Etude théorique et expérimentale de transformations de phase par diffusion dans des aciers sous champ magnétique intense

Abstract : The lithium niobate (LN) is one of the most interesting dielectrical materials for the applications in integrated optics because of its broad range of transparency and its great electro-optical, acousto-optical and nonlinear optical coefficients. The whole of these properties allows in particular the realization of weak losses (0.1 dB/cm) optical waveguides in the wavelengths of telecommunications, of electro-optical integrated modulators with weak order tension, and of integrated second harmonic generator. Our study carries out to characterization of the LN bulk crystals and waveguides by Raman spectroscopy and luminescence polaron. In its congruent composition ([Li] = 48,6 mol{rc) the LN presents specific defects of several species, in particular of the niobium ions in lithium site called antisites (Nbu). The concentration of these defects depends on the composition of the crystal. We studied in particular bulk crystals of composition under congruent ([Li]<48,6 mol%o) and prolonged the linear law which connects the widths of certain Raman lines to the composition of the LN. This law then enables us to finely measure, with the micrometrical scale, the out-diffusion of lithium in the diffused guides titanium and their vicinity. The luminescence of the polaron is observedi n wide band of -700-950nm. This defect corresponds to the state of charge 4+ of the antisite (Nbtio*). The intensity of luminescence polaron depends directly with the degree of chemical reduction of material. The lifetime of luminescence is an important parameter,w hich was measured by a pulsated laser. We also showed the relation which exists between luminescence and the incidental intensity and shown the existence of a value of saturation. Luminescence polaron was studied in crystals of nominally pure, oxidized and reduced bulk LN, then in the waveguides produced by titanium diffusion. We defined the protocols of characterization and the exploitation of measurements allowing to determine the chemical reduction carte of a LN:Ti waveguide and of its vicinity starting from the carte of luminescence. We finally extended the method to the periodically polarized structures (PPLN). In conclusion, we showed that the micro-spectroscopies of Raman diffusion and luminescence polaron are nondestructive, effective and reliable methods to characterize the waveguides and structures PPLN with the scale of the micrometer.
Mots-clés : Acier
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Yudong Zhang. Etude théorique et expérimentale de transformations de phase par diffusion dans des aciers sous champ magnétique intense. Autre [cond-mat.other]. Université Paul Verlaine - Metz, 2004. Français. ⟨NNT : 2004METZ019S⟩. ⟨tel-01750143⟩

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