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von, Ribbeck Hans-Georg. "THz Near-Field Microscopy and Spectroscopy." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2015. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-163917.
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Imaging with THz radiation at nanoscale resolution is highly desirable for specific material investigations that cannot be obtained in other parts of the electromagnetic spectrum. Nevertheless, classical free-space focusing of THz waves is limited to a >100 μm spatial resolution, due to the diffraction limit. However, the scattering- type scanning near-field optical microscopy (s-SNOM) promises to break this diffraction barrier. In this work, the realization of s-SNOM and spectroscopy for the THz spectral region from 30–300 μm (1–10 THz) is presented. This has been accomplished by using two inherently different radiation sources at distinct experimental setups: A femtosecond laser driven photoconductive antenna, emitting pulsed broadband THz radiation from 0.2–2 THz and a free-electron laser (FEL) as narrow-band high-intensity source, tunable from 1.3–10 THz. With the photoconductive antenna system, it was demonstrated for the first time that near-field spectroscopy using broadband THz-pulses, is achievable. Hereby, Terahertz time-domain spectroscopy with a mechanical delay stage (THz-TDS) was realized to obtain spectroscopic s-SNOM information, with an additional asynchronous optical sampling (ASOPS) option for rapid far-field measurements. The near-field spectral capabilities of the microscope are demonstrated with measurements on gold and on variably doped silicon samples. Here it was shown that the spectral response follows the theoretical prediction according to the Drude and the dipole model. While the broadband THz-TDS based s-SNOM in principle allows for the parallel recording of the full spectral response, the weak average power of the THz source ultimately limits the technique to optically investigate selected sample locations only. Therefore, for true THz near-field imaging, a FEL as a high-intensity narrow- band but highly-tunable THz source in combination with the s-SNOM technique, has been explored. Here, the characteristic near-field signatures at wavelengths from 35–230 μm are shown. Moreover, the realization of material sensitive THz near-field imaging is demonstrated by optically resolving, a structured gold rod with a reso- lution of up to 60 nm at 98 μm wavelength. Not only can the gold be distinguished from the silica substrate but moreover parts of the structure have been identified to be residual resin from the fabrication process. Furthermore, in order to explore the resolution capabilities of the technique, the near-fields of patterned gold nano- structures (Fischer pattern) were imaged with a 50 nm resolution at wavelengths up to 230 μm (1.2 THz). Finally, the imaging of a topography-independent optical material contrast of embedded organic structures, at exemplary 150 μm wavelength is shown, thereby demonstrating that the recorded near-field signal alone allows us to identify materials on the nanometer scale. The ability to measure spectroscopic images by THz-s-SNOM, will be of benefit to fundamental research into nanoscale composites, nano-structured conductivity phenomena and metamaterials, and furthermore will enable applications in the chemical and electronics industriesDie Bildgebung mit THz Strahlung im Nanobereich ist höchst wünschenswert für genaue Materialuntersuchungen, welche nicht in anderen Spektralbereichen durchgeführt werden kann. Aufgrund des Beugungslimits ist kann jedoch mit klassischen Methoden keine bessere Auflösung als etwa 100 μm für THz-Strahlung erreicht werden. Die Methode der Streulicht-Nahfeldmikroskopie (s-SNOM) verspricht jedoch dieses Beugungslimit zu durchbrechen. In der vorliegenden Arbeit wird die Realisierung der Nahfeld-Mikroskopie und Spektroskopie im THz-Spektralbereich von 30–1500 μm (0.2–10 THz) präsentiert. Dies wurde mittels zweier grundsätzlich unterschiedlichen Strahlungsquellen an separaten Experimentaufbauten erreicht: Einer photoleitenden Antenne welche gepulste breitbandige THz-Strahlung von 0.2–2 THz emittiert, sowie einem Freie- Elektronen Laser (FEL) als schmalbandige hochleistungs Quelle, durchstimmbar von 1.3–10 THz. Mit dem photoleitenden Antennensystem konnte zum ersten mal demonstriert werden, dass mit breitbandigen THz-Pulsen Nahfeldspektroskopie möglich ist. Dazu wurde die übliche THz-Time-Domain-Spektroskopie (THz-TDS) zur Erhaltung der spektroskopischen s-SNOM Informationen, sowie asynchrones optisches Abtasten (ASOPS) für schnelle Fernfeld Spektroskopie eingesetzt. Die nahfeldspektroskopischen Fähigkeiten des Mikroskops wurden anhand von Messungen an Gold sowie unterschiedlich dotierten Siliziumproben demonstriert. Dabei konnte gezeigt werden, dass die spektrale Antwort den theoretischen Voraussagen des Drude- sowie Dipol Modells folgt. Während das breitband THz-TDS basierte s-SNOM spektroskopische Nahfelduntersuchungen zulässt, limitiert jedoch die schwache Ausgangsleistung der THz-quelle diese Technik insofern, dass praktisch nur Punktspektroskopie an ausgesuchten Probenstellen möglich ist. Für echte nanoskopische Nahfeldbildgebung wurde daher ein FEL als durchstimmbare hochleistungs THz-Quelle in Kombination mit der s-SNOM-Technik erforscht. Hierzu wurden die charakteristischen Nahfeld-Signaturen bei Wellenlängen von 35–230 μm untersucht, gefolgt von die Verwirklichung materialsensitiver THz Nahfeldbildgebung gezeigt an Goldstreifen mit bis zu 60 nm Auflösung. Dabei kann nicht nur das Gold von dem Glassubstrat unterschieden werden, sondern auch Ablagerungen als Überreste des Fabrikationsprozesses identifiziert werden. Um die Grenzen der Auflösungsmöglichkeiten dieser Technik zu sondieren, wurden weiterhin die Nahfelder von gemusterten Gold-Nanostrukturen (Fischer-Pattern) bei Wellenlängen bis zu 230 μm (1.2 THz) abgebildet. Hierbei wurde eine Auflösung von 50 nm festgestellt. Schliesslich konnte der topographieunabhängige Materialkontrast von eingebetteten organischen Strukturen, exemplarisch bei 150 μm Wellenlänge, gezeigt werden. Die Fähigkeit, spektroskopische Aufnahmen mittels der THZ-s-SNOM Technik zu erzeugen, wird der Grundlagenforschung und in der Nanotechnologie zu Gute kommen, und weiterhin Anwendungen in der Chemischen- und Halbleiterindustrie ermöglichen
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Tuo, Mingguang, Min Liang, Jitao Zhang, and Hao Xin. "Time-Domain THz Near-Field Imaging Incorporating Hadamard Multiplexing Method." IEEE, 2016. http://hdl.handle.net/10150/622785.
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Photoconductive antenna (PCA) array based THz near-field imager incorporating Hadamard multiplexing method is developed in this work. By using a 2 × 2 dipole antenna array as the THz transmitter, the system signal-to-noise ratio (SNR) is demonstrated to be improved by a factor of 2 as the theory predicts. Additionally, a 2-D scanning of a metallic structure on a THz-transparent substrate (with a total scanning area of 1 × 1 mm2) is experimentally implemented. Correlation coefficient estimation is made afterwards to quantify the reconstructed image quality.
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Szelc, Jedrzej. "THz imaging and microscopy : a multiplexed near-field TeraHertz microscope." Thesis, University of Southampton, 2011. https://eprints.soton.ac.uk/209643/.
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Stantchev, Rayko Ivanov. "Non-invasive near-field THz imaging using a single pixel detector." Thesis, University of Exeter, 2017. http://hdl.handle.net/10871/27766.
