Smectite, a group of clay minerals, is an interesting material for its strange character as an inorganic material. Its characterization is, however, not yet completed in detail because smectite is commonly low crystalline and fine particle. It has never been found in single crystals. Recently single crystals of smectite have been successfully synthesized from the glass of dehydroxylated Na-mont-morillonite composition under water-saturated conditions at 3-5.5 Gpa and above 1000℃. The smectites were identified by powder X-ray diffraction measurements after the intercalation of ethylene glycol and also in-situ XRD observation at different relative humidities. The thin flakes of smectite dissociated by water were examined by transmission electron microscopy and recognized to be a single crystal in the forms of thin ribbon and lath.
The step growth of silicon on the Si (111) 7×7 surface has been considered within the framework of the dimer-adatom-stacking fault reconstruction model. The atomic steps are classified into U and F steps and growth models of each step have been proposed. Structures and shapes of the growing steps observed with scanning tunneling microscope (STM) and transmission electron microscope (TEM) in the literature are well explained by the proposed model. It is concluded that cancel of the reconstruction of the substrate surface is the most important and rate determing step for the epitaxial grwoth at the steps.
Refractive indices which was measured by means of laser interferometory, were varied along the growth direction. Variation of refractive indices was approximately 1.6×10^<-4> for n_z, and gave rise to change of the phase matching direction up to 0.38 degree. Lattice parameter which was mesured by Bond method X-ray diffraction method, was found to be varied in relation to the change of refractive indices. The reasons about the change of refracive indices and the growth techniques for obtaining the crystals of uniform refractive indices are discussed.
Suitable conditions in LiNbO_3 (LN) single crystal growth by the continuous charging Czochralski (CC-CZ) method were studied, such as the thermal field affected by the vertical position of the inner partition cylinder, mixing situation of the charged material with the melt by means of Fe_2O_3 addition as a tracer and temperature fluctuation in melt affected by the particle size of the charging material. The LN crystal grown from 60 mol% Li_2O melt was annealed in air. Li_3NbO_4 phase appeared as precipitate in the LN. The fact suggests that the Li-sufficient boundary of LN solid solution range is not vertical straight line but curve line toward the Li-deficient side such as the Li-deficient boundary in the phase diagram of the Li_2O-Nb_2O_5 system. The effective nonliner optical coefficient increased as Li content in melt increased. The photore-fraction dependence on Li content in melt with the range from 48.6 to 60 mol% Li_2O was investigated. Birefringence change had a maximum value at 58 mol% Li_2O in melt, which was near the stoichiometric composition. The dependence was not changed by annealing treatment. Therefore the photorefraction depend on the photoconductivity caused by vacancies rather than the Glass constant in the Li-rich melt range.
The properties of Zn doped LiNbO_3, which resits the photorefractive damage, are reviewed mainly from our study. The optically induced change in the birefringence of Zn: LiNbO_3 with 6.6 mol% doping or more can be reduced two orders of magnitude than that in a non-doped crystal. The optical absorption in the ragne of 400-500 nm is reduced and more over the edge is pushed back into the shorter wavelength than non-doped LiNbO_3. These characteristics of Zn: LiNbO_3 resemble to those of Mg: LiNbO_3. The value of birefringence itself and the dependence on the dopant, however, are different from those of Mg: LiNbO_3. The ordinary refractive index is in-creased with Zn content, and the extraordinary one shows nonmonotonic dependence. The change of some physical properties (lattice constant, Curie temperature, extraordinary index, and phase-matching temperature) with a higher concentration of 4.5 mol% of Zn suggests the different siting of Zn in LiNnO_3.
High-quality scandium-doped LiNbO_3 single crystals were grown by the Czochralski technique from congruent melts containing 0.4-4.0 mol%Sc. At a scandium concentration of 1.3mol%, the Curie temperature, refractive indices and absorption edge shift displayed anomalous behavior as a function of scandium concentration. Qualitatively, the behavior was similar to that observed in Mg: LiNbO_3, except the optical damage level increased with increasing scandium concentration. The similarities and differences between Mg and Sc-doped LiNbO_3 are investigated.
LiNbO_3 single crystals with various [Li]/[Mb] ratios were grown by the Czochralski method from melts having compositions varying between 45.5-58.0 mol% Li_2O. From X-ray topography and X-ray rocking curves, the crystal quality of off-congruent crystals was lower than that of the congruent crystal. The optical damage resistance decreases as the crystal composition approaches the stoichiometric value. Also, LiTaO_3 single crystals with low and high purity raw materials and doped with MgO were grown by the Czochralski method. The behavior of the photo-induced birefringence changes were similar to the LN crystal cases, but their damage resistances were weaker than that of LN crystals of the same grade.
A continuous, room temperature laser oscillation at 1.063μm is reported for the first time in Nd-doped ferroelectric LiTaO_3 single crystals, pumped with a Kr laser. A relatively low threshold and high slope-efficiency operation are obtained by using a congruent crystal with a Nd concentration of 〜5×10^<19>/cm^3. Any lasing degradation is not observed, when poled single domain crystals are used. The origin of light scattering is very small ferroelectric do-mains inherent to LiTaO_3 crystals and it is clearly demonstrated the effect of a poling condition on lowering a lasing threshold pump power. The minimum threshold is 9 mW of the absorbed pump power.
5 mol% MgO doped LiNbO_3 thin film optical waveguides were fabricated by liquid phase epitaxy on Z-cut LiTaO_3 substrates using Li_2O-V_2O_5 and Li_2O-B_2O_3 flux systems to investigate a dependence of optical proper-ties on melt composition. Lattice mismatches between LiNbO_3 thin film and LiTaO_3 substrate were measured 5.5×10^<-3> A by X-ray powder deffractometer. Na was add-ed to the flux system to achive lattice matching. 5 mol% MgO doped LiNbO_3 thin films with good surface morpholgy, high crystallinity, nearly stoichiometric singl crystal and single frerroelectric domain were grown on Z-cut LITaO_3 substrates by a flat one-side dipping LPE method. The optical waveguide had a step index for the ordinary and the extraordinary waves. Using Li_2O-B_2O_3 flux system, the optical waveguide with high optical damage resistance and low optical absorption at visible wavelength was obtained.