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The bifunctional possibilities of Tm,Yb:GdVO4@SiO2 core-shell nanoparticles for temperature sensing by using the near-infrared (NIR)-excited upconversion emissions in the first biological window, and biolabeling through the visible emissions they generate, were investigated. The two emission lines located at 700 and 800 nm, that arise from the thermally coupled 3F2,3 and 3H4 energy levels of Tm3+, were used to develop a luminescent thermometer, operating through the Fluorescence Intensity Ratio (FIR) technique, with a very high thermal relative sensitivity. Moreover, since the inert shell surrounding the luminescent active core allows for dispersal of the nanoparticles in water and biological compatible fluids, we investigated the penetration depth that can be realized in biological tissues with their emissions in the NIR range, achieving a value of 0.8 mm when excited at powers of 50 mW. After their internalization in HeLa cells, a low toxicity was observed and the potentiality for biolabelling in the visible range was demonstrated, which facilitated the identification of the location of the nanoparticles inside the cells, and the temperature determination.

A detailed investigation of the dependence of the real time luminescence of Eu3+-doped tin dioxide nanopowders on rare earth site symmetry and host defects is given. Ultrafast spectroscopy shows that host-rare earth energy transfer occurs at a transfer rate of about 1.5×106 s-1, whereas the intrinsic broad band SnO2 emission has a very short build up time, of the order of 60 ps, and a lifetime of hundreds of picoseconds. These results validate the hypothesis that both host and matrix-excited RE emissions are decoupled due to the different origins of the involved physical mechanisms.

We report the magnetic properties and magnetic structure of the zircon-type compound GdVO4, together with the magnetic structure of the isostructural NdVO4. At T≃2.5 K, GdVO4 undergoes a phase transition to antiferromagnetic Gz, driven mainly by the exchange interactions, while the magnetic anisotropy and dipolar interactions are minor contributions. Near the liquid-helium boiling temperature, the magnetocaloric effect of GdVO4 is nearly as large as that of the structurally closely related GdPO4. It is noteworthy that GdVO4 has been recently proposed as a good passive regenerator in Gifford-McMahon cryocoolers, since adding a magnetization-demagnetization stage to the cryocooler refrigeration cycle would increase its efficiency for liquefying helium. NdVO4 is a canted Gz-type antiferromagnet and shows enhancement of the magnetic reflections in neutron diffraction below ca. 500 mK, due to the polarization of the Nd nuclei by the hyperfine field.

The present work displays a thorough study of the optical spectroscopy of multilevel RE-doped tin oxide nanocrystals. Our study provides the structural grounds about the behavior of the RE emission under site-selective excitation as well as the capability of the host to hold the RE ions at the nanocrystal lattice sites and, as a consequence, the possibility to reach the RE excited states by direct host excitation.

The present work gives a detailed investigation of the dependence of the real time luminescence of Eu3+-doped tin dioxide nanopowder on rare earth (RE) site symmetry and host defects. Ultrafast time-resolved analysis of both RE-doped and undoped nanocrystal powder emissions, together with electronic paramagnetic resonance studies, show that host-excited RE emission is associated with RE-induced oxygen vacancies produced by the non-isoelectronic RE-tin site substitution that are decoupled from those producing the bandgap excited emission of the SnO2 matrix. A lower limit for the host-RE energy transfer rate and a model for the excitation mechanism are given.

We developed a new compact, low-cost and non-invasive temperature sensor based on a ratiometric luminescence technique. The setup included a commercial digital color sensor, which collects simultaneously signals in the blue, green and red regions of the electromagnetic spectrum, usually used to assess the quality of computer screens and used for the first time here as a sensor for luminescent thermometry, coupled to an optical system that focuses an excitation laser beam onto luminescent nanoparticles emitting at least in two of these electromagnetic regions, which simplifies considerably the design, alignment and measurement procedures of setups used up to now for the same purpose. The same optical system collects the emission arising from the luminescent nanoparticles and directs it towards the digital color sensor through a dichroic mirror. We probed the potentiality of this setup for luminescence thermometry in the biological range of temperatures using Er,Yb:NaYF, and up to 673 K for microelectronic applications using Tm,Yb:GdVO up-converting nanoparticles. The thermal sensitivity obtained in both cases is similar to that previously reported for the same kinds of nanoparticles using conventional systems. This validates our setup for temperature measurements. Also, we developed new flexible and transparent polymer composites, in which we embedded upconversion luminescent nanoparticles of Er,Yb:NaYF in PDMS, a standard polymer for microfluidic devices used for biomedical studies, which allow fabricating thermometric microfluidic chips in which temperature can be determined using our setup. The thermal sensitivity for these composites is slightly smaller than that of the bare nanoparticles, bur still allowing for precise and fast temperature measurements.

