A model is developed to explain the hysteretic electric field dependence of the electrooptic coefficient in ferroelectric thin films. The reversible electric polarization and the tunable dielectric susceptibility of the ferroelectric thin film are proposed to explain the hysteretic ρ-E (electrooptic coefficient- applied electric field) loop. An empirical model used in ferroelectric capacitors to predict the high frequency C-V curve is utilized here to find the field dependence of the nonlinear susceptibility. The tunable susceptibility can also explain the peaked characteristics of the ρ-E loop. We also show that the linear electrooptic effect in ferroelectric thin films could produce the pseudo-quadratic electrooptic effect on field-induced birefringence as a result of the switchable spontaneous polarization of ferroelectrics. Thus, a careful interpretation of the field-induced birefringence is required to avoid misleading conclusions. This model provides a fundamental understanding to the tunability of the electrooptic coefficient and is useful for the electrooptic characterization of the ferroelectric thin films.
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