Regensburg 2007 – scientific programme

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DF: Fachverband Dielektrische Festkörper

DF 9: Electric, Electromechanical and Optical Properties I

DF 9.8: Talk

Thursday, March 29, 2007, 12:40–13:00, H48

Reduction of optical damage in lithium niobate crystals by thermo-electric oxidization* — •Matthias Falk, Theo Woike, and Karsten Buse — Institute of Physics, University of Bonn, Wegelerstr. 8, 53115 Bonn, Germany

Lithium niobate crystals are a promising material for nonlinear-optical applications, e.g., second harmonic generation. But the conversion efficiency is limited because of unwanted light-induced refractive index changes, the so-called "optical damage". This effect is mainly caused by iron impurities that occur in the valence states Fe²⁺ and Fe³⁺. Electrons from Fe²⁺ are excited by light, redistributed and finally trapped, preferentially in the darker areas of the crystal. Space-charge fields build up and change the refractive index via the electro-optic effect. A method to suppress such index changes, the thermo-electic oxidization treatment, is presented: The crystals are annealed in the presence of an externally applied electrical field. By this the iron impurities are nearly completely oxidized to the Fe³⁺ state, thus there are less photoexcitable electrons in the crystal. Consequently, the optical damage is suppressed by one order of magnitude. Furthermore, it is shown, that for charge compensation of the removed electrons, lithium and hydrogen ions leave the crystal during the treatment.

*Financial support from the Deutsche Forschungsgemeinschaft (FOR 557) and from the Deutsche Telekom AG is gratefully acknowledged.