Dissertations / Theses on the topic 'Photon emission'

Semiconductor quantum dots are promising sources for polarization-entangled photons. As an alternative to the usual cascaded biexciton-exciton emission, direct two-photon emission from the biexciton can be used. With a high-quality optical resonator tuned to half the biexciton energy, a large proportion of the photons can be steered into the two-photon emission channel. In this case the degree of polarization entanglement is inherently insensitive to the exciton fine-structure splitting. In the present work we analyze the biexciton emission with particular emphasis on the influence of coupling of the quantum-dot cavity system to its environment. Especially for a high-quality cavity, the coupling to the surrounding semiconductormaterial can open up additional phonon-assisted decay channels. Our analysis demonstrates that with the cavity tuned to half the biexciton energy, the potentially detrimental influence of the phonons on the polarization entanglement is strongly suppressed-high degrees of entanglement can still be achieved. We further discuss spectral properties and statistics of the emitted twin photons.

Emission computed tomography involves external measurements of gamma photons emitted from within the object under investigation in order to map the radioactive distribution into a two-dimensional array within a slice of interest. Both positron emission tomography (PET) and single photon emission computed tomography (SPECT) constitute the two types of emission computed tomography. PET and SPECT differ radically in almost every aspect of system design; radionuc1ide employed, radiation detectors and arrangement, collimation (electronic, mechanical), processing electronics as well as data acquisition, handling and correction. A prototype scanning-rig incorporating two collimated BOO scintillation detectors has been used to carry out PET experiments utilising 6SOe line sources (positron-emitter) and a perspex phantom of 50-mm in diameter to simulate a small animal i.e. a rat's head. Modifications for the experimental scanning-rig allowed the collection of the singles events in the PET studies in such a way that they could be reconstructed to provide SPECT images for the radioactive distribution under investigation. This property allowed a simultaneous collection of PET and SPECT data for the same object under exactly the same conditions. Two data sets are generated from each tomographic experiment; one is for PET and the other is for SPECT. Each data set is corrected separately for the required corrections i.e. scattering and attenuation before reconstruction, and then two images are produced for each study. The outcome from this work is the comparison between the two images of PET and positron SPECT obtained. The line spread function curves taken for various depths and the image profiles for studies in air and perspex show that PET provides better spatial resolution than positron SPECT. This property of PET is further confirmed by the MTF curves and the fidelity test. Using a collimation aperture of 3- mm wide, the spatial resolution values in air were found to be 3.2 +/- 0.45 mm and 7.4 +/- 0.45 mm FWHM for PET and SPECT respectively. The images of the two line sources with a 10-mm centre-to-centre separation are partially resolved in the SPECT images whereas a sufficient separation between the two sources is achieved in PET. Image combination has been applied in order to obtain a hybrid image which contains the advantages from both PET and SPECT. A straightforward averaging and multiplication of the two images of PET and SPECT were found useful to provide images with enhanced quality. The multiplication process provided images with significantly improved quality for the PE T images. When evaluating the image quality of the line source in air, the fidelity test values are 0.71 and -1.11 for PET and SPECT respectively. The image combination resulted in an image with fidelity values of 0.92 when the two images are multiplied and 0.12 when their averaging was obtained.

An iterative folding method was used to unfold a 6 MV and 18 MV photon en— ergy spectra from depth dose measurements. By developing a response matrix that included photon beam attenuation in water, electron disequilibrium and contamination, and phantom scatter factors; we unfolded the spectra from a Siemens Primus 6 MV and 18 MV radiotherapy photon beams. The unfolding algorithm was initially applied to open fields, where we looked at five different field sizes for each energy. Their resulting spectra were then analyzed, our results showing a broadening of the beam spectra with increasing field size, and an overall increase in the spectral mean energy as a function of field size for both energies. This was attributed, in part, to the increased collimator scatter, SC, arising from the changing field size and the dependence of the amount of flattening filter area exposed on the field size setting. Next we applied the unfolding algorithm to hard and soft wedges for three field sizes, and five different wedge angles. The response matrix had to be modified, adding an extra factor to account for wedge scatter. The results for the hard wedges showed that though effects such as flattening filter and collimator scatter are still present in the wedged spectra, they are dominated by the attenuation properties of the hard wedges, with the wedge angle and field size determining the spectral distribution. For the soft wedges, which are formed via a combination of dose rate adjustment and jaw movement rather than the attenuation of the beam, the results were similar to those of the hard wedges. The field size dominated the spectral distribution, with wedge angle exerting no major effect. For both wedge modes, the spectral change along the wedge gradient axis was examined. It was found that there was a considerable shift in the spectrum from the thin end of the wedge to the thick end. For the hard wedge this was expected, but a similar result was noted for the soft wedge. The effective change in field size in the formation of the soft wedge was noted as the cause of this effect. The unfolding algorithm was also applied to carbon fiber inserts. These devices are routinely used in radiotherapy, and are known to increase surface dose to patients. By analyzing the spectrum of a 6 MV beam attenuated by a carbon fiber insert, we were able to surmise that the increased dose was due in part to a build-up effect, and partially due to a biased attenuation of the linac head—scatter by the carbon fiber insert. Noting that thermoluminescent dosimeters, TLDs, are routinely used for in vivo measurements with soft wedges, we analyzed the effect of the changing spectrum of a soft wedge along the gradient axis on TLDs. By using the unfolded spectrum with EGSnrc Monte Carlo, we were able to show that the TLDs did over respond when measuring dose of a soft wedge. This was due to the changing spectrum of the soft wedge, and was contradictory to common belief concerning soft wedges.

