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We report the synthesis and luminescence properties as well as band structure of a series of layered mixed-anion compounds, including a new layered mixed anion compound, Sr3Sc2Ag2Se2O5. The crystal structure, band structure, and optical properties of this new compound and of related compounds Sr3Sc2Cu2Ch2O5 (Ch:chalcogen = S, Se) are presented. The compounds have a common perovskite-like layer of (Sr3Sc2O5)2+ and different semiconductor layers of (Cu2S2)2−, (Cu2Se2)2− or (Ag2Se2)2−. Similar band structures are found for the valence band maximum (VBM), while the structure of the conduction band minimum (CBM) is different depending on the chalcogen species, resulting in the different band gap energies of the compounds. The band gap energies are estimated to be 3.3, 2.9, and 2.4 eV for Sr3Sc2Cu2S2O5, Sr3Sc2Cu2Se2O5, and Sr3Sc2Ag2Se2O5, respectively. Excitonic luminescence is observed near the band edge for all three compounds, with different wavelengths according to their band gap. These optical observations are consistent with the DFT calculations. The luminescence properties of the system can be controlled by changing the composition of the semiconductor-layers, thereby offering large flexibility in material design advantageous for various applications.

Er3+/Dy3+ co-doped double-clad ZBLAN optical fiber has been used to obtain amplified spontaneous emission (ASE) broadband light sources cladding-pumped by 980-nm multimode laser diode (LD) sources. It has been demonstrated that mid-infrared broadband emission extending from 2515 to 3735 nm was obtained by energy transfer between Er3+ and Dy3+. We experimentally investigated the optimum design of Er3+/Dy3+ co-doped ZBLAN fiber in terms of ion concentration, fiber length, pumping configuration, and pumping power. The ASE output power was more than 2.5 mW when the LD pump power was set at 5 W. To assess its potential for gas sensing applications, the fabricated ASE light source was used to successfully detect methane gas with concentrations at 1% and 5%. The simple and stable construction of our ASE light source is suitable for practical purposes.

The spectral mismatch between the solar spectrum and the sensitivity curve of crystalline silicon (c-Si) solar cell limits energy conversion efficiency of the solar cell. The energy conversion efficiency had approached the Shockley–Queisser's limit, theoretical conversion efficiency of c-Si, of 30%. Luminescent converters, which are expected to show the ideal quantum cutting, have been studied because they have potential to improve c-Si solar cell efficiency up to 38.6%. In this chapter, Yb3+ and lanthanide ion (Ce3+, Pr3+, Nd3+, Eu2+, Eu3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+) co-doped wavelength converters for c-Si solar cell and their UV/blue to 1 μm downconversion processes are described.

Deep-trap persistent luminescent materials, due to their exceptional ability of energy storage and controllable photon release under external stimulation, have attracted considerable attention in the field of optical information storage. Currently, the lack of suitable materials is still the bottleneck that restrains their practical applications. Herein, we successfully synthesized a series of deep-trap persistent luminescent materials Y3Al5–xGaxO12:Ce3+,V3+ (x = 0–3) with a garnet structure and developed novel phosphor-in-glass (PiG) films containing these phosphors. The synthesized PiG films exhibited sufficiently deep traps, narrow trap depth distributions, high trap density, high quantum efficiency, and excellent chemical stability, which solved the problem of chemical stability at high temperatures in the reported phosphor-in-silicone films. Moreover, the trap depth in the phosphors and PiG films could be tailored from 1.2 to 1.6 eV, thanks to the bandgap engineering effect, and the emission color wa...

Persistent luminescence is caused by a charge carrier detrapping process from the carrier traps filled by excitation light. Thus, the carrier storage capacity is an important factor in determining the persistent luminescence intensity. Here, the electron storage capacity was investigated in the YAGG:Ce³⁺-Yb³⁺ transparent ceramic persistent phosphor, in which the Ce³⁺ ion is the luminescence center and the Yb³⁺ ion acts as the electron trap. The number density of the Yb²⁺ electron trapping center was estimated to be approximately 1.6 × 10¹⁸ ions cm⁻³ from the absorption coefficient spectrum of Yb²⁺:4f–5d photochromic absorption center and the Yb-LIII edge XANES spectrum, which means that approximately 12% of the Yb³⁺ ions in the sample were changed to the divalent state after charging. Although the maximum energy density of 0.61 J cm⁻³ was calculated as a storage property of persistent phosphors from the Yb²⁺ number density and the photon energy of Ce³⁺:5d₁−4f luminescence at 520 nm, the actual energy density which was detected as persistent luminescence was 0.011 J cm⁻³. It is suggested that the recombination efficiency of the detrapped electrons from the Yb²⁺ ions with the hole-trapped Ce³⁺ ions is approximately a few percent.

