Theses and Dissertations from UMD
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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 give thesis/dissertation in DRUM
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Item Hypersonic Application of Focused Schlieren and Deflectometry(2010) VanDercreek, Colin Paul; Yu, Kenneth H; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A non-intrusive diagnostic capability for determining the hypersonic shock and boundary layer structure was developed, installed, and successfully tested at the AEDC Hypervelocity Tunnel 9. This customized diagnostic involves a combination of a focused schlieren system, which relies on creating multiple virtual light sources using a Fresnel lens and a source grid, and a deflectometry system, which uses the focused schlieren and a photomultiplier tube. It was used for obtaining spatially resolved images of density gradients with a depth of focus less than one centimeter, while allowing high frequency measurements of density fluctuations. The diagnostic was applied in investigating the second-mode instability waves present in the boundary layer of a sharp-nosed cone submerged in a Mach 10 flow. The waves were successfully imaged and their frequencies were measured even though the flow density was below 0.01 kg/m^3 and the frequencies over 200 kHz. This adds a new capability to hypersonic testing.Item OPTIMAL CONTROL OF OBJECTS ON THE MICRO- AND NANO-SCALE BY ELECTROKINETIC AND ELECTROMAGNETIC MANIPULATION: FOR BIO-SAMPLE PREPARATION, QUANTUM INFORMATION DEVICES AND MAGNETIC DRUG DELIVERY(2010) Probst, Roland; Shapiro, Benjamin; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis I show achievements for precision feedback control of objects inside micro-fluidic systems and for magnetically guided ferrofluids. Essentially, this is about doing flow control, but flow control on the microscale, and further even to nanoscale accuracy, to precisely and robustly manipulate micro and nano-objects (i.e. cells and quantum dots). Target applications include methods to miniaturize the operations of a biological laboratory (lab-on-a-chip), i.e. presenting pathogens to on-chip sensing cells or extracting cells from messy bio-samples such as saliva, urine, or blood; as well as non-biological applications such as deterministically placing quantum dots on photonic crystals to make multi-dot quantum information systems. The particles are steered by creating an electrokinetic fluid flow that carries all the particles from where they are to where they should be at each time step. The control loop comprises sensing, computation, and actuation to steer particles along trajectories. Particle locations are identified in real-time by an optical system and transferred to a control algorithm that then determines the electrode voltages necessary to create a flow field to carry all the particles to their next desired locations. The process repeats at the next time instant. I address following aspects of this technology. First I explain control and vision algorithms for steering single and multiple particles, and show extensions of these algorithms for steering in three dimensional (3D) spaces. Then I show algorithms for calculating power minimum paths for steering multiple particles in actuation constrained environments. With this microfluidic system I steer biological cells and nano particles (quantum dots) to nano meter precision. In the last part of the thesis I develop and experimentally demonstrate two dimensional (2D) manipulation of a single droplet of ferrofluid by feedback control of 4 external electromagnets, with a view towards enabling feedback control of magnetic drug delivery to reach deeper tumors in the long term. To this end, I developed and experimentally demonstrated an optimal control algorithm to effectively manipulate a single ferrofluid droplet by magnetic feedback control. This algorithm was explicitly designed to address the nonlinear and cross-coupled nature of dynamic magnetic actuation and to best exploit available electromagnetic forces for the applications of magnetic drug delivery.Item Application of porous silicon in terahertz technology(2010) Lo, Shu-Zee Alencious; Murphy, Thomas E; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this thesis, we discuss our efforts in developing porous silicon based devices for terahertz signal processing. In the first stage of our research, we demonstrate that porous silicon samples fabricated from highly doped p-type silicon can have adjustable refractive indices ranging from 1.5-2.1 and can exhibit a resistivity that is four orders of magnitude higher than that of the silicon wafer from which they were made. We show that the porous silicon becomes stable and relatively lossless after thermal oxidation. The partially oxidized porous silicon is shown to exhibit a smooth absorption spectrum, with low absorption loss of <10 cm^-1 over the entire terahertz spectrum. As a proof of concept, we fabricated, for the first time, a porous silicon based multilayered Bragg filter with reflectance of 93% and full-width at half-maximum bandwidth of 0.26 THz. Compared with other multilayered filtering techniques, porous silicon has the advantage that it can be easily fabricated, and offers the possibility of forming multilayer and graded index structures for more advanced filters. The large surface area of nanoporous silicon makes it an especially attractive platform for applications in biochemical detection and diagnostics As part of our effort in developing terahertz waveguide for biosensing, we reported the world's first porous silicon based terahertz waveguide using the principle of surface plasmon