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Abstract
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.
Highlights
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
Notable discoveries include conductive domain walls[4], a strain-driven morphotropic phase boundary[5] and a specific magnonic response that can be tuned by epitaxial strain[6] or electric field[7]
Strain engineering[13] is a powerful tool through which, for instance, ferroelectricity is strongly enhanced in BaTiO3, or induced in otherwise non-ferroelectric materials such as SrTiO3
Summary
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. With a bandgap (B2.7 eV) in the visible[8], large birefringence[9] (0.25–0.3), a strong photovoltaic effect[10] and sizeable linear electro-optic coefficients[11], BFO is garnering interest in photonics and plasmonics[12] Most of these physical properties are intimately linked to structural parameters, and may be tuned in thin films by epitaxial strain. We present a combined experimental and theoretical study demonstrating that strain induces a very large change in the refractive index of BFO, which corresponds to an effective elastooptic coefficient larger than in any ferroelectric, and larger than that of quartz[18] This effect is accompanied by a shift of the optical bandgap, reminiscent of pressure-induced changes in light-absorption[19], a phenomenon known as piezochromism in other materials systems[20]. We show how an electric field can be used to toggle between two strain states with different light absorption, corresponding to an electrochromic effect that is intrinsic, reversible and non-volatile
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