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The terehertz radiation potentially has many interesting applications. From air port security, non-destructive evaluations of electronics and space shuttle panels, to non-ionizing photon energies with the potential to detect cancer growths and quality control of pharmaceutical tables, the list of potential applications is vast as shown in chapter 1. However, there is a lack of cheap, robust and efficient THz sources, detectors and modulators. Further, the long wavelengths render micron sized details unseeable with far-field imaging techniques. This has rendered most imaging applications unusable in the real world. This thesis is based around demonstrating an imaging technique that uses a near-field THz modulator to obtain sub-wavelength images. There are five distinct experimental demonstrations that show the full capacity of the imaging technique developed here. Chapter 2 gives an outline of the background physics knowledge needed to understand the entirety of the thesis. An outline of the mathematics used for modellingis given in the latter part of the chapter as well. Chapter 3 gives a background on the THz generation and detection techniques used in our THz-TDS system, optical rectification and electro-optic sampling in ZnTe. Further more, our system is capable of photoexciting a sample in conjunction to it being probed with a THz pulse. For the most part, we photoexcite a silicon wafer in order to use its photoconductive properties to modulate our THz pulse. Our photoexcitation pulse is spatially modulated, via a digital micromirror device, which in turn spatially modulates our THz pulse. This patterned THz pulse can then be used with a single-element detector to perform imaging. How to do this and the type of patterns needed is described in the latter part of chapter 3. Chapter 4 is the first demonstration that photo-induced conductivity in silicon can be used to manipulate evanescent THz fields for sub-wavelength imaging. For this, we imaged a 1D sub-wavelength slit and were able to obtain the slit profile with 65μm (λ/6 at 0.75T Hz) resolution. Chapter 5 demonstrates what limits the resolution in our imaging system. Namely, the distance which the patterned THz pulse propagates to the object from where itwas spatially modulated. We demonstrate 9μm (λ/45 at 0.75T Hz) resolution using an ultra-thin (6μm) silicon wafer. At such sub-wavelength scales polarization becomes an important factor. We show how one can use polarization in order to detect 8μm breaks in a circuit board hidden by 115μm of silicon. Chapter 6 concerns itself with showing how noise affects our images. Further more, our imaging system is compatible with compressed sensing where one can obtain an image using fewer measurements than the number of pixels. We investigate how different under-sampling techniques perform in our system. Note under-sampling at sub-wavelength resolutions, as is done here, is rather unusual and is of yet to be demonstrated for other part of the electro-magnetic spectrum. Chapter 7 shows that one does not need to photoexcite silicon. One can in principle illuminate any material, hence we photoexcite graphene with our spatially modulated optical pulses. This allows us to obtain the THz photoconductive response of our graphene sample with sub-wavelength resolution (75μm ≈ λ/5 at 0.75T Hz). We compare our results with Raman spectra maps. We find a clear correlation between THz photoconductivity and carrier concentration (extracted from Raman). Chapter 8 exploits the full capacity of our imaging system by performing hyper-spectral near-field THz imaging on a biological sample. For this, in our imaged field of view, we measured the full temporal trace of our THz pulse at a sub-wavelength spatial resolution. This has allowed us to extract the frequency dependent permittivity of our biological sample, articular cartilage, over our spectral range (0.2-2T Hz). We find the permittivity to change on a sub-wavelength scale in correlation with changes in the structure of our sample. However, the permittivity extraction procedures that have been developed make a far-field approximation. We mathematically show the presence of the THz near-fields to render the long wavelength spectral parts of our extracted permittivity to be wrong. Chapter 9 is where we conclude and point out the main problem that needs to be addressed in order to make the measurements presented here more accessible to others. Namely, the cost of the laser system powering the THz-TDS and how to further reduce the acquisition time.
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Pan, Yi. "Terahertz time-domain spectroscopy and near-field imaging of microstructured waveguides." Thesis, University of Bath, 2013. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.607613.
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This thesis presents studies of novel terahertz photonic devices, including photoconductive optoelectronic devices and guided-wave components, aimed at the development of next-generation terahertz systems. In chapter 2, a scalable interdigitated THz transmitter is designed to increase the output power and compared with a conventional 50 μm coplanar transmitter. In chapter 3, we compare four different receivers with different antenna geometries in terms of bandwidth and sensitivity. Then we describe a photoconductive near-field detector with a subwavelength aperture and its system integration and characterization. In chapter 4, a parallel metal plate waveguide is designed with an integrated step inside the waveguide that can couple to higher order TM modes efficiently from the TEM mode. In this chapter, we also experimentally and numerically study a 2-dimensionally tapered parallel plate waveguide, by which a free-space THz beam can be focused into a deep subwavelength-scale volume. In chapter 5, a parallel thin dielectric film waveguide is used to explore the guiding mechanism of an antiresonant optical reflection waveguide. Cylindrical silica single capillaries and a microstructured capillary, which guide in a similar way, are characterized in terms of mode profiles and attenuation. In chapter 6, we study oblique transmission through freestanding thin nickel films, which are perforated with periodic conical hole arrays. Surface modes can be supported by both metallic surfaces with different nonlinear dispersion curves, which results in spectral interferences in a near-field region when the surface modes couple out of the waveguide into free space.
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von, Ribbeck Hans-Georg [Verfasser], Lukas [Akademischer Betreuer] Eng, and Thomas [Akademischer Betreuer] Dekorsy. "THz Near-Field Microscopy and Spectroscopy / Hans-Georg von Ribbeck. Gutachter: Lukas Eng ; Thomas Dekorsy. Betreuer: Lukas Eng." Dresden : Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2015. http://d-nb.info/106951828X/34.
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Ben, Mbarek Sofiane. "Etude et réalisation d’antennes à concentration de champ pour la génération et la détection locale de champs électromagnétiques." Thesis, Besançon, 2011. http://www.theses.fr/2011BESA2018.
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L’objectif de cette thèse est le développement des détecteurs pour la microscopie champproche électromagnétique pour deux domaines fréquentiels. Pour le domaine des microondesnous présentons des micro-antennes non conventionnelles basées sur un guidagecoplanaire et l’effet de pointe. Nous pr´esentons les différentes étapes de la conceptionet de la réalisation avec les techniques de micro-fabrication. L’évaluation de leur performancea été obtenue avec une confrontation des résultats de mesure et de cartographie surdes éléments passifs et ceux d’une modélisation d’intégration finie. Pour le domaine desTérahertz, nous avons réalisé des micro-bolométres à température ambiante. Dans le butd’améliorer l’absorption de ces d´etecteurs, leur conception a été basée sur l’étude théoriquede l’absorption d’une onde électromagnétique en incidence normale sur un empilement descouches métalliques et diélectrique. Deux versions ont été réalisées et caractérisées é l’aidedes sources électroniques qui peuvent atteindre 1, 1 THz en continue. Les performancesde ces d´etecteurs en termes de bruit, de sensibilit´e et de temps de r´eponse sont mises enexergueThe objective of this thesis is the development of detectors for near-field microscopy fortwo electromagnetic frequency domains. For microwave domain we present unconventionalmicro-antennas based on coplanar line and point effect. We present the different stages ofthe design and implementation with micro-fabrication technique. The evaluation of theirperformance was obtained with a comparison of measurement results and mapping ofpassive elements and those of a model of finite integration. For the THz domain, we performedroom temperature micro-bolometers. In order to improve the absorption of thesedetectors, their design was based on the theoretical study of the absorption of an electromagneticwave normally incident on a stack of metal and dielectric layers. Two versionswere prepared and characterized using electronic sources that can reach continuous 1,1THz. The performance of these detectors in terms of noise, sensitivity and time responseare highlighted
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Dovelos, Konstantinos. "Terahertz communications: Physical layer enablers and analysis." Doctoral thesis, Universitat Pompeu Fabra, 2021. http://hdl.handle.net/10803/673252.
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Undoubtedly, spectrum scarcity constitutes the main bottleneck of current wireless networks. It is therefore
imperative to move beyond the sub-6 GHz band in order to overcome this limitation. Toward this direction,
terahertz (THz) communication is deemed a promising solution for future wireless systems owing to the
abundant spectrum resources at these frequencies. Despite the prospect of terabit- per-second wireless
links, THz signals suffer from severe propagation losses, which can undermine the communication range
and performance of THz systems. In this dissertation, we tackle this challenge by putting forward two key
physical layer technologies, namely massive multiple-input multiple-output (MIMO) and intelligent
reflecting surfaces (IRSs).
More particularly, this dissertation consists of two parts. In the first part, we thoroughly study the spatialwideband
effect in THz massive MIMO. We commence by demonstrating that conventional narrowband
beamforming/combining leads to substantial performance degradation for large antenna arrays and high
transmission bandwidths. With this in mind, we propose a wideband array architecture based on true-timedelay
and virtual subarrays. For the channel estimation problem, we introduce a wideband dictionary
along with a novel variant of the orthogonal matching pursuit algorithm. Numerical simulations are
provided showcasing that the proposed design enables: i) nearly squint-free beamforming/combining with
a small number of true-time-delay elements; and ii) accurate channel acquisition with reduced pilot
overhead even in the low signal-to-noise-ratio regime.
In the second part, we focus on the fundamentals of IRSs at THz frequencies. Specifically, we show that
an IRS has the potential to improve the energy efficiency of THz MIMO, when it is placed close to one of
the link ends. As a result, electrically large IRSs are expected to operate in the radiating near-field zone,
where the spherical wavefront of the emitted electromagnetic (EM) waves cannot be neglected. To this
end, we introduce a spherical wave channel model by leveraging EM theory, which includes far-field as
special case. Furthermore, we discuss the importance of using EM principles to characterize the path loss
of IRS-aided links, as simplistic models may wrongly estimate the link budget and actual system
performance. Our analysis reveals that: i) conventional far-field beamforming is highly suboptimal in terms
of power gain, and hence beamfocusing is the optimal mode of operation for THz IRSs; and ii) frequencydependent
beamfocusing is required in wideband THz transmissions, as beam squint can substantially
reduce the achievable data rate.
