L'objet de ce travail de thèse est d'étudier un cas particulier de l'interaction lumière-matière dans le cadre du stockage d'information quantique. Nous nous sommes intéressés aux processus appelés transitoires optiques cohérents, et plus précisément à l'écho de photon à deux impulsions. Ce processus est observé dans des cristaux dopés aux ions lanthanides, tel que le Tm3+:YAG. Une fois éclairé par une impulsion faible (le signal à stocker) puis par une impulsion plus intense (servant à la remise en phase des dipôles induits), ce milieu atomique, essentiellement un système à deux niveaux avec une transition large, réemet une troisième impulsion, qui est l'"écho" temporel de la première (chapitre 1). Nous avons observé de tels échos expérimentalement, et ce pour des impulsions de signal faibles. Néanmoins, des effets délétères, dûs à l'impulsion de rephasage et à sa propagation, nous ont permis de conclure sur l'impossibilité d'utiliser ce processus tel quel en tant que protocole de mémoire quantique (chapitre 2). Le troisième chapitre a pour objet l'état des lieux des diverses méthodes dérivées de l'écho de photon à deux impulsions. Notamment, nous examinons la capacité théorique des échos Stark à restituer une éventuelle information quantique. Bien que prometteuse, nous lui avons préféré un autre processus, basé sur la structuration de l'absorption du système en peigne de fréquences atomiques (AFC en anglais). Son étude théorique consistitue la fin du chapitre 3. Le dernier chapitre (4) s'attache à résumer les expériences que nous avons réalisées sur le protocole AFC. Elles ont permis d'observer des échos AFC avec une efficacité proche de 10%. Elles ont également permis de dégager des axes de recherche pour améliorer le protocole, comme l'AFC large- bande, ou en régime de comptage de photons.

Lipid molecules form thin biological membranes that envelop all living cells, and behave as two-dimensional liquid sheets immersed in bulk water. The interactions of such biomembranes with their environment lay the foundation of a plethora of biological processes rooted in the mesoscopic domain - length scales of 1-1000 nm and time scales of 1-1000 ns. Research in this intermediate regime has for a long time been out of reach for conventional experiments, but breakthroughs in computer simulation methods and scattering experimental techniques have made it possible to directly probe static and dynamic properties of biomembranes on these scales. Biomembranes are soft, with a relatively low energy cost of bending, and are thereby influenced by random, thermal fluctuations of individual molecules. Molecular dynamics simulations show how in-plane (density fluctuations) and out-of-plane (undulations) motions are intertwined in the bilayer in the mesoscopic domain. By novel methods, the fluctuation spectra of lipid bilayers can be calculated withdirect Fourier analysis. The interpretation of the fluctuation spectra reveals a picture where density fluctuations and undulations are most pronounced on different length scales, but coalesce in the mesoscopic regime. This analysis has significant consequences for comparison of simulation data to experiments. These new methods merge the molecular fluctuations on small wavelengths, with continuum fluctuations of the elastic membrane sheet on large wavelengths, allowing electron density profiles (EDP) and area per lipid to be extracted from simulations with high accuracy. Molecular dynamics simulations also provide insight on the small-wavelength dynamics of lipid membranes. Rapidly decaying density fluctuations can be described as propagating sound waves in the framework of linearized hydrodynamics, but there is a slow, dispersive, contribution that needs to be described by a stretched exponential over a broad range of length- and time scales - recent experiments suggest that this behavior can prevail even on micrometer length scales. The origin of this behavior is discussed in the context of fluctuations of the bilayer interface and the molecular structure of the bilayer itself. Connections to recent neutron scattering experiments are highlighted. / QC 20111014 / Modelling of biological membranes

Cerebral small vessel disease (SVD) is highly prevalent in older adults and is a predictor of stroke, dementia, and death. SVD is also associated with cognitive dysfunction, gait problems, and urinary incontinence. SVD is diagnosed based on white matter hyperintensities on T2 weighted scans. This thesis investigates the cardiac frequency component of resting state functional magnetic resonance imaging data in young healthy adults, older healthy adults, and older adults with pronounced SVD. A cardiac pulsatility metric is defined, and a tissue type contrast is observed between white matter, grey matter, and cerebrospinal fluid. Aging and disease effects are observed on cardiac pulsatility in white matter. The increased pulsatility may reflect the pathology of venous collagenosis and draining vein stenosis. Developing a better understanding of the etiology of SVD is an important step towards treating the disease.

