Photorefractive damage and photoconductivity in as-grown, reduced and oxidized iron doped lithium niobate (LiNbO3, LN) crystals, in two series of samples with 0.03 and 0.05 wt% iron concentrations, respectively have been studied with the closed-aperture Z-scan technique. The photoconductivity in the various crystals was directly deduced from the dynamic distortion of the beam spot of a red laser light passing throughout the sample, placed at the focal point, followed by a pin-hole and recording in the far field zone behind the crystal. The most important experimental result points out a non-linear increase of the photoconductivity with the laser power density and with the iron concentration in the crystal. To explain these experimental findings, we use the theoretical model taking into account the photoionization of iron ions and intrinsic defects, namely polarons, bipolarons and hole polarons.
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