Ning Da

Luminescence from Bi2+-activated alkali earth borophosphates for white LEDs.

Bi(2+)-doped MBPO(5) (M = Ba(2+), Sr(2+), Ca(2+)), synthesized in air via solid state reaction, are considered as novel orange and red phosphors for white light emitting diodes with improved colour quality. Absorption of Bi(2+) due to (2)P(1/2)-->(2)S(1/2) and (2)P(1/2)-->(2)P(3/2) could be observed and quantified. Excitation to (2)P(3/2) is accompanied by vibronic sidebands, and corresponding emission behaviour is found. The electron-phonon coupling strength increases in the order M = Ba(2+)-->Sr(2+)-->Ca(2+). In the case of MBPO(5):Bi(2+), one-, two- and even three-phonon sidebands could clearly be observed. The crystal structure of all three compounds belongs to space group P3(1)21. Bi(2+) is incorporated on M(2+) sites, and reduction of Bi(3+) to Bi(2+) occurs for reasons of charge compensation. In accordance with crystallographic data, fluorescence decay behaviour indicates that only one type of Bi(2+)-emission centers is present.

Enhanced photoluminescence from mixed-valence Eu-doped nanocrystalline silicate glass ceramics.

Intense photoluminescence was observed from mixed-valence Eu-doped nanocrystalline BaAl₂Si₂O₈/LaBO₃ glass ceramics. For preparation in air, the ratio between Eu3+ and Eu2+ luminescence depends on synthesis temperature. XRD, TEM and IR absorption spectra were employed to characterize the crystallization process and structural properties of the precursor glass and corresponding glass ceramics. When annealed at 950 °C, the material exhibited photoluminescence more than ten times stronger than was found in its glassy state. Spectroscopic data indicate that during such a heat treatment, even in air, a significant part of the Eu3+ ions is reduced to Eu2+. Lifetime of the ⁵D₀ state of Eu3+ increases with increasing heat treatment temperature. Eu3+ species are largely incorporated on La3+ sites in LaBO₃ crystallites whereas Eu2+ locates on Ba2+ sites in the hexacelsian phase. A mechanism for the internal reduction of Eu3+ to Eu2+ is proposed. Spectroscopic properties of the material suggest application in additive luminescent light generation.

Intense red photoluminescence from Mn 2+ -doped (Na + ; Zn 2+ ) sulfophosphate glasses and glass ceramics as LED converters

We report on intense red fluorescence from Mn(2+)-doped sulfophosphate glasses and glass ceramics of the type ZnO-Na(2)O-SO(3)-P(2)O(5). As a hypothesis, controlled internal crystallization of as-melted glasses is achieved on the basis of thermally-induced bimodal separation of an SO(3)-rich phase. Crystal formation is then confined to the relict structure of phase separation. The whole synthesis procedure is performed in air at <or= 800 degrees C. Electron spin resonance and Raman spectroscopy indicate that Mn(2+) species are incorporated on Zn(2+) sites with increasingly ionic character for increasing concentration. Correspondingly, in the glasses, increasing MnO content results in decreasing network polymerization. Stable glasses and continuously increasing emission intensity are observed for relatively high dopant concentration of up to 3 mol.%. Recrystallization of the glass results in strongly increasing emission intensity. Dynamic emission spectroscopy reveals only on type of emission centers in the glassy material, whereas three different centers are observed in the glass ceramic. These are attributed to octahedrally coordinated Mn(2+) in the residual glass phase and in crystalline phosphate and sulfate lattices, respectively. Relatively low crystal field strength results in almost ideal red emission, peaking around 625 nm. Excitation bands lie in the blue-to-green spectral range and exhibit strong overlap. The optimum excitation range matches the emission properties of GaN- and InGaN-based light emitting devices.

All-solid bandgap guiding in tellurite-filled silica photonic crystal fibers

We report all-solid bandgap-guiding fibers formed by pumping molten tellurite glass into silica-air photonic crystal fiber at high pressure. The spectral positions of the guidance bands agree well with multipole simulations and bandgap calculations. The micrometer-diameter tellurite strands are found to contain microheterogeneities (most probably originating from devitrification), which increase the fiber attenuation, although no evidence of crystallization is seen in the bulk tellurite glass. The technique offers a potential route to employing difficult-to-handle glasses, or glasses unsuitable for fiber drawing, in fiber-based amplifiers, modulators, filters, and nonlinear devices.

From plasmonics to supercontinuum generation: Subwavelength scale devices based on hybrid photonic crystal fibers

Photonic crystal fibers (PCFs) consist of arrays of micrometer-size hollow channels extending along the entire fiber length. A new approach to modify the optical properties and to improve the design flexibility of such fibers is to fill the air holes of the array by with different materials, realizing all-solid hybrid fiber structures. In this talk, recent results on silica PCF filled with noble metals, semiconductors or soft glasses are presented. Depending on the actual material being filled into the holes, we observed plasmonic excitations, Mie-resonances and photonic band gaps.