ORIGIN AND CHARACTERISTICS OF INTERNAL FIELDS IN LINBO3 CRYSTALS

We report the presence of an in-built internal field in Z-cut LiNbO3 c rystals of 3.35 KV/mm in the direction of the polarization in a virgin crystal. Upon polarization reversal under an electric field, the inte rnal held tends to realign along the new polarization direction. At ro om temperature, this realignment process was incomplete even after man y days, while on annealing at 250 degrees C for 2 min followed by cool ing down, similar to 80% recovery was observed (from + 3.35 KV/mm to - 2.14 KV/mm). A change in the infrared absorption spectrum of OH- ions in LiNbO3 crystal was also observed upon polarization reversal at roo m temperature. Upon annealing above 200 degrees C for a few minutes fo llowed by cooling to room temperature, the shape of the spectrum parti ally recovers to that of the virgin crystal at room temperature, indic ating a strong correlation between OH- ions and internal field in LiNb O3 crystals. We also show that 180 degrees domains in a polarization r eversed LiNbO3 crystal can be observed by light microscope, suggesting structural changes in the regions close to domain walls.