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Past research has shown that the processing of emotional visual stimuli and visual attention are tightly linked together. In particular, emotional stimuli processing can modulate attention, and, reciprocally, the processing of emotional stimuli can be facilitated or inhibited by attentional processes. However, our understanding of these interactions is still limited, with much work remaining to be done to understand the characteristics of this reciprocal interaction and the different mechanisms that are at play. This thesis presents a series of experiments which use eye-tracking, behavioural and event-related potential (ERP) methods in order to better understand these interactions from a cognitive and neuroscientific point of view. First, the influence of emotional stimuli on eye movements, reflecting overt attention, was investigated. While it is known that the emotional gist of images attracts the eye (Calvo and Lang, 2004), little is known about the influence of emotional content on eye movements in more complex visual environments. Using eye-tracking methods, and by adapting a paradigm originally used to study the influence of semantic inconsistencies in scenes (Loftus and Mackworth, 1978), we found that participants spend more time fixating emotional than neutral targets embedded in visual scenes, but do not fixate them earlier. Emotional targets in scenes were therefore found to hold, but not to attract, the eye. This suggests that due to the complexity of the scenes and the limited processing resources available, the emotional information projected extra-foveally is not processed in such a way that it drives eye movements. Next, in order to better characterise the exogenous deployment of covert attention toward emotional stimuli, a sample of sub-clinically anxious individuals was studied. Anxiety is characterised by a reflexive attentional bias toward threatening stimuli. A dot-probe task (MacLeod et al., 1986) was designed to replicate and extend past findings of this attentional bias. In particular, the experiment was designed to test whether the bias was caused by faster reaction times to fear-congruent probes or slower reaction times to neutral-congruent probes. No attentional bias could be measured. A further analysis of the literature suggests that subliminal cue stimulus presentation, as used in our case, may not generate reliable attentional biases, unlike longer cue presentations. This would suggest that while emotional stimuli can be processed without awareness, further processing may be necessary to trigger reflexive attentional shifts in anxiety. Then the time-course of emotional stimulus processes and its modulation by attention was investigated. Modulations of the very early visual ERP C1 component by emotional stimuli (e.g. Pourtois et al., 2004; Stolarova et al., 2006), but also by visual attention (Kelly et al., 2008), were reported in the literature. A series of three experiments were performed, investigating the interactions between endogenous covert spatial attention and object-based attention with emotional stimuli processing in the C1 time window (50–100 ms). It was found that emotional stimuli modulated the C1 only when they were spatially attended and task-irrelevant. This suggests that whilst spatial attention gates emotional facial processing from the earliest stages, only incidental processing triggers a specific response before 100 ms. Additionally, the results suggest a very early modulation by feature-based attention which is independent from spatial attention. Finally, simulated and actual electroencephalographic data were used to show that modulations of early ERP and event-related field (ERF) components are highly dependent on the high-pass filter used in the pre-processing stage. A survey of the literature found that a large part of ERP/ERF reports (about 40%) use high-pass filters that may bias the results. More particularly, a large proportion of papers reporting very early modulations also use such filters. Consequently, a large part of the literature may need to be re-assessed. The work described in this thesis contributes to a better understanding of the links between emotional stimulus processing and attention at different levels. Using various experimental paradigms, this work confirms that emotional stimuli processing is not ‘automated’, but highly dependent on the focus of attention, even at the earlier stages of visual processing. Furthermore, the uncovered potential bias generated by filtering will help to improve the reliability and precision of research in the ERP/ERF field, and more particularly in studies looking at early effects.

