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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.

Spatially resolved cathodoluminescence (CL) measurements are performed to characterize optical properties and structural quality of ZnO tetrapods. High optical quality of these structures is concluded based on the observation of intense free excitonic (FE) emission at room temperature and a low intensity of the so-called green emission of defect origin. CL mapping performed for individual tetrapods has revealed that the defects responsible for the green emission are predominantly located in core regions (i.e. close to leg junctions) and, therefore, are unlikely to reside near surfaces. Variations in the spectral positions of the FE emission monitored in tetrapod legs of different diameters are also observed and are attributed to the formation of whispering gallery mode polaritons within the tapered microcavity.

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 demonstrate the fabrication and the detection of protein microarrays and nanoarrays, which are tagged by using colloidal CdSe/ZnS quantum dots (QDs), for the purpose of optical readout of those biochips. A microarrayer and a dip-pen nanolithography technique were used for the fabrication of QD-labeled protein microarrays and nanoarrays, respectively. The optical readout of those biochips was conducted by using a microchip scanner and a confocal scanning microscope in the case of the microarrays while an atomic force microscope was employed for characterizing the nanoarrays. We also report a possible way of reducing the ‘doughnut phenomenon’ in microarrays by varying the spot size from 300 μm to 100 μm, wherein the fluorescence intensity distribution becomes more uniform with decreasing spot diameter.

We report three-dimensional fluorescent memory by recording optical bits with irradiation of femtosecond laser pulses at 800 nm and by reading photoluminescence change in Eu3+ doped glass. We produced multi-layered micro-bit patterns and read the blue emission from the 405 and 325 nm excitations due to permanent reduction of Eu3+ to Eu2+ in sodium borate glass by scanning the irradiated region in multilayers.

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.

This research was supported by a grant (M1-02-KR-01- 0001-03-K18-01-025-1-3) from Information Display R and D Center, one of the 21st Century Frontier R and D Program funded by the Ministry of Science and Technology of Korean government.

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 two series of molecular beam epitaxy grown Ga1−xMnxAs epilayers with several different Be doping levels. Two Mn concentrations x were chosen for this study: 0.03 and 0.05, and these values were maintained constant in each series. These samples were characterized by using SQUID and magnetotransport measurements. A systematic increase of the Curie temperature TC was observed in SQUID measurements on the series of Ga1−xMnxAs with x=0.03. The resistivity measured at zero magnetic field shows a local maximum near the Curie temperature, reflecting the effects of critical scattering near TC. The observed increase of TC in Ga1−xMnxAs for this low range of x can be explained by the increase of the free carrier concentrations in the system arising from Be doping. However, in the series of Ga1−xMnxAs with the higher concentration of Mn (x=0.05), the measurements reveal that the TC systematically decreases with increasing Be doping level. We discuss this effect in terms of a fundamental limitation of ...

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.

Temperature dependent photoluminescence and cathodoluminescence (CL) spectroscopies are employed to investigate free exciton (FX) emissions in ZnO tetrapods. The intensity of the no-phonon line is found to be largely suppressed as compared with longitudinal optical phonon assisted transitions, in sharp contrast to bulk ZnO. From spatially resolved CL studies, this suppression is shown to strongly depend on structural morphology of the ZnO tetrapods and becomes most significant within areas with faceted surfaces. A model based on reabsorption due to multiple internal reflections in the vicinity of the FX resonance is suggested to account for the observed effect. Original Publication: Shula Chen, Sun Kyun Lee, Weimin Chen, H X Dong, L Sun, Z H Chen and Irina Buyanova, On the origin of suppression of free exciton no-phonon emission in ZnO tetrapods, 2010, APPLIED PHYSICS LETTERS, (96), 3, 033108. http://dx.doi.org/10.1063/1.3292027 Copyright: American Institute of Physics http://www.aip.org/

A dual-gate-controlled single-electron transistor with coupled dot geometry has been fabricated on a silicon-on-insulator structure. Coupled dots are defined by tunable gates which are designed to separately control the tunneling potential barriers to compensate for disorder due to size fluctuation in quantum dots. The Coulomb-blockade phenomena observed in linear and nonlinear transport regimes were found to be enhanced by the multidot coupling. The Coulomb staircase (nonlinear effect) appears more clearly with the increasing number of coupled dots, indicating definite suppression of the inevitable cotunneling process. In the linear regime, the frequency of Coulomb oscillation was able to be tuned by changing the interdot coupling strength. These results indicate that enhancement of the Coulomb blockade and tunability can be achieved through replacing the traditional single dot by gate-controlled multidots in future single-electron devices.

In this work, we report three-dimensional memory by recording optical bits with irradiation of near-infrared femtosecond laser pulses and by reading photoluminescence change in the blue due to permanent reduction of Eu 3+ to Eu 2+ in sodium borate glasses. We produced a multilayered micro-bit pattern which was read out by detecting the blue emission from the 405 nm excitation with a high S/N ratio. The readout was performed by using a scanning reflection-type confocal microscope.

