Low temperature magnetic force microscopy studies of superconducting niobium films have been undertaken with the goal of studying the interplay between artificial pinning centers and magnetic vortices. / Measurements were performed using a custom built low temperature magnetic force microscope, capable of operation at temperatures ranging from 4.2 K to room temperature. Special attention has been paid to optimizing the instrumentation through a detailed study of the noise characteristics, with particular emphasis placed on achieving a large signal-to-noise ratio and corresponding high force gradient sensitivity. / Magnetic force spectroscopy data has been used to deduce the critical temperature of the superconducting samples, based upon the repulsive Meissner interaction between the magnetic tip and the sample. Images of vortices as a function of applied magnetic field demonstrate the expected linear relation between vortex density and field strength, and confirms that only single vortices, each carrying one flux quantum, are observed. Two different methods are put forward to determine the magnetic penetration depth; one using magnetic force spectroscopy, the other using constant height imaging of vortices. Images of vortices as a function of temperature demonstrate that as temperatures rise, vortices become more easily depinned during the scanning process through interactions with the magnetic field of the tip. Dissipation images of vortices suggest eddy current damping as well as vortex motion within potential wells as major sources of energy loss. Studies on a patterned niobium film show that only interstitial vortices are easily detectable by MFM, but that a strong tip influence results in significant tip induced motion of these vortices around the antidots.

Phase formation and transformation in mechanically alloyed iron-rich Sm-Fe-C is the principal subject of this thesis. Ternary Sm-Fe-C is a complicated system. The strategy was therefore to start with a binary system. A series of mechanically alloyed R$ sb2$Fe$ sb{17}$ powders were investigated for a better understanding of both the Sm-Fe alloy system in general and the Sm$ sb2$Fe$ sb{17}$ compound in particular. The objective was to learn (1) what is the steady-state in the mechanically alloyed R$ sb2$Fe$ sb{17},$ and (2) how the 2-17 structure is formed from the mechanically alloyed precursors. Phase formation and transformation in the mechanically alloyed Sm$ sb2$Fe$ sb{17}$C$ sb{x}$ with various carbon contents was then studied. The objective in this case was to learn (1) how the 2-17 structure with interstitial carbon is formed, (2) what is the maximum C content in the 2-17 structure, the critical content $x sb{c},$ and (3) what phase(s) is (are) formed with $x>x sb{c}.$ / Phase transformation from Sm$ sb2$Fe$ sb{17}$C$ sb{x}$ to Sm$ sb2$Fe$ sb{14}$C was the second subject for study. As required by this study, the grain refinement process was investigated first. The objective was to prepare the nanocrystalline Sm$ sb2$Fe$ sb{17}$C$ sb{x}$ with various grain sizes. Emphasis was on the ball milling of Sm$ sb2$Fe$ sb{17}$/graphite mixture in the hope of forming a nano-scale mixing of Sm$ sb2$Fe$ sb{17}$ and graphite by ball milling. Solid-solid reaction between the Sm$ sb2$Fe$ sb{17}$ and graphite leading to the formation of nanocrystalline Sm$ sb2$Fe$ sb{17}$C$ sb{x}$ was then studied. The phase transformation from Sm$ sb2$Fe$ sb{17}$ was carried out with nanocrystalline Sm$ sb2$Fe$ sb{17}$C$ sb{x}$ samples. Samples prepared by other methods were also studied. The objective was to learn (1) what the transformation product is and (2) what the kinetics of the phase transformation and its grain size dependence are. (Abstract shortened by UMI.)

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2007. / Source: Dissertation Abstracts International, Volume: 69-02, Section: B, page: 1214. Adviser: Jian-Ming Jun. Includes bibliographical references (leaves 122-128) Available on microfilm from Pro Quest Information and Learning.

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2008. / Source: Dissertation Abstracts International, Volume: 69-05, Section: B, page: 3221. Adviser: Phillip Geil. Includes bibliographical references. Available on microfilm from Pro Quest Information and Learning.

Nanophotonic structures provide unique ways to control light and alter its behaviors in ways not possible in macroscopic structures. In this thesis, we explore novel behaviors of light created by nanophotonic structures, with a common theme on resonance effects. The first half of the thesis focuses on a peculiar type of electromagnetic resonance, where the resonance lifetime diverges to infinity. These states, called bound states in the continuum, remain localized in space even though their frequency lie within a continuum of extended modes. We find such states in photonic crystal slabs and the surface of bulk photonic crystals. We show the conditions necessary for them to exist, and provide the first experimental observation of these unusual states. We also show that these states have a topological nature, with conserved and quantized topological charges that govern their generation, evolution, and annihilation. The second half of the thesis concerns light scattering from resonant nanophotonic structures, where resonances can enhance or suppress scattering at particular wavelengths and angles. We show that multiple resonances in one nanostructure and in the same multipole channel generally lead to a scattering dark state where the structure becomes transparent. Based on the coherent interference from multiple scatterers, we show there are geometries that can achieve a sharp structural color where the hue, saturation, and brightness are all viewing-angle independent. We also invent a new type of transparent display based on wavelength-selective light scattering from nanostructures.

