The corona poling process of electro-optic polymers has been
investigated for rodlike aromatic
polyimides doped with nonlinear optical chromophores. The dynamics
of the chromophores in poling
during and after imidization was observed by measuring second harmonic
generation (SHG) from the
polymer. In the course of poling during imidization, the SHG,
observed before imidization, decreases to
zero, reappears, and increases as the temperature increases. When
poling was after imidization, the
SHG intensity increased monotonically with increasing temperature.
The disappearance of SHG in the
poling during imidization is probably due to the known structural
change in the polymer chains during
imidization and the subsequent reorientation of the chromophores caused
by formation of sheets of
polyimides. The characteristics of the samples poled during and
after imidization are nearly the same
in both the extent of the nonlinear susceptibility and their thermal
stability.
A 6-dimensional grand unified theory with the compact space having the topology of a real projective plane, i.e., a 2-sphere with opposite points identified, is considered. The space is locally flat except for two conical singularities where the curvature is concentrated. One supersymmetry is preserved in the effective 4d theory. The unified gauge symmetry, for example SU(5) , is broken only by the non-trivial global topology. In contrast to the Hosotani mechanism, no adjoint Wilson-line modulus associated with this breaking appears. Since, locally, SU(5) remains a good symmetry everywhere, no UV-sensitive threshold corrections arise and SU(5)-violating local operators are forbidden. Doublettriplet splitting can be addressed in the context of a 6d N = 2 super Yang-Mills theory with gauge group SU(6). If this symmetry is first broken to SU(5) at a fixed point and then further reduced to the standard model group in the above non-local way, the two light Higgs doublets of the MSSM are predicted by the group-theoretical and geometrical structure of the model.
Optical and structural properties of glasses and glass-ceramics (GC) obtained by different heat-treatment of Tb and Tb-Yb doped sol-gel derived 30ZrO2-70SiO2 materials were investigated. A glass was formed after treatment at 700 °C whereas devitrification of the media after the treatment at 1000 and 1100 °C, led to the formation of GC containing up to three different crystalline phases, namely, tetragonal ZrO2, Yb-disilicate and cristobalite. The modification of the optical properties through the heat treatment was caused by redistribution of the rare earth elements (REE) among the different phases: both Tb and Yb entered the t-ZrO2 lattice, Yb can also be present in the form of a Yb2Si2O7 crystal. Devitrification led to an increase in Tb→Yb energy transfer efficiency as compared to the glass, though it was higher in the samples heat-treated at 1000 °C than in those treated at 1100 °C. The most intensive Yb3+ luminescence, induced by the energy transfer from the Tb3+ ion, was observed at the interface between t-ZrO2 and the glassy phases, due to the high concentration of REE in this area caused by the inability of ZrO2 to accept larger amounts of the REE. The mechanisms of the Tb→Yb energy transfer vary between different phases of the GC. The results obtained in this study are important for the development of spectral down-converters for potential solar energy applications based on Tb-Yb co-doped glass-ceramics.
In this study, Eu 3+-doped ZrO 2 nanophosphors were obtained by a solgel method with HNO 3 as a catalyst, which led to white powders of small ZrO 2 nanocrystals after low temperature calcination. However, exothermic reactions of NO 3 ¹ were observed during the heat-treatment leading to ZrO 2 crystallization. To avoid such reactions and to understand the initial crystallization of ZrO 2 doped with Eu 3+ ions, we developed an original route in which the xerogel powders were pre-washed in ethanol before immersing them in Eu 3+ solution to remove NO 3 ¹. The effects of heat-treatment on the Eu 3+ photoluminescence (PL), crystallization of ZrO 2 xerogels, and Eu 3+ localization were studied with PL spectroscopy, thermogravimetric analysis and powder X-ray diffraction. It was found that the amorphous ZrO 2 xerogels crystallized in a tetragonal structure, with a small amount of monoclinic ZrO 2 also being precipitated after longer calcination periods. Finally, the Eu 3+ ions on the surface of the ZrO 2 xerogels were found to diffuse into higher symmetric Zr substitutional sites in the tetragonal ZrO 2 matrix after heat-treatment.
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