V.D. Rodríguez

On the local structure of Eu3+ ions in oxyfluoride glasses. Comparison with fluoride and oxide glasses

Broadband and fluorescence line narrowing optical spectroscopic studies have been used to investigate the local environments of Eu3+ ions in lithium fluoroborate glasses. From the vibronic spectra, different borate groups coupled with the Eu3+ ions have been identified. A pulsed tunable dye laser has been used to selectively excite the 5D0 level of the Eu3+ ion and the subsequent 5D0→7F1 fluorescence spectra have been monitored as a function of the exciting wavelength. From these FLN studies, three 7F1 Stark levels have been identified and a C2v orthorhombic symmetry has been assumed in the subsequent calculation of the crystal-field parameters for the different environments occupied by the Eu3+ ions in the glass. The second rank crystal-field parameters have been systematically analyzed for the Eu3+:lithium fluoroborate glass from the site dependent behavior of the 7F1 level splitting. The importance of the J-mixing in the crystal-field analysis has been emphasized. An appropriate method for comparing th...

Optical properties of Er3+ ions in transparent glass ceramics

Abstract A study of optical properties and upconversion processes among Er 3+ ions in oxyfluoride glass and glass ceramic matrix has been carried out. From optical absorption spectra, the oscillator strengths have been obtained for several transitions and they have been used to calculate the Judd–Ofelt parameters. Experimental lifetime values are compared with those obtained with the Judd–Ofelt theory. Different upconversion emissions at 545, 660 and 800 nm have been obtained in Er 3+ doped glass and glass ceramics by exciting at 975 nm. A systematic investigation of the green upconversion is reported with the purpose of determining the involved upconversion mechanisms.

Optical properties of Nd3+ ions in oxyfluoride glasses and glass ceramics comparing different preparation methods

A study of optical properties of Nd3+ doped oxyfluoride glasses and glass ceramics prepared by three different methods has been carried out. These methods start from NdF3, Nd2O3, or a Nd3+ ion solution as doping agent. The alternative preparation method based on a preliminary dissolution of the Nd3+ ions is proposed in order to avoid nonhomogeneous dopant distribution and spontaneous devitrification during glass elaboration. In the frame of the Judd–Ofelt theory, main radiative parameters have been studied: transition probabilities, lifetimes, and stimulated emission cross sections. Fluorescence decay curves have been also analyzed in order to study the final distribution of the Nd3+ ions after the ceramming process, discerning between ions that reside in the fluoride nanocrystals precipitated during heat treatment and those remaining in the glassy phase. The NdF3 based glass ceramics present the best values for spectroscopic parameters as the stimulated cross section of the 4F3/2→4I11/2 laser transition.

Optical spectroscopy analysis of the Eu3+ ions local structure in calcium diborate glasses

Abstract A detailed analysis of the optical properties of Eu3+ ions in calcium diborate glasses has been performed in order to correlate them with the local environment of the lanthanide ions in these glasses. From the excitation and emission spectra under broadband excitation the energy level diagram of the Eu3+ ion and the atomic parameters have been obtained. Moreover, it is found that the Eu3+ ions are predominantly bonded with the bridging and non-bridging oxygens of different borate groups containing the BO3 units. Under site selective excitation in the 7 F 0 → 5 D 0 inhomogeneously broadened peak, valuable information about the splitting of the 7F1 and 7F2 levels and the lifetime of the 5D0 level has been obtained as a function of the excitation wavelength. The linewidth of the low-energy components of the 5 D 0 → 7 F 1 emission shows an increase with the splitting of this level that can be well explained considering non-radiative de-excitation processes towards lower 7F1 Stark levels. From the splitting of the 7F1 and 7F2 levels, the second and fourth rank crystal-field parameters have been calculated as a function of the excitation wavelength. The observed behaviour suggests the existence of an unique class of site for the Eu3+ ions, with a large range of values for the crystal-field strength parameter NV(B2q), from 1000 to 3200 cm−1, that includes weak, medium and strong crystal-field environments for the Eu3+ ions in the calcium diborate glass. The J-mixing is shown to play a relevant role in these calculations for medium and strong crystal-field environments, i.e., for values of the crystal-field strength parameter NV(B2q) over 1400 cm−1.

