We present our recent achievements in the growing and optical characterization of KYb(WO 4 ) 2 ͑hereafter KYbW͒ crystals and demonstrate laser operation in this stoichiometric material. Single crystals of KYbW with optimal crystalline quality have been grown by the top-seeded-solution growth slow-cooling method. The optical anisotropy of this monoclinic crystal has been characterized, locating the tensor of the optical indicatrix and measuring the dispersion of the principal values of the refractive indices as well as the thermo-optic coefficients. Sellmeier equations have been constructed valid in the visible and near-IR spectral range. Raman scattering has been used to determine the phonon energies of KYbW and a simple physical model is applied for classification of the lattice vibration modes. Spectroscopic studies ͑absorption and emission measurements at room and low temperature͒ have been carried out in the spectral region near 1 m characteristic for the ytterbium transition. Energy positions of the Stark sublevels of the ground and the excited state manifolds have been determined and the vibronic substructure has been identified. The intrinsic lifetime of the upper laser level has been measured taking care to suppress the effect of reabsorption and the intrinsic quantum efficiency has been estimated. Lasing has been demonstrated near 1074 nm with 41% slope efficiency at room temperature using a 0.5 mm thin plate of KYbW. This laser material holds great promise for diode pumped high-power lasers, thin disk and waveguide designs as well as for ultrashort ͑ps/fs͒ pulse laser systems.

Non-contact thermometry is essential in biomedical studies requiring thermal sensing and imaging with high thermal and spatial resolutions. In this work, we report the potential use of Er:Yb:NaYF4 and Er:Yb:NaY2F5O up-conversion nanoparticles as thermal sensors by means of lifetime based luminescent thermometry. We demonstrate how Er:Yb:NaY2F5O nanocrystals present a higher thermal sensitivity than the Er:Yb:NaYF4 ones and that their lifetime thermal coefficient is comparable to those corresponding to other nano-sized luminescent systems already used for high resolution lifetime fluorescence thermal sensing. We evaluate the potential use of Er:Yb:NaY2F5O nanoparticles as lifetime based thermal probes by providing the first experimental evidence on sub-tissue lifetime fluorescence thermal sensing by using up-conversion nanoparticles in an ex vivo experiment.

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The crystal structure of monoclinic KLu(WO 4 ) 2 (KLuW) crystals was determined at room temperature by using single-crystal X-ray diffraction data. The unit-cell parameters were a = 10.576 (7), b = 10.214 (7), c = 7.487 (2) Å , = 130.68 (4) , with Z = 4, in space group C2/c. The unit-cell parameters of KLu 1Àx Yb x (WO 4 ) 2 were determined in relation to Yb concentration. Vickers micro-indentations were used to study the microhardness of KLuW. The linear thermal expansion tensor was determined and the principal axis with maximum thermal expansion ( 0 33 = 16.72 Â 10 À6 K À1 ), X 0 3 , was located 13.51 from the c axis. The room-temperature optical tensor was studied in the near-infrared (NIR) and visible range. The principal optical axis with maximum refractive index (n g = 2.113), N g , was located 18.5 from the c axis at 632.8 nm. Undoped and ytterbium-doped KLuW crystals were grown by the TSSG (top-seeded-solution growth) slow-cooling method. The crystals show {110}, { " 1 111}, {010} and {310} faces that basically constitue the habit of the KLuW crystals.

The polarized optical absorption (OA) and photoluminescence of Pr3+ doped KGd(WO4)2 (KGW) single crystals have been measured at selected temperatures between 7 and 300 K. For the studied Pr concentrations, [Pr] = 0.03×1020-1.9×1020 cm-3, a unique site is observed. 74 energy levels were experimentally determined for this site and labelled with the appropriate A or B irreducible representations corresponding to the C2 symmetry of the Gd point site in KGW. The set was fitted to a Hamiltonian of adjustable parameters including free-ion as well as real Bqk and complex Sqk crystal-field parameters. A good simulation of the experimental crystal field energies was achieved with a root mean square deviation σ = 15.3 cm-1. Distortions in the oxygen bonds to Pr3+ are found to contribute to the broadening of some OA bands, particularly those related to the 1D2 multiplet. The OA edge is determined by the interconfigurational 4f→4f15d1 Pr3+ transition peaking at 34 200 cm-1. From the average 300 K OA cross sections the radiative lifetimes of the Pr3+ multiplets have been calculated considering the standard and modified Judd-Ofelt (JO) theories. A better agreement with the experimental results is obtained by the standard theory: the average JO parameters obtained at ω̄2 = 12.0×10-20 cm2, ω̄4 = 8.15×10-20 cm2 and ω̄6 = 2.64×10-20 cm2. Electrons excited to the 3P0 multiplet decay very efficiently to the 1D2 multiplet even at 15 K. In samples with [Pr]⩾0.3×1020 cm-3 the excitation of the 1D2 multiplet decays non-radiatively by an electric dipole-dipole transfer between Pr neighbours.