Controlled reduction of Fe-doped KNbO3 by proton-irradiation

We demonstrate the controlled and reproducible adjustment of the photorefractive properties such as gain and response time in Fe-doped KNbO3 by using MeV proton-irradiation. The reduction is caused by a change in the reduction ratio R = [Fe2+]/[Fe3+] in the irradiated region due to the production of oxygen vacancies during proton-irradiation. We demonstrate that the reduction ratio R can be precisely adjusted over a very large range from R = 0.01 up to R = 40, unmatched by former reduction methods, by simply choosing the appropriate irradiation dose. By applying this technique, we were able to alter the photorefractive gain Gamma from 2.5 cm(-1) up to 34 cm(-1), the absorption constant alpha from 0.06 cm(-1) up to 2.1 cm(-1) and the grating buildup time tau from 1.6 ms down to 34 mu s at an intensity I = 200 W/cm(2) (corresponding to a power of 2.5 mW in the interaction region) in a 1000 ppm Fe-doped KNbO3 crystal, for example. We also present a refined theoretical model for strongly reduced KNbO3, based on earlier work [J. Opt. Sec. Am. B 8 (1991) 2196], together with a set of the basic material parameters for these crystals, which completely explains all the measurements. (C) 1998 Elsevier Science B.V. All rights reserved.