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Waselikowski, Stefan [Verfasser], and Markus [Akademischer Betreuer] Walther. "Investigation of interaction of THz-radiation with metallic subwavelength sized structures in far- and near-field = Untersuchung der Wechselwirkung von THz-Strahlung mit metallischen subwellenlängengroßenStrukturen im Fern- und Nahfeld." Freiburg : Universität, 2014. http://d-nb.info/1123479976/34.
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Beneš, Adam. "Plazmonické antény pro vysoké vlnové délky." Master's thesis, Vysoké učení technické v Brně. Fakulta strojního inženýrství, 2021. http://www.nusl.cz/ntk/nusl-443226.
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Tato diplomová práce se zabývá vlastnostmi plazmonických antén v oblasti vysokých vlnových délek. Důraz je kladen na popis rezonančních vlastností jednotlivých antén i antén uspořádaných do periodických polí. Těžiště práce spočívá v počítačovém modelování navýšení magnetického pole v blízkosti antén, které lze využít ve vysokofrekvenční elektronové paramagnetické rezonanci (HFEPR) k zesílení měřeného signálu. Autor se zabývá kvantifikací zesílení v anténách s odlišnou geometrií a navrhuje i geometrii vlastní. Značná část práce se také věnuje snaze rozlišit příspěvky k navýšení magnetického pole od různých zdrojů při měření HFEPR v uspořádání s dvojitou transmisí záření.
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Kuschewski, Frederik [Verfasser], Lukas M. [Gutachter] Eng, and Hartmut [Gutachter] Roskos. "Investigation of nanometer scale charge carrier density variations with scattering-type scanning near-field microscopy in the THz regime / Frederik Kuschewski ; Gutachter: Lukas M. Eng, Hartmut Roskos." Dresden : Technische Universität Dresden, 2020. http://d-nb.info/1227196806/34.
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Fu, Ying. "Analysis of exposure to electromagnetic fields within the near-field of HF broadcast antennas." Thesis, University of Sheffield, 2013. http://etheses.whiterose.ac.uk/4699/.
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This thesis presents a study of the relationship between field strength and the actual levels of SAR in the human body encountered in close proximity to high gain high power HF broadcast antennas. In order to assess the human exposure against ICNIRP guidelines, it has led to the development of a series of assessments of modelling techniques, mechanical configuration and environment of a transmission site. Due to the complex nature of this EM problem, each chapter of this thesis covers the key developments of particular sub-projects in different areas. The findings of each area correspond to the ICNIRP occupational and public exposure guidelines; they formed the reference parameters in the primary assessments which could apply to a range of similar EM scenarios. The chapters of this thesis do not necessarily follow a chronological order. Various potential influential aspects of assessing the human exposure levels were addressed during continuous problem solving processes, including several issues have not been covered in other research before. There are three areas that have been covered in this thesis, firstly relating whole body SAR to incident plane-wave fields in correlation with ICNIRP basic restriction and reference levels. Second, near-zone incident fields characteristic were assessed by simulations and analyses the results, where some exposure assessment protocols for ICNIRP compliance was derived. Thirdly real-time field measurements were carried out to consolidate the findings from the theoretical modelling analysis. Experimental results are shown to be in agreement with these predicted by theory and in general, the electromagnetic E-field strengths within the near-field of the antenna are found to be significantly below these recommended by ICNIRP.
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Sulaiman, Ali Haidar. "The near-Saturn magnetic field environment." Thesis, Imperial College London, 2015. http://hdl.handle.net/10044/1/44209.
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Shock waves exist throughout the universe and are fundamental to understanding the nature of collisionless plasmas. The complex coupling between charged particles and electromagnetic fields in plasmas give rise to a whole host of mechanisms for dissipation and heating across shock waves, particularly at high Mach numbers. While ongoing studies have investigated these process extensively both theoretically and via simulations, their observations remain few and far between. This thesis presents a study of very high Mach number shocks in a parameter space that has been poorly explored and identifies reformation using in situ magnetic field observations from the Cassini spacecraft at Saturn's bow shock. This gives an insight into quasi-perpendicular shocks across two orders of magnitude in Alfvén Mach number (MA) and spanning Earth-like to Astrophysical-like regimes. The work here shows evidence for cyclic reformation controlled by specular ion reflection occurring at the predicted timescale of ~0.3 τc, where τc is the ion gyroperiod. The relationship between these reformation signatures, magnetic overshoot and variability are also presented. The final part of this thesis characterises the region downstream of Saturn's bow shock, the magnetosheath. The results show a comprehensive overview of the configuration of the magnetic field in a non-axisymmetric magnetosheath. This non-axisymmetry is revealed to have an impact in the rotation of the magnetic field and is significant enough to influence the magnetic shear at the magnetopause.
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Frid, Henrik. "Efficient Computation of the Near-Field Mutual Coupling Between Antennas on Vehicles." Thesis, KTH, Elektroteknisk teori och konstruktion, 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-146903.
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A near-eld generalization of Friis transmission equation has previously been proposed in the literature. Using this generalization, it is possible to calculatethe mutual coupling between two antennas as a weighted integral over the antenna far-fields. In this thesis, a change of variables is used to remove the singularity in the integrand and a normalization of the antenna far-field is suggested to take mismatch and thermal losses into account. The resulting non-singular integral has been implemented in a computer program that can be used to calculate the mutual coupling between two arbitrarily polarized antennas given the antenna far-fields and the geometrical separation between the antennas. The program has several advantages compared to previous programs based on the near-field generalization of Friis transmission equation. Firstly, this program can calculate the mutual coupling between two arbitrarily polarized and oriented antennas whereas previous programs could only be used for linearly polarized and polarization-matched antennas. Secondly, the advantage of the non-singular form is the improved numerical stability. The mutual coupling calculated using this program is demonstrated to agree well with results from full three-dimensional simulations of antennas located in each others near-fields using commercial software. Finally, we investigate for the first time if this integral relation can be used to calculate approximate values of the mutual coupling between antennas on an electrically large vehicle.En närfältsgeneralisering av Friis transmissionsekvation har tidigare foreslagits i litteraturen. Med denna generalisering kan den ömsesidiga kopplingen mellan två antenner beraknas som en viktad integral over antennernas fjärrfältsamplituder. I denna rapport används ett variabelbyte för att ta bort singulariteten i integranden och en normering av fjärrfaltsamplituden föreslås for att ta hänsyn till antennernas förluster. Den resulterande icke-singulära integralen har implementerats i ett datorprogram som kan användas for att beräkna kopplingenmellan två godtyckligt polariserade antenner givet antennernas fjärrfältsamplituder och geometriska separation. Programmet har flera fördelar jämfört med tidigare program som baserats på närfältsgeneraliseringen av Friis transmissionsekvation. För det första kan detta program anvandas för att beräkna kopplingen mellan två godtyckligt polariserade och orienterade antenner då tidigare program har varit skrivna for linjarpolariserade och polarisationsmatchade antenner. Dessutom uppnås en högre numerisk stabilitet eftersom den ickesingulära formen av integralen används. Det demonstreras i flera exempel att kopplingen som beräknats med detta program för antenner som befinner sig i varandras närfält stämmer väl överens med resultat som beräknats med kommersiell mjukvara. Avslutningsvis undersöks om denna närfältsgeneralisering kan användas for att approximativt beräkna kopplingen mellan två antenner på en elektriskt stor farkost.
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Prasse, Brian Shawn. "The determination of far-field backscatter using a near-field calibration array." Thesis, Georgia Institute of Technology, 1999. http://hdl.handle.net/1853/19151.
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Ali, Hassan O. "Near-field computation using the finite element method." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1997. http://www.collectionscanada.ca/obj/s4/f2/dsk2/ftp02/NQ26122.pdf.
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Melville, David O. S. "Planar Lensing Lithography: Enhancing the Optical Near Field." Thesis, University of Canterbury. Electrical and Computer Engineering, 2006. http://hdl.handle.net/10092/1091.
