Setsuhisa Tanabe

Fluorescence properties of Er3+ ions in glass ceramics containing LaF3 nanocrystals

Abstract Transparent oxyfluoride glass ceramics containing LaF3 and ErF3 were prepared and the fluorescent characteristics of Er3+, such as emission spectra and lifetime of excited states were investigated. By heat-treatment of the as-prepared glass above the glass transition temperature, the formation of LaF3 nanocrystals was confirmed by X-ray diffraction (XRD). With increasing annealing temperature and time, the fluorescence intensity at 540 nm and around 1000 nm increased and the lifetime of the 4 S 3/2 level became longer. Also the 540 nm upconversion intensity by 970 nm pumping increased drastically. These results could be attributed to the change of ligand field of Er3+ ions and the decrease of effective phonon energy because of the precipitation of LaF3 nanocrystals and the incorporation of some of Er3+ ions into fluoride.

Optical transitions of rare earth ions for amplifiers: how the local structure works in glass

Abstract Several properties of 4f optical transitions of rare earth ions in glasses utilized as optical amplifiers for telecommunications are discussed. For the Er 3+ : 1.55 μm transition, the role of Judd–Ofelt Ω 6 parameters is presented. With compositional change in structure of aluminosilicate glasses, the changes of the radiative cross section, quantum efficiency, and flatness, is observed, which are especially important for the amplifiers in the wavelength-division-multiplexing (WDM) network system. Moreover, the energy level structures and resultant spectral properties of Pr 3+ , Nd 3+ and Dy 3+ ions, all of which are 1.3 μm-active ions, are compared. The hypersensitivity of Dy 3+ transitions appears especially in chalcogenide glasses, where the non-radiative loss due to multiphonon emissions is minimized. Compositional variations of the branching ratio and radiative cross sections, and their dependence on the Ω 2 parameters, which are affected by the asymmetry of the rare earth ion sites, are discussed.

Relation between the Ω6 intensity parameter of Er3+ ions and the 151Eu isomer shift in oxide glasses

The Ωt intensity parameters (t=2,4,6) of Er3+ ions in several oxide glasses and the isomer shift (IS) of 151Eu Mossbauer spectra in glasses of the same composition were determined. Among these Ωt’s, the Ω6 parameter was found to have a good relation with IS; Ω6 decreases with an increase of IS which reflects the 6s electron density of rare‐earth ions. From the theoretical expression of Ωt, Ω6 is considered to be more affected by the overlap integrals of the 4f and 5d orbitals than Ω2 and Ω4, and to increase with an increase of these overlap integrals. These overlap integrals are supposed to decrease when the 6s electron density is larger, since the 6s electron density shields the 5d electron orbital.

Hydroxyl groups in erbium-doped germanotellurite glasses

Abstract Germanotellurite glasses in the compositions of TeO 2 –GeO 2 –ZnO–Y 2 O 3 –Na 2 O–Er 2 O 3 were prepared by various conditions. The effect of the composition and various drying procedures on the content and the vibration frequency of OH groups was investigated. Among various drying techniques for removing the OH groups, it has been found that bubbling a mixture of dry oxygen gas and carbon tetrachloride shows significant effect on removing OH groups. Also the quenching effect of OH upon the lifetimes of the 4 I 11/2 and 4 I 13/2 levels of Er 3+ in germanotellurite glasses was slight with low Er 3+ and OH content.

Luminescence Characteristics of YAG Glass–Ceramic Phosphor for White LED

Luminescence characteristics of Ce:Y₃Al₅O₁₂ (YAG) glass-ceramic (GC) phosphor for a white LED were investigated. The GC phosphor was obtained by a heat treatment of a Ce-doped SiO₂-Al₂ O₃-Y₂O₃ mother glass between 1300degC and 1500degC for the prescribed time period. The quantum efficiency (QE) of Ce³⁺ fluorescence in the GC materials, the color coordinate, and the luminous flux of electroluminescence of LED composite were evaluated with a blue LED (465 nm) set in an integrating sphere. The QE increased with increasing ceramming temperature of the as-made glass. The color coordinates (<i>x</i>, <i>y</i>) of the composite were increased with increasing thickness of the GC mounted on a blue LED chip. The effect of Gd₂O₃ substitution on the optical properties of the GC materials was also investigated. The excitation and emission wavelengths shifted to longer side up to Gd/(Y + Gd) = 0.40 in molar composition. As a result, the color coordinate of the LED with GdYAG-GC of various thickness shifted to closer to the Planckian locus for the blackbody radiation. These results were explained by partial substitution of Gd³⁺ ions in the precipitated YAG microcrystals, leading to the increase of lattice constant of unit cell, which was confirmed by XRD.

