Tesis sobre el tema "Photon annihilation"

In the face of global warming and shrinking resources of fossil fuels the interest in solar energy has increased in recent years. However, the low energy and cost efficiency of current solar cells has up to this date hindered solar energy from playing a major role on the energy market. Photon upconversion is the process in which light of low energy is converted to high energy photons. Lately, this phenomenon has attracted renewed interest and ongoing research in this field mainly focuses on solar energy applications, solar cells in particular. The aim of this study was to investigate and evaluate amyloid fibrils as nanotemplates for an upconversion system based on the dyes platinum octaetylporphyrin (PtOEP) and 9,10- diphenylanthracene (DPA). This well-known pair of organic dyes upconverts light in the visible spectrum through a mechanism known as sensitized triplet-triplet annihilation. Amyloid fibrils are β-sheet rich protein fibril structures, formed by self-assembly of peptides. Amyloid fibrils were prepared from whey protein isolate using heat and acidic solutions. Dyes were incorporated according to a wellestablished technique, in which dyes are grinded together with the protein in solid state prior to fibrillization. Photophysical properties of pure fibrils and dye-incorporated fibrils were studied using UV-VIS spectroscopy and fluorescence spectroscopy. Atomic force microscopy was further employed to confirm the presence of amyloid fibrils as well as to study fibril structure. Results indicate that amyloid fibrils may not be the optimal host material for the upconversion system PtOEP/DPA. It was found that the absorption and emission spectra of this system overlap to a great deal with that of the fibrils. Though no upconverted emission clearly generated by the dye system was recorded, anti-Stokes emission was indeed observed. Interestingly, this emission appears to be strongly enhanced by the presence of dyes. It is suggested that this emission may be attributed to the protein residues rather than the amyloid structure. Future studies are encouraged to further investigate these remarkable findings.
Intresset för solceller har ökat under de senaste åren, till stor del tillföljd av den globala uppvärmningen och de sinande oljeresurserna. Dagens solceller har dock problem med låg energi- och kostnadseffektivitet, vilket gör att solenergin än så länge har svårt att hävda sig på energimarknaden. Photon upconversion är ett fotofysikaliskt fenomen där fotoner med låg energi omvandlas till fotoner med hög energi. Den senaste tiden har denna process fått förnyat intresse och forskningen inom området har ökat, inte minst med sikte på att integrera processen i solceller och därmed öka dess effektivitet. Målet med denna studie var att undersöka huruvida amyloidfibriller kan användas som stomme för ett photon upconversion-system baserat på platinum-oktaetylporfyrin (PtOEP) och 9,10-difenylantracen (DPA). Dessa två organiska färgämnen är ett välkänt par som konverterar synligt ljus med låg frekvens till mer hög frekvent ljus i det synliga spektrumet, via en mekanism som kallas sensitized triplet-triplet annihilation. Amyloidfibriller är proteinbaserade fiberstrukturer med hög andel β-flak, vilka bildas genom självassociation av peptider. I denna studie skapades amyloidfibriller av vassleprotein genom upphettning i sur lösning. Färgämnena inkorporerades enligt en välbeprövad metod där proteinet mortlas tillsammans med färgämnena i fast tillstånd, innan fibrilleringsprocessen påbörjas. De fotofysikaliska egenskaperna hos fibriller med och utan färgämnen analyserade med UV-VIS samt fluorescensspektroskopi. Atomkraftsmikroskopi användes för att bekräfta att fibriller fanns i proven, samt för att studera dess struktur. De erhållna resultaten antyder att amyloidfibriller inte är ett optimalt material för systemet PtOEP/DPA, delvis på grund av att absorptions- och emissionsspektrumet för systemet överlappar med fibrillernas egna spektrum. Anti-Stokes emission detekterades, men denna är med stor sannolikhet inte orsakad av färgämnena. Dock noterades, intressant nog, att denna emission ökar betydligt i närvaro av färgämnena. En möjlighet är att denna emission är kopplad till monomerer i proteinet snarare än till fibrillstrukturen, eftersom emission observerades hos både nativt och fibrillerat protein. Framtida studier uppmuntras att vidare undersöka dessa effekter.

