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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Subject: search.filters.subject.optical tweezers

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    THE APPLICATIONS OF MULTIPHOTON ABSORPTION POLYMERIZATION
    (2013) Qin, Sijia; Fourkas, John T.; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Among various nano/micro-fabrication techniques, multiphoton absorption polymerization (MAP) stands out for its high resolution and its capability of creating arbitrary three-dimensional micro-structures. This thesis will focus on the applications of, and improvements to, MAP. MAP was used to fabricate polymer double-ring resonators (DRRs) because MAP's high resolution allows for precise control of the coupling gap sizes. Pedestal acrylic DRRs with 33 nm free spectral range and -15 dB isolation were fabricated, and their properties showed qualitative agreement with simulation results. Single-mode, acrylic microring resonator devices were fabricated on a low-index substrate using MAP and were coupled to side-polished fibers as in-line devices. High-finesse spectral notches with low insertion loss were observed at the fiber output. Surface mapping of the polished face of the fiber was accomplished by moving a microring resonator device across and along the fiber core. The optimal coupling region on the polished face of the fiber could be identified through the change of modulation depth. Spectral modulation induced by varying the pressure on a microring resonator device coupled to a side-polished fiber was also investigated. Efficient multiphoton radical generation chemistry has been developed for use in aqueous media and has been applied to the fabrication, manipulation, and assembly of 3-D polymeric and biomolecular structures through a combination of MAP and optical tweezers. The combination of MAP and optical tweezers allows for the realization of structures such as tape-like and rope-like microthreads that can be used in complex microfabrication techniques such as microbraiding and microweaving. These capabilities enhance the toolbox of methods available for the creation of functional microstructures in aqueous media. MAP-fabricated and UV-cured acrylic patterns were treated with reactive ion etching (RIE) to create high-roughness "nanograsses." The nanograss patterns have shown the potential to be used as superhydrophobic materials. The density and dimension of the nanograss depends on the total exposure dose. Different etch angles gave different etch structures.
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    PROBING BIOMECHANICAL PROPERTIES OF SINGLE MOLECULE SYSTEMS USING OPTICAL TWEEZERS
    (2011) Karcz, Adam P.; Seog, Joonil; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Single molecule techniques have provided novel mechanistic insights on biological processes such as protein folding, transcription, and motor protein movement. Using single molecule methods, the distribution of individual molecular behavior is directly measured, which cannot be obtained using conventional bulk approaches. In this study, custom-built optical tweezers with sub-pN force resolution were used to probe the dynamic behavior of DNA:cationic carrier complex. Two histidine-lysine (HK) based polymers (H3K4b vs H3KG4b) were used to compare their condensation behaviors at the single molecular level. The difference between the two HK polymers at the single molecule level may have a significant implication as to why H3KG4b shows much higher gene delivery efficiency than H3K4b. The optical tweezers were also used to probe the unfolding processes of a fragment of F1 RNA. This can be used to characterize secondary structures in RNA, such as hairpins and pseudoknots.
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    Modeling and Experimental Techniques to Demonstrate Nanomanipulation With Optical Tweezers
    (2011) Balijepalli, Arvind K.; Gupta, Satyandra K; LeBrun, Thomas W; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The development of truly three-dimensional nanodevices is currently impeded by the absence of effective prototyping tools at the nanoscale. Optical trapping is well established for flexible three-dimensional manipulation of components at the microscale. However, it has so far not been demonstrated to confine nanoparticles, for long enough time to be useful in nanoassembly applications. Therefore, as part of this work we demonstrate new techniques that successfully extend optical trapping to nanoscale manipulation. In order to extend optical trapping to the nanoscale, we must overcome certain challenges. For the same incident beam power, the optical binding forces acting on a nanoparticle within an optical trap are very weak, in comparison with forces acting on microscale particles. Consequently, due to Brownian motion, the nanoparticle often exits the trap in a very short period of time. We improve the performance of optical traps at the nanoscale by using closed-loop control. Furthermore, we show through laboratory experiments that we are able to localize nanoparticles to the trap using control systems, for sufficient time to be useful in nanoassembly applications, conditions under which a static trap set to the same power as the controller is unable to confine a same-sized particle. Before controlled optical trapping can be demonstrated in the laboratory, key tools must first be developed. We implement Langevin dynamics simulations to model the interaction of nanoparticles with an optical trap. Physically accurate simulations provide a robust platform to test new methods to characterize and improve the performance of optical tweezers at the nanoscale, but depend on accurate trapping force models. Therefore, we have also developed two new laboratory-based force measurement techniques that overcome the drawbacks of conventional force measurements, which do not accurately account for the weak interaction of nanoparticles in an optical trap. Finally, we use numerical simulations to develop new control algorithms that demonstrate significantly enhanced trapping of nanoparticles and implement these techniques in the laboratory. The algorithms and characterization tools developed as part of this work will allow the development of optical trapping instruments that can confine nanoparticles for longer periods of time than is currently possible, for a given beam power. Furthermore, the low average power achieved by the controller makes this technique especially suitable to manipulate biological specimens, but is also generally beneficial to nanoscale prototyping applications. Therefore, capabilities developed as part of this work, and the technology that results from it may enable the prototyping of three-dimensional nanodevices, critically required in many applications.
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    WAVELENGTH DEPENDENCE IN OPTICAL TWEEZERS
    (2010) Hester, Brooke Cranswick; Losert, Wolfgang; Rolston, Steven; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Optical trapping forces are dependent upon the difference between the trap wavelength and the extinction (scattering plus absorption) resonances of a trapped particle. This leads to a wavelength-dependent trapping force, which should allow for the optimization of optical tweezers systems, simply by choosing the best trapping wavelength for a given experiment. Although optical forces due to a near-resonant laser beam have been extensively studied for atoms, the situation for larger particles has not been explored experimentally. The ability to selectively trap certain particles with a given extinction peak may have many practical applications. Here, resonance-based trapping is investigated using nanoshells, particles with a dielectric core and metallic coating that exhibit tunable plasmon resonances, and with silica and polystyrene beads. A measure of the trap strength was realized for single particles trapped in three dimensions, and near-resonant trapping was investigated by measuring the trap strength as a function of trap wavelength. Since the resulting trapping is highly temperature dependent, this necessitated temperature measurements of single optically trapped particles. To make these measurements a new optical tweezer apparatus was designed and constructed; the apparatus has wavelength tunability and was used to study these resonance effects. Optical trap stiffness, which is analogous to the spring constant of a stable trap, is measured for trapped particles that exhibit either single or multiple extinction resonances. The applications of this apparatus are not limited to force spectroscopy. Other measurement systems and techniques could be easily implemented into the custom-built apparatus, allowing for the measurement of various properties of single optically trapped particles as a function of wavelength.