Abstract

Holographic gratings are recorded at room temperature in photorefractive iron-doped congruently-melting lithium-niobate crystals (LiNbO₃). Afterwards the diffraction efficiency of these gratings is measured from time to time to monitor the dark decay. Samples with different iron contents are investigated, and the Fe²⁺/Fe³⁺concentration ratio is varied. Time constants x of the decay range from minutes to years, and the dark conductivities σ_d= εε₀/t are deduced, where ε = 28 is the dielectric constant. The major outcomes are: (1) There is a small, iron-independent background conductivity (τ = 1 year) which arises from mobile ions. (2) For Fe concentrations in excess of about 20×10¹⁸cm^-3(0.05 wt. % Fe₂O₃): The dark conductivity is proportional to the effective trap density $N_{\text{eff }}={\left(1/{\text{C}}_{\text{Fe}}2++1/{\text{C}}_{\text{Fe}}{\text{e}}^{3+}\right)}^{-1}$ (see Fig. 1) and the normalized dark conductivity σ_d/N_effraises exponentially with ${\left({\text{C}}_{\text{Fe}}\right)}^{1/3}$.

© 2000 IEEE