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In 2000, a controversial paper by John Pendry surmised that a slab of negative index material could act as a perfect lens, projecting images with resolution detail beyond the limits of conventional lensing systems. A thin silver slab was his realistic suggestion for a practical near-field superlens - a 'poor-mans perfect lens'. The superlens relied on plasmonic resonances rather than negative refraction to provide imaging. This silver superlens concept was experimentally verified by the author using a novel near-field lithographic technique called Planar Lensing Lithography (PLL), an extension of a previously developed Evanescent Near-Field Optical Lithography (ENFOL) technique. This thesis covers the computational and experimental efforts to test the performance of a silver superlens using PLL, and to compare it with the results produced by ENFOL. The PLL process was developed by creating metal patterned conformable photomasks on glass coverslips and adapting them for use with an available optical exposure system. After sub-diffraction-limited ENFOL results were achieved with this system additional spacer and silver layers were deposited onto the masks to produce a near-field test platform for the silver superlens. Imaging through a silver superlens was achieved in a near-field lithography environment for sub-micron, sub-wavelength, and sub-diffraction-limited features. The performance of PLL masks with 120-, 85-, 60-, and 50-nm thick silver layers was investigated. Features on periods down to 145-nm have been imaged through a 50-nm thick silver layer into a thin photoresist using a broadband mercury arc lamp. The quality of the imaging has been improved by using 365 nm narrowband exposures, however, resolution enhancement was not achieved. Multiple layer silver superlensing has also been experimentally investigated for the first time; it was proposed that a multi-layered superlens could achieve better resolution than a single layer lens for the same total silver thickness. Using a PLL mask with two 30-nm thick silver layers gave 170-nm pitch sub-diffraction-limited resolution, while for a single layer mask with the same total thickness (60 nm) resolution was limited to a 350-nm pitch. The proposed resolution enhancement was verified, however pattern fidelity was reduced, the result of additional surface roughness. Simulation and analytical techniques have been used to investigate and understand vi ABSTRACT the enhancements and limitations of the PLL technique. A Finite-Difference Time- Domain (FDTD) tool was written to produce full-vector numerical simulations and this provided both broad- and narrowband results, allowing image quality as a function of grating period to be investigated. An analytical T-matrix method was also derived to facilitate computationally efficient performance analysis for grating transmission through PLL stacks. Both methods showed that there is a performance advantage for PLL over conventional near-field optical lithography, however, the performance of the system varies greatly with grating period. The advantages of PLL are most prominent for multi-layer lenses. The work of this thesis indicates that the utilisation of plasmonic resonances in PLL and related techniques can enhance the performance of near-field lithography.
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Szewc, Carola. "Molecule interference in the near-field Talbot regime." Thesis, University of Southampton, 2012. https://eprints.soton.ac.uk/343753/.
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We show our progress in setting up a three grating near-field Talbot-Lau interferometer (TLI) for large molecules such as C60. The TLI is the preferred tool for showing the wave nature of large particles, as it accepts beams of poor spatial coherence and because of its scaling properties with increasing mass of the particles. The spatial coherence is prepared by the first grating, it can be seen as an array of many collimation slits. Our setup is highly flexible in terms of the possibility of changing most of the parameters (such as distances between the gratings and grating angles) from the outside of the UHV-chamber. As the interferometer and the molecular beam source are place in separate vacuum chambers, refilling the oven and adjusting the setup parameters to different molecules is possible without opening the interferometer chamber. This is necessary, as the pressure in the interferometer chamber has to be lower than 5.10-7 mbar in order to observe high interference contrast. The TLI is highly sensitive to vibrations, especially in the range of 1 kHz (the inverse time-of-flight of the molecules through the interferometer), which makes vibration damping crucial for the success of observing interference. We observed vibration amplitudes of up to 75 nm at low frequencies (158 Hz). Compared to the inverse time-of-flight of the molecules, these vibrations are not 'seen' by the molecules and are non-critical. We observed interference of C60 with a contrast of 27 %, by scanning the self-image of the diffraction grating. In the present setup, an interference contrast of 60 % should be observed with perfect alignment of the gratings. Time-of-flight measurements showed small modulations of the velocity distribution at for interference expected distances of 2.5 ms. We also show the possibility of reconstructing the Wigner distribution function out of measurements for the case of Talbot interference under experimental conditions.
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Guest, A. R. "The dynamic breakage of Kimberlite in the near field /." [St. Lucia, Qld.], 2004. http://www.library.uq.edu.au/pdfserve.php?image=thesisabs/absthe18507.pdf.
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Jia, Baohua. "A study on the complex evanescent focal region of a high numerical aperture objective and its applications." Australasian Digital Thesis Program, 2006. http://adt.lib.swin.edu.au/public/adt-VSWT20070205.150740/index.html.
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Thesis (PhD) - Swinburne University of Technology, Faculty of Engineering and Industrial Sciences, Centre for Micro-Photonics, 2005.A thesis submitted for the degree of Doctor of Philosophy, Centre for Micro-Photonics, Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, 2005. Typescript. Bibliography: p. 129-142.
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McNab, Sharee J. "Evanescent near-field optical lithography : overcoming the diffraction limit." Thesis, University of Canterbury. Electrical and Electronic Engineering, 2001. http://hdl.handle.net/10092/6655.
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Concepts of optical resolution limits have been transformed in the past two decades with the development of near-field optical microscopy. Resolutions of λ/40 have been demonstrated by taking advantage of additional information present the near field of an object. These resolutions are far higher than what diffraction-limited lens-based optical systems are capable of. Attempts have been made to replicate these resolutions for lithography using a scanning probe based optical equivalent, but these systems suffer from low throughput owing to their serial nature.
A desirable alternative would be replication of all the patterns within a field in a single flood exposure in a manner similar to how optical projection lithography replicates the field of a mask, but with the additional resolution available from working in the near field. This is the basis of evanescent near-field optical lithography, the subject of this thesis. Evanescent near-field optical lithography (ENFOL) brings traditional contact lithography into the near-near field using a combination of conformable masks and ultra-thin photoresists.
This thesis describes a study of ENFOL both experimentally and via electromagnetic simulations to evaluate what the resolution limit might be. The fabrication of membrane masks is described, a key component for the ENFOL exposure. The characteristics of an ENFOL exposure using broad-band light are investigated from exposures into thick resist. These exposures demonstrate the trend of decreasing depth of field as the period of grating structures is reduced.
ENFOL's requirement of a thin imaging photoresist for high resolution lithography complicates the pattern transfer step essential to translate the photoresist image into a useful material for devices. The development of an additive pattern transfer process is described, that utilises a trilayer resist scheme to enable lift-off metallisation. NiCr gratings with periods down to 270nm have been fabricated using this process subsequent to an ENFOL exposure. Wire-grid polarisers consisting of 270nm-period NiCr gratings on glass substrates have been fabricated and their polarisation properties measured at visible wavelengths.
Simulation results of exposures of sub-wavelength grating structures are presented that investigate the fundamental limit to resolution for contact lithography techniques such as ENFOL. A full-vector, rigorous electromagnetic simulation technique, the multiple multipole program is used to provide information about the near field of subwavelength gratings. The potential for λ/20 resolution is indicated; a tantalising prospect for optical lithography and well below the diffraction limit of conventional optical projection-based lithographies. Perhaps the most critical parameter for an evanescent exposure, the depth of field, was characterised and a linear relationship shown between the depth of field and grating period. The effect of parameters such as grating duty cycle, absorber material and thickness on the exposure are observed with the intention to optimise the experimental setup.
Interesting interference phenomena are observed in simulation results for exposures. where the effective exposure wavelength is equivalent to the grating period. In particular a period halving occurs in the transverse magnetic polarisation due to interference of the first diffracted orders. A novel interference technique - evanescent interference lithography is proposed that takes advantage of an enhanced period halving at an exposure wavelength corresponding to a grating resonance.
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Sedaghat, Zohreh. "A near-field study of the probe-sample interaction in near and mid-infrared nanoscopy." Troyes, 2012. http://www.theses.fr/2012TROY0019.
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Cette thèse s’intéresse aux conséquences de l’interaction sonde-échantillon sur la formation des images en microscopie de champ proche à sonde diffusante (s-NSOM). A cette fin, un modèle analytique couramment utilisé est détaillé avant de présenter une méthode électromagnétique plus rigoureuse basée sur l’emploi de fonctions de Green dyadiques. Dans ces deux approches, les diagrammes de rayonnement du système sonde-échantillon sont obtenus et la dépendance du signal aux permittivités de la sonde et de l’échantillon est discutée. Expérimentalement, deux systèmes NSOM ont été mis en oeuvre. Le premier dans l’IR moyen, le second dans le proche IR. Dans l’IR moyen, différents réseaux sub-λ ont été imagés avec des sondes de différentes tailles. Une nette inversion de contraste est observée en deçà d’une certaine taille de sonde. Ce résultat suggère que des résonances induites par la sonde ont été observées, en accord avec la valeur de la fonction diélectrique de la nano-sonde, calculée via le spectre détaillé des modes phononiques en centre de zone. Si dans l’IR moyen les fortes variations de permittivité sont propices aux cartographies d’indice, dans le proche IR une grande partie des applications NSOM concerne la caractérisation de composants optiques intégrés où le but est plutôt d’imager le champ se propageant dans ces composants. Dans une dernière partie nous développons l’utilisation d’une sonde hybride flexible et nous caractérisons un guide d’onde à échange d’ions. La réponse en polarisation de ces sondes développées est alors étudiéeThe probe-sample interaction in scattering type Near-field Scanning Optical Microscopy (s-NSOM) is investigated theoretically and experimentally. To understand this interaction, a commonly used analytical model is detailed as well as a more rigorous method based on a dyadic Green functions. In the simplest cases, both approaches give similar radiation patterns of the coupled probe-sample and the signal dependence to the probe and sample permittivities is then discussed. The case of tipenhanced resonances is notably highlighted. Experimentally, two s-NSOM setups were considered, one in the mid-IR spectral range and one in the near IR. In the former case, we have imaged several sub-λ gratings for different probe sizes. Interestingly, contrast inversions are obtained when the size is decreased suggesting that tip-induced resonances have been observed for the sharpest probe. Such behavior is in agreement with the probe permittivity value calculated via a model based on an elastic continuum medium theory recently developed to take into account phonon confinement. While in mid-IR, the large variation of the dielectric function induced by phonon resonances is favorable to refractive index mapping, in the near-IR most of the application is related to the characterization of integrated optical devices where the goal is mainly to image the field propagating inside the structures. Flexible hybrid probes have been fabricated and used to characterize an ion exchange waveguide. The polarization response of the nano-probes is studied experimentally. The Green technique is used to explain the observed trend
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Woolf, David Nathaniel. "Near-Field Optical Forces: Photonics, Plasmonics and the Casimir Effect." Thesis, Harvard University, 2013. http://dissertations.umi.com/gsas.harvard:10812.
