Chi-Huang Lin

Dynamic Pattern Formation and Beam-Steering Characteristics of Cholesteric Gratings

The dynamic pattern formation and the beam-steering characteristics of cholesteric gratings were studied. Films with a planar cholesteric texture and various thickness to pitch length ratios (d/p) were fabricated. An optical microscope was used to observe the stripe patterns of the cholesteric gratings formed by applying a voltage to the planar films. The micrographs showed that the cholesteric gratings were formed in two different ways, depending on the samples d/p ratio. For samples with 1/2≤d/p≤1.0, the grating stripes simultaneously appeared across the whole sample, and the contrast of the stripes increased with time during formation. For films with d/p≥1.5, the stripes were initiated near the edges, and near the defects on the substrates, and then slowly extended to the whole sample along the rubbing direction. The diffraction measurements showed that the diffracted beams could be steered either electrically or optically only for the latter type of film. These results can be well explained theoretically.

Multidirectional rubbed liquid-crystal cells

The alignment characteristics of the homogeneous liquid-crystal (LC) cell rubbed multidirectionally were examined. LC molecules align along an axis between two different rubbing directions. The rubbing strength, cell thickness, and ambient temperature markedly influence the final orientation of the LC molecules. The orientation of the LC molecules and the transmission of a multidirectionally rubbed cell can be controlled according to the ambient temperature. A model that assumes substrate with various groove densities along various rubbing directions is presented. The elastic constant and the viscosity of the LC molecules are found to be the key factors that influence the orientation of the LC molecules.

Cholesteric Gratings Doped with a Dichroic Dye

We report the results obtained from the studies of cholesteric gratings doped with a dichroic guest-host dye, G206. First, a dye-doped planar cholesteric texture was switched to a fingerprint texture. Using the pump (Ar+ laser)–probe (He–Ne laser) technique, we then studied the diffraction characteristics of the Raman-Nath cholesteric grating. The results showed that the diffraction efficiencies of the first- and second-order diffracted beams decreased and increased, respectively, with increasing power of the pump beam, while those of the zeroth order remained unchanged. In addition, the diffracted beams can be steered using the pump beam. We propose a model based on the thermal effect resulting from the dye absorption of the pump beam. It explains the experimental results qualitatively well.

Variation of nematic liquid crystal on a silver surface

The variation of liquid crystal (LC) molecules on an unaligned surface is complicated and deserves study for the application of dye adsorption. The attenuated total reflection (ATR) technique is a sensitive way to observe the variation of LC director near the silver surface, which can eliminate the guided mode and enhance the sensitivity in the variation of LC directors. The behaviours of LC directors near the silver films are treated as the tilt mode with the aid of the homogeneous alignment layer. The polar anchoring energy coefficient is not constant and is dependent on the variation of LC directors in the azimuthal direction.

Polymer-Stabilized Reflective Fingerprint Cholesteric Texture Grating

This study describes a reflective grating. It is fabricated using a simple process consisting of thermal annealing followed by UV irradiation through a grating photomask. The sample is made from a mixture of cholesteric liquid crystal and prepolymer materials. A cholesteric liquid crystal has a planar texture, which changes under an applied external field, thus forming the electrically switchable reflective grating. The multistability of the grating can be achieved by applying various voltage pulses; the memory effect of the gratings gray-scale persists for two months. The high switching contrast ratio and fast response of the grating are also discussed.

Scattering Light Interference from Liquid Crystal Polymer Dispersion Films

The Quetelet-type ring pattern is observed in liquid crystal polymer dispersion (LCPD) films. The clusters of the polymer network and liquid crystal (LC) domains with different director axes in the LCPD films serve as scatterers. Cells with unidirectional and multidirectional rubbings are fabricated. Experimental results show that the polarization of incident light, the applied voltage and the ambient temperature significantly affect the ring intensities. However, the contribution of the LC domains is not evident until the voltage is applied. Finally, rubbing the cells in multiple directions reveals that measurement of the Quetelet-type ring intensity can be used to readily identify the orientation of the liquid crystals. This finding also reveals that the LCs in an LCPD mixture are aligned closer to the final rubbing direction than are pure LCs in a multidirectional rubbed cell. A simple model was proposed to explain the observations.

Grating Based on Polymer-Stabilized Ferroelectric Liquid Crystal Films

A surface-stabilized ferroelectric liquid crystal (SSFLC) film, doped with a small amount of monomer, BAB-6, was fabricated. The alignment performance and electrooptical effect of SSFLC films polymerized under different polarities of the DC field were examined. A grating was then fabricated by curing a doped SSFLC cell with UV through a photomask. The grating thus formed was found to be a combination of a phase grating and an amplitude grating, between which the grating could be switched by applying a suitable voltage. The response time was in the order of 500–600 µs.

QUÉTELET-TYPE INTERFERENCE FROM LIQUID CRYSTAL POLYMER DISPERSION FILMS

The Quételet-type ring pattern is observed in liquid crystal polymer dispersion (LCPD) films. The clusters of the polymer network and LC domains with different director axes in the LCPD films serve as scatterers. Changes in the first-order ring intensity are probed with a linearly polarized laser beam. Experimental results show that the first-order ring intensity of the Quételettype interference peaks when the polarization of the probe beam is parallel to the director axis of the LC molecules. Also, it is shown that the number of the scatterers, the order parameter of the liquid crystals and the applied electric field affect the ring intensities. A simple model is proposed to explain these observations.

Scattering light interference from liquid crystal polymer dispersion films

The Quetelet-type ring pattern is observed in liquid crystal polymer dispersion (LCPD) films. The clusters of the polymer network and liquid crystal (LC) domains with different director axes in the LCPD films serve as scatterers. Cells with unidirectional and multidirectional rubbins are fabricated. Experimental results show that the polarization of incident light, the applied voltage and the ambient temperature significantly affect the ring intensities. However, the contribution of the LC domains is not evident until the voltage is applied. Finally, rubbing the cells in multiple directions reveals that measurement of the Quetelet-type ring intensity can be used to readily identify the orientation of the liquid crystals. This finding also reveals that the LCs in an LCPD mixture are aligned closer to the final rubbing direction than are pure LCs in a multidirectional rubbed cell. A simple model was proposed to explain the observations.