Fast-switching flexoelectric cholesteric liquid crystal displays that can be operated in two modes: amplitude (flexoelectric effect, in-plane-switching) and phase (dielectric coupling, out-of-plane switching) have been developed. The device comprises of a small amount of polymer network localized at the substrate surfaces and short-pitch cholesteric liquid crystal whose helical axis laid in the direction parallel to the substrates. The polymer network stabilizes the helical axis in the plane parallel to the substrates at zero voltage. The response times for amplitude and phase switching of a polymer-stabilized cholesteric display with 2 microns cell gap are 200 μs at 3.5 V/μm and 3 ms at 12.5 V/μm, respectively. Using a dual-frequency switchable nematic liquid crystal, we are able to improve the contrast of the amplitude switching by obtaining a larger deviation angle of helical axis with a high voltage and in the same time, suppressing the helix unwinding.
We discussed the effect of polymer concentration and localization on flexoelectro-optical device using short pitch cholesterics oriented in uniform lying helix texture. By using a small concentration of photoreactive liquid crystal monomer with various concentrations and selecting the illumination conditions, we have been able to create a localized polymeric network at both substrate surfaces. We can stabilized the two switching modes and eliminating at the same time the effect of the residual birefringence of the polymeric network in the field-unwound state of the sample. The device has two operating modes: amplitude and phase modulation mode, respectively. The amplitude modulation mode is a fast in-plane switching of the device optic axis that enables to achieve a 100 % modulation of the transmitted light intensity whereas the phase mode gives a continuous change of the refractive index and thus of the phase shift of the transmitted light.
We discuss examples involving the development of ordered polymer thin films for application as light bending devices. This paper focus on the preparation of ordered polymer architectures using self-assembled or field-induced optical patterns of various liquid crystal hosts for imprinting or templating polymer or functional materials. The various images of ordered polymer are provided by the polarized optical microscopy (POM) and scanning electron microscopy (SEM) study. Slantwise UV irradiation technique was applied for the formation of the asymmetric polymer patterns. Various UV irradiation angles were used and significant differences in the polymer patterns with respect to the applied angle were observed. Transmission grating patterns are investigated. The electro-optical measurements for the analysis of patterned cells will be presented.
We describe a detailed study of the bend nematic structure stabilized by the polymer network. The formation of phase-separated polymer network in a bend nematic structure is affected by various factors, such as the polymer network morphology, polymer concentration, applied electric field, UV exposure time, and UV intensity. SEM images of the polymer network morphology show a templating effect, where highly ordered nanofibers mimic the bend state of the liquid crystal. At low monomer concentration and low curing voltage, liquid crystal can be oriented into the twist or splay state. High monomer concentration conveys excessive strength of the polymer network on liquid crystal and shows poor electro-optical properties of liquid crystal. With the current nematic host, the best condition of our experiments is 3% monomer concentration and 5V curing voltage. The resulted liquid crystal-polymer composite shows fairly fast optical response to the applied field.