Seong-Woo Suh

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.

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...

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.

Relaxation Dynamics of Nonlinear Optical Chromophores Dispersed in Poled Photo-Polymer Composites

Abstract The relaxation dynamics of nonlinear optical chromophores in photo-polymer composites is studied for a basic understanding of the poling mechanism. The composite films are made up of a photo-polymer containing 0.5 % by weight of 4-dimethylamino-4′-nitrostilbene. They are poled under an external dc electric field at room temperature, followed by UV curing. It is found that for such composite films both the optical retardation and the second harmonic generation (SHG) intensity clearly decay as similar stretched-exponentials in time, and their relaxation behavior is predominantly governed by the period for UV curing. It is suggested that the optical retardation involves mostly the axial order while the SHG intensity is strongly correlated to the degree of polar ordering of the chromophores.

SURFACE POLAR ELECTRO-OPTIC EFFECT INDUCED BY IN-PLANE TWIST IN NEMATIC LIQUID CRYSTALS

Abstract A finite degree of in-plane twist is predicted to produce a surface polar electro-optic (EO) effect in nematic liquid crystals (NLCs). It is found that the flexoelectric coupling with an external electric field essentially renormalizes anchoring energies at the solid-NLC interfaces. Because of the anchoring renormalization by out-of-plane tilt and/or in-plane twist the symmetry of the system is broken, and the polar EO effect is thus induced. The EO modulation exhibits a peak near the Fredericks transition, which agrees well with numerical simulations.

Surface ordering and its influence on electro-optic properties of chiral smectic compounds

Abstract We describe several important aspects of surface smectic ordering prior to the onset of bulk ordering in chiral smectic materials. Above a direct isotropic - smectic A (I-A) and nematic - smectic A (N-A) transition in the bulk, there exists an in-plane molecular tilt that is linearly proportional to an applied electric field. While for the I-A transition, this molecular tilt is relatively confined within the surface smectic layer, for the N-A transition, it can elastically propagate into the bulk nematic phase. For the former case, the temperature dependence of this in-plane molecular tilt is much stronger than that for the latter. Our results suggest that the phase sequence, the range of the smectic A phase, and the magnitude of molecular chirality play a significant role in the nature of the surface smectic A ordering.