Neste trabalho, foram realizadas sínteses dos tungstatos duplos NaLa(WO4)2 (NLW), NaGd(WO4)2 (NGW) e LiLa(WO4)2 (LLW) puros e simplesmente dopados com os íons Nd3+ e Yb3+; comparando-se dois métodos de obteção: o de reação no estado sólido e o sol-gel modificado (Pechini). Verificou-se que o envelhecimento do reagente La2O3 (adsorção de umidade ambiente) é um fator determinante na formação dos materiais desejados. Ambos os métodos testados são análogos para obtenção desses materiais se a proporção estequiométrica dos reagentes for a correta. Um estudo empírico foi realizado para crescimento, a partir da fusão, de fibras cristalinas, através do método de micro-pulling-down (µ-PD) (nos modos de aquecimento resistivo e indutivo), de NLW, NGW e LLW puros e simplesmente dopados com os íons Nd3+ e Yb3+. Neste estudo observou-se a evaporação do líquido durante o processo, o efeito da variação das taxas de puxamento e a interação do líquido com o material do cadinho e com a interface de cristalização. Foram obtidas fibras monocristalinas para todos os compostos. Amostras destas fibras foram caracterizadas por difratometria de raios-X (DRX) para verificação da fase formada, por microscopia ótica (MO) e eletrônica de varredura (MEV) para caracterização do diâmetro das fibras, espectrometria por dispersão em energia (EDS/EDX) para determinação da concentração dos dopantes; e também espectroscopia ótica de absorção no VIS-IR para determinação da incorporação do dopante na forma trivalente. / At the present work, the pure and Nd,Yb-doped alkali rare earth double tungstates NaLa(WO4)2 (NLW), NaGd(WO4)2 (NGW) and LiLa(WO4)2 (LLW) were synthesized. Two synthesis methods were evaluated: the solid state reaction method and the modified sol-gel method (Pechini). Both of them are similar for the synthesis of these materials if stoichiometric proportion of reagents is used. It was verified that the moisture adsorption from air by the reagent La2O3 is a problem in the formation of the studied compounds. An empirical study of growth from the melt of single-crystalline fibers was achieved through the micro-pulling-down (µ-PD) method (in the heating inductive and resistive modes) of pure and Nd,Yb-doped NLW, NGW and LLW. It was observed in this study the liquid evaporation during the process, the effect of different pulling rates, the liquid interaction with the crucible material and the crystallization interface. Single-crystalline fibers were achieved for all compounds studied. Samples were characterized through X-ray powder diffraction (XRD) for phases identification, optical (OM) and scanning electronic microscopy (SEM) for fiber diameter visualization, energy dispersion scattering (EDS/EDX) for determining dopants concentration and also optical absorption spectroscopy in the VIS-IR for determining dopants incorporation in the trivalent state.

Material and structure engineering are increasingly employed in active optical media,in this context defined as media capable of providing laser or/and optical parametric gain. For laser materials, the main aim of the engineering is to tailor the absorption and emission cross sections in order to optimise the laser performance. At the same time, the engineering also results in a collateral modification of the material’s nonlinear response. In the first part of this work, the nonlinear index of refraction is characterised for two crystallographic forms of laser-ion doped and undoped double-tungstate crystals. These laser crystals have broad gain bandwidths, in particular when doped with Yb3+. As shown in this work, the crystals also have large Kerr nonlinearities, where the values vary significantly for different chemical compositions of the crystals. The combination of a broad gain bandwidthand a high Kerr nonlinearity makes the laser-ion doped double tungstates excellent