L’étude de l’interaction entre la lumière et la matière n’a cessé de susciter un intérêt croissant au fil des années. L’amélioration des techniques de fabrication des cavités électromagnétiques permet aujourd’hui de coupler ces cavités à des nanocircuits, se faisant, combinant les champs de l’optique quantique et de la nanoélectronique. À cela s’ajoute enfin la démonstration expérimentale de la possibilité d’utiliser un microscope à effet tunnel comme cavité plasmonique couplée au transport électronique. Cette thèse propose un cadre théorique basé sur l’électrodynamique quantique en cavité, permettant l’étude du couplage entre le transport électronique dans une jonction moléculaire et le champ électromagnétique d’une cavité. L’attention est portée sur le régime de transfert tunnel séquentiel des électrons, auquel est adapté l’utilisation les calculs basés sur l’usage de la matrice densité. Ce régime permet d’établir les equations maîtresses régissant l’évolution temporelle de la matrice densité, ainsi qu’un schéma de calcul numérique du courant électronique et des propriétés statistiques des photons dans la cavité quand il n’est pas possible d’obtenir un résultat analytique. Dans un premier temps, l’attention est portée sur un modele de jonction moléculaire à une orbitale. En effet, l’existence d’un courant électronique signifie que la charge de la molécule fluctue et cette fluctuation se couple au champ électromagnétique de la cavité. L’étude de ce premier système est faite dans le régime, expérimentalement pertinent, de fort taux d’amortissement κ ≥ kBT du mode de la cavité et de couplage lumière–matière arbitrairement élevé. Ce modèle met en évidence l’équivalence du couplage électron–photon et du couv plage électron–phonon pour un unique niveau électronique. Ce couplage électron–phonon est étudié depuis longtemps en nanoélectronique sous le nom de principe Franck–Condon. La caractéristique courant–tension du circuit fait apparaitre une évolution par paliers ou seuils inelastiques, chacun séparé par l’énergie d’un photon. Ce phénomène correspond à une dissipation d’énergie, par émission de photons dans la cavité, médiée par le courant électronique. Pour cette étude, une formule du courant électronique prenant en compte l’effet de l’amortissement de la cavité(facteur de qualité Q ≈ 10) a été dérivée. Cela a permis de montrer que la largeur des sauts du courant est contrôlée par κ plutôt que la température. Ce modèle démontre la possibilité d’obtenir divers régimes d’émission de lumière par passage de courant au sein de la jonction. Pour une importante différence de potentiel entre les électrodes de la jonction, cette théorie prédît un important groupement («bunching») des photons émis dans la cavité. La fonction de corrélation de deux photons à temps égaux g(2)(0) atteint alors une valeur de l’ordre de κ/Γ, où Γ est le taux de transfert tunnel des électrons. En revanche, au premier seuil de transfert inélastique des électrons, cette théorie prédît une émission de lumière non–classique provoquée par le courant électronique moléculaire à un niveau (la jonction se comporte alors comme une source à un photon). Enfin, nous avons montré qu’en présence d’une source de voltage dépendant du temps appliqué à la cavité, le courant dc présente des paliers analogues à ceux obtenus dans le régime Franck–Condon. La théorie développée dans cette thèse est ensuite appliquée à une jonction moléculaire à deux niveaux électroniques. Dans ce scénario, le mode de la cavité se couple à la transition électronique entre les deux orbitales moléculaires. L’effet des fluctuations des charges de chaque orbitale est négligé. Dans ce cadre, nous avons étudié le cas d’un couplage cavité-molécule de type dipolaire électrique. L’attention est portée principalement sur le régime de couplage faible entre le dipole de la molécule et le mode de la cavité. [...]
The study of light–matter interaction has drawn through the years more and more interest. With the improvement of the techniques used for building electromagnetic cavities, it is now possible to couple cavities with nanocircuits merging the fields of quantum optics and nanoelectronics.Not only that, but some experiments also reported the possibility to use a scanning tunneling microscope as a plasmonic cavity coupled with electronic transport. In this thesis a theoretical framework is proposed, based on mesoscopic quantum electrodynamics, for studying the coupling between electronic transport in a molecular junction and the electromagnetic field of a cavity. This thesis focuses on the sequential tunneling regime for the electrons and use density matrix approach. This allows to derive the master equation as well as a computational scheme to compute electronic current and the photon statistic when it is not possible to obtain analytical results. First, a single–level model for the molecule in the junction is studied. Indeed the electronic current induces a fluctuation of the charge on the molecule that couples with the electromagnetic field in the cavity. The investigations on this system are done in the experimentally relevant limit of large damping rate κ for the cavity mode and arbitrary strong light–matter coupling strength. This model shows the equivalence between the electron–photon coupling for a single level and the electron– phonon coupling that has long been studied in nanoelectronics known as the Franck–Condon principle. The current–voltage characteristics show steps, each separated by the energy of a photon, as the electron tunneling dissipate some energy in the cavity mode. In this work a formula has been derived for the electronic current taking into account the damping of the cavity. This allows to show that the width of the current’s steps are controlled by κ rather than the temperature. The single-level junction shows interesting light–emission regimes. At large bias voltage this theory predicts strong photon bunching of the order κ/Γ where Γ is the electronic tunneling rate. However, at the first inelastic threshold the theory predicts current–driven non–classical light emission from the single–level junction. Finally the investigation of the effect of a strong external drive of the cavity on the electronic current shows a quantization of the current that is linked to the Franck–Condon effect. Finally the theory is applied to a double–level model for the molecular junction inspired by quantum optics. In this scenario, the cavity mode couples to the electronic transition between the two states of the molecule. The effect of the charge fluctuations for each single electronic level is neglected. Therefore the coupling is a dipolar coupling in this case. The focus is mainly on the weak coupling regime. The electronic current shows the Rabi splitting due to the hybridization of the cavity mode and the molecule. Electronic tunneling can occur into these hybridized states and is responsible for light emission in the cavity in a iii single tunneling process. Light antibunching is seen in the weak coupling regime since our model predicts that only single photon emission is possible during a tunneling event in this case. Though the intermediate coupling regime is only briefly treated, the strong coupling regime is shown to be similar to two independent single level
El estudio de las interacciones entre luz y materia ha atraído un interés creciente a lo largo de los años. La mejora de las técnicas de fabricación de las cavidades electromagnéticas permite hoy conjugar las cavidades con nanocircuitos, combinando así los campos de la óptica cuántica y de la nanoelectrónica. Se añade a eso la posibilidad de usar un microscopio con efecto túnel a modo de cavidad plasmónica combinada con el transporte electrónico que fue demostrado en numerosas experiencias. Esa tesis propone un cuadro teórico basado en la electrodinámica mesoscópica, permitiendo el estudio de la combinación del transporte electrónico dentro de una unión molecular con el campo electromagnético de una cavidad. El foco se centra en el régimen túnel secuencial de los electrones, a cual está apto el uso de la matriz densidad para los cálculos. Ese régimen permite establecer ecuaciones claves que rigen el desarrollo temporal de la matriz densidad, tal como un esquema de cálculo numérico de la corriente electrónica y de la estadística de los fotones en la cavidad cuando no es posible obtener un resultado analítico. Primero se estudia un modelo de un solo nivel electrónico para la molécula. En efecto, la existencia de una corriente electrónica significa que la carga en la molécula fluctúa y esa fluctuación se combina con el campo electromagnético de la cavidad. El estudio de ese sistema se hace en el limite, experimentalmente pertinente, del ratio alto de la amortiguación κ del modo de la cavidad y del acoplo luz–materia arbitrariamente alto. Ese modelo demuestra la equivalencia del acoplo electrón– fotón para un nivel electrónico y el acoplo electrón–fonón que se ha estudiado desde hace mucho tiempo en el campo de la nanoelectrónica bajo el nombre del principio de Franck–Condon. La característica corriente– tensión del circuito hace aparecer una evolución de escalones, cada uno separado por la energía de un fotón. Eso corresponde a una disipación de energía por parte de los electrones al modo de la cavidad durante el proceso de transporte. En ese trabajo se derivó una ecuación para la corriente electrónica que toma en cuenta el efecto de la amortiguación de la cavidad. Esto demuestra que la anchura de los saltos en la corriente está controlada por κ más que por la temperatura. El modelo de un solo nivel muestra también regímenes inesperados de emisión de luz. En el límite de voltaje alto entre los electrodos de la unión molecular, la teoría predice una agrupación («bunching») de los fotones emitidos dentro de la cavidad. La correlación entre dos fotones emitidos alcanza un valor del orden de κ/Γ donde Γ es el ratio de tunelamiento de los electrones. Sin embargo, en el primer umbral de transferencia inelástica esa teoría iv predice una emisión de luz no-clásica provocada por la corriente electrónica. Por fin, el estudio del impacto de una fuerte excitación externa del modo de la cavidad muestra también una cuantización de la corriente relacionada al efecto Franck–Condon. Finalmente, la teoría desarrollada en esta tesis está aplicada también a una unión molecular de dos niveles electrónicos inspirada de la óptica cuántica. En ese escenario el modo de la cavidad está acoplado con la transición electrónica entre dos orbitales moleculares. El efecto de fluctuaciones de carga en cada orbital no se tiene en cuenta. Entonces en ese marco el acoplo es solo dipolar. Se centra la atención principalmente en el régimen del acoplo débil. La corriente electrónica muestra la huella de oscilaciones de Rabi como resultado de la hibridación del modo de la cavidad con la molécula. El transporte de electrones se puede ocurrir mediante estos estados híbridos. Entonces el traslado de un único electrón es responsable de la emisión de un fotón en la cavidad. Se observa el desagrupamiento («anti-bunching») de la luz emitida

In recent and upcoming years new lasers are being constructed withever higher intensity. These lasers open up the possibility of probingthe high intensity regime of particle physics, which will lead to etherconrming our current models in this regime or the discovery of beyondstandard model physics. However most previous theoretical results in thisarea are based old assumptions about the intensity and shape of the laserpulse that are no longer valid. In this thesis we calculate the tree-levelprobabilities for multi photon emission from an electron propagating inan arbitrary plane wave electromagnetic eld. We show that the classicallimit of our result agrees with the purely classical description of the sameevent. We calculate the soft emission correction to non-linear Comptonscattering. We conclude that our results are infrared divergent and arguethat this will be solved by including loop contributions to the process. Ourresults provide an important component for the theoretical predictions forthe outcome of scattering experiments in high intensity background eld.This thesis will add to the understanding of high intensity QED.

