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The incorporation of oleic acid and oleylamine, acting as organic surfactant coatings for a novel solvothermal synthesis procedure, resulted in the formation of monoclinic KLu(WO4)2 nanocrystals. The formation of this crystalline phase was confirmed structurally from X-ray powder diffraction patterns and Raman vibrational modes, and thermally by differential thermal analysis. The transmission electron microscopy images confirm the nanodimensional size (~12 nm and ~16 nm for microwave-assisted and conventional autoclave solvothermal synthesis) of the particles and no agglomeration, contrary to the traditional modified sol-gel Pechini methodology. Upon doping with holmium (III) and thulium (III) lanthanide ions, these nanocrystals can generate simultaneously photoluminescence and heat, acting as nanothermometers and as photothermal agents in the third biological window, i.e., self-assessed photothermal agents, upon excitation with 808 nm near infrared, lying in the first biological window. The emissions of these nanocrystals, regardless of the solvothermal synthetic methodology applied to synthesize them, are located at 1.45 μm, 1.8 μm and 1.96 μm, attributed to the 3H4 → 3F4 and 3F4 → 3H6 electronic transition of Tm3+ and 5I7 → 5I8 electronic transition of Ho3+, respectively. The self-assessing properties of these nanocrystals are studied as a function of their size and shape and compared to the ones prepared by the modified sol-gel Pechini methodology, revealing that the small nanocrystals obtained by the hydrothermal methods have the ability to generate heat more efficiently, but their capacity to sense temperature is not as good as that of the nanoparticles prepared by the modified sol-gel Pechnini method, revealing that the synthesis method influences the performance of these self-assessed photothermal agents. The self-assessing ability of these nanocrystals in the third biological window is proven via an ex-vivo experiment, achieving thermal knowledge and heat generation at a maximum penetration depth of 2 mm.

Terbium (Tb 3+ ) ions are promising for the development of visible lasers. We report on the spectroscopy of Tb 3+ in the monoclinic KYb(WO 4 ) 2 (KYbW) crystal, including the Judd-Ofelt (J-O) modelling of the transition probabilities, evaluation of the absorption and stimulated-emission (SE) cross-sections with polarized light, and emission lifetime study. Within the strong configuration interaction (SCI) approximation of the J-O theory, the intensity parameters of Tb 3+ are Ω 2 =0.864, Ω 4 =1.657, Ω 6 =1.975 [10 -20 cm 2 ] and Δ=38810 cm –1 . The maximum σ abs for the 7 F 6 → 5 D 4 transition reaches 2.3×10 -21 cm 2 at 486.7 nm and the maximum σ SE for the green emission according to the 5 D 4 → 7 F 5 transition is 2.3×10 -21 cm 2 at 549.3 nm (both for E || N m ). The measured lifetime of the 5 D 4 state for a 1 at% Tb 3+ doping is 395±5 μs. Under blue excitation, Tb: KYbW exhibits intense green emission with the CIE 1931 coordinates of (0.312; 0.644). [ABSTRACT FROM AUTHOR]

We demonstrate the suitability of monoclinic double tungstates (MDTs), KRE(WO4)2, where RE = Gd, Y or Lu, doped with Nd3+, Yb3+, Tm3+, or Ho3+ ions and co-doped with Yb3+-Tm3+, or Tm3+-Ho3+ ion couples, for highly efficient micro-lasers at \sim 1~\mu \textm and at \sim 2~\mu \textm . This is facilitated by the use of high rare-earth doping levels (up to 15 at.% for Tm, 10 at.% for Nd, and 25 at.% for Yb) and a special crystal cut along the \textN\mathrm {g} -axis providing the thermal guiding. Record slope efficiencies for bulk MDT lasers are achieved for each studied ion. A 15 at.% Tm:KLu(WO4)2 laser generated 785 mW at 1957 \ldots 1965 nm with a slope efficiency \eta =77 %. The quantum efficiency for Tm3+ ions amounted to \eta \mathrm {q} = 1.98\pm 0.02 . With a 0.9 mm-thick 25 at.% Yb:KLuW micro-laser, \eta =91 % is achieved, approaching the theoretical limit set by the Stokes shift. [ABSTRACT FROM AUTHOR]

