Nobuhito Ohno

High pressure densification of lithium silicate glasses

Abstract We have carried out neutron diffraction and Raman scattering measurements on the lithium disilicate glass densified by the application of high pressure. The density of the glass increased monotonically with applied pressures up to 6 GPa. A shift of the first sharp diffraction peak (FSDP) toward larger Q vectors was found in the total structure factor S ( Q ) of the neutron diffraction spectra. The Si–O bond length in an SiO 4 tetrahedron elongated by about 0.001 nm after densification under 6 GPa, while the nearest O–O distance did not change. However, an increase in distribution of the O–O distance was found after the densification. A low energy shift of 1080 cm −1 band, which is assigned to Si–O stretching mode, was observed in Raman scattering spectra of the densified glasses. It is consistent with the increase of Si–O bond length. We deduced from these results that the densification is caused by an increase of packing density of SiO 4 tetrahedron, which is accompanied by a distortion of the tetrahedron.

Exciton Transitions in Indium Halides

Reflection, absorption and emission spectra of InBr, InI and InBr 1- x I x mixed crystals have been measured at 2 K by using polarized light. In InBr, a prominent exciton peak is found at 2.48 eV for the polarization E // b , which is on the high energy side of the lowest direct exciton peak at 2.33 eV allowed for E // c . Three square root steps due to indirect excitons have been observed for E // b in contrast to four steps for E // c . Six momentum conserving phonons are confirmed. Luminescence from free and shallow bound indirect excitons is identified in InBr. The first exciton peak in InBr 1- x I x mixed crystals shifts continuously from InI to InBr. Indirect-direct crossover at x =0.5 leads to the conclusion that the lowest energy band gap of InI is of direct type. The indirect transition in InBr is assigned to the transition from S to T point in the Brillouin zone.

Lattice Relaxation Effect Associated with Core Holes in Ionic Crystals Studied by Time-Resolved Luminescence Spectroscopy

Auger-decay-free core luminescence in RbF, CsF, CsCl, CsBr and BaF 2 has been studied under monochromatic vacuum ultraviolet light excitation using a time-resolved technique. Luminescence spectra associated with core holes are carefully separated from those originating in the valence-band excitation. Based on the spectral shape and energy range of the luminescence bands, a lattice relaxation effect following the core hole creation is discussed.

Self-Trapped Exciton Luminescence in Dilated NaI Crystals —Relaxation Process of Excitons in Alkali Halides—

The effect of dilatational stress on the relaxation of excitons in NaI is investigated by means of photoluminescence and decay time measurements. When the lattice spacing is expanded, a new luminescence band, which is ascribed to the self-trapped exciton in off-center configuration, appears around 3.05 eV in addition to the π band at 4.15 eV. The decay profile of the π band, consisting of 1.2-ns and 110-ns decay components, is not affected by the lattice expansion. The 3.05 eV band exhibits two decay components of 6.0 ns and 1.5 µs. These lifetimes are significantly longer than the corresponding values of the π band, indicating that the electron-hole overlap is smaller for the 3.05 eV-luminescent state. The relaxation processes of a free exciton into the self-trapped state in alkali halides are discussed with the help of the present and previous results.

Relationship between refractive index and density of synthetic silica glasses

The effect of impurity and density on refractive index was studied for silica glasses synthesized with direct, sol-gel, and vapor phase axial deposition (VAD) processes. Linear relationships between the refractive index and density n=Cρ+D were found in a wide wavelength region from ultraviolet to infrared. The correlation coefficient C decreased from 0.081±0.007cm3∕g at 1.707μm to 0.061±0.008cm3∕g at 0.238μm. The relationship was different from those of the VAD or direct glasses formed by high-pressure densification, that is, the coefficient increased from 0.200±0.011cm3∕g at 1.707μm to 0.229±0.012cm3∕g at 0.238μm for VAD glass. Chlorine and hydroxyl impurities affected the refractive index; however, these impurities did not cause the linear relationship. After adjusting the index according to the chlorine related effect and the effect of the sample density, the difference between the sample’s refractive indices Δ(n2−1) showed a similar dispersion relation irrespective of the synthetic process. An analysi...

