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

We have measured the temperature dependence of the direct band gap, \(E_g\), in SrTi\(^{16}\)O\(_3\) 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. We demonstrate that these characteristic changes of \(E_g\) can be readily detected even in very thin films of SrTi\(^{16}\)O\(_3\) with a strain-induced ferroelectric order. Optical spectroscopy thus can be used as a relatively simple but sensitive probe of ferroelectric order in very thin films of these titanates and probably also in subsequent multilayers and devices.