We report on the growth, spectroscopic and laser characterization of a novel monoclinic laser crystal, Yb,In:KLu(WO 4 ) 2 (Yb,In:KLuW). The absorption, stimulated-emission and gain cross-section spectra of 3.5 at% Yb, 5.5 at% In-doped KLuW are determined at room temperature with polarized light and compared with those for Yb,In:KYW, as well as singly Yb-doped KLuW and KYW crystals. It is found that the introduction of In results in a decrease of the transition cross-sections and in a spectral broadening of the absorption and emission bands. Such a broadening is more pronounced for light polarization E || N p . For Yb,In:KLuW, the maximum σ abs is 9.9×10 –20 cm 2 at 980.9 nm for E || N m and the corresponding bandwidth of the absorption peak is 3.7 nm. The radiative lifetime for Yb 3+ ions is 237±5 µs. The stimulated-emission cross-sections are σ SE ( m )=2.4×10 –20 cm 2 at 1022.4 nm and σ SE ( p )=1.3×10 –20 cm 2 at 1039.1 nm corresponding to an emission bandwidth of >30 nm and >35 nm, respectively. The diode-pumped N g -cut Yb,In:KLuW microchip laser generated 4.11 W at 1042–1048 nm with a slope efficiency of 78%. The Yb,In:KLuW crystal is very promising for the generation of sub-100 fs pulses in mode-locked lasers due to its broadband emission characteristics. [ABSTRACT FROM AUTHOR]

Abstract: CW mirrorless laser operation at 1840nm at room temperature was observed in buried rib waveguides of KY0.58Gd0.22Lu0.17Tm0.03(WO4)2 fabricated by structuring KY(WO4) substrates by Ar-ion milling and subsequent liquid phase epitaxial growth of the active layer on these substrates. Laser efficiency and laser threshold seems to change with the width of the channels fabricated. [Copyright &y& Elsevier]

Abstract: This paper summarizes the main results we obtained in our laboratories in relation with crystalline layers obtained by Liquid Phase Epitaxial growth of lanthanide doped KLu(WO4)2 and Nb:RbTiOPO4 grown on KLu(WO4)2 and RbTiOPO4 substrates, respectively. Macroscopic defect free epitaxial layers were grown and characterized in terms of their compositional homogeneity, structural stress in the layer/substrate interface and laser and waveguiding performances. [ABSTRACT FROM AUTHOR]

Monoclinic potassium rare-earth double tungstates [KRE(WO4)2, RE = Y, Lu, Yb; KREW] are well suited as hosts for active lanthanide ion (Ln3+) dopants for diode-pumped solid-state lasers, with particular interest in thin-disk laser configurations when they are grown as thin films. Using synchrotron white-beam x-ray topography, we have imaged defects and strain in top-seeded solution-grown (TSSG) bulk substrates of different rare-earth tungstates as well as within Yb3+- and Tm3+-doped epitaxies for thin-disk laser applications grown on these substrates by liquid-phase epitaxy. Higher structural stress in Yb:KYW/KYW epitaxies compared with Yb:KLuW/KLuW epitaxies is found to lower efficiency in laser operation. The quality of Tm:KLuW/KLuW epitaxial films is sensitive to doping level, film thickness, and growth rate. Inhomogeneous stresses within the layers are dominated by lattice-mismatch effects rather than by crystallographic anisotropy. [ABSTRACT FROM AUTHOR]

Abstract: Here we show the results we obtained in the fabrication of planar waveguide lasers based on monoclinic double tungstates doped with Er3+ and Tm3+. We have successfully introduced these ions into the lattice matched KY0.59Gd0.19Lu0.22(WO4)2 epitaxial layers grown on KY(WO4)2 substrates without loss of optical quality and keeping a high refractive index contrast between the epitaxial layer and the substrate. We characterized the waveguiding properties of these epitaxial layers at λ =632.8nm by dark modes spectroscopy, and we showed that these waveguides can support several TE and TM modes. Spectroscopic characterization of the active lanthanide ions in these crystals is also presented. [Copyright &y& Elsevier]

It has been recognized that the monoclinic double tungstates KY(WO4)2, KGd(WO4)2, and KLu(WO4)2 possess a high potential as rare-earth-ion-doped solid-state laser materials, partly due to the high absorption and emission cross sections of rare-earth ions when doped into these materials. Besides, their high refractive indexes make these materials potentially suitable for applications that require optical gain and high power in integrated optics, with rather high integration density. We review the recent advances in the field of bulk lasers in these materials and present our work toward the demonstration of waveguide lasers and their integration with other optical structures on a chip. [ABSTRACT FROM PUBLISHER]