We have developed a novel persistent phosphor of LaAlO3 perovskite doped with Er3+, Cr3+, and Sm3+ (LAO:Er–Cr–Sm), which exhibits long persistent luminescence (PersL) at 1553 nm due to the Er3+:4I13/2 → 4I15/2 transition as well as at 734 nm due to the Cr3+:2E(2G) → 4A2(4F) transition. The intense near-infrared (NIR) PersL bands from Cr3+ and Er3+ match well with the first (NIR-I, 650–950 nm) and third (NIR-III, 1500–1800 nm) biological windows as well as response curves of commercial Si and InGaAs detectors. The photon emission rates of Cr3+ (6.06 × 1016 cps sr−1 m−2) and Er3+ (3.69 × 1016 cps sr−1 m−2) at 60 min after ceasing ultraviolet (UV) illumination were comparable with that of the widely used ZnGa2O4:Cr3+ red persistent phosphor (5.41 × 1016 cps sr−1 m−2). Furthermore, we also show the first in vitro autofluorescence-free imaging through pork tissues by Si/InGaAs cameras monitoring Cr3+/Er3+ PersL, and demonstrate that the Er3+ emission can achieve higher spatial resolution than the Cr3+ emission due to the reduced light scattering at longer wavelengths. It is indicated that this material can act as a promising bio-probe for in vivo optical imaging, especially in the NIR-III biological window.

The development of noncontact thermometers with self-control to specific temperatures to be used as control markers with an additional degree of reliability is a challenge in the field of thermal sensors. Herein, a strategy exploiting the wide tunability of an intrinsic feature of oxide perovskites such as the phase-transition temperature to design a new class of ratiometric luminescent thermometers is introduced. The structural and optical response to the thermal stimuli of LaGaO

Persistent luminescence (PersL) imaging without real-time external excitation has been regarded as the next generation of autofluorescence-free and heating effect-free optical imaging technique. However, in order to acquire improved spatial resolution and deep penetration depth, developing new near-infrared (NIR) persistent phosphors with intense and long duration PersL over 1000 nm is still a challenging task. Herein, by utilizing the persistent energy transfer from Ce3+ to Er3+, we have successfully developed a series of garnet persistent phosphors of Gd3Al5-xGaxO12 (x=2.0, 2.5, 3.0, 3.5) doped with Ce3+, Cr3+ and Er3+ ions (GAGG:Ce-Cr-Er), which exhibit long yellow PersL ranging from 480 nm to 780 nm mainly due to the 5d-4f parity-allowed transition of Ce3+, and NIR PersL in the broad range from 1450 nm to 1670 nm due to the typical Er3+: 4I13/2→4I15/2 transition in garnets. Spectroscopic results of photoluminescence (PL), PersL, thermoluminescence (TL) and fluorescence/PersL decay curves of obtained garnet solid-solutions are discussed in detail, which suggest that GAGG:Ce-Cr-Er ganets with multi-wavelength PersL bands can be a potential candidate for longterm in-vivo optical imaging in the third biological window. Moreover, taking advantage of the Gd3+ host ion with seven unpaired electrons in its 4f shell, enhanced positive contrast for magnetic resonance imaging (MRI) also can be expected using this material as a T1-weighted agent. Thus, the GAGG:Ce-Cr-Er persistent phosphor in the form of nano-particles possesses the possibility as a dual-mode medical diagnosis platform featuring both the deep tissue penetration for in vivo bio-imaging and the high spatial resolution for MRI.

Persistent luminescent properties in Ce3+-Cr3+ codoped Gd3Al5−xGaxO12 garnet (GAGG:Ce-Cr) solid solution have been investigated. The persistent luminescent color is shifted from orange to yellowish green with increasing Ga content because Ce3+: 5d level splitting becomes much weaker. The depth of electron trap introduced by Cr codoping was estimated from the intense thermoluminescence glow peak by the initial rise method. The trap depth decreases from 0.56 eV to 0.29 eV with increasing Ga content. The shift can be explained by downshift of bottom of conduction band. From the persistent luminescence decay curve measurement after ceasing 450 nm blue illumination, the samples with x = 2.5 exhibited the longest persistent luminescence for 405 min until the luminance becomes 2 mcd/m2 in GAGG:Ce-Cr phosphors.