Various semiconductor nanocrystals are currently being studied for their potential in optoelectronic devices such as photovoltaics, lasers, and lighting. Indium phosphide (InP) nanocrystals are particularly noteworthy due to their reduced toxicity and increased environmental sustainability as compared to other semiconductor nanocrystals. Indium phosphide nanocrystals are defect prone but can be passivated with a shell material such as zinc selenide (ZnSe) to mitigate this property and create stable highly luminescent QDs. Due to indium phosphide’s relatively small band gap and the aforementioned stability, InP/ZnSe QDs can be used to make efficient green, red, and near IR emitters. Chapter 1 of this work will begin with an exploration of the structure of indium phosphide zinc selenide quantum dots (QDs). The crystal and electronic structure of InP will be used to form a framework for the interpretation of the absorption and emission properties of InP/ZnSe QDs. This will be done by considering the existing literature on cadmium selenide (CdSe) QD nanocrystals. These two semiconductor nanocrystals share a zinc-blende crystal structure, and both have band structures with two degenerate valence bands at the band edge. In semiconductor QDs charge carriers can be described using an effective mass approximation determined by the curvature of these bands. This will lead to a discussion of envelope functions and fine structure states within those envelope functions. Finally, chapter 1 will include a shortened synthesis of three variations of InP/ZnSe nanoparticles that will be discussed in future chapters. Using the framework created in Chapter 1, Chapter 2 will relate these envelope functions and fine structure states to the absorption and emission of the corresponding transitions. This interpretation will discuss the factors that contribute to the line widths of these particles and will introduce the concept of homogeneous and inhomogeneous distributions in ensemble measurements. This will be explored through the use of the photoluminescence excitation spectrum (PLE) and polarization resolved PLE. In order to fully explain the width of the excitation spectra it will be necessary to recognize and characterize two forms of inhomogeneous broadening, size inhomogeneity and interfacial inhomogeneity. This interfacial inhomogeneity leads to an inhomogeneity in the band offsets of InP and ZnSe which complicates the spectroscopy of these QDs. Chapter 3 will introduce time resolved spectroscopies. Direct comparison with time resolved photoluminescence (TRPL) experiments with radiative lifetimes calculated using the Einstein relations will show that a thermal Boltzmann distribution between the non-emissive, and emissive states are required to relate the two values, as is predicted by the fine structure discussed in chapter 1. Furthermore, the observed radiative lifetime varies across the emission band as predicted by the Einstein relations. This property can be used to determine the extent of aggregation in samples by looking at deviations in radiative lifetime due to Förster resonant energy transfer (FRET).If during the shelling of InP/ZnSe QDs, the presence of indium is not carefully controlled, excess indium can become incorporated into the ZnSe shell. This leads to the formation of indium-based transient traps in the shell. Indeed, most published synthesis do not control for this variable, and thus these traps contribute significantly to the photoluminescent properties of published InP/ZnSe QDs. The indium-based shell transient traps are positioned between the valence band of the InP core, and the ZnSe shell valence band. This has the effect of trapping holes in the shell until they can tunnel into the valence band. Trapped holes have very small overlap with the electron wavefunction, and therefore do not contribute directly to the absorption or emission. Instead, they provide a reservoir state in thermal equilibrium with the emissive bright state, leading to delayed band edge emission. This increases the observed radiative lifetime by up to 30% for larger QDs. The equilibrium formed by process has a larger ratio of holes in the core to shell as opposed to the system short times after the cooling process, which has comparatively more holes in the shell. In higher fluence conditions the InP/ZnSe QDs can absorb a second photon either simultaneously or sequentially. When this happens a biexciton forms. Biexcitons primarily recombine by Auger recombination. Auger recombination is when instead of a biexciton recombining radiatively, two carriers recombine and transfer their momentum to the two remaining carriers. This can create very high energy or “Hot” carriers. These hot carriers are of great interest because they are known to be able to perform chemical reactions that can lead to the degradation and quenching of photoluminescence in QDs. Minimizing or controlling these “hot” carriers is of utmost importance to increasing the stability of InP/ZnSe QDs at high fluences. Biexciton measurements can be measured in TRPL, but the increased time resolution of Transient Absorbance (TA) makes for easier and more accurate measurements. TRPL and TA biexciton measurements will be compared to extract information about the energetic position of the biexciton emitting state as compared to the single exciton emitting state.As mentioned above, trapped holes exhibit minimal overlap with the electron wavefunction, and also interact weakly with the valence band. When two excitons are created if one of the holes cools into a trap state, it will be unable to efficiently undergo Auger recombination. This creates a pseudo-trion, where there are two electrons in the conduction band and a single hole in the valence band. The trapped hole, now held in a trap away from the core, induces an electric field across the QD that decreases the electron-hole interaction and slows the observed radiative lifetime compared to that of the analogous trion. This pseudo trion state that will be referred to as an XT state has an observed radiative lifetime that is a factor of four longer than the biexciton state, enabling precise determination of these rates through the comparison of both time-resolved photoluminescence (TRPL) and transient absorption (TA) measurements. Chapter 4 will consider numerous possibilities regarding the chemical/structural identity of the indium-based transient traps. It will be shown that a zinc vacancy being charge compensated by a substitutional In3+ ion is the most likely candidate. Raman spectroscopy will be used to probe the structural characteristics of the QD and to elucidate the position of excess indium in the QD. This discussion on hole core equilibriums will be expanded to coupled thiols, which can function as hole acceptors, which can trap off holes for extended periods of time, up to hundreds of nanoseconds. InP/ZnSe QDs hold great promise for the development of optoelectronic devices with enhanced environmental sustainability and reduced toxicity compared to other semiconductors. However, the presence of indium-based traps has significant implications for the photophysics and working lifetime of devices making use of InP/ZnSe QDs. Current literature on these particles is naïve to the contributions of these traps and the fingerprints of these traps can be seen in many key papers. This work, and the work it is based on, seeks to carefully disentangle the properties of the InP/ZnSe QD from those contributed by the traps in order to allow for accurate assignments.

