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Active Infrared Nanophotonics in van der Waals Materials
Get PDFTwo-dimensional van der Waals materials have recently been introduced into the field of nanophotonics, creating opportunities to explore novel physics and realize first-of-their kind devices. By reducing the thickness of these materials, novel optical properties emerge due to the introduction of vertical quantum confinement. Unlike most materials, which suffer from a reduction in quality as they are thinned, layered van der Waals materials have naturally passivated surfaces that preserve their performance in monolayer form. Moreover, because the thickness of these materials is below typical charge carrier screening lengths, it is possible to actively control their optical properties with an external gate voltage. By combining these unique properties with the subwavelength control of light-matter interactions provided by nanophotonics, new device architectures can be realized.
In this thesis, we explore van der Waals materials for active infrared nanophotonics, focusing on monolayer graphene and few-layer black phosphorus. Chapter 2 introduces gate-tunable graphene plasmons that interact strongly with their environment and can be combined with an external cavity to reach large absorption strengths in a single atomic layer. Chapter 3 builds on this, using graphene plasmons to control the spectral character and polarization state of thermal radiation. In Chapter 4, we complete the story of actively controlling infrared light using graphene-based structures, introducing graphene into a resonant gold structure to enable active control of phase. By combining these resonant structures together into a multi-pixel array, we realize an actively tunable meta-device for active beam steering in the infrared. In Chapters 5 and 6, we present few layer black phosphorus (BP) as a novel material for active infrared nanophotonics. We study the different electro-optic effects of the material from the visible to mid-infrared. We additionally examine the polarization-dependent response of few-layer BP, observing that we can tune its optical response from being highly anisotropic to nearly isotropic in plane. Finally, Chapter 7 comments on the challenges and opportunities for graphene- and BP-integrated nanophotonic structures and devices.</p
An LES and RANS Study of the Canonical Shock-Turbulence Interaction
Get PDFThe canonical problem of a nearly stationary, nearly planar shockwave passing through isotropic turbulence is investigated within high Reynolds number regimes. The subject flow contains a wide range of turbulent scales and is addressed in Large Eddy Simulation (LES) to relax the otherwise prohibitive computational cost of simulating these flows. Aliasing errors in the LES of the upstream isotropic turbulence are shown to interact with the mean compression of the shock in a problematic matter, and may result in nonphysical behavior such as a reduction in the dissipation rate as the flow crosses the shock. A method for the regularization of LES of shock-turbulence interactions is presented which is constructed to enforce that the energy content in the highest resolved wavenumbers decays as k-5/3, and is computed locally in physical space at low computational cost. The application of the regularization to an existing subgrid scale model is shown to remove high wavenumber errors while maintaining agreement with DNS of forced and decaying isotropic turbulence. Comparisons to analytical models suggest that the regularization significantly improves the ability of the LES to predict amplifications in subgrid terms over the modeled shockwave.
The regularization method is then employed in high resolution LES intended to illustrate the physical behavior of the shocked, turbulent flow. Turbulent statistics downstream of the interaction are provided for a range of weakly compressible upstream turbulent Mach numbers Mt = 0.03 - 0.18, shock Mach numbers Ms = 1.2 - 3.0, and Taylor-based Reynolds numbers Reλ = 20 - 2500. The LES displays minimal Reynolds number effects once an inertial range has developed for Reλ > 100. The inertial range scales of the turbulence are shown to quickly return to isotropy, and downstream of sufficiently strong shocks this process generates a net transfer of energy from transverse into streamwise velocity fluctuations. The streamwise shock displacements are shown to approximately follow a k-11/3 decay with wavenumber as predicted by linear analysis. In conjunction with other statistics this suggests that the instantaneous interaction of the shock with the upstream turbulence proceeds in an approximately linear manner, but nonlinear effects immediately downstream of the shock significantly modify the flow even at the lowest considered turbulent Mach numbers.
