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.