The Na-Yb (or Er) co-substitution of Ca in CaNbGaO (CNGG) laser crystal is studied. In contrast to other garnets whose structural disorder is exclusively based on the presence of differently sized cations on the same crystal sites, Na incorporated in the dodecahedral site (a site also shared by Ca and trivalent lanthanides) creates diverse electric charge distributions over the dodecahedral sublattice, which adds to the disorder associated with Nb and Ga simultaneous occupation of the octahedral and tetrahedral sites. The currently determined cationic compositions of Czochralski grown congruent CNGG and Na-modified CNGG crystals show that Na incorporation reduces the cationic vacancy concentration on dodecahedral and octahedral sites but does not affect that in tetrahedral sites. Physical properties of interest for laser design (optical transmission, elastic constants, hardness, specific heat, thermal conductivity, thermal expansion, refractive index dispersion, group velocity dispersion, and thermo optic coefficients) have been systematically determined at cryogenic temperatures and above room temperature. Na incorporation into CNGG decreases the crystal growth temperature, promotes Yb doping, and importantly, increases the Yb optical bandwidth, offering good prospects for the implementation of ultrashort pulses in mode-locked laser oscillators.

We studied the temperature-dependent luminescence of GdVO nanoparticles co-doped with Er (1 mol %) and Yb (20 mol %) and determined their thermal sensing properties through the fluorescence intensity ratio (FIR) technique. We also analyzed how a silica coating, in a core-shell structure, affects the temperature sensing properties of this material. Spectra were recorded in the range of biological temperatures (298-343 K). The absolute sensitivity for temperature determination calculated for the core-shell nanoparticles is double the one calculated for bare nanoparticles, achieving a thermal resolution of 0.4 K. Moreover, silica-coated nanoparticles show good dispersibility in different solvents, such as water, DMSO, and methanol. Also, they show good luminescence stability without interactions with solvent molecules. Furthermore, we also observed that the silica coating shell prevents progressive heating of the nanoparticles during prolonged excitation periods with the 980 nm laser, preventing effects on their thermometric applications.

Micron-sized bundles of highly crystalline individual nanotubes of Pbcm Vernier-type Yb3+-sensitized Ln3+ (Er, Pr)-doped Y6O5F8 oxyfluorides have been prepared through efficient low-temperature hydrothermal syntheses. Under near infrared (NIR) 978 nm diode laser excitation, the color of the upconverted light from codoped Pr, Er, Yb:Y6O5F8 nanotubes can be selected by the control of the Pr3+ concentration, and by the excitation regime and power density. Samples with low Pr3+ concentration emit predominantly green light, and the selection between bluish-green light and white light has been demonstrated with large Pr3+ concentration (2 mol%), under pulsed or continuous wave excitation, respectively. Involved Er3+ and Pr3+ electronic transitions and mechanisms to populate their corresponding emitting levels have been analyzed for a large number of 2 mol% Yb3+-sensitized, 0.05 mol% Er3+-doped oxyfluoride samples within the Pr3+ concentration span 0.2 - 2 mol% [1]. Furthermore, the ratiometric analysis of the thermal evolution of intensities of NIR excited green upconverted emissions from thermally coupled 2H11/2 and 4S3/2 Er3+ multiplets of Er, Yb:Y6O5F8 indicates thermal sensing potential with better sensitivity S (0.0041 KK-1) than hexagonal Er, Yb:beta-NaYF4(0.0032 K-1) in the temperature range of physiological interest.