LES allows consideration of high Reλ flows, but remains expensive to compute relative to lower cost modeling approaches such as Reynolds-Averaged Navier Stokes (RANS). Conventional RANS models are often not well suited for simulations containing discontinuous features such as shocks and, in an effort to improve the performance of RANS, models for averaged shock corrugation effects and the impact of turbulent entropy or acoustic modes on the energy equation are presented. Unlike previous RANS work that has focused on the modification of turbulent statistics by the shock, the proposed models are introduced to capture the effects of the turbulence on the profiles of primitive variables --- mean density, velocity, and pressure. By producing accurate profiles for the primitive variables, it is shown that the proposed models improve numerical convergence behavior with mesh refinement about a shock, and introduce the physical effects of shock asphericity in a converging shock geometry. These effects are achieved by local closures to turbulent statistics in the averaged Navier-Stokes equations, and can be applied in conjunction with existing Reynolds stress closures that have been constructed for broader applications beyond shock-turbulence interactions.</p
Optoelectronic Design and Prototyping of Spectrum-Splitting Photovoltaics
Get PDFGlobal energy production is dominated by the combustion of fossil fuels but in order to avoid the projected consequences of anthropogenic climate change it is necessary that humankind reduce the carbon intensity of its energy supply. Fortunately the sun supplies a ubiquitous flow of energy of with excellent thermodynamic quality to earth. Massive investment and manufacturing scale has driven the costs of photovoltaic systems to levels competitive with fossil fuel generation, and yet commercial photovoltaic systems convert power from the sun into electricity with less than 20% efficiency. In this thesis we consider the thermodynamic and practical limits to the power conversion efficiency of photovoltaic systems and seek to design systems that address the greatest sources of loss, namely the lack of sub-bandgap absorption and the thermalization of excited carriers. We present several designs of spectrum-splitting systems that utilize optical structures to allocate incident broadband solar radiation into narrower spectral bands which can be converted by multiple distinct photovoltaic cells at greater efficiency. Furthermore, we report on the design and fabrication of thin film III-V single-junction cells at bandgaps spanning the solar spectrum for incorporation within spectrum-splitting systems. These devices were fabricated by utilizing epitaxial lift-off processes from both GaAs and InP wafers as proof of scalability. We additionally report on the fabrication and characterization of series of a spectrum-splitting prototypes. This design featured seven distinct spectral bands with single-junction photovoltaic cells designed to convert them with highest possible efficiency, and the ultimate prototype exhibited an 84.5% spectrum splitting efficiency and 30.2% power conversion efficiency under a standard AM1.5D solar spectrum. We also report a technical pathway to raise the prototype efficiency to a record breaking 45.2%. Finally, we present an optical design of a spectrum-splitting module that is informed by a technoeconomic analysis which drastically reduces the complexity and cost relative to the fabricated prototype.</p
Self-Gluing Formula of the Monopole Invariant and its Application on Symplectic Structures
Get PDFSeiberg-Witten theory has been an important tool in studying a class of 4-manifolds. Moreover, the Seiberg-Witten invariants have been used to compute for simple structures of symplectic manifolds. The normal connected sum operation on 4- manifolds has been used to construct 4-manifolds. In this thesis, we demonstrate how to compute the Seiberg-Witten invariant of 4-manifolds obtained from the normal connected sum operation. In addition, we introduce the application of the formula on the existence of symplectic structures of manifolds given by the normal connected sum.
In Chapter 1, we study the Seiberg-Witten theory for various types of 3- and 4- manifolds. We review the Seiberg-Witten equation and invariants for 4-manifolds with cylindrical ends as well as closed and smooth 4-manifolds . Furthermore, we explain how to compute the Seiberg-Witten invariants for two types of 4-manifolds: the products of a circle and a 3-manifold and sympectic manifolds.
In Chapter 2, we prove that the Seiberg-Witten invariant of a new manifold obtained from the normal connected sum can be represented by the Seiberg-Witten invariant of the original manifolds. In [Tau01], the author has proved the case of the operation along tori. In [MST96], the authors have proved the case of the operation along surfaces with genus at least 2 when the product of the circle and the surface is separating in the ambient 4-manifold. In this thesis, we show the proof of the remaining case.
In Chapter 3, we prove the existence of certain symplectic structures on manifolds obtained from the normal connected sum of two 4-manifolds using the multiple gluing formula stated in Chapter 2. We explain how to construct covering spaces of the manifold and compute the Seiberg-Witten invariant of the covering spaces by the gluing formula. From the relation between the Seiberg-Witten invariants and symplectic structures, we prove the main application.</p
Cyclic Polyolefins via Ring-Expansion Metathesis Polymerization
Get PDFThe synthesis of cyclic polyolefins prepared using a supported, molecular ring-expansion metathesis polymerization catalyst is described. The synthesis of the catalysts, themselves, is described in detail. Additionally, thorough physical characterization of cyclic polymers with comparison to linear polymer analogues is reported.</p
Synthesis and Self-Assembly of Bottlebrush Block Polymers: Molecular Architecture and Materials Design
Get PDFBottlebrush polymers represent a unique molecular architecture and a modular platform for materials design. However, the properties and self-assembly of bottlebrush polymers remain relatively unexplored, in large part due to the synthetic challenges imposed by the sterically demanding architecture. This thesis describes our work to close this gap, connecting (1) the synthesis of polymers with precisely tailored molecular architectures, (2) the study of fundamental structure-property relationships, and (3) the design of functional materials.
