Shin-Ichi Sakata

Two-photon absorption in binary Bi2O3-B2O3 glass at 532 nm

Measurements of two-photon absorption (TPA) coefficients β at 532 nm in binary Bi2O3:B2O3 glasses are reported. The β obtained ranges from 12.9 to 16.4 cm/GW with the larger value observed in higher Bi2O3 glass. The relationship between β and glass composition is discussed in terms of the electronic structure of glasses: β can be scaled with optical band gap.

Preparation of low-temperature Li3VO4 single crystal by floating zone technique

Abstract Single crystals of the low temperature phase β II Li 3 VO 4 were prepared by using a floating zone technique. Three problems, micro-cracks, accumulation of bubbles and the coloration of crystals, were faced during single crystal growth. These problems were investigated in relation with the initial molten zone composition, gas atmosphere and the purity of starting materials, respectively. The obtained crack-free, colorless and transparent β II Li 3 VO 4 single crystal had a diameter of 5 mm and was 5 mm long.

New types of SHG material, Li3VO4 and Li3PO4-type crystals

Abstract Some vanadate materials were investigated as second harmonic generation (SHG) materials for Nd: YAG lasers. Li 3 VO 4 and its derivative crystals, Li 2 ZnSiO 4 and Li 2 MgGeO 4 , were found as a new type of SHG crystal. Their crystal structure belongs to the Li 3 PO 4 type, which is formed of only MO 4 (M is Li, Zn, Si, Ge, V) oxygen tetrahedra. The SHG coefficients, d ij , of the Li 3 VO 4 single crystal was measured by the Maker fringe method and the non-linear coefficient was found to be of the same order as that of KTiOPO 4 . The origin of SHG in Li 3 VO 4 was discussed from its electronic and crystal structure. The covalency of Li 3 VO 4 is important for SHG and, further, the distorted and parallel oriented VO 4 tetrahedra enhance the SHG coefficient.

Development of SiAlON - From Mechanical to Optical Applications

Various rare-earth-doped α-SiAlON powders with high purity were prepared to study mechanical and optical properties of SiAlON-based functional materials in connection with ionic radius and electronic structure of rare-earth elements. Single phase rare-earth-doped α-SiAlON powders were obtained at a temperature as low as 1873 K by heating powder mixtures of rare-earth oxide, AlN and highly active ultrafine amorphous Si3N4. Bending strength of highly dense rare-earth-doped α/β-SiAlON-based ceramics was increased with decreasing radii of rare-earth ions, i.e., Yb-SiAlON-based ceramics exhibited excellent high-temperature strength and oxidation resistance caused by the small ionic radius of ytterbium. As for optical application, α-SiAlON is an excellent host lattice with good thermal and chemical stability for doping rare-earth element which activates photoluminescence. Europium-doped Ca-α-SiAlON phosphor formulated as CaxEuy(Si,Al)12(O,N)16 (where 0<x+y<2) was prepared to obtain high quality phosphor with high brightness and desired emission characteristics. Photoluminescence spectra of the resultant Europium-doped Ca-α-SiAlON exhibited high emission intensity at peak wavelength of 580-600 nm giving the better yellow color tone than Cerium-doped yttrium aluminum garnet for applying white LED. It was demonstrated that nitrides or oxynitrides were the innovative materials for the diverse range of high performance specialty applications.

New Types of Nonlinear Optical Materials: Li3VO4- and Li3PO4-Type Structures

Li3VO4- and Li3PO4-type structure crystals are found as new types of nonlinear optical materials for second-harmonic generation (SHG). Their lattices are formed of only the MO4 oxygen tetrahedra, being different from other typical nonlinear optical materials that have previously been developed.

Titanium Carbide Single-Crystal Tips for High-Resolution Scanning Tunneling Microscopy (STM)

Titanium carbide (TiC) single-crystal tips that were sharpened by cleavage have been used to observe the atomic images of the cleaved surfaces of graphite in air. The obtained atomic images were essentially the same as those observed by the commonly used tungsten and platinum tips. However, the TiC single-crystal tips were much better than the tungsten and platinum tips in that atomic images of high resolution could almost always be observed. It was also found that the TiC single-crystal tips were resistant to the intentional and accidental touching to samples. This indicates that the TiC single-crystal tips are suitable for mechanical microlithography of materials in the nanometer scale.

