Long-range dipolar fields as a tool for nuclear magnetic resonance microscopy
Item
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Title
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Long-range dipolar fields as a tool for nuclear magnetic resonance microscopy
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Identifier
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d_2009_2013:f7fdd302bd08:10159
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identifier
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10406
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Creator
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Dong, Wei,
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Contributor
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Carlos A. Meriles
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Date
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2009
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Language
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English
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Publisher
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City University of New York.
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Subject
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Nuclear physics | Dipolar Field Microscopy | Dipolar fields | NMR microscopy | Optical detection | Optically pumped NMR
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Abstract
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Nuclear Magnetic Resonance (NMR) is widely used today for structural and dynamical studies of the properties of diverse materials. However, due to the relatively low sensitivity of the standard induction detection method, NMR is strongly constrained when probing samples whose effective dimensions are less than a few microns. To overcome these limitations, our novel strategy based on the manipulation of the long-range dipolar interactions between the sample and a hyperpolarized semiconductor tip located close to its surface. These interactions are used to modulate the tip nuclear magnetization in a way proportional to the local sample magnetization. The advantage of this strategy lies in that the highly sensitive detection methods -- e.g., optical detection -- can be used to monitor the semiconductor tip, thus providing the opportunity to indirectly probe the sample neighboring the tip with a favorable signal-to-noise ratio. Because the detected portion of the sample is comparable to the size of the tip, resolution exceeding the currently attainable could be possible.;As an initial demonstration of our methodology we designed an experiment in which a 3 mm diameter distilled water droplet -- playing the role of a sensor -- was used to detect the NMR signal of the sample surrounding the droplet, in this case, silicon oil (Sigma-Aldrich) contained in a 5 mm diameter glass tube. Notice the sample (oil) and the detection center (water) are distinct and discernible objects only connected through long range intermolecular dipolar couplings. A special pulse sequence was designed and applied in the experiment to encode the sample magnetization for detection. By utilizing the Runge-Kutta algorithm, I modeled a 2000 spins ensemble based on the given geometry and numerically calculated the Bloch differential equations of this coupled spins system. Experimental results have a very good agreement with the numerical calculations. This preliminary experiment proves that not only the sample NMR signal can be indirectly detected, also many other sample information -- e.g., relaxation time, sample spectrum, etc. -- are attainable.;Minimizing the short range dipolar couplings is a very crucial part to achieve the final goal of this strategy when the solid state semiconductor was used as the sensor. At the second stage, a modified MREV16 decoupling pulse sequence was designed and applied to greatly reduce the short range dipolar couplings inside the solid state sensor. A 3 mm thick disk GaAs crystal has been chosen as the sensor due to its excellent optical hyperpolarization properties. By acquiring 71Ga NMR signal, I successfully indirectly detected a tiny nuclear dipolar field induced by proton spins from an adjacent organic sample (as small as 7 nT). Optical enhancing the bulk averaged nuclear spin polarization in semiconductors is another critical technique that will be integrated into our strategy. Comparing to the thermal nuclear magnetization, we achieved 2-3 orders of magnitude optical enhancement for 71Ga in GaAs crystal and 3 orders for 125Te in CdTe crystal. Finally optical Faraday rotation will be used as an ultrasensitive detection to incorporate into the strategy. Optical reading of the electronic Larmor frequency shift in the semiconductor by using Time-Resolved Faraday Rotation (TRFR) to probe the sample magnetization change is the basic idea of our optical detection scheme.;Through collaboration with Attocube AG, a leading company specialized on low temperature optical microscopy, our cutting-edge cryogenic optical NMR probe has been finished. Certainly, integrating optical hyperpolarization and optical detection with modern NMR technique is very challenging and requires tremendous work. However given the steady progress in the area of nanotechnology, our strategy's future still appears quite promising.
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Type
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dissertation
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Source
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2009_2013.csv
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degree
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Ph.D.
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Program
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Physics
