Crystal growth, optical characterisation and laser operation of yb3+ in monoclinic double tungstates

El interés y la originalidad del trabajo científico de esta tesis se centra en el crecimiento cristalino, el análisis composicional, estructural, morfológico y óptico de la fase monoclínica de los tungstatos dobles que responden a la fórmula K(RE3+)(WO4)2, donde RE3+ denota iones como Y3+, Gd3+, Yb3+, Er3+ y Lu3+. Estos materiales cristalinos se han estudiado sin dopar y dopados con iones Yb3+ y Er3+ para la generación de radiación láser. La obtención de la fase monoclínica de estos tungstatos dobles se ha realizado mediante el método de crecimiento a partir de solución a alta temperatura Top-Seeded Solution Growth (TSSG) utilizando como solvente el pirotungstato de potasio, K2W2O7, debido a las ventajas que ofrece respecto otros solventes. Se han obtenido monocristales con una muy buena calidad óptica y de tamaño adecuado para su posterior caracterización tanto química como física. La caracterización óptica de estos materiales ha sido la parte más extendida en el trabajo y se ha centrado, por una parte, en la matriz (tungstatos dobles sin dopar) y por otra parte en los iones activos generadores de radiación láser. En cuanto a la matriz se ha llevado a cabo la determinación del rango espectral de transparencia óptica (el cual se extiende entre 0.32 y 5.4 micrómetros), la orientación de los ejes principales ópticos respecto a los ejes cristalográficos y en la medida de los índices de refracción a lo largo de las tres direcciones principales ópticas y para un rango de longitudes de onda que se extiende desde los 450 nm hasta los 1500 nm (índice de refracción alrededor de 2).Por otra parte la caracterización óptica de los iones activos se ha centrado en la espectroscopia del yterbio y del erbio y en los experimentos de laseado realizados sobre los iones yterbio. La espectroscopia concierne, en primer lugar, a la absorción óptica polarizada de los dos iones a temperatura ambiente y a temperaturas criogénicas, 6 K (es destacable el valor del coeficiente de absorción del yterbio, que deriva en una longitud de absorción de tansolo 13.3 micrómetros para el caso del KYb(WO4)2 y en una elevada sección eficaz de emisión del orden de 14x10-20 cm2 a una longitud de onda de 980 nm para la polarización etiquetada como Nm). Dicha longitud de absorción permite pensar en la posibilidad de láseres de disco fino (Thin disk lasers) y guías de onda, y debido a la elevada sección eficaz de absorción, se ha utilizado este ión como sensibilizador de las emisiones del erbio. Además, el rango de longitudes de onda de absorción del yterbio permite la posibilidad de excitación mediante diodo con las ventajas que ello supone. En segundo lugar, la espectroscopia concierne a la luminiscencia de dichos iones desde el punto de vista de la emisión óptica también a temperatura ambiente y bajas temperaturas y a la medida de vidas medias. La emisión del yterbio también destaca por su anchura de las líneas de emisión, muy interesantes para la generación de láser pulsados en régimen de pico- y femtosegundos. Las emisiones del erbio que se han estudiado con mayor profundidad han sido las emisiones con máximo de intensidad a 1530 nm y a 550 nm.La generación de radiación láser a partir de iones yterbio se ha realizado en modo continuo para el caso de KLu(WO4)2 dopado con yterbio y en modo pulsado (Quasi-CW) para el caso en que el ión activo no está presente en cantidades dopantes en la estructura sino substituyendo totalmente a la tierra rara (RE), el KYb(WO4)2. La excitación del yterbio se ha realizado mediante un láser de titanio-zafiro y mediante diodo.The interest and originality of the scientific work of this thesis focuse on the crystal growth, the compositional, structural, morphologycal and optical analysis of the monoclinic phase of the double tungstates K(RE3+)(WO4)2, where RE3+ denotes ions like Y3+, Gd3+, Yb3+, Er3+ and Lu3+. These crystalline materials have been studied without doping and doped with Yb3+ and Er3+ ions for the generation of laser radiation. The obtention of the monoclinic phase of these double tungstates has been realized by means of the Top-Seeded Solution Growth (TSSG) method using the potassium pyrotungstate, K2W2O7, as solvent. This was due to the advantages that it offers in front of other solvents. Single crystals with very high optical quality and suitable size for later chemical and physical characterizations have been obtained. The optical characterization of these materials has been the most extended part of this work and focused, on the one hand, on the host (double tungstates without doping) and, on the other hand, on the active ions. Concerning the host, the determination of the optical spectral range of transparency (that extends from 0.32 to 5.1 micrometers), the orientation of the principal optical axes with respect to the crystallographic axes and the measurement of the refractive indices along the three principal optical directions and for a wavelength range from 450 nm to 1500 nm (refractive index around 2) have been determined.On the other hand, the optical characterization of the active ions focused on the ytterbium and erbium spectroscopy and the laser experiments realized on ytterbium ions. The spectroscopy concerns, in first place, in the polarized optical absorption of the two ions at room temperature and cryogenic temperature, 6 K (it is remarkable the absorption coefficient of ytterbium, that derived in an absorption length of only 13.3 micrometers for the case of KYb(WO4)2, and in a very high absorption cross-section of 14x10-20 cm2 at 980 nm for the polarization labelled Nm). This absorption length allows think in the possibility of Thin-Disk laser designs and waveguides, and due to the high absorption cross-section, this ion has been used as sensitizer of the erbium emissions. Moreover, the range of absorption wavelength for ytterbium allows the possibility of exciting by means of diode. In second place, the spectroscopy involved the luminescence of these ions under the point of view of the optical emission also at room temperature and low temperature and the measurement of the lifetime. The emission of ytterbium is interesting also for the broadening of the emission lines, that is very interesting for the generation of laser pulses in the pico- and femtosecond regimes. The erbium emissions that have been studied deeper have been the 1530 and 550 nm emissions.The generation of laser radiation from ytterbium ions has been realized in continuous-wave mode for the case of KLu(WO4)2 doped with ytterbium and in pulsed mode (Quasi-CW) for the case of KYb(WO4)2. Ytterbium ions have been excited by means of a Titanium-Sapphire laser and a diode

