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The impact of bulk and surface defect states on the vibrational and optical properties of step-graded epitaxial GaAs₁₋ySby (0 ≤ y ≤ 1) materials with and without chemical surface treatment by (NH₄)₂S was investigated. Tunable antimony (Sb) composition GaAs₁₋ySby epitaxial layers, grown by solid source molecular beam epitaxy (MBE), were realized on GaAs and Si substrates by varying key growth parameters (e.g., Sb/Ga flux ratio, growth temperature). Raman and photoluminescence (PL) spectroscopic analysis of (NH₄)₂S-treated GaAs₁₋ySby epitaxial layers revealed composition-independent Raman spectral widths and enhanced PL intensity (1.3x) following (NH₄)₂S surface treatment, indicating bulk defect-minimal epitaxy and a reduction in the surface recombination velocity corresponding to reduced surface defect sites, respectively. Moreover, quantification of the luminescence recombination mechanisms across a range of measurement temperatures and excitation intensities (i.e., varying laser power) indicate the presence of free-electron to neutral acceptor pair or Sb-defect-related recombination pathways, with detectable bulk defect recombination discernible only in binary GaSb PL spectra. In addition, PL analysis of the short- and long-term thermodynamic stability of sulfur-treated GaAs₁₋ySby/Al₂O₃ heterointerfaces revealed an absence of quantifiable atomic interdiffusion or native oxide formation. Leveraging the combined Raman and PL analysis herein, the quality of the heteroepitaxial step-graded epitaxial GaAs₁₋ySby materials can be optimized for optical devices.

We synthesized epitaxial BTO-BFO heterostructure with decreased leakage and simultaneously improved the multiferroic properties. This study provides new direction for ferromagnetic resonance studies, in high quality BTO-BFO films grown on LSMO. We observed small Gilbert damping (=0.004) and the absence of large inhomogeneous broadening, in a film with 80 nm thickness of BTO-BFO on LSMO (110). This fact offers opportunities for employing this material system for spin transfer in multifunctional materials where controlling magnetization by a flow of spin angular momentum, or spin current, is crucial toward developing nanoscale spin-based memory and devices. Magnetic insulators, such as BTO-BFO on LSMO, are potentially excellent candidates for pure spin current without the existence of charge current.

The presence of micro and nano plastics in the environment and their impact on the various life forms within it are of principle concern around the globe. However, whilst a considerable amount of work has been done on the detection of microplastics, many challenges remain in the development of analytical techniques for nanoplastics due to their inherent ultra-small size and ubiquitous shapes. Here, a simple technique is reported based on surface enhanced Raman spectroscopy (SERS) and salt (NaCl) induced aggregation of gold nanoparticles that has been used to detect 100 nm diameter polystyrene (PS) beads. The gold nanoparticles (AuNPs) were synthesized and stabilized by negatively charged sodium citrate. When the PS beads present in a water sample were introduced into the solution of colloidal AuNPs, they interact to each other via hydrophobic interactions and other weak forces (i.e. hydrogen, ionic, and Van der waals forces). Upon an addition of NaCl, the negatively charged ions around the AuNPs are shielded and disturbed, resulting in their aggregation around the PS beads. As a consequence, strong SERS signal enhancement produced by the aggregated AuNPs was observed, and also demonstrated in numerical modelling. Concentrations of 100 nm PS beads as low as 1 part per million (ppm) were measured, and to the best of the author's knowledge, this is the lowest concentration detected for nanoplastics of that size or smaller by such a simple technique that has been reported.

Mechanical metamaterials are novel materials that display unique properties from their underlying microstructure topology rather than the constituent material they are made from. Their effective properties displayed at macroscale depend on the design of their microstructural topology. In this work, two classes of mechanical metamaterials are studied within the 2D-space. The first class is made of trusses, referred to as truss-based mechanical metamaterials. These materials are studied through their application to a beam component, where finite element analysis is performed to determine how truss-based microstructures affect the displacement behavior of the beam. This analysis is further subsidized with the development of a graphical user interface, where users can design a beam made of truss-based microstructures to see how their design affects the beam's behavior. The second class of mechanical metamaterial investigated is made of self-assembled structures, called spinodoids. Their smooth topology makes them less prone to high stress concentrations present in truss-based mechanical metamaterials. A large database of spinodoids is generated in this study. Through data-driven modeling the geometry of the spinodoids is coupled with their Young's modulus value to approach inverse design under uncertainty. To see mechanical metamaterials applied to industry they need to be better understood and thoroughly characterized. Furthermore, more tools that specifically help push the ease in the design of these metamaterials are needed. This work aims to improve the understanding of mechanical metamaterials and develop efficient computational design strategies catered solely for them. Master of Science Mechanical metamaterials are hierarchical materials involving periodically or aperiodically repeating unit cell arrangements in the microscale. The design of the unit cells allows these materials to display unique properties that are not usually found in traditionally manufactured materials. This will enable their use in a multitude of potential engineering applications. The presented study seeks to explore two classes of mechanical metamaterials within the 2D-space, including truss-based architectures and spinodoids. Truss-based mechanical metamaterials are made of trusses arranged in a lattice-like framework, where spinodoids are unit cells that contain smooth structures resulting from mimicking the two phases that coexist in a phase separation process called spinodal decomposition. In this research, computational design strategies are applied to efficiently model and further understand these sub-classes of mechanical metamaterials.

Transport of spin angular momentum (spin currents) in magnetic thin films is important for non-volatile spin-based memory devices and other emerging information technology applications. It is especially important to understand how a spin current propagates across interfaces and how a spin current interacts with magnetic moments. The great interest in devices based on ferromagnetic metals generated intensive theoretical and experimental studies on the basic physics of spin currents for the last few decades. Of particular interest recently is the so-called "pure" electronic spin current, which is carried by electrons and yet unaccompanied by net charge flow, in part because of the prospect of transporting spin with minimal Joule heating. However, in contrast to ferromagnetic metals, spin transport in antiferromagnetic metals, which are promising materials for next-generation magnetic information technology, is not well understood yet. This dissertation addresses the mechanisms of transport by pure spin current in thin-film multilayers incorporating metals with antiferromagnetic order. We focus on two specific materials: (1) CoGd alloys with ferrimagnetic sublattices, which resemble antiferromagnets near the compensation composition, and (2) elemental antiferromagnetic Cr, which can be grown as epitaxial films and hence serve as a model system material. For both the CoGd and Cr studies, spin-valve-like structures of NiFe/Cu/CoGd and NiFe/Cu/Cr/CoFe are prepared to conduct ferromagnetic resonance spin pumping experiments. Precessing magnetization in the NiFe "spin source" pumps a transverse spin current to the adjacent layers. We measure the loss of the spin angular momentum in the "spin sink" layer by measuring the broadening of the resonance linewidth, i.e., the non-local damping enhancement, of the spin source. The antiparallel magnetic moments of Co and Gd sublattices partially cancel out the dephasing of a transverse spin current, thereby resulting in a long spin dephasing length of ≈ 5-6 nm near the magnetic compensation point. We find evidence that the spin current interacts somewhat more strongly with the itinerant transition-metal Co magnetism than the localized rare-earth-metal Gd magnetism in the CoGd alloy. We also examine spin transport via structurally clean antiferromagnetic Cr, epitaxially grown with BCC crystal order. We observe strong spin reflection at the Cu/Cr interface, which is surprising considering that thin layers of Cu and Cr individually are transparent to spin currents carried by electrons. Further, our results indicate other combinations of electrically conductive elemental metals (e.g., Cu/V) can form effective spin-reflecting interfaces. Overall, this thesis advances the basic understanding of spin transport in metallic thin films with and without magnetic order, which can aid the development of next generations of efficient spintronic devices. This work was supported in part by the National Science Foundation, Grant No. DMR-2003914. Doctor of Philosophy Manipulation of electronic flow, i.e., net charge flow, underlies modern electronic devices such as computers, mobile phones, and electric cars. However, the conventional charge transport inevitably results in wasted energy, due to resistive (Joule) heating in the devices. A new research area which uses the electron's spin has recently emerged, namely spintronics. Spintronics uses the spin of electrons rather than just the charge, thereby reducing the dependence on charge flow. The flow of spin angular momentum carried by electrons, i.e., "electronic spin current," underpins numerous phenomena in condensed matter physics. An important example is switching and excitation of magnetic order driven electrically by spin current rather than external magnetic field. Spin currents can interact not only with ferromagnetic order consisting of parallel magnetic moments – but also with antiferromagnetic order consisting of alternating magnetic moments that cancel the net magnetization of the material. Indeed, experiments from the last few years demonstrate the ability to rotate antiferromagnetic order (a.k.a. Néel vector) by spin current, which offers new physics not achievable in ferromagnets, such as ultrafast spin dynamics in the THz regime and superfluid spin transport analogous to superconducting electronic transport. However, interaction of a spin current with antiferromagnetic order is not well understood yet. The aim of this thesis is to build a better understanding of spin currents in antiferromagnetic metals. Specifically, we experimentally study basic spin-current physics in a ferrimagnet (CoGd) and an antiferromagnet (Cr). We choose CoGd because adjusting its chemical composition allows us to easily tune its magnetism from ferromagnet-like (uncompensated magnetization) to antiferromagnet-like (compensated magnetization). In antiferromagnet-like CoGd, we find that the oppositely oriented Co and Gd magnetic moments partially cancel the scrambling (dephasing) of spins, so that the spin current is able to propagate over a longer distance - about 3-4 times more than in ferromagnetic metals. The mechanisms behind the longer spin propagation is somewhat akin to the spin "rephasing" technique for lengthening the lifetime of spin-based qubits for quantum computers, but what is remarkable is that we observe this effect in rather disordered magnetic alloys at room temperature. In the other major project of this thesis, we investigate spin transport through multilayers that contain Cr, a structurally and chemically clean antiferromagnetic material. We find that Cr by itself is a good spin transmitter, i.e., effectively allowing a pure spin current to pass through. Surprisingly, when Cr and Cu (another good spin transmitter) are stacked together, we observe strong reflection of a pure spin current at the interface of Cr and Cu. We find that the antiferromagnetic order in Cr is not responsible for this peculiar spin reflection and that other pairs of spin-transmitting metals (for example, V and Cu) can form spin-reflecting interfaces as well. Our work shows an interesting example of "emergent" phenomena where the interface behaves in a way that is not intuitively expected from the properties of the constituent materials. The basic scientific findings from this thesis may help the development of more efficient information-technology devices that run on spin currents.

