  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.
Here, we apply 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 an electrochromic effect that is reversible,
remanent and not driven by defects. These findings broaden the potential of
multiferroics towards photonics and thin film acousto-optic devices, and
suggest exciting device opportunities arising from the coupling of ferroic,
piezoelectric and optical responses.
