Characterization of iron-doped lithium niobate for holographic storage applications*

A detailed study of eight systematically chosen Fe: LiNb03 crystals is presented. Correlation between the photorefractive sensitivity and various chemical properties of Fe:LiNb03 is investigated in order to ascertain optimum performance of the crystals in holographic storage and display applications. Concentrations of Fe 2+ and Fe 3+ ions have been determined from optical and EPR spectra, while impurities have been detected from x-ray-emission and infrared spectra. Particular emphasis is placed on investigating the dependence on Fe and Fe ion concentrations of the photorefractive sensitivity. The photorefractive sensitivity is shown to depend primarily on the concentration of Fe 2+ ions in Fe: LiNb03 . This fact seems to suggest that Fe ions are the impurity centers responsible for the photorefractive effect in Fe: LiNb03. Spectral dependence of the photorefractive sensitivity and its modification due to oxygen annealing are also reported. Our results indicate that an unannealed Fe:LiNb03 crystal containing 0.05 mo1% Fe with 20-25% of the ions in the Fe 2+ state and the remainder in the Fe3+ state possesses the most favorable photorefractive sensitivity.