High beam-coupling gain and deep- and shallow-trap effects in cobalt-doped barium titanate, BaTiO_3:Co

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Abstract
A series of cobalt-doped barium titanate (BaTiO3) crystals were grown; these crystals have the highest beam-coupling gain reported to date, 7.9 cm−1, measured by using the smallest electro-optic coefficient of BaTiO3. The intensity dependence of the absorption and the gain indicate that there are at least two active photorefractive species. These samples show light-induced absorption and negative absorptive coupling, which is indicative of crystals with deep and shallow traps. However, doping with cobalt diminishes the intensity dependence of the electro-optic gain and increases the electro-optic and absorptive coupling, which indicates that cobalt is a deep-level photorefractive dopant. The higher gain is attributed to an increase of the total effective trap density and to an increase in the deep- and shallow-trap intensity-dependent factor η(I), with higher cobalt doping.