polaritons. The effect of porous silicon film thickness on the propagation of surface plasmons is explained theoretically in this thesis and is found to be in good agreement with experimental results.Item INTERFACIAL SOLVATION AND EXCITED STATE PHOTOPHYSICAL PROPERTIES OF 7-AMINOCOUMARINS AT SILICA/LIQUID INTERFACES(2010) Roy, Debjani; Walker, Robert A; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The properties of solutes adsorbed at interfaces can be very different compared to bulk solution limits. This thesis examines how polar, hydrophilic silica surfaces and different solvents systematically change a solute's equilibrium and dynamic solvation environment at solid/liquid interfaces. The primary tools used in these studies are steady state fluorescence spectroscopy and time correlated single photon counting (TCSPC) -a fluorescence method capable resolving fluorescence emission on the picosecond timescale. To sample adsorbed solutes, TCSPC experiments were carried out in total internal reflection (TIR) geometry. These studies used total of six different 7 aminocoumarin dyes to isolate the effects of molecular and electronic structure on solute photophysical behavior. Fluorescence lifetimes measured in the TIR geometry are compared to the lifetimes of coumarins in bulk solution using different solvents to infer interfacial polarity and excited state solute conformation and dynamics. Steady state emission experiments measuring the behavior of the coumarins adsorbed at silica surfaces from bulk methanol solutions show that all coumarins had a similar affinity &delta G ads &sim &minus 25-30 kJ/mole. Despite these similar adsorption energetics solute structure had a very pronounced effect on the tendency of solutes to aggregate and form multilayers. Our finding suggests that hydrogen bonding donating properties of the silica surface plays a dominant role in determining the interfacial behavior of these solutes. The silica surface also had pronounced effects on the time dependent emission of some solutes. In particular, the strong hydrogen bond donating properties of the silica surface inhibit formation of a planar, charge transfer state through hydrogen bond donation to the solute's amine group. A consequence of this interaction is that the time dependent emission from solutes adsorbed at the surface appears to be more similar to emission from solutes in nonpolar solvation environments. To test the role of solvent identity on the photophysical properties of adsorbed solutes, additional experiments were carried out with a nonpolar solvent (decane), a moderately polar solvent (n decanol) and a polar aprotic solvent (acetonitrile). The results from these studies demonstrated that interfacial solvation depends sensitively on a balance of competing forces including those between the solute and substrate, the solute and solvent and the surface and adjacent solvent.Item Two Experiments with Cold Atoms: I. Application of Bessel Beams for Atom Optics, and II. Spectroscopic Measurements of Rydberg Blockade Effect(2010) Arakelyan, Ilya; Hill, III, Wendell; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this dissertation we report the results of two experimental projects with laser-cooled rubidium atoms: I. Application of Bessel beams for atom optics, and II. Spectroscopic measurements of Rydberg blockade effect. The first part of the thesis is devoted to the development of new elements of atom optics based on blue-detuned high-order Bessel beams. Properties of a 4th order Bessel beam as an atomic guide were investigated for various parameters of the hollow beam, such as the detuning from an atomic resonance, size and the order of the Bessel beam. We extended its application to create more complicated interferometer-type structures by demonstrating a tunnel lock, a novel device that can split an atomic cloud, transport it, delay, and switch its propagation direction between two guides. We reported a first-time demonstration of an atomic beam switch based on the combination of two crossed Bessel beams. We achieved the 30% efficiency of the switch limited by the geometrical overlap between the cloud and the intersection volume of the two tunnels, and investigate the heating processes induced by the switch. We also showed other applications of crossed Bessel beams, such as a 3-D optical trap for atoms confined in the intersection volume of two hollow beams and a splitter of the atomic density. The second part of this dissertation is devoted to the spectroscopic measurements of the Rydberg blockade effect, a conditional suppression of Rydberg excitations depending on the state of a control atom. We assembled a narrow-linewidth, tunable, frequency stabilized laser system at 480 nm to excite laser-cooled rubidium atoms to Rydberg states with a high principal quantum number n ~ 50 through a two-photon transition. We applied the laser system to observe the Autler-Townes splitting of the intermediate 5p state and used the broadening of the resonance features to investigate the enhancement of Rydberg-Rydberg interactions in the presence of an external electric field.Item Visible Light Photorelease of Carboxylate Anions by Mediated Photoinduced Electron Transfer to Pyridinium-based Protecting Groups(2009) Borak, John Brian; Falvey, Daniel E; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The use of sensitized photoinduced electron transfer (PET) to trigger release of redox-active photoremovable protecting groups (PRPGs) allows a broad range of chromophores to be selected that absorb in difference wavelength ranges. Mediated electron transfer (MET) is particularly advantageous as sub-stoichiometric amounts of the often