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SuperCDMS SNOLAB, HVeV Run 3, and Development of KIPM Detectors
Get PDFDark matter is the theorized source of many observed large-scale gravitational effects. It is dark in the sense that it lacks any heretofore measurable direct interaction with the electromagnetic spectrum. Being unable to rely on absorption, reflection, or emission of photons makes studying dark matter particularly challenging. Excluding neutrinos, which fail to explain the observed large-scale effects, dark matter has never been conclusively identified in a local laboratory experiment. There are many proposed models that could explain both our large-scale observations and our lack of local observations while still allowing for the possibility of local observation. Ultra-sensitive direct-detection experiments attempt to make precisely such observations. Confirmed detection of a new stable particle would provide important information for improving our understanding of both dark matter and cosmological models.
The SuperCDMS SNOLAB experiment is a direct-detection experiment designed with an initial focus on particle masses < 10 GeV. The experiment will measure both phonon and ionization signals in kg-scale semiconductor crystals held at cryogenic temperatures. In this thesis, I describe the experiment with emphasis on the ionization readout. I also detail the characterization process I performed on the ionization amplifier's low-power high-electron-mobility transistors (HEMTs).
SuperCDMS high-voltage eV-resolution (HVeV) detectors are gram-scale detectors designed to achieve single electron-hole-pair sensitivity. The first HVeV direct-detection search produced world-leading exclusion limits for dark-matter masses down to ~1 MeV. Here, I present my work analyzing the third search using such detectors. Run 3 was the first to include multiple detectors operated simultaneously and achieved an order of magnitude greater exposure than previous runs. I report the resulting exclusion limits for electron-coupled, dark-photon, and axion-like-particle dark matter.
Lastly, I discuss work performed at Caltech towards the development of kinetic-inductance phonon-mediated (KIPM) dark-matter detectors. KIPM detectors use frequency-multiplexed kinetic inductance detectors (KIDs) and have the potential for excellent event-position reconstruction and background rejection. KIPMs also present a clear path towards sub-eV resolution on event recoil energy. Such detectors could be used as part of a payload upgrade in SuperCDMS SNOLAB. KIPMs could also be used in smaller-scale experiments similar to SuperCDMS HVeV.</p
Strategic Advances in 2D Materials: Low-Temperature Plasma-Enhanced Chemical Vapor Deposition Growth of Graphene and Complementary Insights into MoS₂
Get PDFThis thesis explores the intricate details of the plasma-enhanced chemical vapor deposition (PECVD) technique for growing graphene on various substrates at low temperatures. The research begins by finely optimizing the PECVD growth conditions to produce high-quality graphene on copper ink, which can potentially be used in a wide range of flexible electronics and Internet of Things (IoT) devices. The study also showcases that PECVD is an effective technique for growing graphene directly on electroplated copper over polyimide substrates, which greatly improves the resilience and environmental stability of copper circuits.
Furthermore, the research investigates the possibility of using PECVD to grow graphene on gold, which can be a game-changer in anti-corrosion applications and increase the longevity of gold electrode-based biosensors. The study also makes a significant breakthrough by growing nanocrystalline multilayer graphene on silver in a single step, which demonstrates exceptional oxidation resistance and opens new opportunities for hybrid graphene-silver plasmonic technologies.
Lastly, the thesis examines the potential and complexities of using electrodeposited (ED) copper foil as a graphene growth substrate, showing significant transformations in the properties of the ED copper foil post PECVD process. Towards the latter part of this work, attention is briefly shifted to explore the unique dipole ordering properties of monolayer molybdenum disulfide (MoS2) single crystals, which are synthesized using high-temperature chemical vapor deposition (CVD) and are van der Waals materials like graphene. Although not the main focus, this inclusion offers valuable insights into contrasting attributes and functionalities of graphene and MoS2, especially in areas like high-density data storage and non-volatile memories, and also compares the status of synthesis methods of these two types of van der Waals materials.
Alongside these investigations, the thesis also touches upon the prospects of both large-area PECVD graphene growth and interfacial graphene growth, identifying future paths for research and innovation. This comprehensive study highlights the versatility of low-temperature PECVD for graphene synthesis and provides insights that may reshape research and applications in flexible electronics, biosensing, and beyond. The findings of this research therefore pave ways for researchers, technology developers, and businesses to explore realistic technological applications of graphene and two-dimensional materials in various industries.</p
Localized Fast Radio Bursts Using DSA-110
Get PDFFast Radio Bursts (FRBs) are µs- to ms- scale, energetic (1040~46 erg s-1) bursts detected in the radio frequency (110 MHz to 8 GHz). They primarily originate from extragalactic sources and are likely to originate from compact object sources.
