Ferroelectricity in CaTiO$_{3}$ Single Crystal Surfaces and Thin Films and Probed by Nonlinear Optics and Raman Spectroscopy

Bulk CaTiO$_{3}$ has a centrosymmetric point group and is \textit{not }polar or ferroelectric. However, we present surprising results that show highly regular polar domains in single crystals of CaTiO$_{3}$. Confocal Second Harmonic Generation (SHG) and Raman imaging studies were carried out on perovskite CaTiO$_{3}$ crystal surfaces. They reveal large, crystallographic polar domains at room temperature, with in-plane polarization components delineated by twin walls. SHG analysis indicates that the highest symmetry of the polar surface is $m $(space group P$c)$ with polarization in the $m$ plane. In addition, we present results of the polar domain structure imaged before and after the application of an external electric field. Finally, we present the SHG studies of CaTiO$_{3}$ thin films grown using reactive Molecular Beam Epitaxy (MBE); these films are predicted by theory to be ferroelectric and are shown experimentally, both with SHG and in-plane dielectric measurements, to be ferroelectric for temperatures less than $\sim $150 K with group symmetry \textit{mm}2.