Figure 1: Detection of localized SHG at domain walls in PZT thin films.

(a) Schematic representation of the SHG experimental set-up. Ferroelectric domains (c⁺ and c^– domains) of different geometric shapes are written in the film by applying a bias voltage through a conductive PFM tip. The domain structure imaged by means of PFM is displayed for film regions with (b) right triangle and (c) rectangle-shaped domains. The corresponding nonlinear optical images reveal a localized optical signal at the walls surrounding the (d) triangular and (e) rectangular domains. The bars in all images correspond to a scale of 2 μm. (f) The fundamental wavelength is 800 nm while the localized emission (g) occurs at a wavelength of 400 nm, corresponding to the half of the fundamental wavelength. The scattered dots in h represent the variation of the emission intensity with the power of the fundamental wave. The error bars account for the intensity fluctuation of 10% and the continuous line is a quadratic fit of the experimental data (Supplementary Fig. 1 for more details). The SHG images represent the data recorded at a polarizer angle ϕ=0° integrated over the analyzer angles α. The laboratory coordinate system (x,y,z) displayed in a coincides with the crystallographic axes (X,Y,Z) of tetragonal PZT.