Fixing of photorefractive gratings in KLTN by a 4 K cooldown to the phase transition

Summary form only given. In crystals of potassium lithium tantalate niobate (KLTN), we observed that as the crystals are cooled to the phase transition there is an increase in the conductivity, which disappears immediately below the transition. This fact was the basis to the following holographic fixing process: First, a photorefractive (PR) space charge hologram is written in the crystal in the region where the conductivity is low, at 4 K above T/sub c/. The crystal is then cooled to the phase transition and then warmed up to its initial temperature. Finally an erasing beam is incident on the crystal until the space charge grating is erased. It was predicted that because the ionic species become mobile at the phase transition, a compensation of the PR space charge will occur. The ions will be frozen when the crystal is heated to its original temperature. Thus, after the PR space charge is erased, the frozen space charge will be exposed and the hologram it represents will remain fixed. This process was tested in a KLTN crystal with T/sub c/=170 K, doped with copper and vanadium.