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The luminescence properties of Ce3+-doped GdxY3-xAl5-ySiyO12-yNy (x = 0, 1 and 2; y = 0.25) oxynitride garnet phosphors were investigated and compared with corresponding oxide garnet phosphors (y = 0). The measured photoluminescence spectra exhibit redshift of the emission peak wavelength by the partial nitridation of oxide garnet phosphors because of the strong crystal field splitting and the nephelauxetic effect. Temperature dependence studies of the photoluminescence intensity revealed that the quenching temperature decreased by partial nitridation and Gd substitution. The thermal activation energy, estimated from temperature dependence of the fluorescent lifetime, was found to decrease from 0.85 eV (x = 0, y = 0) to 0.27 eV (x = 2, y = 0.25).

ZnGa2O4:Cr3+ is a well-known deep red (persistent) phosphor with R-lines due to the 2E→4A2 spin-forbidden transition of Cr3+ ions between 690 and 750 nm. Recently, this system has attracted a great deal of interest for in-vivo bio-imaging. In this study, we investigated the temperature dependence of photoluminescence (PL) spectrum of ZnGa2O4:Cr3+ and the possibility of using this material for thermometry. At 80 K, the zero-phonon lines and phonon-side bands of the 2E→4A2 transition were observed in the PL spectrum. At higher temperatures above 250 K, a broad PL band was observed at around 720 nm in addition to the sharp 2E→4A2 luminescence. After determination of the Stokes shift energy between the PL and PL excitation spectra and the increase of broad luminescence band intensity with increasing temperature, we attribute this additional broad luminescence band to the spin-allowed 4T2→4A2 transition of Cr3+ ions. The fluorescence intensity ratio of these emission transitions (4T2→4A2)/(2E→4A2) shows good linearity from 270 K to 700 K in the Arrhenius plot. The activation energy needed for excitation from the 2E state to the 4T2 state of Cr3+ was estimated to be 1744 cm−1. Based on our results, we suggest that the ZnGa2O4:Cr3+ system can work well as a ratiometric thermometer with a high relative sensitivity of 2.8 %K−1 (at 310 K) in a wide temperature range.

We have developed the Ba2SiO4: Eu2+ -Er3+ persistent phosphor. The vacuum referred binding energy (VRBE) diagram of the Ba2SiO4 host with the 4f ground states of divalent and trivalent lanthanide ions in the host band gap was constructed from the obtained spectroscopic data and the Dorenbos's procedure. From the constructed diagram, the VRBE values of the conduction band (CB) bottom and valence band (VB) top were estimated to be −0.66 and −8.27 eV, respectively. From the energy gap between the CB bottom and 4f ground states of divalent lanthanide ions, we selected the Er3+ ion as an efficient co-dopant because the trap depth was estimated to be 0.64 eV, suitable for improving the persistent luminescence duration. The Eu2+-Er3+-codoped sample showed persistent luminescence intensity of over 0.32 mcd/m2 for 7 h duration. The initial persistent luminance intensity of the Eu2+-2%Er3+-codoped sample was found to be 250 times stronger than that of the only Eu2+-doped sample.

We report the synthesis and luminescence properties of new layered mixed-anion compounds, Sr2ScCuSeO3 and Sr3Sc2Cu2Se2O5. Sr2ScCuSeO3 and Sr3Sc2Cu2Se2O5 were synthesized by solid-state reaction and were investigated using x-ray diffraction, diffuse reflectance, and photoluminescence spectroscopies. Both compounds have very similar properties except for their optical band gaps and emission peak energies. At room temperature, Sr2ScCuSeO3 and Sr3Sc2Cu2Se2O5 exhibit optical band gaps energies of 2.9–3.0 eV. The excitonic luminescence with very fast emissions of ∼30 ps were observed in both compounds even at room temperature. This feature of the compounds makes them suitable for fast scintillator applications. Our results suggest that the luminescence properties such as emission peak energies and lifetimes of these layered mixed-anion compounds can be controlled by changing their structure and composition.