Plasmonic nanostructures can enhance light-matter interactions in the subwavelength domain, which is useful for photodetection, light emission, optical biosensing, and spectroscopy. However, conventional plasmonic devices are optimized to operate in a single wavelength band, which is not efficient for wavelength-multiplexed operations and quantum optical applications involving multi-photon nonlinear processes at multiple wavelength bands. Overcoming the limitations of single-resonant plasmonics requires development of plasmonic devices that can enhance the optical interactions at the same locations but at different resonance wavelengths. This dissertation comprehensively studies the theory, design, and applications of such devices, called "multiresonant plasmonic systems with spatial mode overlap". We start by a literature review to elucidate the importance of this topic as well as its current and potential applications. Then, we briefly discuss the fundamentals of plasmonic resonances and mode hybridization to thoroughly explore, classify, and compare the different architectures of the multiresonant plasmonic systems with spatial mode overlap. Also, we establish the black-box coupled mode theory to quantify the coupling of optical modes and analyze the complicated dynamics of optical interactions in multiresonant plasmonic systems. Next, we introduce the nanolaminate plasmonic crystals (NPCs), wafer-scale metamaterials structures that support many (>10) highly-excitable plasmonic modes with spatial overlap across the visible and near-infrared optical bands. The enabling factors behind the NPC's superior performance as multiresonant systems are also theoretically and experimentally investigated. After that, we experimentally demonstrate the NPCs application in simultaneous second harmonic generation and anti-Stokes photoluminescence (ASPL) with controllable nonlinear emission properties. By designing specific non-linear optical experiments and developing advanced ASPL models, this work addresses some important but previously unresolved questions on the ASPL mechanism as well. Finally, we conclude the dissertation by discussing the potential applications of out-of-plane plasmonic systems with spatial mode overlap in wavelength-multiplexed devices and presenting some preliminary results. Doctor of Philosophy Emergence of electronic devices such as cellphones and computers has revolutionized our lifestyles over the past century. By manipulating the flow/storage of electrons at the nanometer scale, electronic components can be very compact, but their speed and energy performance is ultimately limited due to ohmic losses and finite velocity of the electrons. In parallel, photonic devices and circuits have been proposed that by molding the flow of light can overcome the mentioned limitations but are not as integrable as their electronic counterparts. Plasmonics is an emerging research field that combines electronics and photonics using nanostructures that can couple the light waves to the free electrons in metals. By confining the light at deep subwavelength scales, plasmonic devices can highly enhance the light-matter interactions, with applications in ultrafast optical communications, energy-harvesting, optical sensing, and biodetection. Conventionally, plasmonic devices are optimized to operate with a single light color, which limits their performance in wavelength-multiplexed operations and ultrafast non-linear optics. For such applications, it is far more efficient to use the more advanced "multiresonant plasmonic systems with spatial mode overlap" that can enhance the optical interactions at the same locations but for multiple light colors. This dissertation comprehensively studies these systems in terms of the fundamental concepts, design ideas, and applications. Our work advances the plasmonic field from both science and technology perspectives. In particular, we explore and classify the strategies of building multiresonant plasmonic systems with spatial mode overlap for the first time. Also, we establish the black-box coupled mode theory, a novel framework for analysis and design of complicated plasmonic structures with optimized performance. Furthermore, we introduce the "nanolaminate plasmonic crystals" (NPCs), large area and cost-effective devices that can enhance the optical processes for both visible and near-infrared lights. Finally, we demonstrate NPCs ability in simultaneous frequency-doubling and broadband emission of light and come up with advanced theoretical models that can explain the light generation and color conversion in plasmonic devices.

The Fermi Large Area Telescope has observed a gamma-ray excess toward the center of the Galaxy at ~ GeV energies. The spectrum and intensity of the excess are consistent with the annihilation of dark matter with a mass of ~100 GeV and a velocity-averaged cross section of ~ 1e-26 cubic centimeter per second. In the meantime, a population of unresolved millisecond pulsars (MSPs) in the Galactic center remains a possible source of the excess. Furthermore, recent analyses have shown that the excess prefers the spatial morphology of the stellar bulge distribution in the Galactic center, supporting a MSP origin. The new discovery makes it imperative to further study the signals from MSPs. This dissertation studies the gamma-ray emission from Galactic millisecond pulsars to provide new insights into the origin of the Galactic center excess. Using the GALPROP code, we simulate the propagation of e± injected by the putative MSPs in the Galactic bulge and calculate the inverse Compton (IC) emission caused by the e± losing energy in the interstellar radiation field. We find recognizable features in the spatial maps of the IC. Above TeV energies, the IC morphology tends to follow the distribution of the injected e±. Then, we study the Cherenkov Telescope Array (CTA) sensitivity to the IC signal from MSPs. We find that the CTA has the potential to robustly discover the IC signature when the MSP e± injection efficiencies are in the range ≈ 2.9-74.1%. The CTA can also discriminate between an MSP and a dark matter origin for the radiating e± based on their different spatial maps. Next, we analyze the Fermi data from directions of Galactic globular clusters. The globular clusters are thought to be shining in gamma rays because of the MSP population they host. By analyzing their gamma-ray spectra, we reveal evidence for an IC component in the high-energy tail of Fermi data. Based on the IC component in the globular cluster spectra, the e± injection efficiency of millisecond pulsars is estimated to be slightly smaller than 10%. Finally, we study the spatial morphology of the 511 keV signal toward the Galactic center using data from INTEGRAL/SPI. We confirm that the 511 keV signal also traces the old stellar population in the Galactic bulge, which is similar to the Fermi GeV excess. Using a 3D smoothing kernel, we find that the signal is smeared out over a characteristic length scale of 150 ± 50 pc. We show that positron propagation prior to annihilation can explain the overall phenomenology of the 511 keV signal. Doctor of Philosophy Dark matter means matter that does not interact with light; therefore, they are invisible to traditional observations. We know that dark matter exists based on plenty of gravitational evidence: the motions of stars in galaxies, the large-scale structure of the Universe, the temperature fluctuations in the cosmic microwave background. However, we still know very little about the particle nature of dark matter. Detecting dark matter is one of the most extensive missions of modern physics. In indirect detection, the dark matter particles are expected to annihilate or decay in the cosmos, producing messenger particles that include gamma rays, cosmic rays, and neutrinos. Astronomical observations could detect those signals and confirm the nature of dark matter. However, understanding the astrophysical sources is essential for indirect detection of dark matter as they may emit similar signals. For a recent example, the Fermi Large Area Telescope launched by NASA is the most sensitive gamma-ray telescope in the energy range of ~ 100 MeV to ~ 100 GeV. It has detected an excess of gamma-ray signals toward the Galactic center consistent with what we expect from dark matter annihilation. However, millisecond pulsars, a type of fast rotating neutron stars, may also generate similar gamma-ray signals. Therefore, the origin of the signal remains unsettled. In this dissertation, we study different prospective of the gamma-ray emission from the millisecond pulsars in the Milky Way. We first study the inverse Compton signal from the millisecond pulsars in the Galactic bulge, caused by the relativistic e± injected by the millisecond pulsars. We find that the signal traces the original distribution of the e± above TeV energies. Next generation ground-based gamma-ray observatories like the Cherenkov Telescope Array (CTA) could be used to detect the signal. We study the CTA sensitivity to such an inverse Compton signal. We find that CTA can detect the inverse Compton signal from millisecond pulsars and discriminate it from a dark matter signal. We also study the gamma-ray emission from globular clusters in the Milky Way. They are dense collections of old stars orbiting our Galaxy, and they are known for hosting many millisecond pulsars. We reveal evidence for inverse Compton emission from the gamma-ray data of globular clusters. Our discovery helps us better understand the high-energy property of millisecond pulsars. Last, we study the morphology of the Galactic 511 keV signal caused by positron annihilation. Compact objects including millisecond pulsars are potential sources of the positrons. We find that the old stellar distribution with a smearing scale of ~ 150 pc best describes the 511 keV signal. Positron propagation from their sources prior to annihilation could explain the measured smearing scale.

The rapid progress on developing new and improved multifunctional materials, for optoelectronic and spin based phenomena/devices, have increased the importance of the fundamental understanding of their coherent states and nonlinear optical properties. This study is aimed at characterizing, modeling, and controlling the fundamental electronic, phononic, and spin properties of several classes of materials through nonequilibrium and nonlinear light-matter interactions, coupled with a novel design of the material phases, interfaces, and heterostructures. This research directly addresses the Grand Challenges identified in the Basic Energy Sciences Advisory Committee report "Directing Matter and Energy: Five Challenges for Science and the Imagination" (Hemminger, 2007) [1], in particular, the area: "Matter far beyond equilibrium" and addresses the questions, "How do remarkable properties of matter emerge from complex correlations of the atomic or electronic constituents and how can we control these properties?" and "How do we design and perfect atom- and energy-efficient synthesis of revolutionary new forms of matter with tailored properties?". The knowledge gained from these fundamental studies can provide new information for a broad community to provide concepts for the next generation of multifunctional materials and devices, and resulted in several publications and conference presentations. The materials studied in this dissertation included multiferroic BaTiO3-BiFeO3 [2], ferroelectric Pb0.52Zr0.48TiO3 (PZT), InAs/AlAsSb multi-quantum-well [3], lead halide perovskite [4], n-type InAsP films [5, 6], and nanolaminate plasmonic crystals [7]. Probing multiferroics, which are materials that can exhibit ferromagnetic, ferroelectric, and ferroelastic orders simultaneously in a single phase, was a main focus of this study. BiFeO3 (BFO) is the most widely investigated multiferroic due to its high Neel and Curie temperatures and has antiferromagnetic and ferroelectric properties [8]. An inherent drawback of BFO is its large leakage currents. In this project, (1 − x)BaTiO3-(x)BiFeO3, x = 0.725 (BTO-BFO) heterostructures were investigated [9], where the conductivity of the solid solution can be reduced by adding another perovskite material, BaTiO3 [2]. We aimed to study optically induced coherent states in our BTO-BFO structures. Time resolved pumpprobe spectroscopic measurements were performed at room temperature as well as at low temperature (100 K) up to 10 T. Coherent acoustic phonons were observed both in a film and nanorods, resulting in coherent phonon frequencies of 27 and 33 GHz, respectively [2]. Coherent phonon spectroscopy is a sensitive tool to characterize the interfaces and can be employed as an effective ultrasensitive quantum sensor [10]. Furthermore, in the nanorods arrays of BTO-BFO, an additional oscillation with frequency in the range of 8.1 GHz was observed. This frequency is close to a theoretically predicted magnon frequency which could indicate the coexistence of coherent phonons and magnons in the nanorods arrays [2]. In an analogy to photonics which relies on electromagnetic waves, magnonics utilizes spin waves to carry and process information, offering several advantages such as an operation frequency in the THz range. Recently, "a quantum tango" [11] was reported where coupled coherent magnon and phonons modes were formed on a surface patterned ferromagnet. Furthermore, BTO-BFO heterostructures were probed using transient birefringence and magneto-optical Kerr effect spectroscopy. The results demonstrated that the magnetic field dependence of coherent phonons, measured by these two techniques, exhibits more sensitivity to the external magnetic fields compared to the differential reflectivity technique [2]. Moreover, nonlinear optical properties of this structure were investigated via second harmonic generation spectroscopy, where wavelength and polarization dependence of this nonlinear observation will be discussed in this dissertation. As part of this study, another class of multiferroic materials, with strong ferroelectric and piezoelectric properties, Pb0.52Zr0.48TiO3 (PZT) was studied [12]. In this project, the nonlinear optical properties of PZT nanorod arrays were investigated. Clear signatures of second harmonic generations from 490-525 nm (2.38-2.53 eV) at room temperature, were observed. Furthermore, time resolved differential reflectivity measurements were performed to study dynamical properties in the range of 690-1000 nm where multiphoton processes were responsible for the photoexcitations. We compared this excitation scheme, which is sensitive mainly to the surface states, to when the photoexcited energy (∼ 3.1 eV) was close to the bandgap of the nanorods. Our results offer promises for employing these nanostructures in nonlinear photonic applications. Furthermore, the established techniques during my research provided new insights on optical properties of InAs/AlAsSb multi-quantum-well [3], lead halide perovskite [4], n-type InAsP films [5, 6], and nanolaminate plasmonic crystals [7], and the results will be briefly presented in this dissertation. Doctor of Philosophy My research activities have explored multifunctional materials and heterostructures with strongly enhanced coupled electric and magnetic orders and optical properties. In particular, pursuing novel heterostructure designs such as multiferroics can provide control over electric and magnetic ordering in mixed dimensionality. This, together with control at the level from lattice structure to electron spin states can give rise to improved or even qualitatively new and robust materials properties. For example, a better understanding of the phenomena associated with the spin degree of freedom of electrons allows for advancement in spintronic device applications such as storage, logic, and sensors, which are associated with quantum computers and quantum communications [13, 14, 15]. Overarching questions and goals of my activities included: What are the microscopic origins and mechanisms of nonlinear response in strongly coupled nanostructured materials and its relationship to electronic, spin, and lattice degrees of freedom? (2) What are the effects of dimensionality and quantum confinement on optical properties? (3) How do we control and manipulate the coherent states, such as coherent phonons and magnons using external and internal fields, material composition, and morphology to achieve maximal efficiency and tunability? Addressing many of the challenges in the fast-paced technological world requires continued developments of new materials with enhanced optical properties. The knowledge gained from my fundamental studies can provide new information for the next generation of multifunctional materials and devices with advanced optical properties and resulted in several publications and conference presentations.

