Light-induced diffraction in thermally reduced congruent and near-stoichiometric Er:LiNbO₃ crystals

A polarization-isotropic two-colour light-induced diffraction (LID) phenomenon from thermally reduced congruent and near-stoichiometric Er:LiNbO3 crystals was observed when these crystals were exposed to superposed 632.8nm probe and 488nm pump beams. The LID is characterized by expansion of the probe beam spot and the appearance of alternate bright and dark diffraction rings at higher pump light intensities. This phenomenon is absent not only in all other Er-doped crystals which either were as grown or had experienced other post-growth treatments such as vapour transport equilibration (VTE), oxidation and strong annealing but also in a pure VTE-treated crystal even though the crystal was subjected to the same reduction procedure. The LID characteristics of five reduced crystals, which have different Li-to-Nb ratios and different doping levels of Er 3+ , were studied in detail by changing the light intensity, polarization state and wave vector direction of both the pump and the probe beams. The origin of the LID was investigated from the viewpoints of the red fluorescence of Er 3+ excited by a 488nm pump beam, light-induced scattering and light-induced thermal nonlinearity. The results have shown that the thermal nonlinearity mechanism is preferred as it can reasonably explain almost all the observed experimental phenomena.