Locally er-doped near-stoichiometric LiNbO₃ Crystals prepared by standard er diffusion and post-VTE treatment

The feasibility of preparing locally Er-doped near-stoichiometric (NS) LiNbO₃ crystals for integrated optics applications is demonstrated by a two-step process with standard diffusion (1130°C/154 h) of Er metal followed by vapor transport equilibration (VTE) treatment under three different conditions of 1135°C/22 h, 1115°C/50 h, and 1125°C/60 h. Detailed studies on the crystalline phase, Li composition, diffused surface roughness, and emission characteristics of Er³⁺ ions indicate that there is an upper limit on the initial Er metal film thickness: ∼20 nm for an X-cut crystal and ∼30 nm for a Z-cut crystal. When the initial Er film thickness is below this limit, the post-VTE does not induce formation of ErNbO₄ precipitate and the diffused surface retains high quality with a root mean square roughness <3 nm. Depending on the VTE condition adopted, the VTE results in the increase of (Li+Er)/Nb ratio in the diffused layer from congruent point (94.5%) to 97.4%-99.4%. Secondary ion mass spectrometry study shows that the post-VTE does not affect the Gauss nature of the Er profile, but leads to the increase of diffusion depth by as much as 1.6 μm. In comparison with the standard Er diffusion, the post-VTE results in the decrease of Er diffusivity by three to nine times. The higher the VTE temperature is, the lower the Er diffusivity is. In addition, the post-VTE also results in definite reduction of OH^- content in crystal, slight lengthening of lifetime and slight narrowing of linewidth of Er³⁺ emission at 1530 nm.