A. Speghini

Optical spectroscopy of nanocrystalline cubic Y2O3:Er3+ obtained by combustion synthesis

We report the emission and upconversion spectra and the lifetimes of Er3+ doped Y2O3 nanocrystalline and bulk samples. We found that when the nanocrystal and bulk samples of Y1.80Er0.20O3 were excited at 815 nm, the overall emission intensity was stronger for the bulk sample. However, the relative intensity of the (2H11/2, 4S3/2)→4I15/2/4F9/2→4I15/2 transitions is 2:3 and 3:2 for the nanocrystal and bulk sample, respectively. The decay times obtained for the nanocrystalline samples are in general significantly faster than those observed for the bulk sample. We attribute this to the adsorption of CO2 on the surface of the nanocrystalline samples. The effect of Er3+ concentration on the decay time of the nanocrystalline samples is also discussed.

Assessing Single Upconverting Nanoparticle Luminescence by Optical Tweezers.

We report on stable, long-term immobilization and localization of a single colloidal Er(3+)/Yb(3+) codoped upconverting fluorescent nanoparticle (UCNP) by optical trapping with a single infrared laser beam. Contrary to expectations, the single UCNP emission differs from that generated by an assembly of UCNPs. The experimental data reveal that the differences can be explained in terms of modulations caused by radiation-trapping, a phenomenon not considered before but that this work reveals to be of great relevance.

Temperature dependence of Nd3+↔Yb3+ energy transfer in the YAl3(BO3)4 nonlinear laser crystal

The temperature dependence of the Nd3+→Yb3+ energy-transfer rate in the YAl3(BO3)4 nonlinear laser crystal has been investigated from the analysis of fluorescence decay curves recorded in the 10–600 K range. Three different regimes, independent on the dopant concentration, have been observed in the thermal behavior of the Nd3+→Yb3+ energy-transfer rate. By comparing experimental results with theoretical predictions based on the Dexter model [J. Chem. Phys. 21, 836 (1953)], the origin of these different regimes has been explained. In addition, the influence of temperature and of both Nd3+ and Yb3+ concentrations on the Nd3+←Yb3+ energy back-transfer rate has been also investigated, concluding that it is a migration-assisted energy-transfer process. Finally, the populations of both Nd3+ and Yb3+ metastable states achieved after continuous-wave Nd3+ excitation have been calculated and measured and results have been explained in terms of the thermal behavior of both forward- and back-transfer rates.

Structure determination of φ-Bi8Pb5O17 by electron and powder X-ray diffraction

The triclinic crystal structure of f-Bi8Pb5O17, a ionic fast conductor material, has been determined by the synergy of both electron and powder X-ray crystallography. The heavy atom positions were found by direct methods on electron diAraction data and the structure was completed by iterative use of a priori information in direct methods and diAerence Fourier maps on both types of data. Structure refinement was performed by the Rietveld method on powder X-ray data. The results suggest that f-Bi8Pb5O17 is an ordered phase, with Bi and Pb atoms occupying diAerent sites of the lattice, at variance with the other structural phases known for similar composition in the Bi‐Pb‐O phase diagram, which are solid solutions characterised by a wide compositional range. # 2000 Elsevier Science B.V. All rights reserved.

Cr3+→Nd3+ energy transfer in the YAl3(BO3)4 nonlinear laser crystal

The energy transfer between Cr3+ and Nd3+ ions in Cr3+, Nd3+:YAl3(BO3)4 (YAB) codoped crystals has been investigated by means of steady-state and time-resolved laser spectroscopies. The mainly nonradiative character of the energy transfer has been evidenced from the comparison between the emission spectra and the donor fluorescence decays. The analysis of the donor decay profiles has been used to determine the main dipole–dipole character of the transfer, in excellent agreement with the predictions made from the Dexter [J. Chem Phys. 21, 836 (1953)] model, as well as with the dependence of the energy-transfer rates on the acceptor concentration. The temperature behavior of the transfer rate has been analyzed considering the thermal-induced population redistribution between the E2 and T24 states of Cr3+ ions and the existence of the phonon-assisted energy transfer. We have obtained that 80% of the Cr3+→Nd3+ energy transfer in YAB is taking place without the assistance of lattice phonons whereas almost 20% ...

