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

TL;DR

This study uses optical spectroscopy to detect ferroelectric order in bulk and thin film perovskite titanates by observing temperature-dependent changes in the band gap related to the soft phonon mode.

Contribution

It demonstrates that optical spectroscopy can serve as a sensitive, non-invasive method to probe ferroelectric order even in very thin strained films of titanates.

Findings

Band gap exhibits anomalous temperature dependence near ferroelectric transition.

Changes in $E_g$ are linked to the soft phonon mode and electron-phonon interaction.

Optical spectroscopy effectively detects ferroelectric order in thin films.

Abstract

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.