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High-Efficiency Luminescent Solar Concentrators for Photovoltaic Applications
Get PDFDespite an overwhelming abundance of crude solar energy, current photovoltaic systems worldwide harness less than 1% of this available power. As such, emerging solar generation technology must be developed to further spur global adoption -- whereby increased sunlight to power conversion efficiency alongside decreased system costs constitute the primary methods to accomplish this goal. The luminescent solar concentrator (LSC) offers a unique approach to collecting and redirecting large areas of incident light onto small-area solar cells. Relying upon photoluminescent materials (i.e., luminophores) suspended within a dielectric waveguide, the LSC absorbs high energy irradiance and re-emits photons at down-shifted energies into optical waveguide modes.
This thesis presents analytical, computational, and experimental work to illustrate the technical power conversion efficiency limits for LSC-based photovoltaic devices. We begin with a technical description of two LSC numerical models -- a stochastic Monte Carlo ray-trace and a deterministic closed-form approach. We apply these models to quantify the effects of system and component parameters on power conversion efficiency for a number of end-use applications. To validate our modeling and unveil current practical material limits, we fabricate CdSe/CdS and CuInS2/ZnS core/shell quantum dot waveguides hosting embedded InGaP and GaAs photovoltaic cells, respectively. From these measurements, we observe close model-to-experiment matching and report a world-record LSC power conversion efficiency reaching approximately 10% under 1-sun illumination at modest incident to outgoing radiance areas.
Herein we consider four distinct applications for the LSC: (i) single junction LSC devices for terrestrial-based energy generation, (ii) building-integrated LSC form factors for on-site electricity, (iii) multijunction LSC modules for utility-scale installations at high power conversion efficiency, and (iv) ultra-light structures for on-board power in aerospace settings. We organize each chapter according to its end-use application.</p
Open-Circuit Stability and Integration of Silicon Electrodes for Solar Fuels Devices
Get PDFTwo significant challenges that impede the realization of inexpensive, solar-driven water electrolysis involve the corrosion and integration of component materials. For instance, Si is a prominent light absorbing material that readily corrodes in alkaline electrolyte unless subjected to an oxidative potential. Although a protective coating can be applied to mitigate corrosion, the underlying semiconductor remains exposed to electrolyte at pinholes on the protective coating. Illumination slows the dissolution of Si photoanodes further by 2-3 orders of magnitude via oxidation to SiOx. However, Si is still susceptible to corrosion under nighttime conditions and device stability must be maintained regardless of diurnal patterns of sunlight. This thesis explores two approaches to drive Si passivation in the dark at open circuit. First, a protective electrolyte can be introduced to solution that acts as an oxidizing agent to Si. Secondly, a catalytic thin film like NiOx on Si can drive the electrode potential positive by catalyzing O2 in electrolyte. Applying either passivation strategy yielded extended stability of Si photoanodes subjected to simulated day/night cycling. In addition to corrosion, device performance is critically dependent on the integration of component materials. Efficient water splitting requires that at least two semiconductors be connected in series to drive the reaction, while lateral resistance losses in electrolyte preclude large (> cm2) planar photoelectrode areas. Si can be vertically arranged as high aspect ratio microwires that can be embedded in an ion exchange membrane. This assembly can be laminated to a tandem partner arranged in a similar configuration using an electrically conductive interlayer. This thesis additionally investigates the bulk and interfacial properties of Nafion-PEDOT:PSS composite films as a candidate material for this interlayer. After solvent treatment, the composite film exhibited percolation of electrically conductive PEDOT domains even at dilute PEDOT concentrations (~ 0.2 wt%). Despite the presence of an insulating Nafion-rich layer on the surface, the composite forms a low resistance contact to CH3-terminated p-Si, thereby making the composite a viable interlayer for use in a fully integrated, tandem water splitting device.</p
High Sensitivity Time-Varying Systems In Photonics and Electronics
Get PDFIntegrated electronics and photonics have been revolutionizing our daily lives for decades. However, the demand for high-speed communications, low-latency networks, and high-performance optical and electrical sensors continues to grow. In order to keep up with this demand as well as be able to address upcoming and unknown challenges, we need to explore unconventional solutions. Moving away from existing systems and traditional architectures allows us to take a deeper look at these challenges and potentially come up with nontrivial answers. In this thesis, unconventional approaches to implementing high-performance optical and electrical sensors and systems are investigated. Among these unorthodox solutions are time-varying architectures which led to completely new devices, sensors with dramatically improved sensitivity, and the breaking of known trade-offs.
