Yomei Tokuda

Application of femtosecond laser pulses for microfabrication of transparent media

Femtosecond laser microfabrication of 3D optical memories and photonic crystal (PhC) structures in solid glasses and liquid resins are demonstrated. The optical memories can be read out from both transmission and emission images. The PhC structures reveal clear signatures of photonic bandgap (PBG) and microcavity formation.

Gel-melting method for preparation of organically modified siloxane low-melting glasses

Softening or melting behavior of the organically-modified siloxane hybrid gels and glasses in the system of RSiO 3/2 and R 2 SiO 2/2 (R: methyl and phenyl) has been investigated to obtain a new family of low-melting glasses. The RSiO 3/2 and RSiO 3/2 –R 2 SiO 2/2 gels showed softening temperatures around 50–100 °C. The softening temperature of RSiO 3/2 single-component glasses, which were obtained by melting the corresponding gels at a temperature above the softening temperature, increased by heat-treatment at 200 °C, and finally showed no softening behavior. On the other hand, in the PhSiO 3/2 –Ph 2 SiO 2/2 binary glasses, the softening temperatures showed a tendency to saturate after longer heat treatment over 200 h. These facts indicate that the present organically modified siloxane system will be a potential candidate for the low-melting glass.

Waveguide formation in niobium tellurite glasses by pico- and femtosecond laser pulses

Abstract We obtained a temporal and permanent refractive index change in niobium tellurite glasses by using ps- and fs-pulses, respectively. A He–Ne laser beam was guided by a line waveguide structure drawn by the fs-pulses, indicating positive refractive index change. It is suggested that the refractive index change by the ps-pulses is due to an increase in the absorption coefficient while that caused by the fs-pulses results from the rearrangement of the glass structure.

White light emission of Mn-doped SnO-ZnO-P_2O_5 glass containing no rare earth cation

The authors have demonstrated white light emission of rare earth (RE)-free Mn-doped SnO-ZnO-P(2)O(5) glass. The RE-free glass shows white light emission with a high value of quantum efficiency (QE) comparable to conventional crystalline phosphor. It is notable that the high QE value is attained for RE-free transparent glass, and the broad emission can be continuously tuned by both the amount of activator and the composition of the glass. Since this glass possesses low-melting property, we emphasize that the glass phosphor will lead to the development of a novel inorganic white-light-emitting device in combination with a solid state UV light-emitting source.

Correlation between emission property and concentration of Sn^2+ center in the SnO-ZnO-P_2O_5 glass

The authors report on the correlation between the photoluminescence (PL) property and the SnO amount in SnO-ZnO-P2O5 (SZP) glass. In the PL excitation (PLE) spectra of the SZP glass containing Sn2+ emission center, two S1 states, one of which is strongly affected by SnO amount, are assumed to exist. The PLE band closely correlates with the optical band edge originating from Sn2+ species, and they both largely red-shifts with increasing amount of SnO. The emission decay time of the SZP glass decreased with increasing amount of SnO and the internal quantum efficiencies of the SZP glasses containing 1~5 mol% of SnO are comparable to that of MgWO4. It is expected that the composition-dependent S1 state (the lower energy excitation band) governs the quantum efficiency of the SZP glasses.

Correlation between preparation conditions and the photoluminescence properties of Sn2+ centers in ZnO–P2O5 glasses

Photoluminescence (PL) and coordination states of Sn2+ centers in 1SnO–68ZnO–31P2O5 (SZP31) glass prepared under different conditions are examined. When prepared in air, the local coordination state of Sn2+ centers depends on the starting material. In contrast, the PL properties of Sn2+ centers of the glass prepared under inert atmosphere conditions are independent of the starting material because of elimination of the oxidation reaction. 119mSn Mossbauer and X-ray absorption fine structure analyses indicate that most of the Sn2+ centers in the SZP31 glass prepared in an inert atmosphere exist at a SnO-like coordination state, possessing a coordination number greater than 2. It is assumed that the more highly coordinated Sn2+ is the origin of high PL intensity with high quantum efficiency.

High efficient white light emission of rare earth-free MnO–SnO–ZnO–P2O5 glass

White light emission of rare earth (RE)-free Mn-doped SnO–ZnO–P2O5 glass is demonstrated. Glass transition temperature of the obtained glass is below 440°C, which assures the application to sealant of solid state light emitting devices. The RE-free transparent glass shows white light emission with a high value of quantum efficiency (QE) comparable to conventional crystalline phosphor by excitation of deep UV light. The broad emission of RE-free transparent glass can be continuously tuned by both the amount of activator and the composition of the glass without decrease of the QE value.

Structure of organic-inorganic hybrid low-melting glasses from 29Si NMR and ab initio molecular orbital calculations

Abstract The structure of organic–inorganic hybrid glass precursor Me2Si(OPO(OH)2)2 and low-melting glasses SnO–Me2SiO–P2O5 has been examined by 29Si static and MAS NMR, respectively. The 29Si MAS NMR spectra of SnO–Me2SiO–P2O5 were deconvoluted into three Gaussian peaks, whose chemical shifts were located at −3.1, −10.2 and −18.1 ppm. Ab initio molecular orbital calculations have also been carried out for the clusters modeling the structure of precursor and glasses. From the calculations of 29Si NMR shielding constants, the experimental chemical shifts at −10.2, −3.1 and −18.1 ppm were assigned to 29Si atoms in P–O–Si(Me)2–O–P network linkage, in P–O–Si(Me)2–OH terminating structure and in P–O–Si(Me)2–O–Si network linkage, respectively. Based on the area ratio of the assigned three peaks, the network structures around Si constructing SnO–Me2SiO–P2O5 low-melting glasses were discussed in detail.

Photochemical reaction of divalent-germanium center in germanosilicate glasses under intense near-ultraviolet laser excitation: Origin of 5.7 eV band and site selective excitation of divalent-germanium center

The photochemical reaction of Ge2+ in 10GeO2-90SiO2 optical fiber preform and the origin of the 5.7 eV optical absorption band induced by intense ultraviolet laser excitation have been investigated using a near-ultraviolet XeF excimer laser (3.7 eV) as the main excitation source. Based on the previous and present experimental results, it is concluded that the 5.7 eV optical absorption band does not originate from the Ge(2) electron trapped center. In place of this model, we have proposed an alternative structural model for the origin of the 5.7 eV band taking into account the experimental fact that the corresponding center is diamagnetic. In addition, it is found that the site selective excitation of the Ge2+ center can be achieved by using laser sources with different photon energies.

Vibrational dynamics of glassy SiS2 on the basis of molecular orbital calculations

Abstract Ab initio molecular orbital calculations at the Hartree–Fock/3-21G(*) level have been performed on several clusters modeling SiS2 glass. The clusters consist of edge-sharing and/or corner-sharing structural units that can be found in actual glassy SiS2. The experimental Raman spectrum of SiS2 glass was interpreted in terms of vibrational frequencies calculated for these model clusters. The observed Raman bands at 427 and 415 cm −1 were assigned to the breathing motions of S atoms localized in two (or more) and one edge-sharing units, respectively. We also propose that two peaks at 367 and 381 cm −1 occur as a result of an accidental degeneracy or a resonance between the vibrations of the corner- and edge-sharing S atoms. The corner-sharing structure which is isolated from or connected with one edge-sharing also contributes to these peaks around 370 cm −1 .