Cerebral small vessel disease (SVD) is highly prevalent in older adults and is a predictor of stroke, dementia, and death. SVD is also associated with cognitive dysfunction, gait problems, and urinary incontinence. SVD is diagnosed based on white matter hyperintensities on T2 weighted scans. This thesis investigates the cardiac frequency component of resting state functional magnetic resonance imaging data in young healthy adults, older healthy adults, and older adults with pronounced SVD. A cardiac pulsatility metric is defined, and a tissue type contrast is observed between white matter, grey matter, and cerebrospinal fluid. Aging and disease effects are observed on cardiac pulsatility in white matter. The increased pulsatility may reflect the pathology of venous collagenosis and draining vein stenosis. Developing a better understanding of the etiology of SVD is an important step towards treating the disease.

Proton echo planar spectroscopic imaging(PEPSI) is a novel and rapid technique of magnetic resonance spectroscopic imaging(MRSI). To analyze the metabolite in PEPSI by using LCModel, an automatic reconstruction system is necessary. Recently, many researches use graphic processing unit(GPU) to accelerate imaging reconstruction, and Compute Unified Device Architecture(CUDA) is developed by C language, so the programmers can write the program in parallel computing easily. PEPSI data acquisition includes non water suppression and water suppression scans, each scan contains odd and even echoes, these two data are reconstructed separately. The image reconstruction contains k-space filter, time-domain filter, three-dimension fast Fourier transform(FFT), phase correction and combine odd and even data. We use MATLAB, C++ and parallel computing to implement PEPSI reconstruction, and parallel computing applied CUDA which proposed by NVIDIA. In our study, the averaged non water suppression spectroscopic imaging executed by three different programming language are almost the same. In our data scale, the execution time of parallel computing is faster than MATLAB and C++, especially in the FFT step. Therefore, we simulated and compared the performance of one- to three-dimension FFT. Our result shows that accelerating performance of GPU depends on the number of data points according to the performance of FFT and the execution time of single coil PEPSI reconstruction. While the amount of data points is larger than 65536, as demonstrated in our study, parallel computing contribute in terms of computational acceleration.

Traditionally, destructive methods such as coring are used for the condition assessment of an existing concrete structure. Although these methods may yield valid data about the corresponding concrete section, they are quite expensive and time consuming. More important than these, destructive methods damage the structure being investigated and these points usually become focal points for further deterioration. For all these reasons, only a few samples can be collected from a structure and this results in a poor representation of the complete structure. The impact-echo technique is one of the most suitable non-destructive test methods that may be used on concrete for thickness determination or for investigation of possible delaminations in the internal parts of a concrete structure without damaging the surface. It has been observed that reliable results can be obtained quickly. Unlike pulse-echo tests which are commonly used on steel, testing a heterogeneous material like concrete requires the use of low frequency sound waves as in impact-echo, in order to mitigate the effects of paste-aggregate interfaces or small air voids. This method may be used to locate internal cracks or large air voids existing in concrete. It is known that impact-echo has been used successfully on structures with varying geometries and various purposes such as evaluation of concrete pavements, retaining walls and other reinforced concrete sections. Besides the investigation of the internal state, it may also be used when the other side of the section cannot be reached, as in the case of concrete pavements, in order to find the thickness of the section. This is especially important for quality control and for cost calculations. Research conducted in this thesis study was concentrated on the thickness determination of existing concrete pavement sections, produced in the laboratory with dimensions of 1500 x 2000 mm four and varying thicknesses, and the accuracy associated with these results. In order to correctly determine the sensitivity, several other parameters were investigated and optimum ranges were determined for these to be used while on a field test. Among these factors were the steel impactor size, accuracy related to the data acquisition, distance between the impact point and the transducer and the location of the test point. Finally, the accuracy of the impact-echo method for concrete pavement applications was studied. By observing the large number of data points collected, it was found out that an average error of 1.5% exists for a single impact-echo reading regardless of section thickness, but this value reduces to 0.6% when the average of all test results is used while determining pavement thickness. Results of this study show that the impact-echo technique is reliable and may be used with success for the thickness determination of concrete pavements and for locating internal voids.

