Optical absorption characteristics in thermally reduced Er:LiNbO ₃ crystals

Influence of thermal reduction on intrinsic (bipolarons), extrinsic (Er³⁺) defects and OH^- groups in Er:LiNbO₃ crystals, which were as-grown and VTE-treated (VTE: vapor transport equilibration) before being reduced, was studied by measuring the polarised or unpolarised optical absorption in visible and near infrared regions. A wide and strong band extending from the optical absorption edge up to the infrared region and peaking around 500 nm (∼2.5 eV), resulting from the absorption of reduction-induced bipolarons, is observed. Meanwhile, the thermal reduction also induces an additional, relatively much narrow absorption band around 370 nm in a crystal whether it is Er-doped or undoped and whether it is congruent or originally VTE-treated. Both the 500 nm and the 370 nm bands show similar polarisation dependence. The thermal reduction treatment hardly influences Er³⁺ spectroscopic properties such as absorption amplitude, linewidth, peaking position and polarisation dependence. The original VTE effects on the spectroscopic properties of Er:LiNbO₃ crystal are essentially retained still. The thermal reduction has a similar effect on the OH^- absorption to a strong VTE treatment: the removal of the OH ^- groups contained in the crystal.