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Investigation of laser cooling and trapping of atomic silicon: towards the development of a deterministic single ion source
Full text linkIncludes bibliographical references.2023 Spring.The laser cooling and magneto-optical trapping of silicon atoms were investigated experimentally. These are the first steps towards the development of a deterministic single ion source suitable for single ion implantation of a Kane quantum computer. We identified the 3s23p2 3P2 → 3s3p3 3Do3 transition at 221.74nm as a cycling transition suitable for laser cooling. We also identified the 3s23p2 1D2 → 3s3p3 3Do3 at 256.26nm as a repump transition coupling a lower metastable state with the upper cooling state. Two deep ultraviolet (DUV) laser systems were implemented to provide the cooling and repump laser light. Both systems utilized two stage second harmonic generation to quadruple the frequency of a fundamental laser to produce the DUV light. The cooling laser system utilized frequency quadrupling of a tunable cw Ti:Sapphire ring laser to produce up to 90mW at 221.74nm. The repump laser system utilized frequency quadrupling of an external cavity diode laser to produce up to 35mW at 256.26nm. A silicon atomic beam source operating at 1400°C was developed that produced a beam of free silicon atoms for laser studies. The atomic beam characteristics were analyzed, and the velocity distribution was manipulated via laser cooling. Careful spectroscopic studies were performed on the cooling and repump transitions. Frequency references for the DUV lasers were investigated in Te2 and I2 with Doppler free saturated absorption spectroscopy, using the first doubling stage output of the cooling and repump laser, respectively. Specific hyperfine components of the molecular transitions in Te2 and I2, suitable for frequency references, were identified and measured. Locking of the cooling laser on the Te2 reference was demonstrated. A magneto-optic trap (MOT) was implemented in the silicon atomic beam. A CCD optical system to image the fluorescence from atoms in the MOT was developed and achieved single atom ii detection capability. MOT trapping of silicon atoms was attempted. The low flux of atoms in the MOT velocity capture range precluded any observation of trapped atoms. A Zeeman slower, based on a novel design utilizing a variable pitch helical solenoid, was designed, simulated, and constructed to improve the flux of slow atoms. No magneto-optic trap was observed due to insufficient laser power for simultaneous Zeeman slowing and magneto-optic trapping. Investigations were performed for one dimensional laser cooling, via a Zeeman slower, along the atomic beam motion direction. Atomic beam velocity distribution profiles were observed to be modified when the Zeeman slower was on. The parameter space of Zeeman slower currents, laser power and detuning, was explored. A simulation of the atom motion over the 1m long flight path under the influence of the Zeeman slower was carried out and found to agree with the observed results
The protective role of awe on self-focused attention and depressive symptoms
Full text link2023 Spring.Includes bibliographical references.Depression is a public health concern that negatively affects millions of individuals living in the United States. Maladaptive self-focused attention has been found to strongly predict the onset and maintenance of depressive symptoms. Awe is a positive emotion that may protect against this cognitive vulnerability given its ability to elicit the "small self." The present study examined whether awe buffered against the impact of self-focus on depressive symptoms at both the trait and state level. A sample of 286 students were recruited from an undergraduate research pool. Participants completed an online survey that assessed for different measures of self-focus (i.e., first-person singular pronouns, rumination), trait positive emotions, depressive symptoms, and positive and negative affect. One week later, they were randomized to watch an awe-eliciting video or amusement-eliciting video and subsequently completed another survey that assessed for rumination, "small self" feelings, state positive emotions, and positive and negative affect. Results indicated mixed findings. At the trait level, depressive symptoms were positively associated with rumination (but not first-person singular pronouns). This positive association between self-focus and depressive symptoms was attenuated as levels of awe increased when using the measure of rumination, but not the measure of first-person singular pronouns. At the state level, rumination was positively associated with "small self" feelings. In addition, no interaction was found between the effects of the awe inductions and "small self" feelings in relation to rumination. While researchers have proposed that awe has a potential therapeutic role in depression, this study appears to be the first to provide empirical support at the trait level. Future research should examine the relationship between rumination and "small self" feelings or consider more potent elicitors of awe when understanding its effects at the state level
Understanding soil treatment effectiveness in dryland restoration: ecological barriers, contexts, and baseline conditions