candidates to employ for the generation of ultrashort laser pulses by Kerr-lens modelocking. The second part of the work relates to the applications of engineered second-order nonlinear media, which here in particular are periodically-poled KTiOPO4 crystals. Periodic structure engineering of second-order nonlinear crystals on a submicrometre scale opens up for the realisation of novel nonlinear devices. By the use of quasi-phase matching in these structures, it is possible to efficiently downconvert a pump wave into two counterpropagating parametric waves, which leads to a device called a mirrorless optical parametric oscillator. The nonlinear response in these engineered submicrometre structures is such that the parametric wave that propagates in the opposite direction of the pump automatically has a narrow bandwidth, whereas the parametric wave that propagates with the pump essentially is a frequency-shifted replica of the pump wave. The unusual spectral properties andthe tunabilities of mirrorless optical parametric oscillators are investigated. / QC 20120330

Estudou-se neste trabalho a preparação de LiLa(WO4)2:TR3+ (LLW:TR) nas formas de fibras monocristalinas (micro-pulling-down) e de microcristais pó cerâmico (método dos precursores poliméricos). No que se refere às fibras: a taxa de puxamento no crescimento de LiLa(1-x)Eux(WO4)2 para 0≤x≤1 é influenciada pela diferença de raio iônico do Li e das TR; a estrutura tetragonal da scheelita descreve as composições 0≤x≤1; a incorporação do Eu gera distorções na estrutura sem reduzir a simetria local do íon; uma queda de luminescência para x>0,20 é observada; e as estruturas para 0≤x≤1 foram modeladas através de simulação atomística com bastante precisão. No que se refere aos microcristais: o efeito do controle de pH na preparação de LLW:Nd 1,0 mol% foi avaliado; obteve-se aglomerados de morfologia irregular e com tamanhos médios entre 22-48 μm, cujo aumento é maior em função da temperatura do que do tempo de calcinação. No que se refere à construção experimental do diagrama de fases do sistema xLi2W2O7-(1-x)La2W2O9: confirmou-se que o LLW (formado numa região de homogeneidade de 0,48≤x≤0,55) se decompõe peritéticamente a 1000°C; a dopagem por TR influencia sua fusão; dados do crescimento, de DTA e de DRX de fibras de LLW, crescidas com composições baseadas neste diagrama, corroboram o mesmo. O processo de obtenção de fibras foi otimizado, com um excesso mínimo de 1,5 mol% de Li2W2O7. / In this work, the preparation and characterization of LiLa(WO4)2:RE3+ (LLW:RE) microcrystals as single crystal fibers (micro-pulling-down technique) and as powder (polymerizable complex method) was studied. Concerning the fibers: the pulling rate for LiLa(1-x)Eux(WO4)2 (0≤x≤1) is influenced by the difference between the constituents ionic radii; the tetragonal scheelite-like structure describes all compositions; the Eu incorporation distorts the LLW lattice without reducing the dopant local symmetry; a luminescence quenching for x>0.20 was observed; and the structure modelling for 0≤x≤1 by atomistic simulation was carried out with good precision. Concerning the powder microcrystals: the pH control effect on the preparation of Nd:LLW 1,0 mol% was evaluated; they presented irregular morphology and agglomerates with average sizes of 22-48 μm with higher increase for the calcination temperature than for the time. Concerning the experimental construction of the phase diagram of the system xLi2W2O7-(1-x)La2W2O9: it was confirmed that the LLW (formed in the a homogeneity region of 0.48≤x≤0.55) decomposes peritectically at 1000°C; RE-doping influences the LLW melting; growth, DTA and XRD data of LLW fibers (grown from the compositions based on this diagram) confirmed it. The process of fiber obtaining was optimized with a minimum excess of 1.5 mol% of Li2W2O7.

