Appl. Phys. B 68, 795–799 (1999) / DOI 10.1007/s003409901469 Applied Physics B Lasers and Optics  Springer-Verlag 1999 Influence of Zn doping on electrooptical properties and structure parameters of lithium niobate crystals F. Abdi 1 , M. Aillerie 1 , M. Fontana 1 , P. Bourson 1 , T. Volk 2 , B. Maximov 2 , S. Sulyanov 2 , N. Rubinina 3 , M. Wöhlecke 4 1 Laboratoire Mat´ eriaux Optiques à Propri´ ett´ es Sp´ ecifiques, C.L.O.E.S, University of Metz and Sup´ elec, 57070 Metz Cedex, France (E-mail: fontana@ese-metz.fr) 2 Institute of Crystallography, Russian Academy of Sciences, Moscow 117333, Russia 3 Moscow State University 4 Fachbereich Physik, Universität Osnabrück, D 49069 Osnabrück, Germany (E-mail: manfred.woehlecke@physik.uni-osnabrueck.de) Received: 16 Nowember 1998/Revised version: 8 December 1998/Published online: 24 March 1999 Abstract. We report the dependence of the unit cell pa- rameters and the EO coefficients on Zn doping in optical- damage-resistant LiNbO 3 :Zn crystals. Both properties de- pend in a non-monotonic manner on the Zn content. This is accounted for by different types of Zn ion incorporation into the lattice depending on the Zn concentration in the melt. Ex- trema observed in the concentration dependence of the EO coefficients at about 2–3 and 6.4 mol.% Zn correlate with an unusual concentration dependence of the unit cell param- eters a and c. The low-concentration anomalies may be ac- counted for by a decrease of the Li vacancy concentration due to the Zn incorporation into Li sites. Anomalies at high concentrations are obviously due to a partial incorporation of Zn ions on Nb sites, which is reflected in the structure data. Anomalies in the concentration dependence of other op- tical properties at about 6–7 mol.% Zn reported recently are obviously related to a change in the localization of the Zn ions. The combination of high EO coefficients with a reduced optical damage for these concentrations make these crystals attractive for applications as Q-switching or electrooptical modulation. PACS: 78.20.Jq; 61.10.-i; 77.84.Dy Although LiNbO 3 crystals are of great interest for appli- cations in optics due to their large electrooptical (EO) and nonlinear optical coefficients [1, 2], their use in modulators, Q-switches or frequency doublers is hampered by a rela- tively low laser-damage threshold (0.3 GW cm −2 ) [3] and an optical-damage effect, which occurs already at some mW cm −2 [2] (mentioned data belong to undoped congruent crystals). In undoped LiNbO 3 many properties, in particular light-induced charge transport and thus optical damage, de- pend on Li deficiency related intrinsic defects such as antisite Nb Li [4], Li and Nb vacancies. The charge transport can be modified by controlling the concentration of these intrinsic defects. First, the growth of crystals with stoichiometric com- position noticeably affects the optical damage [5, 6]. Second, Dedicated to E. Krätzig on the occasion of his 60th birthday doping with optical-damage-resistant impurities such as Mg, Zn, In or Sc leads to a strong decrease of the optical damage, due to an influence of these ions on the intrinsic defect struc- ture (for references see [7, 8]). Simultaneously some optical properties, particularly refractive indices and phase-matching conditions for various configurations of second-harmonic generation (SHG) related to them [7–9] are influenced by these impurities. This is also interpreted in terms of their influence on intrinsic defects. Although the common sce- nario of the incorporation of these impurities into the LiNbO 3 lattice was sketched [7, 8], experimental studies of optical properties with regard to the microscopic structure are miss- ing. The aim of the present work was to study the influence of Zn doping on electrooptical coefficients and to correlate these optical data with structure ones such as the unit cell parame- ters (a, c and the unit cell volume V ). In order to check the applicability of damage-resistant Zn-doped LiNbO 3 crystals in EO modulation we measured the EO coefficients r c and r 22 as a function of Zn concentration. 1 Experimental We have performed measurements on LiNbO 3 samples grown with the Czochralski technique from a congruent melt (48.5 mol.% Li 2 O and 51.5 mol.% Nb 2 O 5 ). We emphasize that undoped and doped samples were grown from the same charge, thus having the same amount of unintended impu- rity trace. Zn was added to the melt in oxidic form. The Zn concentrations given below correspond to the Zn content in the crystal. This was derived from the melt concentra- tion using the empirical equation proposed by Schlarb et al. for LiNbO 3 :Zn [10]. The optical quality of LiNbO 3 crys- tals doped with Zn up to 7.6 mol.% is comparable to that of nominally pure congruent LiNbO 3 . The crystals do not have growth striations, the nonuniformity of the birefringence along the c axis does not exceed 10 −5 cm −1 , whereas the scattering of the birefringence is about 2 × 10 −6 . As Zn dop- ing slightly shifts the UV band edge to shorter wavelengths [8], the optical transparency of LiNbO 3 :Zn is the same as in undoped crystals.