Yasutomo Arai

Refractive index dispersion, optical transmittance, and Raman scattering of BaTi2O5 glass

The optical and vibrational properties of BaTi2O5 glass prepared by containerless processing were investigated. The refractive index was 2.15 and the Abbe number was 21.5, as measured by the focal method. The glass was transparent from 340 nm to 7.7 μm. From the Raman scattering spectrum, the maximum phonon energy was found to be 829 cm−1. Using these data, fundamental optical parameters such as the optical basicity, the average oscillator strength, and the optical band gap were estimated. These parameters suggest that BaTi2O5 glass is an attractive material with many potential applications such as lenses, windows, nonlinear optics, and phosphors; moreover, these parameters can guide future development of new titanate glass compositions with superior optical properties.

Extrinsic origin of giant permittivity in hexagonal BaTiO3 single crystals: Contributions of interfacial layer and depletion layer

Three dielectric relaxations in hexagonal (h)-BaTiO3 single crystals exhibiting giant permittivity were detected in a frequency range of 100Hz–3GHz and analyzed by an equivalent circuit with three parallel RC elements. A best-fit result indicated that the three dielectric relaxations were the responses of bulk crystal with a capacitance of 1pF, an interfacial layer with a capacitance of 1.4nF, and a depletion layer with a capacitance of 1nF. We confirmed that a giant permittivity exceeding 105 could be achieved by the interfacial layer in the h-BaTiO3 crystal. In addition, a Schottky barrier height at the contacting interface between Cu electrodes and the h-BaTiO3 surface was estimated as 1.56eV from the voltage dependence of capacitance.

Refractive index calculation using the structural properties of La4Ti9O24 glass

A high refractive index of 2.231±0.001 at 1313 nm is achieved in a colorless, transparent spherical La4Ti9O24 glass that was prepared by using an aerodynamic levitation furnace. We also investigate the origin of the high refractive index based on the Lines theory. The physical parameters necessary for the theoretical calculations are the averaged nearest-neighbor Ti–O and La–O distances that are deduced by neutron and synchrotron x-ray diffraction measurement and the partial molar volumes of TiO2 and LaO1.5 in the glass. The sum of the calculated TiO2 and LaO1.5 susceptibilities provides a La4Ti9O24 glass refractive index of 2.244. This is very close to the experimental refractive index of the glass. In addition, the theoretical calculation results suggest that the oxygen fivefold Ti (TiO5) cluster produces greater susceptibilities than the sixfold Ti (TiO6) cluster in the glass.

Thermal Stability and Optical Properties of Er3+ Doped BaTi2O5 Glasses

Er3+ doped BaTi2O5 spherical glasses were prepared using a containerless processing with an aerodynamic levitation furnace. These glasses were found to have high solubility of Er3+ and the substitution ratio of Er3+ for Ba2+ reached 30 %. Differential scanning calorimetry measurements revealed that the glass transition temperature Tg and the crystallization onset temperature Tx shifted to high temperatures and the value of T (= Tx – Tg) increased with substitution of Er3+, indicating that the stability against crystallization improved. We found that these glasses had high refractive indices exceeding 2.1 with low Abbe numbers of approximately 20.

Optical Transmittance and Band Gap of Ferroelectric BaTi2O5 Bulk Glass

Optical transmittance and reflectance on ferroelectric BaTi2O5 glasses prepared recently by a containerless synthesis technique are measured at room temperature in the wavelength range 190–800 nm. The fundamental absorption edge located around 340 nm demonstrates the colourless and transparent character of the glass. The optical band gap of 3.32eV has been estimated. The tail of the optical absorption near the fundamental absorption edge is found to follow the Urbach rule. Our analysis of the experimental spectra supports an indirect allowed interband transition between the valence band formed by O-2p orbitals and the conduction band formed by Ti-3d orbitals.