Estudou-se neste trabalho a preparação de LiLa(WO4)2:TR3+ (LLW:TR) nas formas de fibras monocristalinas (micro-pulling-down) e de microcristais pó cerâmico (método dos precursores poliméricos). No que se refere às fibras: a taxa de puxamento no crescimento de LiLa(1-x)Eux(WO4)2 para 0≤x≤1 é influenciada pela diferença de raio iônico do Li e das TR; a estrutura tetragonal da scheelita descreve as composições 0≤x≤1; a incorporação do Eu gera distorções na estrutura sem reduzir a simetria local do íon; uma queda de luminescência para x>0,20 é observada; e as estruturas para 0≤x≤1 foram modeladas através de simulação atomística com bastante precisão. No que se refere aos microcristais: o efeito do controle de pH na preparação de LLW:Nd 1,0 mol% foi avaliado; obteve-se aglomerados de morfologia irregular e com tamanhos médios entre 22-48 μm, cujo aumento é maior em função da temperatura do que do tempo de calcinação. No que se refere à construção experimental do diagrama de fases do sistema xLi2W2O7-(1-x)La2W2O9: confirmou-se que o LLW (formado numa região de homogeneidade de 0,48≤x≤0,55) se decompõe peritéticamente a 1000°C; a dopagem por TR influencia sua fusão; dados do crescimento, de DTA e de DRX de fibras de LLW, crescidas com composições baseadas neste diagrama, corroboram o mesmo. O processo de obtenção de fibras foi otimizado, com um excesso mínimo de 1,5 mol% de Li2W2O7. / In this work, the preparation and characterization of LiLa(WO4)2:RE3+ (LLW:RE) microcrystals as single crystal fibers (micro-pulling-down technique) and as powder (polymerizable complex method) was studied. Concerning the fibers: the pulling rate for LiLa(1-x)Eux(WO4)2 (0≤x≤1) is influenced by the difference between the constituents ionic radii; the tetragonal scheelite-like structure describes all compositions; the Eu incorporation distorts the LLW lattice without reducing the dopant local symmetry; a luminescence quenching for x>0.20 was observed; and the structure modelling for 0≤x≤1 by atomistic simulation was carried out with good precision. Concerning the powder microcrystals: the pH control effect on the preparation of Nd:LLW 1,0 mol% was evaluated; they presented irregular morphology and agglomerates with average sizes of 22-48 μm with higher increase for the calcination temperature than for the time. Concerning the experimental construction of the phase diagram of the system xLi2W2O7-(1-x)La2W2O9: it was confirmed that the LLW (formed in the a homogeneity region of 0.48≤x≤0.55) decomposes peritectically at 1000°C; RE-doping influences the LLW melting; growth, DTA and XRD data of LLW fibers (grown from the compositions based on this diagram) confirmed it. The process of fiber obtaining was optimized with a minimum excess of 1.5 mol% of Li2W2O7.

Laseres de estado sólido que operan en la región espectral de seguridad ocular alrededor de los 2 micrómetros (2µm) son de elevado interés en la actualidad debido a su potencial aplicación en los campos de la medicina, teledetección remota y como fuentes de bombeo en osciladores ópticos paramétricos (OPOs) para conversión en el infrarrojo medio. La transición láser en las 2 µm es posible en los iones de tulio (Tm) ligeramente por debajo de las 2 µm y en iones de holmio (Ho) ligeramente por encima de las 2 µm. La generación láser en iones de Tm es relativamente simple con la utilización de láseres de diodo como fuentes de bombeo, sin embargo, los láseres basados en Ho se han conseguido tradicionalmente en el pasado mediante el codopaje con Tm o mediante bombeo directo del nivel emisor del Ho. Recientemente, diodos láser que operan a 1.9 µm han aparecido en el mercado con buena eficiencia y con alto potencial para el escalado en potencia de los láseres de Ho. Los láseres de Ho son más apropiados que los láseres de Tm especialmente para aplicaciones médicas por dos razones: La longitud de onda láser ligeramente por encima de las 2 µm, donde el agua (mayor componente del cuerpo humano) presenta una ligera menor absorción que la típica longitud de onda láser del Tm, hace que el láser penetre más en el tejido humano. La segunda razón es que los láseres de Ho pueden operar en régimen pulsado generando mayores energías por pulso que los láseres de Tm debido al mayor tiempo de vida del nivel emisor 5I7 y las aplicaciones médicas requieren por lo general régimen pulsado