The use of three gamma annihilation as a new PET molecular imaging modality which can predict tumour hypoxia was proposed by Kacperski and Spyrou in 2004. The positronium and its annihilation, could then serve as an oxygen-sensitive marker. The relative magnitude of three photon annihilation is increased in an oxygen deficient environment. The main questions addressed in this thesis are the potential use of the lanthanum bromide LaBr3: Ce (5%) scintillator detector for three-photon positron annihilation measurements. Also, the optimum source-to-detector distance that achieves good efficiency and sensitivity of the measurements. The other question is finding the best method to prepare in vitro three types of hypoxic samples (mineral water, defibrinated horse blood and semm). In addition, the suitability of the hydrophilic material is examined in terms of hydration behaviour, radiation and physical properties, for use as a phantom that represents the hypoxic tumour in the three-photon positron annihilation measurements. The main critical question is which the best method that could be used to measure the three-photon positron annihilation yields. Also, how does the 3y/2y ratio vary with different oxygen concentrations in different hydrophilic materials? Lanthanum bromide LaBr3: Ce (5%) scintillator detectors have good fast timing resolution, good stopping power and large light output. The LaBr3:Ce (5%) has very good scintillator characteristics, combining high effective Z and density, fast decay time, light emission wavelengths matching that of commonly available photon detectors and excellent energy resolution (~3% at 662 keY). However, it is highly hygroscopic in nature, making it difficult to produce, but its commercial availability has been gradually increasing in recent times. LaBr3: Ce (5%) has come to be accepted for the superior energy resolution it offers. Therefore, LaBr3: Ce (5%) detectors can be considered as the scintillators of choice for determining the yield of 2-and3-y positron annihilation. The characterisation of the detector used was can-ied out in terms of energy resolution and efficiency. The effect of amplifier parameters on the energy resolution was also studied. The 'geometrical solid angles subtended by the source-to-detector distances play an important role in the sensitivity and accuracy of the detector efficiency measurements. Therefore, the efficiency measurements were investigated at various distances to optimise the geometrical solid angle for the LaBr3: Ce (5%) detector for the three photon annihilation measurements. The experimental data were compared with the GATE simulated results obtained. The objective to study the factors that affect dissolved oxygen (DO) in three types of samples (mineral water, defibrinated horse blood and semm) was to investigate the 3y/2y ratio in hypoxic, nOlIDoxic and hyperoxic conditions of tissues for future application in oncology in the detection and quantification of tumour hypoxia. This was achieved by measuring the DO, pH and temperature before, during and after treating the samples with nitrogen, carbon dioxide and ascorbic acid (AnaeroGen). Two methods for the preparation of the in vitro hypoxic samples were investigated and evaluated. Carbon dioxide proved to be most effective for the reduction of DO in the samples. Blood tended to resist DO reduction since it decreased at a much slower rate than in water and semm. Together with the fact that the oxygen level in blood remained low after the end of exposure to the gases and ascorbic acid suggests that the solubility of oxygen in blood depends upon the concentration of haemoglobin as well as upon ligands such as CO2 • CO2 combines with haemoglobin affecting oxygen binding and fonns bicarbonate which further decreases the affinity of haemoglobin to oxygen Hydrophilic materials.