By developing a time-varying method that we call reciprocal sensitivity enhancement, we demonstrated a nanophotonic optical gyroscope (NOG) for the first time. The efficacy of this method is borne out by its ability to improve the performance of optical gyroscopes by two orders of magnitude. This sensitivity-enhancement method filters out reciprocal imperfections and noise, thereby increasing the overall signal-to-noise ratio. Next, the same approach is used to boost the performance of resonance-based magnetic biosensors. By merging two biosensors and taking advantage of the frequency response of magnetic beads, time-division switching cancels out most of the correlated noise. This solution pushes the sensitivity of this sensor below parts-per-million (PPM) levels for long periods of time — a property which is desirable in many biosensing applications.
Additionally, an electrical scalable router that mitigates line-of-sight issues in next-generation wireless systems is introduced. This novel design does not require any shared timing reference to form a coherent array and uses a time-varying baseband to create a proper true-time delay. Next, we discuss how radiating elements in silicon-photonics platforms can be engineered to create a passive lensless camera. By applying a robust reconstruction algorithm, the captured image can be faithfully recovered. The same concept can be used in multi-mode nanophotonic antennas to alleviate the field-of-view (FOV)-aperture trade-off.
Finally, a hybrid photonic transmitter/receiver architecture, an electrical full-duplex transceiver with one nonreciprocal element, and a nested-ring optical modulator are presented.</p
Quantitative Sequencing and its Application to Studies of the Human Small-Intestine Microbiota
Get PDFOur understanding of the interplay between microbial species and the hosts they live on and in is continually expanding. New insights have focused not only microorganisms that drive specific disease states but also those that help maintain human health. As research drives towards mechanistic understanding of host-microbe relationships new quantitative tools are needed to help interrogate these complex interactions. Chapter I of this thesis discusses formulation of a method for rapid detection of antibiotic resistance in Neisseria gonorrhoeae. Our approach identified RNA signatures from transcriptional profiling of Neisseria gonorrhoeae after 10-minute antibiotic exposure. Utilization of these RNA markers allowed for rapid identification of antibiotic susceptibility or resistance to the antibiotic ciprofloxacin. Chapter II shifts focus to the development of a quantitative sequencing technique for the measurement of absolute taxon abundances in complex microbial communities. Combining the precision of digital PCR with the high-throughput nature of 16S rRNA gene amplicon sequencing allowed for simultaneous quantitative profiling of all bacterial taxa in host-associated microbial communities. We extensively characterized our quantitative sequencing methodology in the presence of high host nucleic acid levels and low microbial loads to understand the limits of quantification and detection in complex sample types. Last, Chapter III applies the quantitative sequencing technology from Chapter II to investigate the microbial community of the human small intestine, specifically the duodenum. Data from the duodenum of 250 individuals revealed a wide range of total microbial loads and a distinct subset of microbes, termed disruptor taxa, that were associated with small intestinal bacterial overgrowth (SIBO) and GI symptom severity.</p
Palladium-Catalyzed Cascade Cyclizations in Natural Product Synthesis: Synthetic Studies of Noraugustamine and Falcatin A
No full textPalladium-catalyzed cascade cyclizations present a powerful strategy for the rapid assembly of polycyclic skeletal frameworks, enabling the efficient synthesis of bioactive and structurally complex natural products. Herein, we review the field of palladium-catalyzed cascade cyclizations in natural product synthesis and describe our application of these transformations toward the total syntheses of noraugustamine and falcatin A.
