426 research outputs found

    Analysis of Dynamic Brain Imaging Data

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    Modern imaging techniques for probing brain function, including functional Magnetic Resonance Imaging, intrinsic and extrinsic contrast optical imaging, and magnetoencephalography, generate large data sets with complex content. In this paper we develop appropriate techniques of analysis and visualization of such imaging data, in order to separate the signal from the noise, as well as to characterize the signal. The techniques developed fall into the general category of multivariate time series analysis, and in particular we extensively use the multitaper framework of spectral analysis. We develop specific protocols for the analysis of fMRI, optical imaging and MEG data, and illustrate the techniques by applications to real data sets generated by these imaging modalities. In general, the analysis protocols involve two distinct stages: `noise' characterization and suppression, and `signal' characterization and visualization. An important general conclusion of our study is the utility of a frequency-based representation, with short, moving analysis windows to account for non-stationarity in the data. Of particular note are (a) the development of a decomposition technique (`space-frequency singular value decomposition') that is shown to be a useful means of characterizing the image data, and (b) the development of an algorithm, based on multitaper methods, for the removal of approximately periodic physiological artifacts arising from cardiac and respiratory sources.Comment: 40 pages; 26 figures with subparts including 3 figures as .gif files. Originally submitted to the neuro-sys archive which was never publicly announced (was 9804003

    Flipping Biological Switches: Solving for Optimal Control: A Dissertation

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    Switches play an important regulatory role at all levels of biology, from molecular switches triggering signaling cascades to cellular switches regulating cell maturation and apoptosis. Medical therapies are often designed to toggle a system from one state to another, achieving a specified health outcome. For instance, small doses of subpathologic viruses activate the immune system’s production of antibodies. Electrical stimulation revert cardiac arrhythmias back to normal sinus rhythm. In all of these examples, a major challenge is finding the optimal stimulus waveform necessary to cause the switch to flip. This thesis develops, validates, and applies a novel model-independent stochastic algorithm, the Extrema Distortion Algorithm (EDA), towards finding the optimal stimulus. We validate the EDA’s performance for the Hodgkin-Huxley model (an empirically validated ionic model of neuronal excitability), the FitzHugh-Nagumo model (an abstract model applied to a wide range of biological systems that that exhibit an oscillatory state and a quiescent state), and the genetic toggle switch (a model of bistable gene expression). We show that the EDA is able to not only find the optimal solution, but also in some cases excel beyond the traditional analytic approaches. Finally, we have computed novel optimal stimulus waveforms for aborting epileptic seizures using the EDA in cellular and network models of epilepsy. This work represents a first step in developing a new class of adaptive algorithms and devices that flip biological switches, revealing basic mechanistic insights and therapeutic applications for a broad range of disorders

    Homodyne detection for laser-interferometric gravitational wave detectors

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    Gravitational waves are ripples of space-time predicted by Einstein\u27s theory of General Relativity. The Laser Interferometer Gravitational-wave Observatory (LIGO), part of a global network of gravitational wave detectors, seeks to detect these waves and study their sources. The LIGO detectors were upgraded in 2008 with the dual goals of increasing the sensitivity (and likelihood of detection) and proving techniques for Advanced LIGO, a major upgrade currently underway. As part of this upgrade, the signal extraction technique was changed from a heterodyne scheme to a form of homodyne detection called DC readout. The DC readout system includes a new optical filter cavity, the output mode cleaner, which removes unwanted optical fields at the interferometer output port. This work describes the implementation and characterization of the new DC readout system and output mode cleaner, including the achieved sensitivity, noise couplings, and servo control systems

    Distributed photovoltaic systems: Utility interface issues and their present status. Intermediate/three-phase systems

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    The interface issues between the intermediate-size Power Conditioning Subsystem (PCS) and the utility are considered. A literature review yielded facts about the status of identified issues