Durante il dottorato ho investigato il processo fotofisico di "upconversion" assistito da annichilazione tripletto-tripletto (TTA-UC) tramite studi di spettroscopia in sistemi profondamente differenti gli uni dagli altri. In TTA-UC radiazione ad alta energia è emessa dalla ricombinazione radiativa dello stato di singoletto eccitato di una molecola emettitore, popolato precedentemente dall'annichilazione dei tripletti di due emettitori. Un sensibilizzatore immagazzina la luce incidente a bassa energia e trasferisce l'eccitazione agli emettitori tramite trasferimento di energia alla Dexter. Poiché il suo funzionamento si basa su tripletti mestastabili, TTA-UC può essere altamente efficiente anche in condizioni di luce non coerente e a bassa energia. Come tale, è particolarmente adatto per dispositivi che sfruttano l'energia solare poiché è in grado di aumentarne l'efficienza di conversione limitando le perdite per trasmissione. Mi sono concentrata su due problemi importanti che tuttora limitano l'impiego di materiali che attuano TTA-UC (upconverters), ossia la limitata capacità di immagazzinare energia dei comuni sensibilizzatori organici e le scarse prestazioni di TTA-UC in upconverters a stato solido, i quali sono più adatti per applicazioni tecnologiche rispetto a sistemi liquidi. Per risolvere il primo problema ho investigato sensibilizzatori ibridi, composti da nanostrutture a semiconduttore decorate con molecole organiche, con ampio assorbimento. Nanocristalli di CdSe drogati con cationi d'oro e decorati con acido antracenico carbossilico si sono dimostrati essere sensibilizzatori ibridi efficienti ed innovativi. Il drogante introduce nel gap energetico dei nanocristalli livelli localizzati su cui le lacune si localizzano sulla scala dei picosecondi, più velocemente dell'estrazione di lacune sul livello HOMO dei leganti. Con tale strategia ho raggiunto l'efficienza di UC del 12%, record per sistemi ibridi. Ho poi mostrato come le proprietà superficiali e fotofisiche di nanoplatelets di CdSe le rendano ottimali candidati in sensibilizzatori ibridi. Ho mostrato che il ricoprimento delle superfici non è omogeneo, ma procede ad isole e l'interazione di "π- π stacking" porta alla formazione di aggregati sulle superfici delle nanoplatelets, con il risultato di ridurre l'energia dei tripletti dei leganti con profonde ripercussioni sulle prestazioni di TTA-UC e sulla scelta della specie emettitrice. Riguardo al secondo problema, ho studiato due upconverters a stato solido, polimeri vetrosi nanostrutturati che mostrano proprietà macroscopiche simili ma realizzati con tecniche differenti. Essi presentano domini liquidi di dimensione inferiore a 50 nm dove le specie che attuano TTA-UC si accumulano, racchiuse in una matrice rigida polimerica che fornisce protezione da ossigeno e qualità ottica eccellenti e stabilità a lungo termine. Il confinamento molecolare permette di aumentare la densità locale di eccitoni aumentando l'efficienza di UC a basse potenze grazie alle ridotte distanze intermolecolari e all'attivazione del regime di TTA-UC confinato.Ho inoltre studiato un nuovo emettitore derivato da perilene, realizzato con lo scopo di aumentarne l'efficienza di fluorescenza. Grazie a questo emettitore ho raggiunto l'efficienza record di UC di 42%, dovuta proprio alla struttura molecolare dell'emettitore che permette di limitare la formazione di aggregati, garantendo un'eccellente efficienza di generazione di singoletti tramite TTA. Infine, ho presentato una prospettiva riguardo alle prestazioni che possono essere raggiunte combinando le due tematiche trattate, ossia inserendo sensibilizzatori ad ampio assorbimento in polimeri nanostrutturati. Trovando il giusto compromesso tra taglia dei domini liquidi e distribuzione dell'energia di eccitazione si raggiungerebbe la massima efficienza di UC a potenze minori dell'irradianza solare, promuovendo lo sviluppo di upconverters a stato solido per tecnologie a energia solare
In my PhD project, I investigated the photophysical process of photon upconversion assisted by triplet-triplet annihilation (sTTA-UC) through spectroscopy studies in a variety of systems, profoundly different on many levels. In sTTA-UC high energy radiation is emitted from the fluorescent recombination of the excited singlet of an emitter molecule, previously populated via annihilation of the metastable triplet states of two emitters. This is a sensitized process since a sensitizer is necessary to harvest the low energy incident light and to transfer the stored energy to the emitters via Dexter energy transfer. Because its functioning relies on long-lived metastable triplets, this process can be highly efficient also under low power, noncoherent light. As such, sTTA-UC is particularly suited for solar applications as it can increase the conversion efficiency by reducing transmission losses. During my studies, I focused on addressing two crucial issues that still limit the application of upconverters in solar technologies, i.e. the limited storage ability of common organic sensitizers and the poor sTTA-UC performance in solid-state upconverters, which are intrinsically better suited than liquid solutions for technological applications. To solve the first problem, I investigated hybrid sensitizers, composed of semiconductor nanostructures decorated with conjugated organic ligands characterized by broadband absorption. CdSe nanocrystals (NCs) doped with gold cations and decorated with 9-anthracene carboxylic acid demonstrated to be efficient innovative broadband hybrid sensitizers. The doping strategy inserts into the NCs energy gap localized hole-accepting states where the holes localize on the picosecond timescale, outpacing hole transfer to the ligand HOMO. With this strategy, I achieved the UC efficiency of 12%, the record performance obtained so far for hybrid upconverters. I then discussed how the CdSe nanoplatelets surface and photophysical properties make them potential optimal light harvesters. My studies on the nanoplatelets-to-ligands energy transfer dependency on the surface ligand density revealed that the surface coverage is not homogeneous but proceeds in an island-like way promoted by π- π stacking and results in the formation of ligands aggregates on the nanoplatelets surfaces, which causes a redshift of the ligand triplet energy with critical repercussions on the sTTA-UC performance and on the emitter selection. To address the second issue, I investigated two solid-state upconverters, i.e. nanostructured glassy polymers that show similar macroscopic properties but fabricated via different approaches. They both feature liquid droplets of mean size less than 50 nm where the upconverting dyes accumulate, embedded in a rigid polymer matrix that grants excellent oxygen protection and optical quality and long-term stability. The dyes confinement allows to increase the effective local excitons density resulting in an enhanced UC efficiency at low excitation intensities, thanks to the reduced intermolecular distances and the activation of the confined sTTA-UC regime. I also introduced a new perylene derivative as emitter, specifically designed to prevent molecular aggregation to maximize its fluorescence efficiency. By employing this emitter, I achieved the record UC efficiency of 42%, which directly stems from the emitter molecular structure, as it limits the formation of aggregates, while guaranteeing excellent singlet generation efficiency upon TTA. I finally presented a perspective of the performances that can be achieved by combining the two topics considered, i.e. loading broadband sensitizers in nanostructured polymers. I highlighted that if the best trade-off between nanostructure size and energy distribution is met the maximum UC efficiency can be achieved at excitation powers orders of magnitude lower that the solar irradiance, therefore promoting the development of real-world solid-state upconverters.

