San-Seong Seomun

A novel property caused by frustration between ferroelectricity and antiferroelectricity and its application to liquid crystal displays-frustoelectricity and V-shaped switching

We have studied the frustration between ferro- and antiferro-electricity in chiral smectic C like liquid crystalline phases, which is not only fundamentally interesting but also very attractive from an application point of view. It causes temperature induced successive phase transitions as characterized by a devils staircase and the thresholdless, hysteresis-free, V-shaped switching induced by an applied electric field. The devils staircase indicates some type of interlayer ordering, while the V-shaped switching suggests considerably diminished tilting correlation. These two are apparently contradictory to each other, but result from the same cause, i.e. the frustration. We have first summarized experimental facts regarding subphases and successive phase transitions observed in many compounds and mixtures, which we believe are related to one another and result from the frustration. We have introduced several different theoretical explanations for these observed facts, and shown that only the axial next nearest neighbor Ising (ANNNI) model can explain almost all of the facts, provided that it is unified with the XY model appropriately. The unified model can make a comprehensive explanation in the most natural way based on the most probable molecular interactions. We have then emphasised that there are several modes regarding the V-shaped switching, because the system becomes so soft with respect to the tilting direction and sense that any additional external or internal force modifies the in-plane local director alignments. For the practically usable ones, we have emphasised the need for some type of randomization in the molecular alignment at the tip of the V and/or the switching process. In particular, the two dimensional (ideally, cylindrically symmetric) azimuthal angle distribution of local in-plane directors around the smectic layer normal is most attractive. Such a randomized state at the tip of the V is thermodynamically unique under a given condition imposed by interfaces. It stays stable even when the smectic layer structure, such as a chevron, changes with temperature. Finally, we have summarized the so-far reported compounds and mixtures for the V-shaped switching and introduced some prototypes of LCDs using them.

Langevin Type Alignment in a Smectic Liquid Crystal Mixture Showing V-Shaped Switching As Studied by Optical Second-Harmonic Generation

Second-harmonic generation (SHG) was observed in a homogeneously aligned smectic C*-like liquid crystal (LC) cell showing V-shaped switching during the application of a triangular wave. At normal incidence of light, abnormally strong SHG signals were observed at about zero electric field. It was confirmed that this phenomenon is characteristic in LCs showing the V-shaped switching, and is not observable in typical ferroelectric and antiferroelectric LCs. The dynamic response of SHG was successfully simulated using the two-dimensional Langevin type potential model.

The Biaxiality of the Chiral Smectic Subphases in Very Thin Freestanding Films

Thick and thin freestanding films for the sample showing the subphases, AF and SmC n * were investigated. The thick film shows optically uniaxiality due to the helix along the smectic layer normal. However, Schlieren textures emerge in the thin film where the helical pitch is less than one pitch. It indicates that the subphases observed have biaxiality. Meanwhile, the decrease of the transmission in the bright brush in Schlieren textures compared with that in the SmC* phase support the distorted Ising model.

Surface molecular alignment by in-plane anchoring in the cell showing the V-shaped switching

The surface molecular alignment in a sandwich cell showing V-shaped switching has been investigated using a photocontrolling method. The results indicate that the molecules on the rubbed surface are aligned parallel to the rubbing direction as in an open cell with an air–liquid crystal boundary. This alignment is due to strong in-plane anchoring, due to which the small twisted state appears along the cell thickness through intralayer molecular interaction.

The Study of the Surface Electroclinic Effect Using Different Aligning Materials

The surface electroclinic effect in sandwich cells was investigated using two aligning materials: nylon and polyimide. The deviation angle of the smectic layer normal from the rubbing direction is much larger (∼ 20°) in the nylon cell than that (∼ 12°) in the polyimide cell. Meanwhile, it is found that the surface electric field to cause the surface electroclinic effect can affect the bulk property even in the SmA phase. It seems to act as a dc bias field to the soft mode.

Alignment instability induced by irradiation of visible light in frustoelectric liquid crystalline cells showing the V-shaped switching

Abstract Irradiation of the powerful visible light from the laser expedites the deterioration of the frustoelectric liquid crystalline cells (Inui mixture) showing the V-shaped switching. This deterioration is caused by the light absorption in the aligning layer, which strongly influences the molecule-surface polar interaction; this cannot be observed in the cell with the transparent aligning material in the used visible light region. This is observed just in the tilted smectic X* phase, not in the SmA phase. Irradiation during the switching leads to hysteresis in the V-shaped pattern and changes even the surface molecular alignment. These results can be explained by the shielding of the surface charge due to the alignment of the spontaneous polarization and the disturbed polar anchoring.