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The coupling of macroscopic objects via the optical near-field can generate strong attractive and repulsive forces. Here, I explore the static and dynamic optomechanical interactions that take place in a geometry consisting of a silicon nanomembrane patterned with a square-lattice photonic crystal suspended above a silicon-on-insulator substrate. This geometry supports a hybridized optical mode formed by the coupling of eigenmodes of the membrane and the silicon substrate layer. This system is capable of generating nanometer-scale deflections at low optical powers for membrane-substrate gaps of less than 200 nm due to the presence of an optical cavity created by the photonic crystal that enhances both the optical force and a force that arises from photo-thermal-mechanical properties of the system. Feedback between Brownian motion of the membrane and the optical and photo-thermal forces lead to dynamic interactions that perturb the mechanical frequency and linewidth in a process known as ``back-action.'' The static and dynamic properties of this system are responsible for optical bistability, mechanical cooling and regenerative oscillations under different initial conditions. Furthermore, solid objects separated by a small distance experience the Casimir force, which results from quantum fluctuations of the electromagnetic field (i.e. virtual photons).The Casimir force supplies a strong nonlinear perturbation to membrane motion when the membrane-substrate separation is less than 150 nm. Taken together, the unique properties of this system makes it an intriguing candidate for transduction, accelerometry, and sensing applications.Engineering and Applied Sciences
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Shepherd, Micah Raymond. "The Effect of Nonlinear Propagation on Near-field Acoustical Holography." Diss., CLICK HERE for online access, 2007. http://contentdm.lib.byu.edu/ETD/image/etd2072.pdf.
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Li, Jianzhong. "Investigation on Near-field Source Localization and the Corresponding Applications." Thesis, Nantes, 2017. http://www.theses.fr/2017NANT4007/document.
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Mes travaux de recherche se sont focalisés sur le traitement d’antenne multi-capteurs et plus particulièrement sur la localisation de sources en champ proche. La localisation de sources a pour objectif d’estimer les paramètres de position des sources. Quand les sources sont proches du réseau de capteurs (situation de champ proche), le front d’onde du signal est sphérique et deux paramètres sont alors nécessaires pour localiser les sources : la direction d’arrivée et la distance entre la source et le réseau de capteurs. Tout d’abord, trois nouvelles méthodes à sous-espace basées sur les statistiques d’ordre supérieur ont été proposées. La première proposition est basée sur une matrice cumulant (du quatrième ordre) non- Hermitienne. Cette méthode permet d’estimer séparément les DDA et les distances avec une seule matrice spécifique et une seule décomposition en éléments propres. Ensuite, nous introduisons dans cette méthode, le principe des méthodes à sous-espace linéaires. Enfin, nous avons proposé d’agrandir virtuellement l’ouverture du réseau de capteurs afin d’améliorer la résolution et la précision dans l’estimation de la distance. Dans un second temps, une nouvelle méthode CS basée sur les statistiques d’ordre supérieur a été proposée. Les simulations ont montré que la méthode proposée possédait une meilleure résolution et une plus grande précision que les méthodes traditionnelles. Enfin, une méthode CS associée à une nouvelle méthode de réduction de bruit a été proposée pour mesurer les épaisseurs d’un milieu stratifié. Plusieurs simulations et une expérimentation ont montré l’efficacité de notre propositionSource localization is a key technology in array signal processing, which is widely applied in radar, geologic prospecting, sonar, electronic surveillance, medical electronics and other fields. Source localization can be classified into far-field source localization and nearfield source localization according to the distance between the sources and the array. Unlike the far-field situation, where each source is parameterized by only the DOA, the near-field signal wavefront is spherical, and both the DOAs and ranges are required to localize near-field sources. First, this dissertation concentrates on the improvements of the MUSIC-based method for near-field source localization. By making full use of the EVD, we make the proposal to estimate the DOAs and ranges in a decoupled way with only one matrix and one EVD. Then we propose a further improvement based on propagator methods. It allows to avoid the EVD and therefore leads to an even lower computational complexity. The third improvement is to increase the number of effective virtual sensors for the range estimation, which expands the aperture and achieves a notable improvement for the range estimation accuracy. In order to apply CS to near-field source localization, we propose a high-order CS method with a pairing step based on clustering. The proposed method can achieve better accuracy and resolution than traditional methods. This work ends with the application of GPR. An enhanced CS method is proposed to carry out the TDE directly in low SNR scenario
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Bechtel, Kyle A. "MAGNETIC RESONANCE IN THE PROXIMITY OF AN INSTABILITY: PERPENDICULAR RESONANCE IN PERMALLOY NEAR THE CRITICAL FIELD." Oxford, Ohio : Miami University, 2009. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=miami1250281122.
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Wall, Alan Thomas. "The Characterization of Military Aircraft Jet Noise Using Near-Field Acoustical Holography Methods." BYU ScholarsArchive, 2013. https://scholarsarchive.byu.edu/etd/7418.
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The noise emissions of jets from full-scale engines installed on military aircraft pose a significant hearing loss risk to military personnel. Noise reduction technologies and the development of operational procedures that minimize noise exposure to personnel are enhanced by the accurate characterization of noise sources within a jet. Hence, more than six decades of research have gone into jet noise measurement and prediction. In the past decade, the noise-source visualization tool near-field acoustical holography (NAH) has been applied to jets. NAH fits a weighted set of expansion wave functions, typically planar, cylindrical, or spherical, to measured sound pressures in the field. NAH measurements were made of a jet from an installed engine on a military aircraft. In the present study, the algorithm of statistically optimized NAH (SONAH) is modified to account for the presence of acoustic reflections from the concrete surface over which the jet was measured. The three dimensional field in the jet vicinity is reconstructed, and information about sources is inferred from reconstructions at the boundary of the turbulent jet flow. Then, a partial field decomposition (PFD) is performed, which represents the total field as the superposition of multiple, independent partial fields. This is the most direct attempt to equate partial fields with independent sources in a jet to date.
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Hakim, Bandar M. "Precise SAR measurements in the near-field of RF antenna systems." College Park, Md. : University of Maryland, 2006. http://hdl.handle.net/1903/3532.
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Thesis (Ph. D.) -- University of Maryland, College Park, 2006.Thesis research directed by: Electrical Engineering. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.
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Aghaei, Morteza. "Near maximum distance separable codes over the field of eleven elements." Thesis, University of Sussex, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.418400.
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McCormick, John. "The use of secondary spatial transforms in near field antenna measurements." Thesis, Open University, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.311859.
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BRANDAO, MARCELO LYRA. "EFFECTS OF OBSTACLES IN THE NEAR-FIELD REGION OF CASSEGRAIN ANTENNAS." PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO, 1985. http://www.maxwell.vrac.puc-rio.br/Busca_etds.php?strSecao=resultado&nrSeq=14269@1.
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A presente tese estudará as modificações introduzidas nos diagramas de radiação de antenas cassegrain, em decorrência da colocação de obstáculos, como cercas, na região de campos próximos.
A análise destas modificações será baseada na Teoria Geométrica da Difração. Definida a geometria do problema, serão desenvolvidos algoritmos para determinação dos pontos de reflexão e difração e, a partir destes pontos, calculados os campos espalhados. A eficiência de diversas configurações de obstáculos, na redução dos campos irradiados em certas direções, será analisada.This work will study the modifications introduced on cassegrain antenna radiation patterns, by putting obstacles, such as fences, in the near-field zone. The analisys of these modifications will be supported on the uniform Geometrical Theory of Diffraction. Defined the geometry of the problem, a mathematical model will be developed for the determination of reflection and diffraction points and, following these, the scattered fields will be computed. The performance of the antenna in the presence of obstacle will be analised.
