Er ³⁺/ Tm ³⁺ co-diffusion characteristics in LiNbO ₃ crystal

Co-diffusion characteristics of Er³⁺ and Tm³⁺ ions in LiNbO₃ crystal have been studied, together with two other properties: doping effect on refractive index of substrate, and Li out-diffusion. Er³⁺/Tm³⁺-codoped LiNbO₃ single-crystals were prepared by co-diffusion of stacked Er- and Tm-metal thin films, coated onto parts of the surfaces of X- and Z-cut congruent LiNbO ₃ crystal plates, at different temperatures and different durations in air. After diffusion, the surface refractive indices of the doped and undoped parts were measured by prism coupling technique, and the surface composition was evaluated from the measured indices. The results show that the dopings have negligible effect on the substrate index and the Li out-diffusion depends mainly on the diffusion temperature. The Er³⁺ and Tm³⁺ profiles were studied using secondary ion mass spectrometry, and a co-diffusion dynamical model is proposed and verified experimentally. From the measured profiles, the temperature-dependent Er³⁺/Tm³⁺ diffusivity and solubility were obtained together with other relevant parameters, such as diffusion constant, activation energy, solubility constant and heat of solution. The Er³⁺ and Tm³⁺ co-diffusion exhibits similar temperature dependent diffusivity, and the dependence is similar to that of single-diffusion. In the co-diffusion case, the total solubility consists of two parts: Er³⁺ and Tm³⁺ parts, there is no anisotropy, and the value is equal to that of single-diffusion case, in which both ions have the similar solubility. The Er³⁺ and Tm³⁺ parts of solubility change relatively with the initial metal film thicknesses and hence have different temperature dependences. Nevertheless, their sum, i.e., the co-diffusion solubility, remains a constant at a given temperature.