Transparent Yb:Sc2O3 ceramics, preparation and laser performance

Abstract Ytterbium-doped scandium oxide ceramics were prepared from commercially available powders. The materials were sintered in high vacuum followed by hot isostatic pressing. Different vacuum sintering temperatures were tested. Laser efficiency of up to 45% and output power about 3 W were obtained. 1. Introduction The use of scandium oxide as a host for solid state lasers has drawn interest in the last decade, since, like YAG, also Sc2O3 doped with Yb3+ is a suitable active gain medium for high-power and high-efficiency diode-pumped solid-state lasers (DPSSL) with relatively wide absorption band suitable for the use of commercially widely available InGaAs diodes. Another advantage is its relatively high thermal conductivity at low dopant concentrations [1]. The production of scandium oxide single crystals is complicated by its high melting point and the usually used micro-pulling-down method [2] does not provide crystals of larger dimensions. Polycrystalline scandium oxide may be prepared by ceramic processing at lower temperatures, and a number of results have been published in the last years. However, the sintering of transparent scandium oxide is not trivial and most of the related works present a process including the synthesis of scandia nanopowders. The presented work focuses on the development of a process for the production of transparent Sc2O3 ceramics with the use of commercial micrometric powders. 2. Experimental Transparent Yb:Sc2O3 ceramics were produced from commercial oxide powders. The mixed powders were pressed into pellets, treated in air and sintered in high vacuum followed by hot isostatic pressing. Different vacuum sintering temperatures were tested. Both transmittance, fluorescence and laser measurements were performed on the produced ceramics. The samples for laser testing were mirror polished, and no coating was applied. Laser performance was characterized using a confocal cavity with V-shaped geometry, longitudinally pumped by a fibre coupled diode laser emitting around 940 nm with maximum power of 20 W. The pump was used in a quasi-continuous excitation and the cooling of the samples is achieved by conduction on one of the sample facets by welding with indium on a water-cooled copper heat sink. 3. Results The sintered samples had a uniform fine microstructure with grain size ranging from about one to ten microns (see Fig. 1), depending on the sintering conditions. Compared to dopant-free Sc2O3 ceramics, where a vacuum pre-sintering in the range 1500 - 1600 °C was sufficient for the preparation transparent material, the addition of Yb increased the necessary temperature by about 100 °C. Laser oscillation with output of more than 3 W and slope efficiency above 45 % were obtained for the sample resulting from the optimized process, as shown in Fig. 2. Fluorescence time decays were slightly lower compared to the values published for single crystals, and are probably caused by concentration quenching or other lattice defects triggering nonradiative decay paths. 4. Conclusions The presented results show the process of preparation of transparent Yb:Sc2O3 ceramics from commercial oxide powders by a combined vacuum and HIP sintering process and the effect of vacuum sintering conditions on the microstructure and optical performance. 5. References [1] A. Yoshikawa, A. Novoselov, Shaped Crystals: Growth by Micro-Pulling-Down Technique, Springer, Berlin, Heidelberg, Chapter 12, 2007. [2] R. Simura, A. Jouini, M. Ji-Hun, A. Brenier, A. Yoshikawa, G. Boulon, T. Fukuda, "Growth and spectroscopic properties of Yb3+-doped Sc2O3 crystals grown by the micro-pulling-down method," Opt. Mater. 30, 18-21 (2007). 6. Acknowledgements The authors gratefully acknowledge the support from the Italian Ministry of Defence under PNRM, Contract No. 8723 of 19/12/2014 (CeMiLAP, Fase 2).