Composition-dependent refractive index dispersion and Sellmeier coefficients for lithium niobate tantalate solid solutions

Understanding and controlling the optical properties of lithium niobate tantalate solid solutions (LiNb1−xTaxO3, 0 ≤ x ≤ 1) is essential for their use in integrated quantum and nonlinear photonics. This material system allows for composition-dependent tuning of key optical parameters such as refractive indices and birefringence. While lithium niobate (LiNbO3, x = 0) and lithium tantalate (LiTaO3, x = 1) are well characterized, reliable dispersion data for intermediate compositions remain scarce, limiting accurate modeling and refractive index engineering. Here, we address this gap by experimentally determining the ordinary and extraordinary refractive indices of LiNb1−xTaxO3 over the spectral range of 405 − 1550 nm using an interferometric technique. As a result, composition-dependent refractive index dispersion and Sellmeier coefficients are derived and discussed in relation to previous studies and structural aspects. In particular, a monotonic decrease of the ordinary refractive index with increasing Tantalum concentration is observed. The present Sellmeier coefficients further allow discussion of zero birefringence dispersion and its composition dependence in lithium niobate tantalate.