Charge transport and the photorefractive effect in doped and undoped lithium tantalate

Holographic techniques were used to study the photorefractive effect in lithium tantalate. Illumination of the crystal with intersecting laser beams (from an argon ion laser) and simultaneous measurements of the photocurrent and the sensitivity to volume phase hologram formation gave detailed information on the charge transport mechanism involved in the photorefractive effect. The dominant charge transport process was found to be directly linked to a bulk spontaneous photocurrent within the crystal. A reverse bias electric field was required to produce zero photocurrent. For long exposures, saturation of the change in refractive index produced by the photorefractive effect was found to be caused by the limiting effects of the associated space charge field. Effects on the holographic sensitivity and the photocurrent caused by doping with iron group impurities and oxidation-reduced treatment of LiTaO3 crystals were studied. Optical absorption measurements were used to help identify the charge states of the impurities.