para evitar daño térmico del tejido. Numerosos óxidos y fluoruros cristalinos han demostrado ser adecuadas matrices para Ho, sin embargo, poca atención se ha puesto en los dobles tungstatos cristalinos de fase monoclínica, de fórmula química KRE(WO4)2, brevemente KREW, donde RE = Y, Gd y Lu conocidos por ser matrices láser muy eficientes para la generación láser a potencias intermedias. Estos cristales anisotrópicos presentan una elevada sección eficaz de absorción y emisión cuando son dopados con iones lantánidos y especialmente para ciertas polarizaciones. Considerando el potencial del Ho y las buenas propiedades de KREW, nuestro trabajo se centra en la investigación de las prestaciones láser en cristales de KREW dopados con Ho utilitzando tanto Tm o Yb como iones sensibilizadores y utilizando fuentes de bombeo emitiendo a 1.9 µm que permiten la excitación directa del nivel emisor. En esta tesis, presentamos los resultados basados en el crecimiento cristalino de monocristales de Ho:KREW, codopajes (Ho,Tm) y (Ho,Yb):KLuW a diferentes concentraciones de Ho, su caracterización en términos de estructura, composición y espectroscopia y finalmente la generación láser alrededor de 2.1 µm. / Eye-safe solid-state lasers that operate in the 2µm spectral range are the subject of interest in the present years because of their potential applications in the field of remote sensing, medicine and as a pump source for Optical Parametric Oscillators (OPOs). Laser transitions around 2 µm are possible in the trivalent lanthanide ions Tm3+ (Tm) (slightly below 2 µm) and Ho3+ (Ho) (slightly above 2 µm). Laser generation in Tm ions is easily achieved with comfortable diode pump sources, however, Ho lasers have usually been achieved in the past either by co-doping the active medium with Tm or by direct pumping of the Ho ions with Tm lasers. Recently, relatively cheap diodes emitting around 1.9 µm are in the market to realize Ho lasers with great potential for power scaling. Ho lasers are more suitable than Tm lasers especially for medical applications because of two reasons: The laser wavelength is slightly above 2µm, where water (main component of human tissue) shows slightly less absorption than the typical wavelength of Tm leading to a deeper penetration in human tissue. The second reason is that Ho lasers can operate in pulsed regime delivering higher energies than Tm lasers due to the longer lifetime of the emitting level 5I7 and medical applications are required to be generally in pulsed regime to avoid thermal damage of human tissue. Many oxide and fluoride crystals were shown to be suitable host for Ho, however little attention was paid to the monoclinic potassium rare earth double tungstate crystal, shortly KRE(WO4)2 or KREW, where RE= Y, Gd, Lu known to be very efficient rare earth solid state hosts for generating intermediate power levels. These anisotropic crystals exhibit very high absorption and emission cross sections when doped with lanthanide ions and especially for selected polarizations. Considering the potentialities of Ho and good properties of KREW, our work focuses in the investigation of the laser performances of a Ho doped KREW either by using Tm or Yb as sensitizers and by using in-band pump sources emitting around 1.9 µm, where the development of compact solid state infrared laser emitting at 2.1 µm for intermediate power levels is followed. Here, in this thesis, we present the results based on growth of single doped Ho:KREW, co-doped (Ho,Tm) and (Ho,Yb):KLuW crystals of several doping concentrations, their characterisation in terms of structure, composition and spectroscopy and finally dedicated for the laser generation around 2.1 µm from these materials, which was highly successful.

