Bond-energy study of photorefractive properties of doped lithium niobate crystals

A bond-energy method is proposed to study the photorefractive behaviors of doped lithium niobate (LN) crystals on the basis of the bond valence model. The dopant occupancy in the LN matrix can be determined by comparing the deviation of its bond energy in different lattice locations at both Li+ and Nb5+ sites. Furthermore, the optical damage resistant (ODR) ions can be easily distinguished from those dopants by the increased bond energy when they are introduced into the Nb5+ sites, which gives us a bond-energy criterion when searching for new ODR ions. The suppression of the photorefractive effect of doped ODR ions more than their threshold concentration can be attributed to the increase of photoconductivity; it has been shown that such a phenomenon results from the decreased concentration and mobility of the charge carriers with increasing bond energy. According to our bond-energy viewpoint, the threshold concentration of ODR ions in both congruent and near-stoichiometric LN crystals can be determined quantitatively via the assumption that all doped LN crystals should endure a same variation of bond energy.