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 KNbO 3 (110) thin films on a SrTiO 3 (100) substrate. From X‐ray diffraction, four possible domain variants in the film growth plane were determined to be KNbO 3 [1 1 0]∥SrTiO 3 [001], [00 1 ], [010], or [0 1 0] 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 o /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 3 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 (2.25π 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.