Electro‐Optic Property of Mg 2+ /Er 3+ ‐Codoped LiNbO 3 Crystal: Mg 2+ Concentration Threshold Effect

A series of Mg 2+ /Er 3+ ‐codoped congruent LiNbO 3 crystals were grown by Czochralski method from the growth melts containing 0.5 mol% Er 2 O 3 while varied MgO content from 0.0 to 7.0 mol%. The unclamped electro‐optic coefficients γ 13 and γ 33 of these crystals were measured by Mach–Zehnder interferometry. Two different voltage‐applying schemes were adopted: one is the DC voltage applied to the crystal via Al films coated onto crystal surfaces and another is via a pair of external Cu slab electrodes. The coefficients measured by the two schemes show similar strong dependence on Mg 2+ concentration. The dependence is non‐monotonous, dramatic, and unusual, and reveals the features of two Mg 2+ concentration thresholds of optical damage: one in the Mg 2+ concentration range of 1.2–2.0 mol% (in crystal) and another in 4.5–5.0 mol%. Around the threshold the electro‐optic coefficient decreases abruptly at first and then recovers quickly, and the coefficient drops by >20% (12%) at the first (second) threshold, which exceeds the error 3% considerably. The dramatic behavior is qualitatively explained on the basis of the EO coefficient model of LiNbO 3 and the defect structure model for Mg 2+ ‐doped LiNbO 3 .