Browsing by Subject "Interferometry"
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Item Advancing a nuclear magnetic resonance force microscopy (NMRFM) probe and simulating NMRFM in thin films(2023-12) Paster, Jeremy W.; Markert, John T.; Lai, Keji; Lewis-Peacock, Jarrod A; Tsoi, Maxim; Marder, MichaelThis research endeavor centers around the development of a nuclear magnetic resonance force microscopy (NMRFM) probe for investigating thin-film samples. Of particular interest is the conducting region that forms when lanthanum aluminate (LAO) is grown epitaxially on strontium titanate (STO). These materials are insulating in their bulk form. We propose NMRFM as a tool to detect whether there is diffusion of atoms across the interface, which could explain the emergence of the conducting region. While conventional scanning probe techniques are constrained to the surface of a sample, NMRFM features the non-invasive and subsurface detection capabilities of conventional nuclear magnetic resonance (NMR) spectroscopy. Unlike conventional NMR, for which a cubic millimeter-sized sample is required to produce a measurable signal, we can readily scale down NMRFM detection sensitivity, extending its application to smaller samples. In combination, these features suggest that NMRFM is well-suited to study the LAO/STO interface. Force detection of nuclear spins is made possible by coupling NMR spin flip sequences to a mechanical oscillator (cantilever). A small magnetic tip deposited on the cantilever establishes a large field gradient and an interaction force with the magnetic moments of the sample nuclei. This tip traces out constant-field slices perpendicular to the magnetic field. Within a particular slice, nuclear spins resonate with the perturbing oscillating field conventionally employed in NMR spectroscopy. We anticipate that evidence of atomic diffusion across the LAO/STO interface is limited to a 10-nanometer region. Before the reconfiguration outlined herein, this NMRFM probe previously resolved a sample whose smallest dimension was 60 microns. To develop this probe for thin films, we adopted the Interrupted OScillating Cyclic Adiabatic Reversal (iOSCAR) protocol. This method distinguishes the minuscule force interaction between the cantilever and the sample by implementing an NMR-induced modulation of the cantilever frequency. iOSCAR generates a distinguishable signal at an established frequency that is far from spurious artifact signals that limited the signal-to-noise ratio of the previous NMRFM protocol. This dissertation involves contextualizing NMR and NMRFM and assesses the need for further experimental investigation of LAO/STO. Furthermore, it details the evolution of an NMRFM probe to enable the exploration of thin-film samples using iOSCAR. While this research project largely involved the creation of experimental components, it concluded by modeling the expected experimental results. We created simulations of thin-film NMRFM, calculating the z component of the sample magnetization near an oscillating cantilever with a magnetic tip. These simulations explore the dynamic interactions between a thin-film sample and a cantilever as sample nuclei undergo magnetic resonance.Item AFM-based measurement of the mechanical properties of thin polymer films and determination of the optical path length of nearly index-matched cavities(2008-05) Wieland, Christopher F., 1980-; Shih, Chih-KangTwo technologies, immersion and imprint lithography, represent important stepping stones for the development of the next generation of lithography tools. However, although the two approaches offer important advantages, both pose many significant technological challenges that must be overcome before they can be successfully implemented. For imprint lithography, special care must be taken when choosing an etch barrier because studies have indicated that some physical material properties may be size dependent. Additionally, regarding immersion lithography, proper image focus requires that the optical path length between the lens and substrate be maintained during the entire writing process. The work described in this document was undertaken to address the two challenges described above. A new mathematical model was developed and used in conjunction with AFM nano-indentation techniques to measure the elastic modulus of adhesive, thin polymer films as a function of the film thickness. It was found that the elastic modulus of the polymer tested did not change appreciably from the value determined using bulk measurement techniques in the thickness range probed. Additionally, a method for monitoring and controlling the optical path length within the gap of a nearly index-matching cavity based on coherent broadband interference was developed. In this method, the spectrum reflected for a cavity illuminated with a modelocked Ti:Sapphire laser was collected and analyzed using Fourier techniques. It was found that this method could determine the optical path length of the cavity, quickly and accurately enough to control a servo-based feedback system to correct deviations in the optical path length in real time when coupled with special computation techniques that minimized unnecessary operations.Item Deformation monitoring using scanning synthetic