Nowadays highly efficient red phosphors with blue or near-UV excitation are strongly required to improve performance of existing white LEDs. With this aim in view, the Eu3+-doped YSiO2N ceramics with the pseudo-wollastonite structure was fabricated and thoroughly studied spectroscopically. The presence of two anions – O2− and N3− − around Eu3+ ions causes a shift of the host valence band (VB) to lower energy and a certain asymmetry of the impurity sites. After illumination by near-UV excitation light (280–360 nm), the YSiO2N:Eu3+ sample exhibited strong red luminescence at around 620 nm assigned to the hypersensitive 5D0→7F2 transition of the Eu3+ ions. Compared with three Eu3+-doped oxides (α-CaSiO3:Eu3+ which has the same pseudo-wollastonite structure, and α- and y-Y2Si2O7:Eu3+ which have the same cation composition), the charge transfer (CT) band was red-shifted by more than 9.5 × 103 cm−1 because the VB top increases. From the Judd-Ofelt analysis, it was found out that the YSiO2N:Eu3+ sample has the large Judd-Ofelt intensity parameter, Ω2, which is correlated with the dopant's site asymmetry. As expected, the mixed-anion coordination which consists of oxide and nitride anions resulted in the strong hypersensitive transition at 620 nm, which is characteristic Eu3+ red luminescence at the lower symmetry site. In addition, the fluorescence lifetime of the Eu3+ luminescence at 0 K (τ0) was estimated experimentally to be 0.664 ms, which was shorter than the radiative lifetime (τR), 1.32 ms, based on the Judd-Ofelt analysis. The experimental quantum yield measured with an integrating sphere, 32.3%, was lower than the internal quantum efficiency, 50.3%, evaluated from the ratio of τ0/τR. The proposed material can be suitable for applications as a red phosphor.

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 report the 5d–4f luminescence of Eu2+ and europium trapped exciton (ETE) luminescence in the different three crystallographic Ca sites in the Eu2+-doped CaAl2O4 and those quenching processes based on the results of the temperature dependence of photoluminescence and photoconductivity. The CaAl2O4:Eu2+ shows the 5d–4f luminescence peaked at 440 nm, 540 nm and the ETE luminescence peaked at 650 nm by UV and blue light excitation at low temperatures. The quenching temperature, at which the luminescence intensity is 50% of the intensity at low temperatures, of 440 nm, 540 nm and 650 nm bands are >300 K, 195 K and 69 K, respectively. The strong quenching for the 650 nm band is caused by the thermal ionization process, which was proved by the photoconductivity measurement. We also construct the vacuum referred binding energy diagram in each Ca site and discuss optical properties from the relative energy location between the 5d state, ETE state and conduction band.

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.

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.

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.

The optical spectroscopy and excited state dynamics of the 5D3 and 5D4 levels of Tb3+ and 5D0 level of Eu3+ have been studied in double phosphate materials having the eulytite disordered cubic structure. In the case of Sr3Tb0.90Eu0.10(PO4)3, Tb3+ → Eu3+ energy transfer is observed upon excitation in the Tb3+ energy levels located in the UV region. This transfer gives rise to strong emission from the 5D0 level of Eu3+, peaking in the red spectral region at 612 nm. The energy transfer efficiency from the 5D4 level of Tb3+ in Sr3Tb0.90Eu0.10(PO4)3 has been evaluated and the luminescence quantum yields of Sr3Tb(PO4)3 and Sr3Tb0.90Eu0.10(PO4)3 have been measured.

The loss mechanism in the Pr3+–Yb3+ codoped glass that showed quantum cutting (QC) phenomenon was investigated and we found that the multi-phonon relaxation from Pr3+:1G4 is the major loss to the quantum cutting process. To reduce the loss, Pr3+–Yb3+ codoped glass ceramics containing SrF2 crystals were prepared and quantum yields (QY) of Yb3+ emission by 440 nm excitation were measured. Only glass ceramics with the highest Yb concentration (x = 2.9) showed higher QY than that of the as-made glass. It is considered that in low Yb concentration Pr3+ and Yb3+ ions were divided into different matrix possibly because of different ionic radius between Pr3+ and Yb3+ ions.

