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 Fröhlich 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.

• Received 3 June 2013

DOI:https://doi.org/10.1103/PhysRevB.88.104110