Long-term assessment of wild boar harvesting and cattle removal for bovine tuberculosis control in free ranging populations

Wild boar is a recognized reservoir of bovine tuberculosis (TB) in the Mediterranean ecosystems, but information is scarce outside of hotspots in southern Spain. We describe the first high-prevalence focus of TB in a non-managed wild boar population in northern Spain and the result of eight years of TB management. Measures implemented for disease control included the control of the local wild boar population through culling and stamping out of a sympatric infected cattle herd. Post-mortem inspection for detection of tuberculosis-like lesions as well as cultures from selected head and cervical lymph nodes was done in 745 wild boar, 355 Iberian ibexes and five cattle between 2004 and 2012. The seasonal prevalence of TB reached 70% amongst adult wild boar and ten different spoligotypes and 13 MIRU-VNTR profiles were detected, although more than half of the isolates were included in the same clonal complex. Only 11% of infected boars had generalized lesions. None of the ibexes were affected, supporting their irrelevance in the epidemiology of TB. An infected cattle herd grazed the zone where 168 of the 197 infected boars were harvested. Cattle removal and wild boar culling together contributed to a decrease in TB prevalence. The need for holistic, sustained over time, intensive and adapted TB control strategies taking into account the multi-host nature of the disease is highlighted. The potential risk for tuberculosis emergence in wildlife scenarios where the risk is assumed to be low should be addressed