Made available in DSpace on 2014-12-17T15:01:26Z (GMT). No. of bitstreams: 1 AndreLLM_DISSERT.pdf: 1863413 bytes, checksum: 1d92759b3bdf5371d233d97ad885e7af (MD5) Previous issue date: 2011-07-21 / Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico / The main goal of this work was to produce nanosized ceramic materials of the family of the tungstates (tungstates of cerium and strontium), and test them for their catalytic activity in processes involving the transformation of methane (CH4). The methodology used for the synthesis of the ceramic powders involved the complexation combining EDTA-citrate. The materials characterization was performed using simple and differential thermogravimetry, x-ray diffraction, transmission electron microscopy, and energy dispersive spectroscopy (EDS). The microstructure analysis was performed using the refinement by the Rietveld method, and the crystallite size and distribution of the materials was elucidate by the Scherrer and Williamson-Hall methods. The conditions of the synthesis process for the three envisaged materials (SrWO4, SrWO4 using tungsten oxide concentrate as raw material, and Ce2(WO4)3) were adjusted to obtain a single phase crystalline material. The catalytic tests were carried out in the presence of methane and synthetic air, which is composed of 21% O2 and 79% N2. The analysis of the conversion of the reaction was done with the aid of an fourier transform infrared device (FTIR). The analysis showed that, structurally, the SrWO4 produced using raw materials of high and poor purity (99% and 92%, respectively) are similar. The ideal parameters of calcination, in the tested range, are temperature of 1000 ?C and time of calcination 5 hours. For the Ce2(WO4)3, the ideal calcination time and are temperature 15 hours and 1000?C, respectively. The Williamson-Hall method provided two different distributions for the crystallite size of each material, whose values ranged between the nanometer and micrometer scales. According to method of Scherrer, all materials produced were composed of nanometric crystallites. The analyses of transmission electron microscopy confirmed the results obtained from the Williamson- Hall method for the crystallite size. The EDS showed an atomic composition for the metals in the SrWO4 that was different of the theoretical composition. With respect to the catalytic tests, all materials were found to be catalytically active, but the reaction process should be further studied and optimized. / O objetivo principal deste trabalho foi produzir materiais cer?micos nanom?tricos da fam?lia dos tungstatos (tungstatos de c?rio e de estr?ncio), e test?-los quanto ?s suas atividades catal?ticas em processos envolvendo a transforma??o do g?s metano (CH4). A metodologia empregada para a s?ntese dos p?s cer?micos envolveu o m?todo de complexa??o combinando EDTA-Citrato. A caracteriza??o dos materiais foi feita atrav?s das t?cnicas de termogravimetria simples e diferencial, difra??o de raios-x, microscopia eletr?nica de transmiss?o, e espectroscopia de energia dispersiva (EDS). A an?lise da microestrutura foi realizada atrav?s do refinamento utilizando o m?todo de Rietveld, e para a estima??o do tamanho de gr?o e de sua distribui??o foram utilizados os m?todos de Scherrer e Williamson-Hall. As condi??es de obten??o dos tr?s materiais desenvolvidos (SrWO4, SrWO4 utilizando concentrado de ?xido de tungst?nio como mat?ria-prima, e Ce2(WO4)3) foram ajustadas para a obten??o de um material cristalino monof?sico. Os testes catal?ticos explorat?rios foram realizados na presen?a dos gases metano e ar sint?tico, composto por 21% de O2 e 79% N2. A an?lise da convers?o foi feita com o aux?lio de um equipamento de infravermelho ? transformada de Fourier. As an?lises mostraram que, estruturalmente, os materiais SrWO4 produzidos com mat?riaprima de elevada pureza (99% de pureza) e oriunda do concentrado de ?xido de tungst?nio (92% de pureza) s?o semelhantes. Seus par?metros ideais de produ??o, dentro do que foi testado, s?o temperatura de calcina??o de 1000?C e tempo de perman?ncia de 5 horas na isoterma. Para o Ce2(WO4)3, as condi??es ideais dentro do que foi testado s?o temperatura de calcina??o de 1000?C e tempo de perman?ncia na isoterma de 15 horas. O m?todo de Williamson-Hall forneceu duas distribui??es diferentes para o tamanho de cristalito de cada material, cujos valores oscilaram entre as escalas nanom?tricas e microm?tricas. Com o m?todo de Scherrer, todos os materiais produzidos possu?ram tamanho de cristalito nanom?trico. As an?lises de microscopia eletr?nica de transmiss?o confirmaram o resultado obtido com o m?todo de Williamson- Hall para o tamanho de cristalito. A EDS indicou a presen?a de diverg?ncias entre a composi??o at?mica met?lica te?rica e experimental do SrWO4. Com rela??o aos testes catal?ticos explorat?rios, os materiais se mostraram ativos por?m o processo reacional ainda precisa ser melhor estudado e otimizado.

