Journal of Luminescence 102–103 (2003) 248–252 Comparative study of the singularity in the optical properties of congruent doped LiNbO 3 crystals F. Jaque a , T.P.J. Han a, *, G. Lifante b a Department of Physics, University of Strathclyde, John Anderson Building, 107 Rottenrow, Glasgow G4 ONG, Scotland, UK b Departamento de F! ısica de Materiales, Universidad Aut ! onoma de Madrid, Madrid 28049, Spain Abstract A comparison of the optical properties such as optical absorption band edge, ordinary and extraordinary refractive indices, powder second harmonic generation and the non-linear coefficient, d 33 , of congruent LiNbO 3 crystals doped with di-, tri- and hexa-valent cations is reported. It has been found that the referred parameters show a clear concentration singularity depending on the valency of the cation. A charge compensation mechanism is proposed to explain the role of the cation. The experimental data of n o and n e are described by a generalised Sellmeier equation considering the different ions sites in the crystal. r 2002 Elsevier Science B.V. All rights reserved. Keywords: LiNbO 3 ; Optical properties; MgO; WO 3 1. Introduction LiNbO 3 is a versatile material with a wide range of application. In un-doped form, it is used for acoustic-optic, piezo-electric, linear, and non- linear optical applications. In doped crystals, in particular with rare earth ions, laser action has been archived in bulk samples and in wave-guide structures [1,2]. Unfortunately, the intense ‘optical damage’, manifestation of the photorefractive effect, suffered under high optical power excitation limits its application to low power devices. The addition of cations such as Mg 2+ , Zn 2+ , and Sc 3+ has demonstrated to reduce this photorefractive effect [3,4]. However, the incorporation of these ions affects its optical and electronic properties. In particular, it has been reported that the refractive indices (ordinary, n o and extraordinary, n e ) in congruent LiNbO 3 samples doped with divalent and trivalent cations showed a singularity for cation concentrations at B4.6 and B3mol%, respectively [5–7]. Using Cr 3+ ions as an optical and paramagnetic probe it has established that in LiNbO 3 co-doped with Mg 2+ or Zn 2+ ions a redistribution of the Cr 3+ ions location takes place for cation concentrations above 4.6%. This redis- tribution was explained assuming that below this critical concentration the cations are incorporated into the Li + sites (antisites, [Nb] Li and Li + vacancies, [V] Li ) in competition with the Cr 3+ ions that are also preferentially incorporated in Li + sites. One consequence of this is that the crystals are uniformly green in colour. However, for MgO or ZnO concentration above this 4.6 mol% threshold all the vacant Li + sites ([Nb] Li *Corresponding author. Fax: +44-141-5483267. E-mail address: t.han@strath.ac.uk (T.P.J. Han). 0022-2313/02/$-see front matter r 2002 Elsevier Science B.V. All rights reserved. PII:S0022-2313(02)00507-0