Nonlinear parametric inverse problems appear in many applications in science and engineering. We focus on diffuse optical tomography (DOT) in medical imaging. DOT aims to recover an unknown image of interest, such as the absorption coefficient in tissue to locate tumors in the body. Using a mathematical (forward) model to predict measurements given a parametrization of the tissue, we minimize the misfit between predicted and actual measurements up to a given noise level. The main computational bottleneck in such inverse problems is the repeated evaluation of this large-scale forward model, which corresponds to solving large linear systems for each source and frequency at each optimization step. Moreover, to efficiently compute derivative information, we need to solve, repeatedly, linear systems with the adjoint for each detector and frequency. As rapid advances in technology allow for large numbers of sources and detectors, these problems become computationally prohibitive. In this thesis, we introduce two methods to drastically reduce this cost. To efficiently implement Newton methods, we extend the use of simultaneous random sources to reduce the number of linear system solves to include simultaneous random detectors. Moreover, we combine simultaneous random sources and detectors with optimized ones that lead to faster convergence and more accurate solutions. We can use reduced order models (ROM) to drastically reduce the size of the linear systems to be solved in each optimization step while still solving the inverse problem accurately. However, the construction of the ROM bases still incurs a substantial cost. We propose to use randomization to drastically reduce the number of large linear solves needed for constructing the global ROM bases without degrading the accuracy of the solution to the inversion problem. We demonstrate the efficiency of these approaches with 2-dimensional and 3-dimensional examples from DOT; however, our methods have the potential to be useful for other applications as well.

The rendering of different shapes of just a single sample of a concentric double quantum ring is demonstrated realizable with a terahertz laser field, that in turn, allows the manipulation of electronic and optical properties of a sample. It is shown that by changing the intensity or frequency of laser field, one can come to a new set of degenerated levels in double quantum rings and switch the charge distribution between the rings. In addition, depending on the direction of an additional static electric field, the linear and quadratic quantum confined Stark effects are observed. The absorption spectrum shifts and the additive absorption coefficient variations affected by laser and electric fields are discussed. Finally, anisotropic electronic and optical properties of isotropic concentric double quantum rings are modeled with the help of terahertz laser field.

InGaN/GaN quantum dots (QDs) are theoretically predicted to have reduced density of dislocations' smaller polarization field, and larger electron-hole overlap, which make them potentially useful for realizing high performance light emitters. The objective of this doctoral research was to study the growth and characterization of InGaN/GaN QDs, investigate the role of nonradiative Auger recombination in the efficiency droop of quantum dot LEDs, and design and characterize high performance green InGaN/GaN QD LEDs and lasers. We have also investigated the role of interface roughness in InGaN/GaN multi-quantum wells and this parameter has been correlated with the quantum efficiency. Reproducible high-level p-type doping of GaN, which has always been a shortcoming, has been demonstrated under N-rich growth conditions, with the highest doping level of 2.1x1018 cm-3. Self-organized InGaN/GaN QDs with a dot density of 2-5x1010 cm-2 have been grown and characterized. Subsequently, we have demonstrated InGaN/GaN QD LEDs with a very small blue shift (4.8 nm) of the peak emission with increasing injection, and a reduced efficiency droop. Furthermore, we have demonstrated hole tunnel injection InGaN/GaN QD LEDs, for the first time, which effectively minimizes hot carriers and ensures uniform injection of holes in all the QD layers. The current density at maximum efficiency is 201.2A/cm2, compared to ∼20 A/cm2 for MQW LEDs. Finally, we have characterized the Auger recombination process in InGaN/GaN QWs, QDs, and defect-free nanowires (NWs). The origin of the abnormally high Auger recombination in the nitride heterostructures is attributed to defect related Auger recombination. The Auger coefficient is low, as predicted (C a= 4.1x10-33cm6·s -1), in the defect-free nanowires and LEDs made with these nanowires do not exhibit any efficiency droop upto injection levels of 400A/cm 2. The first InGaN/GaN quantum dot lasers emitting in the green have also been demonstrated using focused ion beam etching to create the laser facets (mirrors). Emission above threshold is characterized by a peak at 524 nm (green) and linewidth of 0.7 nm. The lowest measured threshold current density is 1.2 kA/cm2 at 278 K. The slope and wall plug efficiencies are 0.74 W/A and ∼1.1%, respectively.

