Electronic structure and optical index damage of iron-doped lithium niobate

Abstract

The properties of Fe²⁺, Fe³⁺, and other 3dⁿ ionic impurities in the LiNbO₃ structure are investigated with a view to elucidating the mechanism of laser‐induced reversible refractive index damage. Polarized optical absorption spectra of Fe²⁺ in LiNbO₃ and LiTaO₃ are reported and interpreted. The 2.66 eV band responsible for laser damage in Fe‐doped LiNbO₃ is identified as Fe²⁺ → Nb⁵⁺ intervalence transfer. The Fe d ε level lies in the 3.7 eV band gap at about 0.6 eV below the conduction band, and the activation energy for thermal Fe²⁺ → Nb⁵⁺ electron transfer is estimated to be about 0.9 eV, in reasonable agreement with the value of 1.3 ± 0.2 eV observed for thermal bleaching of Fe²⁺ centers in x‐irradiated LiNbO₃. Particular advantages of intervalence transfer as a mechanism for initiating optical index damage are noted. The observed electric field gradients in pure LiNbO₃ and LiTaO₃ are related to the B₂⁰ ligand‐field parameter of a 3dⁿ (n ≠ 5) impurity ion. Comparison with experimental estimates of B₂⁰ may provide an indication of the site of the impurity ion, as in the case of Cr³⁺ which appears to favor the Nb/Ta site. Magnetic susceptibility measurements in the range 4.2–300°K are reported for oxidized and reduced Fe‐doped LiNbO₃. It is demonstrated that susceptibility studies may be used to estimate the concentrations of Fe²⁺ and Fe³⁺ in LiNbO₃, and that in reduced LiNbO₃ the Nb d ε electrons show Pauli paramagnetism characteristic of metallic behavior. It is argued that the system will be nonmetallic at the lower conduction electron concentrations encountered during optical damaging and bleaching of unreduced crystals, and that low electron mobility may be a significant factor in the thermal processes observed in optically damaged LiNbO₃.