Electronic structure of second harmonic generation crystal Li3VO4

The origin of second harmonic generation (SHG) of Li3VO4 was investigated from the viewpoint of the band structure by using the tight‐binding method. The tight‐binding parameters were optimized to reproduce the density of states (DOS) obtained from x ray photoelectron spectroscopy and the optical band gap. Although Li3PO4 has the same crystal structure as Li3VO4, it shows no SHG. To explain the difference in optical nonlinearity we compared the electronic structures of Li3VO4 and Li3PO4, in particular at the bottom of conduction band (CB) and the top of valence band (VB), since they are known to play a primary role in SHG. In Li3PO4, the bottom of CB consists of P 3s and O 2p orbitals and the top of VB is composed of O 2p orbitals. These electronic structures result in a relatively low DOS at the bottom of CB and a wide band gap in Li3PO4. On the other hand, in Li3VO4, both bottom of CB and top of VB are composed of V 3d and O 2p. The preferential contribution of V 3d orbitals to the band edge states lead...

OPTICAL ABSORPTION AND ELECTRON SPIN RESONANCE STUDIES OF NONLINEAR OPTICAL CRYSTAL LI3VO4 GROWN BY THE FLOATING-ZONE METHOD

The optically nonlinear Li3VO4 crystal grown by the floating-zone (FZ) method has been characterized by optical absorption and electron spin resonance (ESR) spectroscopy. The as-grown single crystals grown in gaseous argon exhibit a broad absorption extending from the uv to near-infrared light regions and an absorption band at about 3 eV. Postoxidation reduces the whole optical absorption and reveals an additional 1.9 eV band. ESR detects several signals associated with transition-metal ions in as-grown and reduced crystals, i.e., Fe3+, Mn2+ and Cr3+ ions. The Mn2+ and Cr3+ signals are absent in well-oxidized crystals, while the Fe3+ signal shows no significant change. The reduced crystals have three absorption bands associated with transition-metal impurities and oxygen vacancies. The diffuse reflectance spectra of transition-metal-doped Li3VO4 polycrystals were also measured to examine the effect of the presence of Fe3+, Mn2+ and Cr3+. The origin of the optical absorption in as-grown, oxidized and reduced Li3VO4 are discussed in terms of unexpectedly doped iron-group transition-metals and oxygen vacancies. The 3 eV and 1.9 eV absorption bands and the broad band extending from the uv to near-infrared regions are suppressed in the well-oxidized crystals subjected to repeated zone refining, indicating that multiple zone refining and complete oxidation are important in obtaining optical-quality Li3VO4 single crystals.

Point defects in irradiated Li3VO4 single crystal

Radiation‐induced defects in a new second harmonic generation material, lithium vanadate single crystal, have been studied by electron paramagnetic resonance (EPR) and optical absorption measurements. EPR detects two intense signals due to intrinsic O− and impurity‐associated CO−3 trapped‐hole centers at 77 K, but no such signal in the case of trapped‐electron centers. The EPR signal of impurity Fe3+ ions is suppressed by 77 K irradiation. Low‐temperature irradiation produces three optical absorption bands peaking at 3.14, 2.0, and 1.56 eV. It has been concluded that some of the free electrons produced by irradiation are trapped by Fe3+ ions at low temperatures. After annealing at room temperature all the optical absorption bands disappear, although the EPR signals of CO−3 trapped‐hole centers and unidentified electron‐type centers at g=1.96 remain stable up to 330 K.

Tensile and bending behavior of melt growth Al2O3/YAG composite at ultra high temperatures (1773-2023K)

The deformation and fracture behavior at 1773-2023K of the unidirectionally solidified eutectic Al2O3/YAG (Yttrium-Aluminum Garnet with the composition of Y3Al5O13) ceramic composite was investigated. The stress-stain curve and strength of unnotched and notched specimens, measured by bending and tensile tests, showed that (a) both unnotched and notched specimens fractured in a brittle manner at low temperatures and at high displacement speeds, but in a ductile manner at high temperatures and at low displacement speeds, and (b) the notched strength increased, reaching maximum, and decreased with increasing temperature and decreasing displacement speed. The increase in the notched strength with increasing temperature and decreasing displacement speed up to the maximum value was accounted for by the increase in plastic zone size ahead of the notch, and the decrease with further increasing temperature and decreasing displacement speed by the loss of the stress carrying capacity of the yielded ligament, based on the finite element analysis.