Pyroelectrically induced photorefractive damage in magnesium-doped lithium niobate crystals

Beam distortion due to photorefraction limits the usability of lithium niobate (LiNbO3 ) crystals for frequency con- version applications. To prevent beam distortion in LiNbO3, 5 mol:% magnesium-doped LiNbO3 (MgO:LN) is usually used. However, we show that strong beam distortion of green laser light can occur within seconds in MgO:LN, starting at light intensity levels in the 100 mW/cm^2 regime, if the crystal is heated by several degrees Celsius during or before illumination. Beam distortion does not occur in undoped congruent LiNbO3 (CLN) under the same conditions. We show that the pyroelectric effect together with an elevated photoconductivity compared to that of CLN causes this beam distortion and that this effect also influences frequency conversion experiments in the infrared even if no external heating is applied.