Small-signal dynamic response of semiconductor quantum dot (QD) lasers with asymmetric barrier layers was studied. Semiconductor lasers are used in many communication systems. Fiber optic communication systems use semiconductor lasers in order to transmit information. DVD and Blu-ray disk players feature semiconductor lasers as their readout source. Barcode readers and laser pointers also use semiconductor lasers. A medical application of semiconductor lasers is for minor soft tissue procedures. Semiconductor lasers are also used to pump solid-state and fiber lasers. Semiconductor lasers are able to transmit telephone, internet, and television signals through fiber optic cables over long distances. The amount of information able to be transferred is directly related to the bandwidth of the laser. By introducing asymmetric barrier layers, the modulation bandwidth of the laser will improve, allowing for more information to be transferred. Also, by introducing asymmetric barrier layers, the output power will be unrestricted, meaning as more current is applied to the system, the laser will get more powerful. An optimum pumping current was found which maximized modulation bandwidth at -3dB, and is lower in QD lasers with asymmetric barrier layers (ABL) as opposed to conventional QD lasers. Modulation bandwidth was found to increase with cross section of carrier capture before reaching an asymptote. Both surface density of QDs and cavity length had optimum values which maximized modulation bandwidth. Relative QD size fluctuation was considered in order to see how variation in QD sizes effects the modulation bandwidth of the semiconductor QD laser with ABLs. These calculations give a good starting point for fabricating semiconductor QD lasers with ABLs featuring the largest modulation bandwidth possible for fiber optic communication systems. In semiconductor QD lasers, the electrons and holes may be captured into excited states within the QDs, rather than the ground state. The particles may also jump from the ground state up to an excited state, or drop from the excited state to the ground state. Recombination of electron-hole pairs can occur from the ground state to the ground state or from an excited state to an excited state. In the situation if the capture of charge carriers into the ground state in QDs takes place via the excited-state, then this two-step capture process makes the output power from ground-state lasing to saturate in conventional QD lasers. By using ABLs in the QD laser, it is predicted that the output power of ground-state lasing will continue to rise with applied current, as the ABLs will stop the electrons and holes from recombining in the optical confinement layer. Thus, ABL QD lasers will be able to be used in applications that require large energy outputs. Master of Science Semiconductor lasers (also known as diode lasers) have been used in numerous applications ranging from communication to medical applications. Among all applications of diode lasers, of particular importance is their use for high speed transmission of information and data in fiber optic communication systems. This is accomplished by direct conversion of the diode laser input (electrical current) to its output (optical power). Direct modulation of the laser optical output through varying electrical current helps cut costs by not requiring other expensive equipment in order to perform modulation. The performance of conventional semiconductor lasers suffers from parasitic recombination outside of the active region – an unwanted process that consumes a considerable fraction of the laser input (injection current) while not contributing to the useful output and thus damaging its performance. Asymmetric barrier layers were proposed as a way to suppress parasitic recombination in semiconductor lasers. In this study, the optimal conditions for semiconductor quantum dot lasers with asymmetric barrier layers were calculated in order to maximize their modulation bandwidth – the parameter that determines the highest speed of efficient information transmission. This includes finding the optimal values of the dc component of the pump current, quantum dot surface density and size fluctuations, and cavity length. As compared to conventional quantum dot lasers, the optimal dc current maximizing the modulation bandwidth is shown to be considerably lower in quantum dot lasers with asymmetric barrier layers thus proving their outperforming efficiency. In the presence of extra states in quantum dots in conventional lasers, the optical output of needed ground-state lasing may be heavily impacted – it may remain almost unchanged with increasing the laser input current. As opposed to conventional lasers, the output power of ground-state lasing in devices with asymmetric barrier layers will continue growing as more input current is applied to the system.

Large-scale optoelectronic integration is limited by the lack of efficient light sources and broadband photodetectors, which could be integrated with the silicon complementary metal-oxide-semiconductor (CMOS) technology. Persistent efforts continue to achieve efficient light emission as well as broadband photodetection from silicon in extending the silicon technology into fully integrated optoelectronic circuits. Recent breakthroughs, including the demonstration of high-speed optical modulators, photodetectors, and waveguides in silicon, have brought the concept of transition from electrical to optical interconnects closer to realization. The on-chip light sources based on silicon are still a key challenge due to the indirect bandgap of silicon that impedes coherent light sources. To overcome this issue, we have studied, fabricated, and characterized nanostructures including single semiconductor epilayers, multiple quantum wells, and graphene-semiconductor heterostructures to develop coherent light sources and photodetectors in silicon. To develop coherent light sources, we reported the demonstration of room-temperature lasing at the technologically crucial 1.5 m wavelength range from Er-doped GaN epilayers and Er-doped GaN multiple-quantum wells grown on silicon and sapphire. The realization of room-temperature lasing at the minimum loss window of optical fiber and in the eye-safe wavelength region of 1.5 m is highly sought-after for use in many applications in various fields including defense, industrial processing, communication, medicine, spectroscopy and imaging. The results laid the foundation for achieving hybrid GaN-Si lasers providing a new pathway towards full photonic integration for silicon optoelectronics. Silicon photodiodes contribute a large portion in the photodetector market. However, silicon photodetectors are sensitive in the UV to near infrared region. Photodetection in the mid-infrared is based on thermal radiation detectors, narrow bandgap materials (InGaAs, HgCdTe) semiconductors, photo-ionization of shallow impurities in semiconductors (Si:As, Ge:Ga), and quantum well structures. Such technology requires complicated fabrication processes or cryogenic operation, resulting in manufacturing costs and severe integration issues. To develop broadband photodetectors, we focus on graphene photodetectors on silicon. Graphene generates photocarriers by absorbing photons in a broadband spectrum from the deep-ultraviolet to the terahertz region. Graphene can be realized as the next generation broadband photodetection material, especially in the infrared to terahertz region. Here, we have demonstrated high-performance hybrid photodetectors operating from the deep-ultraviolet to the mid-infrared region with high sensitivity and ultrafast response by coupling graphene with a p-type semiconductor photosensitizer, nitrogen-doped Ta2O5 thin film. Doctor of Philosophy According to Moor's law, the number of transistors per die area doubles every 18 months with no increase in power consumption, which means that digital devices including smart phones and computers will become significantly faster and more energy-efficient than those of the previous generation. Photons (light) travel with the highest speed permitted by the known law of physics. The idea of optical interconnects, using photons instead of electrons, enables faster data transfer. Two important elements of the integrated circuits (ICs) based on photons are the coherent light source (laser) and the photodetector. We investigated the optical properties of erbium doped gallium nitride epilayers and multiple quantum wells grown on silicon and sapphire and demonstrated lasing from these materials at 1.5 µm. We also fabricated and characterized graphene photodetectors that can detect the light from the deep ultraviolet to the mid-infrared region. The results provided a new pathway towards full photonic integration for silicon optoelectronics. Besides, they are the heart of many important applications ranging from gas sensing, aerospace sensors and systems, thermal imaging, biomedical imaging, infrared spectroscopy, and lidar-to-optical telecommunications.

