Defect doping and characterization in oxide single crystals using femtosecond laser
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Defect doping and characterization in oxide single crystals using femtosecond laser Shingo Kanehira1, Chiwon Moon2, Eita Tochigi3, Naoya Shibata3, Yuichi Ikuhara3, Kiyotaka Miura2, and Kazuyuki Hirao2 1
Society-Academia Collaboration for Innovation, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan 2 Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan 3 Institute of Engineering Innovation, School of Engineering, The University of Tokyo, 2-11-16, Yayoi, Bunkyo-ku, Tokyo 113-8656, Japan
ABSTRACT Defects inside oxide ceramics of magnesium oxide (MgO) and sapphire (Al2O3) were formed using femtosecond laser irradiation. The laser irradiance in these ceramics forms various defects classified as dislocations, twins, or nano cracks near the focal point due to the increase of temperature and successive generation of shockwave within several picoseconds. The morphology of defects mainly depends on the crystal structure; dense dislocations inside MgO with a rock-salt type structure, and nanocracks, dislocations, or twins in sapphire with a corundum structure. The TEM analysis revealed that the dense dislocations formed on the two slip planes {110} to form a cross-shaped pattern when the laser beam focused on the (100) plane in MgO. In case of sapphire crystal, cracks propagated from the focal point that has an amorphous structure along R-planes {11-02}, which had the lowest fracture surface energy. The nano crack transformed into the aligned void and dislocation structure during the heat treatment over 1573K, which is one of the crack healing effect. INTRODUCTION Interface has an important role in the crystal because it affects or dominates characteristics of the bulk material. For example, Transmission Electron Microscope (TEM) reveals that additive segregates at grain boundary and forms covalent bond between matrix and additive. [1] The effective addition of impurities, for example, rare-earth elements (Eu3+, Y3+, Lu3+) in Al2O3 polycrystal increased the mechanical or electrical properties dramatically. [2] Such an extensive analysis technique using high-resolution TEM or STEM accelerates the progress of material science in a wide area. In addition, it is clear that not only additive or impurities but also various defects dominate the characteristics of bulk materials. These defects have an important role in luminescence of glass or crystals, mechanical or electrical properties. Therefore, we also need to analyze the defect inside the materials extensively to clarify the mechanism of destruction or another characteristics in detail. Femtosecond laser irradiation has an extensive ability for various material processing. For example, there is many examples such as formation of voids or rare-earth cluster, precipitation of semiconductor particles such as silicon, -BBO crystal growth at selected area, writing of
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waveguide, coupler, and so on. Another characteristic of the laser is that we can choose the focal point not only at the surface but also i
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