Ko-Ting Cheng

Electrically switchable and thermally erasable biphotonic holographic gratings in dye-doped liquid crystal films

This study investigates high-resolution photoinduced biphotonic holographic gratings in azo-dye-doped liquid crystal films. A biphotonic grating (BG) is formed under the illumination of one linearly polarized green light with the simultaneous irradiation of an interference pattern created by two linearly polarized red lights. This study ascribes the formation of this grating to two mechanisms. One mechanism is the green-light-inducing strong dye absorption followed by adsorption through the trans–cis isomerization; the other mechanism is the inhibition effect of adsorption induced by the red light through the cis–trans inverse isomerization. These produce a twisted nematic structure-modulated pattern, which, in turn, causes the BG. Additional experiments demonstrate that the formed BGs are electrically switchable and thermally erasable.

Electrically switchable and optically rewritable reflective Fresnel zone plate in dye-doped cholesteric liquid crystals

This work demonstrates a reflective Fresnel zone plate based on dye-doped cholesteric liquid crystals (DDCLC) using the photo-induced realignment technique. Illumination of a DDCLC film with a laser beam through a Fresnel-zone-plate mask yields a reflective lens with binary-amplitude structures - planar and focal conic textures, which reflect and scatter probed light, respectively. The formed lens persists without any external disturbance, and its focusing efficiency, analyzed using circularly polarized light, is ~ 23.7%, which almost equals the measured diffraction efficiency of the used Fresnel-zone-plate mask (~ 25.6%). The lens is thermally erasable, rewritable and switchable between focusing and defocusing states, upon application of a voltage.

Polarizer-free, electrically switchable and optically rewritable displays based on dye-doped polymer-dispersed liquid crystals.

This work demonstrates the feasibility of a polarizer-free, electrically switchable and optically rewritable display based on dye-doped polymer-dispersed liquid crystals (DD-PDLCs). Experimental results indicate that the doped dyes are homeotropically adsorbed onto the polymer film when an appropriate AC voltage is applied during patterning. The adsorbed dyes in the illuminated region then align the liquid crystals homeotropically, and produce a transparent pattern in the scattering background without any polarizer. Notably, the adsorbed dyes can be erased and readsorbed using thermal and optical treatments, respectively. The switching time of the fabricated display is of the order of milliseconds, and the contrast ratio is approximately 30.

Variable liquid crystal pretilt angles generated by photoalignment in homeotropically aligned azo dye-doped liquid crystals

This letter demonstrates the feasibility of producing variable liquid crystal (LC) pretilt angles using light-induced ripple structures (LIRSs) in homeotropically aligned azo dye-doped liquid crystals (ADDLCs). Illuminating homeotropically aligned ADDLCs with a linearly polarized light for a suitable period yields the LIRSs which provide LCs an anisotropic homogeneous anchoring force. Experimentally, the effective alignment force produced by the homeotropic alignment layer and the LIRSs determines the LC pretilt angle (24° to 63.5°), defined from the normal to the substrate. A no-bias pi cell for liquid crystal displays is demonstrated using this approach.

Fast Optical Recording of Polarization Holographic Grating Based on an Azo-Dye-Doped Polymer-Ball-Type Polymer-Dispersed Liquid Crystal Film

This work reports a fast optical recording material based on an azo-dye-doped polymer-ball-type polymer-dispersed liquid crystal film. A memory polarization holographic grating is written on the film and formed in � 2 ms using two writing beams derived froma Q-switched Nd:YAG SHG pulse laser ( � w ¼ 532 nm) with duration of 20 ns and a total energy density of � 15 mJ/cm 2 . The polarization grating (PG) is created fromthe photo-excited dye m olecules. These photo-excited dye molecules undergo three-dimensional rotation, diffusion, then adsorb on the surface of the polymer balls, and finally induce the reorientation of the LCs. Polarization analysis of the PG reveals that it diffracts the linearly polarized incident light into beams with different polarizations. A model based on the Jones matrix method is also proposed and closely fits the experiment results. Experimental results indicate that the PG is electrically switched (� 3 V/mm). The PG can be erased partially by thermal treatment. We speculate that the weakly-adsorbed dyes desorb from the surfaces of the polymer balls owing to thermal disturbance, causing partial grating erasion. [DOI: 10.1143/JJAP.42.4406]

