On the effect of structural and symmetrical features of potassium titanyl phosphate crystals with different contents of
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ICAL PROPERTIES OF CRYSTALS
On the Effect of Structural and Symmetrical Features of Potassium Titanyl Phosphate Crystals with Different Contents of Niobium, Antimony, and Zirconium on the Second-Harmonic Intensity V. A. Ivanov, V. A. Burdov, M. O. Marychev, D. N. Titaev, M. A. Faddeev, and E. V. Chuprunov Nizhni Novgorod State University, pr. Gagarina 23, Nizhni Novgorod, 603950 Russia e-mail: [email protected] Received May 2, 2007
Abstract—A model is proposed, which shows that, at small deviations from the centrosymmetric state of the atomic structure, the quadratic nonlinear susceptibility of a crystal monotonically decreases with approach of the degree of central symmetry η 1 [ϕ(r)] of the electric potential function of the crystal structure to unity. The quadratic nonlinear susceptibility of K1 – xTi1 – xNbxOPO4 (x = 0, 0.02, 0.04, 0.11), K1 – xTi1 – xSbxOPO4 (x = 0.01, 0.07, 0.17), and KTi1 – xZrxOPO4 (x = 0.03, 0.04) crystals has been measured. The degree of central symmetry η 1 [ϕ(r)] has been calculated for the structures of K1 – xTi1 – xNbxOPO4 (x = 0, 0.04, 0.11), K1 – xTi1 – xSbxOPO4 (x = 0.01, 0.07, 0.17), and KTi1 – xZrxOPO4 (x = 0.03, 0.04) crystals. It is shown that, at η 1 [ϕ(r)] > 0.7, the relationship between the quadratic nonlinear susceptibility of the investigated crystals and the degree of their central symmetry η 1 [ϕ(r)] is in qualitative agreement with the proposed model. PACS numbers: 61.50.Ah, 42.65-k DOI: 10.1134/S1063774508040202
INTRODUCTION Calculation of the output of the second harmonic of laser radiation from crystals on the basis of their atomic structure is a difficult problem. It is urgent to search for some features of the crystal structure that can affect the nonlinear susceptibility and make it possible to predict prospects of using particular crystals as nonlinear. It is known that the second harmonic generation (SHG) is related to the acentricity of the atomic structure as a whole. In some studies [1–5], the effect of the degree of acentricity of individual structural fragments on the SHG was investigated. The models considered in [1] are based on the assumption that the SHG intensity in crystal structures that are similar to potassium titanyl phosphate (KTP) is mainly affected by the polarization of some part of the electron density of the crystal that is responsible for the chemical bond between the atoms of the oxygen octahedron and metal (Ti or a substituent atom), entering the composition of a KTP-like crystal. In some models, reported in [1–5], it is suggested that, the larger the difference between the metal–oxygen bond lengths, the stronger the SHG. Stucky et al., who proposed other models [1], showed that the SHG intensity is affected by the degree of polarization of chemical bonds. However, it was noted in [1] that these mod-
els can be used only for crystals with a simple structure. In addition, it was noted that, although the metal–oxygen bond lengths affect the SHG intensity, investigation should not be restricted only to some part of the structure, neglectin
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