Surface ablation of RbTiOPO4 by femtosecond laser

We report here the results obtained in surface ablation of RbTiOPO4 single crystals by femtosecond laser. We fabricated and characterized one-dimensional (1D) diffraction gratings with different lattice spacings of 15 and 20 μm, and with a sub-modulation of the period introduced in the later. The optical and electronic microscopy characterization and filling factor analysis of these diffraction gratings are reported. We also show that the roughness generated on the grooves by the ablation process can be improved by chemical etching.This work was partially funded by the European Commission under the Seventh Framework Program under Project Cleanspace FP7-SPACE-2010-1-GA-263044, supported by the Spanish Government under Projects PI09/90527, TEC2009-09551, AECID A/024560/09, FIS2009-09522, HOPE CSD2007-00007 and SAUUL CSD2007-00013 (Consolider-Ingenio 2010), by Catalan Government under Projects 2009SGR235 and 2009SGR549, by Junta de Castilla y León under Project GR27, and by the Research Center on Engineering of Materials and Systems (EMaS) of the URV. J.J.C. is supported by the Education and Science Ministry of Spain and European Social Fund under the Ramón y Cajal program, RYC2006-858. We also acknowledge support from the Centro de Laseres Pulsados, CLPU, Salamanca, Spain

Modelling of graphene Q-switched Tm lasers

DOI: 10.1016/j.optcom.2016.12.023 URL: http://www.sciencedirect.com/science/article/pii/S0030401816310847 Filiació URV: SI Inclòs a la memòria: SIWe report on a model of diode-pumped Thulium lasers passively Q-switched by a graphene saturable absorber applicable also for any other ¿fast¿ saturable absorber. It reasonably predicts the dependence of the pulse duration, pulse energy and pulse repetition frequency on the absorbed power. The model is applied in the present work for a Tm: KLuW microchip laser passively Q-switched with a multi-layer graphene saturable absorber. The laser generates ~1 W at 1926 nm with a slope efficiency of 39%. Stable 190 ns /4.1 ¿J pulses are achieved at a pulse repetition frequency of 260 kHz. The potential of graphene for the generation of few-ns pulses at ~2 µm is discussed

Europium doping in monoclinic KYb(WO4)2 crystal

DOI: 10.1016/j.jlumin.2016.11.046 URL: http://www.sciencedirect.com/science/article/pii/S0022231316313291 Filiació URV: SI Inclòs a la memòria: SIWe report on a detailed spectroscopic study of Eu3+ ions in the monoclinic KYb(WO4)2 crystal. The polarized room and low-temperature absorption spectra are measured. The maximum ¿abs corresponding to the 7F1 ¿ 5D1 transition is 1.32×10¿20 cm2 at 534.2 nm with a bandwidth of 0.7 nm (for E||Nm). The Stark sub-levels of the excited mulitplets are determined. A Judd-Ofelt analysis is applied to the Eu3+:KYb(WO4)2 crystal to determine the probability of spontaneous transitions, radiative lifetimes and luminescence branching ratios. Within the strong configuration interaction (SCI) approximation, the intensity parameters are ¿2=4.757, ¿4=2.295, ¿6=1.644 [10¿20 cm2] and ¿f=50160 cm¿1. The radiative lifetime of the 5D0 state is 351 µs. The maximum stimulated-emission cross-section corresponding to the 7F1 ¿ 5D1 transition is 1.44×10¿20 cm2 at 703.2 nm (for E||Nm). Under UV excitation, the Eu3+:KYb(WO4)2 crystal provides intense red photoluminescence with CIE coordinates, x=0.675, y=0.325

Indium-modified Yb:KLu(WO4)2 crystal: Growth, spectroscopy and laser operation

DOI: 10.1016/j.jlumin.2016.11.018 URL: http://www.sciencedirect.com/science/article/pii/S0022231316311516 Filiació URV: SI Inclòs a la memòria: SIWe report on the growth, spectroscopic and laser characterization of a novel monoclinic laser crystal, Yb,In:KLu(WO4)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 ||Np. For Yb,In:KLuW, the maximum σabs is 9.9×10–20 cm2 at 980.9 nm for E ||Nm and the corresponding bandwidth of the absorption peak is 3.7 nm. The radiative lifetime for Yb3+ ions is 237±5 µs. The stimulated-emission cross-sections are σSE(m)=2.4×10–20 cm2 at 1022.4 nm and σSE(p)=1.3×10–20 cm2 at 1039.1 nm corresponding to an emission bandwidth of >30 nm and >35 nm, respectively. The diode-pumped Ng-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

