The successful integration of ferroelectrics into nanoscale devices relies on our ability to engineer and control polarization states in the application relevant ultrathin regime. Here I will show how nonlinear optics enables the in-situ investigation of the ferroelectric polarization during the thin film epitaxial growth. Beyond the unprecedented access to the emergence of ferroelectricity and domain formation during the epitaxial deposition, we investigate transient polarization states originating from the evolving charge-screening environment of the oxide thin film growth process. We show the impact of lattice chemistry, depolarizing field-related effects in-situ and identify routes towards the establishment of robust polarization states in the ultrathin regime. Finally, we show how light can be used beyond its classical role of a probe and present reversible optical control of polarization states in ferroelectric heterostructures. Our work provides new insights dealing with the physics of ultrathin ferroelectrics and further control of ferroelectric-based heterostructure.

Nanoscale electrostatic control of oxide interfaces enables physical phenomena and exotic functionalities beyond the realm of the bulk material, including spatially confined superconductivity, multiferroicity, and topological properties. Here, we exploit the spontaneously forming charged sheets of layered materials to engineer the ordering of electric dipoles in ferroelectric oxide heterostructures. In our layered ferroelectric Aurivillius-type Bi(n+1)Fe(n-3)Ti(3)O(3n+3) (BFTO) model system, the naturally forming charged (Bi2O2)2+ planes locally polarize the perovskite matrix constituting the unit cell. We reveal the dynamical evolution of this polarizing effect during the thin film growth using in-situ optical second harmonic generation (ISHG) in combination with reflection high-energy electron diffraction monitoring. We observe in real-time the resulting formation of the characteristic Aurivillius antipolar ordering along the direction perpendicular to the Aurivillius layering. Next, we use the polarizing (Bi2O2)2+ stacking as a versatile electrostatic environment for tuning the properties of functional oxide thin films. Specifically, we insert multiferroic BiFeO3 (BFO) into the Aurivillius scaffold and stabilize a ferrielectric-like electric-dipole order in the final heterostructure while maintaining the antiferromagnetic order of the BFO. Hence, in contrast to the conventional approach focusing on the depolarizing-field-tuning in ferroelectric superlattices, our work brings the use of polarizing charged layers as an additional strategy towards dielectric engineering of technologically relevant epitaxial thin films.

Ferroelectric thin films hold significant promise in modern nanoelectronics, demanding precise control over their domain structure. While electrical field-driven switching is currently employed, it often leads to undesirable side effects. Conversely, mechanical switching offers a voltage-free alternative but faces challenges in thicker films.

Recent breakthroughs have demonstrated stable mechanical switching in films up to 200 nm thick, attributed to the presence of nanocavities. The ability to manipulate domains within films of such considerable thickness was attributed to the presence of nanocavities observed within the sample bulk, positioned in close proximity to the film surface. These void-like nanoscale defects were believed to function as pinning centers for the domain.

Here, we present a phase-field investigation dedicated to comprehending the pivotal role played by nanocavities in enabling mechanical domain switching in thicker ferroelectric films.
We systematically analyze the impact of various cavity parameters on the stability of mechanical switching, elucidating the complex interplay between applied pressure, cavity size, depth from the film surface, and dielectric properties. These investigations are reported in stability diagrams, providing a clear understanding of the essential conditions for successful nanocavity-assisted mechanical switching. Our findings reveal the intricate interplay between these factors and outline the conditions for stable mechanical switching.
Furthermore, phase-field simulations showcase the energetic mechanisms governing nanocavity-assisted mechanical switching, emphasizing the pivotal role of these defects as pinning centers. As such, the evolution of polarization under tip-induced pressure is illustrated, providing insights into the underlying energetic mechanisms governing ferroelectric switching stability.

This study elucidates nanocavity-assisted mechanical control of polarization, thus enabling mechanical switching across substantial film thicknesses. This investigation not only enhances our fundamental understanding but also paves the way for the precise control of polarization within void-assisted switching scenarios. We envision these insights holding promise for the design and optimization of films tailored for mechanical switching applications, positioning nanocavity-assisted domain switching as a key player in the advancement of ferroelectric thin film technology.

In the last decade hafnia and zirconia films have attracted tremendous attention arising from the discovery of ferroelectricity at the nanoscale, which can enable the downscaling of the next-generation of non-volatile memory and energy storage devices. In this presentation an overview regarding our most recent results on this topic will be discussed. Special attention will be given to the possibility for achieving orthorhombic and rhombohedral ZrO2 films through substrate orientation control. The present work combines experimental structural studies with density-functional theory (DFT) calculations to disclose the phases in the ZrO2 thin films grown by ion-beam sputtering deposition technique on Nb:SrTiO3 substrates. Scanning probe microscopy techniques and macroscopic polarization-electric field hysteresis loops show ferroelectric behavior in these films. Interestingly, the studied films show a ferroelectric behavior per se.