ORIGIN OF THERMAL FIXING IN PHOTOREFRACTIVE LITHIUM-NIOBATE CRYSTALS

Macroscopic light patterns are thermally fixed in photorefractive iron - or copper-doped LiNbO3 crystals with different hydrogen concentratio ns. Spatially resolved absorption measurements reveal unambiguously th at migrating hydrogen ions compensate for the space-charge fields at e nhanced temperatures if the hydrogen concentration is large enough. Ho wever, in crystals with small hydrogen concentrations thermal fixing i s also possible and another compensation mechanism appears. Partial de polarization of the crystals does not contribute to the charge compens ation processes. The refractive-index pattern is developed by homogene ous illumination and measured by an interferometric technique. The dev elopment process is described considering the spatially modulated conc entrations of filled and empty electron traps, which cause spatially m odulated currents, even for homogeneous illumination.