Photon energy upconversion (UC), a process that can convert two or more photons with low energy to a single photon of higher energy, has the potential for overcoming the thermodynamic efficiency limits of sunlight-powered devices and processes. An attractive route to lowering the incident power density for UC lies in harnessing energy transfer through triplet-triplet annihilation (TTA). To maximize energy migration in multicomponent TTA-assisted UC systems, triplet exciton diffusivity of the chromophores within an inert medium is of paramount importance, especially in a solid-state matrix for practical device integration. In this thesis, low-threshold sensitized UC systems were fabricated and demonstrated by a photo-induced interfacial polymerization within a coaxial-flow microfluidic channel and in combination with nanostructured optical semiconductors. Dual-phase structured uniform UC capsules allow for the highly efficient bimolecular interactions required for TTA-based upconversion, as well as mechanical strength for integrity and stability. Through controlled interfacial photopolymerization, diffusive energy transfer-driven photoluminescence in a bi-molecular UC system was explored with concomitant tuning of the capsule properties. We believe that this core-shell structure has significance not only for enabling promising applications in photovoltaic devices and photochromic displays, but also for providing a useful platform for photocatalytic and photosensor units. Furthermore, for improving photon upconverted emission, a photonic crystal was integrated as an optical structure consisting of monodisperse inorganic colloidal nanoparticles and polymer resin. The constructively enhanced reflected light allows for the reuse of solar photons over a broad spectrum, resulting in an increase in the power conversion efficiency of a dye-sensitized solar cell as much as 15-20 %.