The exact nature of the sources and the emission mechanisms remain inconclusive.
The first FRB was not confirmed until 2007. By early 2023, over 600 FRBs have been reported, and about 40 FRBs have been associated with an individual host galaxy. The rapid growth in sample size has helped to greatly narrow down the number of source models. Neutron stars, especially magnetars, have been the most popular source candidate, although other possibilities still remain.
In this thesis, we explore a few observational methods to study the potential FRB source and host environments. In Chapter 2, we demonstrated a method to constrain the energy ratio emitted from the FRB's multiwavelength transient counterparts as compared to the FRB energy themselves. We used the existing multiwavelength transient blind survey database and the current FRB population fluence distribution to produce tighter constraints than most targeted surveys. In Chapter 3, we investigated whether or not the persistent radio source associated with FRB 121102 could be an AGN using the VLA monitoring data and a new Keck optical spectrum. We constrained the emission source radius to be 1017~18 cm based on the low level of variability in the VLA radio flux measurements presented in this work as compared to the Galactic scintillation theory and other published results by VLBI. We estimated the mass of the potential black hole to be ≾104~5 M☉ based on the Hα line width in the Keck spectrum. We concluded that the source is unlikely an AGN based on the size, mass, and radio luminosity, and that the persistent radio source could be explained by an isolated neutron star with a pulsar wind nebula. In Chapter 4, we showed the burst morphology of a sample of 21 FRBs detected by DSA-110 during part of the commissioning period in 2022, including 16 localized FRBs with optical spectra. We explored the potential correlation between burst morphology and host properties. We found a strong correlation between the host Hα luminosity and FRB burst energy that is likely a result of observational selection effects. We measured the scintillation timescales and found most of them close to the predicted Galactic scintillation timescales. In Chapter 5, we summarize the thesis and very briefly discussed potential extensions of the above methods for the study of FRB sources.</p
Structural Analysis of MurG Interactions with Substrates, Inhibitors, and MraY
Get PDFThe peptidoglycan layer in bacterial cells is a popular target for antibiotic development. The membrane protein MraY and peripheral membrane protein MurG are part of critical steps in the synthesis of peptidoglycan. Lipid I, a lipid precursor formed by MraY, is recognized by MurG through its soluble domain. Currently, there is no structure of MurG with bound Lipid I, and the residues required for this interaction have not been conclusively defined. Crystallographic methods and Cryo-Electron Microscopy were applied to study the interactions between MurG and the soluble domain of Lipid I by binding Park’s Nucleotide, Lipid II, or a Lipid I analog were used to study the interactions of MurG and MraY with the aforementioned substrates. By adding Park’s Nucleotide, Murgocil, Lipid II, the Lipid I analog, or a combination of the listed additives to concentrated MurG, crystals formed under optimized conditions. We aim to obtain electron-density maps from these techniques to model the structure of MurG
Innovations in Wireless Bioelectronics for Precision Medicine, from Sustainable Sweat Sensing to Ingestible Gut Monitoring
No full textBiofluids, constituting about 60% of the human body, serve as treasure troves of biomarkers such as metabolites and electrolytes, shedding light on individual health conditions. Although blood and urine tests have been routinely utilized, they are limited by their invasive and episodic nature. However, the promise of continuous and noninvasive access to other biofluids like sweat, GI fluids, and saliva paves the way for real-time, onsite health monitoring. This thesis delves into the untapped potential of wearable sensors and noninvasive biofluid analysis, emphasizing the importance of continuous and sustainable monitoring for predictive personal healthcare. Chapter 1 introduces the paradigm of biofluid sensing, focusing on sweat as a key candidate for personalized healthcare applications. Chapter 2 delves into the physiology of sweat glands, highlighting the composition of sweat and the mechanisms behind sweat extraction, either through natural exercise or iontophoretic stimulation. Chapter 3 embarks on the development of innovative sensors designed for detecting clinically pertinent biomarkers in sweat, a step forward in predictive health analytics. In Chapter 4, the spotlight is on system integration, as the study emphasizes the need for miniaturized and reliable wireless sensor devices that ensure minimal discomfort and maximum reliability. Chapters 5 and 6 delve into strategies for sustainably powering wearable devices from energy harvested from body motions and from ambient light, respectively. The final chapter, Chapter 7, extrapolates the aforementioned technologies for the realm of ingestible devices, adapting them for electrochemical sensing in alternate media, primarily gastrointestinal fluids. This allows for enhanced detection of