aperture radar interferometry(2009-05) Gudipati, Krishna Vikas, 1979-; Buckley, Sean M.This dissertation provides the first demonstration of scanning synthetic aperture radar (ScanSAR) advanced interferometry processing for measuring surface deformation. ScanSAR data are synthesized from ERS-1/2 stripmap SAR images over known deformation in Phoenix, Arizona. The strategy is to construct a burst pattern similar to Envisat ScanSAR data and to create a realistic variable-burst synchronization scenario in which any image pair has at least 50% burst overlap. The Small Baseline Subsets technique is applied to the synthesized data to demonstrate ScanSAR time series analysis for a scenario generally conducive for interferometry. The same processing approach is employed with the stripmap data to validate the results. The differences in ScanSAR and stripmap velocities have a mean and standard deviation of 0.02±0.02 cm/year. 96.3% and 99.1% of the velocity differences are within ±0.1 cm/year and ±0.2 cm/year, respectively. The RMS deviations between the ScanSAR and stripmap displacement estimates are 0.40±0.30 cm. 68.5% and 94.6% of the differences are within ±0.5 cm and ±1.0 cm, respectively. The Permanent Scatterer (PS) technique also is adapted and applied to the synthesized data to demonstrate the presence of PS in ScanSAR data. The atmospheric and nonlinear motion phase derived from a PS analysis of stripmap data are removed from the ScanSAR interferograms. Even for this idealized scenario, the final PS identification yields fewer ScanSAR PS (10 PS/km²) than the stripmap PS results (312 PS/km² or 15.6 PS/km² at the ScanSAR pixel resolution). Based on the calculated likelihood of finding multiple stripmap PS within a ScanSAR pixel, it is concluded that the ScanSAR single scatterer PS model is flawed. A model is introduced that considers multiple PS within a ScanSAR pixel. The search for two PS per pixel yields 120 PS/km². The ScanSAR and stripmap PS velocity differences mean is zero and standard deviation is 0.02 cm/year. However, while the differences between the ScanSAR and stripmap PS DEM error estimates are zero-mean, they have a 7-meter standard deviation. One possible explanation for this relatively large deviation is the differencing of the wrong ScanSAR and stripmap PS as the result of a misalignment between the ScanSAR and stripmap images.Item Design of an optical microelectromechanical-system microphone with sub 15-dBA noise floor(2018-05-02) Kim, Donghwan, 1981-; Hall, Neal A.; Hamilton, Mark F.; Ho, Paul S.; Wilson, Preston S.; Yu, Edward T.This research work presents the modeling, fabrication, and characterization of the optical microphone. The optical microphone detects diaphragm displacement due to input sound pressure, using an interferometric-based displacement detection scheme instead of using capacitive readout technique, which is extensively used in commercial microelectromechanical-system microphones. The optical-based transduction mechanism enables a backplate design with an extremely high perforation density, which in-turn drastically reduces the backplate flow resistance, which is a dominant noise source in miniaturized microphones. Therefore, an accurate estimation of the backplate-induced flow resistance is a critical step to predict signal-to-noise ratio precisely. A flow resistance modeling technique via computational fluid dynamics is presented in this work. A prototype backplate is fabricated for a verification of the flow-resistance modeling technique. A 22.0-dBA noise floor is demonstrated using the prototype backplate, which is 6-dB better than state-of-the-art commercial capacitive MEMS microphones. Design of experiments were performed with the verified microphone model to illustrate design implications toward sub 15-dBA optical microphone. The design-of-experiments study focused on various microphone components including diaphragm compliance, acoustical low cut-off frequency, back-cavity volume, inlet port and vent to show how each parameter affect to the microphone signal-to-noise ratio and acoustic overload point. Finally, a force-feedback optical microphone concept is presented to achieve a higher acoustic overload pressure, which is defined by 10% total harmonic distortion, using a Si membrane with piezoelectric thin-film actuators. A feasibility study was performed to explore the concept of a force-feedback optical microphone, including a fabrication of the minimalistic backplate with high aspect-ratio spokes and Si membrane with piezoelectric-film actuators at Microelectronics Research Center at The University of Texas at Austin.Item Estimating high resolution atmospheric phase screens from differential InSAR measurements(2010-05) Yang, Dochul; Buckley, Sean M.; Tapley, Byron D.; Schutz, Bob E.; Lightsey, Glenn; Wilson, Clark R.Atmospheric artifacts superimposed on interferometric synthetic aperture radar (InSAR) measurements have the potential to greatly impede the accurate estimation of deformation signals. The research presented in this dissertation demonstrates a novel InSAR time series algorithm, called HiRAPS algorithm, for effectively estimating high resolution atmospheric phase screens (APS) from differential InSAR measurements. In summary, the HiRAPS algorithm utilizes short