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Transparent and crack-free Bi12GeO20 single crystals have been grown by the pulling-down method with a continuous powder feed system. Bi-rich feed powder having a composition of 14.1mol% GeO2 is favorable to obtain stoichiometric crystals. Striation-free crystals with an increased transmittance are grown under a small temperature gradient. The shape of the solid-liquid interface during the crystal growth is nearly flat, which is advantageous to avoid core formation. Average dislocation density is estimated to be 2×104/cm2, which is comparable to that of Bi12GeO20 crystals grown by the conventional Czochralski method.

This review deals with effects of oxygen partial pressures on the crystal growth of oxides containing transition elements. Rutile (TiO_2) and chromium-doped forsterite (Cr: Mg_2SiO_4) single crystals are successfully grown by the floating zone method. Rutile single crystals are grown under a low oxygen partial pressure of about 10^3 Pa to avoid the formation of low-angle grain boundaries. Zirconium-doping is effective to grow rutile single crystals without low-angle grain boundaries and bubble inclusions at a high growth rate of 10 mm/h under a high oxygen partial pressure of 10^5 Pa. Cr^-rich Cr: Mg_2SiO_4 single crystals are grown under a high oxygen partial pressure of 1-2 10^5 Pa, which can not be realized in the conventional Czochralski method using an iridium crucible.

MgTa2O6 single crystals, which adopt trirutile structure, were successfully grown by a floating zone method, and their optical properties were invetigated. The as-grown crystals were black and opaque, but subsequent annealing in air from 1200° to 800°C rendered the crystals pale-yellow and transparent. The refractive indices for ordinary and extraordinary rays were 2.07 and 2.18 under white light, respectively, and the birefringence was 0.11. The interference figure observed with a polarizing microscope was typical one for an optically uniaxial crystal, which indicates that the crystal was optically uniform. Transmittance in the wavelength range of 500 to 2000nm was about 75%, which approximately corresponded to the theoretical value calculated from the refractive index. The absorption edge of MgTa2O6 was at 290nm.

26aD03, Tm:GdVO_4 single crystals were successfully grown by the floating zone method. All the grown crystals had no cracks and no inclusions for any dopant concentration. Laser oscillation was observed for the 3 at%-doped crystal at 1920nm with the pumping wavelength of 806nm. The slope efficiency was 38% with the threshold of about 400mW.