Full text link2023 Spring.Includes bibliographical references.Land degradation is one of the greatest environmental issues our planet faces today, with over 33% of Earth's soils currently degraded. Drylands are especially vulnerable to soil degradation given their history of intensive land use and desertification. However, dryland restoration can be very difficult, and often fails when seeding is used as a sole treatment. Soil-based restoration, which includes abiotic treatments like organic amendments and water collection pits, and biotic treatments like microbial inoculation, may be needed for ecosystem recovery in drylands. Compared to plant-based restoration, however, less is known about how and when to use active soil restoration for optimal results. To improve our understanding of how to best use active soil restoration to restore degraded drylands, we conducted two research studies: (1) a global meta-analysis of dryland soil restoration treatment effectiveness across environmental gradients (Chapter 1), and (2) a regional field study comparing microbial communities across degraded, intact, and revegetated dryland sites to understand baseline conditions and when active soil restoration (e.g., inoculation) may be needed to improve soil conditions (Chapter 2). For project 1, we generated a global database from 155 publications and 1,403 unique studies of responses of soil health variables [i.e., aggregate stability, bulk density, soil moisture, soil organic carbon, soil nitrogen, mycorrhizal colonization, and basal respiration] to soil restoration relative to untreated controls. We then used quantitative meta-analysis techniques to analyze soil restoration effect sizes. In Chapter 2, we collected soil samples from paired reference, degraded, and revegetated plots across seven different dryland sites across the southwestern United States, sequenced the 16S and ITS rRNA gene regions from extracted DNA for bacteria/archaeal and fungal communities (respectively), and analyzed differences in microbial community composition among samples. Results from the meta-analysis suggested that active soil restoration generally improves soil health and is most effective in arid, fine-textured soils. Organic amendments were most effective at increasing soil organic carbon, while fungi inoculation treatments were most effective at increasing mycorrhizal colonization. From the regional microbiome study, we found that soil microbial communities differ between paired degraded and intact sites, and that degraded sites have lower abundances of biocrust-forming bacteria and dark septate endophytic fungi, which are both indicative of reference/intact conditions, making these taxa potential targets for inoculation treatments. However, we found that microbial communities do not differ between degraded and revegetated sites, suggesting that degraded sites may require active interventions beyond revegetation, such as direct microbial inoculation, to replenish microbial communities. These findings advance understanding of the effects of dryland degradation and restoration on soil health and have actionable implications for improving restoration decision-making, and thus improve outcomes in dryland restoration
Hurricanes make the best bouquets
No full text2023 Spring.Hurricanes Make the Best Bouquets is a collection of short stories which explores love, loss, isolation, inner strength, and the process and shape of forgiveness. These nine stories reimagine the domestic drama as a place where displaced men and women—mothers, daughters, sisters, divorcees and one widower—search for answers in decomposition, orange blossom perfume, and strange apparitions. From the top of a lighthouse to a Visalia Super 8 to a Salvation Army window display, the heartache these characters experience in the wake of abandonment, death, and self-discovery asks them to adapt to new realities and confront the meaning of home and belonging
Luminescence measurements inform a strategy for unlocking the full potential of CdTe-based photovoltaics
Full text linkIncludes bibliographical references.2023 Spring.Cadmium telluride (CdTe) photovoltaics are characterized by simplicity and speed of fabrication with low usage of materials, all of which translate into low cost. These significant advantages have earned CdTe the second-highest adoption rate of all photovoltaic technologies. However, conversion efficiencies, while functional, are significantly lower than the theoretical limit for this material. This discrepancy is almost entirely a discrepancy in voltage, and the so-called "voltage deficit" of CdTe has stubbornly persisted for decades. While many strategies are being pursued to attempt to reduce the voltage deficit, this issue is fundamentally one of excessive nonradiative recombination due to defects within the absorber material, as will be demonstrated in this dissertation. Recombination is evaluated primarily by luminescence measurements, and as such this class of measurements is particularly relevant to the challenges faced by CdTe research today. The rate of recombination is parameterized by the carrier lifetime, and time-resolved photoluminescence (TRPL) is the most common method of determining this parameter in CdTe. Historically, accurate determination of bulk lifetime was as simple as extracting the time constant of the slowest component of a TRPL decay. However, significant gains in material passivation and doping over the last few years have both decreased the relative influence of trap-assisted recombination and increased the influence of p-n junction fields on TRPL measurements. Consequently, when measurements are performed on complete cells, extracting the tail time constant from a TRPL decay no longer necessarily gives an accurate representation of the bulk material lifetime, and the result is distorted by field effect contributions. This fact is not necessarily well-known by the CdTe community, and extraction of the tail time constant is still the most common way to report lifetimes, even in measurements on complete state-of-the-art cells. This dissertation demonstrates the skewing effects of junction fields, and identifies under which conditions they manifest and how. To probe field effects, external electrical bias was incorporated during TRPL measurements, which allows fairly precise manipulation of fields. Biased TRPL measurements were performed on a variety of samples, and a model was developed to