The scheelite type tungstates MWO4 have been studied for a long time due to their optical properties. The main property is the luminescence, both intrinsic and extrinsic (when doped with trivalent lanthanide ions, Ln3+). Another group of scheelite- typed are the double tungstates, ALn(WO4)2. The main feature of these tungstates is a structural disorder involving a random distribution of the ions A (alkali metals) and Ln in the crystal lattice that may influence the luminescence of the material. In literature there are several models to explain both intrinsic and extrinsic luminescence, as recombination of self-trapped excitons, MO and/or WO3 vacancies, stoichiometry deviation, other phases, oxygen at interstitial site, oxygen vacancies and M ion vacancies. As the main technology applications associated with these tungstates are such optics fiber, solid state lasers, scintillators in detectors and recently as white LEDs, it is necessary to better understand and possibly solve or dominate the many physical problems that surround them. Then, using computer simulation based on a model in which the ions are treated as charged spheres interacting through interaction potentials which aim to minimize the lattice energy, tungstates have their perfect and defective crystal lattices simulated to try to elucidate the defect mechanism that dominates and/or contributes for luminescence and its consequences. Using static computer simulation we have as main results: a) 21 different tungstates were modeled using a single set of potential parameters taking into account the covalency of the (WO4)2- group. This covalent interaction affects the behavior of defects, where (WO4)2- groups can be directly connected by an oxygen ion at an interstitial site; b) the charge compensation for extrinsic defects is via interstitial oxygen. When codoped, the codopant ionic radius directly influences the solution energy; c) the simulated energy levels for SrWO4:Eu3+ were compared with recent experimental studies and are in agreement, pointing two different symmetries to the Eu site and d) simulation of holes and electrons in these tungstates reveals that n-type conductivity is expected. / Os tungstatos tipo scheelita MWO4 vêm sendo estudados há bastante tempo devido às suas propriedades ópticas. A principal é a luminescência, tanto intrínseca quanto extrínseca (quando dopados com íons lantanídeos trivalentes, Ln3+). Outro grupo de tungstatos tipo scheelita são os duplos, ALn(WO4)2. A principal característica deste tungstatos é uma desordem estrutural, envolvendo uma distribuição aleatória dos íons A (metais alcalinos) e Ln na rede que pode influenciar a luminescência deste material. Na literatura há diversos modelos para explicar tanto a luminescência intrínseca quanto a extrínseca, como recombinação de éxcitons auto-armadilhados, vacâncias de MO e/ou WO3, desvio de estequiometria, outras fases, oxigênio em um sítio intersticial e vacâncias de oxigênio e vacâncias do íon M. Como as principais aplicações tecnológicas associadas a estes tungstatos, são como fibras ópticas, lasers do estado sólido, cintiladores em detectores e recentemente como LEDs brancos, faz-se necessário entender melhor e se possível solucionar ou dominar os diversos problemas físicos que os cercam. Então, usando simulação computacional baseada em um modelo em que os íons são considerados como esferas carregadas interagindo entre si através de potenciais de interação que visam minimizar a energia da rede, os tungstatos têm suas redes cristalinas perfeitas e defeituosas simuladas para procurar elucidar o mecanismo de defeito que domina(m) e/ou contribui(em) para a luminescência e quais suas consequências. Usando a simulação computacional estática temos como principais resultados: a) 21 tungstatos diferentes foram modelados usando um único conjunto de parâmetros dos potenciais levando em conta a covalência do grupo (WO4)2-. Esta interação covalente afeta o comportamento dos defeitos, onde grupos de (WO4)2- podem ser diretamente ligados por um íon de oxigênio em um sítio intersticial; b) a compensação de cargas para defeitos extrínsecos é via oxigênio intersticial. Quando codopados, o raio iônico do codopante influencia diretamente na energia de solução; c) os níveis de energia simulados para o SrWO4:Eu3+ e comparados com trabalhos experimentais recentes estão em acordo, apontando duas simetrias diferentes para o sítio de Eu e d) a simulação de buracos e elétrons nestes tungstatos revela que condutividade tipo n é esperada.

