A study of optical absorption, ESR spectra and photorefraction in LiNbO₃: Mg+Fe crystals

Abstract Transition metal iron has been chosen as a “marking impurity”, and two kinds of iron-doped LiNbO3 crystals have been grown. Their Li/Nb and contents of MgO in the growing melt are 0.945+0 mol% MgO and 0.945+6 mol% MgO, respectively. The amount of Fe2O3 in the crystals is close to eachother. The measurements of optical absorption edge, OH− absorption bands and Fe3+ ESR spectra in LiNbO3:Fe and LiNbO3:Mg+Fe were made. As experimental results showed, the significant difference between them revealed that the ionic environment of Fe3+ cation in the two kinds of iron-doped crystals is different. According to this, we suggested that the Fe3+ cation should substitute for Nb in LiNbO3Mg+Fe crystal rather than substitute for Li in LiNbO3:Fe crystal. Based on this arguement, we can explain qualitatively the experimental results of opticalabsorption and ESR measurements. The photorefraction in the iron-doped LiNbO3 crystals have been observed and estimated qualitatively by using a focused Ar Laser beam. As the experimental results show, the resistance to the optical-damage of congruent LiNbO3 with 6.0 mol.%MgO added to the growing melt can be greatly improved, even through photorefraction sensitive iron impurity was doped.