Optical spectroscopy of Cr³⁺ in LiNbO₃: chromium concentration and crystal composition effects

Lithium niobate (LiNbO₃, LN) is a widespread optical material used in electro-optical, photorefractive and laser applications. Recently, new techniques of crystal growth allowed the achievement of crystals with different compositions. The possibility of preparing stoichiometric samples renewed the interest on studying the basic properties of this material. This study is devoted to the optical characterisation of LiNbO₃ crystals doped Cr³⁺ as well as the determination and the analysis of the influence of composition and chromium concentration on the optical properties. We analysed in a nearly stoichiometric LN:Cr crystal the different Cr³⁺ centres by specifying their schemes of octahedral crystal field and the Cr³⁺ optical transitions. Our results lead to distinguish three Cr³⁺ centres: a main Cr centre corresponding to the low crystal field scheme and two Cr centres corresponding to the high crystal field scheme. The effect of composition and chromium concentration are determined on the LN:Cr crystals optical features (intensity, width, position...) and are interpreted owing to a model taking into account the evolution of the concentration of intrinsic defects in function of Cr doping. A survey of the results gained by optical spectroscopy and EPR, on the same set of LN:Cr samples, is also achieved. (author)