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Nano-sized Ca3Co4−xNixO9+δ (0 ≤ x ≤ 0.3) thermoelectric powders are synthesized by using the solution combustion method, with aspartic acid as a combustion fuel. The synthesized Ca3Co4−xNixO9+δ nano-sized powders exhibit a spherical-like shape and a smooth surface. Higher Ni content results in a smaller grain size and a higher porosity, resulting in a decrease in the electrical conductivity. However, the Seebeck coefficient of Ni-added Ca3Co4O9 is much higher than that of Ca3Co4O9. The highest power factor (1.4 × 10−4 Wm−1K−2), which is more than nine times larger than that of Ca3Co4O9, is attained for Ca3Co0.38Ni0.2O9+δ at 800 °C. The addition of a small amount of Ni is highly effective in improving the thermoelectric properties of Ca3Co4O9. We believe that Ca3Co4−xNixO9+δ is a potential p-type thermoelectric material for renewable energy conversion.

The spectroscopic characterizations of sodium borate glasses codoped with Ce 3+ and Ag + prepared by melt quench method are done using absorption, excitation and emission spectra. The nonradiative energy transfer between Ag + and Ce 3+ ions process is observed. With fs-laser irradiation, we formed silver aggregates and studied efficient resonant energy transfer from Ce 3+ ions to silver aggregates by analyzing the emission of Ce 3+ ions and the absorption of the silver aggregates.

We report the fabrication of a nanocrystal (NC) doped polymer tip at the end of a single-mode optical fiber with active polymerization technique. The tip is created by immersing the fiber in a photosensitive epoxy precursor containing colloidal CdSe–ZnS NCs and polymerizing the epoxy using laser light introduced through the fiber. The laser power was found to have a strong influence on the tip height and shape. As the laser beam progressed from the fiber end, the power was reduced by conversion and absorption, resulting in a cone-shaped tip. The shape of the extreme end of the tip was sensitive to the transmitted beam pattern, indicating that the shape of the tip is independent of the substrate fiber. A blue shift and reduction in decay time suggest that the emission properties of the NCs are affected by immobilization in the epoxy, possibly because of oxidation at the NC surface.

In this report, an ultrafast laser was used to produce persistent change of valence states of Mn ions in sodium borate glasses. Glass composition of sodium borate in this study was 85B 2 O 3 –15Na 2 O. We observed a broad emission at 600 nm of the transition 4 T 1 – 6 A 1 of doped Mn 2+ ions with excitation at 325 nm in an unirradiated area. Femtosecond laser irradiation produced additional strong blue emission at 440 nm and reduced the emission intensity from Mn 2+ ions. The irradiation produced the absorption band of Mn 3+ at 550 nm, resulting in color change of the glass. The blue band was measured as a function of irradiation exposure for samples with various concentrations of Mn ions. The valence state change of Mn 2+ to Mn 3+ was suppressed in Mn and Cr codoped glasses.

We report photovoltaic constant and photoconductivity of Ce,Mn:LiNbO 3 crystals. Using the DC electric field dependence of non-phase-matched second-harmonic generation, photovoltaic constant and photoconductivity were deduced from an analysis of the observed relaxation behavior of space-charge field at two different light intensities for photorefraction. In Ce,Mn:LiNbO 3 crystal, the photovoltaic constant and photoconductivity were determined to be 1.0 × 10 −8 A/W and 1.1 × 10 −16 Ω −1 cm −1 at the intensity of 0.3 mW/cm 2 , respectively. The photovoltaic constant at 514 nm is at least three times larger compared to that in iron doped LiNbO 3 .

We have observed ultraviolet-induced visible absorption and performed two-color holographic recording at green, red, and near-infrared light in thulium doped near-stoichiometric LiNbO3 crystal. The diffraction efficiency and sensitivity of the recording process at all wavelengths are increased by ultraviolet gating light. Characteristics of recording in the visible are explained by a two-center model. Two-color recording in the near-infrared exhibits significant improvements of the fixed diffraction efficiency and the role of small polarons is discussed in the visible and near-infrared recording with ultraviolet sensitizing light. � 2004 Elsevier B.V. All rights reserved.

Optical spectroscopy of Tm 3+ -doped 59H 3 BO 3 –20ZnO–20LiF–1Tm 2 O 3 glass is presented. From the measured absorption spectra, radiative parameters were determined using the Judd–Ofelt (JO) theory. Excitation in the range of 750–800 nm resulted in a frequency up-converted emission at 453 nm ( 1 D 2 → 3 F 4 ). Excitation spectra and intensity dependence of the up-converted fluorescence have been investigated and a mechanism has been proposed that explains up-conversion process.

