UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a given thesis/dissertation in DRUM.
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Item Transmission Spectra of Rb 87 atoms near an Optical Nanofiber.(2016) Patterson, Burkley D.; Orozco, Luis A; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We present measurements of the transmission spectra of 87Rb atoms at 780 nm in the vicinity of a nanofiber. A uniform distribution of fixed atoms around a nanofiber should produce a spectrum that is broadened towards the red due to shifts from the van der Waals potential. If the atoms are free, this also produces an attractive force that accelerates them until they collide with the fiber which depletes the steady-state density of near-surface atoms. It is for this reason that measurements of the van der Waals interaction are sparse. We confirm this by measuring the spectrum cold atoms from a magneto-optical trap around the fiber, revealing a symmetric line shape with nearly the natural linewidth of the transition. When we use an auxiliary 750 nm laser we are able to controllably desorb a steady flux of atoms from the fiber that reside near the surface (less than 50 nm) long enough to feel the van der Walls interaction and produce an asymmetric spectrum. We quantify the spectral asymmetry as a function of 750 nm laser power and find a maximum. Our model, which that takes into account the change in the density distribution, qualitatively explains the observations. In the future this can be used as a tool to more comprehensively study atom-surface interactions.Item Optical nanofiber fabrication and analysis towards coupling atoms to superconducting qubits(2014) Hoffman, Jonathan; Orozco, Luis A.; Rolston, Steven L.; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We describe advancements towards coupling superconducting qubits to neutral atoms. To produce a measurably large coupling, the atoms will need to be on the order of a few micrometers away from the qubit. A consequence of combining superconducting qubits and atoms is addressing their operational constraints, such as the deleterious light effects on superconducting systems and the magnetic field sensitivity of superconducting qubits. Our group proposes the use of optical-nanofiber-based optical dipole traps to confine atoms near the superconductor. Optical nanofibers (ONFs) have high-intensity evanescent waves that require less power than equivalent standard dipole traps. This thesis focuses on the fabrication and analysis of the behavior of ONFs. First we present the construction of the pulling apparatus. We outline the necessary steps for a typical pull, detailing the cleaning and alignment process. Then we examine the quality of the fibers by measuring their transmission and comparing our results to other reported measurements, demonstrating a two-order of magnitude decrease in loss. Next we present the modal evolution in ONFs using simulations and spectrogram analysis. We identify crucial elements to improve the transmission and demonstrate understanding of the modal dynamics during the pull. Then we study higher-order modes (HOMs) with ONFs using the first excited TE01, TM01, and HE21 modes. We demonstrate transmissions greater than 97% for 780 nm light when we launch the first excited LP11 family of modes through fibers with a 350 nm waist. This setup enables us to launch these three modes with high purity at the output, where less than 1% of the light is coupled to the fundamental mode. We then focus on the identification of modes on the ONF waist. First we use Rayleigh scattering to identify the modal content of an ONF. Bulk optics can convert the modes in the ONF, and we observe the controllable conversion of superpositions of modes. Finally, we use an evanescently-coupled tapered optical fiber probe that allows for the identification of the fundamental mode beating with HOMs and compare the results to simulations.Item Processing-Structure-Microstructure-Property Relationships in Polymer Nanocomposites(2008-01-31) Kota, Arun Kumar; Bruck, Hugh A; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The optimal development of polymer nanocomposites using carbon nanotube (CNTs) and carbon nanofiber (CNFs) fillers requires a complete understanding of processing-structure-property relationships. The purpose of this understanding is to determine the optimal approach for processing polymer nanocomposites with engineered microstructures and enhanced material properties. In this research, two processing techniques were investigated: solvent processing and twin screw extrusion. The former is a batch process which employs mixing a polymer solution with a filler suspension using long mixing times and low levels of shear mixing. The latter is a continuous process that mixes polymer melts with solid nanoscale ingredients using high levels of shear mixing for a short mixing time. Previous studies conducted on polymer-CNT/CNF using these processes have focused mainly on processing-microstructure and structure-property relationships using one technique or the other. This research focuses on understanding the processing-property relationships by comparing the structure-property relationships resulting from the two processes. Furthermore, the effect of ingredients and processing parameters within each process on microstructure and structure-property relationships was investigated. The microstructural features, namely, distribution of agglomerates, dispersion, alignment, and aspect ratio of the filler were studied using optical, scanning electron, confocal and transmission electron microscopy, respectively. The composition of the filler was determined using thermogravimetric analysis. The electrical, rheological, thermo-oxidative and mechanical properties of the composites were also investigated. Many significant insights related to processing-structure-property relationships were obtained including: (a) deagglomeration is a critical combination of the magnitude of shear rate and the residence time, (b) the structure-property relationships can be modeled using a new methodology based on the degree of percolation by representing the material as an interpenetrating phase composite, (c) annealing can re-establish interconnectivity and improve electrical properties, (d) the degree of dispersion can be resolved using thermogravimetric analysis, and (e) increasing extrusion speed inhibits thermal decomposition and begins to asymptotically increase strength and stiffness through reduction in aspect ratio and size of agglomerates. Finally, a new combinatorial approach was developed for rapidly determining processing-structure relationships of polymer nanocomposites. This dissertation has broad implications in the processing of high performance and multifunctional polymer nanocomposites, combinatorial materials science, and histopathology.