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We have studied gray-tracking induced by a pulsed and polarized 532-nm laser beam in flux grown KTiOPO 4 (KTP) crystals. Transmission spectra measured under polarized light give different results: gray-tracking leads to an increase in the initial anisotropy of the linear optical properties of KTP, and the polar axis is the most sensitive to this process. The dynamics of relaxation of gray-tracking is anisotropic and depends on the wavelength under analysis. We show a possible induced modification of the crystal surface and also the existence of an intensity above which gray-tracking reaches the saturation point. We then measure the temperature above which gray-tracking no longer exists.

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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.