Our approach to the Amaryllidaceae alkaloid noraugustamine was driven by the simultaneous disconnection of a C–C and a C–N bond, with the aim of forming both bonds and two of the target’s six rings in a single step. A radical cascade cyclization delivered noraugustamine but displayed poor regioselectivity for 6-exo-trig versus 7-endo-trig cyclization. Improved regioselectivity was achieved using a palladium-catalyzed Heck cyclization, leading to the development of a novel oxidative Heck/aza-Wacker cascade forming both of the desired bonds with good yield and selectivity. This transformation and the general lessons taken from this work should find broad utility in the design of cascade cyclizations toward alkaloids of similar complexity.
We also investigated a palladium-catalyzed carboetherification cascade toward the synthesis of the central five- and seven-membered rings of the myrsinane diterpene falcatin A. In this case, competitive C–O coupling, olefin insertion, and cyclopropanation hindered our efforts to develop the proposed transformation in a simplified model system. A stereoselective bromoetherification and a nickel-catalyzed Nozaki–Hiyama–Kishi reaction were ultimately successful, forming the targeted rings. Efforts to synthesize a fully elaborated cyclization substrate, translate the key steps from the model system, and complete the synthesis of falcatin A are ongoing.</p
Microwave-to-Optical Transduction Using Rare-Earth Ions
Get PDFSuperconducting qubits that operate at microwave frequencies are one of the most promising platforms for quantum information processing. However, connecting distant processors with microwave photons is challenging since microwave photons suffer from thermal noise and large propagation losses in room temperature components.
Conversely, optical photons within the telecommunications band are known to have extremely low loss in optical fiber and the thermal noise is minuscule at room temperature. In order to interface superconducting qubits with room temperature optical photons, a quantum transducer is required that can convert photons between microwave and optical frequencies.
This thesis describes the development of a microwave-to-optical transducer using an ensemble of erbium ions, doped within a yttrium orthovanadate crystal, that are simultaneously coupled to a superconducting microwave resonator and a photonic crystal optical resonator. The erbium ions have spin transitions that couple to the microwave resonator and optical transitions at telecom wavelengths that couple to the optical resonator.</p
Emotion Experience from Stories, Videos and Everyday Life: Structure and Individual Differences
Get PDFMost studies of emotion have as their subject matter the emotion experiences that people can describe and rate. By contrast to this approach from psychology, studies in animals, and some biological studies in humans, focus on behavior and its adaptive function. These two literatures typically use very different corresponding features by which to characterize emotion: categories or dimensions describing feelings for which we have convenient words, for the former (e.g., happiness, pleasantness), and functional properties for the latter (e.g., persistence, generalizability, approachabil- ity). In this thesis I use both sets of ratings, and I ask whether the latter, biologically inspired features could also be used to characterize people’s emotion experiences, and might reveal novel dimensions of variability. They also typically use different sets of stimuli to induce the emotions: lexical stimuli in which participants are asked to imagine something hypothetical are common in human studies; ecologically valid stimuli that at least the subjects cannot distinguish from the real world are common in animal studies. Here I used three domains of stimuli: stories, videos, and real-life experiences, in the same set of participants, permitting a unique comparison.
I took advantage of a sample of approximately 1000 Americans who were surveyed longitudinally over the internet during the COVID-19 pandemic. I collected ratings of emotion experiences evoked by three classes of stimuli: a validated set of short stories, a validated set of short videos, and actual experiences in real life across multiple waves. I found that all three types of emotion experiences could be characterized by low dimensional spaces, with the first two factors that accounted for most of the variance in people’s ratings corresponding to the dimensions of valence and arousal, in line with prior work. However, I discovered additional novel factors related to generalizability (the extent to which an emotion experience is shared across many different situations and occurrences) or modularity (the extent to which an emotion experience is unique to specific situations). The findings show that emotion features not usually assessed in humans can be recovered from subjective ratings of their experiences. I argue for a revision of current dimensional theories of emotion: they have been incomplete because they were restricted to ratings entrenched in how we think of our conscious experience, and the typical English words we use to describe it. The new dimensions validate some theories of emotion and offer hope for linking psychological studies in humans with behavioral or neurobiological work across species. I also characterized the distributions of the three types of emotion experiences and found that emotions were distributed along continuous gradients, with no well-separated clusters even for emotions belonging to the six basic emotion categories.
