A high-transparency and large-size single crystal, up to 0.5 cm 3 , of the piezoelectric phase of GeO 2 was grown by the top seeded solution growth method from a high-temperature solution using K 2 Mo 4 O 13 as solvent. The obtained volume makes this flux-grown GeO 2 single crystal, with the metastable α-quartz like structure, the largest reported in the literature to our knowledge. Several oriented plates, X-, Y-, and Z-cut according to the dielectric frame, were obtained from the grown crystal, which exhibits a typical hexagonal morphology. The presence of hydroxyl groups as chemical impurities, known to damage the piezoelectric property, was not detected by infrared spectroscopy in transmission mode or Raman spectroscopy on the resulting oriented plates of α-GeO 2 . The effect of a prolonged annealing (up to five months) at high-temperature (800−900 °C) was followed by Raman spectroscopy: no structural evolution as well as no macroscopic modification of the transparency or the morphology of the α-GeO 2 single crystal were observed. These results were consistent with a high optical quality crystal as checked by UV−vis−NIR spectroscopy.

We report on continuous-wave lasing of Yb(3+) at room temperature in the noncentrosymmetric RbTiOPO(4) crystal, codoped with Nb(5+), for all three possible polarizations. A maximum output power of 154 mW at 1050 nm was obtained for an absorbed power of 386 mW. The highest slope efficiency reached approximately 60% and the lowest threshold (with respect to the absorbed power) was 18 mW. The laser was tunable from 1009 to 1081 nm.

When dealing with crystals showing self-frequency conversion properties, RbTiOPO4 doped with Nb5+ (RTP:Nb) is a better matrix than RTP for hosting luminescent ions in higher concentration. Therefore, we investigated the structural and the non-linear optical properties of RTP:Nb and RTP crystals for comparison. We used a new crystal growth process for RTP:Nb, which provides isometric inclusion- and crack-free crystals. Structural studies show that Nb5+ ions are located in Ti positions only, which is confirmed by optical absorption and polarized Raman scattering studies. Moreover, the transparency domain of RTP:Nb is affected in the UV edge and in the IR region by the presence of Nb5+ ions. Finally, we found that the fundamental wavelengths of type II angular non-critical phase-matching for second-harmonic generation along the x and y principal axes are smaller in RTP:Nb than in RTP, approaching those of KTP.

Erbium (Er)- and Ytterbium (Yb)-codoped monoclinic KLu(WO4)2 single crystals were grown by top seeded solution growth-slow cooling method for several different doping concentrations. Growth parameters have been optimized to obtain macrodefect-free single crystals. Er energy levels involved in the 4I13/2-->4I15/2 were determined by 6 K polarized optical absorption. The maximum emission cross section for this electronic transition has been evaluated, being 2.85x10(-20) cm2 for E||Nm at 1535 nm. Laser oscillation in the 1.5 microm range was obtained by pumping the Yb ion at 980 nm and sensitizing Er. The maximum output power achieved was 152 mW, with 1.2% slope efficiency.

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