We have developed a novel glass–ceramic material containing Er 3+ :YAG micro-crystals as the only crystalline phase by simple crystallization process of a glass in an Y 2 O 3 –Al 2 O 3 –SiO 2 -based system. While Y 2 Si 2 O 7 , Y 4.67 (SiO 4 ) 3 O, LiAlSiO 4 and LiAlSi 2 O 6 crystal phases were observed in glass–ceramics by lower crystallizing temperatures, single-phase YAG micro-crystals were precipitated by crystallizing the glass above 1350 °C. The YAG particle sizes were around 5 μm in average and dispersed homogeneously in the glass-matrix. From the energy dispersive X-ray analysis, the Er ions were found to be introduced selectively in the YAG micro-crystals. By 970 nm excitation, the glass ceramic showed a broad-band photoluminescence in the 1450–1670 nm range, which is characteristic to the 4 I 13/2 → 4 I 15/2 transition in the Er:YAG crystal, compared with the conventional silica-based glasses, and each emission line-width was broader than that of Er:YAG polycrystal.

Er3+-doped YAG (Er:YAG) nanocrystals were prepared by a glycothermal synthesis, and the effect of heat-treatment on the size of the nanocrystals and their fluorescence properties in optical-telecommunication-band were examined. The crystallite size of the as-prepared nanocrystals was around 32 nm, which was evaluated from the broadening of X-ray diffraction peaks. When the heat-treatment temperature was 600 °C, the crystallite size was almost independent of the heat-treating time, while it increased with time at 800 and 1000 °C. The emission intensity of the 4I13/2 → 4I15/2 transition and the lifetime of the 4I13/2 level increased with increasing heat-treatment temperature and time. The decrease in the organic residues due to the combustion during the heat treatment at 600 °C and the decrease in the specific surface area induced by the heat treatment at 800 and 1000 °C are responsible for the observed phenomena, i.e., the increase in the emission intensity and lifetime. Quantum efficiencies of Er3+ ions in the nanocrystals were also evaluated based on an assumption that the radiative decay rates of the nanocrystals are equal to that of the bulk crystal. The relationship between the quantum efficiency and the specific surface area of the nanocrystals were discussed with a simple model, which led to the conclusion that Er3+ ions in the surface region with thickness about 10 nm do not work well as luminescence centers due to the many defects. The Er:YAG nanocrystals prepared by the glycothermal method had emission linewidth broader than the polycrystalline bulk crystal prepared by a solid-state reaction.

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.

Racah parameters of Cr4+ ions in alumino-silicate glasses have been calculated, indicating most Cr4+ ions are located at low-field sites. Using three-peak Gauss fitting for the luminescence lineshape of Cr4+:alumino-silicate glass at low temperatures, the existence of Cr4+ at high-field sites has been demonstrated for the first time in the alumino-silicate glass. The temperature dependence of the emission of low-Cr2O3 glass shows blue shift because most of the Cr4+ ions are in the low-field sites; in which the Cr4+ ions show the large difference of the thermal population at different excited states due to the non-radiative processes. On the other hand, the concentration dependence of the emission of Cr4+ ions in alumino-silicate glass shows red shift, caused by the energy transfer between Cr4+ ions in different crystal-field sites, and subsequently the mechanism of energy transfer between Cr4+ ions at low-field sites and at high-field sites has been proposed.

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.

Concentration and temperature dependences of lifetime of Yb 3+ : 2 F 5/2 level were investigated in tellurite glasses doped with up to 6 mol% Yb 2 O 3 . The lifetime obtained by 1.02 μm emission from the Yb 3+ ions at room temperature was insensitive to the Yb 2 O 3 concentration in singly doped glasses. However, the Yb-lifetime showed strong concentration quenching with the presence of only 0.1 mol% Pr 2 O 3 . The energy transfer efficiency from Yb to Pr in a codoped glass was found to be more than 90% even in the glass of [Yb]/[Pr] > 100. The Yb-lifetime in singly doped glass increased and that in codoped glass decreased with increasing temperature. These results can be explained by assuming rapid energy migration between Yb ions and promotion of Yb 3+ : 2 F 5/2 → Pr 3+ : 1 G 4 transfer at higher temperature.