A novel capacitor devicewith a high, frequency‐independent dielectric constant has been fabricated by a simple powder processing route. A mixture of microparticles of ferroelectric BaTiO3 and de‐passivated metallic nickel was sintered to yield percolative composites in which the system of conducting particles separated by insulator layers (see Figure for an SEM image) exhibits a dielectric constant higher than that of either component.

Single crystals of Yb 3+ -doped NaGd͑WO 4 ͒ 2 with up to 20 mol % ytterbium content have been grown by the Czochralski technique in air or in N 2 +O 2 atmosphere and cooled to room temperature at different rates ͑4-250°C/h͒. Only the noncentrosymmetric tetragonal space group I4 accounts for all reflections observed in the single crystal x-ray diffraction analysis. The distortion of this symmetry with respect to the centrosymmetric tetragonal space group I4 1 / a is much lower for crystals cooled at a fast rate. Na + , Gd 3+ , and Yb 3+ ions share the two nonequivalent 2b and 2d sites of the I4 structure, but Yb 3+ ͑and Gd 3+ ͒ ions are found preferentially in the 2b site. Optical spectroscopy at low ͑5 K͒ temperature provides additional evidence of the existence of these two sites contributing to the line broadening. The comparison with the 2 F 7/2 ͑n͒ and 2 F 5/2 ͑nЈ͒ Stark energy levels calculated using the crystallographic Yb-O bond distances allows to correlate the experimental optical bands with the 2b and 2d sites. As a novel uniaxial laser host for Yb 3+ , NaGd͑WO 4 ͒ 2 is characterized also with respect to its transparency, band-edge, refractive indices, and main optical phonons. Continuous-wave Yb 3+ -laser operation is studied at room temperature both under Ti:sapphire and diode laser pumping. A maximum slope efficiency of 77% with respect to the absorbed power is achieved for the polarization by Ti:sapphire laser pumping in a three-mirror cavity with Brewster geometry. The emission is tunable in the 1014-1079 nm spectral range with an intracavity Lyot filter. Passive mode locking of this laser produces 120 fs long pulses at 1037.5 nm with an average power of 360 mW at Ϸ97 MHz repetition rate. Using uncoated samples of Yb: NaGd͑WO 4 ͒ 2 at normal incidence in simple two-mirror cavities, output powers as high as 1.45 W and slope efficiencies as high as 51% are achieved with different diode laser pump sources.

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.

Mild hydrothermal preparations using nitrate and chloride reagents as Gd3+ and Yb3+ sources lead to the synthesis of NaGd1−x Yb x (WO4)2 (0.001≤ x ≤ 0.5) particles with tetragonal scheelite-like structure phase. Nearly neutral pH ∼7.5 conditions ensure the stability of this crystalline phase over a wide range of reaction times. Synthetical routes with both kind of reagents yield basically the same particle morphology sequences, although using Gd(Yb)-chlorides the presence of nanorods is more evident, whereas using Gd(Yb)-nitrates the faster growth rate favors well-defined micrometer-sized octahedral particles. The spectroscopic properties of Yb3+ in NaGd1−x Yb x (WO4)2 synthesized micrometer-sized octahedra are equivalent to those obtained in bulk single crystals, showing a single exponential photoluminescence decay and 2F5/2 lifetime τ ≈ 330 μs for 0.001 ≤ x ≤ 0.005 doped samples. The nanoparticles and nanorods formed with Gd(Yb)-nitrates and pH < 7 by short time annealing as well as in Gd(Yb)-chloride preparations show a strong reduction of the Yb3+ lifetime with regards to that observed in micrometer-sized octahedra or in single crystals, leading to nonexponential fluorescence decays when both nano- and micrometer-sized particles coexist. The use of raw Yb2O3 with increasing purity level in the hydrothermal process yields materials that show an increase in Yb3+ lifetime toward its radiative value.

Double tungstate and molybdate compounds with the general formulae MT(WO 4 ) 2 and MT(MoO 4 ) 2 where M is a monovalent alkali cation (Li-Cs) and T is a trivalent cation (Y, La or rare earth Ln) exhibit ordered phases with separate sites for M and T cations and disordered phases where M and T cations are randomly distributed over the same cationic sublattice [1]. Some of the optically passive ordered phases like the monoclinic KGd(WO 4 ) 2 , KY(WO 4 ) 2 , and KLu(WO 4 ) 2 are established laser hosts with very large absorption and emission cross sections of the active dopant. The sodium compounds NaT(WO 4 ) 2 and NaT(MoO 4 ) 2 represent disordered phases with tetragonal structure at room temperature. For T = Y, La, Ce-Er, they exhibit also a congruent melting character. Hence their growth with active Ln-dopants by the Czochralski method is very attractive for the synthesis of novel crystalline laser materials.The revived interest in tetragonal sodium double tungstates and molybdates is due to their potential to ensure larger tunability and bandwidths in mode-locked diodepumped solid-state lasers in comparison to ordered crystals. This is especially true for doping with Yb 3+ which, due to the stronger electron-phonon coupling to the lattice, exhibits intrinsically broader linewidths than the Nd 3+ ion. Hence, the requirements to the pump laser diodes are reduced and Yb-doped disordered hosts hold a greater promise for the generation of mode-locked pulses shorter than 100 fs. It should be added that Yb 3+ possesses longer energy-storage lifetime and smaller quantum defect than Nd 3+ , and that it can be pumped by the optically more robust InGaAs laser diodes operating in the 900 -1000 nm spectral range [2]. Finally, the relatively simple two-manifold structure of Yb 3+ prohibits excited state absorption, up-conversion and cross-relaxation processes.The sodium lanthanum crystals NaLa(WO 4 ) 2 (NaLaW) and NaLa(MoO 4 ) 2 (NaLaMo) were studied in the past as room temperature hosts only for Nd 3+ lasers. Lamp-pumped pulsed laser operation of Nd: NaLaW was reported for the 4F 3/2 -4I 11/2 transition at 1063.5 nm [3,4] and for the 4F 3/2 -4I 13/2 transition at 1335.5 nm [5]. Lamp-pumped operation of Nd:NaLaMo was demonstrated not only in the pulsed regime at 1059.5 -1065.3 nm [6, 7] and 1338 -1344 nm [5] but also in the continuous-wave (cw) regime at 1065.3 nm [8]. In addition NaLaMo, which is an efficient Raman active medium [9], when doped with Nd 3+ , was shown to be an efficient self-converting Raman crystal both in the picosecond [10] and in the nanosecond [11] regime.Here we report on the cw laser performance of Yb :NaLaW and Yb:NaLaMo crystals at room temperature. Note that the isostructural NaGd(WO 4 ) 2 or NaGdW, was the first and so far only disordered laser crystal of this type for which room-temperature cw laser operation could be demonstrated with Yb 3+ doping [12].Single crystals of disordered NaLa(WO 4 ) 2 and NaLa(MoO 4 ) 2 doped with Yb 3+ are grown by the Czochralski method from the melt. Continuous-wave...

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.

Room-temperature continuous-wave (cw) laser operation is demonstrated with the newly developed Yb:NaLa(WO 4 ) 2 disordered crystal by end-pumping with a fiber-coupled diode laser. A maximum output power of 330 mW is obtained with an optical efficiency of 4.9% and a slope efficiency of 6.3% with respect to the incident pump power. The efficiencies in terms of the absorbed pump power are roughly three times higher. Sellmeier dispersion curves for the ordinary and extraordinary refractive indices of the NaLa(WO 4 ) 2 host are reported along with crystallographic and spectroscopic properties related to the Yb 3+ -doping.

No abstract