Terahertz (THz) detectors are used in a broad range of applications, including homeland security, biomedical imaging, and space science. A major limitation of the current THz detectors is their low operating temperature (120K). Quantum dot intersublevel photodetectors (QDIPs) and quantum ring intersubband detectors (QRIDs) have potential to operate at high temperatures with desirable performance characteristics, due to the three-dimensional confinement in the detector active regions. This dissertation is devoted to developing THz QDIPs and QRIDs operating at elevated temperatures. An In0.4Ga0.6As/GaAs multi-quantum dot layer intersublevel detector with response peaks in the THz range has been demonstrated. At 150K, the response in the 15--5.4 THz range has a peak responsivity of 0.05A/W and specific detectivity D* of 2x107Jones for an applied bias of -2V. An InAs/GaAs QRID has been demonstrated with 3 distinct response peaks at -6.5, 10, and 12.5THz. The THz response is observed up to T=120K. At 80K, the responsivity of the peaks vary from 0.07A/W to 0.02A/W. The investigation of the multicolor characteristics of these detectors has also been conducted. Multicolor detection plays an important role in high-resolution imaging tasks. To elucidate the multi-response peaks, the electron energy levels in the QDs and QRs have been calculated by an 8 band k·p method and an effective mass method, respectively. Addtionally, a resonant tunnel double barrier structure as a means to select these multi-detection peaks has been developed and applied in a QDIP design.

Optimization algorithms utilize known information about the system to identify solutions that are more efficient and meet the requirements of the user. The algorithms require an objective function, or formula (linear or nonlinear) that models what the user is looking to optimize, in order to begin the search for a more feasible solution. Because optimization problems can involve either linearor non-linear functions, various algorithms have been created that can locate optimum solutions faster depending on the type of objective function being optimized.This research focuses on optimizing an aircrafts thermal management system by using one such algorithm. This was performed in a three step process: initial research and testing, algorithm search method implementation, and post processing and analysis. The aircraft was modeled using complex Matlab Simulink block diagrams to simulate the thermal response of the system for any given type of mission. Using the provided parametric data, areas of user control within the model were located and optimization methods for these areas were devised. The function characterizing the fuel feed temperatures was chosen as the objective function to be minimized. Baseline data proved the function to be nonlinear. Optimization software incorporating a genetic algorithm (GA) was chosen since they are known to be best suited for nonlinear objective functions. Optimization method implementation results showed a decrease in fuel temperature and convergence times. Data pulled from the GA detailed feasible fuel drainage sequences that would reduce fuel temperatures to 132F from the baseline temperature of 143F. Currently, methods using smaller drain sequences have been unable to match these results due to the coarse control over the fuel drainage these sequences provide. Because numerous computations are ran during each test, only feasible sequences shown to decrease the temperature were validated. Results show a need for physical hardware testing to verify the computational results shown.

Infrared detectors have a wide range of imaging applications, including medical diagnosis, thermal imaging, and night vision. A typical infrared camera includes a liquid-nitrogen cooling system. The cost of infrared cameras can be reduced significantly, and their weight can also be decreased, if these cooling systems are replaced by thermo-electric coolers, or completely removed. This necessitates that the detectors can operate at high temperatures (≥150K). Quantum dot infrared photodetectors (QDIPs), with three-dimensional confined quantum dots in the active regions, have the potential for high temperature operation. In the present study, QDIPs have been designed, fabricated and characterized. The results demonstrate that QDIPs have great potential for high temperature operation. Quantum dot infrared photodetectors, with multiple In0.4Ga 0.6As/GaAs quantum dot layers in the active region have been investigated. The spectral response of these devices exhibits three wavelength absorption peaks, centered at 3.5, 7.5, and 22mum respectively. Good performance (J dark∼10-4A/cm2, Rpeak=0.15A/W, D*∼1010cm.Hz1/2/W, T=80K, Vbias=2.0V) was observed for a 30-dot layer device. The highest operating temperature of a 20-dot layer device is 120K while that of 30-dot layer device is 200K. A novel QDIP, in which a resonant tunneling filter is incorporated with each quantum dot layer in the active region of the device, has been studied theoretically and experimentally. In this tunnel QDIP (T-QDIP) the double barrier resonant tunneling heterostructure selectively transmits the photoexcited carriers while blocking the carriers, with a broad energy distribution, which contribute to the dark current. A significant suppression of the dark current, by almost two orders of magnitude, is theoretically calculated and experimentally measured in these devices. Room temperature operation for a QDIP was demonstrated for the first time with the tunnel device with good performance (Rpeak=0.16A/W D*∼107cm.Hz 1/2/W, T=300K, Vbias=2.0V). Terahertz detection with the tunnel QDIPs was also demonstrated for the first time with InAlAs/GaAs quantum dots in the active region. These devices show a long wavelength cut-off at ∼75mum (4THz) and high temperature operation (150K).

