Zr⁴⁺/Ti⁴⁺ codiffusion characteristics in lithium niobate

Zr⁴⁺/Ti⁴⁺-codoped LiNbO₃ plates were prepared by local codiffusion of stacked ZrO₂ and Ti metal films coated onto Z-cut congruent LiNbO₃ substrates in wet O₂ at 1060°C. The metal and oxide films have different thicknesses and coating sequences. After diffusion, the Zr⁴⁺ doping effect on the refractive index of LiNbO₃ and the Li₂O out-diffusion issue were studied by the prism coupling technique. The codiffusion characteristics of Zr⁴⁺ and Ti⁴⁺ were studied by secondary ion mass spectrometry. The results show that the Zr⁴⁺ doping has little contribution to the refractive index of the crystal. Li₂O out-diffusion is not measurable. In the Zr⁴⁺-only diffusion case, the diffusivity of Zr⁴⁺ is four times smaller than that of Ti⁴⁺. In the Zr⁴⁺/Ti⁴⁺ codiffusion case, the Ti⁴⁺ codiffusion assists the Zr⁴⁺ diffusion. The Zr⁴⁺ diffusivity increases linearly by two more times with the increase in initial Ti film thickness from 0 to 200 nm. On the other hand, the Zr⁴⁺ affects the Ti⁴⁺ diffusion little. Neither the ZrO₂ film thickness nor the coating sequence of Ti metal and ZrO₂ oxide films influences the diffusivity of the two ions. All the codiffusion characteristics are explained. A Zr⁴⁺/Ti⁴⁺ codiffusion model is suggested that consists of two independent diffusion equations with a Zr⁴⁺ diffusivity dependent of Ti⁴⁺ concentration and a constant Ti⁴⁺ diffusivity. In addition, the existence of a waveguide in the Zr⁴⁺/Ti⁴⁺-codoped layer is verified experimentally, and the optical-damage-resistant feature of the waveguide is verified by two-beam hologram recording experimental results.