E.W.J.L. Oomen

Europium (III) in oxide glasses: Dependence of the emission spectrum upon glass composition

Abstract Several Eu3+-doped silicate, germanate, borate and phosphate glasses have been investigated in order to study the dependence of the Eu3+ emission spectrum upon the host composition. In these glasses, variations in the Eu3+ emission spectrum are considered to be restricted to differences in the intensities of the 5 D 0 → 7 F 2 and 5 D 0 → 7 F 4 , bands, which depend on the Judd-Ofelt parameters Ω 2 and Ω 4 respectively. In germanate glasses, the network-former ions (exchanging Ge by Si), the modifier ions and the modifier concentration have been varied. Evaluation of the Eu3+ emission in these glasses reveals that the 5 D 0 → 7 F 2 intensity ( Ω 2 ) depends on short-range effects such as the covalency of the Eu3+ ligands and structural changes in the vicinity of the Eu3+ ion. The 5 D 0 → 7 F 4 intensity ( Ω 4 ) seems to depend mainly on long-range effects (related to bulk properties of the glass). Studies of (99 1 2 − x) B 2 O 3 ·x Li 2 O · 1 2 Eu 2 O 3 glasses reveal a rather large difference in Ω 2 when the network former units change from tetrahedral BO4 units to BO3 triangles. For phosphate glasses hardly any trends are observed between the Eu3+ emission bands and the glass composition. This is ascribed to the structural differences between phosphate glasses and other oxide glasses.

Up-conversion of red light into blue light in thulium doped fluorozirconate glasses

Abstract Up-conversion of red light with a wavelength of about 650 nm in Tm3+ doed flourozirconate glasses results in blue light. This blue light consists of two emission bands; one at 450 nm which is ascribed to the 1D2→3H4 transition, the others at 475 nm is ascribed to the 1G4→3H6 transition. The emission intensities of both bands vary quadratically with the excitation power. The up-conversion mechanisms for these bands are elucidated from the results of the (up-conversion) emission and excitation spectra and decay time measurements of fluoride glasses with different Tm3+ concentrations. For glasses with a Tm3+ concentration of 0.2 mol% and greater, cross-relaxation processes occur which decrease the up-conversion efficiency. These cross-relaxation processes turn out to be very efficient in Tm3+ doped fluoride glasses.

The incorporation of Rhodamine B in silica sol-gel layers

Abstract The effect of the experimental conditions of the sol-gel process on the incorporation of the dye Rhodamine B in a silica layer and the consequent optical-, leaching- and light fastness-properties are reported. If at least 20 alkoxy silane groups per molecule Rhodamine B are available and if these silanes are sufficiently hydrolyzed, the dye is completely encapsulated by a three dimensional ‘cage’ of SiO bonds. If this encapsulation is not the case, the dye is not properly incorporated, which results in leaching of the dye when the coating is immersed in water and in reduction of light fastness. A low acid concentration in the coating solution results in the incorporation of large amounts of Rhodamine B clusters in the silica layer. These clusters have a light absorption (maximum at 510 nm) different than incorporated monomers of Rhodamine B (maximum 560 nm).

On the origin of the red emission band from erbium doped fluoride glasses excited with 800 nm

Abstract Upconversion of 800 nm excitation in erbium doped fluoride glasses results in a strong green emission and a weak red emission. The origin of the red emission is elucidated. For fluoride glasses with an erbium concentration up to about 1 mol% it originates from the nonradiative relaxation of the green emitting 4S 3 2 level to the red emitting 4F 9 2 level. This results in a quadratic dependence of the red emission intensity on the excitation power, Pex. For glasses with an erbium concentration higher than 1 mol% the population of the red emitting level is mainly due to a cross-relaxation process between two erbium ions; one in the 4S 3 2 and the other in the 4I 9 2 state. In this case the red emission intensity depends on (Pex) 3 2 .

Upconversion in Tm3+-doped ZBLAN glasses

Abstract Upconversion emission spectra (at 650 nm excitation) and excitation spectra (from 620–680 nm) are presented for Tm-doped ZBLAN glasses. Two blue emission bands become apparent with maxima at 450 and 480 nm. The proposed upconversion mechanism explains the spectra and is in agreement with the observed quadratic dependence of the intensities of both emission bands with excitation power. Cross-relaxation processes, decreasing the upconversion efficiency, become active for Tm concentrations above 0.2 mol%.

The effect of TEOG on the hydrolysis-condensation mechanism of a two-step sol-gel process of TEOS

The effect of the addition of tetraethoxygermane (TEOG) in the basic step of a two-step silica sol-gel process on the hydrolysis-condensation mechanism of tetraethoxysilane (TEOS) was investigated. Gelation time versus hydrolysis time curves were recorded for mixtures of TEOS, ethanol and water to which TEOG was added. It is found that an ethoxy group of TEOG reacts with silanols under ethanol formation. The rate of this reaction is almost independent of the base concentration. Because unhydrolysed TEOG has four reactive groups, it is efficient in cross-linking chains, which implies that in general its addition results in shorter gelation times. This effect becomes more apparent for mixtures with low water content. At high TEOG: silanol ratios, however, an increase in the gelation time is found to be due to the capturing of silanols by TEOG. The cross-linking effect of TEOG becomes weaker if it is partly hydrolysed before addition to the TEOS mixture.

Hydrolysis-condensation mechanism of a two-step sol-gel process of mixtures of TEOS and TEOG

Abstract The effect of the addition of tetraethoxygermanate (TEOG) on the hydrolysis-condensation mechanism of a two-step silica sol-gel process was investigated. Gelation time versus hydrolysis time curves were recorded for mixtures of TEOS, ethanol and water to which TEOG was added. It was found that TEOG reacts with silanols under ethnaol formation. Because TEOG has four reactive groups, it is efficient in cross-linking chains, which implies that, in general, its addition results in shorter gelation times. At high TEOG concentrations, an unexpected dissolution of the previously obtained gel was observed.

The influence of cross-linking on gel formation

Abstract Besides the rate of the hydrolysis and condensation reactions, cross-linking also has a strong influence on gel formation of tetraethoxysilane (TEOS). Parameters which have an effect on cross-linking, as the water concentration, the functionality of the precursor and the addition of tetraethoxygermane (TEOG) are investigated. Gelation time versus hydrolysis time curves were recorded of different TEOS, ethanol, water systems with different water concentrations and systems to which different alkyl-substituted silanes and TEOG were added. It is found that the polymerization reaction of TEOS is dominated by silanol-silanol condensation whereas for condensation of TEOG an ethoxy group reacts with silanol under ethanol formation. It is also found that the gelation process of mixtures to which an alkyl-substituted silane is added, is slower than that of TEOS itself although both the hydrolysis rate and the condensation rate of each of these compounds are higher.

Upconversion in rare-earth-doped fluoride glasses

Er3+ doped heavy metal fluoride glasses are studied by use of the upconversion process because they can efficiently convert 800 nm light from a diode laser into visible light. The dynamics of the upconversion processes and the efficiencies of these processes vary with composition and preparation route of the heavy metal fluoride glasses. The dynamics of the upconversion processes in these glasses is discussed, and the dependence on rare-earth concentration, fluoride glass composition, and preparation conditions are discussed. A practical application of this work, the construction of an upconversion laser, is also described.