“…The experimental data are easy to collect, but the data analysis requires extensive modelling and fitting [186]. Ferroelectric materials have so far been investigated with ex-situ SE exclusively, involving measurements of dielectric function [187,188], optical conductivity [188], band gap [188][189][190][191][192][193][194][195], ferroelectric phase transition [193,196,197], critical thickness for strain relaxation [198], refractive index, and extinction coefficient [188-192, 194, 199]. A recent report of in situ SE monitoring during the growth of Ruddlesden-Popper phase shows that deposition of each epitaxial layer can be detected with submonolayer precision [200], confirming the potential of this technique for monitoring the growth of polar thin films in the near future.…”