costly sensitizer (relative to the amount of protected substrate) can be combined with an excess amount of an inexpensive electron donor. Thus, the sensitizer acts as an electron shuttle between the donor and the protecting group to initiate release. The development of improved MET release systems using visible light as the trigger is the focus of the current work. The N-alkylpicolinium (NAP) group has demonstrated its utility as an aqueous-compatible PET-based PRPG, releasing protected substrates upon one electron reduction. Adaptation of MET PRPG release to visible light absorbing mediators began with employing ketocoumarin dyes that primarily form excited triplet states. These chromophores demonstrated high rates of release of NAP-protected carboxylates using sub-stoichiometric concentrations of mediator. Subsequently, nanomolar concentrations of gold nanoparticles were used to mediate electron transfer to NAP-protected compounds. This system exhibited rapid deprotection with very high release quantum efficiencies. In an effort to use highly stable visible-light-absorbing metal-centered dyes with modest redox properties, the NAP group has been synthetically modified to adjust its reduction potential to more positive values. Photolysis of solutions containing the protected substrate, a large excess of an electron donor, and substoichiometric amounts of the dye tris(bipyridyl)ruthenium(II) released the free carboxylates in high yields while photodegradation of the chromophore was minimal. To demonstrate the utility of the NAP group, a quasi-reversible photorheological fluid has been developed based on the formation and disruption of aqueous micelles. In solutions containing the surfactant cetyltrimethylammonium bromide, visible light photorelease of a carboxylate additive from the NAP-ester derivative induces a 105 increase in solution viscosity due to the formation of an interpenetrating micelle network. Subsequent irradiation of the viscoelastic fluid with UV light induces a cis-trans isomerization within the released carboxylate thereby disrupting the micelle network and decreasing solution viscosity by 102.5.Item Photon Pair Production from a Hot Atomic Ensemble in the Diamond Configuration(2009) Willis, Richard Thomas; Rolston, Steven; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis discusses four-wave mixing (4WM) in a warm ensemble of rubidium using the diamond configuration level structure. Both classical 4WM and non- classical photon-pair production are investigated. Quantum information science has spawned a great amount of experimental work on the interaction of light with collective modes of excitation in atomic ensem- bles. Plans to build quantum networks and quantum repeaters with atom ensembles take advantage of nonlinear interactions to produce and store non-classical states of light. These technologies will require photon sources that not only generate non- classical light, but also resonant, narrow band light. Here we investigate a system which could be used as such a source. We take advantage of the 4WM interaction in a warm ensemble of Rubidium atoms. Our scheme utilizes the diamond energy level configuration which, in ru- bidium, allows for correlated pairs at telecommunications wavelengths. We start by examining the properties of classical 4WM in the system. We measure the reso- nance structure and see that it can be understood in terms of velocity class selective resonant enhancement and power splitting effects. The efficiency of the process is low and limited by linear absorption of the pumps. Our observations agree with a semi-classical Maxwell-Bloch theoretical treatment. Next we observe pair generation by spontaneous 4WM from the warm ensem- ble. The temporal profile of the cross-correlation function (CCF) for the photons depends on pump-laser power and detuning. This allows us to produce biphotons with controllable spectra. A simple quantum optical theoretical treatment based on linear filtering gives qualitative agreement with the data. We show that the photon pairs are polarization entangled, clearly violating Bell's Inequality. A perturbative quantum optical treatment predicts the polariza- tion state of the pairs and agrees with our measurements. We analyze the photon statistics of the source and find the largest violation of the two beam Cauchy-Schwarz inequality from a warm atomic source yet. We cast the system as a heralded sin- gle photon source at telecommunications wavelengths and see that it is competitive with other systems in terms of spectral brightness.Item EFFICIENT SIMULATION OF ELECTRON TRAPPING IN LASER AND PLASMA WAKEFIELD ACCELERATION(2009) Morshed, Sepehr; Antonsen, Thomas M; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Plasma based laser Wakefield accelerators (LWFA) have been a subject of interest in the plasma community for many years. In LWFA schemes the laser pulse must propagate several centimeters and maintain its coherence over this distance, which corresponds to many Rayleigh lengths. These Wakefields and their effect on the laser can be simulated in the quasistatic approximation. The 2D, cylindrically symmetric, quasistatic simulation code, WAKE is an efficient tool for the modeling of short-pulse laser propagation in under dense plasmas [P. Mora & T.M. Antonsen Phys. Plasmas 4, 1997]. The quasistatic approximation, which assumes that the driver and its wakefields are undisturbed during the transit time of plasma electrons, through the pulse, cannot, however, treat electron trapping and beam loading. Here we modify WAKE to include the effects of electron trapping and beam loading by introducing a population of beam electrons. Background