Optical probe, local fields, and Lorentz factor in ferroelectrics

An optical probe is suggested that allows measurements of the local field and Lorentz factor ($L$) in ferroelectric medium. The copolymer poly (vinyliden fluoride/trifluoroethylene) is mixed with Pd-tetraphenylporphyrin (TPP-Pd) that has a very narrow absorption band. Thus TPP-Pd serves as a molecular optical probe of the local field. During the switching of the electric field lower than the coercive one the factor $L$ of a non-polarized ferroelectric mixture is found to be of about $1/3$ that corresponds to the random distribution of molecular dipoles in the ferroelectric. With increasing field the dipole orientation acquires a lower symmetry and $L$ tends to zero as predicted by lattice sum calculations for vinyliden-fluoride. The knowledge of the field dependence of $L$ and the usage of the optical probe makes it possible to measure directly the local and macroscopic fields in the individual elements of various ferroelectric-dielectric heterostructures.