Tilt-polarization coupling, external fields

The electroclinic effect is a result of the coupling between tilt and polarization. The polarization Pe, which is induced by an external field E in the smectic- phase of chiral molecules, consists of a part Po which is present in every dielectric (orientation and electronic polarization), and a part Pg which is due to the P-6 coupling and should show a similar behavior as the induced tilt angle 9. While it is difficult to separate Pg and Pq exactly, measurements of the total polarization in the smectic- phase and around the smectic- -smectic-C transition indicate at least qualitatively that Pg and 9 show very similar behavior [65]. 

Due to special symmetry conditions, the presence of chiral molecules in tilted smectic phases leads to the appearance of a spontaneous electric polarization within each smectic layer [1], [2], Magnitude and direction of the polarization are coupled to the magnitude and direction of the molecular tilt giving rise to imique properties of tilted as well as orthogonal smectic phases of chiral molecules in external electric fields. In this chapter, the basic properties of smectic-C and smectic- l phases of chiral low-molecular weight molecules are summarized, mainly from the viewpoint of physics. 

The two switching effects are dis rfayed in Figure 83. The electroclinic effect (a) has been discussed in Sectioo 0. The flekt-induced helical unwinding (b) is caused by the coupling of the spontaneous polarization Pj to an external held. Both effects lead to a rotatioo of the optical axis in the film plane. The sample is placed between crossed polarizers and, due to the rotatioa of the optical axis, the intensity of the transmitted light can be modulated by the applied field. The induced tilt of the optical axis has been found to be a linear function of the applied field. In the case of short-pitch ferroelectric PDLC, the tilt can be about 13 at an at lied field of 10 V/pjn (Fig. M). 

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