The Scanning Tunnelling Microscope is an established surface science tool, combining unprecedented resolution with real space mapping. One of its major drawbacks, however, is that it gives no chemical information, but Photon Emission is able to probe the inelastic channel, which for metals contain invaluable chemical information. It has already been shown that flat metal surfaces produce differing emission spectra. Real surfaces may not be flat, so it is important to known how curvature changes the yield of photons, and the emission spectrum. For this reason, a two sphere model was developed. The mechanism for Photon Emission, that of excitation of localised surface plasmons followed by either radiation or dielectric loss, was split into separate problems so that the dependence of the overall photon emission on materials and curvature is clear. Experiments were performed on small silver particles, and for what was later believed to be a silver tip it was found that the Photon Emission was approximately proportional to both the tin and particle radius. Light emission was observed from clusters containing as few as ˜30 atoms, and the first chemically specific photon maps were presented, which distinguished silver particles from carbide deposits. Emission was also seen on the Si(111)-7x7 surface. The new theory compared well with spectra from flat surfaces, and when extended to two spheres it also showed that the Photon Emission was approximately proportional to both the tip and particle radius. It was predicted that the onset of emission would occur at a lower tip bias for gold particles than for silver particles, and this was confirmed experimentally. Metal-specific spectra will be produced if the plasmon modes do not move in energy when curvature varies, staying close to the travelling surface plasmon energy. This should occur for small radius tips and particles, and the movement of modes should be reduced for tungsten tips.

Our goal is to develop a single-photon emission computed tomography (SPECT) system for dynamic cardiac imaging so that heart disease may be more accurately evaluated. We have developed multiple, stationary, modular scintillation cameras that allow for dynamic imaging because of large detector area, large collection efficiency, high count-rate capability, and no motion of detector, collimator, or aperture. We make use of coded-aperture pinhole arrays because they increase photon-collection efficiency. The coded apertures allow for overlapping projections or multiplexing of an object onto the detector face. We have designed a novel collimation system that allows for an increased number of pinhole projections without substantial multiplexing. This new method is called "subslicing". We verified the subslice concept both in computer simulation and with our 16-module ring imaging system. Comparison of results with and without subslicing shows that the new approach substantially reduces artifacts in the image reconstruction. (Abstract shortened with permission of author.)

On demand single-photon and entangled-photon sources are key building-blocks for many proposed photonic quantum technologies. For practical device applications, epitaxially grown quantum dots (QDs) are of increasing importance due to their bright photon emission with sharp line width. Particularly, they are solid-state systems and can be easily embedded within a light-emitting diode (LED) to achieve electrically driven sources. Therefore, one would expect a full-fledged optoelectronic quantum network that is running on macroscopically separated, QD-based single- and entangled-photon devices. An all-electrically operated wavelength-tunable on demand single-photon source (SPS) is demonstrated first. The device consists of a LED in the form of self-assembled InGaAs QDs containing nanomembrane integrated onto a piezoelectric crystal. Triggered single photons are generated via injection of ultra-short electrical pulses into the diode, while their energy can be precisely tuned over a broad range of about 4.8 meV by varying the voltage applied to the piezoelectric crystal. High speed operation of this single-photon emitting diode up to 0.8 GHz is demonstrated. In the second part of this thesis, a fast strain-tunable entangled-light-emitting diode (ELED) is demonstrated. It has been shown that the fine structure splitting of the exciton can be effectively overcome by employing a specific anisotropic strain field. By injecting ultra-fast electrical pulses to the diode, electrically triggered entangled-photon emission with high degree of entanglement is successfully realized. A statistical investigation reveals that more than 30% of the QDs in the strain-tunable quantum LED emit polarization-entangled photon-pairs with entanglement-fidelities up to f+ = 0.83(5). Driven at the highest operation speed ever reported so far (400 MHz), the strain-tunable quantum LED emerges as unique devices for high-data rate entangled-photon applications. In the end of this thesis, on demand and wavelength-tunable LH single-photon emission from strain engineered GaAs QDs is demonstrated. Fourier-transform spectroscopy is performed, from which the coherence time of the LH single-photon emission is studied. It is envisioned that this new type of LH exciton-based SPS can be applied to realize an all-semiconductor based quantum interface in the foreseeable distributed quantum networks.

Thesis (Ph.D.) - University of Glasgow, 1990.
BLL. DX192708. Ph.D. thesis submitted to the Department of Clinical Physics and Bio-Engineering, University of Glasgow, 1990. Includes bibliographical references.

In single-photon emission computed tomography (SPECT), the goal is to estimate the biodistribution of radioactive pharmaceuticals that have been administered to a patient. Various degrading physical effects contribute to inaccuracies in the estimation process. In this dissertation, we examine the effects of photon attenuation and noisy projection data on SPECT image reconstructions. In addition, image-quality assessment is performed to determine the best reconstruction method for a given detection task. Accurate quantitation of the activity distribution is desirable for detection and estimation tasks in SPECT. Photon attenuation hinders our ability to obtain good quantitative estimates; therefore effective correction techniques need to be developed. For the case of a convex body outline and a uniformly attenuating medium, the attenuation problem is modelled by the exponential Radon transform. In this dissertation, we mathematically describe the three, previously published, analytical inverses to this forward problem, known as the methods of Bellini, Inouye, and Tretiak & Metz. Also included are a new analytic inverse developed recently by Metz and the widely used approximate inverse of Chang. The physical properties of photon decay and collection cause random fluctuations to be introduced into the measured projection data. During the reconstruction process, the noise structure is altered by filtering and backprojection. By characterizing the noise in the reconstructions, optimal reconstruction and restoration filters may be derived. In this study, we examine the behavior of the noise as propagated through the attenuation correction and reconstruction process for a uniformly emitting and attenuating disk object. The results demonstrate that SPECT image noise is globally nonstationary, although it is locally stationary at the center of the disk for the Bellini, Chang, Scaled Filtered-Backprojection, new Metz, and Tretiak & Metz methods. The Inouye method yielded the highest noise variance at the disk's center, while the Tretiak & Metz method increased noise variance drastically toward the edge of the disk. In addition to noise characterization, we seek the best attenuation correction method for clinical use. Image quality is defined by the ability of an observer to perform a specified task given a class of images. Psychophysical studies were performed with human observers for the tasks of detecting a 10% contrast cold disk signal on a uniformly emitting and attenuating circular background, where the signal was located either at the center or near the edge of the disk. The ideal, non-prewhitening and region-of-interest model observers were evaluated as image-quality measures, with the goal of finding an observer that correlated well with human performance. For the center-location task, we found that the human observers did not perform significantly better with any particular method. Also, the model observers were shown to be invariant across correction method. For the edge-location task, the human observers performed statistically significantly worse with the method of Tretiak & Metz. This behavior was also reflected in the non-prewhitening and region-of-interest model-observer signal-to-noise ratios.

The attenuation of photons within the body has been recognised as the major limiting factor hindering the ability of single photon emission computed tomography (SPECT) as a quantitative technique. This thesis investigates several aspects of an emission-transmission SPECT system using the Monte Carlo method and experimental techniques. The system was based on a rotating gamma camera fitted with a parallel hole collimator. The simulation of a transmission study was performed using a simple non-uniform mathematical phantom with two different external sources, a collimated line source and a flood source. The results showed that the attenuation maps were highly dependent on the geometry and photon energy of the source. The collimated line source produced improved image quality with lower statistical noise than the flood source. The results showed that, when high atomic number elements are present in the tissue composition, the attenuation coefficients at different energies are related through a second order polynomial transformation. If the object under study is formed of soft tissue equivalent materials, a linear transformation holds. The attenuation maps generated in the transmission study were used to correct for non-uniform attenuation compensation of an emission phantom. The results showed that non-uniform attenuation compensation improved image quality and reduced noise when compared to data without attenuation compensation. The presence of scattered photons in the emission data reduced the quality of the images and precluded accurate quantification. Absolute quantification was performed using the percent air sensitivity criterion. The largest difference between the theoretical and the Monte Carlo simulated images was approximately 8%. An emission-transmission myocardial perfusion study was simulated using an anthropomorphic phantom. Two photon energies of clinical interest were used, 75 keV and 140 keV, corresponding to the main photon emission energies of 201Tl and 99mTC. The results showed that 99mTc provided better image quality than 201Tl. Non-uniform attenuation compensation produced a very good agreement between the theoretical prediction and the simulation when scatter-free data were considered. The results presented in this thesis indicate that it is not possible to accomplish accurate attenuation compensation in general situations if scatter correction is not applied.

