Origin of thermal fixing in photorefractive lithium niobate crystals

Macroscopic light patterns are thermally fixed in photorefractive iron- or copper-doped ${\mathrm{LiNbO}}_3$ crystals with different hydrogen concentrations. Spatially resolved absorption measurements reveal unambiguously that migrating hydrogen ions compensate for the space-charge fields at enhanced temperatures if the hydrogen concentration is large enough. However, in crystals with small hydrogen concentrations thermal fixing is also possible and another compensation mechanism appears. Partial depolarization of the crystals does not contribute to the charge compensation processes. The refractive-index pattern is developed by homogeneous illumination and measured by an interferometric technique. The development process is described considering the spatially modulated concentrations of filled and empty electron traps, which cause spatially modulated currents, even for homogeneous illumination.