Influence of Li-rich vapor transport equilibration on surface refractive index and Mg concentration of MgO (5 mol%)-doped LiNbO_3 crystal

摘要

The Li-rich vapor transport equilibration (VTE) effect on surface refractive index of MgO (5 mol%)-doped LiNbO.3 crystal is studied at the wavelengths of 473, 632.8 and 1536 nm. The results show that in the early stage of VTE the ordinary index n_0 decreases substantially with the VTE duration t, then recovers slightly, and tends to a constant for t > 80 h. The extraordinary index n_e shows a similar VTE effect but reveals a relatively weak dependence on the VTE duration. Secondary ion mass spectrometry analysis shows that the VTE induces Mg diffusion within the crystal and hence an inhomogeneous Mg depth profile, depending on the VTE duration. As the VTE is prolonged the Mg homo geneity is improved. The surface Mg concentration, deter mined from the measured surface n_0 and energy dispersive X-ray analysis, shows a substantial VTE duration depen dence, which is responsible for the substantial VTE effect on surface index. Neutron activation analysis shows that the VTE hardly induces the MgO loss from the crystal. It ap pears that the Mg ions counter diffuse to the crystal surface in the early stage of VTE and then diffuse slowly back into the bulk of the crystal toward equilibrium as Li concentra tion tends to equilibrium. Gravimetric measurement shows that the Li_2O content in the crystal increases with the VTE duration in the early stage of VTE and saturates for t > 60 h, and the Li_2O content increase in the saturation regime is about 2.25 mol%.