A study of the Compton Scattering Camera in application to single photon emission computed tomography has been carried out and is presented in this thesis. It is shown that conventional gamma camera technology is fundamentally restricted by the collimator. The collimator places restrictions on camera sensitivity and on achievable reconstructed spatial resolution. The Compton scattering camera is proposed as an alternative to conventional gamma camera technology. The Compton scattering camera uses the process of Compton scattering to determine where a photon has come from. This results in a significant improvement to camera sensitivity and opens up the potential for improved spatial resolving power. An analytical model describing the Compton scattering camera is applied and studied. Suggestions are made regarding the design of a practical camera. It is shown that while a significant gain in sensitivity is achieved by the use of the alternative camera, many more measurement data bins are generated. The latter leads to poor photon counts in each data bin in situations encountered in practical medical imaging. Suggestions are made as to how to combat this problem. The use of Compton scattering to localise a photon direction vector leads to a new and complicated image reconstruction problem. Iterative algorithms have been devised for the image reconstruction by others and are reviewed herein. Progress towards developing a theory of direct image reconstruction is reported. To this end the notion of the cone-surface projection is introduced. It is shown that a certain subset of data, namely the restricted cone-surface projection, is invertible leading to a formula giving the source distribution in terms of the projections. Hence, the reconstruction problem, in the absence of noise uncertainties, is overspecified. Two possible reconstruction paths are outlined. One is direct reconstruction based on the restricted cone-surface projections. The other forms the cone-beam projections from the cone-surface projections and employs standard cone-beam reconstruction. Results of computer simulation of the reconstruction paths are reported. The direct reconstruction is robust with respect to some angular uncertainty and missing low angle scattering. The formation of the parallel-ray projection from the restricted cone-surface projection appears to be adversely susceptible to such measurement uncertainty. These studies are carried out without accounting for photon noise. For the algorithms to be practically useful, it is necessary that the theorems relating to the restricted cone-surface projection be generalised to cover the full cone-surface projection. Suggestions as to how this may be done are given. To perform the direct reconstruction from the restricted cone-surface projection. it is necessary to numerically evaluate the Hankel transform. The literature on Hankel transform algorithms is somewhat fragmented; a review of Hankel transform algorithms is therefore presented. Results of testing of a variety of zero-order Hankel transform algorithms are reported. It is found that algorithms based on the trapezoidal rule and the back-projection method with Fourier interpolation lead to the most accurate Hankel transforms in general. Unfortunately these algorithms are relatively computationally expensive. More efficient algorithms, such as the projection-slice method with Hansen and Law's method of evaluating the Abel transform, may be suitable in applications where less accuracy can be tolerated.

Generare fotoni veloci al passaggio di radiazione ionizzante sta diventando un requisito essenziale per le future generazioni di rivelatori basati sugli scintillatori, sia per quanto riguarda la fisica delle alte energie (HEP) che per quanto riguarda la tomografia ad emissione di positroni con capacità di time of flight (TOF-PET). Lo stato dell'arte per quanto riguarda la risoluzione temporale di sistemi in coincidenza ha raggiunto i 100 ps FWHM con eccitazione tramite fotoni da 511 keV, lettura a singolo canale, mentre 160 ps sono stati ottenuti nei calorimetri HEP multicanale. Scendere sotto il limite dei 20 ps di risoluzione in coincidenza (CTR) porterebbe ad un analisi più efficiente in fase di dignosi: questo obiettivo può essere raggiunto aumentando la statistica di conteggio dei fotoni oppure riducendo il tempo di salita e di decadimento dell'impulso di scintillazione. La prima parte della tesi di dottorato si è concentrata sull'impatto del segnale ottico di scintillazione come il fattore chiave per il miglioramento delle caratteristiche temporali del sistema. I parametri principali, cioè il light yield (proporzionale al numero di foto-elettroni rivelati Npe), il tempo di salita e di decadimento (τr, τd), sono stati presi in considerazione per quantificare il loro contributo al CTR e, in funzione di questo, indagare nuove soluzioni per migliorarlo. Nella seconda parte della tesi, è stato identificata e caratterizzata una nuova classe di nanocristalli semiconduttori (NCs) con soppressione di effetto Auger, in grado di fornire caratteristiche temporali migliori in seguito all'eccitazione da parte di radiazione ionizzante. L'implementazione di scintillatori basati su nanocristalli come nuova generazione di rivelatori veloci di particelle viene esplorata seguendo due linee principali: (1) nanocristalli come parte di una eterostruttura e (2) nanocomposti, dove i nanocristalli sono inseriti in una matrice ospite. I risultati principali sono stati la determinazione del light yield intrinseco di cristalli LYSO, eccitati con elettroni, risultato essere 40'000 ± 3%(stat.) ± 9%(syst.) ph/MeV. Questo numero impone un limite al valore che Npe può teoricamente raggiungere usando fotorivelatori di ultima generazione. L'estrazione di tutta la luce creata in uno scintillatore e la sua conversione in fotoelettroni con un efficienza del 100% può portare a un miglioramento del CTR di un solo fattore 2, invece che del fattore 10 richiesto. Il tempo di salita e di decadimento del segnale di scintillazione è statro inoltre studiato. Due classi di NCs sono stati studiati con successo: CdSe bidimensionali (NPLs) e sferici dSe/CdS core/giant shell quantum dots(GS QDs). E' stato dimostrato che il rate di emissione di questi NCs sotto eccitazione di raggi X pulsati è molto più veloce rispetto ai tradizionali meccanismi presenti negli scintillatori classici, cioè la transizione 5d-4f e la densità di fotoelettroni nei primi 100 ps del segnale è un ordine di magnitudine più alto. I CdSe NPLs hanno un tempo di decadimento effettivo di 77 ps e CdSe/CdS GS QDs mostrano un valore di 849 ps. Inoltre sono state misurate le ZNO:ga nanopolveri (τd=500 ps) inserite in una matrice di polystirene tremite eccitazione di fotoni da 511 keV in coincidenza. Ciò a portato a valori di CTR di 200 ps, utilizzando la rivelazione di soli 30 fotoelettroni. Per concludere, l'attuale contributo propone un possibile metodo percorribile alle tecnologie per ottenere risoluzioni temporali sotto i 20 ps. E' stato dimostrato il grande potenziale di sviluppo della tecnologia dei NC. Una ricerca più accurate in questa direzione è un'opzione valida per ottenere risoluzioni temporali veloci in ambiti come HEP e medical imaging.
Generating a prompt response to the passage of ionizing particles has emerged as a critical requirement for next-generation scintillator-based radiation detectors in high energy physics (HEP) and time-of-flight positron emission tomography (TOF-PET). State-of-the-art time resolution values are of the order of 100 ps FWHM for 511 keV gamma excitation, single channel readout and 160 ps for multichannel HEP calorimeters. Going down to sub-20 ps coincidence time resolution (CTR) implies a major technological challenge that could be overcome by increasing photostatistic, reducing rise and decay-time or using an intense prompt signal as a time tag in the readout chain. The first part of the doctoral thesis will focus on the impact of the scintillating crystal optical signal as the key factor to reduce timing. The main parameters, i.e. light yield (proportional to the number of detected photoelectrons Npe), rise- and decay-time (τr, τd) will be put into perspective in order to know their present contribution to the CTR and therefore, find ways to improve their current values. In the second part of the manuscript, we identify and characterize a new class of Auger suppressed semiconductor nanocrytals (NCs) with enhanced timing characteristics under ionizing radiation. The implementation of nanocrystal-based scintillators as a new generation of ultrafast particle detectors is explored using two main lines: (1) nanocrystals as part of a heterostructure and (2) nanocomposites, where the NCs are embedded in a matrix host. Relevant results are the determination of LYSO intrinsic light yield under electron excitation with values of 40’000 ± 3%(stat.) ± 9%(syst.) ph/MeV. This number sets a limit to the value that Npe could theoretically reach using state-of-the-art photodetectors. Extracting all the light created in the scintillator and converting this light into photoelectrons with 100% efficiency could bring a CTR improvement of only a factor 2, instead of a factor of 10 as needed. The rise- and decay-time of the scintillating signal was also investigated. Two classes of NCs were successfully studied: two-dimensional CdSe nanoplatelets (NPLs) and spherical CdSe/CdS core/giant shell quantum dots (GS QDs). We demonstrate that the emission rates of these NCs under pulsed X-ray excitation are much faster than traditional mechanisms in bulk scintillators, i.e. 5d-4f transitions and the photoelectron density in the first 100 ps of the signal is one order of magnitude higher. CdSe NPLs have a sub-100 ps effective decay time of 77 ps and CdSe/CdS GS QDs exhibit a sub-ns value of 849 ps. Further, the first generation of ZnO:Ga nanopowders (τd=500 ps) embedded in a polystyrene host matrix were measured in coincidence under 511 keV gamma excitation, yielding CTR values of 200 ps for only 30 pe detected. In conclusion, the present contribution proposes a feasible way to overcome the technological challenge that sub-20 ps time resolution implies. We demonstrate the large development potential of NC technology and justify the rigorous research in this field to make NCs a viable option for superfast timing in domains such as HEP and medical imaging.