substantiate and better explain the results. It was found that the same characteristics which enable good performance (high lifetime, doping, and mobility) are the same which add complexity to TRPL interpretation. It was also found that field effects can be effectively suppressed by significant forward bias, leading to far more accurate determination of bulk lifetime. TRPL and external radiative efficiency (ERE) luminescence measurement results have indicated very low rates of nonradiative recombination and associated very high lifetime for some CdTe-based materials deposited at Colorado State University, particularly the cadmium selenium telluride (CdSeTe) alloy. While these attributes should allow voltages approaching 1 V and efficiencies on the order of 25%, when incorporated into "traditional" cell architectures these materials typically achieve middling performance at best, and often no performance at all. To unlock the great potential indicated by luminescence measurements, a different cell architecture is proposed which aims to accommodate these materials and take advantage of their characteristics. In an n-i-p configuration, an intrinsic absorber material is sandwiched between two carrier-selective contacts, at least one of which must be transparent. This design eliminates the requirement that the absorber be doped, which penalizes lifetime. Based on the findings of modeling reported here, undoped CdSeTe appears to be an ideal intrinsic layer material. The currently-utilized SnO2:F/MgZnO front contact appears to be excellent as the n-type electron-selective layer. The one missing component is the p-type hole-selective layer; modeling in this dissertation describes in detail what attributes are required of this material. Most important is band alignment with CdSeTe, which should produce a valence band offset as close to zero as possible, and a conduction band offset which forms a sufficiently high electron barrier. Sufficient p-type doping is also quite important. Based on these criteria, ZnTe was identified as a suitable candidate material, and several cells were fabricated with this architecture. While preliminary cells achieved relatively poor performance compared with traditional designs, J-V curves were surprisingly well-behaved, and the almost immediate achievement of functioning cells using an entirely new approach is promising. Luminescence characterization of these structures identified several areas for improvement, namely the use of a p-type dopant other than copper and the replacement of ZnTe with another material with similar band structure but more compatible lattice constant
Phytoalexin deficient4 (PAD4): a plant defense regulatory gene with distinct alternative splicing patterns in tomato (Solanum lycopersicum) and soybean (Glycine max)
Full text link2023 Spring.Includes bibliographical references.Alternative splicing is an important post-transcriptional regulatory mechanism that contributes to a plant's ability to perceive and respond to a variety of biotic and abiotic stressors. Alternative splicing has a documented role in plant immunity, as many R genes, which are important for plant defense against specialized pathogens, undergo alternative splicing in response to pathogen perception. Despite this, the role of alternative splicing in other components of plant defense responses is not well documented. As transcriptome data diversify to include more species and conditions, the extent of alternative splicing in plants has become apparent. PHYTOALEXIN DEFICIENT4 (PAD4), plays an integral role in plant defense signaling to biotic stressors, and in regulating responses to abiotic stresses. PAD4 undergoes alternative splicing in Soybean (Glycine max). Additionally, the expression pattern of Glycine max PAD4, GmPAD4, and its splice variant GmPAD4-AS1 are further characterized in early growth stages. We hypothesize PAD4 produces full-length and alternatively spliced transcripts in multiple species, and that PAD4 gene structure may influence the occurrence of alternatively spliced transcripts. Here we characterize alternative splicing of PAD4 in tomato (Solanum lycopersicum), identifying two splice variants. We also investigate the conservation of PAD4 intron-exon structure conservation across diverse species. PAD4 expression patterns are characterized using available expression data
Subspace and network averaging for computer vision and bioinformatics
Full text linkIncludes bibliographical references.2023 Spring.Finding a central prototype (a.k.a. average) from a cluster of points in high dimensional space has broad applications to complex problems like action clustering in computer vision or gene co-expression module representation in bioinformatics. A central prototype of a set of points may be cast as the solution to an optimization problem that either minimizes distance or maximizes similarity between the prototype and each point in the cluster. In this dissertation we offer four novel prototypes for a cluster of points: the flag median, maximally correlated flag, cluster expression vector and eigengene subspace. We will formalize the flag median and the maximally correlated flag using subspace representations for data, specifically the Grasmann and flag manifolds. In addition to introducing these prototypes, we will derive a novel algorithm which can be used to calculate subspace prototypes: FlagIRLS. The third and fourth prototypes, the cluster expression vector and eigengene subspace, are inspired by problems involving gene cluster (e.g., pathway or module) representations. The cluster expression vector leverages connections within networks of genes whereas the eigengene subspace is computed using Principal Component Analysis (PCA). In this work we will explore the theoretical under-pinnings of these prototypes, find algorithms to compute and them to computer vision and biological data sets