    Polarisation properties of exciton-polaritons in semiconductor microcavities

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    Interactions of exciton-polaritons in semiconductor microcavities and the resulting polarisation dynamics are investigated theoretically. Within the coboson framework of polariton-polariton scattering, it is shown that the matrix element of direct Coulomb scattering is proportional to the transferred momentum, q, cubed in the limit of small q. In the same limit, the magnitude of superexchange/exchange interactions can be considered constant. These results are applied to the elastic circle geometry, where a system of equations describing the steady-state pseudospin components is derived. It is shown, that for this geometry, polariton-polariton scattering can account for the generation of circularly/linearly polarised final states from linearly/circularly polarised initial states, depolarisation and the generation of spin currents. In the low density regime polaritons are good bosons and the dynamics of polariton Bose-Einstein condensation (BEC) are investigated. A stochastic model is derived, resulting in a Langevin type equation describing the time dynamics of the condensate spinor order parameter. The build up in condensate polarisation degree is shown to evidence macroscopic ground state population, while the stochastic choice of polarisation vector evidences the symmetry breaking nature of the phase transition. The decrease of polarisation degree above threshold is demonstrated to be a consequence of polariton-polariton interactions, a result which is complemented by recent experimental work. The stochastic model is extended to include Josephson coupling of spatially separate condensates. The coupling results in polarisation and phase correlations between the condensates, explaining the polarisation locking and spatial coherence seen experimentally. Finally, the effect of polarisation pinning by local effective fields is examined

    Emergence of Collective Light Scattering in Atomic \u3csup\u3e87\u3c/sup\u3eRb Samples

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    Over the past half century, atomic ensembles have been used to create sensors, clocks, and quantum information systems. As these devices become more compact, and as the number of atoms increases to improve the sensitivity for detection, the atomic samples are increasing in density and optical depth. As such, the spectroscopic properties of the atomic media are modified due to interactions among the particles in the ensemble. We report investigation of near-resonance light scattering from a cold atomic sample of 87Rb. Initially prepared in a magneto-optical trap, the atoms are loaded into a far-off-resonance optical dipole trap (FORT) in which the ensemble has a temperature near 100 mK and initial Gaussian radii of approximately 3 mm and 280 mm in the transverse and longitudinal directions, respectively. With atomic densities in the range of 1010 - 1013 atoms/cm3, measurements are made on the F=2 -\u3e F\u27=3 nearly closed hyperfine transition. The experimental geometry consists of projecting a near-resonance collimated laser beam onto the entire volume of the FORT and detecting the diffusely scattered light. The measured scattered light intensity as a function of detuning, atomic number, and sample size suggests that collective light scattering depends on the optical depth of the system

    Optimisation of nonlinear photonic devices: design of optical fibre spectra and plasmonic systems

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    El propósito de esta tesis es diseñar y optimizar dispositivos fotónicos en el régimen no lineal. En particular, se han elegido dos tipos de dispositivos, que se clasifican según los fenómenos físicos de interés. La primera clase corresponde a fibras convencionales o de cristal fotónico, diseñadas para que la dinámica temporal de los paquetes de onda que se propagan en su interior genere espectros con las características deseadas, en el contexto del supercontinuo. La segunda clase explota la fenomenología espacial asociada a las ondas electromagnéticas que se propagan sobre la superficie de un metal. Estas ondas permiten, desde diseñar dispositivos tipo chip fotónico cuyas dimensiones típicas están muy por debajo de la longitud de onda de la luz, hasta la generación de estados no lineales híbridos de dinámica singular. Todos estos efectos tienen lugar dentro del marco proporcionado por las ecuaciones de Maxwell macroscópicas, las cuales han sido resueltas numéricamente. En algunos casos se emplean grandes aproximaciones teóricas para estudiar sistemas 1D, mientras que en otros se integran directamente en 3D. En el caso en el que la optimización del dispositivo resulta no trivial tras haber adquirido un conocimiento teórico profundo del mismo, se emplea una novedosa herramienta numérica que nace de la combinación de algoritmos genéticos con plataforma Grid.Milián Enrique, C. (2012). Optimisation of nonlinear photonic devices: design of optical fibre spectra and plasmonic systems [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/14670Palanci
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