Optical nanoantennas confine light on the nanoscale, enabling strong light-matter interactions with potential for ultra-compact optical devices. Apart from the direct applications in optical nanoscopy and sensing, nanoantennas can strongly enhance the spontaneous emission rate of single photon emitters. A nanoantenna, acting as a nanocavity, enables high photon output due to its high radiative losses associated with plasmonic resonances and thus pave the way for compact, bright, and pure single photon sources with applications in quantum technologies. The sub-wavelength field localization at the nanoantenna causes a vectorial field distribution with non-zero field components in all dimensions. The efficiency of coupling between an emitter such as single molecule and nanoantenna will thus strongly depend on the spatial overlap of the emitter's dipole with the nanoantenna field. In order to achieve such coupling, careful nanopositioning of the emitter, in its location as well as the orientation, within the nanocavity field is required. Therefore, the full vectorial characterization of the emitter-nanoantenna is crucial to maximize the coupling strength. This thesis addresses the aforementioned issues and studies the controlled nanoscale interaction of a single molecule with a resonant nanoantenna. To this end we first fabricate nanoantennas on the vertex of a fiber tip using focused-ion-beam milling and use a near-field technique for the nanopositioning control. In the first experiment, we investigate the excitation properties of a resonant dipole nanoantenna by mapping its vectorial near-field distribution with molecular resolution. The nanoantenna tip is scanned over specifically selected single molecules to map x-, y-, and z-field components. In addition to characterization of the vectorial field, we show the apparent position of the molecule shifts up to 20 nm depending on their orientation with important implications for localization microscopy. Next, our unprecedented experimental approach allows us to examine an often overlooked, but important near-field concept of nanoantennas, local interference. The highly structured vectorial amplitude and phase distribution of the nanoantenna overlaps with the exciting far-field to create this local interference. We perform a detailed study through direct observation and show an example of exploiting this local interference to shape and control the near-field of the nanoantenna. Lastly, we quantify the vectorial interaction of a molecule-nanoantenna cavity system by mapping the coupling strength g with 5 nm spatial resolution. We show that for a molecule's optimal position at the nanoantenna, the coupling rate reaches up to g_max= 206 GHz, much higher than for any conventional cavities. Such a large coupling provides ideal conditions for fast and pure non-classical photon emission, enabling a single photon source with an emission rate above 1 GHz at room-temperature
Nanoantenas ópticas confinan la luz a una escala nanométrica, permitiendo interacciones fuertes entre la luz y la materia con potencial en dispositivos ópticos ultra-compactos. Aparte de las aplicaciones directas en nanoscopía y detección, nanoantenas pueden aumentar fuertemente la tasa de emisión espontánea de un emisor de fotón único. Una nanoantena, actuando como una nanocavidad, permite un rendimiento elevado de fotones debido a sus pérdidas radiativas asociadas con las resonancias plasmónicas y por tanto allana el camino para fuentes de fotón único compactas y brillantes con aplicaciones en tecnologías cuánticas. La localización sublongitud de onda de campos en la nanoantena crea una distribución vectorial de campos con componentes no nulas en todas las dimensiones. La eficiencia del acoplamiento entre un emisor como una molécula única y una nanoantena dependerá en gran medida de la superposición espacial del momento dipolar del emisor y el campo de la nanoantena. Con el fin de lograr este acoplamiento, el posicionamiento preciso del emisor, tanto su ubicación como su orientación, dentro de la nanocavidad es necesario. Por tanto, la caracterización vectorial del sistema emisor-nanoantena es crucial para maximizar la fuerza de su acoplamiento. Esta tesis aborda los temas antes mencionados y estudia la interacción controlada de una molécula única junto con una nanoantena. Con este fin primero fabricamos las nanoantenas en el vértice de una fibra óptica utilizando un haz de iones enfocado fresado y usamos una técnica de campo cercano para controlar el nanoposicionamiento. En el primer experimento, investigamos las propiedades de excitación de una nanoantena dipolar resonante al mapear su distribución de campo cercano con una resolución molecular. La punta de la nanoantena se escanea sobre moléculas únicas seleccionadas específicamente para mapear las componentes x-, y-, y z-del campo. Además de la caracterización del campo vectorial, se demuestra que la posición aparente de la molécula se desplaza hasta arriba 20 nm, dependiendo de su orientación, con importantes implicaciones para la microscopía de localización. A continuación, nuestro enfoque experimental sin precedentes nos permite examinar un concepto de campo cercano menudo pasado por alto, pero uno importante para las nanoantenas, la interferencia local. La distribución vectorial altamente estructurada de la amplitud y fase de la nanoantena se solapa con el campo lejano excitante para crear esta interferencia local. Realizamos un estudio detallado a través de la observación directa y damos un ejemplo de cómo aprovechar de esta interferencia local para dar forma y controlar el campo cercano de la nanoantena. Por último, se cuantifica la interacción vectorial de un sistema molécula-nanoantena mapeando la fuerza de acoplamiento g con una resolución espacial de 5 nm. Demostramos que para la posición óptima de una molécula en la nanoantena, la tasa de acoplamiento llega hasta g_max = 206 GHz, muy superior a todas las cavidades convencionales. Tal acoplamiento grande proporciona las condiciones ideales para la emisión de fotones no clásicas de forma rápida y puro, haciendo posible una fuente de fotones únicas con una tasa de emisión por encima de 1 GHz a temperatura ambiente

A new technique to investigate the phenomenon of two-photon emission in atomic nuclei has been developed and an experiment performed at the Chalmers University of Technology, Gothenburg. The long-lived first excited state in 7²GE was populated by the inelastic scattering of a pulsed neutron flux.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemistry, 2014.
This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (pages 137-152).
The photoluminescence spectrum of an ensemble of emitters is the result of the homogeneous "natural" spectra of single emitters subjected to interparticle inhomogeneities and perturbations from the environment. For semiconductor nanocrystals (NCs), efforts to tune ensemble linewidths for optical applications have focused primarily on eliminating sample inhomogeneities because conventional single-molecule methods cannot reliably build accurate ensemble-level statistics for single-particle linewidths. Photon-correlation Fourier spectroscopy in solution (S-PCFS) offers a unique approach for investigating single-nanocrystal spectra with large sample statistics, without user selection bias, with high signal-to-noise ratios, and at fast timescales. With S-PCFS, we directly and quantitatively deconstruct the ensemble spectra of nanocrystals into contributions from the average single-NC homogeneous linewidth, spectral dynamics, and sample inhomogeneity. First, we discovered that single NCs at room temperature, in contrast to cryogenic temperatures, do not exhibit spectral dynamics on sub-millisecond timescales. Second, the linewidths of these homogeneous spectra were found to vary significantly from batch to batch and subject to synthetic control. Our findings crystallize our understanding of the synthetic challenges facing underdeveloped nanomaterials such as InP and InAs nanocrystals and introduce new avenues for the synthetic optimization of fluorescent nanoparticles. Finally, we have made strides toward understanding the underlying physical processes responsible for the homogeneous spectra of single nanocrystals at room temperature. Through careful synthetic control over the nanocrystal structure and composition, we have been able to understand changes in the homogeneous spectral linewidth in terms of exciton-phonon coupling. Combined with a simple spectral lineshape model, we have worked towards quantitatively understanding exciton-phonon coupling with respect to specific nanocrystal structural and composition parameters.
by Jian Cui.
Ph. D.

