Influence of Li-rich vapor transport equilibration on surface refractive index and Mg concentration of MgO (5 mol%)-doped LiNbO₃ crystal

The Li-rich vapor transport equilibration (VTE) effect on surface refractive index of MgO (5 mol%)-doped LiNbO₃ 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 _o 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 homogeneity is improved. The surface Mg concentration, determined from the measured surface n _o and energy dispersive X-ray analysis, shows a substantial VTE duration dependence, 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 appears 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 concentration tends to equilibrium. Gravimetric measurement shows that the Li₂O content in the crystal increases with the VTE duration in the early stage of VTE and saturates for t>60 h, and the Li₂O content increase in the saturation regime is about 2.25 mol%.