gastrointestinal diseases and a deeper understanding of the intricate gut-brain axis. The ultimate vision of this research is to equip individuals with wearable and ingestible sensors that can seamlessly monitor a broad spectrum of clinically relevant biomarkers. This continuous monitoring, coupled with data analytics, will potentially catalyze a shift from reactive to predictive healthcare, ushering in an era of personalized therapeutic interventions. As wearable sweat and ingestible sensors become mainstream, a confluence of biosensing mechanisms, materials science, and flexible electronics is anticipated enable continuous and unobtrusive acquisition of clinically relevant biomarkers over prolonged periods and large populations, further refining the nexus between health monitoring and precision medicine
Efforts Towards C-C Bond Formations: From Ni Catalysis to Transition-Metal Free Electrolysis
Get PDFThe selective construction of C-C bonds has been a critical challenge in modern synthetic organic chemistry. Among the numerous methodologies developed, cross-coupling remains an attractive strategy for direct C-C bond formation. Herein, a diverse range of cross-coupling reactions for C-C bond formations are investigated from different perspectives. First, the mechanism of a Ni/cyano-box-catalyzed asymmetric Suzuki alkynylation is studied. The existing data is consistent with a radical chain pathway that is previously proposed for other Ni-catalyzed enantioselective cross-coupling reactions. Next, moving on from the traditional electrophile-nucleophile cross-couplings, we explore Ni-catalyzed reductive coupling of alkyl halides with internal olefins in the presence of a hydrosilane. With judicious choice of the directing group, hydroalkylation of internal olefins can be achieved with high regio- and enantioselectivity. Following that, an electrochemically driven, transition-metal free cross-electrophile coupling reaction is explored as a greener alternative to constructive C(sp³)-C(sp³) bonds. Specifically, we focus on improving the Mg sacrificial anode performance in these electroreductive systems. By carefully choosing the electrolyte composition, we are able to manipulate the metal electrode interfaces for a more effective counter electrode. Finally, Al stripping in ethereal solvents is investigated for its application as a sacrificial anode in reductive electrosynthesis. Inspired by Al corrosion chemistry, we are able to achieve bulk Al stripping in THF-based electrolyte by incorporating halide co-supporting electrolytes.</p
Subtractive Photonics in Bulk CMOS
Get PDFMuch of humanity's technological advancement over the last few decades may be attributed to exponentially increasing computing power, the bedrock of which is bulk CMOS technology. Exponentially increasing data rates in communications have also played an important role, facilitated by advancements in fiber optics and integrated photonics. However, efforts to capitalize on the complementary strengths of these two domains by merging them, an idea first envisioned almost 40 years ago, have so far proven inadequate. All previous attempts to integrate photonics in bulk CMOS have required either expensive process modification or resulted in waveguides with high loss.
In this thesis, we discuss our investigations of a new method of integrating photonics into bulk CMOS, which we call the method of subtractive photonics. This method entails forming waveguides out of the back-end interconnect of an electronic chip. The interconnect metal is designed to wrap around dielectric channels such that when the metal is etched away, suspended dielectric waveguides remain. Although this method introduces a large, previously untapped design space, since there are many interconnect layers that can be used in photonic structures, it also introduces certain severe constraints. This thesis explores some of the possibilities this design space opens up, as well as some of the challenges involved in designing photonics in a process intended only for electronics. As part of this exploration, we demonstrate waveguides with an upper bound on loss that is significantly lower than the best previously published waveguide loss for unmodified bulk CMOS. We also demonstrate the first measurements of waveguide loss at visible and near-visible wavelengths in unmodified bulk CMOS, as well as the first measurements of waveguide coupled photodiodes in unmodified bulk CMOS. These proof-of-concept results may pave the way towards fully integrated electronic-photonic systems in unmodified bulk CMOS.</p
Single Rare-Earth Ions in Solid-State Hosts: A Platform for Quantum Networks
Get PDFSolid-state defects have emerged as leading candidates for quantum network nodes due to their compatibility with scalable device engineering and local nuclear spins for quantum processing. Rare-earth ions in crystalline hosts are particularly attractive due to their long optical and spin coherence times at cryogenic temperatures. However, until recently, detection and utilization of single rare-earth ions in quantum technologies has been hindered by their inherently weak optical transitions. In this thesis I present progress towards realizing a novel quantum network node architecture using single ¹7¹Yb³⁺ ions in YVO₄, coupled to a nanophotonic cavity.