Cooperative energy transfer in Yb3+–Tb3+ codoped silica sol-gel glasses

Optical properties of Yb3+–Tb3+ codoped silica sol-gel samples have been studied after the gel to glass transition. Different upconversion emissions have been observed under near infrared excitation at about 1 μm. The Tb3+ ions are excited by means of energy transfer processes from Yb3+ ions. The temporal evolution of the blue-green upconversion emissions coming from Tb3+ ions and their dependence on the excitation intensity at about 1 μm has been studied. The experimental results are in good agreement with a cooperative resonant energy transfer mechanism from Yb3+ ions. An efficient backtransfer process is observed from Tb3+ ions towards Yb3+ ions. The upconversion efficiency, which is limited by this backtransfer process, has been obtained and compared with other upconversion results in similar matrix.

Preparation and luminescence of bulk oxyfluoride glasses doped with Ag nanoclusters

Bulk oxyfluoride glasses doped with Ag nanoclusters have been prepared using the melt quenching technique. When pumped in the absorption band of Ag nanoclusters between 300 to 500 nm, these glasses emit a very broad luminescence band covering all the visible range with a weak tail extending into the near infrared. The maximum of the luminescence band and its color shifts to the blue with a shortening of the excitation wavelength and an increasing ratio of oxide to fluoride components, resulting in white color luminescence at a particular ratio of oxide to fluoride; with a quantum yield above 20%.

Optical nanoheater based on the Yb3+-Er3+ co-doped nanoparticles.

Yb(3+)-Er(3+) co-doped fluoride nanoparticles have been prepared. When pumped by 975 nm laser diode into absorption band of Yb(3+), the laser-induced temperature rise up to 800 degrees C has been detected in the nanoparticles by measuring the ratio of the intensities of the thermalised up-conversion luminescence bands (2)H(11/2)-->(4)I(15/2) and (4)S(3/2)-->(4)I(15/2) of Er(3+). These results show that a controlled optical heating of the nanoparticles and their surrounding nano-volumes can be realised, while the location and temperature rise of the nanoparticles and heated nano-volumes can be detected distantly by means of luminescence.

Optical properties and cross relaxation among Sm3+ ions in fluorzincate glasses

Optical absorption and emission spectra are presented for Sm3+ ions in ZnF2−CdF2-based glasses (0.1, 1.5 and 2.5 mol% of Sm3+). The measured oscillator strengths and branching ratios for several transitions are compared with values obtained with The Judd-Ofelt theory. Energy transfer among Sm3+ ions has been investigated. The decay curves are nonexponential when the concentration is 1.5 or 2.5 mol%. It can be explained by cross relaxation between ions of Sm3+. Energy migration does not play a relevant role in these decay processes.

Energy transfer with migration. Generalization of the Yokota–Tanimoto model for any kind of multipole interaction

The dynamics of luminescence decay is analyzed within the general scenario of randomly distributed ions with whatever multipole interaction, taking into account energy transfer and migration processes among them. By applying Pade approximants to the Allinger and Blumen expression for the excited state population, a simple expression has been obtained which can be easily fitted to experimental data. This expression can be considered a generalization of the dipole–dipole Yokota–Tanimoto equation. On the other hand, the decay curves from the 5D3 level of the Tb3+ ions in fluorozincate glasses have been measured for different Tb3+ concentrations. The results are analyzed in the framework of the proposed expression.

Increase of the 800 nm excited Tm3+ blue upconversion emission in fluoroindate glasses by codoping with Yb3+ ions

Intense Tm3+ blue upconversion emission has been observed in Tm3+–Yb3+ codoped fluoroindate glasses under direct excitation into Tm3+ ions with a diode laser at 796 nm. The dependence of the intensity of this upconversion emission on the Tm3+ and Yb3+ concentration has been studied in order to determine the optimum ion concentrations and the involved mechanisms. The blue upconversion emission is highly increased with the Yb3+ concentration. For fixed Yb3+ concentrations, the maximum upconversion efficiency was obtained with Tm3+ concentration of about 0.5 mol%. For this optimum Tm3+ concentration, the upconversion emission intensity is increased by a factor about 100 by codoping with 2.25 mol% of Yb3+. The results are well explained by a proposed rate equation model.