We have developed Pr3+–Cr3+ and Tb3+–Cr3+ co-doped Y3Al2Ga3O12 (YAGG) persistent phosphors with orange (Pr3+) and light green (Tb3+) persistent luminescence. The duration of their persistent luminance (of 0.32 mcd m−2) can last up to about 8 h and 12 h, respectively. On the other hand, the persistent luminescence intensities of Nd3+–Cr3+ and Dy3+–Cr3+ co-doped YAGG samples are much weaker than those of the Pr3+–Cr3+ and Tb3+–Cr3+ co-doped ones, and the dominant persistent luminescence is mainly due to the deep-red transition of Cr3+. The mechanisms of different persistent luminescence behaviors of these four samples can be explained by the vacuum referred binding energy (VRBE) diagram of lanthanide ions in the YAGG host. The stability of hole traps at the ground states of Pr3+, Nd3+, Tb3+ and Dy3+ ions and the possibility to collaborate with Cr3+ electron traps to induce persistent luminescence are discussed in detail.

Persistent phosphors have the ability to emit light long after the excitation has ended, typically by using thermal energy to liberate previously trapped charges. Alternatively, also photons can be used for the detrapping, leading to optically stimulated luminescence (OSL). This particular field of phosphor research has seen a strong expansion over the past two decades, with a steady growth of the materials library, an improved structural and luminescence characterization and the development of novel applications. Despite this success, clear challenges lie ahead in terms of a deeper understanding of the trapping mechanism and an associated optimization of the energy storage capacity being crucial for many applications. This focus issue “Persistent and Photostimulable Phosphors” within Optical Materials Express features papers presented at the third International Workshop on Persistent and Photostimulable Phosphors (IWPPP 2015) held at the University of Texas at Arlington.

By utilizing efficient persistent energy transfer from Ce3+ to Er3+, we have successfully developed a novel garnet persistent phosphor of Y3Al2Ga3O12 doped with Er3+, Ce3+, Cr3+ ions (YAGG:Er–Ce–Cr) exhibiting long (>10 h) near-infrared (NIR) persistent luminescence (PersL) in the broad range from 1450 nm to 1670 nm due to the typical Er3+:4I13/2 → 4I15/2 transition in garnet. The NIR PersL bands of Er3+ match well with the third bio-imaging window (NIR-III, approximately from 1500 nm to 1800 nm) and the response curve of InGaAs detectors. The photon emission rate (8.33 × 1017 cps Sr−1 m−2) of the YAGG:Er–Ce–Cr persistent phosphor at 10 min after ceasing blue light illumination was over two times higher than that of the widely used ZnGa2O4:Cr3+ deep-red persistent phosphor (3.30 × 1017 cps Sr−1 m−2). We also show the first PersL imaging by a commercial InGaAs camera monitoring Er3+ emission indicating that this material can be a promising candidate for in vivo bio-imaging in the NIR-III window.

Ce-doped 34Li2O–5MgO–10Al2O3–51SiO2 glass was prepared by the conventional melt quenching method and compared with commercially available material, Li-glass (Saint Gobain) on their optical and scintillation properties. They exhibited 80% transmittance at wavelength longer than 350 nm. Photoluminescence (PL) emission peaks of the prepared glass and Li-glass resulted 385 and 420 nm, respectively. Temperature dependences of PL of these materials were examined from 25 to 300 K and they showed different responses. In X-ray induced scintillation spectra, emission peaks were similar to those in PL. Primary scintillation decay time of the prepared glass was largely faster than the conventional Li-glass and resulted few ns. In thermally stimulated luminescence glow curve, the prepared glass showed a deeper trap and it showed better afterglow property. Finally, 252Cf neutron induced light yield was evaluated and the light yield of the prepared glass was around 10% of that of Li-glass.

We investigated the persistent luminescence (PersL) properties of a rare earth-free Mn2+ doped AlN (AlN:Mn) red phosphor together with a commercial SrAl2O4:Eu2+, Dy3+ green persistent phosphor as a reference. Similar to its photoluminescence (PL) spectrum, the PersL spectrum of the AlN:Mn phosphor exhibited a red emission band centered at 600 nm due to the Mn2+: 4T1(4G) → 6A1(6S) transition with a relatively narrow full width at half maximum (FWHM) of 43 nm. The luminance of AlN:Mn powders was 0.65 mcd/m2 at 60 min after ceasing ultraviolet (UV) illumination, and its duration upon 0.32 mcd/m2 could reach over 110 min. An extremely broad thermoluminescence (TL) glow curve was observed ranging from 100 K to 600 K and peaked at around 310 K, indicating a wide trap distribution in this material.

