# Probing Ferroic States in Oxide Thin Films Using Optical Second Harmonic Generation
## 4 Future Perspectives
### Probing Ferroelectric Heterostructures during the Growth Process  
Significant advances have been made in the growth quality of thin film oxide ferroelectrics. Still, the process has been lacking control of the polar state in multilayer architectures. Most of the technological applications for ferroic thin films require a deterministic control of the domain state. Ferroelectric tunnel junctions require the stabilization of a single-domain state to maximize the polarization-orientation dependent change in conduction between the switched states [133; 134]. In the case of tetragonal ferroelectric-based memory, the presence of the as-grown in-plane domains hinders the switching process of the out-of-plane polarization state [2; 96]. Eliminating the in-plane domains in these systems would thus be of interest [135]. On the other hand, in multiferroic heterostructures aiming towards the electrical control of magnetization, a specific domain configuration triggers deterministic switching events [111]. To achieve full polarization control of a heterostructure, one needs to be able to monitor the polarization during the growth process. Most of the classical ferroelectric epitaxial strained systems are in fact grown in their polar phase [33; 61; 136]. However, none of the traditional thin-film characterization techniques can be implemented in situ due to their invasive nature and the extreme conditions during deposition, such as high substrate temperatures and pressure variations. Synchrotron \(x\)-ray-based techniques, which are sensitive to periodicity in polar systems, i.e., lattice parameter and periodic domain splitting, have been used to show that past a critical thickness, BTO thin films experience an increased tetragonality during the growth process [21; 136]. A schematic of the experimental setup is shown in Figure 17a,b with the corresponding measurements. These pioneering experiments revealed the emergence of the polar state during the growth and broadened the investigation of ferroelectric materials in situ. This technique, however, requires high intensity flux and is limited to highly periodic patterns. SHG is sensitive to symmetry breaking at the emergence of all ferroic orders, which makes it an ideal technique for studying not only single- and complex-domain states but also the transition from one to another. Its non-invasive and non-destructive nature renders the optical coupling into a growth chamber conceivable [61; 137; 138; 139]. Aside from a few attempts to characterize growth of metallic ferromagnets [138; 140] and its use as a surface reaction probe [141], the SHG potential as oxide growth diagnostic tool during the pulsed laser deposition process has only recently been demonstrated [61]. A schematic of the in situ SHG (ISHG) experimental setup is shown in Figure 17c. Figure 16: Three-dimensional structure of domain walls in ferroelectric crystals. (**a**) SHG image of periodic Bloch domain walls in LiTaO\({}_{3}\) crystals. Reprinted from [122]. (**b**) Three dimensional SHG microscopy image of domains walls in TGS. Reproduced with permission from [127], Copyright Wiley-VCH Verlag GmbH and Co. KGaA, 2017. Direct access to the thin film polarization during its growth is achieved in real time using ISHG. The emergence of the ferroelectric polarization in BTO is shown in Figure 17d. The ability to probe multilayer systems [61; 120] opens new avenues towards understanding the complex interplay between strain, interfaces, and electrostatics in oxide superlattices. Interfering signals originating from each polar constituent of a superlattice result in destructive or constructive interference depending on their relative polarization orientation. For instance, Figure 17e shows a succession of increasing ISHG intensity during the growth of ferroelectric BFO layers with parallel polarization orientation. The growth of oppositely polarized layers results in a decrease of the signal [61].