Sin-Doo Lee

FAST LINEAR ELECTRO-OPTICAL SWITCHING PROPERTIES OF POLYMER-DISPERSED FERROELECTRIC LIQUID CRYSTALS

A fast linear electro‐optic effect is demonstrated in ferroelectric liquid crystal/polymer composites suitable for display applications. Both the ferroelectric liquid crystal (FLC) and the polymer matrix are oriented by the rubbing process which remarkably improves the electro‐optic characteristics of the composites. Depending on the phase transition sequence and the pitch of the FLC, the size as well as the shape of the droplets vary in the polymer matrix. The associated relaxation dynamics can be understood within a simplified model which contains the essential features of the experimental data. The switching times are on the order of a hundred μs.

Novel electro-optic effect associated with a homeotropic to twisted-planar transition in nematic liquid crystals

A homeotropic to twisted‐planar (HTP) structural transition is demonstrated in a homeotropically aligned nematic liquid crystal with negative dielectric anisotropy. This HTP transition results in a novel electro‐optic effect useful for extremely high contrast, wide viewing, and achromatic display applications. The HTP architecture is found to be a promising candidate for use in the active addressing scheme. The role of the surface anchoring and molecular chirality on the HTP transition is also discussed.

Ferroelectric Liquid Crystalline Ordering of Rigid Rods with Dipolar Interactions

Abstract A mean-field model for the ferroelectric ordering of hard sphero-cylinders with dipolar interaction is developed within the Onsager formalism. By performing accurate numerical analyses, a complete phase diagram is obtained in terms of the dipolar strength, the number density, and the pressure of the spherocylinders, which exhibits a usual isotropic-nematic, a nematic-ferroelectric nematic, and a direct isotropic-ferroelectric nematic phase transitions. These results are in reasonable agreement with the recent computer simulations. It is also found that there exists always a direct isotropic-ferroelectric nematic phase transition above a critical dipolar strength.

Precise determination of the cholesteric pitch of a chiral liquid crystal in a circularly aligned configuration

We propose a novel method of measuring the cholesteric pitch of a liquid crystal (LC) in a circularly aligned configuration. The LC cell is made with one homogeneously rubbed surface and one circularly rubbed surface so that it produces a disclination line passing through the singularity created by the circular rubbing. This disclination line is associated with two domains with different twist orientations. In such a geometry, the relationship between the angular deviation of the disclination line and the cholesteric pitch is analytically derived. The concept of a homogeneous-circular configuration is adopted to accurately measure the cholesteric pitch of LC in a broad range from 0.5 to 500 μm.

Nonlinear optical anisotropy associated with polar ordering in ferroelectric liquid crystals

Abstract We have developed a new experimental method of measuring the second harmonic generation (SHG) from ferroelectric liquid crystals (FLCs). The measurements are made on a homogeneously aligned FLC cell at an oblique incidence to the surface normal as a function of the azimuthal angle of rotation. This method is simple and powerful to study both the molecular orientation and the associated nonlinear optical (NLO) properties of FLCs. The absolute values of the NLO coefficients are determined self-consistently from both the SHG intensity profiles and the Maker fringes obtained for the same FLC cell. The anomaly observed in the electric field dependence is described in terms of the third-order optical nonlinearity of FLCs.

Homeotropic to Twisted Planar Transition in Nematic Liquid Crystals with Negative Dielectric Amsotropy

We have studied the effect of molecular chirality and cell gap on the electro-optic (EO) modulation associated with a homeotropic to twisted-planar (HTP) transition. It is found that an optimum con...

Optical observations of layer ordering in helial smectic A* liquid crystals

Abstract We report the first optical microscopic observation of characteristic textures and the associated layer ordering in a recently discovered, helical smectic A* phase on a multiple domain substrate. The experimental results are quite unusual but consistent with a mean-field model of the twisted grain boundary phase.

Continuous Ferroelectric Switching in Twisted Smectic C* Liquid Crystals

Abstract We report on the uniform alignment and continuous ferroelectric switching of a twisted smectic C* liquid crystals (LCs). In contrast to surface stabilized ferroelectric LCs, a twisted smectic C* (Sm C*) structure exhibits essentially no threshold field and produces intrinsic gray scales. For constructing such a twisted structure, the symmetry in molecular tilt with respect to the rubbing axis is externally imposed by the surface forces. The symmetry required for producing a uniformly twisted structure is that the twist angle between two rubbing axes on the substrates must be exactly twice of the molecular tilt. One important parameter is the existence of the smectic A state in the phase sequence, which facilitates to the uniform growth of the smectic layers. We perform numerical simulations to obtain the director profiles and the resultant electro-optic modulation as a function of the electric field in the twisted Sm C* structure. It is found that the experimental data agree well with the numeric...

Critical behavior of the field-induced molecular tilt near the nematic-chiral-smectic-A transition

Abstract We present experimental results for the critical behavior of the field-induced molecular tilt near the nematic-smectic-A transition in several chiral liquid crystals. Our results suggests that the smectic A order at a solid substrate significantly contributes to the electroclinic effect close to the phase transition even when the bulk is in the nematic phase.

Interplay between dielectric and ferroelectric couplings in twisted smectic C* liquid crystals

Abstract We study the physical mechanism for a continuous electro-optic effect in twisted smectic C* (TS) liquid crystals. The basic principle combines the fast speed of ferroelectric switching and an analog optical transfer of a conventional twisted nematic structure. For numerical simulations, the total free energy as a combination of nematic-like elastic, ferroelectric, dielectric, and surface energy terms is adopted, and the modified Ritz and relaxation methods are implemented to find the director profiles and the resultant optical transmission through the TS structure as a function of an external electric field. Numerical results show thai a delicate interplay between the ferroelectric and dielectric couplings results in asymmetric deformations of the TS structure, which amplifies the molecular switching on one surface and reduces it on the other. The polar anchoring strength relative to the nonpolar one is an essential factor for correctly describing the experimental results.