Influence of MgO codoping on Er concentration in congruent LiNbO ₃ crystal: Mg threshold concentration effect

Congruent Er:Mg:LiNbO₃ crystals were grown by the Czochralski method from the melts containing fixed 0.5 mol% Er₂O₃ while varied MgO content ranged from 0.0 to 8.0 mol%. The Mg and Er contents in the crystals were determined by neutron activation analysis. In the presence of Er codopant, the Mg threshold concentration with respect to optical damage is determined from the measured OH absorption spectra. The results show that the Er codopant has less effect on both the Mg threshold concentration and Mg segregation coefficient, which is within 1.13-1.23 as the Mg concentration is below the threshold while within 0.88-0.98 when above the threshold, consistent with the only MgO doping case. On the other hand, the practical Er concentration in the crystal is closely related to the Mg content and shows definite Mg threshold effect. Below the threshold, the Er concentration decreases linearly with the increased Mg concentration in the crystal; above the threshold, the decrease is more remarkable and follows another linear function. The Mg concentration effect on the Er segregation coefficient is discussed from the viewpoint of the Mg doping effect on the solubility of Er ions in the crystal.