Determination of piezoelectric and flexoelectric polarization in ferroelectric liquid crystals.

A method of determining the local polarization in chiral smectic liquid crystals is proposed. The method consists of simultaneous measurement of the electric permittivity and depth of electro-optic modulation in a helical liquid crystal using weak ac electric field. The spontaneous polarization of a single smectic layer determined in this way contains both the flexoelectric and piezoelectric component. On the other hand, the polarization measured using strong electric fields in a switching experiment contains the piezoelectric component only. The comparison of polarization obtained using strong and weak fields makes it possible to determine the value of flexoelectric polarization. This kind of measurement has been performed for two ferroelectric liquid crystals having quite small absolute values of the spontaneous polarization (few nC/cm2). It turned out that in both cases, the flexoelectric polarization is of the same order of magnitude as the piezoelectric one.