Proopiomelanocortin (POMC) is a large multipeptide precursor found in the anterior (AL) and intermediate lobes (IL) of the pituitary, the arcuate nucleus and the nucleus of the solitary tract. Secretion of POMC by pituitary cells is differentially regulated between the two lobes. The coordination of POMC mRNA levels with manipulations that alter release have been examined. Environmental stress, which produced parallel increases in POMC mRNA levels and plasma POMC peptides, was found to be a potent stimulator of both measures. Intermediate lobe melanotrophs were also found to respond to catecholaminergic stimuli by increasing POMC mRNA levels in response to dopamine antagonists and to beta-adrenergic agonists. These receptors were found to produce additive increases when ligands for both were administered together. A number of studies were performed to measure POMC translation/POMC mRNA levels and to determine if increased POMC mRNA levels were required to support increased POMC translation. Neither acute in vitro challenges or in vivo manipulations were effective in increasing POMC translational efficiency as a function of POMC mRNA. Two weeks of swim stress, which increased IL lobe POMC mRNA levels 3-fold, led to a decrease in translational efficiency, due perhaps to overloading of the ribosomes. In situ hybridization was performed to examine changes in POMC mRNA levels while retaining an anatomical context. Changes detected in IL POMC mRNA by in situ hybridization following administration of haloperidol paralleled those detected by blot hybridization. Resolution of single cells was also demonstrated in AL corticotrophs and in cells of the arcuate nucleus. These studies have demonstrated a coordination between POMC mRNA levels and secretory activity induced by environmental and pharmacological stimuli that are known to increase secretion of POMC peptides. This increase in POMC mRNA was shown in the IL to be necessary for increased translation of POMC which is required to replenish secreted peptides. This indicates that determination of mRNA levels can serve as an indicator of cellular response. In situ hybridization was shown to constitute a quantitative method for detecting changes in mRNA while retaining an anatomical context. This should prove useful in studying a heterogeneous tissue such as the brain.

ATPase activity which is stimulated by submicromolar concentrations of Ca('2+) was identified and characterized in human placental microvillous brush border membranes. The enzyme exhibited an apparent K(,m) for free Ca('2+) of 18.3 (+OR-) 3.7 nM and a V(,max) of 233.0 (+OR-) 30.0 nmol/min/mg protein. Studies using trans-1,2-diaminocyclohexane-N,N,N',N',-tetraacetic acid (CDTA) showed this enzyme to require Mg('2+) for maximal activity. The high affinity Ca('2+)-ATPase was unaffected by up to 100 (mu)M concentrations of vanadate, but was sensitive to trifluoperazine inhibition (IC(,50) < 50 (mu)M). The enzyme was not stimulated by the addition of up to 10 (mu)g calmodulin, but this lack of effect may be related to the endogenous calmodulin content of the membrane preparation. High affinity Ca('2+)-ATPase was significantly inhibited by 1,1-Bis(p-chlorophenyl)-2,2,2-trichloroethane (DDT) and its specific homologs at 10 (mu)M concentrations. In addition to the high affinity ATPase, an ATP-dependent Ca('2+) uptake was characterized in human placental microvillous brush border membranes. Multiple freeze-thaw cycles greatly diminished uptake activity by 90%, indicating an intravesicular accumulation of Ca('2+). Kinetic studies indicate an apparent K(,m) for calcium of 0.22 (+OR-) 0.04 M and a V(,max) of 441 (+OR-) 137 pmoles/min/mg protein at 37(DEGREES)C. Ca('2+) uptake was shown to have a optimum pH of 7.2, and was unaffected by the addition of oxalate, consistent with a plasma membrane origin of uptake. The activity was temperature dependent and was significantly inhibited by trifluoperazine (IC(,50) = 100 uM) and vanadate (IC(,50) = 100 uM). DDT and its specific homologs were shown to inhibit Ca('2+) uptake, but to a lesser degree than that seen with the high affinity Ca('2+)-ATPase. Thus, human term placental brush border membranes have both a Ca('2+) transport activity and a calcium-stimulated ATPase activity with similar biochemical characteristics. It is suggested that the high affinity Ca('2+)-ATPase is the mechanism responsible for Ca('2+) uptake in this membrane fraction, which functions to maintain the low intracellular level of free Ca('2+) in placental syncytiotrophoblasts.