Transfer of Excitation Energy from Pr3+ to Gd3+ in YF3:Pr3+,Gd3+

Luminescence and excitation spectra for YF3:Gd3+, YF3:Pr3+, and YF3:Pr3+,Gd3+ have been studied in the vacuum ultraviolet (VUV) spectral region at room temperature. In YF3:Gd3+, Gd3+ ions absorb VUV light ranging from 150 to 200 nm due to 4 f–4 f transitions, yielding an ultraviolet (UV) luminescence line at 311 nm originating from the 4 f–4 f transition (6P7/2→8S7/2 state). In YF3:Pr3+,Gd3+, Pr3+ ions absorb the VUV light (150–200 nm) due to 4 f–5d transitions, and the absorption gives rise to the UV luminescence of Gd3+ ions that is much stronger than that of YF3:Gd3+. In this paper, we discuss the energy transfer process from Pr3+ to Gd3+ ions in YF3:Pr3+,Gd3+ excited by VUV light.

Effect of surfactant Sb on carrier lifetime in GaInP epilayers

Samples of Ga0.52In0.48P grown on (001) GaAs with small amounts of surfactant Sb were investigated using time-resolved photoluminescence. All samples show a luminescence that may be fit to a two-stage exponential decay with a fast and a slow lifetime. For growth without Sb (Sb/III(v)=0), the sample shows a strong CuPtB ordering and a fast component lifetime of 7 ns. As the Sb concentration is increased, the degree of order is reduced, with a consequent increase in band gap energy. In the highest band gap material, produced at Sb/III(v)=0.016, the fast lifetime is 2.9 ns, an order of magnitude larger than published values for GaInP disordered by misorienting the substrate. Increasing the Sb further causes the band gap energy to decrease due to the onset of composition modulation. At Sb/III(v)=0.064, the fast component lifetime decreases to 0.79 ns. Samples grown with Sb/III(v)>0.016 show a lifetime that depends on energy and is fit well by a model of localized excitons.

Effect of Dilatational Strain on the Self-Trapped Exciton Luminescence of Alkali Halides

The effect of dilatational strain on the self-trapped exciton (STE) luminescence of alkali halides is investigated at 11 K by using thin crystals grown inside a narrow gap of the quartz cell. In strained NaBr, new luminescence is found to appear around 3.13 eV in addition to the well-known π emission. This luminescence has an excitation threshold at the absorption band associated with spin-triplet exciton states, and exhibits a long lifetime of 103 µs. The same emission band can be produced by causing electrons to recombine with V K centers. It is pointed out that an expansion of the lattice parameter leads to an STE configuration of low symmetry, from which the 3.13 eV luminescence occurs. It is further proved that the dilatational strain has a considerable effect on the STE luminescence in RbI, but not in KBr. These results are discussed in terms of the off-center model for the STE.

Optical Properties of Orthorhombic Thallous Iodide

Reflection spectra of a single crystal of orthorhombic TlI have been measured for the polarization \(\bm{E}//\bm{c}\) and \(\bm{E}//\bm{a}\) over a range of photon energy from 2.6 to 4.1 eV at 2 K. The spectra show pronounced anisotropy between these polarizations. The lowest exciton transition at 2.87 eV is confirmed to occur in a cationic sublattice (Tl + 6 s →6 p ). However, the admixture of the cation s orbital in the top valence band is found to be not so large as in cubic thallous halides. The binding energy of the first exciton is estimated to be 17.1 meV.

Magneto-Reflectance of Excitons in Indium Halides

Magneto-reflectance of the direct excitons in InI and InBr is investigated in magnetic fields up to 15T at LHeT. Oscillatory structure which corresponds to the n s exciton states appears in magnetic fields in both halides. Their exciton energies are characterized by a hydrogenic exciton series in spite of the strongly anisotropic media. The binding energy of the exciton is determined as 4.2 meV, the reduced mass as 0.17 m 0 and the dielectric constant effective to the exciton binding as 24 in InI. They are 11.6 meV, 0.26 m 0 and 18 in InBr. In InI, the energy of the n s exciton state in the low field connects smoothly to that of the N = n - 2 Landau-like state in the high field region. Moreover, in InI, the oscillatory structure is accompanied by a subsidiary peak on the high energy side in high fields.