Chapter 1 introduces key concepts related to polymer architecture and block polymer phase behavior. Recent developments in the synthesis and self-assembly of bottlebrush block polymers are highlighted in order to frame the work presented in Chapters 2–6.
Chapter 2 introduces a versatile strategy to design polymer architectures with arbitrary side chain chemistry and connectivity. Simultaneous control over the molecular weight, grafting density, and graft distribution can be achieved via living ring-opening metathesis polymerization (ROMP). Copolymerizing a macromonomer and a small-molecule co-monomer provides access to well-defined polymers spanning the linear, comb, and bottlebrush regimes. This design strategy creates new opportunities for molecular and materials design.
Chapter 3 explores the physical consequences of varying the grafting density and graft distribution in two contexts: block polymer self-assembly and linear rheological properties. The molecular architecture strongly influences packing demands and therefore the conformations of the backbone and side chains. Collectively, these studies represent progress toward a universal model connecting the chemistry and conformations of graft polymers.
Chapter 4 discusses the phase behavior of ABA' and ABC bottlebrush triblock terpolymers. Low-χ interactions between the end blocks promote organization into a unique mixed-domain lamellar morphology, LAMP. X-ray scattering experiments reveal an unusual trend: the domain spacing strongly decreases with increasing total molecular weight. Insights into this behavior provide new opportunities for block polymer design with potential consequences spanning all self-assembling soft materials.
Chapter 5 describes other physical consequences of low-χ block polymer design. The ternary phase diagrams for ABC, ACB, and BAC bottlebrush triblock terpolymers reveal the influences of low-χ A/C interactions, frustration, and the molecular architecture. Potential non-equilibrium effects and crystallization in these bottlebrush polymers will also be discussed.
Chapter 6 describes applications of bottlebrush polymers as functional materials. Self-assembly enables mesoscale structural control over many materials properties, such as reflectivity, conductivity, and modulus. The synthetic methods (Chapter 2) and physical insights (Chapters 3−5) provided in previous chapters illustrate opportunities for materials design. We will discuss AB brush diblock polymers that self-assemble to photonic crystals and ABA brush triblock copolymers in solid polymer electrolytes.</p
Resolvent-Based Modeling of Flows in a Channel
Get PDFThis thesis concerns the continued development of the resolvent framework (McKeon and Sharma, 2010) to model wall-bounded turbulent flows. Herein, we introduce novel modifications and extensions of the framework to improve the compact representation of flows in a channel. In particular, inspired by ideas rooted in classical linear stability theory, we introduce a decomposition of the velocity field into Orr-Sommerfeld (OS) and Squire (SQ) modes in a nonlinear context via the resolvent operator. We demonstrate through the analysis of a number of exact coherent states (ECS) of the Navier-Stokes equations (NSE) in Couette and Poiseuille flow that this decomposition offers a significant improvement in the low-dimensional representation of these flows. With this efficient basis, we are able to develop through the notion of interaction coefficients a method to compute accurate, self-consistent solutions of the NSE with knowledge of only the mean velocity profile. We also highlight the role of the solenoidal component of the nonlinear forcing in the solution process. In addition, the resolvent framework is extended to the analysis of 2D/3C flows. This approach, again applied to ECS, sheds light on the underlying scale interactions which sustain these solutions. Notably, it reveals that lower branch ECS can be effectively described in their entirety with a single resolvent response mode. This discovery is leveraged to construct a method to compute accurate approximations of ECS starting from a laminar profile using a single parameter model. This thesis also utilizes a constant time-step DNS of a turbulent channel to perform a direct characterization of the nonlinear forcing terms. We compute power spectra and confirm that the nonlinear forcing has a non-trivial signature in the wavenumber-frequency domain. We also compute and analyze spectra for the OS/SQ vorticity and discuss the potential benefit of this decomposition technique to the study of fully turbulent flows as well.</p
Therapeutic Opportunities and Approaches to Sequence Control for Nucleic Acids
Get PDFRNA interference (RNAi) is a powerful mechanism to regulate gene expression. A key feature of RNAi is its sequence specificity: a short interfering RNA (siRNA) assembles into the RNA induced silencing complex (RISC) and then targets cellular transcripts complementary to the siRNA for degradation. RNAi has been adapted for therapeutic applications, but is challenged by the need to identify unique target transcripts for each disease that are both effective and result in few off-target effects. This challenge could be eased if siRNAs could be activated only and specifically in diseased cells. If this were the case, rather than targeting a new transcript for each new disease, the same cellular housekeeping genes could be reused. Targeting housekeeping genes would result in greater potency, both effectively treating the disease and requiring less drug for treatment, alleviating problems associated with toxicity and delivery. A new class of nucleic acid therapeutics called conditional siRNAs (Cond-siRNA) is designed to act in this environment-specific manner. The first part of this thesis uses molecular dynamics simulations to understand the structure of Cond-siRNA and to suggest improvements in future designs.