Epitaxial monoclinic double tungstate composites based on the strongly anisotropic KLu(WO4)2 (KLuW) were grown with high crystalline quality and laser operation of ytterbium was demonstrated for the first time. Highly efficient CW laser emission of an Yb:KLuW-KLuW crystal was achieved near 1030 nm. The 100-μm-thick Yb:KLuW layer was pumped at wavelengths near 980 nm by a tapered diode laser as well as by a Ti:sapphire laser. More than 500 mW of CW output power and slope efficiencies up to 66% were obtained at room temperature without cooling. [ABSTRACT FROM AUTHOR]

The strongly anisotropic monoclinic double tungstate crystal KLu(WO4)2 was doped with Yb3+ ions and shown to be a highly efficient active laser medium in the 1-μm spectral range for continuous-wave room-temperature operation, very suitable for pumping with InGaAs laser diodes. [ABSTRACT FROM AUTHOR]

The strongly anisotropic monoclinic double tungstate crystal KLu(WO4)2 was doped with Yb3+ ions and shown to be a highly efficient active laser medium in the 1-μm spectral range for continuous-wave room-temperature operation, very suitable for pumping with InGaAs laser diodes. [ABSTRACT FROM AUTHOR]

The incorporation of oleic acid and oleylamine, acting as organic surfactant coatings for a novel solvothermal synthesis procedure, resulted in the formation of monoclinic KLu(WO4)2 nanocrystals. The formation of this crystalline phase was confirmed structurally from X-ray powder diffraction patterns and Raman vibrational modes, and thermally by differential thermal analysis. The transmission electron microscopy images confirm the nanodimensional size (~12 nm and ~16 nm for microwave-assisted and conventional autoclave solvothermal synthesis) of the particles and no agglomeration, contrary to the traditional modified sol-gel Pechini methodology. Upon doping with holmium (III) and thulium (III) lanthanide ions, these nanocrystals can generate simultaneously photoluminescence and heat, acting as nanothermometers and as photothermal agents in the third biological window, i.e., self-assessed photothermal agents, upon excitation with 808 nm near infrared, lying in the first biological window. The emissions of these nanocrystals, regardless of the solvothermal synthetic methodology applied to synthesize them, are located at 1.45 μm, 1.8 μm and 1.96 μm, attributed to the 3H4 → 3F4 and 3F4 → 3H6 electronic transition of Tm3+ and 5I7 → 5I8 electronic transition of Ho3+, respectively. The self-assessing properties of these nanocrystals are studied as a function of their size and shape and compared to the ones prepared by the modified sol-gel Pechini methodology, revealing that the small nanocrystals obtained by the hydrothermal methods have the ability to generate heat more efficiently, but their capacity to sense temperature is not as good as that of the nanoparticles prepared by the modified sol-gel Pechnini method, revealing that the synthesis method influences the performance of these self-assessed photothermal agents. The self-assessing ability of these nanocrystals in the third biological window is proven via an ex-vivo experiment, achieving thermal knowledge and heat generation at a maximum penetration depth of 2 mm. [ABSTRACT FROM AUTHOR]

Abstract: Crystal growth and spectroscopic study of the new compound Yb3+:LiLu(WO4)2 have been undertaken for the first time. Preliminary growth using flux method with Li2W2O7 as solvent allows us to obtain small single crystals. As a promising laser crystal, Yb3+:LiLu(WO4)2 have characteristics of a low concentration quenching effect and low laser threshold owing to the existence of covalent bands in WO4 group. Its absorption peak at 977nm, corresponding to 2F7/2(1)→ 2F5/2(5) transition, is suitable for laser diode pumping. Emission spectrum shows large bands extending from 900 to 1100nm, five peaks centred respectively at 959, 978, 997, 1025 and 1056nm are well resolved. The FWHM of the most intense emission band located at 997nm is about 20nm. Energy Stark levels are: 0, 201, 469 and 749cm−1 for the 2F7/2 component and: 10,230, 10,427 and 10,890cm−1 for the 2F5/2 component. The overall splitting of 2F7/2 ground-state manifold is 749cm−1, which is larger than that observed in similar systems such as Yb3+:LiLa(WO4)2 (632cm−1). Such large crystal-field splitting of the 2F7/2 ground-state will be advantageous to lower laser thermal effect and re-absorption losses. As we can see, the stark electronic levels are characterized by relatively large crystal-field splitting favourable to laser crystal applications in the near infrared spectral region. [Copyright &y& Elsevier]