I discuss experimental progress towards developing a material with an isotropic, negative index of refraction at optical frequencies. The simplest way to make such a material is to create a metafluid, or a disordered collection of subwavelength, isotropic electromagnetic resonators. Small clusters of metal particles, such as tetrahedra, serve as these constituents. What is needed are methods for manufacturing these structures with high precision and in sufficient yield that their resonances are identical. Jonathan Fan et al. [Science, 328 (5982), 1135-1138, 2010] demonstrated that colloidal self-assembly is a means of preparing electromagnetic resonators from metal nanoparticles. However, the resonances are sensitive to the separation gaps between particles. Standard synthesis routes for metal nanoparticles yield crystals or nanoshells that are inadequate for metafluids due to polydispersity, faceting, and thermal instabilities. To ensure that the separation gaps and resonances are uniform, more monodisperse spherical particles are needed. An additional challenge is the self-assembly of tetrahedral clusters in high yield from these particles. In self-assembly approaches that others have examined previously, the yield of any particular type of cluster is low. In this dissertation I present solutions to several of these problems, developed in collaboration with my research group and others. We demonstrate that slow chemical etching can transform octahedral gold crystals into ultrasmooth, monodisperse nanospheres. The particles can serve as seeds for the growth of larger octahedra which can in turn be etched. The size of the gold nanospheres can therefore be adjusted as desired. We further show that in colloidal mixtures of two sphere species that strongly bind to one another, the sphere size ratio determines the size distribution of self-assembled clusters. At a critical size ratio, tetrahedral clusters assemble in high yield. We explain the experimentally observed 90% yield with a nonequilibrium “random parking” model based on irreversible binding. Simulations based on this model reveal that 100% yield of tetrahedra is possible in principle. Finally, we combine these results and present methods for the self-assembly and purification of tetrahedral plasmonic nanoclusters, the simplest building blocks for isotropic metafluids. / Physics

The steady, supersonic outflow from the Sun we call the solar wind was first posited in the 1950s and initial theories rightly linked the acceleration of the wind to the existence of the million-degree solar corona. Still today, the wind acceleration mechanisms and the coronal heating processes remain unsolved challenges in solar physics. In this work, I seek to answer a portion of the mystery by focusing on a particular acceleration process: Alfven waves launched by the motion of magnetic field footpoints in the photosphere. The entire corona is threaded with magnetic loops and flux tubes that open up into the heliosphere. I have sought a better understanding of the role these magnetic fields play in determining solar wind properties in open flux tubes. After an introduction of relevant material, I discuss my parameter study of magnetic field profiles and the statistical understanding we can draw from the resulting steady-state wind. In the chapter following, I describe how I extended this work to consider time dependence in the turbulent heating by Alfven waves in three dimensional simulations. The bursty nature of this heating led to a natural next step that expands my work to include not only the theoretical, but also a project to analyze observations of small network jets in the chromosphere and transition region, and the underlying photospheric magnetic field that forms thresholds in jet production. In summary, this work takes a broad look at the extent to which Alfven-wave-driven turbulent heating can explain measured solar wind properties and other observed phenomena. / Astronomy

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This research provides a proof of concept and background theory for the physics behind the state-of-the-art ultra-fast plasmonic spiking neurons (PSN), which can serve as a primary synaptic device for developing a platform for fast neural computing. Such a plasmonic-powered computing system allows localized AI with ultra-fast operation speed. The designed architecture for a plasmonic spiking neuron (PSN) presented in this thesis is a photonic integrated nanodevice consisting of two electro-optic and optoelectronic active components and works based on their coupling. The electro-optic active structure incorporated a periodic array of seeded quantum nanorods sandwiched between two electrodes and positioned at a near-field distance from the topmost metal layer of a sub-wavelength metal-oxide multilayer metamaterial. Three of the metal layers of the metamaterials form the active optoelectronic component. The device operates based on the coupling of the two active components through optical complex modes supported by the multilayer and switching between two of them. Both action and resting potentials occur through subsequent quantum and extraordinary photonics phenomena. These phenomena include the generation of plasmonic high-k complex modes, switching between the modes by enhanced quantum-confined stark effect, decay of the plasmonic excitations in each metal layer into hot-electrons, and collecting hot-electrons by the optoelectronic component. The underlying principles and functionality of the plasmonic spiking neuron are illustrated using computer simulation.