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Yb3+-Lasermedien glänzen mit hoher Effizienz und relativ geringer thermischer Last, besonders in Laseroszillatoren und -verstärkern mittlerer bis hoher Leistung. Modenkopplung von Yb3+-Lasersystemen ermöglicht Subpikosekunden-Pulse bei hoher mittlerer Ausgangsleistung. Diese Arbeit widmet sich zwei Gruppen der vielversprechendsten neuen Yb3+-aktivierten Laserkristalle: den strukturell analogen, monoklinen Doppelwolframaten Yb:KGd(WO4)2 (Yb:KGW), Yb:KY(WO4)2 (Yb:KYW) und KYb(WO4)2 (KYbW) und den Yb3+-dotierten Sesquioxiden, vertreten durch Yb:Sc2O3 (Yb:Skandia). Die spektroskopischen Daten von KYbW, darunter eine extrem kurze 1/e-Absorptionslänge von 13 Mikrometern bei 981 nm, wurden im Rahmen der Dissertation vermessen. Die Lasereigenschaften niedrig Yb3+-dotierter Wolframate im Dauerstrich (cw)- und modengekoppelten Betrieb wurden in Lasern moderater Ausgangsleistung untersucht. Ultrakurzpuls-Erzeugung mit Yb:KYW, Yb:KGW und Yb:Glas wurde in einem passiv modengekoppelten Laser verglichen. Dabei wurde eine relativ hohe Lasereffizienz erreicht, Dank einer Trapezlaserdiode als Pumpquelle mit exzellenter Strahlqualität. Quasi-cw- und cw-Laserbetrieb von Yb3+ in hochdotierten und stöchiometrischen Wolframatkristallen wurden untersucht. Diese Materialien sind interessant für Mikrochip- und Scheibenlaser. Wichtige Fragestellungen waren die Kristallqualität und die Hitzeentwicklung bei hohen Yb3+-Konzentrationen. Erstmals wurde Lasertätigkeit von Wolframaten mit Yb3+-Konzentrationen >>20% und schließlich, mit KYbW, cw-Laserbetrieb eines stöchiometrischen Yb-Lasermaterials demonstriert. Weiterhin wurde mit KYbW der kleinste bisher für einen Laserkristall gemessene Laserquantendefekt erzielt, 1,6%. Unter Benutzung eines Yb:Sc2O3-Lasermediums wurde erstmals modengekoppelter Betrieb eines Oszillators mit Sequioxid-Laserkristall gezeigt. Betrieb mit nichtsolitonen- und solitonenartiger Pulsformung sowie mit Ti:Saphir-Laser oder Trapezlaserdiode als Pumplaser wurden untersucht. Mit einem Ti:Saphir-gepumpten Yb:Skandia-Laser wurde eine Konversionseffizienz von 47 % bezogen auf die absorbierte Pumpleistung erreicht, der bisher höchste Wert für einen modengekoppelten Yb3+-basierten Laser. / Yb3+ laser media excel with high efficiency and relatively low heat load, especially in medium to high power laser oscillators and amplifiers. Mode-locking of Yb3+ laser systems can provide subpicosecond pulse durations at high average power. This work deals with two groups of the most promising novel Yb3+-activated laser crystals: Yb3+-activated monoclinic double tungstates, namely the isostructural crystals Yb:KGd(WO4)2 (Yb:KGW), Yb:KY(WO4)2 (Yb:KYW), and KYb(WO4)2 (KYbW), and Yb3+-doped sesquioxides, represented by Yb:Sc2O3 (Yb:scandia). Spectroscopic data of KYbW were investigated as part of this thesis, finding an extremely short 1/e-absorption length of 13 micrometers at 981 nm. Continuous-wave (cw) and mode-locked laser performance of moderate-average-power lasers based on lowly Yb3+-doped tungstates were examined. Ultrashort pulse generation with Yb:KYW, Yb:KGW, and Yb:glass was compared in a passively mode-locked laser. A relatively high mode-locked laser efficiency was achieved due to a tapered diode pump laser with excellent beam quality. Quasi-cw and cw lasing of Yb3+ in highly doped and stoichiometric tungstate crystals were investigated. These materials are interesting for microchip and thin-disk lasers. Important issues were crystal quality and heat generation at high Yb3+ concentrations. For the first time, laser operation of tungstates with a Yb3+ concentration >>20% and finally, with KYbW, cw lasing of a stoichiometric Yb laser material was achieved. Furthermore, with KYbW, the smallest laser quantum defect ever for a laser crystal was demonstrated, 1.6%. Using a Yb:Sc2O3 laser medium, for the first time mode locking of an oscillator using a sesquioxide laser crystal was realized. Laser regimes with non-solitonlike and solitonlike pulse shaping were investigated, using a Ti:Sapphire laser and a tapered laser diode as pump sources. With a Ti:Sapphire-laser-pumped Yb:scandia laser the highest conversion efficiency with respect to absorbed pump power for any mode-locked Yb3+-based laser was achieved, 47%.