First, we demonstrate coherent operation of single ¹7¹Yb³⁺ ions as optically addressed qubits. To do this, we leverage first order insensitivity of optical and spin transitions to electric and magnetic fields, thereby protecting the qubits from environmental noise. We demonstrate initialization, high fidelity control and readout of a hyperfine spin qubit with long quantum storage times. We also characterize the optical transitions and find a lifetime-limited echo coherence, thereby enabling a coherent spin-photon interface.
Next, we focus on realizing an auxiliary quantum register. The high-fidelity spin control of our ¹7¹Yb³⁺ qubit is leveraged to access local nuclear spins. These spins comprise a dense ensemble which serves as a deterministic quantum resource. We utilize Hamiltonian engineering to generate tailored interactions, enabling polarization, coherent control and preparation of many-body nuclear spin states. Finally, we implement a spin-wave based memory protocol and demonstrate storage and retrieval of quantum states.
Moving beyond a single quantum node, in the final section of this thesis we will realize a small-scale quantum network using this platform. As a first step we demonstrate time-resolved quantum interference between photons emitted by ions in two separate devices. Then, we demonstrate a novel heralded entanglement protocol which incorporates optical dynamical decoupling and frequency erasure via precise photon detection. This protocol counteracts both static and dynamic inhomogeneity in the ions’ optical transition frequencies, thereby enabling entanglement generation between any pair of qubits in a scalable fashion.
These results showcase single rare-earth ions as a promising platform for the future quantum internet.</p
Some Computer Studies of Membrane Proteins, Molecular Chaperones, and Color
Get PDFThis thesis shares a series of stories on seemingly disparate topics united by my efforts and love of computers. Initially, I discuss how the challenge of membrane protein expression provided an initial impetus for research. I channeled efforts towards developing a predictive (machine-learning) model for heterologous overexpression in E. coli. While we made strides to extend this model to other systems (not discussed here), my time was refocused onto questions of more fundamental biochemical interest: the biogenesis of tail-anchored membrane proteins. I built structural, predictive, and phylogenetic models to better understand how the C-terminal domain of co-chaperone Sgt2 functioned, refined the definition of the wider Sti1 family which includes Sgt2-C, and extended our understanding of those features of tail-anchored proteins that determine successful targeting in Yeast and Human cells. I developed a deep phylogeny of Get3, a chaperone involved in tail-anchored protein biogenesis, and helped specifically place Get3 proteins of photosynthesising organisms into evolutionary context. Along the way, I developed a parallel and compelling theme around data visualization, specifically around the use of colormaps across the life sciences. In particular, I built an application to screen and notify preprint authors when their manuscript had poor colormap usage. This was the first time automated software has been used to help authors improve their work at the preprint stage, an area that has grown significantly since my initial work. Finally, I brought together structural biology and data visualization by making perceptually uniform colormaps available in popular molecular visualization software tools to advocate for more thoughtful color usage in the field
Love and War: Control of Female Social Behaviors by the Hypothalamus
Get PDFAggressive and mating behaviors, crucial for survival, are inherently programmed in the brain and are orchestrated by genetically defined cell types within subcortical circuits. Despite the hard-wired nature, females exhibit a remarkable ability to flexibly adjust these behaviors to according to their reproductive states. The neural mechanisms governing the stable control and adaptive regulation of these behaviors remained unclear. Moreover, given the inherently complex and dynamic nature social interactions, the dynamics of the underlying motivational states and their encoding in the female brain was largely unknown.
Addressing these knowledge gaps in my thesis, I initially undertook a dissection of the subcortical circuits and genetically defined cell types involved in the control of female aggressive and mating behaviors. Using single-cell RNA sequencing and optogenetic perturbations, I identified distinct transcriptomic cell types in the ventromedial hypothalamus: α cells governing mating and ß cells regulating aggression. Furthermore, longitudinal monitoring of their activity during the transition from virginity to motherhood revealed that ß cells became more responsive to social cues, resulting a shift from mating to aggression. In a second line of investigation, I delved into the dynamics of female mating and its neural encoding. By monitoring single-cell activity in receptive females and applying dynamical system modeling to neural activity, I uncovered that α cell formed line attractor dynamics, encoding a sexual aroused state during mating. Additionally, longitudinal monitoring of activity across different hormonal states revealed population dynamics displaying receptivity state-dependent patterns across the estrus cycle. A third aspect of my research explored comprehensive changes in gene expression patterns in circuits influenced by hormones. Through comparative analysis of transcriptomic profiles in the ventromedial hypothalamus at different hormonal states. I identified qualitative changes in cell types within mating-activated α cells, correlated with sexual receptivity. These studies significantly contribute to our understanding of the neural basis controlling aggressive and mating behaviors, shedding light on their flexible regulation by physiological conditions in females.</p