We report on the up-converted emissions at 367, 458 and 483 nm in thulium doped lutetium aluminum garnet (LuAG) under infrared excitation in the range of 730-800 nm. We also observe blue and strong green emission in thulium and holmium codoped LuAG with near infrared excitation. Excitation spectra, power dependence, temporal behavior and temperature dependence of up-converted emissions were measured to explain the up-conversion mechanisms. Several cross-relaxation mechanisms and excited state absorption are found to be responsible for the observed up-conversion processes.

We investigated the photoluminescence properties of near-stoichiometric LiNbO3 crystals doped with Tb only and double doped with Tb and Fe. Excitation spectra for the emission from the 5D4 state in both crystals have been studied in the range 200-500 nm. We observed two broad ultraviolet absorption bands and discussed their relations to Tb ions. Temperature dependence of the 5D4 emission intensity and lifetime showed strong quenching below room temperature.

We report photochromic properties of manganese and cerium co-doped lithium niobate crystals that is a candidate for nonvolatile holographic storage. The reduction and ultraviolet-induced absorption spectra showed the charge state changes of dopant ions in co-doped crystals. We also report the dynamics of the light-induced absorption and grating formations. The crystal with relatively higher cerium concentration than manganese has produced large persistent diffraction efficiency under 1 W/cm2 recording intensity and 25 mW/cm2 sensitization.

Pure titanium dioxide (TiO2) thin films were deposited on single-crystal Si(100) substrates by laser ablation. We investigated the effects of ambient gas (O2 or Ar), pressures, and substrate temperatures on film quality. From the annealing experiment of the deposited TiO2 thin film under Ar or O2 ambient gas, we see the chemical effect of ambient gas on film quality. The crystallinity of the deposited TiO2 thin film is best at 700 °C in the substrate temperature range attempted, 400–700 °C, and at pressures of 0.1 Torr and below. The rutile phase is dominant under most experimental conditions. Only under very extreme conditions did we obtain a thin film of the anatase phase.

Comparative analysis of the structural and optical properties of ZnO thin films, grown on oxygen (O)- and hydrogen (H)-plasma treated sapphire substrates (denoted by samples O and H, respectively), by plasma-assisted molecular beam epitaxy is reported. We found that sample O has better crystal structure quality as compared to sample H by means of high-resolution X-ray diffraction, atomic force microscopy, and Hall measurements. We observed higher intensity ratio of excitonic emission to deep-level emission, longer radiative recombination lifetime, and lower stimulated emission threshold for sample O compared to sample H. From the results, we conclude that the O-plasma treatment on sapphire substrates may result in a decrease in the number of oxygen vacancies and better lattice matching conditions, thereby leading to an enhancement of the crystal quality and the optical properties of ZnO thin films grown on O-plasma treated sapphire substrates.

In this work, we report permanent reduction of Eu 3+ to Eu 2+ in sodium borate glasses by irradiation of near-infrared femtosecond laser. Glass composition of sodium borate was 85B 2 O 3 –15Na 2 O. The glasses were doped with 0.05, 0.1, and 0.5 mol% Eu 2 O 3 . Absorption and fluorescence dynamics were studied to investigate valence state change of europium ions and the energy transfer between Eu 2+ and Eu 3+ ions. As the femtosecond laser intensity or exposure time increases, the emission band at 400 nm becomes stronger. However, the photoreduction efficiency decreases as the dopant concentration increases. We discuss the photoreduction mechanism under multiphoton absorption.

Influence of the size of indium clusters on optical properties of green-light-emitting InGaN quantum wells (QWs) was investigated by photoluminescence (PL), cathodoluminescence, PL excitation, and time-resolved PL techniques. Low luminescence efficiency was observed for green-light-emitting InGaN QWs with micron-sized indium clusters, in contrast to the case of InGaN QWs with submicron-sized small indium segregation. Both the thermal activation energy and the carrier lifetime dramatically decreased, while a large Stokes-like shift between absorption edge and PL peak energy was still observed for the InGaN QWs with micron-sized indium clusters. These facts indicate that the effective potential barrier between radiative and nonradiative channels (thus effective carrier localization) rapidly decreases due to the formation of micron-sized large indium clusters possessing a number of nonradiative centers, leading to significant luminescence degradation.

