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 [1(1) over bar0$] parallel to SrTiO3 [001], [00(1) over bar], [010], or [0(1) over bar0$] denoted as variants X+, X-, Y+, and Y-, respectively. From theoretical modeling of the SHG output from the film, the ratios of nonlinear coefficients d(0)/d(32) and d(24)/d(0) in the film were determined to be 2.19 +/- 0.12 and 0.46 +/- 0.26, respectively, in good agreement with the bulk KNbO, crystals. The term d(0) is defined as d(15) + (d(31) + d(33))/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 (2.25 pi mm(2)) 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.