Identifying and localising tumor hypoxia in cancer patients is a challenge in oncology imaging. There are many ongoing clinical trials using conventional positron emission tomography (PET) scans and PET agents as cellular markers for detection of tumour hypoxia depending on the concept of the basic physics of 2gamma annihilation. However, Kacperski and Spyrou (2004) proposed, for the first time, to use 3gamma annihilation as a new PET molecular imaging modality, where the positronium and its annihilation, could serve as an oxygen-sensitive marker. The effective yield of 3gamma annihilation depends on the rates of formation and quenching. The formation of positronium is not only sensitive to the physics parameters, but also behaves as an active chemical particle. A hypoxic cell is a microenviromnent which has an inadequate amount of oxygen. Oxygen is known to be a strong positronium quencher where 2gamma annihilation replaces the 3gamma process. It is thus possible for hypoxic cells to be characterised by higher 3gamma rates than those cells which are well oxygenated. The measurement of 3gamma annihilation to differentiate between oxygenated and non-oxygenated biological samples in order to extract useful information in PET for oncology is a challenging project. It opens up very interesting applications in nuclear medicine imaging as the 3gamma yield has not been measured before in biological tissue and in particular hypoxic tumour cells. The project is multidisciplinary involving physics, biology and chemistry. There are many factors which affect the dissolved oxygen in mineral water, defibrinated horse blood and serum samples. These factors and the challenges to prepare in-vitro hypoxic samples have been experimentally measured using polarography with different chemical reactions e.g. carbon dioxide, nitrogen and AnaeroGen(TM). Results have shown that polarography was more suitable than colorimetery in the measurement of dissolved oxygen in blood due to the high absorption characteristic of blood. AnaeroGen(TM) is the method of choice, unlike carbon dioxide or nitrogen treatment, for preparing hypoxic samples due to the good agreement of the behaviour of the oxygen reduction as in the oxyhaemoglobin dissociation curve which is caused by a decrease in pH, an increase in partial pressure of carbon dioxide and an increase in temperature. The 3gamma yield was measured in normoxic and hypoxic environments using the triple coincidence measurement of three high-energy resolution detectors (HPGe). The AnaeroGen(TM) was used to generate a hypoxic environment. The percentage of the coincidence events qualified as 3gamma was 26.5% higher in the hypoxic environment. This reseach work is a step towards the application of the novel modality of 3gamma PET which in conjunction with conventional 2gamma PET could serve as a non-invasive oxygen sensitive marker. The combination of 3gamma and 2gamma coincidences in nuclear medicine imaging systems may contribute important information for the development and validation of appropriate hypoxia markers. The relative 3gamma/2gamma yield was measured for a positron emitter 22Na with the new generation of scintillator LaC13:Ce and LaBr3:Ce detectors, which had been characterised together with NaI(Tl) and HPGe detectors. The experimental focus was on measuring the relative 3gamma/2gamma yield in different samples by applying the peak-to-peak and the peak-to-valley methods. The value of the ratio 3gamma/2gamma depends on the specimen and is of the order of 10-2. The relative 3gamma/2gamma yields obtained for the peak to peak method in the silica sample were, for example, (3.41+/-0.18)x10 -2, (2.98+/-0.13)x10-2, (4.01+/-0.16)x10 -2 and (2.12+/-0.14)x10-2 for LaBr3:Ce, LaCl3:Ce, NaI(Tl) and HPGe detectors, respectively. The results show that the lanthanum based crystals, LaBr3:Ce and LaCl3:Ce, have the potential to replace NaI(Tl) and HPGe due to both good energy resolution and good detection efficiency and can be the scintillator of choice for determining the yield of 3gamma. The peak-to-valley method was applied to measure the relative yield of 3gamma/2gamma positron annihilation using 18F in 11 haematological samples of different oxygenation levels. The relative 3gamma/2gamma yield was found to vary as much as 11% between the components investigated.

