Matthias Rössle, C. N. Wang, P. Marsik, Meghdad Yazdi-Rizi, K. W. Kim, Adam Dubroka, Ivan Marozau, C. W. Schneider, J. Humlicek, D. Baeriswyl, C. Bernhard

• We have measured the temperature dependence of the direct band gap Eg in SrTiO3 and BaTiO3 and related materials with quantum-paraelectric and ferroelectric properties using optical spectroscopy. We show that Eg exhibits an anomalous temperature dependence with pronounced changes in the vicinity of the ferroelectric transition that can be accounted for in terms of the Frohlich electron-phonon interaction with an optical phonon mode, the so-called soft mode. In addition, we demonstrate that these characteristic changes of Eg can be readily detected even in very thin films of SrTiO3 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.