In general the acoustic wave is used as a detecting tool in the ocean, its application placing a sound source into ocean, then the sound may impinge involves the target by wave propagation in the ocean. Due to the reflection and scattering effect of target, part of acoustic energy will be received by transducer through the path of reflection. The goal of target identification can be achieved by signal processing finally. If a submarine wish to avoid the detection by sonar system , it should attenuate the acoustic energy . Therefore the reflected signal can not be analyzed and distinguished by sonar system .The area of underwater acoustic attenuation has been researched for camouflaging submarine purpose for many years. There are two acoustic energy attenuation methods to reduce the reflective wave and transmitted wave. One is active attenuation control, which is to understand how the destructive interference of incident acoustic wave could be achieved for acoustic energy attenuation purposes. The other one is passive acoustic attenuation technique, which rely on the attenuation performance of underwater acoustic material to reduce the acoustic energy of incident wave. To be evaluated the acoustic absorption efficiency of material. Although the efficiency of active attenuation control is better compared with passive acoustic attenuation technique, the development of active attenuation control have not been highly pursued in the commercial market for underwater application, due to the limitations in piezo-composite technology. The cost of installation and maintenance is also higher in active control. This thesis studied the acoustic absorptive material based on passive acoustic attenuation technique . It could be attenuated the acoustic energy and spectrum of reflection and transmitted wave. Therefore, the signal can not be analyzed and distinguishing by sonar system. According to Alberich acoustic absorption coating, their designs have the inherent problem of degradation under hydrostatic pressure and temperature. Thus, the objective of this thesis is to study the characteristics of the acoustic absorptive material at various water depth where the hydrostatic pressure are different. To measure the characteristics of acoustic material, an experimental system is setup, and the standard measuring method and criterion is also studied for future experimental reference. Furthermore, the different measurement parameters are discussed for accuracy of experimental results. There are five specimens tested in this experiment. The specimens are mainly made of neoprene and sawdust mixture and marked as A1¡BA2¡BA3¡BA4¡Band A5 respectively. The composites of these specimens are analyzed by x-ray diffraction meter. The physical properties and the acoustic absorption in airborne were measured before underwater hydrostatic pressure applied on these specimens. The physical properties show that the impedance of these specimens is very close to acoustic impedance of the water. Therefore, the specimen may be considered an acoustic isolator in the air. To reduce the boundaries interference, such as reflection, diffraction and scattering signal. The pulse sound is used as sound source in this underwater experiment. Moreover, the gating system is applied to capture the proper signals for analysis. The echo reduction and insertion loss are measured in the 11 to 30 kHz frequency region for acoustic absorption evaluation in this experiment. The performance of experiment is found that specimen has the echo reduction about 10 dB and the insertion loss about 15 dB at 1 bar hydrostatic pressure. But when the hydrostatic pressure was increased to 5 bar, the echo reduction and insertion loss were both decreased by 3 dB. In addition, when the hydrostatic pressure was loaded at 10 bar, the echo reduction was decreased by 8 dB, and the insertion loss was decreased by 5 dB. It became evident that the efficiency of acoustic absorption is degraded under the higher hydrostatic pressure.

The internal tide in GPSC¡]Gaoping submarine canyon¡^is the main factor controlling the movement of watermasses. In order to improve our understanding on the compact of suspended sediment exchange in and out GPSC, the data used in this study are collected from four cruises of field observations using research vessel OR3. Instruments deployed include ADCPs¡BEK500¡BCTD and vertical string of temperature loggers. The collected data are analyzed through a variety of time series analysis technique, such as harmonic analysis¡BFFT and EOF. The results show that¡]1¡^the echo intensity recorded by ADCP through calibration could reduce the decay of echo with the distance. The results seem useful to apply in watermass behavior studies.¡]2¡^ Echo intensity with calibration were comparable with signal recorded by EK500 which could be validated to each other. These observations were related to sediment resuspension influenced by internal tide. ¡]3¡^There were two layers of large turbidity, at the depth of canyon edge and near the bottom of canyon, both were fluctuated with two interval tidal frequency.¡]4¡^Another band of echo intensity fluctuations, not directly correlate to sediment resuspension, was likely due to vertical migration, of zooplankton or biology effects.

The development of low-frequency sonar systems, using for instance a network of autonomous systems in unmanned vehicles, provides a practical means for bistatic measurements (i.e. when the source and receiver are widely separated) allowing for multiple viewpoints of the target of interest. Time-frequency analysis, in particular Wigner-Ville analysis, takes advantage of the evolution time dependent aspect of the echo spectrum to differentiate a man-made target (e.g. elastic spherical shell) from a natural one of the similar shape (e.g. solid). A key energetic feature of fluid loaded and thin spherical shell is the coincidence pattern, or mid-frequency enhancement echoes (MFE), that result from antisymmetric Lamb-waves propagating around the circumference of the shell. This thesis investigates numerically the bistatic variations of the MFE (with respect to the monostatic configuration) using the Wigner-Ville analysis. The observed time-frequency shifts of the MFE are modeled using a previously derived quantitative ray theory for spherical shell's scattering. Additionally, the advantage of an optimal array beamformer, based on joint time delays and frequency shifts (over a conventional time-delay beamformer) is illustrated for enhancing the detection of the MFE recorded across a bistatic receiver array.

Thesis (Ph. D.)--University of Missouri-Columbia, 2002. / Typescript. Vita. Includes bibliographical references (leaves 137-143). Also available on the Internet.