The control of optical fields is usually achieved through the electro-optic or acousto-optic effect in single-crystal ferroelectric or polar compounds such as LiNbO3 or quartz. In recent years, tremendous progress has been made in ferroelectric oxide thin film technology – a field which is now a strong driving force in areas such as electronics, spintronics, and photovoltaics. In our recent study published in Nature Communications, we have applied epitaxial strain engineering to tune the optical response of BiFeO3 thin films, and find a very large variation of the optical index with strain, corresponding to an effective elasto-optic coefficient larger than that of quartz. We observe a concomitant strain-driven variation in light absorption – reminiscent of piezochromism – which we show can be manipulated by an electric field. This constitutes a novel electrochromic effect that is reversible, remanent, and not driven by defects. These findings broaden the potential of multiferroics toward photonics and thin film acousto-optic devices, and suggest exciting device opportunities arising from the coupling of ferroic, piezoelectric, and optical responses.
Large elasto-optic effect and reversible electrochromism in multiferroic BiFeO3
D. Sando et al, Nature Commun. 7, 10718 (2016)