Structural analysis: from grain to atom": Yb-doped YAG polycrystalline optical ceramics

The recent development of ceramic laser materials is probably the most important innovation in the field of laser material fabrication technology. Large-scale laser materials are necessary for high-power laser systems; this can be achieved with polycrystals ('ceramics'), mainly cubic garnets. To optimize and obtain the required high optical quality, links between elaboration conditions, microstructure and optical properties must be established. From a general point of view, the optical quality, in terms of transparency and laser output, is directly linked to the quality of the materials microstructure: any grain-boundary (GB) segregation or second-phase, significant porosity or heterogeneity can lead to a decrease of performances, due essentially to undesirable wave scattering sources which will reduce the laser efficiency [1, 2]. In this context, a multiscale structural characterization is required in order to identify phases, heterogeneities from the micron to the atomic scale. A very important class of materials for laser applications (i.e. diode-pumped solid-state lasers) concerns Yttrium Aluminium Garnets (YAG: Y3Al5O12) doped with rare earth elements. In this work, we will focus on Yb-doped YAG compounds elaborated by a classical high temperature sintering route. Both Scanning Electron Microscopy (SEM) and Transmission EM (TEM) have been carried out on such systems, and 3 main topics will be considered: i) to limit the undesirable 'thermal lensing effect' (see §.3.1), a pure-YAG / Yb-doped YAG composite material has been sintered, and electron microscopy has been used in order to evaluate and measure the microstructure in terms of Yb concentration gradient. SEM and EDX were first used for a large scale characterization, then TEM was performed on a FIB lamellae extracted in the interfacial region where the gradient was identified. ii) owing to usual sintering aids (e.g. SiO2), second phases can occur at GBs, and High Resolution TEM plus nano-probe EDX analysis were performed in order to measure segregation effects and study any eventual second phases, in relation with the elaboration conditions. iii) in the perspective of evaluating the possibility of High Resolution imaging to reveal the presence of dopants (i.e., isolated ions Yb3+) in the YAG lattice, both Conventional and Cs-Corrected HREM observations have been undertaken.