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Etude des perturbations optiques et du crazing dans les cristaux de DKDP

Abstract : KDP (KH2PO4) and deuterated DKDP (K(H1-xDx)2PO4) crystals are widely used for their nonlinear optical properties as frequency converters or Pockels cells. The development of a system for rapid growth under stationary conditions led to obtain DKDP crystals in shorter time with homogeneous distribution of deuterium. However, the first uncoated optics cut from DKDP crystals grown with such system showed significant perturbations of the laser wave front (optical distortions) as well as crazing (formation of cracks on the surface due to the isotopic H/D exchange), which may render problematic the utilization of these crystals as optics for high energy lasers. Recently, it has been shown that crazing could be promoted by the presence of silicate impurities incorporated into specific parts of the crystal: the vicinal sectors, shallow. To obtain DKDP crystals with optimal optical properties it was thus of major importance to determine the origin of the optical distortions, their eventual correlation with crazing and identify the mechanism responsible: nature of impurity, its gradient between sectors and what part does the isotopic composition play. We have confirmed for KDP the direct correlations observed in DKDP between vicinal sectors, contrasts in X-ray topography and optical distortions. Complications specific to DKDP related to deuterium losses were observed: crazing and surface alteration takes place only in the vicinal sectors previously identified. So even if inhomogeneous deuterium losses in DKDP may cause optical distortion by itself and should thus be avoided in coated optics, other causes exist. Hence, gradients of traces impurities inside the crystal appear to be at the origin of several phenomena leading to optical distortions. To improve optical quality, growth parameters, as temperature, supersaturation and pH, are modified to identify optimal conditions. X-ray topography obtained for each parameters demonstrate that a crystal grown at acid pH havs a very low density of defects. Simultaneously, ICP-MS is used to identify impurity incorporating inside vicinal sector shallow, revealing a correlation between Ca2+ concentration and structural defects in X-ray topography.
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Juliette Piquard. Etude des perturbations optiques et du crazing dans les cristaux de DKDP. Chimie organique. Université Grenoble Alpes, 2017. Français. ⟨NNT : 2017GREAI097⟩. ⟨tel-01725390⟩

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