time span differential interferograms and rearranges components of existing advanced InSAR techniques to identify a higher density of scatterers used to create the APS. The improved scatterer density allows one to estimate high spatial frequency atmospheric signals not recovered from existing InSAR time series techniques. The HiRAPS algorithm was tested with simulated and actual data, which contain phase contributions from linear and nonlinear deformation, topographic height errors, and atmospheric artifacts. Simulated differential interferograms were generated to have the same spatial and temporal baselines as the actual differential interferograms formed from RADARSAT-1 data over Phoenix, Arizona. The APS superimposed on simulated differential interferograms were then estimated and compared to simulated APS. The root mean square error (RMSE) between the estimated and simulated APS was calculated to qualitatively assess the different values obtained. The RMSE was 0.26 radians when utilizing the HiRAPS algorithm, compared to an RMSE value of 0.39 radians using an implementation of the permanent scatterer (PS) algorithm. The HiRAPS algorithm also showed its applicability for estimating high spatial frequency atmospheric signals for actual data. Sixty-six SAR images, starting from October 5, 2002 and spanning 5 years, were processed for this research. The APS pixel density obtained using the HiRAPS algorithm was 253 pixels per square kilometer, compared to 14 pixels per square kilometer utilizing the PS algorithm. The APS superimposed on the differential interferograms were estimated with both the proposed and PS algorithms. High resolution APS were estimated with the HiRAPS algorithm, whereas only low resolution APS were obtained with the PS algorithm. After estimating and removing estimated APS, the phase stability of APS-free differential interferograms was examined by identifying the permanent scatterers (PS). The final density of identified PS obtained with the HiRAPS algorithm was 453 PS per square kilometer, whereas the density of detected PS using the generic PS algorithm was 381 PS per square kilometer. The maximum difference in the deformation time series between the HiRAPS algorithm and the PS algorithm was less than 6 mm. However, the HiRAPS algorithm resulted in less apparent noise in the time series than the PS algorithm due to the precise estimation of APS.Item Experimental studies of high energy density silicon using ultra-fast lasers(2007) Grigsby, Will Robert, 1978-; Ditmire, Todd R.Understanding material behavior under extreme conditions is an important area of research in physics and material science. One method to study the behavior of materials under these conditions is to drive a strong shock wave through a material and watch its response. In many cases the material response is complicated by phase transitions such as lattice restructuring (Barker 1975; Mabire and Hereil 2000; Swift, Tierney et al. 2005) and melting (Asay 1975; Elias, Chapron et al. 1988; Werdiger, Eliezer et al. 1999; Mabire and Hereil 2000; Swift, Tierney et al. 2005). To study these dynamics we are using lasers in high time resolution pump-probe experiments to develop a real time diagnostic on the phase of a shocked material. This technique enables probing of the entire phase history of a material as it shock compresses and releases. In addition to linear reflectivity and ultra-fast 2D displacement interferometry, we developed a melting diagnostics based on the non-linear optical technique of third harmonic generation (THG) using a circularly polarized laser pulse. This diagnostic resolves the less than 300 fs melting transition of laser excited Si and GaAs, and it also detects a response in shock compressed silicon. Our results show that Si remains crystalline during compression of an elastic 100 kbar shock wave. Results from Si shocked to higher pressures (> 300 kbar) indicate a decrease in THG, suggesting some level of disordering or unexplained phase change.Item Radar interferometry measurement of land subsidence(2000-08) Buckley, Sean Monroe, 1970-; Tapley, Byron D.Radar interferometry is a remote sensing technique which has been developed over the past two decades for the measurement of earth-surface topography and deformation. This investigation considers the application of radar interferometry to the study of urban land subsidence. Radar data collected by the European Earth Resource Satellites (ERS-1 and ERS-2) over Phoenix, Arizona and Houston, Texas are used to create a time series of interferograms spanning the 1990s. Deformation maps are validated with traditional subsidence measurements where available. Comparisons between interferograms confirm the existence and continued development of several subsidence features, some of which were not identified with traditional measurement techniques. Appropriate land use, land cover as well as atmospheric conditions are all critical to the successful application of the interferometry technique. This ix investigation provides methods for assessing the atmospheric contamination associated with and the environmental conditions under which radar interferometry may be used to monitor slow, continuous deformation. Phoenix, Arizona and Houston, Texas have witnessed significant urban