We report a new material, BaCN2:Eu2+ for a very sensitive optical pressure sensor, 50 times more sensitive than ruby. Photoluminescence spectra of the BaCN2:Eu2+ phosphor was measured under hydrostatic pressures from ambient pressure to 5.34 GPa at room temperature. The peak wavelength of the luminescence was drastically red-shifted at a rate of 19 nm GPa−1, which is approximately 50 times larger than that of the ruby, most commonly used as a pressure sensor in the high-pressure experiments. This large shift of the luminescence wavelength is suitable for application in optical pressure sensors for the high-pressure experiments without a high-resolution monochromator.

Persistent luminescence (PersL) imaging without real-time external excitation has been regarded as the next generation of autofluorescence-free optical imaging technology. However, to achieve improved imaging resolution and deep tissue penetration, developing new near-infrared (NIR) persistent phosphors with intense and long duration PersL over 1000 nm is still a challenging but urgent task in this field. Herein, making use of the persistent energy transfer process from Cr3+ to Er3+, we report a novel garnet persistent phosphor of Y3Al2Ga3O12 codoped with Er3+ and Cr3+ (YAGG:Er–Cr), which shows intense Cr3+ PersL (∼690 nm) in the deep red region matching well with the first biological window (NIR-I, 650–950 nm) and Er3+ PersL (∼1532 nm) in the NIR region matching well with the third biological window (NIR-III, 1500–1800 nm). The optical imaging through raw-pork tissues (thickness of 1 cm) suggests that the emission band of Er3+ can achieve higher spatial resolution and more accurate signal location than t...

The increasing interest in the development of ratiometric optical thermal sensors has led to a wide variety of new systems with promising properties. Among them, singly-doped ratiometric thermomete...

Yttrium aluminum garnet (YAG) doped with Ce3+ ion is known as an excellent phosphor for light-emitting diode (LED), usually used as a powder form dispersed in organic resins. We have developed translucent glass-ceramics (GC) of YAG: Ce3+ microcrystals in 2004. The GC sheet with half millimeter thick can work efficiently to make identical emission spectra with conventional white LED when combined with a blue LED. This report reviews the development history of the GC materials and impact for all inorganic solution for solid-state lighting.

Light-based technologies are strongly supported by various glass materials in general. This article focuses on glasses containing rare-earth elements, doped fibers, as well as their photonic functions such as optical amplifications in fiber telecommunication. Scientific progress and applications of rare-earth spectroscopy and technologies of doped glasses and fibers in recent 60 years have been more than dramatic. Development of various laser diodes has been strongly linked with realization of novel devices of rare-earth-doped glasses. In addition to many milestones listed in the resolution of International Year of Light, we should notice that 2015 is also the 50th anniversary of the erbium-doped glass laser, which is later transformed into the optical fiber amplifier, the enabler of today's information society.

A luminescent Eu, Dy: SrAl2O4 glass-ceramics with high transparency in the visible region was successfully synthesized using the frozen sorbet technique with the control of O2 partial pressure (PO2) for the oxidation of Eu2+ ions. The glass-ceramics include Eu2+, Eu3+, and Dy3+ ions, and thus exhibits three characteristic types of emission bands, 4f–5d at around 520 nm (Eu2+ ions), 4f–4f at 610 nm (Eu3+ ions), and 480 nm (Dy3+ ions). The Eu, Dy: SrAl2O4 glass-ceramics provide remarkable long-persistent luminescence under dark condition. The glass-ceramics also exhibits color-changing luminescence in the visible region based on their remarkable light storage properties. The luminescent Eu, Dy: SrAl2O4 glass-ceramics using the frozen sorbet technique with control of PO2 are promising materials for application in novel photonic and light storage materials.

In the past few years, red to near-infrared persistent phosphors activated by transition metals (Cr 3+ or Mn 2+ ) received much attention. The research was triggered by a recently proposed in vivo imaging application and it may show more significance in the future. In this review, we try to make a full list of known Cr 3+ or Mn 2+ -activated compounds showing red to near-infrared persistent luminescence, which may be useful for further exploration of new compounds. We also notice that radiance (in units of W sr −1 m −2 ) is even more suitable than luminance (in units of cd m −2 ) to quantify the brightness of red to near-infrared persistent phosphors for the in vivo imaging application. Finally, we discuss the mechanism of the persistent luminescence by taking an example of Cr 3+ -activated Zn 0.98 (Ga 1 − x Al x ) 2 O 3.98 persistent phosphors.

