Characterization of iron substitution process in Fe:LiNbO 3 single crystal fibers by polaron measurements P. Bourson a, * , M. Aillerie a , M. Cochez b , M. Ferriol b , Y. Zhang a , L. Guilbert a a Laboratoire Mat eriaux Optiques Photonique et Syst  eme (MOPS), CNRS FRE 2304, Universit e de Metz––Sup elec, 2 rue E. Belin, F-57070 Metz, France b Laboratoire de Chimie et Applications, EA no. 3471, Universit e de Metz, rue Victor Demange, F-57500 Saint Avold, France Received 8 December 2002; accepted 24 February 2003 Abstract The growth of iron-doped single-crystal fibers of lithium niobate was performed by the Laser Heated Pedestal Growth Technique for different Fe 2 O 3 contents in the feed rods. We used the polaron luminescence to explain the processes of iron substitution in iron-doped single-crystal fibers of lithium niobate. The interpretation of the polaron behavior as a function of iron concentration confirms several predicted effects as the decrease of the global amount of vacancies and the predominant role of the niobium in its polaronic or bipolaronic forms in the LN lattice. Ó 2003 Elsevier B.V. All rights reserved. PACS: 81.10.Fq; 78.30.Hv; 42.70.Mp Keywords: Crystal fiber; Laser heated pedestal growth; Polaron; Luminescence; Lithium niobate 1. Introduction In a previous work [1], we investigated the in- fluence of the dopant concentration on the OH  absorption band in Fe-doped LiNbO 3 (LN) single crystal fibers. The OH  absorption band exhibits a decreasing intensity as the iron concentration increases even if in the range investigated, iron appears not sufficient to significantly perturb the ionic environment of protons in the structure of the single crystal fibers of congruent lithium niobate. We have interpreted this phenomenon through a high temperature reaction scheme occurring at the melt atmosphere interface and involving the partial dissociation of LiNbO 3 and oxidation/reduction processes leading to the decrease of the OH  and the increasing presence of Fe 2þ ions in the crystal lattice as a function of Fe 3þ concentration. In this paper, we analyse our last results on the characterization by polaron measurements of the iron substitution process in Fe-doped LiNbO 3 single crystal fibers. We performed our measure- ments on the same crystal fiber series as in [1] to eliminate all undesired growth influences in the analysis of the peculiar role of iron ions in the behavior of LN crystals. 2. Crystal growth The crystal fibers investigated during this work were grown in the Laser Heated Pedestal Growth * Corresponding author. E-mail address: bourson@ese-metz.fr (P. Bourson). 0925-3467/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-3467(03)00113-7 Optical Materials 24 (2003) 111–116 www.elsevier.com/locate/optmat