In this thesis the main aspects of three photon positron annihilation processes and their potential use in medical imaging have been investigated as a positron emission tomography technique. The main objectives are focused on: three-photon positron annihilation measurement and imaging, analytical modelling and Monte Carlo simulation and the evaluation of the detection system requirements. A novel method as proof-of-principle of the three photon positron annihilation imaging concept based on a triple coincidence imaging technique using high energy resolution semiconductor detectors has been introduced. It has been shown that a simple system of three high-energy resolution detectors is able to produce images of three photon positron annihilations events. The full energy photopeak detected of the true 3y events can be easily identified in the spectrum. Although the sensitivity is small due to a very small solid angle (~0.05 str) subtended by the detectors and rather poor detection efficiency, it is a first step towards a scanner capable of a new imaging modality. This method has been investigated using Monte Carlo simulation results and experimental data acquired. Further a new three-photon yield measurement method based on three-photon positron annihilation imaging technique with correction for scattered and random events is proposed. The feasibility of this approach has been verified using experiments and compared to existing methods. Results show that this method is more accurate with better scatter correction due to electronic collimation than others but it has also some limitations. In order to obtain quantitative information from the detection system it is necessary to establish mathematical or analytical models, which describe the system. This was achieved for the triple coincidence condition. The count rate of single and triple detected events was investigated. Results have shown differences due to scatter and random events estimation. The effect of semiconductor detectors properties on three-photon image quality and scanner design was also investigated. It was in addition shown that computer simulations can be effectively used to predict the image quality and background noise for a particular scanner design. Important characteristics which affect scanner performance were evaluated. The effect of detector and scanner size on spatial resolution of three photon images was discussed. It was found that by reducing scanner size spatial resolution was improved for three-photon positron annihilation imaging as for conventional two photon-positron annihilation. Variation of scanner size (scanner diameter) affects the point spread function of the three photon positron annihilation image profile and introduces a combination of errors due to photon energy and detection position. To introduce the three-photon technique in a dedicated PET system, high- energy resolution detectors are needed to improve the quality of the image and reduce the noise due to scattered events arising from Compton scattering which do not correspond to 3y events. Semiconductor detectors, particularly CZT which have good energy resolution, significantly better stopping power and can be used at room temperature are proposed as the detectors of choice for the new detection system. Therefore, images of two photon-positron annihilation can be mapped with those of 3- photon events and new valuable information can be extracted. This information will be valuable to treatments involving external beam radiotherapy and may also be of use in brachytherapy.

Photon upconversion by sensitized triplet–triplet annihilation (UC-STTA) is a photophysical process that facilitates the conversion of two low-energy photons into a single high-energy photon. A low-energy photon is absorbed by a sensitizer molecule that produces a triplet excited state which is transferred to an emitter molecule. When two emitter triplet states encounter each other, TTA can take place to produce a singlet excited state which decays by emission of a high-energy (upconverted) photon. While traditional single-threshold dye-sensitized solar cells (DSSCs) have a maximum efficiency limit of ca. 30%, it has been predicted theoretically that implementation of UC-STTA in DSSCs could increase that efficiency to more than 40%. A possible way to implement UC-STTA into DSSCs, would be to replace the standard sensi- tized nanostructured TiO2 photoanodes by upconverting ones loaded with emitter molecules. Following TTA, the excited emitter molecule would be quenched by injection of a high-energy electron into the conduction band of the TiO2. To explore the practical aspects of this strategy for a highly efficient DSSC, in this thesis UC-STTA is studied in model systems based on nanostructured ZrO2 films. These ZrO2 films are a good proxy for the TiO2 films used in DSSCs, and allow for relatively easy optimization and study of UC-STTA by allowing measurements of the upconverted photons without the complications of electron injection into the film. Herein it is experimentally proven that UC-STTA is viable on nanostructured metal oxide films under non-coherent irradiation with intensities comparable to sunlight. Two different system architectures are studied, differing in the position of the molecular components involved in the UC-STTA mechanism. Both architectures have the emitter molecules adsorbed onto the ZrO2 surface, but the sensitizers are positioned either in solution around the nanostructure, or co-adsorbed with the emitters onto the ZrO2 surface. A set of challenges in the study and optimization of the UC-STTA process is identified for each type of system. Proposals are also given for how to further improve the understanding and UC-STTA optimization of these systems toward application in DSSCs to overcome the present solar energy conversion efficiency limit.

The major advantage of position sensitive radiation detector systems lies in their ability to non invasively map the regional distribution of the emitted radiation in real-time. Three of such detector systems were studied in this thesis, gamma-cameras, positron cameras and CMOS image sensors. A number of physical factors associated to these detectors degrade the qualitative and quantitative properties of the obtained images. These blurring factors could be divided into two groups. The first group consists of the general degrading factors inherent to the physical interaction processes of radiation with matter, such as scatter and attenuation processes which are common to all three detectors The second group consists of specific factors inherent to the particular radiation detection properties of the used detector which have to be separately studied for each detector system. Therefore, the aim of this thesis was devoted to the development of computational methods to enable quantitative molecular imaging in PET, SPET and in vivo patient dosimetry with CMOS image sensors.

The first task was to develop a novel quantitative dual isotope method for simultaneous assessments of regional lung ventilation and perfusion using a SPET technique. This method included correction routines for photon scattering, non uniform attenuation at two different photon energies (140 and 392 keV) and organ outline. This quantitative method was validated both with phantom experiments and physiological studies on healthy subjects.