A structural investigation of Mg(PO3)2, Zn(PO3)2 and Pb(PO3)2 glasses using molecular dynamics simulation

An investigation into the structure of three metaphosphate glasses, Mg(PO3)2, Zn(PO3)2 and Pb(PO3)2, using molecular dynamics simulations is presented. The phosphate network was found to consist of long linear chains of PO4 tetrahedral units. This type of structure exists in the three metaphosphate glasses, indicating that the cation modifier has little effect on the structure of the phosphate backbone. A study of the metal networks revealed that the modifiers field strength plays a significant role in determining the short range order of the metal ions. The results indicate that the lead ion occupies several different environments compared with those occupied by the magnesium or zinc ions.

Structural investigation of NaPO3 glass using molecular dynamics simulation

A molecular dynamics (MD) study of sodium phosphate glass was performed in order to determine the short range structure. The structure of the glass was analysed and the results compared to those found experimentally using X-ray diffraction. The results from the simulation revealed two phosphorus–oxygen interatomic bond distances as well as a predominance of Q2 species which is in agreement with what is expected from the metaphosphate glass structure. The local environment of the sodium ion of the simulated glass was found to be different from that observed in a sodium disilicate glass. This can be explained by the higher degree of cross-linking within the silicate structure.

Inverted opal luminescent Ce-doped silica glasses

Inverted opal Ce-doped silica glasses (Ce : Si molar ratio 1 ⋅ 10−3 were prepared by a sol-gel method using opals of latex microspheres as templates. The rare earth is homogeneously dispersed in silica host matrix, as evidenced by the absence of segregated CeO2, instead present in monolithic Ce-doped SG with the same cerium content. This suggests that the nanometric dimensions of bridges and junctions of the host matrix in the inverted opal structures favor the RE distribution avoiding the possible segregation of CeO2.

Annealing effect on optical barrier in ion-implanted tellurite glass waveguides

Slab waveguides were fabricated in tellurite glasses by means of nitrogen ion implantation, using a wide range of ion doses (from 5x1012 to 8x1016 ions/cm2), in order to investigate their effects on the induced refractive index change. The results of the characterization, carried out by means of dark-line spectroscopy, show the presence of an optical barrier with a decrease of the refractive index at the end of range. Annealing post-implantation process was performed at different temperatures on higher doses (≥1016 ions/cm2) ion implanted slab waveguides and the characterization showed a decrease of the barrier effect probably due to a corresponding reduction of the defects inside the glass matrix.

Characterisation of slab waveguides, fabricated in CaF2 and Er-doped tungsten-tellurite glass by MeV energy N+ ion implantation, using spectroscopic ellipsometry and m-line spectroscopy

Slab waveguides were fabricated in Er-doped tungsten-tellurite glass and CaF2 crystal samples via ion implantation. Waveguides were fabricated by implantation of MeV energy N+ ions at the Van de Graaff accelerator of the Research Institute for Particle and Nuclear Physics, Budapest, Hungary. Part of the samples was annealed. Implantations were carried out at energies of 1.5 MeV (tungsten-tellurite glass) and 3.5 MeV (CaF2). The implanted doses were between 5 x 1012 and 8 x 1016 ions/cm2. Refractive index profile of the waveguides was measured using SOPRA ES4G and Woollam M-2000DI spectroscopic ellipsometers at the Research Institute for Technical Physics and Materials Science, Budapest. Functionality of the waveguides was tested using a home-made instrument (COMPASSO), based on m-line spectroscopy and prism coupling technique, which was developed at the Materials and Photonics Devices Laboratory (MDF Lab.) of the Institute of Applied Physics in Sesto Fiorentino, Italy. Results of both types of measurements were compared to depth distributions of nuclear damage in the samples, calculated by SRIM 2007 code. Thicknesses of the guiding layer and of the implanted barrier obtained by spectroscopic ellipsometry correspond well to SRIM simulations. Irradiationinduced refractive index modulation saturated around a dose of 8 x 1016 ions/cm2 in tungsten-tellurite glass. Annealing of the implanted waveguides resulted in a reduction of the propagation loss, but also reduced the number of supported guiding modes at the lower doses. We report on the first working waveguides fabricated in an alkali earth halide crystal implanted by MeV energy medium-mass ions.