My thesis presents two additional topics that capitalize on my unique sample: the emotions experienced during the COVID pandemic, and individual differences. For example, I also found that resilience buffered individuals against the effect of loneliness on depression, and that people who had tested positive for COVID felt more morally disgusted towards acts of violating social norms. I also explored the association between psychological traits and differences in emotion experiences both in terms of the magnitudes of the ratings and the overall correlation structure across scales. Again, the richness of my dataset reveals a number of associations that are theoretically interesting and that will be of relevance to understanding mood and anxiety disorders as well.
All of the data will be made publicly available, and the core parts of many of the investigations were pre-registered.</p
Exploring Earth's Core-Mantle Boundary with Multi-Technique Approaches
Get PDFEarth's core-mantle boundary (CMB) is the most extreme interface of the planet's interior. It regulates the flow of heat out of the core and in doing so influences the two internal engines of our dynamic habitable planet: convection in the solid mantle and the magnetic geodynamo in the core. Seismic observations of the CMB have revealed a complex landscape of heterogeneous multi-scale structures that likely play key roles in Earth's internal dynamics and may hold memory of Earth's ancient past. Many details of the compositions and properties of these structures, however, are essentially unknown. In this thesis, I deploy a suite of experimental techniques and interdisciplinary approaches to constrain the temperature and phase relations of the CMB, properties that affect dynamics of the mantle and core. In particular, I focus my study on ultralow velocity zones (ULVZs) - the most extreme and perhaps least well understood structures in the lowermost mantle. I first quantitatively show that these structures, originally posited to be areas of partial melt, can be well explained as solid FeO-rich formations given seismic, geodynamic, and mineralogical constraints. To further explore the viability of such solid FeO-rich structures, I develop a multi-technique approach combining two in-situ synchrotron-based methods, one sensitive to crystal structure and another to atomic dynamics, to study the high-pressure melting of iron-bearing materials. With this approach, I place new constraints on the core-mantle boundary temperature by measuring the melting temperature of a candidate core-forming alloy (Fe0.8Ni0.1Si0.1) at high pressures, finding that the addition of silicon to an Fe0.9Ni0.1 core can reduce CMB temperatures to ~3500 K. I then measure the melting of Fe0.94O, finding a melting temperature of 4140 ± 110 K at CMB pressure, demonstrating the stability of solid FeO-rich ULVZs in the lowermost mantle. The melting experiments show strong agreement between the two independent techniques, helping to address sources of large discrepancies in previous high-pressure melting experiments. Reported high conductivity for iron-rich (Mg,Fe)O at CMB conditions may provide a mechanism for upwelling promoted by solid conductive ULVZs at roots of major hotspot plumes. As a whole, the thesis advances our understanding of the compositions and origins of ultralow velocity zones and, more broadly, the physical properties of Earth's core-mantle boundary region.</p
Foundations and Applications of Single-Cell RNA Sequencing
Get PDFSingle-cell RNA-sequencing is an experimental technique for studying cellular gene expression, with a multitude of engineering challenges. These challenges transcend the boundaries of traditional academic disciplines and the field of mechanical engineering, that aims to address roadblocks in critical technologies towards engineering our environment, is central to this endeavor.
This thesis addresses three engineering challenges that must be met in order to realize the goal of bringing single-cell RNA sequencing to the clinic. The first is scalable cellular isolation and sampling. Chapter 2 describes the poseidon and colosseum instruments that enable massive scale single-cell isolation and collection. They each have novel design elements that reduce cost and enable modularity, at a similar accuracy to expensive commercial alternatives.
