Electro-optic effect in ferroelectric thin films: contributions of strain and domain structure

Recently, novel thin-film-based plasmonic electro-optic (EO) modulators, which are compact, energy-saving, and high-speed, have attracted a great deal of attention. Unlike traditional EO modulators, a plasmonic waveguide will allow us to use a thin film much thinner than the wavelength of light. So far, most of such devices use non-linear polymers though they have long-term stability problem. Therefore, it will be superior if the ferroelectric thin films, which show large EO coefficients such as BaTiO3, can be used. However, it is widely observed that the EO coefficients of ferroelectric thin films are often different from those of the bulks.

In this presentation, we show that the strain and domain structures of ferroelectric thin films remarkably affect their EO properties. 1) In (001)-epitaxial (Ba0.5Sr0.5)TiO3 (BST) thin films, the EO response is enhanced toward the ferroelectric-to-paraelectric phase transition temperature modified by the strain. Phenomenological thermodynamic model reasonably supports our experimental results. 2) In (111)-epitaxial Pb(Zr0.7Ti0.3)O3 (PZT) thin films, where two-types of domains having different polarization component along the direction normal to the substrate coexist, the EO response is maximized near the coercive electric field of the film. These experimental findings suggest that the strain and domain engineering of ferroelectric thin films are powerful tools for manipulating their EO properties.