With the development of controlled and predictable nanoparticle fabrication, assembling multiple nano-objects into larger functional nanostructure has attracted increasing attention. As the most basic structure, assembly of one-dimensional (1D) structures is a good model for investigating the assembly mechanism of a nanostructure's formation from individual particles. In this dissertation, the dynamics and the growth mechanism of anisotropic 1D nanostructures is investigated. In our first study, we demonstrate a simple method for assembling superparamagnetic nanoparticles (SPIONs) into structure-controlled 1D chains in a rotating magnetic field. The length of the SPION chains can be well described by an exponential distribution, as is also seen in SPION chains in a static field. In addition, the maximum chain length is limited by the field's rotational speed, as is seen in micro-sized beads forming chains in a rotating field. However, due to a combination of thermal fluctuations and hydrodynamic forces, the chain length in our case is shorter than either limit. In addition to chain length, the disorder of chains was also studied. Because of the friction between particles, kinetic potential traps prevent relaxation to the global free energy minimum. The traps are too deep to be overcome through thermal fluctuations, and assemblies captured by the kinetic traps therefore form disordered chains. We demonstrate that this disorder gradually heals over a timescale of tens of minutes and that the healing process can be promoted by increasing particle concentration or solution ionic strength, suggesting that the chain growth process provides the energy required to overcome the kinetic trapping. Next, we introduce a novel optical technique we term Quantitative Optical Anisotropy Imaging (QOAI). A fast and precise single-particle characterizing technique for anisotropic nanostructures, QOAI allows real-time tracking of particle orientation as well as the spectroscopic characterization of polarizabilities of nanoparticles on a microsecond timescale. The abilities of QOAI are demonstrated by the detection and the characterization of single gold nanorods. We also show that single particle diffusions and the process of particle binding to a wall can be tracked through QOAI. The rotational diffusivities of gold nanorods near the wall were determined by autocorrelation analysis, which shows that the diffusivity in the polar direction is slightly smaller than in the azimuthal direction. This result demonstrates that a detailed correlation analysis with QOAI may provide the opportunity to analyze both the translational and rotational motion of particles simultaneously, enabling true 3-dimensional orientation tracking. Finally, optical methods including QOAI are applied to the investigation of magnetic assembly, demonstrating that optical anisotropy is generated during particle binding, which can be used as a probe of the magnetic assembly process. QOAI is employed to track the dynamics of magnetic clusters in real time, attempting to capture insights on the self-assembly of the magnetic nanoparticles. By turning the external magnetic field on and off, the processes of combining superparamagnetic colloidal nanoparticle clusters into chain assemblies are monitored along with the chain growth. This fast and orientation-sensitive single-particle measurement opens the door to detailed studies of self-assembly away from equilibrium. Doctor of Philosophy Nanotechnology is the study and application of phenomena at the nanoscale, which is between 1 and 100 nm. Due to quantum effects, nanomaterials exhibit many interesting properties that cannot be found in bulk materials and are highly influenced by the shape of the nanostructures. One of the most promising strategies for forming complex nanostructures is to use smaller nanoparticles as building blocks. Therefore, significant efforts have been spent on the studies of the fabrication and modeling of the assembly of nanostructures. As a good starting point for analyzing the mechanism of self-assembly, we focus on the most basic structure, one-dimensional (1D) nanowires and chains. First, we demonstrate a simple method to fabricate one-dimensional magnetic chains from spherical magnetic nanoparticles in a rotating magnetic field. The growth mechanism of the nanochains is investigated, indicating the theory developed for chains formed with larger beads is not applicable at the nanoscale, and additional factors, such as the effect of temperature, need to be considered. Second, we introduce a fast, sensitive optical technique for characterizing anisotropic nanostructures. Because of their unique optical properties, gold nanorods are used to demonstrate the capabilities of the optical system. Not only static properties (orientation, aspect ratio), but also dynamics properties (rotational motion), of single gold nanorods are characterized quantitatively. Finally, this optical technique is extended to preliminary work on characterizing magnetic chain assembly. The processes of magnetic cluster binding and dissociation in a magnetic field are monitored and analyzed.

Recently, the resistance switching random access memory (ReRAM) in several MIM systems has been studied extensively for applications to the next generation non-volatile memory (NVM) devices and memristors since the scaling of conventional memories based on floating gate MOSFETs is getting increasingly difficult. ReRAM is being considered one of the most promising candidates for next generation non-volatile memory due to its relatively high switching speed, superior scalability, low power consumption, good retention and simple fabrication method. Cu/TaOX/Pt resistive switching device is a very good candidate due to its well performance and well characterization. However, since platinum (Pt) acting as the inert electrode is not economical efficient for industrial production, a compatible replacement of Pt is highly desirable. The device property of Co based resistive switching devices has been explored in this work. Compared with Pt devices, electric characterization of the fabricated Cu/TaOX/Co devices exhibits very similar FORM, SET and RESET voltages for Cu conductive filaments. However, for the oxygen vacancy (VO) filament the Co device has a significant smaller FORM, SET and RESET voltages of VO filament, which can be partly attributed to the work function difference between Pt and Co of 1.35 V and partly to the impaired integrity properties of Co vs Pt inert electrode. The limit of SET-RESET operations is mainly due to the geometrical shape of the Cu conductive filament is more cylindered rather than Cone-like shape as well as the high Joules heat dissipation. What’s more, ReRAM is also the most promising candidate for a flexible memory, as a variety of materials can be used both inorganics, organics and even hybrid nanocomposites. Besides inorganic ReRAM device, we also fabricated an organic ReRAM device with the structure Cu doped PANI-CSA/O-AA/Al. We have manufactured ReRAM based on Cu-doped PANI-CSA polymer electrode, O-AA as the polymer solid electrolyte and Al as the bottom electrode for the first time. This polymer device shows a significantly lower forming voltage than inorganic ReRAM devices such as Cu/TaOX/Pt. Our results also demonstrate that our organic ReRAM is a promising candidate for inexpensive candidate for inexpensive and environmentally friendly memory devices. We have demonstrated that the FORM operation of the polymer devices depends on the concentration of Cu+ ions as well as the thickness of the polymer electrode. M.S. Although the scaling of conventional memories such as volatile dynamic random access memory (DRAM) and non-volatile flash technology is becoming increasingly difficult, new types of non-volatile memories, such as resistive switching memories, have recently attracted the attention of both industry and academia. Resistive switching memory is considered as the next generation non-volatile memory because of its excellent scalability, high switching speed, simple structure and low power consumption. What’s more, ReRAM is also a promising candidate for a flexible memory, as a variety of materials can be used both inorganics, organics and even hybrid nanocomposites. ReRAM shows unique nanoionics based filamentary switching mechanism. Besides the nonvolatile memory applications, resistive switching devices implement the formation of a memristor, which is the fourth basic electrical component and can be used for neuromorphic computing. First, we report the device property of Co based resistive switching devices with a structure of Cu/TaOX/Co layers. The I-V characteristics of the manufactured Cu/TaOX/Co devices shows very similar FORM, SET and RESET voltages for Cu conductive filaments compared with Pt device. However, the Co device has a significant smaller FORM, SET and RESET voltages for oxygen vacancy (VO) filaments, which can be partly attributed to the work function difference between Pt and Co of 13.5 eV and partly to the impaired integrity properties of Co vs Pt inert electrode. The main reason for the limit of SET-RESET operations is that high Joules heat dissipation. With high Joules heat accumulation, the maximum switching cycles of Co devices is up to 8 times, while in the case of Pt cases, it is almost unlimited. Secondly, we fabricated an organic ReRAM device with the structure Cu-doped PANI-CSA/O-AA/Al. Cu-doped PANI-CSA polymer electrode has been introduced for the first time as the top polymer electrode of a ReRAM device. Compared to inorganic ReRAM device, this polymer device can be operated at a significantly lower forming voltage than inorganic devices such as Cu/TaOX/Pt. We have demonstrated that our organic ReRAM is a promising candidate for environmentally friendly and flexible memory devices. Our results demonstrate the FORM operation of the polymer devices depend on the concentration of Cu+ ions as well as the thickness of the polymer top layer.

GaN has the potential to revolutionize the high power electronics industry, enabling high voltage applications and better power conversion efficiency due to its intrinsic material properties and newly available high purity bulk substrates. However, unintentional impurity incorporation needs to be reduced. This reduction can be accomplished by reducing the source of contamination and exploration of extreme growth conditions which reduce the incorporation of these contaminants. Newly available bulk substrates with low threading dislocations allow for better study of material properties, as opposed to material whose properties are dominated by structural and chemical defects. In addition, very thick films can be grown without cracking due to exact lattice and thermal expansion coefficient match. Through chemical and electrical measurements, this work aims to find growth conditions which reduces contamination without a severe impact on growth rate, which is an important factor from an industry standpoint. The proposed thicknesses of these devices are on the order of one hundred microns and requires tight control of the intentional dopants. Doctor of Philosophy GaN is a compound semiconductor which has the potential to revolutionize the high power electronics industry, enabling new applications and energy savings due to its inherent material properties. However, material quality and purity requires improvement. This improvement can be accomplished by reducing contamination and growing under extreme conditions. Newly available bulk substrates with low defects allow for better study of material properties. In addition, very thick films can be grown without cracking on these substrates due to exact lattice and thermal expansion coefficient match. Through chemical and electrical measurements, this work aims to find optimal growth conditions for high purity GaN without a severe impact on growth rate, which is an important factor from an industry standpoint. The proposed thicknesses of these devices are on the order of one hundred microns and requires tight control of impurities.

High temperature waste heat recovery has been gaining attention in recent years as it forms one of the largest sources of available energy. A rapid development of thermoelectric (TE) materials that can directly convert heat into electricity through the Seebeck effect, opens promising pathway for harvesting the thermal energy from the surroundings. In order to harvest the high-quality waste heat at elevated temperature, excellent thermal and mechanical stability of the TE materials is critical for a sustainable energy harvesting. In this respect, half-Heusler (hH) alloys are one of the promising high-temperature TE materials due to their high dimensionless thermoelectric figure of merit (zT) along with excellent mechanical and thermal stability. This dissertation demonstrates novel hH compositions and microstructures for the waste heat recovery systems. Focus in the thesis is on development of high performance hH TE materials with excellent in-air thermal stability at high temperatures (>700K). This will allow manufacturing of high efficiency and durable high temperature thermoelectric generators (TEGs). In chapter 3 and 4, a comprehensive optimization of n-type MNiSn and p-type MCoSb (M = Hf, Zr, and Ti) compounds is investigated through systematic control of processing parameters during melting and sintering. The synthesis conditions were controlled to achieve the phase purity, desired microstructure and the enhanced charge-carrier transport. Optimized n-type and p-type compositions are found to exhibit zTmax ~ 1 at 773 K. Chapter 5 describes breakthrough in decoupling of TE parameters in n-type half-Heusler (hH) alloys through multi-scale nanocomposite architecture with tungsten nanoinclusions. The tungsten nanoparticles not only assist electron injection, thereby improving electrical conductivity, but also enhance the Seebeck coefficient through energy filtering effect. The microstructure comprises of disordered phases with feature sizes at multiple length scales, which assists in effective scattering of heat-carrying phonons over diverse mean-free-path ranges. Cumulatively, these effects are shown to result in outstanding thermoelectric performance of zTmax ~ 1.4 at 773 K and zTavg ~ 0.93 between 300 and 973 K. In order to deploy TE materials into a thermal energy conversion device, it is essential to understand the transformation behavior under thermal cycling at high temperatures. In-air thermal stability of the hH compositions is demonstrated in chapter 6. All the optimized compositions are found to be stable below 673 K in-air condition. The n-type MNiSn and p-type NbFeSb compounds were found to show good thermal stability even at higher temperatures (>773K), whereas MCoSb compounds did not exhibit similar level of stability. Building upon the improved material performance and thermal stability, uni-coupled TE generators are demonstrated that exhibit high power density of 13.81 W⸱cm-2 and conversion efficiency of 10.9 % under a temperature difference of 674 K. The uni-couple TEG device shows stable performance for more than 150 hours at 873 K in air. These results are very promising for deployment of TE materials in waste heat recovery systems. Doctor of Philosophy Based on the 2012 international energy agency (IEA) report, global waste heat energy is estimated to be in the range of 246 Exajoule (1 EJ = 10¹⁸ J). Tapping even small fraction of this wasted energy through thermal energy harvesting techniques will allow us to generate significant magnitude of green energy. Thermoelectrics (TEs) are one of the most promising thermal energy conversion materials as they offer cost-effective and environmentally friendly option with solid-state silent operation and scalability. Among many different options for high temperature TE materials, half-Heusler system is one of the leading candidates as it has the potential to provide high performance and thermal stability at temperatures as high as 873 K. The progress in developing practical half-Heusler materials has been limited for last two decades. Despite many publications, the maximum figure of merit (zT) of n-type half-Heusler materials has been stagnant (zT ~ 1.0). Further, there has been a lack of focus towards module development that can operate under realistic conditions. This dissertation provides comprehensive studies on novel thermoelectric compositions and nanocomposites that are suitable for manufacturing of high temperature modules. Microstructural architectures proposed here provide the ability to tailor electronic transport and phonon scattering beyond the commonly demonstrated regimes. Optimized materials were successfully implemented in efficient and stable thermoelectric generator exhibiting power density on the order of 13.81 W⸱cm⁻² , which is 1400 % higher than that of the fuel cell (~1 W⸱cm⁻² ).

