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We measured the refractive indices of Zn/In-co-doped lithium niobate in the wavelength range of 400 to 1200 nm, using samples with 5.5 mol% ZnO and various additional concentrations of Inz03. The results can be well described by a generalized Sellmeier equation which takes into account the defect structure of the material. The proposed Sellmeier equation allows the treatment of nonlinear optical effects; calculated phase-matching conditions for second-harmonic generation are in good agreement with experimental results. INT RO DC CTIONLithium niobate is presently one of the most important materials for optical devices. Lsually crystals with a distinct index of refraction are required; often applications are hampcred by the photorefractive effect. also known as optical damage l . Both thc refractive index and the sensitivity to light illumination are influenced by the crystal composition or -to be more specific -by the number of Nb antisites in the crystal 2. Besides the techniques which improve the stoichiometry without any dopirig ' 3 4 , ', dopants like Mg I , Zn 6, In and Sc a are known to reduce the optical damage. As each of these dopants is known to increase the birefringence of lithium niobatc (thus the applicability for nonlinear optics) only up to a certain threshold conccntration, it is important to study the effect of co-doping with more than one rnatcrial. EXPERIMENTAL DETAILSFor our investigations we choose lithium niobate co-doped with Zn and In, as both of' these dopants allow growing crystals of excellent optical quality. The crystals were grown by the Czochralsky technique from a congruent melt (48.5 mol % L i 2 0 and 51.5 mol% Nb205) to which ZnO and In203 was added. Crystals with a fixed ZnO concentration of 5.5 mol% (slightly lower than the threshold value) and In coriceritrations in the range 0 -3 mol% were investigated.For the refractive index measurements we used an interferometric method, recently described in detail ' , l o , which yields an accuracy of about An = ti x provided the optical homogeneity of the sample is sufficient. Homogeneity tests by [903]/269 Downloaded by [Michigan State University] at 13:59 09 April 2015 270/[904] u. SCHLARB, B. MATZAS, A. REICHERT ET AL.means of spatially resolved second harmonic measurements show that the composition varies less than 0.05 mol%, a value well below the composition uncertainty. RESULTS AND DISCUSSIONBoth the ordinary and the extraordinary index of refraction are shown in Fig. 1 for five selected wavelengths in the range 400 -1200 nm as a function of the additional In content in the crystal. The results can be well described by a generalized Sellmeier equation which takes into account the defect structure of Li-deficient LiNbOs with a di-or trivalent dopant (Zn or In). For details of the derivation the reader is referred to Schlarb and Betzler ll.The generalized Sellmeier equation is defined as

Induced noncolinear frequency doubling (INCFD) is presented as a new characterization technique for electro-optic crystals. Using the interaction of two noncolinear laser beams for the generation of second harmonic light, the method yields a three-dimensional spatial resolution. First experimental results of INCFD measurements on composition gradients in lithium niobate and on the spatial orientations of 90o-domain walls in potassium niobate are shown.

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