By examining continuous data from the Poker Flat Incoherent Scatter Radar (PFISR) in Poker Flat, Alaska, short-lived enhancements in the F-region ion temperature, or "Tᵢ spikes", were discovered in the evening while the radar was on the equatorward edge of the high latitude convection region. These enhancements were several hundred Kelvin above the background temperature, would last less than 15 minutes and were preceded by sharp depletions in plasma density (of roughly one half). Though they were mostly detected in the summer, 25 events throughout a whole year of data were identi ed in which the spike occurred within 1.5 hours of the density drop. By examining the location of PFISR at the time of the enhancements, as well as the conditions under which these spikes occurred, it was concluded that these enhancements were the result of electric elds increasing the frictional heating between ions and neutrals. By then examining geophysical data, it was found that these events were temporal and related to changes in magnetic indices. One possible explanation for the observations is that the electric eld is at its strongest near the plasmapause during substorms. Another more likely possibility is that during substorms the region of sunward ion convection expands into a region in the evening side where the neutral gas moves in a direction opposite to the ions, thereby enhancing the frictional heating rate.

An explicit algorithm for the extrapolation of one-way wavefields is proposed which combines recent developments in information theory and theoretical signal processing with the physics of wave propagation. Because of excessive memory requirements, explicit formulations for wave propagation have proven to be a challenge in {3-D}. By using ideas from ``compressed sensing'', we are able to formulate the (inverse) wavefield extrapolation problem on small subsets of the data volume, thereby reducing the size of the operators. According {to} compressed sensing theory, signals can successfully be recovered from an imcomplete set of measurements when the measurement basis is incoherent} with the representation in which the wavefield is sparse. In this new approach, the eigenfunctions of the Helmholtz operator are recognized as a basis that is incoherent with curvelets that are known to compress seismic wavefields. By casting the wavefield extrapolation problem in this framework, wavefields can successfully be extrapolated in the modal domain via a computationally cheaper operatoion. A proof of principle for the ``compressed sensing'' method is given for wavefield extrapolation in 2-D. The results show that our method is stable and produces identical results compared to the direct application of the full extrapolation operator.

La mise en forme de lumière blanche incohérente à l’aide d’un nouveau composant est proposée dans cette thèse. L'objectif était de réaliser une carte d'irradiance arbitraire sur un écran à l’aide d’une structure originale contenant seulement des facettes, légèrement inclinées par rapport à la direction d'origine selon leurs axes propres. Une approche basée sur l’optique géométrique a été utilisée pour concevoir et analyser la structure proposée. Celle-ci est constituée d’une matrice de facettes jouant le rôle de déflecteur. Nous avons étudié le cas en transmission et en réflexion. L’obtention des angles d’inclinaison s’est faite avec deux approches différentes : l'optimisation sous Zemax et le calcul analytique. Plusieurs critères de qualité ont été proposés pour comparer la carte d'irradiance. Le tolérancement a démontré qu’il est plus intéressant de travailler en transmission qu’en réflexion. Une réalisation a été faite avec succès en utilisant une technologie additive innovante. / Beam shaping of incoherent white light with a large spectrum is proposed in this PhD thesis. The objective was to realize an arbitrary irradiance map on a target plane using a faceted structure. To maintain the design result within the geometrical optics domain, large facet element dimensions are required to obtain usable results. Each facet element can slightly tilt along its own axes to deflect the incident light, either by reflection, either by transmission. The calculation of the tilt angles is made by an analytical approach, and also by automatic optimization with Zemax. Several quality factors are proposed in order to qualify the illumination/irradiance chart on the screen. Because of the required tolerances on the fabrication technique, we show that it is more interesting to design a transmissive structure than a reflective one. With a new additive technology, a structure is realized successfully, showing the interest of the concept.

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A técnica de eco de fótons com luz temporalmente incoerente (EFLI) foi utilizada neste trabalho para a medida do tempo de relaxação transversal T2 do Iodeto de 3-3´-Dietiloxadicarbonocianina (DODCI) como função da temperatura. Nestes experimentos foi utilizado um laser de corante de banda larga, bombeado pelo 2&#176 harmônico de um laser de Nd+3 : YAG Q-switched. Este laser operou com os corantes Kiton Red 620 e rodamina 640, cujos máximos do espectro de emissão estão respectivamente em 598 e 610 nm. O tempo de relaxação T2, que é proporcional ao inverso da largura de linha homogênea, segue uma dependência funcional com a temperatura do tipo T-1,9. Encontramos o valor de T2 entre 0 e 30fs para &#955 = 598nm e entre 30 e 590fs para &#955 = 610nm, no intervalo de temperatura entre 300 e 60k. Os perfis das medidas de EFLI podem ser descritos por um modelo baseado num sistema quântico de dois níveis / The photon echo with incoherent light technique (EFLI) has been used in this work for the measurement of the transverse relaxation time T2 in 3-3´-Dietiloxadicarboncyanine Iodide (DODCI) as a function of the temperature. A broad-band dye laser, pumped by the second harmonic of a Q-switched Nd+3 : YAG laser, was used in this experiment. The laser used Kiton Red 620 and rodamine 640 dyes, whose maxima output power are respectively around 598 and 610nm. The relaxation time T2 , which is inversely proportional to the homogeneous linewidth, depends on the temperature according to a T-1,9 Law. We found the value of T2 ranging from 0 to 30fs at 598nm and from 30 to 590fs at 610nm in the temperature range between 300 and 60K. The EFLI profiles can be described by means of a two-level quantum system model

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In spite of being the subject of intense research, several key but complex questions on the nonequilibrium physics of correlated quantum systems remain controversial. For example, the nature of particle and energy transport in different interacting regimes, the relevance of integrability and the impact of environmental coupling are still under active debate. These problems can now be approached numerically, due to the development of powerful algorithms which allow the efficient simulation of the dynamics of correlated systems. In the present thesis we study numerically and analytically the transport properties of low-dimensional quantum systems. In particular, we consider the steady-state spin and energy conduction through XXZ boundary-driven spin-1/2 chains. In the first part, we analyse the transport through chains with only coherent processes in the bulk. For spin transport induced by a magnetisation imbalance between the boundaries, previously identified ballistic, diffusive and negative differential conductivity regimes are reproduced. We provide a comprehensive explanation of the latter. The energy conduction induced by this driving scheme features the same properties as spin transport. For thermally-driven chains, we discuss the nature of energy transport and the emergence of local thermal states when the integrability of the Hamiltonian is broken. In the second part of the thesis we analyse the effect of bulk incoherent effects on the transport properties previously discussed. First we find that for weak particle-particle interactions, pure dephasing degrades spin and energy conduction. In contrast, for strong interactions dephasing induces a significant transport enhancement. We identify the underlying mechanism and discuss its generality. Finally, motivated by the lattice structure of several organic conductors, we study the interplay between coherent and incoherent processes in systems of weakly-coupled chains. We find an enhancement effect due to incoherent interchain hopping, stronger than that by dephasing, which increases with the chain length and relates to superdiffusive transport.

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