development over the past century. The residential, industrial and agricultural water supply of these metropolitan areas is provided primarily from groundwater pumped from vast aquifer systems underlying the regions. The pumping has depressurized and subsequently compacted the aquifers. Consequently, several meters of land subsidence has occurred since groundwater extraction began, resulting in millions of dollars of damage to urban infrastructure. Considering the vast urban development continuing in these metropolitan areas, the demand on the groundwater supply and the associated land subsidence remain critical problems to be addressed by city planners. Monitoring groundwater-use and subsidence patterns in these regions over time provides valuable information which can be used to mitigate infrastructure damage as well as infer subsurface properties. Three techniques have been used historically to monitor subsidence in these regions: releveling of benchmark surveys, the use of extensometer measurements, and the use of GPS measurements. However, these measurements provide spatially-limited point measurements of deformation. In contrast, spaceborne repeat-pass differential radar interferometry, a technique developed over the past decade, provides subtle earth-surface deformation measurements at significantly improved spatial resolution as compared with traditional subsidence monitoring techniques.Item Radar interferometry measurement of land subsidence in El Paso, Texas(2004) Leuro, Erick; Wilson, Clark R.; Buckley, Sean M.This work presents the application of radar interferometry to detect land subsidence associated with water pumping in El Paso, Texas and adjacent areas. Geological and hydrological information are compared with the radar information to validate the results. An error treatment of the measurements is performed using the singular value decomposition technique. Synthetic aperture radar interferometry is a proven remote sensing technique to detect ground deformation in a three-dimensional scale with millimetric precision. It has been applied successfully in earthquake monitoring, volcano deformation, glacier movement and aquifer compaction. El Paso and Ciudad Juarez are located over the Hueco Bolson aquifer, an unconsolidated alluvial aquifer that consists of gravel, sand, silt and clay. Because of increased water pumping since the early 20th century, the water table has changed and subsidence has occurred. Measurements of land subsidence are reported from the 1950s, 1970s, and 1980s. This work considers subsidence in the 1990sItem Time reversal and plane-wave decomposition in seismic interferometry, inversion and imaging(2012-12) Tao, Yi, active 2012; Sen, Mrinal K.This thesis concerns the study of time reversal and plane-wave decomposition in various geophysical applications. Time reversal is a key step in seismic interferometry, reverse time migration and full waveform inversion. The plane-wave transform, also known as the tau-p transform or slant-stack, can separate waves based on their ray parameters or their emergence angles at the surface. I propose a new approach to retrieve virtual full-wave seismic responses from crosscorrelating recorded seismic data in the plane-wave domain. Unlike a traditional approach where the correlogram is obtained from crosscorrelating recorded data, which contains the full range of ray parameters, this method directly chooses common ray parameters to cancel overlapping ray paths. Thus, it can sometime avoid spurious arrivals when the acquisition requirement of seismic interferometry is not strictly met. I demonstrate the method with synthetic examples and an ocean bottom seismometer data example. I show a multi-scale application of plane-wave based full waveform inversion (FWI) with the aid of frequency domain forward modeling. FWI uses the two-way wave-equation to produce high-resolution velocity models for seismic imaging. This technique is implemented by an adjoint-state approach, which viii involves a time-reversal propagation of the residual wavefield at receivers, similar to seismic interferometry. With a plane-wave transformed gather, we can decompose the data by ray parameters and iteratively update the velocity model with selected ray parameters. This encoding approach can significantly reduce the number of shots and receivers required in gradient and Hessian calculations. Borrowing the idea of minimizing different data residual norms in FWI, I study the effect of different scaling methods to the receiver wavefield in the reverse time migration. I show that this type of scaling is able to significantly suppress outliers compared to conventional algorithms. I also show that scaling by its absolute norm generally produces better results than other approaches. I propose a robust stochastic time-lapse seismic inversion strategy with an application of monitoring Cranfield CO2 injection site. This workflow involves two steps. The first step is the baseline inversion using a hybrid starting model that combines a fractal prior and the low-frequency prior from well log data. The second step is to use a double-difference inversion scheme to focus on the local areas where time-lapse changes have occurred. Synthetic data and field data show the effectiveness of this method.