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.

From the ultraviolet induced absorption, we have found a broad blue band and little involvement of Pr ions in ultraviolet illumination. Ultraviolet excited photoluminescence and the excitation spectra for visible emission were also measured in Pr:LiNbO3. We estimated photovoltaic constant and photoconductivity by measuring photovoltaic current and voltage with different boundary conditions. With ultraviolet gated short-circuit holographic recording, we have measured enhanced sensitivity and diffraction efficiency.

By rotating the reference beam instead of a photopolymer, we combined rotational multiplexing with angular or shift multiplexing for high capacity hologram storage in DuPont's HRF-150-100 photopolymer material. We used the spherical reference and signal waves for angular and rotational as well as shift and rotation multiplexing. To get uniform diffraction efficiency, we used the time-scheduled exposure during recording.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

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.

This study examines the influence of the defect properties on internal quantum efficiency (IQE) in blue InGaN-based light emitting diodes (LEDs). The defect parameter is introduced for estimating defect properties among defect density, size, and defect type that are strongly correlated with IQE in InGaN LED. The value of IQE can be expressed by the value of the defect parameter, which was obtained from transmission electron microscopy and cathodoluminescence measurement.

Time-resolved photoluminescence (PL) is employed to characterize optical quality of ZnO tetrapods. PL decay of free excitons (FE) is concluded to contain two components with time constants of 1 and 14 ns at room temperature. The fast PL decay is attributed to nonradiative recombination whereas the slow decay is suggested to mainly represent FE radiative lifetime, based on correlation between thermally induced increases in the PL linewidth and FE lifetimes. The results underline superior optical quality of the tetrapods as the decay time of the slow PL component is comparable to the longest lifetimes reported to date for ZnO.

An efficient quenching of the photoluminescence spectrum was observed for the quantum dot (QD)-multiwalled carbon nanotubes (MWCNTs) nanocomposite. While the lifetime of the blue emission from ZnSe shell was significantly reduced, it did not exhibit any obvious change in the red emission from CdSe core-crystal. This difference revealed two competing charge transfer processes between the CdSe core-crystal, the ZnSe shell and the MWCNTs. Current-voltage measurements on devices fabricated utilizing CdSe QD-MWCNT nanocomposites showed photovoltaic (PV) effect under ultraviolet light illumination. Charge transfer mechanisms of the PV cells are described on the basis of the experimental results.

We have investigated the effects of well strain on optical and device characteristics of AlGaInP-based multi-quantum well (MQW) light-emitting diodes (LEDs) according to the variation of well strain and the corresponding change of well thickness in order to maintain the operating wavelengths. As the compressive well strain increased from +0.04 to +0.41% and the corresponding well thickness decreased from 7.2 to 4.5 nm, the relative photoluminescence (PL) intensity increased from 1.0 to 1.21, and the radiative carrier lifetime measured from the time-resolved PL (TRPL) was shortened from 4.43 to 1.77 ns. The compressive well strain may cause the strain-induced heavy-hole-assisted recombination process, followed by reduction of the radiative decay time due to a reduced effective-heavy-hole mass. In addition, a thinner well thickness may enhance the spatial carrier confinement in the well regions, which greatly improved light output power of the LEDs.

In situ growth of ZnO quantum dots (QDs) on the surface of multiwalled carbon nanotubes (MWCNTs) was realized via a mild solution-process method, which resulted in an improvement in photoinduced charge separation and transport of carriers to the collecting electrode. The charge transfer efficiency was significantly increased by more than 90% due to the conjugation of ZnO QDs with MWCNTs, as confirmed by photoluminescence measurements. Ultraviolet photovoltaic cells based on the charge transfer at the ZnO QD-MWCNT heterostructures were fabricated, and their power conversion efficiency was measured to be above 1%.

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.

To investigate the feasibility of a high density optical memory in photopolymers, we used twophoton absorption of near-infrared femtosecond laser pulses to perform bit-wise three-dimensional storage. We monitored the transmitted probe beam, which was modulated by the change of refractive index caused by the femtosecond laser irradiation. The size of the spots generated by the focused Gaussian beam depended on the laser intensity and exposure time. The minimum lateral and longitudinal sizes of a spot were estimated as 0.5 and 5 μm, respectively.

A lateral gate-controlled double dot structure in Si has been fabricated for studying coupled two qubits. Nonequilibrium single-electron tunneling measurements at 1.4K show that the second Coulomb peak, associated with a two-electron occupation, splits into two side peaks, and that their separation displays a strong magnetic-field dependence for various interdot coupling constants. Moreover, for some fixed magnetic fields, the separation of the side peaks decays exponentially as a function of the interdot coupling. We attribute this behavior to electron spin exchange and spin swapping between singlet and triplet states in the coupled double dot in the presence of a magnetic field.

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.