Upconversion fluorescences of TeO2- and Ga2O3-based oxide glasses containing Er3+

Abstract Upconversion-pumped fluorescence was observed for the first time in Er2O3-doped oxide glasses even at room temperature without any sensitizing ions such as Yb3+ in the systems of TeO2- and Ga2O3-based oxide glasses. A diode laser operating at 798 nm was used as the pumping source for stepwise excitation. The upconversion-pumped fluorescence was confirmed by the quadratic changes of fluorescence intensity against the pumping power. It is suggested that these oxide glasses can become one candidate for a high temperature upconversion device.

Analysis of Ce3+ luminescence quenching in solid solutions between Y3Al5O12 and Y3Ga5O12 by temperature dependence of photoconductivity measurement

Photocurrent excitation spectra were measured to investigate the quenching in the garnet solid solutions. Intense photocurrent excitation bands attributed to the lowest 5d1 and the second lowest 5d2 levels were observed in the Ce-doped Y3Al2Ga3O12 (Ce:YAGG) and Y3Ga5O12 (Ce:YGG). Based on the results of temperature dependence of photoconductivity, the 5d1 and 5d2 levels in the Ce:YAGG are found to be located below and within the conduction band, respectively, while both levels in the Ce:YGG are found to be located within its conduction band located at lower energy levels. In addition, the threshold of photoionization from the 4f level of Ce3+ to the conduction band in the Ce:YAGG and Ce:YGG were estimated to be 3.2, and 2.8 eV, respectively. We conclude that the main quenching process in the Ce:YAGG is caused by the thermally stimulated ionization process with activation energy of 90 meV from the 5d1 to the conduction band, and that in the Ce:YGG is caused by the direct ionization process from the 5d levels to the conduction band.

YAG glass-ceramic phosphor for white LED (I): background and development

We have developed a Ce:YAG (Y3Al5O12) glass-ceramic phosphor for the white LED. The glass-ceramic phosphor was obtained by a heat treatment of a Ce-doped SiO2-Al2O3-Y2O3 mother glass between 1200°C and 1500°C for the prescribed time of period. We confirmed that, by XRD measurements, only YAG crystal precipitated in the mother glass after the heat treatment. It was shown from SEM observation that the YAG crystals with a grain size of approximately 20μm were uniformly dispersed in the glass matrix. The yellow emission, around 540nm in wavelength, was observed from the glass-ceramic phosphor, when it was excited by a blue LED (465nm). The white light due to the mix of yellow and blue light was observed from the glass-ceramic plate with a thickness of 0.5mm. The YAG glass-ceramic phosphor showed a high-temperature resistance and a good performance in a damp heat test. Moreover, a higher thermal conductivity of 2.18 Wm-1K-1 and bending strength of 125MPa were observed compared with a conventional soda-lime glass or an epoxy resin. In addition, since the YAG glass-ceramic phosphor can be formed in a plate-like shape, there is no need to be sealed in resins for the fabrication of the LED devices. Therefore, it is expected that this newly developed glass-ceramic phosphor is a promising candidate for the realization of resin-free, high-temperature and high-humidity resistant, long-life white LED devices.

Simultaneous Tailoring of Phase Evolution and Dopant Distribution in the Glassy Phase for Controllable Luminescence

Construction of an active composite with multicolor visible and broadband near-infrared luminescence is of great technological importance for various applications, including three-dimensional (3D) display, broadband telecommunication, and tunable lasers. The major challenge is the effective management of energy transfer between different dopants in composite. Here we present an in situ strategy for controlling energy transfer between multiple active centers via simultaneous tailoring of the evolution of phases and the distribution of dopants in the glassy phase. We show that the orderly precipitation of Ga(2)O(3) and LaF(3) nanocrystals and the selective incorporation of Ni(2+) and Er(3+) into them can be achieved. The obtained composite shows unique multicolor visible and broadband near-infrared emission. Possible mechanisms for the selective doping phenomenon are proposed, based on thorough structural and optical characterizations and crystal-field calculation results. Moreover, the strategy can be successfully extended to accomplish space-selective control of multicolor luminescence by employing the modulated stimulation field. The results suggest that the strategy could be applied to fabricate a multifunctional light source with a broad range of important host/activator combinations and to construct various types of three-dimensional active microstructures.

Phonon sideband of Eu3+ in sodium borate glasses

Abstract The phonon sideband (PSB) of Eu3+ associated with the 7F0-5D2 transition was investigated for the B2O3Na2O glasses doped with 1 mol% Eu2O3. It was found that the coupling of the phonon mode with Eu3+ ions in the glasses was through B-O− or B-O stretching vibration of BO3 units and vibrations of BO4− units in tetra- and diborate groups. The results showed that B-O− bonds present around Eu3+ ions, which have the highest phonon energy, contributed to the nonradiative decay by multiphonon relaxation of Eu3+ ions even in low-alkali borate glasses. Moreover, the electron-phonon coupling strength of each borate group was found to be influenced by the fraction present and by the site selectivity of Eu3+ ions for the group.