Fátima Esteban-Betegón

Tunable laser operation of ytterbium in disordered single crystals of Yb:NaGd(WO4)2.

Lasing of Yb3+ in a disordered single crystal host, NaGd(WO4)2, is reported. Pump efficiencies as high as 20% and slope efficiencies as high as 30% are achieved for both sigma- and pi-polarizations with Ti:sapphire laser pumping. The emission of Yb:NaGd(WO4)2 is centered near 1030 nm. Tunability between 1016 and 1049 nm is obtained with a Lyot filter.

Zirconia/nickel interfaces in micro- and nanocomposites

Abstract Experimental tests for zirconia/nickel composites determined lower toughness than expected. In this regard, the combination of ab initio density functional theory calculations and high-resolution transmission electron microscopy observations allowed to describe the nature of the zirconia/nickel interface and to justify the absence of any toughening mechanism by grain bridging of the metal particles. In the particular case of nanocomposites, enhanced values of hardness were found. Hardness increases have been justified by a novel model which takes into account the Hall — Petch effect and mean particle size according to the percolation theory. It has been found that this model is not exclusive to zirconia/nickel but can be extended to several other superhard composites.

Efficient mid-infrared laser operation of Li 3 Lu 3-x Tm x Ba 2 (MoO 4 ) 8 disordered crystal

Tm-doped Li(3)Lu(3)Ba(2)(MoO(4))(8) monoclinic (C2/c) crystals were grown by the TSSG-method. Details of the crystal growth and Tm(3+) spectroscopy are presented. 514 mW of laser light at 1940 nm was obtained with 71.4% of slope efficiency in quasi-cw operation mode. The laser was tuned in the 1853-2009 nm range. The crystal shows local disorder due to the shared occupancy by Li and Lu of the same 8f lattice site, this confers potential applications for mode-locked sub-200 fs laser pulses.

Hydrothermal Tm3+-Lu2O3 nanorods with highly efficient 2 μm emission.

Cubic Ia3Tm-Lu(2)O(3) porous nanorods of ∼45 μm length and 90 nm diameter have been prepared with precise compositions through a soft hydrothermal route (i.e., autogenic pressure, neutral pH, and 185 °C for 24 h) by using chloride reagents. For these nanorods, room temperature excitation and photoluminescence spectra of Tm(3+) multiplets related to the eye-safe (3)F(4)→(3)H(6) laser transition at ∼1.85-2.05 μm are similar to those of bulk crystals. Room-temperature luminescence decays of (3)H(4) and (3)F(4) exhibit nonexponential dynamics analytically reproduced by the sum of two exponential regimes, which are ascribed to the different rates of nonradiative relaxations in defects at the surface and in the body of the nanocrystals, respectively. Measured fluorescence lifetimes τ ∼ 200-260 μs and τ ∼ 2.3-2.9 ms for (3)H(4) and (3)F(4), respectively, in 0.2% mol Tm-Lu(2)O(3) nanorods, are considerably larger than in previous nanocrystalline Tm-doped sesquioxides, and they are close to values of bulk sesquioxide crystals with equivalent Tm(3+) content.

Electric Field Enhancement and Conduction Mechanisms in Ni/BaTiO3 Percolative Composites

Ni-BaTiO3 composites have revealed to be an optimal system to prepare materials with very high permittivity. In this sense, an exhaustive characterization of these materials under strong electric fields is mandatory. In this article, leakage currents of such composites, with nickel contents very close to the percolation threshold (fc = 0.31), have been measured, analyzed and identified to present Schottky and tunneling conductivity mechanisms, for moderate and strong applied field regimes, respectively. According to the Schottky model, internal fields resulted to be largely enhanced (enhancement factors as large as 104) with respect to the monolithic BaTiO3 ceramics.

From porous to dense Tm3+–Lu2O3 micro- and nanosized crystalline morphologies designed through hydrothermal precursors: assessment on infrared emission properties

Hydrothermal syntheses at 185 °C for 24 h yield the pure cubic Ia micro- and nanosized Tm3+-doped Lu2O3 crystalline sesquioxide, with morphologies that are tailored through the crystallographic phase of the hydrothermal precursors generated by the reaction conditions. Observed morphologies have been described as porous stacked bricks, square nanosheets, bundled porous nanorods, microsized porous cubooctahedra, nanowires and non-porous tablets. The prepared 0.5 mol% Tm3+-doped Lu2O3 shows in all cases excitation (3H6 → 3H4) and emission (3H4 → 3F4 and 3F4 → 3H6) spectra of Tm3+ similar to those observed in single crystals. Room temperature luminescence decays of 3H4 exhibit non-exponential dynamics analytically reproduced by the sum of two exponential regimes, which are ascribed to the different rates of non-radiative relaxations in defects at the surface and in the body of the nanocrystals. For non-porous tablets the value of the measured long-lived 3H4 Tm3+ fluorescence lifetime τ is ∼240 μs, i.e., close to τ values measured in Lu2O3 single crystals with a similar Tm3+ doping level.

Dielectric and Ferroelectric Properties of BaTiO 3 /Ni Cermets Under High Electric Fields

Dielectric properties of ferroelectric-ceramic/metal exhibit extraordinary high values of effective dielectric permittivity. We have prepared BT/Ni ceramics with Ni composition from f = 0 to f = 0.3 and we have studied the dependence of its dielectric permittivity, and its ferroelectric properties under high electric fields. In these composites, the ferroelectric character of the matrix is strongly modified by the metallic particles as a consequence of the large increase of linear dielectric permittivity while the non-linear part remains nearly unchanged. In samples with low or moderate nickel content, ferroelectric switching still can be detected. Samples with Ni concentrations close to the percolation threshold which have giant linear dielectric permittivity, become paraelectric. In these latter, leakage current also plays an important role.

Tunable CW laser operation of Tm 3+ in locally disordered NaLa(WO 4 ) 2

We report now on lasing of Tm³⁺ in the isostructural host NaLa(WO₄)₂, shortly NaLaW, which was grown for the present work by the Czochralski method, with 5 mol % Tm substitution in the melt.