Studying dynamics of proteins in their biological milieu such as water is interesting because of their strong absorption in the terahertz range that contain information on their global and sub-global collective vibrational modes (conformational dynamics) and global dynamical correlations among solvent water molecules and proteins. In addition, water molecules dynamics within protein solvation layers play a major role in enzyme activity. However, due to the strong absorption of water in the gigahertz-to-terahertz frequencies, it is challenging to study the properties of the solvent dynamics as well as the conformational changes of protein in water. In response, we have developed a highly sensitive megahertz-to-terahertz dielectric spectroscopy system to probe the hydration shells as well as large-scale dynamics of these biomolecules. Thereby, we have deduced the conformation flexibility of proteins and compare the hydration dynamics around proteins to understand the effects of surface-mediated solvent dynamics, relationships among different measures of interfacial solvent dynamics, and protein-mediated solvent dynamics based on the complex dielectric response from 50 MHz up to 2 THz by using the system we developed. Comparing these assets of various proteins in different classes helps us shed light on the macromolecular dynamics in a biologically relevant water environment. Doctor of Philosophy Proteins are complicated biomolecules that exist in all living creatures and they are, mostly, involved in building up structures and cell functions in various biological systems. Not only their existence but also their complex movements and dynamics are vital to cell functions in living beings. Until recently, their chemical functions and dynamics have been extremely challenging to investigate and track in their native environments. Thanks to various efforts by researchers all over the world to learn more about their convoluted behavior, new techniques have arisen to study these properties. We, as a part of this community, have been able to develop highly sensitive megahertz-to-terahertz dielectric spectroscopy system to probe proteins and other biomolecules dynamics in picosecond to microsecond range. Using our benchmark system, we have been able to map the detailed dynamical properties of biomolecules as well as their exclusive hydration shell characterizations. In this work, we gathered details about three well-known proteins and biomolecules by studying their dielectric responses. Thus, we have been able to discuss the movements, relaxation processes and hydration shell properties of these molecules in liquid water as their basic native environment.

In this dissertation, I extend the analysis of human dynamics to human movements in online activities. My work starts with a discussion of the human information foraging process based on three large collections of empirical search click-through logs collected in different time periods. With the analogy of viewing the click-through on search engine result pages as a random walk, a variety of quantities like the distributions of step length and waiting time as well as mean-squared displacements, correlations and entropies are discussed. Notable differences between the different logs reveal an increased efficiency of the search engines, which is found to be related to the vanishing of the heavy-tailed characteristics of step lengths in newer logs as well as the switch from superdiffusion to normal diffusion in the diffusive processes of the random walks. In the language of foraging, the newer logs indicate that online searches overwhelmingly yield local searches, whereas for the older logs the foraging processes are a combination of local searches and relocation phases that are power-law distributed. The investigation highlights the presence of intermittent search processes in online searches, where phases of local explorations are separated by power-law distributed relocation jumps. In the second part of this dissertation I focus on an in-depth analysis of online gambling behaviors. For this analysis the collected empirical gambling logs reveal the wide existence of heavy-tailed statistics in various quantities in different online gambling games. For example, when players are allowed to choose arbitrary bet values, the bet values present log-normal distributions, meanwhile if they are restricted to use items as wagers, the distribution becomes truncated power laws. Under the analogy of viewing the net change of income of each player as a random walk, the mean-squared displacement and first-passage time distribution of these net income random walks both exhibit anomalous diffusion. In particular, in an online lottery game the mean-squared displacement presents a crossover from a superdiffusive to a normal diffusive regime, which is reproduced using simulations and explained analytically. This investigation also reveals the scaling characteristics and probability reweighting in risk attitude of online gamblers, which may help to interpret behaviors in economic systems. This work was supported by the US National Science Foundation through grants DMR-1205309 and DMR-1606814. Ph. D. Humans are complex, meanwhile understanding the complex human behaviors is of crucial importance in solving many social problems. In recent years, socio physicists have made substantial progress in human dynamics research. In this dissertation, I extend this type of analysis to human movements in online activities. My work starts with a discussion of the human information foraging process. This investigation is based on empirical search logs and an analogy of viewing the click-through on search engine result pages as a random walk. With an increased efficiency of the search engines, the heavy-tailed characteristics of step lengths disappear, and the diffusive processes of the random walkers switch from superdiffusion to normal diffusion. In the language of foraging, the newer logs indicate that online searches overwhelmingly yield local searches, whereas for the older logs the foraging processes are a combination of local searches and relocation phases that are power-law distributed. The investigation highlights the presence of intermittent search processes in online searches, where phases of local explorations are separated by power-law distributed relocation jumps. In the second part of this dissertation I focus on an in-depth analysis of online gambling behaviors, where the collected empirical gambling logs reveal the wide existence of heavy-tailed statistics in various quantities. Using an analogy of viewing the net change of income of each player as a random walk, the mean-squared displacement and first-passage time distribution of these net income random walks exhibit anomalous diffusion. This investigation also reveals the scaling characteristics and probability reweighting in risk attitude of online gamblers, which may help to interpret behaviors in economic systems. This work was supported by the US National Science Foundation through grants DMR-1205309 and DMR-1606814.

The absorption of solid state materials in complex photonic and optoelectronic devices overlap in the visible spectrum. Due to the overlap of spectral features, ultrafast measurements of charge carrier dynamics and transport is obscured. Here, the element specificity of transient extreme ultraviolet (XUV) spectroscopy is advanced as a probe for studying photoexcited charge transport in multiple-material junctions. The core-hole excited by the XUV transitions also imparts structural information on to the probed electronic transition. Transient XUV can therefore measure electron and averaged phonon dynamics for each elemental species in a junction. Application to polaron measurement in α-Fe_2O_3, valley-specific scattering in Si, and charge transfer in a nanoscale Ni-TiO_2-Si junction will be discussed.

We numerically investigate the dynamical properties of driven magnetic flux vortices in disordered type-II superconductors for a variety of temperatures, types of disorder and sample thicknesses. We do so with the aid of Langevin molecular dynamics simulations of a coarsegrained elastic line model of flux vortices in the extreme London limit. Some original findings of this doctoral work include the discovery that flux vortices driven through random point disorder show simple aging following drive quenches from the moving lattice state to both the pinned glassy state (non-universal aging) and near the critical depinning region (universal aging); estimations of experimentally consistent critical scaling exponents for the continuous depinning phase transition of vortices in three dimensions; and an estimation of the boundary curve separating regions of linear and non-linear electrical transport for flux lines driven through planar defects via novel direct measurements of vortex excitations. Ph. D. The works contained in this dissertation were undertaken with the goal of better understanding the dynamics of driven magnetic flux lines in type-II superconductors under different conditions of temperature, material defects and sample thickness. The investigations were conducted with the aid of computer simulations of the flux lines which preserve physical aspects of the system relevant to long-time dynamics while discarding irrelevant microscopic details. As a result of this work, we found (among other things) that when driven by electric currents, flux lines display very different dynamics depending on the strength of the current. When the current is weak, the material defects strongly pin the flux lines leaving them in a disordered glassy state. Sufficiently high current overpowers the defect pinning and results in the flux lines forming into a highly ordered crystal-like structure. In the intermediate critical current regime, the competing forces become comparable resulting in very large fluctuations of the flux lines and a critical slowing down of the flux line dynamics.