Microchip laser operation of Yb-doped gallium garnets

Open access DOI: 10.1364/OME.6.000046 URL: https://www.osapublishing.org/ome/abstract.cfm?uri=ome-6-1-46 Filiació URV: SIContinuous-wave microchip laser operation and thermal lensing are studied for Yb-doped gallium garnets, Yb:LuGG, Yb:YGG, Yb:CNGG and Yb:CLNGG under diode-pumping at ~932 and 969 nm. It is shown that although thermal the conductivity of Ga garnets is lower than that of Yb:YAG, the compromised thermo-optic properties, high absorption in the zero-phonon line and low internal loss make the ordered Yb:YGG and Yb:LuGG crystals to be promising for compact highly efficient microchip lasers. In particular, Yb:LuGG microchip laser generated 8.97 W of output power with a slope efficiency η = 75% and 9.31 W with η = 65%, for pumping at 932 and 969 nm, respectively. Multi-watt output in the range 1039–1078 nm is emitted for different transmission of the output coupler. The sensitivity factor of the thermal lens for this crystal is 2.1 m−1/W (pumping at 969 nm with a pump waist radius of 100 μm) and the estimated thermal conductivity is 5.8 ± 0.5 W/mK. Power scaling of Yb:CNGG and Yb:CLNGG microchip lasers is limited by poor thermo-optic properties and high internal losses. Ordered Ga garnets show good prospects for the development of passively Q-switched microchip lasers with high pulse energies

Thulium doped LuAG ceramics for passively mode locked lasers

DOI: 10.1364/OE.25.007084 URL: https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-6-7084 Filiació URV: SIPassive mode-locking of a thulium doped Lu3Al5O12 ceramic laser is demonstrated at 2022 nm. By applying different near surface GaSb-based saturable absorber mirrors, stable self-starting mode-locked operation with pulse durations between 2 and 4 picoseconds was achieved at a repetition rate of 92 MHz. The SESAM mode-locked Tm:LuAG ceramic laser exhibits an excellent stability with a fundamental beat note extinction ratio of 80 dB above the noise level. Furthermore, spectroscopic properties of Tm:LuAG ceramics at room temperature are presented

Crystal growth, optical spectroscopy and laser action of Tm3+-doped monoclinic magnesium tungstate

DOI: 10.1364/OE.25.003682 URL: https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-4-3682 Filiació URV: SI MemòriaWe report on the crystal growth, spectroscopic investigation and laser performance of Tm3+-doped monoclinic magnesium tungstate (Tm:MgWO4), for the first time, to the best of our knowledge. A high-quality crystal has been grown by the top seeded solution growth method. The relevant spectroscopic properties are characterized in terms of absorption, luminescence and Raman spectroscopy. Judd-Ofelt (J-O) analysis is performed to evaluate the spontaneous emission probabilities and the radiative lifetimes. The absorption, stimulated-emission and gain cross-section spectra are determined for the principal light polarizations. The first laser action in the 2 ¿m spectral range is demonstrated in the regime of continuous-wave operation with a maximum output power of 775 mW and a slope efficiency of 39%

Graphene Q-switched Tm:KY(WO4)2 waveguide laser

DOI: 10.1088/1555-6611/aa5c15 URL: http://iopscience.iop.org/article/10.1088/1555-6611/aa5c15/meta;jsessionid=400897093FD1F1097EB0E6468F81AD1D.c4.iopscience.cld.iop.org Filiació URV: SIWe report on the first Tm3+-doped double tungstate waveguide laser passively Q-switched by a graphene saturable absorber using a 12.4 ¿m-thick 3 at.% Tm:KY0.58Gd0.22Lu0.17(WO4)2 epitaxial layer grown on a (0 1 0)-oriented pure KY(WO4)2 substrate. This laser generated 5.8 nJ/195 ns pulses at 1831.8 nm corresponding to a pulse repetition frequency of 1.13 MHz. These are the shortest pulses achieved in passively Q-switched Tm waveguide lasers. The laser slope efficiency was 9% and the Q-switching conversion efficiency reached 45%. Graphene is promising for the generation of ns pulses at ¿2 ¿m in Tm3+-doped double tungstate waveguide lasers operating in the MHz-range