An important goal of single photon emission tomography (SPECT) is the determination of absolute regional radionuclide concentration as a function of time. Quantitative and qualitative studies of SPECT with regard to clinical application is the object of this work. Three basic approaches for image reconstruction and factors which affect the choice of a reconstruction algorithm have been reviewed, discussed and the reconstruction techniques, GRADY and CBP evaluated, based on computer modelling. A sophisticated package of computational subroutines, RECLBL, for image reconstruction and for generation of phantoms, which was fully implemented on PRIME was used throughout this study. Two different systems, a rotating gamma-camera and a prototype scanning-rig have been used to carry out tomography experiments with different phantoms in emission and transmission mode. Performance assessment and reproducibility of the gamma-camera was tested prior to the experimental work. SPECT studies are generally hampered for a number of reasons, the most severe being attenuation and scattering. The effect of scattered photons on image quality was discussed, three distinct techniques were utilised to correct the images and results were compared. Determination of the depth of the source, Am-241 and Tc-99m in the attenuating media, water and TEMEX by analysing the spectroscopic data base on the SPR and spatial resolution was studied, results revealed that both techniques had the same range of depth sensitivity. A method of simultaneous emission and transmission tomography was developed to correct the images for attenuation. The reproducibility of the technique was examined. Results showed that the technique is able to present a promising and a practical approach to more accurate quantitative SPECT imaging. A procedure to evaluate images, under certain conditions has been defined, its properties were evaluated using computer modelling as well as real data. Usefulness of the odd sampling technique to improve image quality has been investigated and is recommended.

In dieser Forschungsarbeit wird das selbstorganisierte Wachstum von InP/InGaP-Quantenpunkten (QP) sowie ihre optischen und strukturellen Eigenschaften untersucht. Die QP wurden auf GaAsgitterangepasstem InGaP gewachsen.Selbstorganisierte InP-QP werden mittels Gasquellen-Molekularstrahlepitaxie gewachsen, wobei die InP-Abscheidungsrate uber einen weiten Bereich variiert wird. Bei besonders geringer Wachstumsratevon rund 0,01 Atomlagen/s wird eine Flachendichte von 1 QP/μm2 erreicht. Die daraus resultierenden InP QP, konnen einzeln charakterisiert werden ohne vorher das Substrat lithografisch behandeln zu mussen. Sowohl exzitonische als auch biexzitonische Emission kann dabei an einzelnen QPn als Doublett mit einer Feinstrukturaufspaltung von 320μeV beobachtet warden. Hanbury-Brown-Twiss Korrelationsmessungen der exzitonischen Emission unter Dauerstrichanregung zeigen Antibunching mit einem Autokorrelationskoeffizienten von g(2)(0)=0.2. Dieses System liee sich beispielsweise als Einzelphotonenquelle in Anwendungsbereichen wie der Quantenkryptographie einsetzen. Daruber hinaus wird die Bildung wohlgeordneter Quantenpunktketten auf GaAs (001)-Substraten unter Ausnutzung einer selbstorganisierten InGaP-Oberflachenwellung demonstriert. Diese Anordnung basiert weder auf gestapelten Quantenpunktschichten noch einem intentionalen Substratschragschnitt. Die Strukturen warden mittels polarisationsabhangiger Photolumineszenzspektroskopie sowie Transmissionselektronenmikroskopie untersucht. Die Lumineszenz der InGaP-Matrix ist in eine kristallografische Richtung polarisiert, bedingt durch anisotrope Verspannung, welche ihrerseits aus der lateralen Variation der Materialzusammensetzung entsteht. Photolumineszenzmessungen der QP zeigen eine lineare Polarisation entlang [-110], der Richtung der Ketten. Der Polarisationsgrad liegt bei 66%. Diese optische Anisotropie wird direkt in einer Heterostruktur hervorgerufen, die lediglich eine Quantenpunktschicht beinhaltet.
In this work the growth of self-assembled InP/InGaP quantum dots, as well as their optical and structural properties are presented and discussed. The QDs were grown on InGaP, lattice matched to GaAs.Self-assembled InP quantum dots are grown using gas-source molecular beam epitaxy over a wide range of InP deposition rates, using an ultra-low growth rate of about 0.01 atomic monolayers/s, a quantum-dot density of 1 dot/μm2 is realized. The resulting isolated InP quantum dots are individually characterized without the need for lithographical patterning and masks on the substrate. Both excitionic and biexcitonic emissions are observed from single dots, appearing as doublets with a fine-structure splitting of 320 μeV. Hanbury Brown-Twiss correlation measurements for the excitonic emission under cw excitation show anti-bunching behavior with an autocorrelation value of g(2)(0)=0.2. This system is applicable as a single-photon source for applications such as quantum cryptography. The formation of well-ordered chains of InP quantum dots on GaAs (001) substrates by using self-organized InGaP surface undulations as a template is also demonstrated. The ordering requires neither stacked layers of quantum dots nor substrate misorientation. The structures are investigated by polarization-dependent photoluminescence together with transmission electron microscopy. Luminescence from the InGaP matrix is polarized in one crystallographic direction due to anisotropic strain arising from a lateral compositional modulation. The photoluminescence measurements show enhanced linear polarization in the alignment direction of quantum dots, [-110]. A polarization degree of 66% is observed. The optical anisotropy is achieved with a straightforward heterostructure, requiring only a single layer of QDs.

Thesis (Ph. D.)--University of Missouri-Columbia, 2005.
The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Vita. "December 2005" Includes bibliographical references.

Although gold nanoparticles (GNPs) are promising probes for biological imaging because of their attracting optical properties and bio-friendly nature, properties of the multi-photon (MP) emission from GNP aggregates produced by a short-wave infrared (SWIR) laser have not been examined. In this paper, characterization of MP emission from aggregated 50 nm GNPs excited by a femtosecond (fs) laser at 1560 nm is discussed with respect to aggregate structures. The key technique in this work is single particle spectroscopy. A pattern matching technique is applied to correlate MP emission and SEM images, which includes an optimization processes to maximize cross correlation coefficients between a binary microscope image and a binary SEM image with respect to xy displacement, image rotation angle, and image magnification. Once optimization is completed, emission spots are matched to the SEM image, which clarifies GNP ordering and emission properties of each aggregate. Correlation results showed that GNP aggregates have stronger MP emission than single GNPs. By combining the pattern matching technique with spectroscopy, MP emission spectrum is characterized for each GNP aggregate. A broad spectrum in the visible region and near infrared (NIR) region is obtained from GNP dimers, unlike previously reported surface plasmon enhanced emission spectrum.

