Optical second harmonic signature of phase coexistence in ferroelectric|dielectric heterostructures

Abstract

An exotic coexistence of polar states is known to occur in $\text{ferroelectric}|\text{dielectric} {\mathrm{PbTiO}}_{3}|{\mathrm{SrTiO}}_{3}$ ($\mathrm{PTO}|\mathrm{STO}$) heterostructures. In the PTO layers with a thickness of 10--20 unit cells, in-plane-polarized regions order alongside the so-called vortex phase with a whirl-like arrangement of electric dipoles. We investigate the optical signature of these polar phases noninvasively using optical second harmonic generation (SHG) on $\mathrm{PTO}|\mathrm{STO}$-based heterostructures. We identify the phase coexistence down to a single $\mathrm{PTO}|\mathrm{STO}$ bilayer. By comparing the SHG yield in dependence of the number of PTO layers, we further find that interlayer coupling plays an important role in setting the final polarization state. Our nonlinear optical experiments demonstrate the potential of this noninvasive approach for the identification and understanding of complex noncollinear ordering of dipole moments in oxide heterostructures and lays the groundwork for their operando investigation.