Yttrium Aluminum Garnet (YAG, Y₃Al₅O₁₂) is an excellent host for high power solid state lasers due to its high mechanical and optical properties [1]. This matrix allows the incorporation in solid solution of luminescent ions such as rare-earth elements (e.g. Nd³⁺, Yb³⁺, Ho³⁺…) or transition metals (e.g. Cr³⁺, Cr⁴⁺). In this context, tetravalent chromium-doped yttrium aluminum garnet (Cr⁴⁺:YAG) is an interesting saturable absorber of infrared lasers (emitting wavelength of 1.064 μm), owing to absorption bands of Cr⁴⁺ ion ranging from 800 nm to 1200 nm [2].
Since the 90’s, many works have demonstrated the possibility to manufacture transparent YAG ceramics with optical quality comparable to single crystals grown by the Czochralski method [3,4]. Nevertheless, a drastic control of all the steps of the ceramic elaboration process remains critical to obtain the optical quality required for laser applications. In the case of Cr⁴⁺:YAG ceramics, one more requirement is needed to ensure its functionality, i.e. enough conversion efficiency of trivalent to tetravalent chromium ion (Cr³⁺ à Cr⁴⁺) must be reached. 
Chromium doped YAG ceramics originally include Cr atoms in its trivalent state. Therefore, a careful choice of additives promoting both chromium conversion and densification of the material is necessary for its intended application. It has been shown that the addition of divalent charge compensators like Mg²⁺ or Ca²⁺ is required to promote the chromium conversion and to maintain charge balance in YAG structure [5].
The purpose of this work is to determine the influence of additives (e.g. calcium and magnesium) on the microstructural features (e.g. porosity, grain size, secondary phase, grain boundary composition) of YAG ceramics in order to clarify the sintering mechanisms. In addition, thanks to the high flexibility of ceramic processes, composite ceramics with Cr doping gradient have been produced. As a result, laser performances of Cr⁴⁺:YAG components as saturable absorber in high peak power laser device have been measured and compared to single-crystal properties (e.g. saturation fluence, pulse duration).
 
[1]          A. Goldstein, A. Krell, Transparent Ceramics at 50: Progress Made and Further Prospects, J. Am. Ceram. Soc. 99 (2016) 3173–3197. https://doi.org/10.1111/jace.14553.
[2]          H. Eilers, W.M. Dennis, W.M. Yen, S. Kuck, K. Peterman, G. Huber, W. Jia, Performance of a Cr:YAG laser, IEEE J. Quantum Electron. 29 (1993) 2508–2512. https://doi.org/10.1109/3.247708.
[3]          J. Lu, T. Murai, K. Takaichi, T. Uematsu, K. Misawa, M. Prabhu, J. Xu, K. Ueda, H. Yagi, T. Yanagitani, A.A. Kaminskii, A. Kudryashov, 72 W Nd:Y₃Al₅O₁₂ ceramic laser, Appl Phys Lett. 78 (2001) 3586–3588.
[4]          A. Ikesue, T. Kinoshita, K. Kamata, K. Yoshida, Fabrication and Optical Properties of High-Performance Polycrystalline Nd:YAG Ceramics for Solid-State Lasers, J Am Ceram Soc. 78 (1995) 1033–1040. https://doi.org/10.1111/j.1151-2916.1995.tb08433.x.
[5]          H. Yagi, K. Takaichi, K. Ueda, T. Yanagitani, A.A. Kaminskii, Influence of annealing conditions on the optical properties of chromium-doped ceramic Y₃Al₅O₁₂, Opt. Mater. 29 (2006) 392–396. https://doi.org/10.1016/j.optmat.2005.08.035.