In recent years, Silicon Photomultipliers (SiPMs) have been increasingly used as photo-detectors in Positron Emission Tomography (PET) application, which is a nuclear imaging tech-nique that is used to accurately image biochemical processes inside the human body. A SiPM is composed by an array of parallel connected micro-cells of Single Photon Avalanche Diodes (SPADs), and can be classified mainly into two categories Digital-SiPMs (D-SiPMs) and Analog-SiPMs (A-SiPMs). In A-SiPMs, all the microcells share the same bias voltage and have a common readout line. Through custom manufacturing process, the performance of A-SiPMs can be exten-sively optimized. With respect to A-SiPMs, D-SiPMs are composed by many SPAD pixels, each one containing one SPAD and local front-end circuitry. The digital SiPM takes advantage of CMOS technology to perform a 1-b direct A/D conversion per SPAD thus providing a fully digi-tal implementation. On the other hand, SPADs fabricated in CMOS process typically suffer high noise since the critical SPAD implants can hardly be optimized by using the standard CMOS process flow. The main activities carried out within this PhD thesis have been focused on two critical as-pects relevant for the optimization of PET systems performance: (I) the improvement of the per-formance of SPAD in D-SiPMs and (II) the development of high-performance A-SiPM readout application specific integrated circuit (ASIC). Concerning the first point, novel SPADs have been fabricated in CMOS 150-nm technology targeting at low noise, high sensitivity and excellent timing jitter. Three structures with different shapes, geometries and diameters, have been implemented in three test chips. Measurement re-sults of one p+/n-well SPAD array show a 0.4Hz/µm2 dark count noise, 0.85% afterpulsing for a dead time of 150ns at 3V excess bias. The photon detection probability is about 31% at 450nm wavelength at 5V excess bias. The SPAD exhibits a timing jitter of 82ps (FWHM) under a blue laser, which is potentially suitable for D-SiPMs in PET application. The second objective of this PhD work was to develop A-SiPM readout ASIC for PET appli-cation. To utilize the high intrinsic time resolution of A-SiPMs, the development of specialized, highly integrated readout electronics is required. Therefore, two ASICs, first chip with test struc-tures and 16 channels and the second chip with 32 channels, have been developed in 150-nm CMOS technology, with the aim of developing a compact A-SiPM module. The performance of the second chip has been validated by using 3 × 3 × 5 mm3 LYSO crystals coupled to 4 × 4 mm2 SiPMs (FBK-NUV-HD). The measurements show an energy resolution of 14.7% FWHM for the detection of 511 keV photons and the coincidence time resolution is 433ps (FWHM). To improve the timing resolution, part of the PhD work was carried on Stanford University, focused on char-acterization of A-SiPMs and analysis of noise contribution.

In recent years, Silicon Photomultipliers (SiPMs) have been increasingly used as photo-detectors in Positron Emission Tomography (PET) application, which is a nuclear imaging tech-nique that is used to accurately image biochemical processes inside the human body. A SiPM is composed by an array of parallel connected micro-cells of Single Photon Avalanche Diodes (SPADs), and can be classified mainly into two categories Digital-SiPMs (D-SiPMs) and Analog-SiPMs (A-SiPMs). In A-SiPMs, all the microcells share the same bias voltage and have a common readout line. Through custom manufacturing process, the performance of A-SiPMs can be exten-sively optimized. With respect to A-SiPMs, D-SiPMs are composed by many SPAD pixels, each one containing one SPAD and local front-end circuitry. The digital SiPM takes advantage of CMOS technology to perform a 1-b direct A/D conversion per SPAD thus providing a fully digi-tal implementation. On the other hand, SPADs fabricated in CMOS process typically suffer high noise since the critical SPAD implants can hardly be optimized by using the standard CMOS process flow. The main activities carried out within this PhD thesis have been focused on two critical as-pects relevant for the optimization of PET systems performance: (I) the improvement of the per-formance of SPAD in D-SiPMs and (II) the development of high-performance A-SiPM readout application specific integrated circuit (ASIC). Concerning the first point, novel SPADs have been fabricated in CMOS 150-nm technology targeting at low noise, high sensitivity and excellent timing jitter. Three structures with different shapes, geometries and diameters, have been implemented in three test chips. Measurement re-sults of one p+/n-well SPAD array show a 0.4Hz/µm2 dark count noise, 0.85% afterpulsing for a dead time of 150ns at 3V excess bias. The photon detection probability is about 31% at 450nm wavelength at 5V excess bias. The SPAD exhibits a timing jitter of 82ps (FWHM) under a blue laser, which is potentially suitable for D-SiPMs in PET application. The second objective of this PhD work was to develop A-SiPM readout ASIC for PET appli-cation. To utilize the high intrinsic time resolution of A-SiPMs, the development of specialized, highly integrated readout electronics is required. Therefore, two ASICs, first chip with test struc-tures and 16 channels and the second chip with 32 channels, have been developed in 150-nm CMOS technology, with the aim of developing a compact A-SiPM module. The performance of the second chip has been validated by using 3 × 3 × 5 mm3 LYSO crystals coupled to 4 × 4 mm2 SiPMs (FBK-NUV-HD). The measurements show an energy resolution of 14.7% FWHM for the detection of 511 keV photons and the coincidence time resolution is 433ps (FWHM). To improve the timing resolution, part of the PhD work was carried on Stanford University, focused on char-acterization of A-SiPMs and analysis of noise contribution.

Thesis (Ph.D.)--University of Missouri-Columbia, 2005.
The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file viewed on (November 15, 2006) Vita. Includes bibliographical references.

Thesis (Ph. D.)--University of Oregon, 2004.
Typescript. Includes vita and abstract. Includes bibliographical references (leaves 102 -106). Also available for download via the World Wide Web; free to University of Oregon users.

The purpose of this thesis is to investigate the nonlinear phenomena known as doubly-stimulated, non-degenerate two-photon emission (ND-2PE) in Gallium Arsenide (GaAs). 2PE refers to the simultaneous emission of two-photons as electrons move from the conduction band in a direct gap semiconductor to the valence band. Following the same path for describing one-photon emission (1PE) we describe 2PE as a product of the irradiance, I, and the negative of the loss which in this case is two-photon absorption, [alpha]2, the negative coming from the population inversion. We attempt to observe 2PE by using a frequency non-degenerate pump-probe experiment in which a third beam optically excites a 4 µm thick GaAs sample. We use non- degenerate beams in hopes of utilizing the 3-orders of magnitude enhancement seen in two- photon absorption (2PA) by going to extreme nondegeneracy (END) to enhance 2PE. GaAs is chosen due to the availability of the appropriate wavelengths, the maturity of the GaAs technology, its use in optoelectronic devices and its ability to be electrically pumped. During the experimental development we learn how to effectively etch and manipulate thin GaAs samples and model the transmission spectrum of these samples using thin film transmission matrices. We are able to match the measured transmission spectrum with the theoretical transmission spectrum. Here we etch the bulk GaAs left on the sample leaving only the 4 µm thickness of molecular beam epitaxial grown GaAs plus additional layers of aluminum gallium arsenide (AlGaAs). These samples were grown for us by Professor Gregory Salamo of the University of Arkansas. Using the pump-probe experiment on the 4 µm GaAs sample, we measure the change of the 2PA due to the presence of optically excited carriers. The goal is to reduce the 2PA signal to zero and then invert the 2PA signal indicating an increase in transmission indicative of 2PE when the population is inverted. Our results show that we achieve a 45% reduction in the 2PA signal in a 4 μm thick GaAs sample due to the excited carriers. Unfortunately, we currently cannot experimentally determine whether the reduction is strictly due to free-carrier absorption (FCA) of our pump or possibly due to a change in the two-photon absorption coefficient. We measure the transmission of various wavelengths around the bang gap of GaAs as a function of excitation wavelength and achieve a transmittance of ~80% which we attribute to possibly be one photon gain (1PG) at 880 nm. We also go to cryogenic temperatures to concentrate the carriers near the bottom of the conduction band and improve the theoretical gain coefficient for 2PE. Unfortunately, we do not observe a measurable change in 2PA with the addition of optically excited carriers. Along with FCA of our infrared pump we suspect that the difficulties in this first set of experiments are also a result or radiative recombination due to amplified spontaneous emission reducing our free carrier density along with the fact that 4 µm is too thick for uniform excitation. We now have 1 µm samples from Professor Gregory Salamo which we hope will give better and more definitive results.
M.S.
Masters
Optics and Photonics
Optics and Photonics
Optics

