Light-induced charge-transport properties of photorefractive barium-calcium-titanate crystals doped with iron

Nominally pure and iron doped, as-grown, and thermally annealed photorefrac tive barium-calcium-titanate crystals of the congruently melting compositio n Ba0.23Ca0.77TiO3 (BCT) are investigated by holographic and conventional e lectrical techniques. Refractive-index changes, two-beam-coupling gains, ph otoconductivities, dark conductivities, and bulk-photovoltaic current densi ties are measured. As-grown and oxidized crystals are hole conductive and a t usual illumination conditions (light wavelength 514.5 nm, light intensity between 0.1 and 1 W/cm(2)) all measured properties are excellently describ ed by an one-center charge-transport model. The effective electrooptic coef ficient r(333) is only about 30 pm/V and thus much smaller than the value o btained from interferometric measurements. Two-beam-coupling gains as high as 7 cm(-1) are achieved. Doping with iron increases considerably the effec tive trap density, and bulk-photovoltaic fields of the order of some kilovo lts per centimeter are observed in iron-doped crystals. Typical response ti mes of iron-doped, as-grown, or oxidized crystals are about 0.5 s at 1 W/cm (2). Reduction yields electron-conductive BCT. The dark storage time increa ses from 6 min in the as-grown state to 3 h upon a slight reduction treatme nt, but decreases for strongly reduced samples. The investigation reveals t hat BCT will become a very promising alternative to barium-titanate crystal s (BaTiO3) for many applications. (C) 2000 American Institute of Physics. [ S0021-8979(00)04910-0].