The second task was to develop and clinically apply a quantitative method for tumour to background activity uptake measurements using planar mammo-scintigraphy, with partial volume compensation.

The third stage was to produce several computational models to assess the spatial resolution limitations in PET from the positron range, the annihilation photon non-collineairy and the photon depth of interaction.

Finally, a quantitative image processing method for a CMOS image sensor for applications in ion beam therapy dosimetry was developed.

From the obtained phantom and physiological results it was concluded that the methodologies developed for the simultaneous measurement of the lung ventilation and perfusion and for the quantification of the tumour malignancy grade in breast carcinoma were both accurate. Further, the obtained models for the influence that the positron range in various human tissues, and the photon emission non-collinearity and depth of interaction have on PET image spatial resolution, could be used both to optimise future PET camera designs and spatial resolution recovery algorithms. Finally, it was shown that the proton fluence rate in a proton therapy beam could be monitored and visualised by using a simple and inexpensive CMOS image sensor.

La technique originale de spectroscopie par saturation d'absorption, dite holeburning spectral, est mise en oeuvre pour étudier l'élargissement homogène de transitions optiques à 1,55 μm dans deux types de nanostructures. Pour les boîtes quantiques GaN/AlN, notre expérience constitue la première mesure directe de la largeur homogène de la transition intrabande s − pz. Des études en puissance démontrent le rôle prédominant des processus Auger dans la relaxation de population des niveaux. Le profil spectral d'absorption homogène s'avère gaussien. La forte augmentation de la largeur homogène entre 5K et 30K suggère des mécanismes de décohérence autres que le couplage aux phonons acoustiques, comme la diffusion spectrale. Dans le cas des nanotubes de carbone, notre dispositif expérimental permet d'étudier finement l'évolution du spectre d'absorption homogène de la transition électronique fondamentale sur une large gamme de puissance et pour des températures allant de 5K à 300 K. Les études en puissance mettent en évidence quantitativement la contribution prédominante de l'élargissement collisionnel et la contribution marginale de la réduction de force d'oscillateur au signal nonlinéaire. Deux processus d'interaction à deux excitons sont analysés : l'annihilation exciton-exciton (EEA) et la diffusion exciton-exciton (EES), et nous révélons la nature hybride Wannier-Frenkel particulière des excitons dans les nanotubes de carbone. Finalement, nous étudions le déphasage assisté par phonons et nous mettons en évidence les caractéristiques du couplage exciton-phonon, liées au caractère unidimensionnel.

In this thesis work, after a brief introduction of some basic concepts of quantum optics, some experimental techniques useful for the manipulation of quantum states of light are presented. In particular, I focused on two transformations operated at the single-photon level, like the addition and the subtraction of a single photon from a travelling light beam. Both these methods share a probabilistic approach, that is their implementation has a non unitary success probability. Anyway, the detection of an additional photon in an ancillary mode exactly announces when the operation has been actually implemented. That's why this type of implementation is generally called heralded. Starting from these basic operations, a way to implement some of their superpositions is then analyzed. In the second part of this thesis, I describe two state-engineering experiments based on these fundamental operations. The first one realizes a so-called universal orthogonalizer, an universal strategy that, given an arbitrary input state, can be used to produce at the output an orthogonal one. The adjective universal refers to the fact that the procedure works equally well for arbitrary input fields. The other experiment uses coherent superpositions of basic quantum operations to emulate a strong Kerr nonlinearity at the few-photon level, and could be used to implement new logic gates for quantum computation. The third part of this thesis is dedicated to a different experimental approach, in some sense complementary to state engineering: mode engineering. Indeed, while in state engineering the purpose is the detection of an unknown state after its manipulation, in mode engineering experiments the perspective is completely reversed: the final state is assumed to be known and what has to be inferred are its spatio-temporal features. By following this alternative experimental approach, the interaction of broadband single photons with resonant atomic vapours is analyzed by observing the traces left on the photon temporal shape.

