Green-induced infrared absorption ͑GRIIRA͒ was investigated by a photothermal technique for undoped and Mg-doped LiNbO 3 crystals that have different Li/Nb ratios. Threshold effect on GRIIRA was found, threshold MgO concentrations being the same for GRIIRA and photorefraction. We suggest that GRIIRA is associated with the formation of the small polaron that is located on Nb antisite defect. The remarkable decrease of GRIIRA in Mg:LiNbO 3 can then be attributed to the elimination of this intrinsic defect, Nb in Li, following the incorporation of Mg on Li sites. For nonlinear optical applications, LiNbO 3 doped with MgO at concentrations over threshold has a combined advantage of having almost no GRIIRA and photorefraction.

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