In this dissertation, I have verified that the striking purple color of the intermetallic compound AuAl₂, also known as purple gold, originates from surface plasmons (SPs). This contrasts to a previous assumption that this color is due to an interband absorption transition. The existence of SPs was demonstrated by launching them in thin AuAl2 films in the Kretschmann configuration, which enables us to measure the SP dispersion relation. I observed that the SP energy in thin films of purple gold is around 2.1 eV, comparable to previous work on the dielectric function of this material. Furthermore, SP sensing using AuAl₂ also shows the ability to measure the change in the refractive index of standard sucrose solution. AuAl₂ in nanoparticle form is also discussed in terms of plasmonic applications, where Mie scattering theory predicts that the particle bears nearly uniform absorption over the entire visible spectrum with an order magnitude higher than a lightabsorbing carbonaceous particle. The second topic of this dissertation focuses on plasmon enhanced fluorescence in gold nanoparticles (Au NPs). Here, I investigated the distance-dependent fluorescence emission of rhodamine green 110 fluorophores from Au NPs with tunable spacers. These spacers consist of polyelectrolyte multilayers (PEMs) consisting of poly(allylamine hydrochloride) and poly(styrene sulfonate) assembled at pH 8.4. The distance between Au NPs and fluorophores was varied by changing the ambient pH from 3 to 10 and back, which causes the swelling and deswelling of PEM spacer. Maximum fluorescence intensity with 4.0-fold enhancement was observed with 7-layer coated Au NPs at ambient pH 10 referenced to pH 3. The last topic of this dissertation examines a novel approach to assemble nanoparticles, in particular, dimers of gold nanospheres (NSs). 16 nm and 60 nm diameter NSs were connected using photocleavable molecules as linkers. I showed that the orientation of the dimers can be controlled with the polarization of UV illumination that cleaves the linkers, making dipolar patches. This type of assembly provides a simple method with potential applications in multiple contexts, such as biomedicine and nanorobotics. PHD This dissertation covers three related topics. The first is an investigation of the optical properties of the unusually colored purple gold, which is a blend of gold and aluminum with the chemical formula is AuAl₂. This compound is interesting in that the origin of this color is different from most other metals. In the case of gold, for example, the metal gold is yellow color by absorbing the blue component from white light, leaving behind yellow color reflected light. The blue light is absorbed by electrons that change their state from a lower energy to a higher one. In purple gold, the color results from a different phenomenon known as “surface plasmons.” Surface plasmons are waves consisting of many electrons that move back and forth near an interface between a metal and an electrical insulator. The energy of surface plasmons in purple gold is low and corresponds to the purple color in this compound. Recently, published theoretical work supports the possibility of surface plasmons in purple gold. In this dissertation, I experimentally verify the presence of surface plasmons in purple gold. To launch surface plasmons, light was reflected off of a purple gold film deposited on the hypotenuse of a prism with varying angles of incidence. Surface plasmons can be observed by the sudden dimming of reflected light. From this, I was able to extract the surface plasmon dispersion relation, which is the relation between the inverse of the wavelength and the energy of the surface plasmons. In addition, I computed the light absorption properties of purple v gold when it is used in a nanoparticle form. The computational result showed that small purple gold nanoparticles absorb light very well, which may be useful in photothermal cancer therapy and solar steam generation. The second dissertation topic comprises a study of fluorescent molecules. These are compounds that reemit light with a different and redder color than the color of the light that illuminates them. In this experiment, green fluorescent molecules were placed near the surface of gold nanoparticles to observe how the brightness of the light emission is affected by the distance between the molecule and the metal. The underlying mechanism is based on localized surface plasmon resonances in gold nanoparticles. Localized surface plasmon resonances are waves consisting of many electrons that oscillate inside the particle, and they only occur when light at certain frequency illuminate the particle. On the resonance, the particle also exhibits the brighter light around the particle’s surface but the dimmer light away from the particle’s surface. The light enhancement from the particle can change the light emission of the fluorescent molecules. If the fluorescent molecules were placed in the range of localized surface plasmon resonances, the light emission is increased owing to the brighter light from the particle. However, if the fluorescent molecules were placed further away from the range of localized surface plasmon resonances, the light emission is decreased owing to the dimmer light from the particle. The distance between the surface of gold nanoparticle and the fluorescent molecules was varied by wrapping the gold particles with ultra-thin films of different plastic polymers before attaching fluorescent molecules to the surface of the films. These polymer films have the property that they swell and shrink when the acidity and basicity of the solution of gold particles changes, which allows me to vary the distance between the gold particles and fluorescent molecules. The results showed that the observed light gets dimmer when the solution is more acidic. On the other hand, the brighter light is noticed when the solution is more basic, and this observation is repeatable many times. Moreover, my work differs from other published works vi in that the particles with the polymer films are more robust and stable than the other particles. This allows more design flexibility and suggests applications in biomedical or environmental research where the particles can be used to locally measure properties, such as acidity in confined spacers such as living cells. It may be possible to use this technique for tumor cells in our body or toxic pollutants in the air or water. The last dissertation topic involves assembling nanoparticles to build them into larger structures. In this experiment, I fabricated particle dimers that consisted of two gold nanospheres of different sizes. They were attached together by using small molecules that are sensitive to ultraviolet (UV) light, where these molecules allow small gold nanospheres to be attached to large gold nanospheres only in those locations on the large nanospheres that have been illuminated with a sufficient amount of UV light. To achieve this alignment, UV light with a linear polarization (a specific electric field direction) was used to select the area on the large nanospheres where the UV light was particularly intense and therefore able to break the molecules, leaving positively charged surface patches on the spheres. This results in the electrostatic attraction between the positive patches on the large gold nanospheres and the negatively charged small gold nanospheres. With this method, I was able to make dimers of nanospheres in a preferred alignment by changing the polarization of UV light. The experimental results showed a good yield of dipolar patches, which allows multifunctional nanostructures with applications in nanomedicine, optical sensing, nanoelectronics, etc.

The objective of this study was to expand on a method previously established in the lab for predicting subcortical structures using functional magnetic resonance imaging (fMRI) data restricted to the cortical surface. Our goal is to enhance the utility of low cost, portable imaging modalities, such as functional near infrared spectroscopy (fNIRS), which is limited in signal penetration depth. Previous work in the lab successfully employed functional connectivity to predict ten resting state networks and six anatomically de fined structures from the outer 10 mm layer of cortex using resting state fMRI data. The novelty of this study was two-fold: we chose to predict the functionally de fined region fusiform face area (FFA), and we utilized the functional connectivity of both resting state and task activation. Right FFA was identi ed for 27 subjects using a general linear model of a functional localizer tasks, and the average time series were extracted from right FFA and used as training and testing labels in support vector regression (SVR) models. Both resting state and task data decoded activity in right FFA above chance, both within and between run types. Our method is not specific to resting state, potentially broadening the scope of research questions depth-limited techniques can address. We observed a similarity in our accuracy cross-validation to previous work in the lab. We characterized this relationship between prediction accuracy and spatial signal-to-noise (SNR). We found that this relationship varied between resting state and task, as well as the functionality of features included in SVR modeling. Master of Science We used functional magnetic resonance imaging (fMRI) to predict activity in a deep brain region based on activity along the brain surface. This would increase the type of brain function a person could study using alternative methods that are less costly and easier to use, but can only detect signals along the surface. We were able to use this method to predict the fusiform face are, a region in the brain that responds more strongly to face images than other types of images. We also found a relationship between the quality of spatial information in the brain and the accuracy of predictions. This relationship differed depending on the types of brain regions were used to build the models, as well as whether the subjects were performing a task or rest scan.

The overall energy consumption of portable devices has been projected to triple over the next decade, growing to match the total power generated by the European Union and Canada by 2025. The rise of the internet-of-things (IoT) and ubiquitous and embedded computing has resulted in an exponential increase in such devices, wherein projections estimate that 50 billion smart devices will be connected and online by 2020. In order to alleviate the associated stresses placed on power generation and distribution networks, a holistic approach must be taken to conserve energy usage in electronic devices from the component to the circuit level. An effective approach to reduce power dissipation has been a continual reduction in operating voltage, thereby quadratically down-scaling active power dissipation. However, as state-of-the-art silicon (Si) complimentary metal-oxide-semiconductor (CMOS) field-effect transistors (FETs) enter sub-threshold operation in the ultra-low supply voltage regime, their drive current is noticeable degraded. Therefore, new energy-efficient MOSFETs and circuit architectures must be introduced. In this work, tunnel FETs (TFETs), which operate leveraging quantum mechanical tunneling, are investigated. A comprehensive investigation detailing electronic materials, to novel TFET device designs, to memory and logic digital circuits based upon those TFETs is provided in this work. Combined, these advances offer a computing platform that could save considerable energy and reduce power consumption in next-generation, ultra-low voltage applications. Ph. D.

This dissertation focuses on projects where optical techniques were employed to characterize novel materials, developing concepts toward next generation of devices. The materials that I studied included InAsP, InMnSb and InMnAs, and BT-BCN. I have employed several advanced time resolved and magneto-optical techniques to explore unexplored properties of these structures. The first class of the materials were the ternary alloys InAsP. The electron g-factor of InAsP can be tuned, even allowing for g=0, making InAsP an ideal candidate for quantum communication devices. Furthermore, InAsP shows promises for opto-electronics and spintronics, where the development of devices requires extensive knowledge of carrier and spin dynamics. Thus, I have performed time and polarization resolved pump-probe spectroscopy on InAsP with various compositions. The carrier and spin relaxation time in these structures were observed and demonstrated tunability to the excitation wavelengths, composition and temperature. The sensitivity to these parameters provide several avenues to control carrier and spin dynamics in InAsP alloys. The second project focused on the ferromagnetic narrow gap semiconductors InMnAs and InMnSb. The incorporation of Mn can lead to ferromagnetic behavior of InMnAs and InMnSb, and enhance the g-factors, making them ideal candidates for spintronics devices. When grown using Molecular Beam Epitaxy (MBE), the Curie temperature (textit{$T_c$}) of these structures is textless 100 K, however structures grown using Metalorganic Vapor phase Epitaxy (MOVPE) have textit{$T_c$} textgreater 300 K. Magnetic circular dichroism was performed on MOVPE grown InMnAs and InMnSb. Comparison of the experimental results with the theoretical calculations provides a direct method to map the band structure, including the temperature dependence of the spin-orbit split-off band to conduction band transition and g-factors, as well as the estimated sp-d electron/hole coupling parameters. My final project was on the lead-free ferroelectric BT-BCN. Ferroelectric materials are being investigated for high speed, density, nonvolatile and energy efficient memory devices; however, commercial ferroelectric memories typically contain lead, and use a destructive reading method. Reflectometry measurements were used in order to determine the refractive index of BT-BCN with varying thicknesses, which can provide a means to nondestructively read ferroelectric memory through optical methods. Ph. D.

We study game theoretic ecological models with cyclic competition in the case where the strategies can be mixed or pure. For both projects, reported in [49] and [50], we employ Monte Carlo simulations to study finite systems. In chapter 3 the results of a previously published paper [49] are presented and expanded upon, where we study the extinction time of four cyclically competing species on different lattice structures using Lotka-Volterra dynamics. We find that the extinction time of a well mixed system goes linearly with respect to the system size and that the probability distribution approximately takes the shape of a shifted exponential. However, this is not true for when spatial structure is added to the model. In that case we find that instead the probability distribution takes on a non-trivial shape with two characteristic slopes and that the mean goes as a power law with an exponent greater than one. This is attributed to neutral species pairs, species who do not interact, forming domains and coarsening. In chapter 4 the results of [50] are reported and expanded, where we allow agents to choose cyclically competing strategies out of a distribution. We first study the case of three strategies and find through both simulation and mean field equations that the probability distributions of the agents synchronize and oscillate with time in the limit where the agents probability distributions can be approximated as continuous. However, when we simulate the system on a one-dimensional lattice and the probability distributions are small and discretized, it is found that there is a drastic transition in stability, where the average extinction time of a strategy goes from being a power law with respect to system size to an exponential. This transition can also be observed in space time images with the emergence of tile patterns. We also look into the case of four cyclically competing strategies and find results similar to that of [49], such as the coarsening of neutral domains. However, the transition from power law to exponential for the average extinction time seen for three strategies is not observed, but we do find a transition from one power law to another with a different slope. This work was supported by the United States National Science Foundation through grants DMR-0904999 and DMR-1205309. Ph. D.

