Fig. 4

Engineering ferroelectric multilayers with user-defined up/down polarization sequence II. a A-site growth and upwards polarization of BiFeO₃ (BFO) is achieved by insertion of a TiO₂ monolayer between SrRuO₃ (SRO) and BFO or by substituting SRO by MnO₂-terminated La_0.7Sr_0.3MnO₃. B-site growth and downwards polarization of BFO is achieved by SrO self-termination of the SRO. b Sketch of multilayer with varying direction of polarization in different layers. The second harmonic generation contributions from these layers may interfere constructively or destructively depending on the respective in situ second harmonic generation (ISHG) phase (φ↓ or φ↑ with φ↓ – φ↑ = 180°). c Piezoresponse force microscopy (PFM) scan confirming the up-orientation and reversibility of polarization for an A-site-grown BFO film of 20 unit cells. d PFM scan confirming the down-orientation and reversibility of polarization for a B-site-grown BFO film of 20 unit cells. In c, d, the scale bar corresponds to 1 µm. e ISHG from a sequence of oppositely polarized ferroelectric BFO layers (open green circles: up polarization, open green squares: down polarization). The opposite orientation manifests as destructive interference of the ISHG contributions from the two layers. The inset emphasizes the identical critical thickness for the emergence of the polarization in both BFO layers (lines are guides to the eye). Further deposition of down-polarized BFO results in a recovery of the ISHG signal as shown and discussed in Supplementary Fig. 3