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Effets photo-induits coopératifs: du photomagnétisme sous irradiation continue aux phénomènes ultrarapides- étude par spectroscopie et diffraction X.

Abstract : The control with ultra-short laser pulses of the collective and concerted transformation of molecules driving a macroscopic state switching on an ultra-fast time scale in solid state opens new prospects in materials science. The goal is to realize at the material level what happens at the molecular level in femtochemistry. These processes are highly cooperative and highly non-linear, leading to self-amplification and self-organization within the material, a so-called photo-induced phase transition with a new long range order (structural, magnetic, ferroelectric...). Two families of molecular compounds have been studied here: first of all, spin transition materials changing from a diamagnetic state over to a paramagnetic state under the effect of temperature or under continuous laser excitation. It concerns photo-active molecular bistability prototype materials in solid state, whose switching has been studied during X-ray diffraction, optical reflectivity and magnetism experiments. Then we have studied charge-transfer molecular systems, prototype compounds for ultrafast photo-induced phase transitions: insulator-metal, neutral-ionic... As well as ultrafast optical experiments, time-resolved X-ray crystallography is a key technique in order to follow at the atomic level the different steps of the photo-induced transformation and thus to observe the involved mechanisms. We have underlined a process of photo-formation of one-dimensional nano-domains of lattice-relaxed charge-transfer excitations, governing the photo-induced phase transition of the molecular charge-transfer complex TTF-CA by the first time-resolved diffuse scattering measurements. Moreover, a new femtosecond laser-plasma source and a optical pump-probe spectroscopy set-up with a highly sensitive detecting system have been developed in this work. The results presented here will be an illustration of the present scientific challenges existing on the one hand with the development of projects of major scale (new ultrafast sources) and on the other hand with ultrafast photo-switching.
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Submitted on : Wednesday, April 25, 2007 - 8:00:00 AM
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David Glijer. Effets photo-induits coopératifs: du photomagnétisme sous irradiation continue aux phénomènes ultrarapides- étude par spectroscopie et diffraction X.. Physique [physics]. Ecole Polytechnique X, 2006. Français. ⟨pastel-00002257⟩

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