plasma electrons that are beginning to start their oscillation around the radial axis and have energy above some threshold are removed from the background plasma and promoted to "beam" electrons. The population of beam electrons which are no longer subject to the quasistatic approximation, are treated without approximation and provide their own electromagnetic field that acts upon the background plasma. The algorithm is benchmarked to OSIRIS (a standard particle in cell code) simulations which makes no quasistatic approximation. We also have done simulation and comparison of results for centimeter scale GeV electron accelerator experiments from LBNL. These modifications to WAKE provide a tool for simulating GeV laser or plasma wakefield acceleration on desktop computers.Item MULTIPHOTON ABSORPTION: FABRICATION, FUNCTIONALIZATION AND APPLICATIONS(2009) Li, Linjie; Fourkas, John T; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Despite the remarkable progress in micro/nano-scale fabrication that has occurred over the last decades, feature sizes are still restricted by the diffraction limit. The resolution in conventional photolithography is generally constrained to approximately one quarter of a wavelength (lamda) of the light used. Multiphoton absorption polymerization (MAP) offers another option for high-resolution fabrication. Using nonlinear optical and chemical effects, MAP can generate features with a transverse dimension as small as 80 nm using 800-nm laser excitation. MAP has the additional capability of fabricating arbitrary 3D structures, which is essential in many applications. Details of MAP fabrication setup and process are described in this thesis. Novel optical devices have been fabricated with MAP. One drawback of MAP is that the resolution in axial direction remains about three to five times poorer because of the shape of the laser focal point. A novel technique called Resolution Augmentation through Photo-Induced Deactivation (RAPID) lithography has been developed to overcome this issue. With RAPID, resolution of 40 nm in axial direction has been achieved. The aspect ratio of the volume element of MAP has been reduced from about 3 to 0.5. Selective functionalization of polymeric microstructure has been performed in two ways. In the first approach, microstructures are fabricated with hybrid resists that permits the chemical functionality only applies to one material. The second method is able to pattern both binary and gray-scale functionalities onto polymer surface. The density of the surface functional groups is determined by the intensity of the exposed light. The nonlinear novelty of multiphoton absorption has not only been realized in MAP, it also shows promise for multiphoton absorption based microscopy. Photoluminescence from noble metal nanostructures has been used for two-photon imaging of living cells. Multiphoton Absorption Induced Luminescence (MAIL) has been used to monitor the targeting and endocytosis of goldnanoparticles to human umbilical vein endothelial cells. Field-enhanced phenomena have been studied with MAIL and MAP.Item Studies of atomic properties of francium and rubidium.(2009) Perez Galvan, Adrian; Orozco, Luis A; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)High precision measurements of atomic properties are excellent probes for elec- troweak interaction studies at the lowest possible energy range. The extraction of standard model coupling constants relies on a unique combination of experimen- tal measurements and theoretical atomic structure calculations. It is only through stringent comparison between experimental and theoretical values of atomic prop- erties that a successful experiment can take place. Francium, with its heavy nucleus and alkali structure that makes it amenable to laser cooling and trapping, stands as an ideal test bed for such studies. Our group has successfully created, trapped and cooled several isotopes of francium, the heaviest of the alkalies, and demonstrated that precision studies of atomic properties, such as the measurement of the 8S1/2 excited state lifetime of 210Fr presented here, are feasible. Further work in our program of electroweak studies requires a better control of the electromagnetic environment observed by the sample of cold atoms as well as a lower background pressure (10-10 torr or better). We have designed and adapted to our previous setup a new &ldquo science &rdquo vacuum chamber that fulfills these requirements and the transport system that will transfer the francium atoms to the new chamber. We use this new experimental setup as well as a rubidium glass cell to perform precision studies of atomic and nuclear properties of rubidium. Spectroscopic studies of the most abundant isotopes of rubidium, 87Rb and 85Rb, are a vital component in our program. Performing measurements in rubidium allows us to do extensive and rigorous searches of systematics that can be later extrapolated to francium. We present a precision lifetime measurement of the 5D3/2 state of 87Rb and a measurement of hyperfine splittings of the 6S1/2 level of 87Rb and 85Rb. The quality of the data of the latter allows us to observe a hyperfine anomaly attributed to an isotopic difference of the magnetization distribution in the nucleus i.e. the Bohr-Weisskopf effect. The measurements we present in this work complement each other in exploring the behavior of the valence electron at different distances from the nucleus. In addition, they constitute excellent tests for the predictions of ab initio calculations using many body perturbation theory and bolster our confidence on the reliability of the experimental and theoretical tools needed for our work.