Investigation of the oxidation state of Fe in stoichiometric Fe:LiNbO3 for digital holographic recording

We investigated the oxidation state of Fe in stoichiometric LiNbO3 (SLN) in comparison with congruent LiNbO3 (CLN) from the viewpoint of digital holographic data storage. The dual oxidation stales (Fe2+ acid Fe3+) were controlled by changing Fe-doping levels and thermal annealing conditions after growth. We found that the photorefractive (PR) sensitivity of SLN under extraordinary polarization is more than 50% higher than that of CLN, which is more than 10 times higher than that of CLN under the generally used ordinary polarization. This high performance in PR sensitivity can be explained by the increased Pockels constant and photoconductivity. We could demonstrate high performance digital recording with fast writing speed and low bit-error rate using stoichiometric LiNbO3 under extraordinary polarization. We also observed that SLN showed a smaller erasure time constant than that of CLN because of increased photoconductivity. Consequently, the "M/# value" was almost the same as that of CLN. The M/# value is the metric for multiplexing and is proportional to the product of PR sensitivity and erasure time constant.