There is a special need to develop a dosimetry technique with a large-dynamic range and high-spatial resolution to characterize the microstructured X-ray beams used in microbeam radiation therapy (MRT) for cancer. We report the synthesis and characterization of oxyfluoride glass-ceramic (SiO2–Al2O3–CaF2–CaO–SmF3) plates, which contain trivalent-samarium-doped calcium fluoride (CaF2:Sm3+) nanocrystals, for use as a dosimetric detector material, particularly for MRT applications. Our approach utilizes the extent of Sm3+→Sm2+ valence reduction caused by X-ray irradiation as a probe of the X-ray dose delivered; and confocal fluorescent microscopy is used to read out the distribution of valence reduction through the photoluminescence (PL) signal. Our study showed that the Sm3+→Sm2+ valence reduction takes place in CaF2 nanocrystals, but not in the glass matrix. The Sm2+ shows PL emission predominantly due to the fast 4f55d1→7F0 transition, which allows us to read out the detector plate at a high scanning speed. Further, our experiments showed that the detection dose range reaches several thousands of grays, and X-ray dose distribution is detected at a micrometer scale. In addition, the Sm2+ signal can be erased either by heating the irradiated sample at a suitable high temperature or by exposing it to UV light; and the erased glass-ceramic plate is reusable. The new Sm-doped oxyfluoride glass-ceramic with CaF2 nanocrystals reported in this work shows potential for practical use in high-dose and high-resolution dosimetry for MRT.

Ag nanoparticles (AgNPs) solutions were prepared and grown by photochemical reaction using white light sources, specifically such as a Xe lamp, the sunlight, and an incandescent lamp. Dichroism, also called the “Lycurgus effect”, was observed in the obtained AgNPs solutions that were irradiated with white light for several days. The color of the transmitted light through the AgNPs solutions that were irradiated with the Xe lamp, the sunlight, and the incandescent lamp were orange, red, and blue, respectively, whereas the color of scattered light was greenish in all the samples. Based on the results of the transmittance, reflectance spectra, and scanning electron microscope images, the dichroic phenomenon is mainly caused by localized surface plasmon resonance light scattering of large triangular prismatic and decahedral AgNPs. In addition, we successfully prepared a dried silica gel that contains AgNPs with a dichroic property similar to the Lycurgus cup. This was fabricated by the solidification of the AgNPs solutions that were prepared under sunlight irradiation by a sol–gel method.

The temperature and compositional dependence of luminescence and photoconductivity were studied in Ce 3+ -doped garnet ceramics with the compositions of (Ce 0.005 Y 0.995 ) 3 Sc 2 Al 3− x Ga x O 12 ( x = 0, 1, 2, 3). With increasing Ga content in the garnet host, the Ce 3+ luminescence from the lowest 5d level (5d 1 ) was blue-shifted because the crystal field splitting of the 5d levels decreased. In addition, the quantum yield of the Ce 3+ luminescence at 300 K decreased and the luminescence quenching temperature of the Ce 3+ luminescence decreased with increasing Ga content. Although a photocurrent was not observed in the x = 0 sample even at 600 K, the photocurrent excitation bands corresponding to the lowest (5d 1 ) and the second lowest (5d 2 ) 5d levels were observed in the Ga-substituted garnet samples ( x = 1, 2, 3) even at lower temperatures. Following the analysis of the activation energy of the photocurrent, the relative energies of the Ce 3+ 5d levels and the conduction band of the garnet host are discussed.

Quantum cutting of 0.44 μm photon into 1.0 μm and 1.3 μm photons was demonstrated in Pr 3+ –Yb 3+ codoped oxyfluoride glass ceramics by spectroscopic investigation from blue to near-infrared region. The Pr 3+ –Yb 3+ codoped sample showed energy transfer (Pr 3+ : 3 P 0 → 1 G 4 )→(Yb 3+ : 2 F 5/2 ← 2 F 7/2 ). Both under 0.44 μm and 0.94 μm excitations, the codoped sample showed Pr 3+ : 1 G 4 → 3 H 4 luminescence at 1.3 μm, as well as Yb 3+ : 2 F 5/2 → 2 F 7/2 luminescence at 1.0 μm. The relative intensity of (Pr 3+ :1.3 μm/Yb 3+ :1.0 μm) by 0.44 μm excitation was much higher than that by 0.94 μm excitation. The mechanism of downconversion was proposed as a quantum cutting process by one-step energy transfer (Pr 3+ : 3 P 0 → 1 G 4 )→(Yb 3+ : 2 F 5/2 ← 2 F 7/2 ), which results in generation of two near-infrared photons via the Pr 3+ : 1 G 4 → 3 H 5 and Yb 3+ : 2 F 5/2 → 2 F 7/2 transitions.

