Domain structure-second harmonic generation correlation in potassium niobate thin films deposited on a strontium titanate substrate

The experimental measurement of second harmonic generation (SHG) and theoretical correlation of the SHG output to domain microstructure is presented for ferroelectric KNbO₃ (110) thin films on a SrTiO₃ (100) substrate. From X-ray diffraction, four possible domain variants in the film growth plane were determined to be KNbO₃ [110]∥SrTiO₃ [001], [001], [010], or [010] denoted as variants X+, X-, Y+, and Y-, respectively. From theoretical modeling of the SHG output from the film, the ratios of nonlinear coefficients d₀/d₃₂ and d₂₄/d₀ in the film were determined to be 2.19 ± 0.12 and 0.46 ± 0.26, respectively, in good agreement with the bulk KNbO₃ crystals. The term d₀ is defined as d₁₅ + (d₃₁ + d₃₃)/2. The SHG can also distinguish between X+ and X-, as well as Y+ and Y-, domains, which was not possible by X-ray diffraction. The ratio of net area fraction (|A^X+ - A^X-|/|A^Y+ - A^Y-|) was determined to be 2.76 ± 1.06 in the probe area (2.25π mm²) where A^X+, A^X-, A^Y+, and A^Y- are the area fractions of domain variants X+, X-, Y+, and Y-, respectively, in the film growth plane.