We have studied the electronic structures of Al/MgF2/tris-(8-hydroxyquinoline)aluminum (Alq3) interface using UV and x-ray photoelectron spectroscopy [(UPS) and (XPS), respectively]. The UPS revealed that the valence peak shift occurred with MgF2 deposition before Al was deposited and was independent of the gap state formation. The XPS core level peaks indicated that the MgF2 strongly interacted with Al and O atoms in Alq3 even before Al was deposited, and the deposition of Al caused a slight change to the N 1s core level peak. These results indicate that the interaction mechanism in Al/MgF2/Alq3 is different from those found in Al/LiF/Alq3 and other metal/Alq3.

We observed femtosecond laser-induced photoluminescence (PL) change in Sm-doped sodium-borate glass. Recording and readout of the multi-layer dot patterns by use of this spectral change are here discussed

We report ultraviolet-induced transient and stable absorption and two-color holographic recording at 514, 633, and 830 nm in thulium doped stoichiometric LiNbO 3 crystal. The gating light at 355 nm increased the diffraction efficiency and sensitivity. We found the optimum gating-recording intensity ratio for maximum diffraction efficiency and weak nonvolatile holographic storage with gating.

The broad photoluminescent emission from Ce:Sr0.6Ba0.4Nb2O6 has been observed at 550 nm with excitation in the blue. Photoluminescent excitation spectrum has revealed a trivalent cerium absorption band that is assumed to be a 4f-5d transition. Temperature dependence of the photoluminescence spectra and its lifetimes in 15 - 365 K showed the existence of two different trap centers in the material. We have also investigated the photoluminescence during two-wave mixing experiment to study trap centers.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Summary form only given. Planar type InGaAs/InP based avalanche photodiodes (APDs) are widely used for 2.5 Gbit/s optical communication system have also been recently developed for 10-Gbit/s applications. There exists a critical problem of edge breakdown in a planar APD. To overcome this problem, they make a smooth junction front using etching process so that the curvature become large or employ guard rings to disperse dense equipotential lines at the periphery. Floating guard rings (FGRs) were studied extensively by Y. Liu and S.R. Forrest (1992). But, there is lack of accordance between experimental and calculated optimum FGR's dimension and also lack of experimental data showing why and how well FGR's suppress the peripheral electric field. We fabricated various planar APDs with FGRs of different dimensions to systematically see the role of FGRs and to determine optimized condition.

Spot size converter (SSC)-SOA consists of a slightly tensile bulk 0.2 /spl mu/m thick InGaAsP (/spl lambda/=1.55 /spl mu/m) active layer and 0.1 /spl mu/m thick InGaAsP (/spl lambda/=1.3 /spl mu/m) waveguides, which were grown by selective area growth (SAG) using MOCVD to implement SSC. The total device length is 1500 /spl mu/m. The thickness enhancement factor was greater than 3 by using SiO/sub 2/ mask. The SAG method with lateral tapering technique provided a typical beam divergences of 8/spl deg//spl times/15/spl deg/. To reduce facet reflectivity, window regions of 20 /spl mu/m length were introduced and the waveguides were tilted by 7/spl deg/. Both facets were antireflection coated to minimize the reflection from the cleaved facet. The gain measurement was done by coupling lensed fibers to both sides. To obtain the higher coupling efficiency in SOA module, taper lensed fiber with AR coating were used. Larger than 22 dB of the fiber to fiber gain was obtained for -20 dBm input power and 150 mA at 1540 nm.

A Si-based coupled double dot has been studied for its application to two-qubit gate. The authors manipulated electron number of each dot by using its adjacent side gate and finally observed a honeycomb charge-stability pattern, demonstrating interdot capacitive coupling. From the honeycomb diagram the capacitance-related interdot coupling parameters were extracted. Moreover, a fine structure in a conductance trace near the triple point of the honeycomb, where the tunnel coupling is maximized, was measured for finite bias, and its dependence on the interdot coupling was attributed to the spin exchange between the two dots.

The 1.5μm emission characteristics of stoichiometric LiNbO3 crystals with different Er3+ doping levels of 0.2, 0.5, 1.0, and 2.0mol% were investigated. All of the emission decays could be fitted by a single exponential curve. The lifetimes of the 1.5μm transition (I13∕24→I15∕24) were measured to be 3.0–3.2ms, which are about 25% longer than those reported for conventional Er-doped congruent LiNbO3. Our analysis showed that the longer lifetime in stoichiometric LiNbO3 is likely to originate from the reduced density of intrinsic defects.