The second challenge is the rapid preprocessing of single-cell RNA-sequencing data. Chapter 3 describes the kallisto | bustools command-line tools that make scalable scRNAseq analysis fast and efficient. These tools implement novel algorithms for sequence read-alignment, barcode error correction, and molecular counting that helps resolve ambiguities in sequence mapping.
The third challenge is refining gene expression data to the isoform level. This refinement is crucial for understanding transcriptional regulation and the effects of alternative splicing in biological processes. Towards that end, I have extended the kallisto | bustools workflow to process full-length scRNAseq data taking advantage of expectation maximization algorithm to disambiguate sequence alignments. Chapter four describes how I used these tools to assemble the first ever spatially-resolved single-cell isoform atlas, and in particular one of great interest in the neuroscience community (the mouse primary motor cortex) with data generated with three RNA-sequencing assays.</p
Waveguide Quantum Electrodynamics with Superconducting Slow-Light Waveguide Circuits
Get PDFWaveguide quantum electrodynamics (QED) refers to the study of quantum emitters (qubits) coupled to a single mode waveguide - a 1D electromagnetic reservoir with a continuum of states. This paradigmatic quantum-optical system can serve as a test-bed for experimental investigations in many-body physics, quantum non-linear optics, reservoir engineering, non-Markovian physics, quantum networks, and quantum computing. While such a system can be realized in a variety of physical platforms, superconducting quantum circuits are well suited to the study of waveguide QED due their readily available strong light-matter interaction strengths.
Of particular interest is the ability to tailor the dispersion relation and modal properties of the waveguide beyond that of a conventional waveguide with linear dispersion. For example, through periodic modulation of the geometry of a waveguide, or through the fabrication of an array of coupled resonant elements, novel electromagnetic responses can be engineered. These include spectral constriction of the 1D continuum to a transmission band of finite bandwidth, enhanced or suppressed emission rates of quantum emitters into the waveguide that are dependent on their frequencies, and extreme slowing of the velocity of light. Such attributes of dispersive waveguides can be leveraged to substantially enrich the physics and applications of qubit-waveguide systems.
In this thesis, we demonstrate the design, fabrication, and characterization of a slow-light waveguide (SLWG) comprised of an array of coupled lumped-element superconducting microwave resonators, and present on various experiments involving superconducting transmon qubits coupled to the SLWG. We investigate the physics of a qubit strongly coupled to the SLWG reservoir by tuning its frequency across the passband of this waveguide, where we find substantial changes to the qubit emission rate, along with oscillatory energy relaxation of the qubit resulting from the beating of bound and radiative dressed qubit-photon states. Further, upon addition of a reflective boundary to one end of the waveguide, we observe revivals in the qubit population on a timescale 30 times longer than the inverse of the qubit's emission rate, corresponding to the round-trip travel time of an emitted photon.
In addition, we show how we leveraged the ability to induce this non-Markovian time-delayed feedback via the SLWG's long delay to generate multidimensional cluster states of itinerant microwave photonic qubits. By utilizing the SLWG as a delay line with 240 ns round-trip delay, a single flux-tunable transmon qubit as a quantum emitter, and a second auxiliary transmon as a switchable mirror, we achieve rapid, shaped emission of entangled photon wavepackets, and effect time-delayed feedback within the waveguide between previously emitted photons and the emitter qubit. We leverage these capabilities to generate a 2D cluster state of four photons with 70% fidelity, as verified by tomographic reconstruction of the quantum state. We conclude by discussing directly realizable novel follow-up experiments that involve a continuously driven qubit in the presence of time-delayed feedback, and discuss how our cluster-state generation scheme could be straightforwardly extended to generation of even larger multidimensional cluster states, thereby enabling utilization of such states for quantum information processing techniques in the microwave domain.</p