Recent experiments have shown that junctions consisting of individual single-molecule magnets (SMMs) bridged between two electrodes can be fabricated in three-terminal devices, and that the characteristic magnetic anisotropy of the SMMs can be affected by electrons tunneling through the molecule. Vibrational modes of the SMM can couple to electronic charge and spin degrees of freedom, and this coupling also influences the magnetic and transport properties of the SMM. The effect of electron-vibron coupling on transport has been extensively studied in small molecules, but not yet for junctions of SMMs. The goals of this thesis will be two-fold: to present a novel approach for studying the effects of this electron-vibron coupling on transport through SMMs that utilizes both density functional theory calculations and model Hamiltonian construction and analysis, and to present a software framework based on this hybrid approach for the simulation of transport across user-defined SMMs. The results of these simulations will indicate a characteristic suppression of the current at low energies that is strongly dependent on the overall electron-vibron coupling strength and number of molecular vibrational modes considered. Master of Science

The obligate stem parasite Cuscuta pentagona is able to take up host plant mRNA through a specialized organ known as the haustorium. Direct cell-to-cell symplastic connections between two different organisms are rare, and the translocation mechanisms and fate of these RNAs in the parasite is not understood. To characterize this phenomenon, mobile Arabidopsis and tomato mRNAs were identified from microarray and transcriptome sequencing projects and quantified in the host-parasite system. Mobile RNAs were quantified using real time (qRT)-PCR and were found to vary substantially in their rate of uptake and distribution in the parasite. Transcripts of tomato Gibberellic Acid Insensitive (SlGAI) and Cathepsin D Protease Inhibitor (SlPI) can be traced over 30-cm of parasite stem. SlPI was abundant in the C. pentagona stem, but the number of copies decreased substantially within the first eight hours post detachment. Additional studies of mobile RNAs from Arabidopsis, Translationally Controlled Tumor Protein (AtTCTP), Auxin Response Factor (AtARF) and a Salt-inducible Zinc Finger Protein (AtSZFP) supported the idea that mRNA molecules differ in their mechanisms of uptake and mobility between host and parasite. Known phloem-mobile RNAs (SlGAI and AtTCTP) have uptake patterns that differ from each other as well as from other RNAs that are not reported to be phloem mobile (SlPI and AtSZF1). The function of RNAs in plants extend beyond protein translation to include post transcriptional gene silencing or long distance signaling, and mobile RNA in C. pentagona systems offers novel insights into this aspect of plant biology. Studies of cell-to-cell trafficking of RNAs and other macromolecules would be facilitated by the ability to manipulate individual cells. To this end, work was initiated to explore alternative approaches to understanding single cell biology using laser-mediated approaches. Optoperforation, or the use of multiphoton processes to form quasi-free electron plasmas to initiate transient pore formation in plasma membranes, has been demonstrated, but not in cells of an intact plant. This work details a protocol for optoperforation of Arabidopsis epidermal cells to allow for uptake of external dye-labeled dextrans and retention for up to 72 hours, and has the potential for transformation and molecular tagging applications. Ph. D.

The chemical reaction of the first excited electronic state of molecular nitrogen, N₂(A), with ground state atomic oxygen is an important contributor to thermospheric nitric oxide (NO). The importance is assessed by including this reaction in a one-dimensional photochemical model. The method is to scale the photoelectron impact ionization rate of molecular nitrogen by a Gaussian centered near 100 km. Large uncertainties remain in the temperature dependence and branching ratios of many reactions important to NO production and loss. Similarly large uncertainties are present in the solar soft x-ray irradiance, known to be the fundamental driver of the low-latitude NO. To illustrate, it is shown that the equatorial, midday NO density measured by the Student Nitric Oxide Explorer (SNOE) satellite near the Solar Cycle 23 maximum can be recovered by the model to within the 20% measurement uncertainties using two rather different but equally reasonable chemical schemes, each with their own solar soft-xray irradiance parameterizations. Including the N₂(A) changes the NO production rate by an average of 11%, but the NO density changes by a much larger 44%. This is explained by tracing the direct, indirect, and catalytic contributions of N₂(A) to NO, finding them to contribute 40%, 33%, and 27% respectively. The contribution of N₂(A) relative to the total NO production and loss is assessed by tracing both back to their origins in the primary photoabsorption and photoelectron impact processes. The photoelectron impact ionization of N₂ is shown to be the main driver of the midday NO production while the photoelectron impact dissociation of N₂ is the main NO destroyer. The net photoelectron impact excitation rate of N₂, which is responsible for the N₂(A) production, is larger than the ionization and dissociation rates and thus potentially very important. Although the conservative assumptions regarding the level-specific NO yield from the N₂(A)+O reaction results in N₂(A) being a somewhat minor contributor, N₂(A) production is found to be the most efficient producer of NO among the thermospheric energy deposition processes. Ph. D.

Recent years have seen remarkable progress in our understanding of physical aging in nondisordered systems with slow, i.e. glassy-like dynamics. In many systems a single dynamical length L(t), that grows as a power-law of time t or, in much more complicated cases, as a logarithmic function of t, governs the dynamics out of equilibrium. In the aging or dynamical scaling regime, these systems are best characterized by two-times quantities, like dynamical correlation and response functions, that transform in a specific way under a dynamical scale transformation. The resulting dynamical scaling functions and the associated non-equilibrium exponents are often found to be universal and to depend only on some global features of the system under investigation. We discuss three different types of systems with simple and complex aging properties, namely reaction diffusion systems with a power growth law, driven diffusive systems with a logarithmic growth law, and a non-equilibrium polymer network that is supposed to capture important properties of the cytoskeleton of living cells. For the reaction diffusion systems, our study focuses on systems with reversible reaction diffusion and we study two-times functions in systems with power law growth. For the driven diffusive systems, we focus on the ABC model and a related domain model and measure two- times quantities in systems undergoing logarithmic growth. For the polymer network model, we explain in some detail its relationship with the cytoskeleton, an organelle that is responsible for the shape and locomotion of cells. Our study of this system sheds new light on the non- equilibrium relaxation properties of the cytoskeleton by investigating through a power law growth of a coarse grained length in our system. Ph. D.

Many modern technologies are enabled by the use of thin films and/or nanostructured composite materials. For example, many thermoelectric devices, solar cells, power electronics, thermal barrier coatings, and hard disk drives contain nanostructured materials where the thermal conductivity of the material is a critical parameter for the device performance. At the nanoscale, the mean free path and wavelength of heat carriers may become comparable to or smaller than the size of a nanostructured material and/or device. For nanostructured materials made from semiconductors and insulators, the additional phonon scattering mechanisms associated with the high density of interfaces and boundaries introduces additional resistances that can significantly change the thermal conductivity of the material as compared to a macroscale counterpart. Thus, better understanding and control of nanoscale heat conduction in solids is important scientifically and for the engineering applications mentioned above. In this dissertation, I discuss my work in two areas dealing with nanoscale thermal transport: (1) I describe my development and advancement of important thermal characterization tools for measurements of thermal and thermoelectric properties of a variety of materials from thin films to nanostructured bulk systems, and (2) I discuss my measurements on several materials systems done with these characterization tools. First, I describe the development, assembly, and modification of a time-domain thermoreflectance (TDTR) system that we use to measure the thermal conductivity and the interface thermal conductance of a variety of samples including nanocrystalline alloys of Ni-Fe and Co-P, bulk metallic glasses, and other thin films. Next, a unique thermoelectric measurement system was designed and assembled for measurements of electrical resistivity and thermopower of thermoelectric materials in the temperature range of 20 to 350 °C. Finally, a commercial Anter Flashline 3000 thermal diffusivity measurement system is used to measure the thermal diffusivitiy and heat capacity of bulk materials at high temperatures. With regards to the specific experiments, I examine the thermal conductivity and interface thermal conductance of two different types of nanocrystalline metallic alloys of nickel-iron and cobalt-phosphorus. I find that the thermal conductivity of the nanocrystalline alloys is reduced by a factor of approximately two from the thermal conductivity measured on metallic alloys with larger grain sizes. With subsequent molecular dynamics simulations performed by a collaborator, and my own electrical conductivity measurements, we determine that this strong reduction in thermal conductivity is the result of increased electron scattering at the grain boundaries, and that the phonon component of the thermal conductivity is largely unchanged by the grain boundaries. We also examine four complex bulk metallic glass (BMG) materials with compositions of Zr₅₀Cu₄₀Al₁₀, Cu46.25Zr44.25Al7.5Er₂, Fe₄₈Cr₁₅Mo₁₄C₁₅B₆Er₂, and Ti41.5Zr2.5Hf₅Cu42.5Ni7.5Si₁. From these measurements, I find that the addition of even a small percentage of heavy atoms (i.e. Hf and Er) into complex disordered BMG structures can create a significant reduction in the phonon thermal conductivity of these materials. This work also indicates that the addition of these heavy atoms does not disrupt electron transport to the degree with which thermal transport is reduced. Ph. D.