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Dennis, Ian Robert Howard. "Passive ranging of near field targets in the audio frequency band." Thesis, University of St Andrews, 1997. http://hdl.handle.net/10023/13610.
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The use of passive ranging techniques for imaging targets situated in the near field for the audio frequency band is presented in this thesis. A three port interferometer used in millimetre waves is introduced and it is shown that with some approximations, it can be used to passively image targets in the medium-far field. The geometry of this system is then adapted for use in the near field in the audio frequency band, without the need for making any approximations. A model is then constructed to demonstrate the theory. The design, construction and testing of the new system for use in an anechoic chamber is then presented. This thesis also introduces a number of digital signal processing techniques that take full advantage of the recent increase in computing power and decrease in costs. These techniques are then used to perform frequency analysis on real data obtained experimentally from the new three port system. A number of different experiments and results are then presented. A loudspeaker is used as a test source and it is shown that the two separate drive units can be distinguished very accurately with the system. The whole system is then fully characterized and the optimal system setup conditions are found. Finally some ideas for future work and possible applications are presented. It is shown that the system would make an ideal tool for testing the spatial coherence properties of loudspeakers.
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Alexander, William Nathan. "Normalization of Roughness Noise on the Near-Field Wall Pressure Spectrum." Thesis, Virginia Tech, 2009. http://hdl.handle.net/10919/33643.
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Roughness noise can be a significant contributor of sound in low Mach number, high Reynolds number flows. Only a small amount of experimental research has been conducted to analyze roughness noise because of its often low energy levels that are hard to isolate even in a laboratory setting. This study details efforts to scale the roughness noise while independently varying roughness size and edge velocity. Measurements were taken in the Virginia Tech Anechoic Wall Jet Facility for stochastic rough surfaces varying from hydrodynamically smooth to fully rough as well as deterministic rough surfaces including 1mm and 3mm hemispheres and a 2D wavy wall. Inner and outer variable normalizations were applied to recorded far field data in an attempt to find specific driving variables of the roughness noise. Also, a newly formulated derivation that attempts to scale the far field sound from a single point wall pressure measurement was used to collapse the far field noise. From the results, the inner and outer variable scalings were unable to collapse the noise generated by all velocities and roughness sizes. The changing spectral shapes of noise generated by rough surfaces with significantly varying wavenumber spectra make it impossible to scale the produced noise using the proposed inner and outer variable scalings. They use only one a single scaling value for the entire frequency range of each spectrum. The analyzed wall pressure normalization, which is inherently frequency dependent, produces a tight collapse within the uncertainty of the measurements for all rough surfaces studied except the larger hemispherical roughness which had individual elements that dominated the surrounding region of the wall pressure microphone. This indicates that the roughness generated noise is directly proportional to the wall pressure spectrum. The collapsed data displayed a slope of Ï ^2, the expected dipole efficiency factor. This is the clearest confirmation to date that the roughness noise source is of a dipole nature.Master of Science
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Desabrais, Kenneth J. "Velocity field measurements in the near wake of a parachute canopy." Link to electronic thesis, 2002. http://www.wpi.edu/Pubs/ETD/Available/etd-0426102-134658.
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Thesis (Ph. D.)--Worcester Polytechnic Institute.Keywords: parachute shedding characteristics; near wake evolution; parachute inflation; canopy breathing; velocity field measurements. Includes bibliographical references (p. 126-131).
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Shu, Qingying. "Statistical modelling of the near-Earth magnetic field in space weather." Thesis, University of Glasgow, 2018. http://theses.gla.ac.uk/8937/.
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Space weather refers to electromagnetic disturbances in the near-Earth environment as a result of the Sun-Earth interaction. Severe space weather events such as magnetic storms can cause disruption to a wide range of technologies and infrastructure, including communications systems, electronic circuits and power grids. Because of its high potential impact, space weather has been included in the UK National Risk Register since 2011. Space weather monitoring and early magnetic storm detection can be used to mitigate risk in sensitive technological systems. The aim of this project is to investigate the electromagnetic disturbances in the near-Earth environment through developing statistical models that quantifies the variations and uncertainties in the near-Earth magnetic field. Data of the near-Earth magnetic field arise from in-situ satellite measurements and computer model outputs. The Cluster II mission (Escoubet et al., 2001a) has four satellites that provide in-situ measurements of the near-Earth magnetic field at time-varying locations along their trajectories. The computer model consists of an internal part that calculates the magnetic field sourced from Earth itself and an external part that estimates the magnetic field resulting from the Sun-Earth interaction. These magnetic fields, termed as the internal field and the external field, add up to the total magnetic field. Numerical estimates of the internal field and the external field are obtained respectively from the IGRF-11 model (Finlay et al., 2010) and the Tysganenko-96 (T96) model (Tsyganenko, 2013) given the times and the locations as inputs. The IGRF model outputs are invariant to space weather conditions whereas the T96 model outputs change with the input space weather parameters. The time-varying space weather parameters for T96 model include the solar wind ram pressure, the y and the z-components of the interplanetary magnetic field, and the disturbance storm time index. These parameters are the estimated time series of the solar wind conditions at the magnetopause, i.e. the boundary of the magnetosphere on the day-side, and the disturbance level at the Earth’s surface. Real-time values of the T96 model input parameters are available at hourly resolution from https://omniweb.gsfc.nasa.gov/. The overall aim of the thesis is to build spatio-temporal models that can be used to understand uncertainties and constraints leveraged from 3D mathematical models of space weather events. These spatio-temporal models can be then used to help understand the design parameters that need to be varied in building a precise and reliable sensor network. Chapter 1 provides an introduction to space weather in terms of the near-Earth magnetic field environment. Beginning with an overview of the near-Earth magnetic field environment, Chapter 2 describes the sources for generating in-situ satellite measurements and computer model outputs, namely the Cluster II mission, the IGRF model, and the T96 model. The process of sampling the magnetic field data from the different data sources and the space-time dependence in the hourly sampled magnetic field data are also included in this Chapter. Converting the space-time structure in the magnetic field data into a time series structure with a function relating the position in space to time, Chapter 3 explores the temporal variations in the sampled in-situ satellite measurements. Through a hierarchical approach, the satellite measurements are related to the computer model outputs. This chapter proposes statistical methods for dealing with the non-stationary features, temporal autocorrelation, and volatility present in the time series data. With the aim of better characterising the electromagnetic environment around the Earth, Chapter 4 develops time-series models of the near-Earth magnetic field utilising in-situ (CLUSTER) magnetic field data. Regression models linking the CLUSTER satellite observations and two physical models of the magnetic field (T96 and IGRF) are fit to each orbit in the period 2003-2013. The time series of model parameter estimates are then analysed to examine any long term patterns, variations and associations to storm indices. In addition to explaining how the two physical models calibrate with the observed satellite measurements, these statistical models capture the inherent volatility in the magnetic field, and allow us to identify other factors associated with the magnetic field variation, such as the relative position of each satellite relative to the Earth and the Sun. Mixed-effect models that include these factors are constructed for parameters estimated from the regression models for evaluating the performance of the two computer models. Following the calibration of the computer models against the satellite measurements, Chapter 5 investigates how these computer models allow us to investigate the association between the variations in near-Earth magnetic field and storms. To identify the signatures of storm onsets in different locations in the magnetosphere, change-point detection methods are considered for time series magnetic field signals generated from the computer models along various feasible satellite orbits. The detection results inform on potential sampling strategies of the near-Earth magnetic field to be predictive of storms through selecting achievable satellite orbits for placing satellite sensors and detecting changes in the time series magnetic signals. Chapter 6 provides of a summary of the main finding within this thesis, identifies some limitations of the work carried out in the main chapters, and include a discussion of future research. An Appendix provides details of coordinate transformation for converting the time and position dependent magnetic field data into an appropriate coordinate system.
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Eftekhar, Ali Asghar. "Nanoscale light-matter interactions in the near-field of high-Q microresonators." Diss., Georgia Institute of Technology, 2011. http://hdl.handle.net/1853/45900.
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The light-matter interaction in the near-field of high-Q resonators in SOI and SiN platforms is studied. The interactions of high-Q traveling-wave resonators with both resonant and non-resonant nanoparticles are studied and different applications based on this enhanced interactions in near-field such as high-resolution imaging of mode profile of high-Q resonators, label-free sensing, optical trapping, and SERS sensing are investigated. A near-field imaging system for the investigation of the near-field phenomena in the near-field of high-Q resonators is realized. A new technique for high-resolution imaging of the optical modes in high-Q resonators based on the near-field perturbation is developed that enables to achieve a very high resolution (< 10 nm) near-field image. The prospect of the high Q resonators on SOI platform for highly multiplexed label-free sensing and the effect of different phenomena such as the analyte drift and diffusion and the binding kinetics are studied. Also, the possibility of enhancing nanoparticle binding to the sensor surface using optical trapping is investigated and the dynamic of a nanoparticle in the high-Q resonator optical trap is studied. Furthermore, the interaction between a resonant nanoparticle with a high-Q microdisk resonator and its application for SERS sensing is studied. A model for interaction of resonant nanoparticles with high-Q resonators is developed and the optimal parameters for the design of coupled microdisk resonator and a plasmonic nanoparticle are calculated. The possible of resonant plasmonic nanoparticle trapping and alignment in an SiN microdisk resonator optical trap is also shown.
