Journal of the Korean Physical Society, Vol. 32, No. , February 1998, pp. S1056∼S1059 Dynamics of Polarization Reversal in the Surface Layer of Ferroelectric Liquid Crystals Ju-Hyun Lee, Jae-Hoon Kim ∗ and Sin-Doo Lee School of Electrical Engineering, Seoul National University, Seoul 151-742, Korea Using a triangular wave method, we have studied the polarization reversal dynamics in the surface layer of a ferroelectric liquid crystal in a surface-stabilized geometry. It is found that there exist two distinct current peaks associated with two types of molecular reorientation processes in the polarization reversal. A simple model for the two processes occurred in the bulk and the surface layer is developed to describe the physical origin of the second peak under an external electric field. The non-polar and polar surface anchoring energies are estimated as 6.0 × 10 −1 erg/cm 2 and 3.7 × 10 −2 erg/cm 2 , respectively. I. INTRODUCTION Ferroelectric liquid crystals (FLCs) exhibit a variety of structures depending on the anisotropic interactions at FLC/solid interfaces, the aligning procedure, and ex- ternal fields. Particularly, knowledge of the interfacial interactions is of great importance for understanding the nature of the molecular reorientation and the polariza- tion reversal occurred in the surface layer of FLCs. For instance, the molecular orientation of a surface-stabilized ferroelectric liquid crystal (SSFLC), characterized by two energetically degenerate states, is governed by the inter- facial interactions that tend to unwind the intrinsic helix of FLCs [1]. In the presence of an external electric field E, one of the two states of SSFLC is preferred, depend- ing on the polarity of E, by means of a ferroelectric cou- pling with E. The molecular switching of FLCs has been extensively studied by polarization reversal [2], strobo- scopic micrography [3,4] and electro-optic measurements [5]. Among them, the polarization reversal revealed two current peaks in the SSFLC geometry [6]. Although the physical origin of the second peak is somewhat related to the chevron structures and/or the interfaces of FLCs [7–9], a complete picture remains to be explored. In the present work, we describe how the interfacial interactions influence the dynamics of the polarization reversal and the associated molecular reorientation pro- cesses of FLCs. The experimental results together with numerical simulations in a simple model clearly indicate that the second current peak in the polarization rever- sal comes from the surface layer of FLCs. The effect of the chevron structures on the polarization reversal is also discussed. II. EXPERIMENTAL The FLC material used in this study was SCE 12 of British Drug House. The phase transition sequence is as follows : isotropic → (118.0 ◦ C) → cholesteric →(78.8 ◦ C) → smectic A → (64.0 ◦ C) → smectic C* → (-20.0 ◦ C) → crystalline. The sample cell was made up of conduc- tive indium-tin-oxide coated glasses which were treated with poly(1,4-butylene terephthalate) (PBT) containing aromatic rings which resemble the cores of the liquid crystal (LC) molecules. The thickness of the polymer layer was about 300 ˚ A, and both glass surfaces of the cell were unidirectionally rubbed so as to give a planar orientation. The cell gap was maintained by glass spac- ers of 3 µm thick, and the effective electrode area was 0.64 cm 2 . The FLC was filled in the isotropic state, and cooled into the mesophase. Electric contacts were made directly to the internal surfaces of the glasses to apply an external electric field. The sample cell was mounted in a home-made mi- crofurnace for temperature control and the temperature fluctuations were approximately 0.02 ◦ C. For uniform alignment of both the FLC molecules and smectic layers, the sample cell was cooled down at a rate of 0.1 ◦ C/min into the ferroelectric, smectic C* phase. In addition, the square wave voltage of 100 Hz was applied to the cell in the vicinity of the smectic A - smectic C* (Sm A - Sm C*) phase transition temperature [10]. No apparent chevron structures were observed in the electrode area of the cell. For measuring the spontaneous polarization of the FLC sample, we employed a triangular wave method to mon- itor the shape and the time evolution of the polarization current peaks as a function of the applied voltage. III. THE POLARIZATION REVERSAL IN THE SURFACE LAYER We first describe the polarization current peaks in -S1056-