Bioengineering like the work done in the development of Cond-siRNAs depends on the existence of tools that make work simple, fast, cheap, and reproducible. In the case of nucleic acids, de novo synthesis of custom constructs is a fundamental tool. While approaches to synthesis have improved immensely since their inception, increasing ambition demands increasingly powerful tools. As target constructs get longer, the synthesis can become intractably complicated, slowing the process, increasing costs, and making it less likely to be replicated by others. The source of complexity in nucleic acid synthesis is the inability to directly synthesize long fragments without errors. Finding a new means of sequence-controlled synthesis that results in fewer errors and perhaps allows for correction could address this challenge. The second part of this thesis looks at using graphene as a mask for patterning the deposition of molecules on a surface with an eye towards arranging and coupling reactants in a sequence-specific way.</p
A Measurement of νe Appearance and νμ Disappearance Neutrino Oscillations with the NOvA Experiment
Get PDFNOvA is a two-detector, neutrino oscillation experiment on an 810 km baseline in the NuMI beam at Fermilab. NuMI is an off-axis, narrow-band beam centered near 1.8 GeV and is configurable to send a high-purity νμ or -νμ neutrino flux which passes through the NOvA far detector, sensitive to νμ and -νμ disappearance and νe and -νe appearance. The NOvA near and far detectors are functionally identical, liquid scintillator trackers. They are finely segmented relative to the radiation length in the detectors allowing for the detailed reconstruction of electromagnetic shower necessary to identify νe CC events.
Through a joint analysis of the appearance and disappearance oscillation channels using neutrino and anti-neutrino data, NOvA constrains the allowed parameter space in δCP, the neutrino mass hierarchy, sin2θ23, and Δm232. We observe 58 and 18 appearance candidates on an expected background of 15.06 and 5.32 events in neutrino and anti-neutrino mode, respectively. This observation gives a 1.77σ preference for the normal mass hierarchy and 1.7σ preference for θ23 in the upper octant. NOvA will maintain its position as a long-baseline oscillation experiment with leading sensitivity for several more years in which time analysis will further disambiguate the fundamental neutrino parameters that determine oscillation behavior.</p
Moral Distinctions between Passive and Active Euthanasia
Get PDFMorally speaking, what distinguishes passive from active euthanasia? Is there even a moral distinction? Before we can answer these questions, it will serve us well to get a sense of what either type of euthanasia involves. Euthanasia is often termed 'mercy killing' or 'assisted suicide.' It is the intentional ending of a patient’s life to ease his pain and suffering (typically caused by some terminal illness). Euthanasia can be classified as passive or active. Passive euthanasia involves withholding common treatments (drugs, operations, respirators etc.) necessary for a patient to continue living. Active euthanasia, on the other hand, involves the use of lethal substances or forces (e.g. a lethal injection) to kill the patient. The prima facie distinction between active and passive euthanasia is that the former involves killing a patient, while the latter involves letting the patient die. Thus, some philosophers suggest that by asking whether there is a moral distinction between active and passive euthanasia, we are really asking whether there is a moral distinction between ‘killing’ and 'letting die.' With that said, solving this age-old 'killing' versus ‘letting die’ moral dilemma is far beyond the scope of this paper. However, I believe we need not fully resolve the dilemma in order to gain insight into the moral differences between active and passive euthanasia