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Brown, Wayne. "Characterisation of low energy ions observed near and during the substorm expansion phase onset." Thesis, University of Warwick, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.302621.
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Coelho, Sergio Luis Villares. "The dynamics of the near field of entraining jets in cross-flows." Thesis, University of Cambridge, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.292919.
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Singer, Farah. "Influence of the nonlocal effects on the near-field radiative heat transfer." Thesis, Poitiers, 2014. http://www.theses.fr/2014POIT2338.
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Dans ce mémoire de thèse, nous étudions la validité de quelques modèles non locaux de la permittivité diélectrique (PD) dans le calcul du coefficient de transfert de chaleur par rayonnement (CTCR) entre deux matériaux diélectriques, semi- infinies, plans et parallèles, et séparés par un espace vide de largeur d.Dans les études théoriques antérieures, il a été montré que lorsque l'on considère un modèle local de la PD, le transfert de chaleur par rayonnement en champ proche (TCRCP) suit une loi 1/d² quand d devient de l'ordre ou inférieure à quelques centaines de nanomètres. Cette divergence non physique constitue la faille majeure du modèle local. Plusieurs efforts ont été fournis afin de développer un nouveau modèle de la PD qui tient compte des effets nonlocaux. Aucune correction non locale pour le TCRCP n’a été abordée dans le passé dans le cas des diélectriques. Cependant dans le cas des métaux, un travail complet a été effectué en utilisant le modèle non local de Lindhard-Mermin de la PD.Nos travaux portent sur l'étude de quatre modèles différents de la PD nonlocale. Nous exploitons ces modèles pour le calcul du CTCR entre deux plans de 6H-SiC. Nous montrons que le CTCR sature quand d tend vers zéro. La distance du début de saturation dépend grandement des paramètres clés de chaque modèleIn this thesis, we study the validity of few nonlocal models of the dielectric permittivity in the calculation of the radiative heat transfer coefficient (RHTC) between two semi-infinite parallel dielectric planes separated by a vacuum gap of width d.In past theoretical studies, it has been shown that upon considering a local model of the dielectric permittivity, near field radiative heat transfer (NFRHT) between two dielectric materials follows a 1/d2 law when d is of the order or less than few hundreds of nanometers. This nonphysical diverging increase has been the bottleneck of the local model. Overwhelming efforts have been deployed in order to come up with a new model in which the nonlocal effects of the dielectric permittivity are taken into account. To the best of our knowledge, no nonlocal correction to the NFRHT has been addressed in the past in the case of dielectrics. In the case of metals however, an important and complete work has been performed using the Lindhard-Mermin nonlocal dielectric permittivity model.Our work focuses on studying four different nonlocal models of the dielectric permittivity and on using them in the calculation of the RHTC between two solid semi-infinite parallel planes of 6H-SiC. We show that the RHTC saturates as the separation distance d tend to zero. The distance at which saturation starts to take place depends on key parameters involved in each model
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Bird, Jonathan C. "The Formation and Evolution of Disk Galaxies." The Ohio State University, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=osu1345571232.
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Laroussi, Rédouane. "Far-field predictions from near-field measurements using the three dimensional finite element and boundary integral methods." Thesis, University of Ottawa (Canada), 1992. http://hdl.handle.net/10393/7861.
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Characterizing antenna designs in their operational modes has been a classic problem for electromagnetic engineers for many years. The far-field pattern is of great importance in antenna characterization, but in many cases, it is impossible or at least impractical to measure directly. This thesis presents a numerical technique whereby near-field measurements can be transformed into far-field solutions. The finite element method (FEM), and an integral formulation, solution to the scattering problem, are used separately to solve the open boundary wave propagation problem in three dimensions. The near-field of a known source is sampled over a mesh of points and used as data for both methods. The integral formulation yields the far-field solution for homogeneous problems without the need for space discretization. The finite element method uses tetrahedral elements for discretizing the domain of interest and second order interpolation functions. It is complemented by the application of an absorbing boundary condition (ABC) which allows truncation of the domain. Classical ABCs of the local type are applied to the three dimensional problem. A new non-local ABC is obtained from the integral formulation and is coupled to the finite element approach. Computed field strengths obtained with the different approaches are compared with the analytical results obtained from the excitation source's analytical expression. The agreement was very good with maximum error percentages of about 10% for the FEM method and 2.5% for the integral formulation.
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Pavel, Michael D. "Magnetic fields in the milky way: near-infrared polarimetry." Thesis, Boston University, 2013. https://hdl.handle.net/2144/12831.
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Thesis (Ph.D.)--Boston UniversityAstronomers have a limited understanding of the large-scale structure of the Galactic magnetic field and its role in the evolution of the interstellar medium (ISM). This understanding derives primarily from Faraday rotation and synchrotron observations which do not probe the cool, dusty ISM. To advance our knowledge of the Galactic magnetic field, this dissertation reports on the application of a different method, near-infrared (NIR) polarization of background starlight, to place new observational constraints on the nature of the Galactic magnetic field and to study the field's role in the evolution of interstellar material. A radiative transfer computer code was developed to predict all-sky starlight polarization observations. Starlight polarimetry predictions were made for several different dynamo-driven magnetic field geometries, assuming that magnetically-aligned interstellar dust grains polarize background starlight. New NIR starlight polarimetry measurements in the outer Galaxy were tested against these predictions. These observations favor disk-symmetric magnetic fields while rejecting disk-antisymmetric magnetic fields. This result contradicts some previous interpretations of all-sky, radio Faraday rotation measurements. The Galactic magnetic pitch angle is constrained to p = -6 ± 2°. The physical orientations of Galactic HII regions, traced by mid-infrared emission, are compared to the large-scale, disk-symmetric Galactic magnetic field geometry derived above. Hydrogen recombination line spectra towards these same objects revealed that many possessed turbulent linewidths. If fluid turbulence decays with time, then it may be used as a relative age indicator. A trend is seen between magnetic alignment and the degree of turbulence in the HII region. This result leads to the development of an observationally-driven HII region magnetic evolutionary sequence. Resolved polarimetry across the face of the galaxy M51 was measured for comparison with the internal, edge-on view of the Milky Way seen from Earth. Strong upper limits ( < 0.05% at a resolution of 0.6 arcseconds) were placed on the degree of NIR polarization across the face of M51. These results were combined with resolved optical polarimetry measurements from the literature. Normal polarization mechanisms cannot explain the observed polarization dependence on wavelength.
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Mcnaughton, James. "Turbulence modelling in the near-field of an axial flow tidal turbine in Code_Saturne." Thesis, University of Manchester, 2013. https://www.research.manchester.ac.uk/portal/en/theses/turbulence-modelling-in-the-nearfield-of-an-axial-flow-tidal-turbine-in-codesaturne(54c9b513-2472-4c61-a521-8ef73428465b).html.
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This Thesis presents simulation of flow past laboratory-scale and full-scale tidal stream turbines (TST) using EDF's open-source CFD solver Code_Saturne. The work shows that detailed results may be obtained with confidence and that greater information on the loading and wake structure is available than other methods, such as blade element momentum theory.Results are obtained using a new sliding-mesh method that has been implemented in Code_Saturne as part of this work. The sliding-mesh method uses internal Dirichlet boundary conditions with values on the interface prescribed via a halo-point method. Parallel performance is optimised by a carefully-chosen method of exchanging information between specific processes. Validation is provided for flow past a rotating cylinder and a sphere.For the laboratory-scale TST, Reynolds-Averaged Navier-Stokes models are used to model turbulence. The k-omega-SST and Launder-Reece-Rodi (LRR) models yield good agreement with experimental values of power and thrust coefficients as a function of tip-speed ratio (TSR). The standard k-epsilon model is shown to perform poorly due to an overprediction of turbulent kinetic energy upstream of the rotor plane. The k-omega-SST model is then used to examine wake behaviour for parametric studies of turbulence intensity and TSR. Increased turbulence levels are shown to reduce the downstream propagation of the wake because of increased mixing. The near wake is influenced by the TSR, whilst the far wake is independent of TSR.The predicted effect of tidal conditions typical of the EMEC test site are considered for flow past Tidal Generation Limited's 1MW TST. The effect of sheared-velocity profiles leads to an increase in loading on an individual turbine blade at the point of a rotation where velocity shear is greatest. The effect of increased yaw angle leads to large fluctuations of the power coefficient, but smaller fluctuations of the thrust coefficient. Mean values of thrust and power decrease as a function of the cosine of the yaw angle and yaw angle squared respectively.
