Structure-Optical Property Correlation of Epitaxial Potassium Niobate Thin Films Deposited on Magnesium Oxide (100) Substrates Using a Strontium Titanate Transition Layer

Epitaxial thin films of orthorhombic KNbO₃ (100)/(010) were deposited on MgO (100) single-crystal substrates by pulsed laser ablation using an ε120 Å SrTiO₃ (110) transition layer in between. From X-ray diffraction, the orientation relationships were determined as (010) 100] or (100) 010] KNb0₃∥ (110) 110] SrTiO₃∥ (100) 011] MgO and (010) 001] or (100) 001] KNbO₃∥ (110) 001] SrTiO₃∥ (100) 011] MgO. The measured film refractive indices at 632.8 nm were 2.213 ± 0.003, 2.278 ± 0.003, and 2.285 ± 0.003, respectively, along MgO 100], 011], and 011] directions. A model was developed to correlate the measured effective refractive indices of the film to area fractions of domain variants in the film. Using the model, the area fraction of domains with their polarization axis normal to the substrate was estimated to be 60.0 ± 2.7%.