Nanoscale materials have great applications in many areas. One of these applications is for manufacturing ultra-compact and efficient sensors for chemical and biological molecule detection. Noble metals, such as gold (Au) and silver (Ag), because of their distinguished optical property"localized surface plasmon resonances (LSPRs) that exhibit low loss, are ideal materials to fabricate these nanoscale plasmonic particles or structures. This work addresses the synthesis, characterization, and sensing applications of Au and Ag nanoparticles (NPs). The progress on certain subjects related to our work"NP synthesis, surface functionalization, Au sphere-film structure and two-photon fluorescence"are reviewed in Chapter 1. We also show the calculation results of LSPRs of Au nanosphere suspensions using Mie theory. The measured extinction spectra of Au nanosphere suspensions agree with the calculated results very well. Chapter 2 is a chapter describing the chemical synthesis of a variety of NPs, such as Ag prisms and cubes, Au spheres, rods, and bipyramids. These experiments involved different synthetic mechanisms and methods which enabled us to prepare NPs with desired shapes and optical properties. To put these NPs into application, it is desirable and sometimes necessary to functionalize their surfaces. In Chapter 3, we present the functionalization of Ag cubes with poly(allylamine hydrochloride) (PAH) and poly(allylamine hydrochloride)-dithiocarbamate (PAH-DTC), which follows our previous work on Au NPs. The purpose of studying Ag instead of Au is to use the stronger plasmonic enhancement in Ag when applied to two-photon imaging applications. However, we found that PAH-DTC shrank the Ag cubes. We also functionalized the cationic hexadecyltrimethylammonium bromide (CTAB)-stabilized Au NRs with anionic poly(sodium 4-styrenesulfonate) (PSS). Coated with the strong polyelectrolyte PSS, the NRs become more manageable and can be stable for over six months and are easily immobilized onto positively charged substrate. We put PSS-functionalized Au NPs into use and studied their adsorption process onto PAH-coated optical fiber tapers by monitoring the transmission light through the fiber. When the diameter of the fiber taper gets smaller, stronger coupling occurred between transmitted light inside the taper and the Au NPs on the taper surface (cylinder). This coupling resulted in a loss of the guided light at the plasmon resonance wavelength of the NPs. By monitoring this loss, we can study the adsorption rate of Au NPs onto the fiber. In Chapter 4, we used Au nanospheres to study the adsorption rate on substrates with different curvatures. We also established a theoretical model to explain this phenomenon for cylindrical surface as well as planar and spherical surfaces. Our results fit well with the theory, which predicts that particle adsorption rates depend strongly on surface geometry, and can exceed the planar surface deposition rate by over two orders of magnitude when the diffusion length of the particle is large compared to the surface curvature. In Chapter 5, we studied the optical properties of Au nanospheres separated from a thick Au film by a polyelectrolyte multilayer (PEM) film assembled from PAH and PSS under specific pH condition. The PEM film undergoes swelling and shrinking when the environmental pH is changed as a result of charging and discharging of the polyelectrolytes. Therefore, the PEM film provides an efficient means to tune the distance between Au spheres and Au film. The extinction peak blue-shifted as much as 100 nm when the pH of the water changed from pH 10 to pH 3 for 100 nm diameter Au spheres on a PEM film assembled at pH 9.5. Our preliminary estimates that the gap between sphere and surface can be as small as a few nm even though the film itself is tens of nm thick when it is not constrained by Au spheres. We studied two-photon excitation fluorescence (TPEF) from Ag triangles in Chapter 6. The triangles were fabricated by nanosphere lithography, which used convective self-assembly to make the nanosphere mask. The LSPRs of the nanotriangles were tuned to be in the 800--900 nm range to match with the Ti:Sapphire pulse laser at 880 nm. We found that certain spots on the fluorescence images gave rise to larger fluorescence intensity than rest of the area. SEM imaging reveals that the unusually bright spots seen on the surface were related to regions where the triangles transformed to spherical particles. The larger intensity is tentatively ascribed to the plasmon resonance of those spherical particles in ~400 nm range. Ph. D.

Qweak will determine the weak charge of the proton, QpW, via an asymmetry measurement of parity-violating elastic electron-proton scattering at low four momentum transfer to a precision of 4%. QpW has firm Standard Model prediction and is related to the weak mixing angle, sin20W, a well-defined Standard Model parameter. Qweak will probe a subset of new physics to the TeV mass scale and test the Standard Model. The details of how this measurement was performed and the analysis of the 25% elastic dataset will be presented in this thesis. Also, an analysis of an auxiliary measurement of the parity-violating asymmetry in the N >> Δ transition is presented. It is used as a systematic inelastic background correction in the elastic analysis and to extract information about the hadronic parity violation through the low energy constant, dΔ. The elastic asymmetry at Q2 = 0:0252 ± 0:0007 GeV2 was measured to be Aep = -265 ± 40 ± 22 ± 68 ppb (stat., sys., and blinding). Extrapolated to Q2 = 0, the value of the proton's weak charge was measured to be QpW = 0:077 ± 0:019 (stat. and sys.) ±0:026 (blinding). This is within 1 o of the Standard Model prediction of QpW = 0:0705 ± 0:0008. The N >> Δ inelastic asymmetry at Q2 = 0:02078 ± 0:0005 GeV2 and W = 1205 MeV was measured to be Ainel = -3:03 ± 0:65 ± 0:73 ± 0:07 ppm (stat., sys., and blinding). This result constrains the low energy constant to be dΔ = 5:8 ± 22gπ, and, if the result of the G0 experiment is included, dΔ = 5:8 ± 17gπ. This result rules out suggested large values of dΔ motivated by radiative hyperon decays. The elastic measurement is the first direct measurement of the weak charge of the proton while the inelastic measurement is only the second measurement of the neutral current excitation of the Δ resonance. It is currently the best constraint for the low energy constant, dΔ. Ph. D.

As the switching rates in electronic and optoelectronic devices are pushed to even higher frequencies, it is crucial to probe carrier dynamics in semiconductors on femtosecond timescales. Time resolved spectroscopy is an excellent tool to probe the relaxation dynamics of photoexcited carriers; where after the initial photoexcitation, the nonequilibrium population of electrons and holes relax by a series of scattering processes including carrier-carrier and carrier-phonon scattering. Probing carrier and spin relaxation dynamics in InAs and InSb based narrow-gap semiconductors is crucial to understand the different scattering mechanisms related to the systems. Similar studies in InSb quantum wells are also intriguing, especially for their scientifically unique features (such as small effective mass, large g-factor etc). Our time resolved techniques demonstrated tunability of carrier and spin dynamics which might be important for charge and spin based devices. The samples studied in this work were provided by the groups of Prof. Wessels (Northwestern University) and Prof. Santos (University of Oklahoma). Theoretical calculations were performed by the group of Prof. Stanton (University of Florida). The THz measurements were performed at Wright State University in collaboration with Prof. Jason Deibel. This work has been supported by the National Science Foundation through grants Career Award DMR-0846834, AFOSR Young Investigator Program 06NE231. A portion of this work was performed at the National High Magnetic Field Laboratory (in collaboration with Dr. Stephen McGill), which is supported by National Science Foundation Cooperative Agreement No. DMR-0654118, the State of Florida, and the U.S. Department of Energy. Ph. D.

The goal of this project was to study some unexplored optical and magneto-optical properties of the newest member of III-V ferromagnetic structures, InMnSb, as well as InSb films and InSb/AlInSb quantum wells. The emphasis was on dynamical aspects such as charge and spin dynamics in order to address several important issues of the spin-related phenomena. The objectives in this project were to: 1) understand charge/spin dynamics in NGS with different confinement potentials, 2) study phenomena such as interband photo-galvanic effects, in order to generate spin polarized current, 3) probe the effect of magnetic impurities on the spin/charge dynamics. This thesis describes three experiments: detection and measurement of spin polarized photocurrents in InSb films and quantum wells arising from the circular photogalvanic effect, and measurements of the carrier and spin relaxation in InSb and InMnSb structures by magneto-optical Kerr effect and differential transmission. The samples for our studies have been provided by Prof. Heremans at Virginia Tech, Prof. Santos at the University of Oklahoma, and Prof. Furdyna at the University of Notre Dame. Ph. D.

The use of ultracold neutrons (UCN) to study fundamental parameters such as the neutron lifetime and decay correlations in polarized neutron beta decay are poised to make significant contributions to our understand of the Standard Model and its extensions. To this end, the UCNA experiment is pursuing a precision measurement (0.2%) of the angular correlation between the neutron spin and the direction of emission of the electron in polarized neutron decay (the ``A'' asymmetry). The UCNA experiment makes use of the spallation-driven solid deuterium (SD2) UCN source at the Los Alamos Neutron Science Center (LANSCE). The UCN leave the source and are 100% polarized by passing through a strong magnetic field before their decay is observed by a very sensitive electron spectrometer. UCN guides facilitate the transfer of UCN from the source to the spectrometer. Common guide materials include stainless steel, copper, aluminum, and quartz. Often a thin film is applied to these components to increase their ability to transport/bottle and preserve the polarization of UCN. In the region of the SD2 UCN source, nickel-58 films are applied, whereas once the UCN are polarized, diamond-like carbon (DLC) films are employed. This dissertation covers the application, process developments, and characterization of these coatings. In addition a study concerning the surface finish resulting from the mechanical polishing and electropolishing of the guides that make up the UCNA beamline is presented. Ph. D.

The purpose of this thesis is to make a broadband antireflection coating configuration known as a quarter-wave stack consisting of one layer of titania and of one layer of silica nanoparticles. We utilize much that is already known about silica nanoparticle deposition. The first objective of this thesis is deposition and characterization of titania nanoparticle films deposited on glass microscope slides by a technique known as Ionic Self-Assembled Multilayers or ISAM deposition. This technique takes advantage of the electrostatic attraction between oppositely charged materials and ideally results in a uniform nanoparticle film whose thickness and optical properties can be tightly controlled. Deposition of a quarter-wave stack based on ISAM deposition of silica and titania nanoparticles is significantly simpler and less expensive than alternative deposition methods. Initial attempts to deposit titania films were unsuccessful because of excess diffuse scattering due to inhomogeneities in the film. In order to reduce diffuse scattering, two approaches were considered. The first approach was to improve the deposition process itself by experimenting with different values of deposition parameters such as solution pH and solution molarity. The other approach focused on removing the large nanoparticle aggregates from the colloidal solutions of titania nanoparticles that were suspected to be responsible for rough film surfaces resulting in diffuse scattering. This approach was successful. In addition, evidence suggested that surface roughness contributed more to diffuse scattering than the bulk of the films. After minimizing diffuse scattering from titania nanoparticle films, we used known results from research on silica nanoparticle films to deposit quarter-wave stacks consisting of one layer of titania nanoparticles with high refractive index and one layer of silica nanoparticles with low refractive index. This contrast in refractive indices is a desirable characteristic of quarter-wave stacks. The thicknesses and refractive indices of the two layers in the quarter-wave stacks were measured by ellipsometry and compared to the nominal thicknesses of these layers. Finally, the reflectance was derived from a model of the quarter-wave stack and was compared to the measured reflectance. It was found that construction of a quarter-wave stack by ISAM is possible but that it will be necessary to acquire data from more experiments. Ph. D.

Fluctuations are of great importance in systems of small length and energy scales. Measuring the pulling of single molecules or the stationary fiow of mesospheres dragged through a viscous media enables the direct analysis of work and entropy distributions. These probability distributions are the result of a large number of repetitions of the same experiment. Due to the small scale of these experiments, the outcome can vary significantly from one realization to the next. Strong theoretical predictions exist, collectively called Fluctuation Theorems, that restrict the shape of these distributions due to an underlying time reversal symmetry of the microscopic dynamics. Fluctuation Theorems are the strongest existing statements on the entropy production of systems that are out of equilibrium. Being the most important ingredient for the Fluctuation Theorems, the probability distribution of the entropy change is itself of great interest. Using numerically exact methods we characterize entropy distributions for various stochastic reaction-diffusion systems that present different properties in their underlying dynamics. We investigate these systems in their steady states and in cases where time dependent forces act on them. This study allows us to clarify the connection between the microscopic rules and the resulting entropy production. The present work also adds to the discussion of the steady state properties of stationary probabilities and discusses a non-equilibrium current amplitude that allows us to quantify the distance from equilibrium. The presented results are part of a greater endeavor to find common rules that will eventually lead to a general understanding of non-equilibrium systems. Ph. D.