Optical probe of ferroelectric order in bulk and thin-film perovskite titanates

Abstract

We have measured the temperature dependence of the direct band gap ${E}_{g}$ in SrTiO${}_{3}$ and BaTiO${}_{3}$ and related materials with quantum-paraelectric and ferroelectric properties using optical spectroscopy. We show that ${E}_{g}$ exhibits an anomalous temperature dependence with pronounced changes in the vicinity of the ferroelectric transition that can be accounted for in terms of the Fr\"ohlich electron-phonon interaction with an optical phonon mode, the so-called soft mode. In addition, we demonstrate that these characteristic changes of ${E}_{g}$ can be readily detected even in very thin films of SrTiO${}_{3}$ with a strain-induced ferroelectric order. Optical spectroscopy thus can be used as a rather sensitive probe of ferroelectric order in very thin films of these titanates and probably also in subsequent multilayers and devices.