Detailed spectroscopic analysis of the electronic configuration of Cr3+ in Bi2Ga4O9 is reported. The material exhibits unique luminescent properties arising from the crystal field experienced by Cr3+, with simultaneous strong sharp and broadband near-infrared emissions from the 2E and 4T2 excited states, in a wide range of temperature. The system displays dual near-infrared emission characterized by a remarkable thermal sensitivity over the whole explored range of temperatures, reaching a value of 0.7%·K–1 in the physiological range. Moreover, the possibility to absorb and emit in the first biological window, allows one to consider the system as a new promising candidate for ratiometric fluorescent thermal sensing in biotechnological applications.

Optical and optoelectronic properties of Ce3+ doped Mg3Y2Ge3-xSixO12 (Ce:MYGSG) (x=0, 0.5, 1.0, 1.5) inverse garnet phosphors were investigated. Since inverse garnets have various Ce3+ sites with different crystal field strength, the Ce:MYGSG shows inhomogeneous broadening in PL spectra. Quantum efficiency is at most 20%, which is much lower than that of Ce3+ doped Y3Al5O12 (Ce:YAG). Temperature dependence of PL intensity revealed that thermal relaxation via the cross point of the 5d1 and 4f levels is one of the quenching process in the Ce:MYGSG. By the photoconductivity measurements, thermal ionization and photoionization occur more easily in the Ge4+ rich sample. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical and electronic properties of the SrAl2O4:Eu2+-Dy3+ phosphor were investigated to analyze the mechanisms of its strong and long persistent luminescence. From the temperature dependence of photocurrent excitation spectra, the relative energy locations between the 5d levels of Eu2+ in two different sites and the conduction band were estimated. While most of the 5d levels of Eu2+ at the Sr site with weak crystal field are located within the conduction band, only the 5d levels of Eu2+ above 3.1 eV (400 nm) at the Sr site with strong crystal field are located within the conduction band. Based on the analysis of electron transfer from some electron traps, we assume that some electrons are transferred by a quantum tunnelling path. In addition, the persistent luminescence performance is good by the excitation of the usual blue LED (450 nm), composing white LEDs for the general illumination. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Transparent Ce:YAG ceramic phosphors were synthesized from the oxide powder which was produced by co-preparation method of the hydroxides. The Ce:YAG ceramics had a broad emission band peaked at 530 nm due to the 5d → 4f transition of Ce 3+ . The transmittances of the samples obtained were 70–87% at 800 nm. The absorption coefficient and emission intensity of Ce 3+ were increased with increasing thickness. Under 465 nm LED excitation, the color coordinates of the Ce:YAG ceramics shifted from the blue region to yellow region with increasing sample thickness, passing nearby the theoretical white point in the chromaticity diagram. The highest value of luminous efficacy of the ceramic white LED was 73.5 lm/W.

In Ce:YAG–GC, Ce 3+ ions, incorporated in the Y sites of the precipitated Ce:YAG micro crystals, emit yellow fluorescent light when they are optically excited by a blue LED. White light emission can be obtained by mixing the exciting blue light and the fluorescent yellow light. In the emission process of Ce:YAG– GC, light scattering generally occurs at the phase boundary between the precipitated Ce:YAG micro crystals and the glassmatrix phase due to the difference in refractive index. Since the light scattering changes the direction of both the exciting blue light and the fluorescent yellow light, this directly affects the luminous efficacy of Ce:YAG–GC phosphor. In this study, the interdependence between Ce:YAG particle si ze, luminous efficacy, and the light scattering coefficient were investigated in order to improve the luminous efficacy of Ce:YAG–GC phosphor. Since the luminous efficacy should be affected by the light scattering, as mentioned above, an understanding of light scattering phenomena is critical for the development of Ce:YAG–GC phosphor with higher luminous efficacy. 2. Experimental

The surface plasmon (SP) excited infrared-to-visible upconversion luminescence has been studied in the oxyfluoride glass ceramics containing Er 3+ -doped fluoride nano-crystals. Upconversion luminescence was observed at glass ceramics. Quantum yield of upconversion luminescence increased with increasing heat-treatment temperature. The transparent glass ceramics heat-treated at 700 °C was covered with a 50 nm gold film, and then attached to an SF10 prism with index-matching oil to make an attenuated total reflection (ATR) illumination. The intense upconversion luminescence bands at 540 and 660 nm were observed at the SP resonance angle by using p -polarized laser beam. The apparent dependences of the upconversion efficiency on laser polarization and on incident angle demonstrated the SP excited upconversion of Er 3+ .

