Diffusivity of Er3+ into LiNbO3 as a function of stoichiometry

Diffusivity of Er3+ into LiNbO3 crystal has been studied as a function of Li2 O content and crystal orientation. The Li2 O content in single-crystal LiNbO3 plates was adjusted by Li-rich vapor transport equilibration (VTE) technique. Five X -cut and five Z -cut congruent substrates were VTE treated at first at 1100 °C for different durations ranging from 6 to 74 h, and then were subjected to a special post-VTE anneal procedure at the same temperature for 60 h to obtain homogeneous Li2 O content. The Li2 O content was determined from the measured fundamental optical absorption edge. Its homogeneity is verified by measuring the optical absorption edge as a function of the crystal thickness. After the composition characterization, the substrates were coated with 45 nm thick Er film and annealed at 1130 °C for 40 h in argon atmosphere. The Er3+ profile was analyzed by secondary ion mass spectrometry. The results show that the Er3+ diffusivity decreases monoexponentially with the increase in the Li2 O content in both cases of X - and Z -cut. As the Li2 O content is increased from the congruent point (48.4 mol %) to the near-stoichiometric composition (∼49.9 mol %), the diffusivity decreases from 0.107 (0.072) to 0.017 (0.018) μ m2 /h for the Z (X) -cut substrate. The diffusion shows considerable anisotropy for the lower Li2 O content and the diffusivity in a Z -cut substrate is larger. The anisotropy weakens gradually with the increase in the Li2 O content, and the diffusivities in the X - and Z -cut substrates ultimately tend to a same value (∼0.02 μ m2 /h) at the Li-rich phase boundary. After the Er diffusion procedure, endface coupling experiment was carried out to check if a planar waveguide layer was formed near the Er-diffused surface due to the Li outdiffusion. The results show that the Li outdiffusion did not accompany the Er diffusion procedure.