The dependence of tumour growth and metastasis on blood vessels makes tumour angiogenesis a rational target for therapy. Imaging of αvβ3 expression could potentially be used as a biomarker and an early indicator of efficacy of antiangiogenic treatments at a molecular level. Research efforts have mainly focused on the development of RGD-based radiolabelled αvβ3 inhibitors suitable for PET and SPECT imaging modalities that, owing to their high sensitivity, represent the most powerful tool for monitoring in vivo tumour angiogenesis. The aim of this multidisciplinary project was the design, synthesis and biological evaluation of novel αvβ3 ligands as molecular imaging probes. Three classes of integrin antagonists were designed: 1) triazole-based RGD mimetics that can be isotopically-labelled with tritium, fluorine and iodine radioisotopes by means of highly practical procedures, 2) RGD peptidomimetics incorporating the metabolically stable 2,2,2-trifluoroethylamine function as a peptide bond bioisostere and 3) RGD cyclopeptides conjugated with FDR, a novel prosthetic group allowing glycosylation and 18F-fluorination of aminooxy-functionalised molecules in one synthetic step. RGD-based strategies have also been used for selective tumour delivery of chemotherapeutic agents. A number of cytotoxic drugs have been conjugated to RGD peptides, providing experimental evidence that αvβ3 targeted chemotherapy strategies could be used as a powerful tool to reduce the toxicity and augment the therapeutic window of existing cytotoxic agents. In this work, we described the rational design of a novel targeted cytotoxic conjugate containing a triazole-based RGD peptidomimetic as tumour-homing motif of the potent antimitotic agent, paclitaxel. Preliminary in vitro studies were performed to assess the therapeutic potential of this targeted cytotoxic construct.

This thesis presents the template-based quantitative perfusion SPECT (TQPS) method, which is designed for the semi-quantitative analysis in SPECT myocardial perfusion imaging (MPI). Unlike traditional methods employing normal patient databases as the healthy standard when quantifying myocardial perfusion defects, the proposed method utilizes a patient-specific template for its healthy standard. In doing so, TQPS aims to overcome a number of the limitations associated with the non-patient-specific nature of normal patient databases. The TQPS method begins with the construction of a template, which is a 3D digital model of the patient’s healthy heart, using the SPECT reconstructed image. The template is then projected, reconstructed and sampled into the bulls-eye map domain. A ratio of the patient and template bulls-eye images produces a final corrected image in which a patient-specific threshold is applied to identify perfusion defects. Traditional semi-quantitative cardiac measurements, such as the summed stress score and perfusion defect extent were employed for the analysis. This thesis presents the investigation of TQPS in three phases: method evaluation, optimization, and validation. The first two phases focused on controlled simulation studies in which the assessment was based on how well TQPS was able to quantify myocardial perfusion defects relative to the truth. In these studies, the method was able to spatially define perfusion defects with a sensitivity and specificity of 79% and 76%, respectively, while estimating the global perfusion defect size to within 3% of the truth. In the third phase, the aim was to clinically evaluate TQPS relative to an established commercial method (QPS). TQPS exhibited improved specificity relative to the commercial method for the detection of significant coronary artery disease in the left anterior descending artery. The sensitivities for detecting 70% stenosis or greater in the LAD, LCX and RCA territories for QPS and TQPS were 60%, 82%, 75%, and 88%, 94%, 75%, respectively. In summary, the TQPS method was able to accurately quantify myocardial perfusion defects in SPECT MPI, while exhibiting considerable advantages over a traditional normal database method, particularly in the LAD coronary territory.

Many contemporary experimental QCD results achieve greater accuracy in measurement than equivalent theoretical predictions calculated at leading order. Therefore it is necessary to consider next to leading order (NLO) predictions for many processes in order to compare experiment with theory. Accurate theoretical predictions are also important in order to reduce the uncertainty in QCD parameters such as the coupling constant a, and to test whether QCD is in fact the correct theory to describe the strong interaction. With NLO results it is also possible to separate different clustering algorithms and test non-perturbative effects. This thesis concentrates on the techniques necessary for the calculation of NLO observables from the processes e(^+)e(^-) → 4 jets and pp → γ + X. We formulate a new version of the hybrid subtraction scheme based on the colour antenna structure of the final state to evaluate the necessary phase space integrals for the 4 jet process. The scheme is universal and can be applied to any QCD processes. The general purpose Monte Carlo EERAD2 which incorporates this new technique is compared with both experimental data gathered by the DELPHI collaboration and other groups which have reported similar calculations. A Monte Carlo written for the process pp → γ + X requires a knowledge of the non- perturbative photon fragmentation function, D(_γ), and the second half of this thesis concentrates on a calculation of this process using the ALEPH measurement of D(_γ) based on a democratic algorithm. The Monte Carlo DPRAD incorporates these techniques and results from it are compared with data from the Tevatron.

Following radiation excitation, exoelectrons which are emitted from the surface of a phosphor material on heating are produced from a depth of the order of ten nanometres thick. This permits the preparation of thin film TSEE dosimeters. The energies of these electrons are so small that only a multiplying device can be used to detect them. The study therefore involves building and developing a dosimetry system based on this principle. An energy band model has been used to describe the mechanism of exoelectron emission. The application of this model to experimental data shows that it can predict with accuracy the exoelectron energies and their frequencies of escape from traps which are eventually emptied by thermal stimulation. Deviations from theoretically predicted values are described and partially interpreted. A TSEE reader system has been developed which employs an electronic quenched gas-flow GM counter, ohmic regulated heating system and thermocouple for temperature measurements. The counting circuit is optimised for maximum pulse rate (25KHz for a resolution time of 40μsec), speed and minimum background electronic noise. The heating system can be controlled within 0.1°C/s - 10°C/s and the dosimeter actual temperature, accurately located to within ±2^oC. The whole set of TSEE data is accumulated on a BBC microcomputer. The most intense emitter, BeO was selected from among the commonly used exoelectron phosphors. The physical and chemical treatments of Na₂O and Li₂O doped thin film BeO which enhance TSEE dosimetry properties are considered. The description of preparation of the thin films is given. Characteristics of most importance to TSEE dosimetry obtained are as follows; a dose reproducibility within ±2.92&37, good linearity of response even from background level 10^-5Gy to 1Gy and an energy independence of response. The field in which this dosimetry system has proved very useful is in the study of interface dosimetry. It provides the necessary spatial resolution required for measurements in the steep dose gradient encountered between two dissimilar media of lead-; tin-; copper-; aluminium-; perspex; and carbon. From the results of measurements information can be gleaned about the finite dimensions of the interfaces for photon energies below 1.25MeV for which reliable information is scanty.

Seizures are thought to arise primarily from the cerebral cortex. However, the propagation and behavioral manifestations of seizures involve a network of both cortical and subcortical structures. The medial thalamus and upper brainstem reticular formation are crucial areas for the maintenance of normal consciousness. Bilateral involvement of these structures may be responsible for loss of consciousness during partial seizures. Therefore, we sought to investigate the role of the medial thalamus and brainstem in seizures. We performed SPECT ictal-interictal difference imaging co-registered with high-resolution MRI scans to localize regions of cerebral blood flow changes in patients undergoing inpatient monitoring for epilepsy. Ictal-interictal SPECT scans from 43 seizures in 40 patients were analyzed. The medial thalami showed SPECT difference imaging changes of >20% in 18 patients. Of patients with medial thalamic changes, the majority (13 of 18) had seizure onset in the temporal lobe, while only 1 had confirmed onset in extratemporal structures, and the remainder were non-localized. In contrast, in the 22 patients without >20% SPECT changes in the medial thalami, 6 had extratemporal onset, 6 had temporal onset, and the remainder were non-localized. In patients with temporal lobe seizures, the side of greater medial thalamic and brainstem reticular formation involvement was strongly related to SPECT injection timing such that there was a sequential pattern of ipsilateral followed by contralateral changes. Brainstem structures showed >20% SPECT changes in 27 of 43 seizures with no clear relation to temporal or extratemporal onset. We conclude that the medial thalamus is preferentially involved in seizures arising from the temporal lobes, possibly reflecting the strong connections between limbic temporal structures and the medial thalamus. Sequential involvement of ipsilateral followed by contralateral structures in the medial thalamus and upper brainstem may explain how seizures produce peri-ictal loss of consciousness despite incomplete involvement of the cerebral cortex.