Particular thermally induced phase separation of liquid crystal and poly(N-vinyl carbazole) films and its application

In the current study, a method of particular thermally induced phase separation (TIPS) of liquid crystals (LCs) and polymers is presented. The method involves a combination of dissolution process and TIPS. The LCs and poly(N-vinyl carbazole) (PVK) play the roles of solvent and solute, respectively, during the processes of particular TIPS. The nematic LC sample fabricated by two substrates coated with uniform PVK films is heated and then cooled, generating the rough PVK layers onto the surfaces of the substrates. The LC sample having rough PVK layers produces micron-sized, multiple domains of disordered LCs that can scatter incident light. Additionally, an application of a scattering mode light shutter, having the advantages of low driving voltage, polarization-independent scattering, fast response, high contrast ratio, and being polarizer free, is reported.

Binary liquid crystal alignments based on photoalignment in azo dye-doped liquid crystals and their application

This work demonstrates the feasibility of binary liquid crystal (LC) alignments, in which two forms of LC alignment are in a single pixel or a specific area using a surface-treated alignment layer and a photoalignment film of the adsorption of azo dyes onto the polymer surface in azo dye-doped liquid crystals. Binary LC alignments that involve two of common LC alignments, which are homogeneous, homeotropic, hybrid, and twisted nematic alignments, are initially demonstrated. Then, a binary LC alignment that comprises hybrid and homeotropic alignments is adopted to fabricate a viewing-angle-dependent liquid crystal display, which displays two images by changing viewing angles.

Direct optical switching of bistable cholesteric textures in chiral azobenzene-doped liquid crystals

Bistable cholesteric textures (i.e., planar and focal conic textures) in chiral azobenzene-doped liquid crystals (LCs) is demonstrated. Chiral azobenzene is a chiral dopant with optically tuned helical twisting power that results from the photo-isomerization between trans- and cis-isomers via exposure to UV or visible light. The pitch length of the material can be optically and repeatedly elongated and shortened. With regard to free energy, LCs tend to be stable at planar (focal conic) textures when pitch length is elongated (shortened) by exposure to UV (visible) light. Thus, direct optical switchable LC displays are investigated.

Radial liquid crystal alignment based on circular rubbing of a substrate coated with poly(N-vinyl carbazole) film

This paper presents a simple method to produce radial liquid crystal (LC) alignment layers using circular rubbing of poly(N-vinyl carbazole) (PVK) films. The produced layer can be used for fabricating axially symmetric homogeneous?radial, homeotropic?radial and radial?radial LC alignment devices by combining a rubbed PVK-coated substrate with another one with a desired LC alignment layer. The transmittance?voltage curves of the fabricated LC devices at various positions are measured to examine the uniformity of the alignment effect. Additionally, the PVK film does not absorb visible light, and can be operated at high temperatures.

Designs of High Color Purity RGB Color Filter for Liquid Crystal Displays Applications Using Fabry–Perot Etalons

Two designs of high color purity RGB color filter (RGB-CF) using Fabry-Perot etalon were proposed. First, various transmission spectra for corresponding pixel arrays were achieved by optimizing the orientation of fixed polymerized liquid crystal polymers with high birefringence. The designed high color purity RGB-CF is believed to have potential for display applications. Second, three primary transmissive peaks for each pixel were achieved with the optimized air gap. Compensated CFs were adopted to enhance color purity by absorbing undesirable transmissive light. The second design is to overcome the limitation of the cell gap in the first design. Additionally, two approaches to improve light utilization efficiency were discussed.