Abstract

We report for the first time on proton-irradiation as a method to precisely adjust the reduction ratio, i.e. the ratio R = [Fe²⁺]/[Fe³⁺], of Fe:KNbO₃ over three orders of magnitude. By applying this technique, we were able to alter the photorefractive gain Γ from 2.5 cm⁻¹ up to 34 cm⁻¹, the absorption constant α from 0.06 cm⁻¹ up to 2.1 cm⁻¹ and the grating buildup time τ from 1.6 ms down to 34 µs at an intensity I = 200 W/cm² in a 1000 ppm Fe-doped KNbO₃ crystal, for example [1]. The major advantage of ion-irradiation is that it is a very homogeneous and a very reproducible process, which can be applied in subsequent steps on the same sample, to increase the reduction ratio continuously from R = 0.01 up to R = 40, by simply implanting protons of the appropriate dose. Figure 1 shows the change of the photorefractive gain Γ and the absorption constant α as a function of the increasing reduction ratio R for a 10'000 ppm Fe-doped KNbO₃ crystal. Further experimental results and a refined theoretical model for strongly reduced KNbO₃, together with a set of the basic material parameters for these crystals, which completely explains all the measurements, will be presented at the Conference. Finally it is worth to mention, that this method is quite universally applicable to many other photorefractive crystals, such as BaTiO₃ and SBN, and dopants, such as Rh and Mn.

© 1998 IEEE