In this contribution we report what it is, to the best of our knowledge, the first femtosecond mode-locked laser operation of In-modified KY(WO4)2 crystals doped with Yb3+. The presence of indium, co-substituting yttrium position, introduces disorder in the crystal structure. KLn(WO4)2 monoclinic crystals, with Ln = Y, Gd or Lu, are well established laser crystals characterized by very large absorption and emission cross sections in the Nm crystal direction [1]. In particular, the laser properties of Yb-doped KY(WO4)2 crystal have been object of study in the past [2-4]. In this crystal, Yb3+ sits a single position and the main origin of its band broadening is the interaction with the vibrating lattice environment. Due to the small ionic radius of In3+ replacing the larger Y3+ cation, disorder is introduced in the crystal structure. This disorder induces inhomogeneous broadening of the Yb3+ optical bands and therefore allows to produce ultrashort laser pulses.

Nanoparticulate films with high up-conversion emission (UC) properties were prepared by spray-deposition of nanometer-sized YVO4:Yb,Er particles. The optical properties of YVO4:Yb,Er were optimized upon annealing before the film deposition in order to get the highest possible UC signal in the considered type of system. Thanks to a simple model and some time-resolved spectroscopic investigations, the contribution of the scattering to the UC signal could be separated from the intrinsic properties (crystallinity, surface defects) of the material. The films obtained by this technique present the advantages of having both high UC and good transparency.

Monoclinic Yb-sensitized (Tm, Ho)-doped KLu(WO4)2 nanocrystals of ∼100 nm size have been synthesized by the modified Pechini sol-gel method. Their diode laser near-infrared (∼980 nm) excited upconversion emission properties have been characterized at power densities in the range 30-355 W cm-2. Bright white light composed of blue ∼475 nm, green ∼540 nm, and red ∼650 nm emissions, corresponding to Tm 3+ 1G4 → 3H6, Ho 3+ 5S2, 5F4 → 5I8, and Ho3+ 5F5 → 5I8 electronic transitions, respectively, was generated by adjusting the Yb, Tm, and Ho contents in KLu(WO4)2 nanocrystalline samples. Chromaticity coordinates of the emitted white light can be tuned by modifying the excitation power density. The effect of Tm and Ho on the luminescence dynamics has been described by analyzing the upconverted emission intensity dependence on the excitation power, as well as from Stokes and decay time measurements. The effect on upconversion properties of further codoping with Eu in these (Tm, Ho, Yb)-doped KLu(WO4)2 nanocrystals has also been studied.

KIn0.07Yb0.13Y0.8(WO4)2 monoclinic C2/c single crystals were grown by the TSSG technique. 500 mW of output power were obtained at 300 K with slope efficiency of 69%. The laser emission was tunable over 1020-1047 nm.

We report randomlaser action in ground powders of NdxY1−xAl3(BO3)4 (x = 0.5–1) lasercrystals under nanosecond pulse excitation at 802 nm. The dependence of the randomlaser threshold energy on the neodymium concentration and focusing conditions is investigated. The slope efficiency of the output randomlaser emission is determined as a function of Nd3+ concentration.

QC 20120208

The characterization of the coefficient of the nonlinear optical Kerr effect, the nonlinear refractive index (n(2)), of several femtosecond laser crystals with compositions derived by total or partial replacement of D(2+) in DXO(4), X = Mo or W, is presented. Tetragonal (space group I4) Na-based double tungstates NaT(WO(4))(2) (T = Y, La, Gd, Lu and Bi) and double molybdate NaY(MoO(4))(2), as well as the monoclinic (space group C2/c) Li(3)Gd(3)Ba(2)(MoO(4))(8) crystals, have been measured by the z-scan technique. All these crystals present structural local disorder, and among them the tetragonal ones exhibit significant n(2) values, which should allow their efficient laser pulsed operation by Kerr-lens mode locking, especially NaBi(WO(4))(2), 68x10(-16) cm(2)/W (for sigma light), which is about twice than for the others. This feature is attributed to the high polarizability associated to the lone electron pair of Bi(3+)., QC 20120208

In this work, we calculate in two different ways the transport mean-free-paths in NdxY1-xAl(BO3)4 laser crystal powders by using the diffuse spectral reflectance and transmittance of the powders and the absorption coefficient of the crystal materials. Similar results have been obtained from both methods.