The present work reports the study of noncoherent photon upconversion (NCPU) via triplet-triplet annihilation (TTA) in polymer systems. This upconversion mechanism has application in photovoltaic devices through the utilization of sub-band gap photons for potentially enhanced power conversion efficiencies. First, homomolecular TTA was studied in zinc tetraphenylporphyrin (ZnTPP) in polymer matrices. Here, ZnTPP acts as both the sensitizer and upconverting emitter as TTA yields an S2 excited porphyrin. Use of poly(methyl methacrylate) (PMMA) as the host polymer demonstrates aggregation-driven upconverted fluorescence (UC) by TTA (TTA-UC). The dye-loading ratio of the precursor solution was varied, controlling the degree of pre-aggregation. Power-dependence studies of the champion film demonstrated that TTA-UC is occurring toward the strong annihilation kinetic limit. A sub-linear dependence of upconverted fluorescence on film thickness was observed in this system. The ZnTPP study was extended to polymers possessing low glass transition temperatures, representing molecular diffusion-driven TTA-UC. Upconverted fluorescence was not observed in ZnTPP in a polyurethane (PU) matrix, likely due to coordination of the PU to the axial position of the Zn2+ ion. Low intensity NCPU via homomolecular TTA was observed in ZnTPP in a poly(ethylene glycol) (PEG) matrix, but the kinetic limit was not determined due to film photodegradation. Dye-loading studies revealed that porphyrin self-quenching was evident at low dye concentrations. Likely reasons for the low upconverted fluorescence intensities realized are this self-quenching and the possibility of PEG coordination to the Zn2+ ion, though it is believed self-quenching is the dominant parasitic effect. Strategies to determine the effect and extent of polymer coordination to the Zn2+ ion are discussed. The study of polymer-based NCPU is extended to a pair of macromolecules, each containing a single ruthenium tris(bipyridine) (Ru(bpy)3) core and multiple pendant arms, which in turn, each contain eight 9,10-diphenylanthracene (DPA) moieties. A power-dependence study of NCPU in this system is reported, and TTA-UC in the weak annihilation kinetic limit was observed. Upconverted fluorescence quantum yields vary linearly with excitation power in both polymers, consistent with the observed kinetic limit. Stern-Volmer experiments have compared the quenching of Ru(bpy)3 phosphorescence (Ph) by monomeric and polymeric DPA. These data show an enhancement in quenching rate constant for the DPA polymer (pDPA). Kinetic analysis of the Ru-DPA polymers has revealed that the energy scheme realized in this system is intrachain TTET from Ru(bpy)3 core to DPA emitter followed by interchain TTA between excited DPA moieties. Low intensity upconverted fluorescence is observed in Ru-DPA containing thin films. Based on the results presented, the requirements of future photophysically-active polymers are discussed with regards to meaningful application in photovoltaics.

The photophysical properties of a series of Zn (II) porphyrins adsorbed onto a semiconductor were investigated using steady-state absorbance and emission measurements. The ability of the porphyrins to undergo triplet-triplet annihilation (TTA), a photophysical process through which photons in the red and near-infrared (NIR) regions of the optical spectrum can be converted into higher energy photons (upconversion), was explored. Aggregation capabilities were determined to verify possibility of these molecules to undergo triplet-triplet annihilation (TTA). TTA has significant potential for increasing the efficiency of dye-sensitized solar cells (DSSCs) by upconverting photons in the energy rich NIR region of the solar spectrum. A key requirement for efficient TTA is aggregation of the sensitizer dye, and in this thesis, we have examined the aggregation of porphyrins in TiO2-based sol-gel films. Solution phase absorption and emission studies were conducted using zinc (II) tetraphenylporphyrin and three of its functionalized derivatives, tetra(4-aminophenyl)porphyrin Zn(II), tetra(4-carboxyphenyl)porphyrin Zn(II), and tetra(4-sulfonatophenyl)porphyrin Zn(II), to evaluate their potential as DSSC sensitizers on TiO2 thin films. Mesoporous TiO2 thin films were synthesized, using a polymer-templating sol-gel route, and characterized with tunneling electron microscopy (TEM), atomic force microscopy (AFM), and UV-Vis absorbance measurements. Spectroscopy measurements were also carried out on porphyrin-sensitized TiO2 thin films and compared to solution-based results. A simple DSSC was constructed and used to further explore the application of zinc (II) porphyrin sensitizers in photovoltaic applications.