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Kwok, Sai Kit. "The investigation of near field couplings between circuit elements on dielectric boards /." access full-text access abstract and table of contents, 2005. http://libweb.cityu.edu.hk/cgi-bin/ezdb/thesis.pl?phd-ee-b1988736xa.pdf.
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Thesis (Ph. D.)--City University of Hong Kong, 2005."Submitted to Department of Electronic Engineering in partial fulfillment of the requirements for the degree of Doctor of Philosophy." Includes bibliographical references (leaves 115-128).
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Artvin, Zafer. "Fabrication Of Nanostructured Samples For The Investigation Of Near Field Radiation Transfer." Master's thesis, METU, 2012. http://etd.lib.metu.edu.tr/upload/12614826/index.pdf.
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Radiative heat transfer in nanostructures with sub-wavelength dimensions can exceed that predicted by Planck's blackbody distribution. This increased effect is due to the tunneling of infrared radiation between nanogaps, and can allow the eventual development of nano-thermo-photo-voltaic (Nano-TPV) cells for energy generation from low temperature heat sources. Although near field radiation effects have been discussed for many years, experimental verification of these effects is very limited so far. In this study, silica coated silicon wafer sample chips have been manufactured by using MEMS fabrication methods for testing the near field radiation effects. A variety of samples with 1×
1, 2×
2 and 5×
5 mm2 area, and with 25 nm, 50 nm, 100 nm and 200 nm (nano-gap) separations have been prepared. 3D structures with vacuum gaps have been obtained by bonding of the silica coated wafers. The samples have been tested in an experimental setup by a collaborative group at Ö
zyegin University, Istanbul. An increase in the net radiation heat transfer with decreasing nano-gap size has been reported by the Ö
zyegin group who used these samples in a parallel study. The thesis outlines the micro-fabrication techniques used for the sample preparation. Also, the manufacturing problems we have faced during this research program are discussed.
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Yoxall, Edward. "Applications of scattering-type scanning near-field optical microscopy in the infrared." Thesis, Imperial College London, 2013. http://hdl.handle.net/10044/1/23637.
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This thesis is split into two broad sections. These are defined by the various applications of scattering-type near-field optical microscopy (s-SNOM) in different parts of the electromagnetic spectrum; the near-infrared (700 - 1000nm) and the mid-infrared (6 - 10um). S-SNOM is a means of imaging surfaces at resolutions well below the diffraction limit - the level of recorded detail does not depend on the wavelength of light (as it does with traditional optical microscopy), but instead on the sharpness of a probe (usually around 10nm), meaning an image resolution approaching a thousandth of a wavelength in the mid-infrared. For the work presented in the near-infrared, the focus lies with the modelling and mapping of various plasmonic resonances supported by metallic nanostructures. These resonances have the ability to "squeeze" light into substantially sub-wavelength volumes which is useful for a variety of applications ranging from cancer treatments to molecular sensing. The mid-infrared section starts with the implementation of a pulsed quantum cascade laser (QCL) as the system's light source. This presents some instrumentation challenges as all s-SNOM imaging to date has been conducted with continuous-wave (CW) lasers. Using a pulsed laser also raises some significant signal-to-noise implications which are quantified and discussed. In terms of the experimental applications of such a setup, the first steps towards ultra-high resolution infrared chemical spectroscopy are made by studying the epithelial cells of an oesophageal biopsy. The thesis concludes with an examination of the major noise sources faced by s-SNOM, and makes a number of recommendations on how their effects can be mitigated.
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Zhang, Zihao. "Investigating the far- and near-field thermal radiation in carbon-based nanomaterials." Diss., Georgia Institute of Technology, 2015. http://hdl.handle.net/1853/54433.
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Two classes of carbon nanomaterials—carbon nanotubes and graphene—have promoted the advancement of nanoelectronics, quantum computing, chemical sensing and storage, thermal management, and optoelectronic components. Studies of the thermal radiative properties of carbon nanotube thin film arrays and simple graphene hybrid structures reveal some of the most exciting characteristic electromagnetic interactions of an unusual sort of material, called hyperbolic metamaterials. The features and results on these materials in the context of both far-field and near-field radiation are presented in this dissertation.
Due to the optically dark nature of pyrolytic carbon in the wavelength range from visible to infrared, it has been suggested vertically aligned carbon nanotube (VACNT) coatings may serve as effective radiative absorbers. The spectral optical constants of VACNT are modeled using the effective medium theory (EMT), which is based on the anisotropic permittivity components of graphite. The effects of other EMT parameters such as volume filling ratio and local filament alignment factor are explored. Low reflectance and high absorptance are observed up to the far-infrared and wide range of oblique incidence angles. The radiative properties of tilt-aligned carbon nanotube (TACNT) thin films are illustrated. Energy streamlines by tracing the Poynting vectors are used to show a self-collimation effect within the TACNT thin films, meaning infrared light can be transmitted along the axes of CNT filaments.
Graphene, a single layer sheet of carbon atoms, produces variable conductance in the terahertz frequency regime by tailoring the applied voltage gating or doping. Periodically embedding between dielectric spacers, the substitution of graphene provides low radiative attenuation compared to traditional metal-dielectric multilayers. The hyperbolic nature, namely negative angle of refraction, is tested on the graphene-dielectric multilayers imposed with varying levels of doping. EMT should be valid for graphene-dielectric multilayers due to the nanometers-thick layers compared to the characteristic wavelength of infrared light. For metal- or semiconductor-dielectric multilayers with thicker or lossier layers, EMT may not hold. The validity of EMT for these multilayers is better understood by comparing against the radiative properties determined by layered medium optics.
When bodies of different temperatures are separated by a nanometers-size vacuum gap, thermal radiation is enhanced several-fold over that of blackbodies. This phenomenon can be used to develop more efficient thermophotovoltaic devices. Due to their hyperbolic nature, VACNT and graphite are demonstrated to further increase evanescent wave tunneling. The heat flux between these materials separated by vacuum gaps smaller than a micron is vastly improved over traditional semiconductor materials. A hybrid structure composed of VACNT substrates covered by doped graphene is analyzed and is shown to further improve the heat flux, due to the surface plasmon polariton coupling between the graphene sheets.
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Allinson, Sarah Jane. "Investigation of inorganic colloids in the near-field of a waste repository." Thesis, Loughborough University, 2004. https://dspace.lboro.ac.uk/2134/36074.
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The major concern in nuclear waste management is its safe disposal. Models are used to predict the long-term fate and mobility of radionuclides and other contaminants through the environment. Information from these models is used to assess the long-term safety of a waste disposal site. To accurately predict the fate of contaminants these models must contain all the parameters that affect contaminant transport through the geosphere. Colloids can be found in all natural surface and ground waters and have large surface areas onto which contaminants can sorb. They have been identified as being potentially important in the transport of radionuclides in groundwater and could significantly affect any performance assessment of a repository. Colloid mediated transport is generally not included in the predictive models although their influence could be significant. This report describes firstly the results of a groundwater sampling campaign undertaken at the Drigg site in Cumbria which focussed on the colloidal content of the waters and their associated radionuclides.
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Barton, Catherine Arundel. "The measurement, partitioning and near-field modeling of perfluorooctanoate (PFO) in air." Access to citation, abstract and download form provided by ProQuest Information and Learning Company; downloadable PDF file, 191 p, 2008. http://proquest.umi.com/pqdweb?did=1601522501&sid=9&Fmt=2&clientId=8331&RQT=309&VName=PQD.
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Moulton, Carey Lloyd. "The Near Field Acoustics of the Active Noise Control of a Model Propeller." NCSU, 2000. http://www.lib.ncsu.edu/theses/available/etd-20000726-233230.
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The Near field properties of a 6" radius model propeller, and the Active Noise Control(ANC) of the Blade Passing Tone (BPT) have been studied. The study has been careful toidentify and separate the effects of the hydrodynamic near field and the geometric near field. The near and far field radiation regions of a model propeller were consideredseparately. Firstly to determine if far field control affects the near field noise levels and secondly to determine how the far field noise levels are affected by near field control. It has been found that active noise control (ANC) of the propeller blade passing tone (BPT)can be achieved within the geometric near field, however the extent of the effective control is small. The near field optimized ANC can result in as much as 10.5 dB ofreduction of the BPT, however this noise reduction can only be achieved over a small spatial region. For this experimental arrangement there is no effective ANC directlybehind the propeller. Whenever ANC is optimized for near field noise it results in far field noise levels of up to 15.2 dB above the uncontrolled far field propeller levels. The far field optimized control shows no detrimental effects in the near field noise environment.