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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We perform angular non-critical phase-matching third-harmonic generation (THG) experiments in (KTP). From these experiments, we determine the magnitude and sign of the two phase-matched nonlinear coefficients of by a complete analysis of phase-matching properties of the cascaded quadratic and pure cubic processes, the resolution of the coupled propagation equations and by the calculation of the quadratic and cubic dielectric susceptibility tensor by an extended bond charge model; we find: and . We also demonstrate that the two studied THG are mainly governed by the cubic interaction.

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