We investigated optical power budget of Bi 2 O 3 -based erbium-doped fiber (BIEDF). Lateral spontaneous emissions and scattering laser powers in the BIEDF were measured quantitatively by using an integrating sphere. Compared with the power of amplified spontaneous emission and signal detected at the output fiber end, it was found that considerable powers were consumed by the laterally emitting lights. As an optically undetected loss limits power conversion efficiency (PCE) of the fiber amplifier, the effect of nonradiative decay from the termination level of pump excited state absorption (pump ESA) was estimated from decay rate analyses of the relevant levels. The nonradiative loss was comparable to amplified signal power in the BIEDF when pumped with a 980-nm LD. Nonradiative decay following cooperative upconversion (CUP) process is also discussed using rate equations analysis.

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 (x, y) 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.

Under the pumping of violet lighting emitting diode, quantum yields for multichannel transition emissions have been determined in Sm 3+ -doped heavy metal tellurite glass for the first time. For the derivation, the necessary fluorescence spectra were measured and calibrated in an integrating sphere connected to a CCD detector with a 400 μm-core optical fiber. The spectral power distribution of the sample was derived from the measured spectra first, and then the quantum yields of the visible emissions of Sm 3+ were calculated based on the distribution. The total quantum yield for four emission transitions of Sm 3+ in visible region is 4.07%. Integrating sphere with a CCD detector is proven to be a reliable and reproducible method to characterize luminescence and laser materials.

Quantum yields for multichannel transition emissions have been determined in a Eu3+-doped heavy metal tellurite glass under the pumping of a violet light emitting diode (LED) for the first time. To achieve this goal, the necessary fluorescence spectra were measured and calibrated in an integrating sphere, which was connected to a CCD detector with a 400 µm core optical fibre. The spectral power distribution of the sample under the violet LED pumping was derived from the measured spectra first; then the quantum yields for the visible emissions of Eu3+ were calculated based on the distribution, and the total quantum yield is 4.36%. For accurate measurements, an integrating sphere coupled with a CCD detector is proven to be a reliable and reproducible way to characterize luminescence and laser materials.

Tm 3+ /Yb 3+ -doped alkali-barium-bismuth-tellurite (LKBBT) glasses have been fabricated and characterized. Density, refractive index, optical absorption, absorption and emission cross-sections of Yb 3+ , Judd–Ofelt parameters and spontaneous transition probabilities of Tm 3+ have been measured and calculated, respectively. Intense blue three-photon upconversion fluorescence and near-infrared two-photon upconversion fluorescence were investigated under the excitation of a 980 nm diode laser at room temperature. Wide infrared transmission window, high refractive index and strong blue three-photon upconversion emission of Tm 3+ indicate that Tm 3+ /Yb 3+ co-doped LKBBT glasses are promising upconversion optical and laser materials.

Eu 3+ doped alkali–barium–bismuth–tellurite (Eu 3+ :LKBBT) glasses were prepared by conventional melt quenching. Twelve emission bands including infrequent blue and green bands are observed and they almost cover whole visible spectral region under violet light radiation. The blue and green emissions of Eu 3+ rarely appeared in oxide glasses before, but they have been clearly recorded in Eu 3+ :LKBBT glasses even in the case of high concentration doping of Eu 3+ . The analysis based on spontaneous-radiative rate, energy gap and Raman scattering reveals that the obtaining of the abundant multichannel emissions of Eu 3+ is due to the higher refractive index and the lower phonon energy in LKBBT glass system.

We demonstrate high-performance and wideband-bismuth-oxide-based erbium-doped fiber amplifiers (BI-EDFAs) with multi-stage configurations. Cascade configurations that gain flattening filters and isolators inserted between the doped fibers are applied in order to suppress amplified spontaneous emission (ASE). As a result of the optimization of the fiber length, the gain characteristics of the two-stage BI-EDFAs were improved in extended L-band region. The BI-EDFAs for four-channel coarse wavelength division multiplexing (CWDM) systems ranging from 1550–1610 nm are then presented. Three-stage configurations are applied, and a gain of 20 dB and an output power of 20 dB m are simultaneously achieved with practical pumping condition.