Charge Density Study on Phase Transition in BaTi2O5 Ferroelectric

The accurate crystal structure of BaTi2O5 ferroelectric has been derived at the charge density levels from 100 to 900 K in both the paraelectric and ferroelectric phases. High-energy synchrotron-radiation powder-diffraction data were taken and analyzed by the maximum entropy method (MEM)/Rietveld method. Three independent Ti–O6 octahedra in the structure of BaTi2O5 are distorted, two of which can be regarded as having five-oxygen coordination in both phases. The structural change associated with the ferroelectric phase transition is characterized by the occurrence of extra distortion in Ti1–O6, which is the least distorted octahedron above TC. The spontaneous polarization calculated from the determined crystal structure is Ps = 15 µC/cm2 at room temperature. The contribution of the distortion of Ti1–O6 to Ps is about 80%.

Spherical sapphire single-crystal synthesis by aerodynamic levitation with high growth rate

This article describes a technique for forming spherical Al2O3 single crystals by an aerodynamic levitator by applying the unidirectional solidification method. A levitation method intrinsically produces a spherical material. In addition, both the focused laser beam heating and the gas flows to the sample trapped in the levitator generate a steep temperature gradient in the molten sample, enabling the sample to grow with high growth rate. Spherical sapphire single-crystal production consists of two processes. First step is to solidify the molten sample unidirectionally from polycrystalline parts at the bottom of the sample. This sample was almost a single crystal with polycrystalline parts remaining just around the bottom. The crystal is then revolved before the second step. In the second step the revolved crystal was remelted from the polycrystalline part and solidified from the seed, resulting in complete single-crystal production. The x-ray Laue and crossed nicol experiment results for the Al2O3 spheri...

Structural-relaxation-induced high refractive indices of Ba1−xCaxTi2O5 glasses

Ba1−xAxTi2O5 (A = Mg2+, Ca2+, or Sr2+) spherical glasses were prepared by containerless processing with an aerodynamic levitation furnace. The glass-forming region was x ≤ 0.05 for A = Mg2+ and Sr2+, but it ranged up to x = 0.90 for A = Ca2+. The impact of this exceptionally large amount of Ca2+ substituted for Ba2+ on the optical properties of high refractive index of BaTi2O5 glass was investigated. Although all glasses were colorless and transparent, the optical band gap decreased as the Ca2+ content increased. The refractive index increased linearly from 2.10 to 2.18 with increasing Ca2+ substitution, while the Abbe number decreased. The optical parameters estimated from the experimental results indicated that Ca2+ substitution had a direct impact on the Ti–O bonding state and relaxed the distorted Ti–O polyhedra, which was confirmed by Raman scattering spectra. These results demonstrate that changing the local structure by substituting Ca2+ for Ba2+ can control the physical properties of high refractive index BaTi2O5 glass. The improved physical properties of Ba1−xCaxTi2O5 spherical glasses suggest that such glasses are suitable for making small optical components.

Structure and Physical Properties of Metastable Hexagonal LuFeO3

Metastable Lu x FeO 3 polycrystals (0.65 < x < 1.35) have been synthesized by containerless processing. Synchrotron-radiation powder diffraction analysis and scanning electron microscope observation have shown them to be in three metastable phases. The crystal structure of one of these phases was assigned to a polar space group P6 3 cm with the composition of LuFeO 3 , which has a similar structure as that of hexagonal YMnO 3 . A characteristic dielectric relaxation with strong frequency dispersion of the hexagonal LuFeO 3 was observed above room temperature.

Giant Second Harmonic Generation from Metastable BaTi2O5

Giant second harmonic generation (SHG) was observed while metastable α-BaTi2O5 crystallized from BaTi2O5 glass. The observed SHG confirmed that α-BaTi2O5 had a large polarization and a non-centrosymmetric structure. The optical anomaly suggested that the alignment of polar nanocrystals of α-BaTi2O5 was the cause of the observed giant dielectric response. Furthermore, the giant SH intensity of α-BaTi2O5 could be obtained at room temperature by quenching the crystallized α-BaTi2O5. The thermal stability of α-BaTi2O5 was evaluated by in situ monitoring of the SH intensity above the crystallization temperature. These results demonstrate that α-BaTi2O5 is a promising material for electro-optical applications, even at high temperatures.