Fig. 1

Emergence of polarization in ultrathin tetragonal BaTiO₃ films at 650 °C. a In situ second harmonic generation (ISHG) intensity dependence of BaTiO₃ (BTO) film thickness during the pulsed laser deposition growth of epitaxial BaTiO₃|SrRuO₃||SrTiO₃. The dashed line indicates the intensity of the surface-induced background ISHG signal on the order of 1%. A schematic of the ISHG setup is shown as inset. b Reflection high energy electron diffraction (RHEED) measured in parallel to the ISHG yield. c Unit-cell (u.c.) polarization as a function of thickness derived by deconvolution from the background ISHG signal (Methods section) and normalization of the square root of the remaining polarization-related ISHG intensity to the film thickness. Data below 1 u.c. are not plotted due to artifacts resulting from the normalization of zero-signal noise. In the inset, the arrow indicates the polarization direction of the BTO single-domain film. d Topography and out-of-plane piezoresponse force microscopy scan (PFM OOP)  of a BTO film of 30 u.c. that was tip-voltage-poled in a quadratic box-in-box region (see inset). b, d Do not reveal any evidence for probe-laser damage to the film surface, laser-induced polarization switching or changes in the ferroelectric poling behavior. The scale bar corresponds to 1 µm