Tm 3+ -doped tellurite glass fiber was fabricated and the optical properties, such as upconversion fluorescence were investigated by a single- and a dual-wavelength pumping scheme. From the blue upconversion characteristics of the fiber, the completely different pumping mechanisms of Tm 3+ was shown between 1.05 and 1.4 μm pump as a main pump for the S + , S-band amplification. The ESA to the 1 G 4 level hardly occurs by the 1.4 μm pump, while intense blue upconversion was observed by the 1.05 μm pump.

Optical properties and local structure of rare-earth ions in glasses for broadband optical fiber amplifiers are reviewed. In a series of Er-doped glasses with broad 1.5 μm emission, heavy-metal oxide glasses are attracting great interest. Anomalous compositional dependences of optical properties of Er-doped antimony silicate glasses are shown. Structural implication of small compositional dependence observed for the Judd–Ofelt Ω 6 parameter and local phonon energy are discussed.

Optical and material properties of (75−x)TeO2–xGeO2–20ZnO–5Na2O–0.1Tm2O3 glasses were investigated as candidate materials for an S-band Tm-doped fiber amplifier (TDFA). With increasing GeO2 content, the lifetime and the quantum efficiency of the 1.46 μm emission decreased slightly, while the emission bandwidth, the Vickers hardness, and thermal stability of the glass improved monotonically. Above 20 mol% GeO2, the quantum efficiency decreased more rapidly with increasing GeO2. We conclude that addition of a small amount of germania may improve material properties without deteriorating the optical properties of doped Tm3+, and thus the germanotellurite fiber may be a more reliable material for the S-band TDFA in wavelength-division-multiplexing telecommunication.

Transparent glass ceramics containing CaF2 nano-crystals co-doped with Er–Yb were developed by heat treatment of glasses in the system SiO2–Al2O3–CaF2–LnF3 (Ln=Er, Yb). The crystal size of CaF2 increased with increasing heat-treatment temperature. Upconversion emission intensities at 660 and 540 nm increased dramatically with increasing heat-treatment temperature. The cathode luminescence mapping indicates that the visible Er-luminescence arises mainly from the precipitated CaF2 nano-crystals with a phonon energy lower than that of a silicate matrix. It is concluded that both Er and Yb were concentrated in the CaF2 nano-crystals, and the quantum efficiency of Er3+-luminescence and the energy transfer efficiency from Yb3+ were considerably improved after ceramization.

Phase-selective excitation by a field-emission electron gun was performed for an Er 3+ -doped Y 3 Al 5 O 12 (YAG) glass-ceramics, and the emission spectra due to the 4 S 3/2 → 4 I 15/2 transition of Er 3+ were measured. Powder X-ray diffraction analysis revealed that a single phase of YAG was precipitated in the glass-ceramics. Field-emission scanning electron microscope (FE-SEM) images indicated that two phases existed in the glass-ceramics. There was a large difference in the intensity of cathodoluminescence (CL) between these two phases. All of the spectral peaks of the phase with higher CL intensity were attributed to the 4 S 3/2 → 4 I 15/2 transition of Er 3+ in YAG crystal. The phase with lower CL intensity is considered to be composed of the glass phase including a small amount of YAG crystals. The CL from Er 3+ in the glass phase could not be detected, implying that almost all of Er 3+ ions are incorporated into YAG phase.

Gain characteristics of Tm-doped fiber amplifier were investigated by pumping with 1.05 μm and tunable L-band lasers. Red-shift and improved gain were observed by the pumping wavelengths in the 1.54–1.65 μm range. The gain values in the 1.44–1.50 μm range were improved by optimized pumping wavelengths and the optimized wavelengths were varied with the signal wavelength.

Optical properties and amplifier characteristics of Tm-doped fluoride fiber were investigated by pumping with 1.05 μm- and tunable L-band-lasers. Red-shift and improved gain in the S-band were observed by the pumping wavelengths in 1.55–1.64 μm in addition to the main 1.05 μm pump. The pump-power dependencies of gains were varied depending on the signal wavelengths in 1.44–1.52 μm region. The results were discussed with the laser level structures of the self-terminating transition of Tm 3+ ion for optimization of the S-band amplifier in the wavelength-division-multiplexing telecommunication.