the Mueller matrices for coherent and incoherent scattering events. Verification studies were performed comparing results from Mercury against analytical, experimental, and computational results. In all cases, Mercury showed agreement with the expected results. It was also shown that polarization effects are present and potentially significant even in cases where an initial beam is completely unpolarized. Adding polarization effects slowed the code between 10-50% depending on the particular problem. Mercury can now accurately model polarized light.Item The Institutional Consequences of Congressional Polarization(2010-07-14) Ilderton, Nathan A.Polarization, defined as the ideological distance between the Democrat and Republican parties in Congress, has increased dramatically in Congress since the 1970s. Research on polarization in the U.S. Congress primarily focuses on the sources of this increase. Relatively little work has been done on the consequences of polarization for Congress? relationship with the president and the passage of legislation. This dissertation corrects this omission by examining the influence of polarization on several key aspects of the legislative process. It examines the impact of polarization on the interaction between Congress and the president, including the president?s strategy in supporting or opposing legislation and the success the president has on bills when he takes a position. It also examines the effect polarization has on the overall passage of legislation. An empirical examination was undertaken using significant bills in Congress over a sixty year time period (1947-2006). The results indicate that the effects of polarization on the legislative process are contingent upon the presence of divided government, defined as times when the president and a majority of members of Congress are from different parties, and the chamber of Congress under examination. As polarization increases, the president is more likely to support legislation and be successful when his party controls Congress, but he opposes more legislation and is less successful as polarization increases under divided government. Legislative gridlock, the inability of Congress to pass important or innovative legislation, tends to decrease in both the House and Senate as polarization increases under unified government. However, as polarization increases under divided government the overall passage of bills into law decreases. The dissertation also offers an improved method for modeling the impact of divided government on gridlock. Prior studies model divided government without regard for whether the president takes a position on a given bill. This study shows that when the president takes a position on a bill under divided government the probability it passes decreases, but the probability of passage increases when the president does not take a position. This finding implies that previous research may underestimate the true effects of divided government on gridlock.